Book of Abstract

Collective Electronic Excitations in 2D FP7 INDEX Conference Scuola Normale Superiore | Pisa IT September 20-24 | 2015

Organizing Scientific Committee François Dubin, ICFO (ES), Vittorio Pellegrini, Scuola Normale Superiore & IIT (IT), Aron Pinczuk, Columbia University (USA), David Ritchie, University of Cambridge (UK), Massimo Rontani, Cnr Nano (IT), Thomas Satzoukidis, Scuola Normale Superiore (IT), Masha Vladimirova, CNRS (FR). Organizing Committee Thomas Satzoukidis, Elisa Guidi, Gisella Chiné, Scuola Normale Superiore (IT), Luisa Neri, CNR NANO (IT).

Websites http://web.nano.cnr.it/CEE2D/ http://indexitn.coulomb.univ-montp2.fr/

CEE2D | PISA IT | 2015 3

INDEX

PROGRAM 5

POSTER SESSION 9

ABSTRACTS OF TALKS 11

ABSTRACTS OF POSTERS 40

INDEX

PROGRAM

POSTER SESSION

ABSTRACTS OF TALKS

ABSTRACTS OF POSTERS

CEE2D | PISA IT | 2015 5 PROGRAM Sunday September 20th

18:30 Welcome cocktail, Sala Gran Priore in Scuola Normale Superiore.

Monday September 21st

08:30-09:15 Registration

09:15-09:30 Conference opening: V. Pellegrini, M. Rontani

Aleksey K. Fedorov, Russian Quantum Center, Moscow (RU) & LPTMS, CNRS (FR)* 09:30-10:15 Roton Phenomena of weakly interacting dipolar excitons in a semiconductor layer

Anton Nalitov, Clermont Ferrand University (FR) 10:15-10:45 Topological polariton states and Kibble-Zurek mechanism in zigzag chains of pillar microcavities

10:45-11:15 Coffee Break

Cory Dean, Columbia University (US)* 11:15-12:00 Hofstadter’s Butterfly in the strongly interacting regime

Massimo Rontani, CNR NANO S3, Modena (IT)* 12:00-12:45 Giant orbital magnetic moment and spin-orbit coupling of a carbon nanotube as an excitonic insulator

12:45-14:30 Lunch Break

Serguei Andreev, CNRS, Orsay (FR) 14:30-15:00 Resonantly paired gas of dipolar excitons

Fedor Fedickhin, Laboratoire Charles Coulomb, University of Montpellier (FR)* 15:00-15:30 Transport of indirect excitons in polar GaN/AlGaN quantum well structures grown on sapphire and GaN substrates

Dario Ballarini, NNL Istituto di Nanotecnologie CNR, Lecce (IT) 15:30-16:00 Non linear interactions in high-speed organic polariton flow at room temperature

16:00-16:30 Coffee Break

Andrea Gamucci, Laboratorio Nest Scuola Normale Superiore, Pisa (IT)* 16:30-17:15 Evidence for electron-hole pairing in graphene-GaAs double layers

Guido Pupillo, CNRS Strasbourg (FR) 17:15-17:45 Cavity enhanced transport of excitons and charge

6 www.web.nano.cnr.it/CEE2D Tuesday September 22nd Yuliya Kuznetsova, Columbia University, New York, NY (US)* 09:00-09:45 Artificial Graphene: Lattices in Nano-patterned Semiconductors

François Dubin, ICFO (ES)* 09:45-10:30 Looking for the dark exciton condensate

10:30-11:00 Coffee Break

Daniele Sanvitto, NNL Istituto di Nanotecnologie CNR (IT)* 11:00-11:45 Quantum fluid dynamics of polariton condensates

Ursula Wurstbauer, Technische Universität München TUM & Nanosystems 11:45-12:30 Initiative Munich NIM (DE)* Confocal shift interferometry of coherent emission from trapped dipolar excitons

Free afternoon, social event, dinner

Wednesday September 23rd

Jeremy Baumberg, University of Cambridge (UK)* 09:00-09:45 Ultralow Energy Switching of Ferromagnetic Condensates in Semiconductor Microcavities

Masha Vladimirova, CNRS & Université de Montpellier (FR)* 09:45-10:30 Spin coherence in coupled quantum wells amended by in-plane magnetic field

10:30-11:00 Coffee break

11:00-11:45 Alexey Kavokin, University of Southampton (UK)*

Jelena Klinovaja, University of Basel (CH)* 11:45-12:30 Engineering Topological Quantum States: From 1D to 2D

12:30-14:30 Lunch break

Peristera Andreakou, Laboratoire de Photonique et Nanostructures, CNRS, Marcoussis (FR)* 14:30-15:15 Transport of indirect excitons in coupled quantum wells: Basic physics and devices

Karsten Leding Vendelbjerg, Cnr Nano S3 & University of Modena & Reggio 15:15-15:45 Emilia, Modena (IT) Manipulation of spin transfer torque using light

CEE2D | PISA IT | 2015 7

15:45-16:15 Coffee break

16:15-18:30 Poster session

18:30-20:00 SB meeting Index Project

20:00-22:00 INDEX dinner

Thursday September 24th Rainer Mahrt, IBM Research Zurich (CH)* 09:00-09:45 Bose-Einstein Condensation in a Polymer: Towards Quantum Simulation

Ugo Siciliani de Cumis, University of Cambridge (UK) 09:45-10:15 Electronic transport and optical properties of electrically-generated electron- hole bilayers with minimum inter-layer separation

Vanik Shahnazaryan, University of Iceland, Reykjavik (IS) 10:15-10:45 Adiabatic preparation of a cold exciton condensate

10:45-11:15 Coffee break

Marco Polini, IIT Graphene Labs & Nest Scuola Normale Superiore, Pisa (IT)* 11:15-12:00 Collective non-local transport in graphene heterostructures

Federico Grasselli, University of Modena & Reggio Emilia & Cnr Nano S3, 12:00-12:30 Modena (IT) Time-Dependent Quantum Dynamics of 2D Spatially Indirect Excitons 12:30-14:30 Lunch break

Carlo Andrea Rozzi, CNR NANO S3, Modena (IT)* 14:30-15:15 Ultrafast dynamics in light-harvesting and photovoltaics: a theoretical and experimental investigation

Gabriel Gil, Cnr Nano S3 & University of Modena & Reggio Emilia, Modena (IT) 15:15-15:45 Excitation energy transfer in hybrid nano-systems by a multi-scale beyond- Förster approach

15:45 Conference closing, discussion remarks

* Invited Speaker

8 www.web.nano.cnr.it/CEE2D

INDEX

PROGRAM

POSTER SESSION

ABSTRACTS OF TALKS

ABSTRACTS OF POSTERS

CEE2D | PISA IT | 2015 9 POSTER SESSION

Room-Temperature Rectification In Graphene/LAO/STO Heterostructures 40 I. Aliaj, E. di Gennaro, V. Miseikis, I. Torre, A. Gamucci, C. Coletti, M. Polini, F. M. Granozio, V. Pellegrini, F. Beltram, and S. Roddaro

Macroscopic Occupation Of Dark Excitonic States In A Trap 41 Mussie Beian, Mathieu Alloing, Romain Anankine, Edmond Cambril, Carmen Gomez Carbonell, Aristide Lemaître, and François Dubin

Lateral Transport Of Indirect Exciton Spins In Double Quantum Well Structures 42 S. Büyükköse, C. Hubert, A. Violante, and P. V. Santos

Oxide Heterostructures As A Possible Innovative Road For Studying Indirect Excitons 43 L. Maritato, A. Galdi, C. Sacco, and D.G. Schlom

Cavity Polaritons Under The Influence Of The Landau Quantization, Rashba Spin-Orbit 44 Coupling, Zeeman Splitting And Gyrotropy Effects S. A. Moskalenko, I.V. Podlesny, E.V. Dumanov, and M.A. Liberman

Magnetoexciton In Nanoring Of Non-Uniform Thickness 45 L. C. Porras and I. D. Mikhailov

Biased Controlled Dipole Oriented Polariton Bistability 46 Pramod K Sharma, Simeon I. Tsintzos, Gabriel Christmann, Zacharias Hatzopoulos, Jeremy J Baumberg, and Pavlos G Savvidis

Exciton Dynamics In Disk-Like Quantum Dots With A Magnetic Impurity 47 V. Moldoveanu, I. V. Dinu, R. Dragomir, and B. Tanatar

Quantum Rings And Quantum Dots: Optical Properties And Annealing Process 48 M. Triki, D. Elmaghraoui, and S. Jaziri

10 www.web.nano.cnr.it/CEE2D

INDEX

PROGRAM

POSTER SESSION

ABSTRACTS OF TALKS

ABSTRACTS OF POSTERS

CEE2D | PISA IT | 2015 11 ROTON PHENOMENA OF WEAKLY INTERACTING DIPOLAR EXCITONS IN A SEMICONDUCTOR LAYER

A.K. Fedorov1,2, I.L. Kurbakov3, and. Yu.E. Lozovik3

1Russian Quantum Center, Skolkovo, Moscow 143025, Russia 2LPTMS, CNRS, Univ Paris Sud, Universite Paris-Saclay, Orsay 91405, France 3Institute of Spectroscopy, Russian Academy of Sciences, Troitsk, Moscow 142190, Russia

Many-body systems with dipole-dipole interaction provide an interface between physics of strongly and weakly correlated quantum matter. The anisotropy and the region of attraction of the dipole- dipole interaction potential provide a set of interesting many-body phenomena. In the limit of strongly correlated system of in-plane dipoles, the ground state of the system has the chain structure; the 3D system of parallel dipoles has the chain structure as well.

Being typical for strongly correlated systems, local minimum in a non-monotonic excitation spectrum – roton-maxon character – originally observed in liquid helium appears in a weakly interacting gas [1]. This facts itself opens fascinating a prospective for revealing of non- conventional structural properties of dipolar condensates close to the threshold of instability.

In this contribution, we predict the formation of the roton-maxon excitation spectrum and the roton instability effect for a weakly correlated Bose gas of dipolar excitons [2]. The formation of the roton minimum and the roton instability is the result of important features of the dipole- dipole interaction – anisotropy and attraction region – as well as the layer geometry, which passes through unstable 3D and stable 2D regimes. According to numerical estimations, the threshold of the roton instability for Bose-Einstein condensed exciton gas with roton-maxon spectrum is achievable experimentally in GaAs semiconductor layers.

References [1] L. Santos, G.V. Shlyapnikov, and M. Lewenstein, Phys. Rev. Lett. 90, 250403 (2003). [2] A.K. Fedorov, I.L. Kurbakov, and Yu.E. Lozovik, Phys. Rev. B 90, 165430 (2014).

12 www.web.nano.cnr.it/CEE2D TOPOLOGICAL POLARITON STATES AND KIBBLE-ZUREK MECHANISM IN ZIGZAG CHAINS OF PILLAR MICROCAVITIES

D. D. Solnyshkov, A. V. Nalitov, and G. Malpuech

Institut Pascal, PHOTON-N2, Université Clermont Auvergne, CNRS, 4 Avenue Blaise Pascal, 63178 Aubière Cedex, France.

We theoretically consider a zigzag chain of coupled pillar microcavities, taking into account the polarization. We show that the TE-TM splitting leads to the appearance of topological polariton states at the edges of the chain. Their polarization depends on the parity of the number of pillars. This follows from a tight-binding model and is confirmed by full numerical simulations. A random choice of polarization of the bulk states during the condensate formation is equivalent to dimerization of polymer chains. We show that topological defects appear as domain walls between polarization domains, analogous to Su-Shriffer-Heeger solitons in polymers. Their density in a polariton condensate is governed by the condensation quench speed via Kibble-Zurek mechanism.

Reference [1] D. D. Solnyshkov, A. V. Nalitov and G. Malpuech, arXiv:1506.04626 (2015).

CEE2D | PISA IT | 2015 13 HOFSTADTER’S BUTTERFLY IN THE STRONGLY INTERACTING REGIME

Cory Dean

Columbia University, New York City, NY, USA

In 1976, Douglas Hofstadter predicted that in the presence of both a strong magnetic field, and a spatially varying periodic potential, Bloch electrons confined to a 2D quantum well exhibit a self- similar fractal energy spectrum known as the "Hofstadter's Butterfly". In subsequent years, experimental discovery of the quantum Hall effect gave birth to an expansive field of research into 2D electronic systems in the presence of a magnetic field, however, direct confirmation of the fractal spectrum remained elusive. Recently we demonstrated that graphene, in which Bloch electrons can be described by Dirac fermions, provides a new opportunity to investigate this nearly 40 year old problem. In this talk I will discuss the experimental realization of Hofstader's butterfly by exploiting nano-scale interfacial effects between graphene and hexagonal boron nitride substrates, together with application of extremely high magnetic fields. Utilizing newly developed techniques to fabricate ultra-clean graphene devices, I will additionally discus the capability to probe for the first time the effect of strong electron interactions , within the fractal Hofstadter spectrum.

14 www.web.nano.cnr.it/CEE2D GIANT ORBITAL MAGNETIC MOMENT AND SPIN-ORBIT COUPLING OF A CARBON NANOTUBE AS AN EXCITONIC INSULATOR

M. Rontani

CNR-NANO, Research Center S3, Via Campi 213a 41125 Modena Italy, [email protected]

We suggest that an undoped carbon nanotube might be an excitonic insulator---the long-sought phase of matter proposed by Keldysh, Kohn, and others fifty years ago. We show theoretically that the condensation of triplet excitons, driven by intervalley exchange interaction, spontaneously occurs at equilibrium if the tube radius is sufficiently small [1]. Our claim contradicts previous studies that neglected the coupling between K and K’ valleys [2]. We predict that the signatures of exciton condensation are its sizable contributions to (i) the energy gap, (ii) the spin-orbit interaction, and (iii) the magnetic moment per electron. The increase of the gap might have already been measured, albeit attributed to the Mott-Hubbard insulating state [3], whereas giant values of spin- orbit energy splittings were recently reported with no explanation [4]. The enhancement of the quasiparticle magnetic moment is a pair-breaking effect that counteracts the weak paramagnetism of the ground-state condensate of excitons. This property could rationalize the anomalous magnitude of magnetic moments recently observed in different ultraclean devices close to charge neutrality [5].

Figure 1. Orbital magnetic moment (left) and spin-orbit energy gap (right) vs radius of a nominally metallic carbon nanotube. The inter-valley exchange interaction w2 drives the excitonic insulator phase enhancing both observables with respect to their non-interacting values (respectively the straight line in the left panel and the dashed line in the right panel). This work was supported by EU-FP7 Marie Curie ITN INDEX and MIUR PRIN MEMO.

References [1] M. Rontani, Phys. Rev. B 90, 195415 (2014). [2] T. Ando, J. Phys. Soc. Jpn. 66, 1066 (1997); R. R. Hartmann et al., Phys. Rev. B 84, 0354437 (2011). [3] V. V. Deshpande et al., Science 323, 106 (2009). [4] G. A. Steele et al., Nat. Commun. 4, 1573 (2013). [5] E. A. Laird et al., Rev. Mod. Phys. (2015), in press, available at arXiv:1403.6113.

CEE2D | PISA IT | 2015 15 RESONANTLY PAIRED GAS OF DIPOLAR EXCITONS

Sergey Andreev

Laboratoire de la Physique Théorique et des Modèles Statistiques (LPTMS), CNRS, Orsay, France ITMO University, St. Petersburg, Russia

The phenomenon of resonant pairing lies at the heart of superconductivity in metals. Here Cooper pairs of fermionic particles – electrons, can Bose-Einstein condense to form a superfluid state. Advances in the field of ultracold atomic gases have allowed a thorough study of resonantly paired fermionic superfluids [1] and have also inspired theorists to envisage their bosonic counterpart [2]. However, experimental realization of a bosonic analog of the Bardeen-Cooper-Schrieffer (BCS) superconductor have so far been precluded by a short lifetime of the atomic molecules. In this talk I suggest that the quantum molecular phase of bosons may already have been observed in the experiments on long-living dipolar excitons in semiconductor quantum wells. A striking feature discovered in these experiments is the spontaneous fragmentation of the photoluminescent ring of cold excitons into an array of bright spots, dubbed the Macroscopically Ordered Exciton State (MOES) [3]. I develop my earlier theoretical idea [4,5] to show, that the MOES appears as a result of an interplay between the dipolar repulsion and resonant pairing of excitons. The molecular phase occupies the central part of each spot and can be distinguished by shift-interferometry measurements of the exciton photoluminescence.

References [1] See V. Gurarie and L. Radzihovsky, Ann. Phys. 322, 2 (2007) and references therein. [2] L. Radzihovsky, J. Park, and P. B. Weichman Phys. Rev. Lett. 92, 160402 (2004). [3] L. V. Butov, A. C. Gossard, and D. S. Chemla, Nature (London) 418, 751 (2002). [4] S. V. Andreev, Phys. Rev. Lett. 110, 146401 (2013). [5] To be published in Rapid Communication section of PRB in 2015.

16 www.web.nano.cnr.it/CEE2D TRANSPORT OF INDIRECT EXCITONS IN POLAR GAN/ALGAN QUANTUM WELL STRUCTURES GROWN ON SAPPHIRE AND GAN SUBSTRATES

F. Fedichkin1, B. Jouault1, M. Vladimirova1, T. Guillet1, C. Brimont1, P. Valvin1, T. Bretagnon1, N. Grandjean2, and P. Lefebvre1

1Laboratoire Charles Coulomb, CNRS-INP-UMR 5221, Université Montpellier 2, F-34095 Montpellier, France 2Institute of Condensed Matter Physics, EPFL, CH-1015 Lausanne, Switzerland

Spatially resolved PL patterns measured at 10 K for 7 m-wide GaN/Al0.19Ga0.81N QWs grown on GaN (a) and sapphire (b) substrates, respectively. The PL blue-shift is proportional to the exciton density, the spatial dependence of which can therefore be determined. In (c) and (d), we plotted the full widths at half maximum of this dependence, versus temperature, for different excitation powers. Clearly, for the GaN substrate, we observe pure excitonic propagation, free of the secondary, parasitic PL. Moreover, contrary to the case of sapphire substrate, suppression of nonradiative recombination, with GaN substrate, allows for an excitonic propagation above 30-35 μm, that is not quenched by increasing temperature. An indirect exciton (IX) is a quasiparticle consisting of an electron and a hole spatially separated in two different planes of a quantum nanostructure, thus exhibiting a strongly dipolar character. Current research on transport properties of IXs is opening a pathway to the development of novel optoelectronic devices, which have already been demonstrated in GaAs-based heterostructures [1- 3]. Applying the same ideas to IXs in wide-band gap polar quantum wells (QWs) is particularly promising because of much larger exciton binding energies and natural dipoles induced by strong built-in electric fields. We have recently studied the transport of IXs in GaN/AlGaN QWs grown on sapphire substrates, at temperatures up to 80 K [4], by mapping the micro-photoluminescence (μ- PL) signal obtained under intense, point excitation. The low-temperature PL recorded at long distances from the excitation spot (30 < r < 100μm) turned out to be a secondary PL, excited by the light emitted at the central spot, guided along the plane, due to the refractive index contrast between the layer and the substrate. At higher temperatures, this signal is rapidly quenched and the distance reached by the measurable PL is limited by recombination of excitons at non-radiative defects. Using GaN substrates instead of sapphire should both suppress the secondary emission and the nonradiative recombination, by reducing dislocation densities by 3-4 orders of magnitude. In this work, we compare exciton propagations in two GaN/Al0.19Ga0.81N QWs of identical structures, except for the substrates, respectively of GaN and sapphire. For the GaN substrate, we indeed observe the mere propagation of excitons up to 35 μm away from the excitation spot and up to 250 K (see below). We propose a drift/diffusion modelling of exciton transport, accounting for dipole- dipole repulsion in high-density regions and for disorder along the sample plane.

Spatially resolved PL patterns measured at 10 K for 7 m-wide GaN/Al0.19Ga0.81N QWs grown on GaN (a) and sapphire (b) substrates, respectively. The PL blue-shift is proportional to the exciton density, the spatial dependence of which can therefore be determined. In (c) and (d), we plotted the full widths at half maximum of this dependence, versus temperature, for different excitation powers. Clearly, for the GaN substrate, we observe pure excitonic propagation, free of the secondary, parasitic PL. Moreover, contrary to the case of sapphire substrate, suppression of nonradiative recombination, with GaN substrate, allows for an excitonic propagation above 30-35 μm, that is not quenched by increasing temperature.

CEE2D | PISA IT | 2015 17

References [1] Y. Y. Kuznetsova, M. Remeika, A. A. High, A. T. Hammack, L. V. Butov, M. Hanson, and A. C. Gossard, Optics Letters 35 (10), 1587 (2010). [2] A.A. High, E.E. Novitskaya, L.V. Butov, and A.C. Gossard, Science 321, 229 (2008). [3] A.A. High, A.T. Hammack, L.V. Butov, and A.C. Gossard, Optics Letters 32, 2466 (2007). [4] F. Fedichkin, P. Andreakou, B. Jouault, M. Vladimirova, T. Guillet, C. Brimont, P. Valvin, T. Bretagnon, A. Dussaigne, N. Grandjean, P. Lefebvre, Phys. Rev. B 91, 205424 (2015).

18 www.web.nano.cnr.it/CEE2D NON LINEAR INTERACTIONS IN HIGH-SPEED ORGANIC POLARITON FLOW AT ROOM TEMPERATURE

G. Lerario1, D. Ballarini1, A. Fieramosca1, A. Cannavale1, A. Genco1, F. Mangione1, S. Gambino1,2, L. Dominici1,2, M. De Giorgi1, G. Gigli1,3, and D. Sanvitto1

1NNL, Istituto di Nanotecnologie - CNR, Via Arnesano, 73100, Lecce, Italy 2CBN-IIT, Istituto Italiano di Tecnologia, Via Barsanti, 73100, Lecce, Italy 3Università del Salento, Via Arnesano, 73100 Lecce, Italy

One efficient way to dress photons with interactions is through their strong-coupling with the electronic dipole of excitons. The physics of strong-coupling regime between the electromagnetic and the excitonic fields in semiconductors is described by new eigenmodes of the coupled system - exciton-polaritons - which allowed the observation of condensation and superfluidity in microcavity structures.[1] Once polaritons have been put in motion in the plane of the cavity, the combination of small mass and non-linear interactions provides a fertile ground also for investigations on all-optical devices and circuits.[2-4] However, polariton propagation has been observed so far only in GaAs- based microcavities at cryogenic temperature. Here we demonstrate exciton-polaritons propagating at room temperature for long distances (≈ 150 micron) and high group velocity (50% the speed of light) in an organic layer coupled to a Bloch Surface Wave (BSW). Crucially, this is the first time that, in organic materials, the energy resonance of the propagating mode is controlled by increasing or decreasing the polariton density through resonant excitation. This is of potential relevance for the implementation of fast switches and polariton devices, but also for the effect of nonlinear interactions in a wide range of accelerating optical wave-packets.[5] The BSW is a non-radiative surface optical mode located within the band gap of the periodic mirror and which, coupled to a thin layer of small organic molecules, assures long lifetime and fast group velocities for the Bloch Surface Wave-Polaritons (BSWP).[6] The optical measurements have been performed through leakage microscopy of the evanescent waves from the top of the structure. The typical anticrossing behaviour is observed from the photoluminescence dispersion under non- resonant continuous-wave excitation as shown in Fig. 1a. The blue-shift of the BSWP energy is reversible, red-shifting for lower excitation power. In Fig. 1b the low-power (blue line) and high- power (red line) reflectance spectra are shown. In Fig. 1c, an experimental image of propagating BSWP is shown, with the polariton flow evidenced by the presence of few defects on the sample surface.

References [1] T. Byrnes, N. Y. Kim, and Y. Yamamoto, Nat. Phys. 10, 803 (2014). [2] I. Carusotto and C. Ciuti, Rev. Mod. Phys. 85, 299 (2013). [3] D. Ballarini, M. De Giorgi, E. Cancellieri, R. Houdré, E. Giacobino, R. Cingolani, A. Bramati, G. Gigli, and D. Sanvitto, Nat. Comm. 4, 1778 (2013). [4] T. C. H. Liew, A. V. Kavokin, and I. A. Shelykh, Phys. Rev. Lett. 101, 016402 (2008). [5] P. Zhang, Y. Hu, T. Li, D. Cannan, X. Yin, R. Morandotti, Z. Chen, and X. Zhang, Phys. Rev. Lett. 109, 193901 (2012). [6] G. Lerario, A. Cannavale, D. Ballarini, L. Dominici, M. De Giorgi, M. Liscidini, D. Gerace, D. Sanvitto, and G. Gigli, Optics Letters 39, 2068 (2014).

CEE2D | PISA IT | 2015 19

Figure 1: a) Dispersion relation (energy vs in-plane momentum) showing the coupled BSWP (dashed white line superimposed to the experimental data as obtained from PL under non-resonant excitation), the light line (red line), the exciton (yellow line) and the bare BSW (green line). b) Reflectance measurements showing the BSWP resonance at low power (blue line) and high power (red line), showing a blueshift of about 1.8 meV. c) Real space two-dimensional experimental image of BSWP propagation. The polariton flow, propagating downwards, is highlighted by the presence of few defects on the surface of the sample structure.

20 www.web.nano.cnr.it/CEE2D EVIDENCE FOR ELECTRON-HOLE PAIRING IN GRAPHENE-GAAS DOUBLE LAYERS

A. Gamucci1,2, D. Spirito1,2, M. Carrega1, B. Karmakar1, A. Lombardo3, M. Bruna3, L. N. Pfeiffer4, K. W. West4, A. C. Ferrari3, M. Polini1,2, V. Pellegrini1,2

1National Enterprise for Nanoscience and Nanotechnology (NEST), Istituto Nanoscienze-Consiglio Nazionale delle Ricerche and Scuola Normale Superiore, I-56126 Pisa, Italy 2Istituto Italiano di Tecnologia, Graphene labs, Via Morego 30, I-16163 Genova, Italy 3Cambridge Graphene Centre, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0FA, UK 4Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08540, USA

Spatially-separated two-dimensional systems of electrons and holes are predicted to condense below a critical temperature into a neutral superfluid state of electron-hole pairs, called “exciton condensate”. Evidence of this scenario will be presented in systems of massless Dirac holes in a graphene flake in close proximity of electrons hosted in a gallium arsenide quantum well. A logarithmic enhancement of Coulomb drag at zero magnetic field and temperatures below 5 K, which we attribute to pairing fluctuations extending above the temperature for exciton condensation, has been found experimentally. Our Dirac-Schrödinger hybrid system offers a new benchmark to study superfluidity in reduced spatial dimensions.

CEE2D | PISA IT | 2015 21 CAVITY ENHANCED TRANSPORT OF EXCITONS AND CHARGE

J. Schachenmayer1, C. Genes2, E. Tignone3, G. Pupillo3

1JILA, NIST, Department of Physics, University of Colorado, USA 2Institut für Theoretische Physik, Universitaet Innsbruck, Austria 3University of Strasbourg and CNRS, Strasbourg, France

We show that exciton-type transport in certain materials can be dramatically modified by their inclusion in an optical cavity: the modification of the electromagnetic vacuum mode structure introduced by the cavity leads to transport via delocalized polariton modes rather than through tunneling processes in the material itself. This can help overcome exponential suppression of transmission properties as a function of the system size in the case of disorder and other imperfections. We exemplify massive improvement of transmission for excitonic wave-packets through a cavity, as well as enhancement of steady-state exciton currents under incoherent pumping. These results may have implications for experiments of exciton transport in disordered organic materials [1].

We further report on recent experiments and theory [2] on charge transport in disordered molecular semiconductors strongly coupled to the vacuum field on plasmonic structures: the coupling to the electromagnetic field enhances charge transport by an order of magnitude at resonance in the coupled state. A theoretical model is presented that confirms the delocalization of the wave- functions of the light-hybridized states and the consequences on conductivity. While this is a proof- of-principle study, conductivity mediated by light-matter hybridized states is easy to implement and we expect that it will be used to improve organic devices.

References [1] J. Schachenmayer, C. Genes, E. Tignone, and G. Pupillo, Phys. Rev. Lett. 114, 196403 (2015). [2] E. Orgiu, J. George, J. A. Hutchison, E. Devaux, J. F. Dayen, B. Doudin, F. Stellacci, C. Genet, J. Schachenmayer, C. Genes, G. Pupillo, P. Samori, and T. W. Ebbesen, arXiv:1409.1900, Nature Materials (in press, 2015).

22 www.web.nano.cnr.it/CEE2D ARTIFICIAL GRAPHENE: LATTICES IN NANO-PATTERNED SEMICONDUCTORS*

Yuliya Kuznetsova

Dept. of Physics, Columbia University, New York, NY, USA

Charge carriers in graphene behave as massless Dirac fermions (MDFs) with a linear energy- momentum dispersion. MDFs provide a platform for the studies of quasiparticles with relativistic- like dispersion and of topological protection in the presence of a large spin-orbit coupling. Engineered lattices with a honeycomb topology, called artificial graphene (AG), form a condensed- matter platform for exploration of the new physics that emerges due to the tunable interplay between topology and quasiparticle interactions. In the research reported here, artificial graphene is realized by superimposing a small lattice constant (in this work, as small as 40 nm) honeycomb potential on a two-dimensional electron gas hosted by a GaAs quantum well.

Optical methods form an effective probe of the band structure and other properties of electrons in the engineered potential. Resonant inelastic light scattering spectra reveal transitions in artificial graphene that are interpreted in terms of joint density for transitions between the AG bands. The agreement of observed transitions with calculated joint density of states of the AG band structure offers evidence of the occurrence of Dirac bands and MDFs in the artificial lattice. The ability to create small-period artificial graphene lattices in semiconductors opens new, highly tunable pathways for the exploration of fundamental condensed matter science, including the possibility of access to novel topological phases in semiconductors with strong spin-orbit coupling.

This work is done in collaboration with: Sheng Wang, Diego Scarabelli, Loren N. Pfeiffer, Ken West, Geoff C. Gardner, Michael J. Manfra, Vittorio Pellegrini, Shalom J. Wind, and Aron Pinczuk.

(*) Supported by the US Department of Energy, Basic Energy Sciences award DE-SC0010695.

CEE2D | PISA IT | 2015 23 LOOKING FOR THE DARK EXCITON CONDENSATE

François Dubin

ICFO-The Institute of Photonic Sciences, Av. Carl Friedrich Gauss, num. 3, 08860 Castelldefels (Barcelona), Spain

Predicted in the 1960’s, Bose-Einstein condensation (BEC) of semiconductor excitons remains an open question both theoretically and experimentally. This situation was not foreseen since excitons are light mass quasi-particles made by a Coulomb bound electron-hole pair. BEC is thus expected to occur below a few Kelvins, i.e. directly accessible by standard cryogeny. Nevertheless, it has so far not be signalled by the long expected strong photoluminescence emitted by a condensed population of low energy bright excitons. Indeed, recent theoretical works have pointed out that the excitons ground-state being optically inactive, i.e. dark, BEC has then to be signalled by a macroscopic population of dark excitons [1]. These can naturally not be directly observed optically, nevertheless here we report experiments that provide strong evidence for the dark state condensation of excitons. These experiments are performed in a microscopic trap where the overall exciton density is controlled deterministically while the bath temperature is varied. Thus, quantum statistical signatures are resolved and the quantum phase transition is mapped [2].

References [1] M. Combescot et al., Phys. Rev. Lett. 99, 176403 (2007). [2] M. Beian et al., arXiv:1506.08020 (2015).

24 www.web.nano.cnr.it/CEE2D QUANTUM FLUID DYNAMICS OF POLARITON CONDENSATES

Daniele Sanvitto

NNL, Istituto di Nanotecnologie, via per Arnesano, Lecce (Italy)

Polaritons are one of the most striking quasi-particles, made of light and matter, which have recently demonstrated phase transition to a Bose-Einstein condensate state in semi-conductors [1]. The physics of these objects has been investigated with several impressive achievements, from superfluidity [2], lasing at room temperature [3], solitons [4], optical logic [5], black holes [6], and many others. Here we will discuss many aspects related to polariton condensation with the peculiarity to exhibit out of thermal equilibrium. In particular thanks to the relative ease for generation and detection of polariton states we will get insight on the intrinsic dual nature of its photon and exciton components and their oscillating behaviour under specific excitation conditions [7,8]. Using ultrafast techniques we can observe the dynamics of a quantum fluid under direct injection, which will span from vortices of different spin nature to the appearance of a surprising diffraction-limited self-localisation of the polariton condensate–independent of the pump dimension–similar to the backjet of a water droplet splashing into a pond [9,10]. Interestingly, a similar self-focusing effect was also observed under non-resonant excitation on the top of a potential hill, followed by a ballistic expansion of a multimode condensate outside of the excitation region. These results show that much is left to explore in the high-density and ultrafast dynamics of polaritons, with a striking and unique phenomenology that could open new areas of research and applications not accessible to their atomic counterparts.

References [1] J. Kasprzak et al., Nature 443, 409 (2006). [2] A. Amo et al., Nature 457, 291 (2009). [3] P. Bhattacharya et al., Phys. Rev. Lett. 112, (2014). [4] M. Sich et al., Nat. Photon. 6, 50 (2011). [5] D. Ballarini et al., Nat. Commun. 4, 1778 (2013). [6] H. S. Nguyen et al., Phys. Rev. Lett. 114, 036402 (2015). [7] L. Dominici et al., Phys. Rev. Lett. 113, 226401 (2014). [8] D. Colas et al., Light: Science & Applications (2015). [9] L. Dominici et al., ArXiv13093083. [10] L. Dominici et al., ArXiv14030487.

CEE2D | PISA IT | 2015 25 CONFOCAL SHIFT INTERFEROMETRY OF COHERENT EMISSION FROM TRAPPED DIPOLAR EXCITONS

U. Wurstbauer1,2, J. Repp1,2,3, G. J. Schinner2,3, E. Schubert2,3, A. K. Rai4, D. Reuter4,5, A. D. Wieck4, J. P. Kotthaus3, and A. W. Holleitner1,2

1Walter Schottky Institut and Physik-Department, Am Coulombwall 4a, Technische Universität München, D-85748 Garching, Germany 2Nanosystems Initiative Munich (NIM), Schellingstr. 4, 80799 München, Germany 3Center for NanoScience and Fakultät für Physik, Ludwig-Maximilians-Universität, Geschwister- Scholl-Platz 1, 80539 München, Germany 4Angewandte Festkörperphysik, Ruhr-Universität Bochum, Universitätsstraße 150, 44780 Bochum, Germany 5Department Physik, Universität Paderborn, 33098 Paderborn, Germany

In coupled double quantum wells (CDQW) photogenerated and spatially indirect excitons can be efficiently manipulated via gate-voltage-induced control of the quantum confined Stark Effect (QCSE). On an InGaAs-GaAs CDQW heterostructure we have realized electrostatically widely tunable trapping devices for dipolar indirect excitons. Utilizing the QCSE resonantly excited direct excitons transform into indirect excitons and are collected via electrostatically shaped energy landscapes. With their electron and hole confined to two different quantum wells, these indirect excitons exhibit a large dipole moment and long lifetimes of about 100 ns. We introduce a confocal shift-interferometer based on optical fibers [1]. The presented spectroscopy allows measuring coherence maps of luminescent samples with a high spatial resolution even at cryogenic temperatures. We apply the spectroscopy onto electrostatically trapped, dipolar excitons in a semiconductor double quantum well. We find that the measured spatial coherence length of the excitonic emission coincides with the point spread function of the confocal setup. The results are consistent with a temporal coherence of the excitonic emission down to temperatures of 250 mK.

Reference [1] J. Repp et al., Applied Physics Letters 105, 241101 (2014).

26 www.web.nano.cnr.it/CEE2D ULTRALOW ENERGY SWITCHING OF FERROMAGNETIC CONDENSATES IN SEMICONDUCTOR MICROCAVITIES

A. Dreismann1*, H. Ohadi1, Y. G. Rubo2, Y. del Valle-Inclan Redondo1, S. I. Tsintzos3, Z Hatzopoulos3, P. G. Savvidis1,3, and Jeremy J. Baumberg1*

1Department of Physics, Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom, [email protected], [email protected] 2Instituto de Energías Renovables, Universidad Nacional Autónoma de México, Temixco, Morelos, 62580, Mexico 3Foundation for Research and Technology–Hellas, Institute of Electronic Structure and Laser, 71110 Heraklion, Crete, Greece

A key requirement for the implementation of polariton-based spin-optotronic devices is the electrical control of polariton spins. While the flow of polariton condensates can be guided using all-optical methods, electrical fields or semiconductor post-processing,[1] the manipulation of polariton spins is still restricted to optical means. Here we demonstrate for the first time the ability to control the spin of a polariton condensate with external electric fields [2].

Fig. 1: (a) Illustration of a trapped polariton condensate: four linearly polarised laser beams induce a potential landscape, giving rise to a trapped condensate at the centre of the excitation pattern. (b) In the absence of an external electric field, the condensate stochastically adopts left- or right-circular polarisation for each realisation of the experiment. If a field is applied, the condensate polarisation changes depending on the magnitude and the polarity of the field. (c) Bias-pulses are employed to realise an ultra-low energy optoelectronic spin-switch.

We create trapped condensates that are spatially separated from the excitonic reservoir by patterning the optical excitation into four laser spots, giving rise to a confining potential (Fig. 1a). As shown recently [3], this geometry allows the observation of a variety of different polarisation states under linear non-resonant excitation, ranging from the pinned linear polarisation reported in the literature to strong stochastic circular polarisation. By applying an electrical field perpendicular to the quantum-well plane we are able precisely tune the polarisation of the condensate emission (Fig. 1b), demonstrating the direct electrical control of the condensate spin. We utilise this phenomenon to realise an electrical spin-switch, operating at record ultra-low switching energies of order attojoules (Fig. 1c) and switching speeds that are only limited by the condensate dynamics.

References [1] P. Tsosis et. al., Phys. Rev. Applied 2, 014002 (2014). [2] A. Dreismann et al., Nat.Phys. Science (2015). [3] H. Ohadi et. al., Phys. Rev. X 5, 031002 (2015).

CEE2D | PISA IT | 2015 27 SPIN COHERENCE IN COUPLED QUANTUM WELLS AMENDED BY IN-PLANE MAGNETIC FIELD

P. Andreakou1, A. Mikhailov, S. Cronenberger1, D. Scalbert1, A. Nalitov2, N. A. Gippius3, A. V. Kavokin4, M. Nawrocki5, L. V. Butov6, K. L. Campman7, A. C. Gossard7, and M. Vladimirova1*

1Laboratoire Charles Coulomb, UMR 5221 CNRS/ Université de Montpellier, France 2Institut Pascal, PHOTON-N2, Université Blaise Pascal, CNRS, Aubière Cedex, France 3Skolkovo Institute of Science and Technology, Skolkovo, Russia 4School of Physics and Astronomy, University of Southampton, Southampton, United Kingdom 5Institute of Experimental Physics, Warsaw University, Poland 6Department of Physics, University of California at San Diego, USA 7Materials Department, University of California at Santa Barbara, USA

Semiconductor coupled quantum wells (CQWs) offer an excellent laboratory for studying both intra-well direct excitons (DX) and inter-well indirect excitons (IX), as well as their interactions and spin dynamics. The presentation will focus on the pump-probe spectroscopy, a powerful tool of nonlinear optics, that we have applied to biased CQWs. We will show how DX and IX spin and population dynamics, as well as the spin polarization of residual electrons may be detected via the modulation of reflectivity and Kerr rotation spectra [1, 2]. Spin relaxation and decoherence mechanisms for DX, IX and electron gas will be discussed. An unusual enhancement of the exciton spin coherence time with in-plane magnetic field in unbiased CQWs will be presented.

References [1] A. V. Nalitov, M. Vladimirova, A. V. Kavokin, L. V. Butov, and N. A. Gippius, Phys. Rev. B 89, 155309 (2013). [2] P. Andreakou, S. Cronenberger, D. Scalbert, A. Nalitov, N. A. Gippius, A. V. Kavokin, J. R. Leonard, L. V. Butov, K. L. Campman, A. C. Gossard, and M. Vladimirova, Phys. Rev. B 91, 125437 (2015).

28 www.web.nano.cnr.it/CEE2D ENGINEERING TOPOLOGICAL QUANTUM STATES: FROM 1D TO 2D

Jelena Klinovaja

Department of Physics, University of Basel, Klingelbergstrasse 82, Basel, CH-4056 Basel, Switzerland

I will present results on exotic bound states in one-dimensional (Majorana fermions and parafermions) and two-dimensional (edge states in topological insulators) condensed matter systems that have attracted wide attention due to their promise of non-Abelian statistics believed to be useful for topological quantum computing. I discuss systems in which topological properties could be engineered per demand. For example, Majorana fermions can emerge in hybrid systems with proximity pairing in which the usually weak Rashba spin-orbit interaction is replaced by magnetic textures. Here, I will discuss candidate materials such as semiconducting nanowires [1-2], graphene nanoribbons [3], atomic magnetic chains or magnetic semiconductors [4]. One further goal is to go beyond Majorana fermions and to identify systems that can host quasiparticles with more powerful non-Abelian statistics such as parafermions in double wires coupled by crossed Andreev reflections [5,6]. In the second part of my talk, I will focus on 'strip of stripes model' consisting of weakly coupled one-dimensional wires [6-8], where interaction effects in the wires can be treated non- perturbatively via bosonization. I will demonstrate that such systems can exhibit the integer or fractional quantum Hall effect [6], spin Hall effect [7], and anomalous Hall effect [8]. In the fractional regimes, the quasiparticles have fractional charges and non-trivial Abelian braid statistics.

References [1] J. Klinovaja and D. Loss, Phys. Rev. B 86, 085408 (2012). [2] D. Rainis, L. Trifunovic, J. Klinovaja, and D. Loss, Phys. Rev. B 87, 024515 (2013). [3] J. Klinovaja and D. Loss, Phys. Rev. X 3, 011008 (2013); J. Klinovaja and D. Loss, Phys. Rev. B 88, 075404 (2013). [4] J. Klinovaja, P. Stano, A. Yazdani, and D. Loss, Phys. Rev. Lett. 111, 186805 (2013). [5] J. Klinovaja and D. Loss, Phys. Rev. B 90, 045118 (2014). [6] J. Klinovaja, A. Yacoby, and D. Loss, Phys. Rev. B 90, 155447 (2014). [7] J. Klinovaja and D. Loss, Phys. Rev. Lett. 111, 196401 (2013); J. Klinovaja and D. Loss, Eur. Phys. J. B 87, 171 (2014). [8] J. Klinovaja and Y. Tserkovnyak, Phys. Rev. B 90, 115426 (2014). [9] J. Klinovaja, Y. Tserkovnyak, and D. Loss, Phys. Rev. B 91, 085426 (2015).

CEE2D | PISA IT | 2015 29 TRANSPORT OF INDIRECT EXCITONS IN COUPLED QUANTUM WELLS: BASIC PHYSICS AND DEVICES

Peristera Andreakou

Laboratoire de Photonique et Nanostructures, CNRS, Route de Nozay, 91460 Marcoussis, France

An indirect exciton (IXs) is a bound pair of an electron and a hole, which are located in separated layers of a heterostructure. The spatial overlap of electron and hole wavefunctions is controlled by the voltage applied to the top design and ground plane electrode. Therefore, the lifetimes of indirect excitons can exceed those of direct excitons by orders of magnitude. Long lifetimes allow IXs to travel over sufficiently large distances to accommodate devices as well as to cool down to lattice temperature for the observation of condensed matter phenomena.

My talk focuses on recent advances in a variety of excitonic devices based on IXs. The first part of my talk explores devices based on a coupled quantum well system. These devices take advantage of the controllable transport of IXs through modulated electrostatic landscapes. Excitonic diodes, transistors and routers are presented [1]. Such devices promise to effectively eliminate the time delay between signal processing and optical communication, a significant advantage over electronic transistors. My presentation also includes the development of devices such as electrostatic traps and stirring potentials that allow for studies of exciton condensates analogous of atomic condensates but much simpler to achieve in a lab [2, 3].

A different system for the study of IXs is CdSe/CdS colloidal heterostructures. These quantum rods are utilized as gain medium to realize a single mode, single exciton whispering gallery mode microlaser [4].

References [1] P. Andreakou, S. V. Poltavtsey, J. R. Leonard, E. V. Calman, M. Remeika, Y. Y. Kuznetsova, L. V. Butov, J. Wilkes, M. Hanson, A. C. Gossard, Appl. Phys. Lett. 104, 091101 (2014). [2] Y. Y. Kuznetsova, P. Andreakou, M. W. Hasling, J. R. Leonard, E. V. Calman, C. J. Dorow, L. V. Butov, M. Hanson, A. C. Gossard, Optics Letters 40, 589 (2015). [3] M. W. Hasling, Y. Y. Kuznetsova, P. Andreakou, J. R. Leonard, E. V. Calman, C. Dorow, L. V. Butov, M. Hanson, A. C. Gossard, Journal of Appl. Phys. 117, 023108 (2015). [4] C. Grivas, C. Li, P. Andreakou, P. Wang, M. Ding, G. Brambilla, L. Manna, P. G. Lagoudakis, Nature Communication 4, 2376 (2013).

30 www.web.nano.cnr.it/CEE2D MANIPULATION OF SPIN TRANSFER TORQUE USING LIGHT

K. L. Vendelbjerg1, L. J. Sham2, and M. Rontani1

1CNR-NANO, Modena, Italy 2Dept of Physics, University of California San Diego, California

We show that the spin transfer torque induced by a spin-polarized current on a nanomagnet as the current flows through a semiconductor-nanomagnet-semiconductor junction is externally controlled by shining the junction off-resonantly with a strong laser beam. The excitonic coherence driven by the laser dresses the virtual electron-hole pairs coupling conduction and valence bands and inducing an evanescent state in the proximity of the nanomagnet. The Fano-like quantum interference between this localized state and the continuum spectrum is different in the two spin channels and hence it dramatically alters the spin transport, leading to the coherent control of the spin transfer torque. This work is supported by EU-FP7 Marie Curie Initial Training Network INDEX.

CEE2D | PISA IT | 2015 31 BOSE-EINSTEIN CONDENSATION IN A POLYMER: TOWARDS QUANTUM SIMULATION

Johannes Plumhof1, Lijian Mai1, Thilo Stöferle1, Ullrich Scherf2, and Rainer F. Mahrt1

1IBM Research Zurich, Säumerstrasse 4, CH-8803 Rüschlikon, Switzerland 2Bergische Universität Wuppertal,Fachgebiet Makromolekulare Chemie, Gaußstraße 20, 42119 Wuppertal, Germany

During recent years polaritonics has emerged as a new field of solid-state physics based on the unique quantum properties of mixed light-matter quasiparticles, so called exciton-polaritons. Recent discoveries of Bose-Einstein condensation (BEC) and superfluidity provide opportunities to harness these coherent quantum effects in a new generation of opto-electronic devices. Until now, BECs have been realized either with laser-cooled gases at nano-Kelvin temperatures or with high-quality semiconductor crystals produced by only a few laboratories worldwide. By utilizing the extremely large oscillator strength, exciton binding energy and saturation density of organic semiconductors we demonstrate BEC at room temperature with an amorphous spin-coated polymer film embedded in a Fabry-Pérot microcavity. Since no crystal growth is involved, our approach radically reduces the complexity of experiments to investigate BEC physics and paves the way for a new generation of opto-electronic devices, taking advantage of the processibility and flexibility of polymers. Finally, experiments on sub-micron sized defect cavities and possible ways

32 www.web.nano.cnr.it/CEE2D ELECTRONIC TRANSPORT AND OPTICAL PROPERTIES OF ELECTRICALLY-GENERATED ELECTRON-HOLE BILAYERS WITH MINIMUM INTER-LAYER SEPARATION

U. Siciliani de Cumis1, J. Waldie1, A. F. Croxall1, J. Llandro, H. E. Beere1, I. Farrer1, D. A. Ritchie1, A. Perali, D. Neilson2, R. Anankin3, M. Beian3, F. Dubin3, I. Aliaj4, T. Satzoukidis4, and V. Pellegrini4

1Cavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge CB3 0HE, United Kingdom 2University of Camerino, Madonna delle Carceri, 9, 62032, Camerino (MC), Italy 3INP, Université Pierre et Marie Curie, 4 place Jussieu, 75005 Paris, France 4NEST, Scuola Normale Superiore, Piazza San Silvestro 12, 56127 Pisa, Italy

The sustained improvement in semiconductor growth techniques over the past 30 years has allowed electrons and holes to be confined in layers so thin that they can be considered as quasi two- dimensional systems. Further interesting and more complex possibilities arise when electrons and holes are confined in two adjacent 2D layers kept apart by distance of few nanometres so that particle recombination is hindered by the presence of a potential barrier. The behaviour of the particles in such electron-hole bilayers is the product of the relative contributions of inter-layer and intra-layer Coulomb interactions. In particular, when the distance between the layers is comparable to the average separation between particles in the same layer, novel phases are predicted to emerge like a supersolid Wigner crystal [1] or an excitonic superfluid phase of indirect excitons [2] with a BEC to BCS crossover [3]. Indeed, a BEC of electron-hole pairs has been recently observed in an optically-generated electron-hole bilayer system [4]. Electrically-generated and independently-contacted electron-hole bilayers have been intensively studied in the recent past, with a particular focus on transport measurements, such as the Coulomb drag technique, which is used to probe directly the inter-layer interactions [5], [6]. We present Coulomb drag measurements of electron-hole bilayers generated in a GaAs/AlGaAs double quantum well structure with an interlayer barrier thickness of just 5 nm, the narrowest ever realised in such a system. Recent theoretical calculations suggest that our systems is tantalisingly close to the transition to an electron-hole superfluid state [7]. We discuss the evidence, in both the Coulomb drag and the sheet resistivities, for electron-hole pairing correlations in these bilayers and present some recent studies focusing on the optical properties of these structures.

References [1] Y. Joglekar, A. Balatsky, and S. Das Sarma, Phys. Rev. B 74, 233302 (2006). [2] Y. E. Lozovik, and V. I. Yudson, J.E.T.P. 44 (1976). [3] P. Pieri, D. Neilson, and G. Strinati. Phys. Rev. B 75, 113301 (2007). [4] A. A. High, J. R. Leonard, M. Remeika, L.V. Butov, M. Hanson, and A. C. Gossard, Nano Lett. 12, 2605 (2012). [5] A. F Croxall, K. Das Gupta, C. Nicoll, M. Thangaraj, H. E. Beere, I. Farrer, D. A. Ritchie, and M. Pepper, Phys. Rev. Lett. 101, 246801 (2008). [6] J. A. Seamons, C. P. Morath, J. L. Reno, and M. P. Lilly. Phys. Rev. Lett. 102, 026804 (2009). [7] D. Neilson, A. Perali, A.F. Croxall, to be published.

CEE2D | PISA IT | 2015 33 ADIABATIC PREPARATION OF A COLD EXCITON CONDENSATE

V. Shahnazaryan1,2, O. Kyriienko3, and I. Shelykh1,4,5

1Science Institute, University of Iceland, Dunhagi-3, IS-107, Reykjavik, Iceland 2Institute of Mathematics and High Technologies, Russian-Armenian (Slavonic) University, Hovsep Emin 123, 0051, Yerevan, Armenia 3Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, DK-2100 Copenhagen, Denmark 4Division of Physics and Applied Physics, Nanyang Technological University 637371, Singapore 5ITMO University, St. Petersburg 197101, Russia

We propose a scheme for the controllable preparation of a cold indirect exciton condensate by means of optical pump based on Landau-Zener bosonic transfer in a dipolariton system. Dipolaritons are bosonic quasiparticles which arise from the coupling between cavity photon (C), direct exciton (DX), and indirect exciton (IX) modes, and appear in a double quantum well embedded in a semiconductor microcavity (see Fig.1). Controlling the detuning between modes of the system, the limiting cases of exciton-polaritons and indirect excitons can be realized. The protocol is based on several steps. First stage corresponds to initial preparation of polariton condensate with high cavity photon and direct exciton fractions, while indirect exciton mode is located high in energy at zero external voltage (Fig. 2a). Next, applying electric field the IX energy is lowered to far red-detuned value, where adiabatic following of the lower dipolariton mode converts particles to indirect excitons with inherited coherence properties (Fig. 2b). The following allows for generation of a spatially localized cold exciton gas, on the contrary to currently used methods, where IX cloud appears due to diffusion of carriers from spatially separated electron- and hole-rich areas. Finally, to reduce residual effects of cavity an optical incoherent pump of polaritonic reservoir states shall be switched off during the transfer event. We analyzed the population transfer for various sets of parameters and switching conditions, and demonstrated that adiabatic cold exciton preparation is experimentally feasible in currently existing setups. The main results of investigation were published in the paper [1].

Figure 1. (a): Sketch of the dipolaritonic system, representing double quantum well (DQW) structure embedded in a microcavity formed by distributed Bragg reflectors (DBRs). (b): Schematic representation of dispersion of lower (LP), middle (MP), and upper (UP) dipolaritons. Incoherent optical pump creates carriers at high energy, which relax to LP reservoir, and consequently scatter to a macroscopically coherent ground state.

34 www.web.nano.cnr.it/CEE2D

Figure 2. (a): Time dependence of energies of the modes. The bias applied to the system causes linear decrease of IX energy in t = 1 ns to t = 1.5 ns window, up to far red-detuned value. The dashed red line corresponds to time dependence of the pump intensity (in arbitrary units). (b): The evolution of occupations of the modes, where NC, NDX, NIX denote the occupations of cavity, direct exciton, and indirect exciton modes, correspondingly. At the first stage (t < 1 ns) the formation of polariton condensate takes place. Next, continuous change of an applied bias drives the system through an avoided crossing, leading to the transfer of polariton occupation to an indirect exciton mode.

Reference [1] V. Shahnazaryan, O. Kyriienko, and I. Shelykh, Physical Review B 91, 085302 (2015).

CEE2D | PISA IT | 2015 35 COLLECTIVE NON-LOCAL TRANSPORT IN GRAPHENE HETEROSTRUCTURES

Marco Polini

Istituto Italiano di Tecnologia, Graphene Labs, Via Morego 30, I-16163, Genova (Italy) NEST, Scuola Normale Superiore, I-56126, Pisa (Italy)

Graphene sheets encapsulated in thin slabs of hexagonal boron nitride display micrometer-scale ballistic transport [1] and ultra-high room-temperature mobilities [2,3], which are solely limited by graphene acoustic phonons [4]. In this talk I will first try and convince you that transport at sufficiently high temperatures in such samples can be described by the theory of hydrodynamics [5,6], which is non-local and non-linear. I will then present results of recent combined experimental and theoretical work in which the hydrodynamic shear viscosity of the massless Dirac fermion liquid hosted by the encapsulated graphene sheet is accessed through non-local dc transport experiments in a multi-terminal Hall bar [7,8]. I will conclude by highlighting the implications of hydrodynamic flow for all-electrical detection of graphene Terahertz plasmons [9].

References [1] A.S. Mayorov et al., Nano Lett. 11, 2396 (2011). [2] L. Wang et al., Science 342, 614 (2013). [3] A. Woessner et al., Nature Mater. 14, 421 (2015). [4] A. Principi, M. Carrega, M.B. Lundeberg, A. Woessner, F.H.L. Koppens, G. Vignale, and M. Polini, Phys. Rev. B 90, 165408 (2014). [5] L.D. Landau and E.M. Lifshitz, Course of Theoretical Physics: Fluid Mechanics (Pergamon, New York, 1987). [6] A. Tomadin, G. Vignale, and M. Polini, Phys. Rev. Lett. 113, 235901 (2014). [7] D. Bandurin, I. Torre, R.K. Kumar, M. Ben Shalom, A. Tomadin, A. Principi, G.H. Auton, E. Khestanova, K.S. NovoseIov, I.V. Grigorieva, L.A. Ponomarenko, A.K. Geim, and M. Polini, submitted. [8] I. Torre, A. Tomadin, A.K. Geim, and M. Polini, arXiv:1508.00363. [9] I. Torre, A. Tomadin, R. Krahne, V. Pellegrini, and M. Polini, Phys. Rev. B 91, 081402(R) (2015).

36 www.web.nano.cnr.it/CEE2D TIME-DEPENDENT QUANTUM DYNAMICS OF 2D SPATIALLY INDIRECT EXCITONS

Federico Grasselli1,2, Andrea Bertoni2, and Guido Goldoni1,2

1Dept. of Physics, Informatics and Mathematics, Univ. of Modena and Reggio Emilia 2CNR-NANO S3, Via Campi 213/a, Modena, Italy

We study the quantum dynamics of a single spatially indirect exciton (IX) in prototypical potential landscapes. The internal relative quantum dynamics of the correlated electron-hole pair is fully taken into account. The two particles, localized in different layers, can be independently gated. The quantum evolution of the electron-hole pair is then realized by means of a numerical solver of the time dependent Schroedinger equation, based on the split-step Fourier method, which relies on a Suzuki-Trotter factorization of the evolution operator [1]. Such method is numerically exact, unitary, and can be also extended to possibly time dependent external potentials, no matter their strength or range. On the basis of the results obtained from a minimal 1D model [2], for which we observed a wide variety of typical two-particle scattering processes (dissociation of the IX, excitation to higher internal levels, dwelling around the potential edge, periodic transmission of the IX wave packet), we developed a more quantitative and realistic 2D-, i.e. 4-degree-of-freedom- (4DoF), model. A massive parallelization of the code has been required in order to cope with the huge amount of data and operation that a 4DoF quantum problem require. We performed simulations of scattering events, considering different potential energy configurations and incidence angles, in order to have a deeper insight into such a composite-quantum-particle- collision event, and to emphasizing the main differences between the 4DoF model and the ‘’rigid IX’’ one, often employed in the literature, for which the IX relative motion is frozen in its ground state [Figure].

Figure: Scattering on electron potential well. Comparison between the rigid IX model (left) and the fully 4DoF one (right) at the same time, t=12ps. Centre of mass probability density (green-scale), and external potential are shown.

References [1] A. Bertoni, J. Comput. Electron. 2, 291 (2003). [2] F. Grasselli, A. Bertoni, and G. Goldoni, J. Chem. Phys. 142, 034701 (2015).

CEE2D | PISA IT | 2015 37 ULTRAFAST DYNAMICS IN LIGHT-HARVESTING AND PHOTOVOLTAICS: A THEORETICAL AND EXPERIMENTAL INVESTIGATION

Carlo Andrea Rozzi

CNR NANO S3, via G. Campi 213A, 41125 Modena, Italy

It is known that the primary steps of photoinduced energy and charge transfer may occur on extremely fast time scales in many natural and man-made compounds that perform conversion of sunlight into chemical or electrical energy. These processes have traditionally been interpreted in terms of the incoherent kinetics of optical excitations and of charge hopping, but recently signatures of quantum coherence were observed in energy transfer in photosynthetic bacteria and algae [1,2]. We have studied the early steps of photoinduced charge separation in reference systems for artificial photosynthesis and photovoltaics by combining Time-dependent Density Functional Theory simulations of the quantum dynamics and high time resolution femtosecond spectroscopy. Our results show that the coherent coupling between electronic and nuclear degrees of freedom is of key importance for charge delocalization and transfer in both of covalently and non-covalently bonded systems [3,4]. We have exploited the results of our research to design, synthesize and characterize a novel molecular scaffold for photovoltaic applications [5].

References [1] G. S. Engel et al., Nature 446, 782-786 (2007). [2] E. Collini et al., Nature 463, 644-647 (2010). [3] C. A. Rozzi, S. Falke, et al., Nat. Comm. 4, 1602 (2013). [4] S. Falke, C. A. Rozzi, et al., Science, 344, 1001 (2014). [5] S. Pittalis et al., Adv. Func. Mat. (2014).

38 www.web.nano.cnr.it/CEE2D EXCITATION ENERGY TRANSFER IN HYBRID NANO-SYSTEMS BY A MULTI-SCALE BEYOND-FORSTER APPROACH

G. Gil1,2, A. Delgado1, A. Bertoni1, G. Goldoni1, and S. Corni1

1CNR Institute for Nanosciences, Center S3, Modena, Italy 2Department of Physics, Informatics and Mathematics, University of Modena and Reggio Emilia, Modena, Italy

We developed a new multi-scale approach to treat excitation energy transfer in hybrid nano-systems composed by an organic molecule nearby a semiconductor nanoparticle, a class of nano-materials of growing relevance in diverse fields, from artificial photosynthesis to nanomedicine. Since such systems may be composed of millions of atoms, their optical excitations are often described by ab- initio methods selecting a small active optical center, comprising the molecule and a cluster of tens/hundreds of semiconductor atoms. However, this may not be appropriate in many situations, e.g., when the optical excitations of the molecule are resonant with those of the nanoparticle, which is extended over the entire nano-system. In our method, optical excitations of each subsystem is accurately described within a quantum- mechanical approach at the appropriate level of description, that is a state-of-the-art Time- Dependent Density Functional Theory description for the molecule and an accurate -yet semi- empirical- envelope-function based Configuration Interaction description of excited electron-hole pairs (excitons) for the nanoparticle. Energy transfer from one subsystem to the other is described here by a beyond-Förster [1] approach which considers the interaction between all the transition multipole moments of the nanoparticle and the transition dipole moment of the molecule, an approximation which is accurate for molecule-nanoparticle distances a few times the molecular size. This method allows for an accurate description of the dynamical correlations inside each segment. This novel approach is applied to a case study relevant to photodynamic therapy, where the molecule is a free-base porphyrin and the nanoparticle is a core/shell CdSe/ZnS semiconductor quantum dot [2].

References [1] D.L. Andrews, C. Curutchet, G.D. Scholes, Laser Photonics Rev. 5 1, 114-123 (2011). [2] J.M. Tsay, M. Trzoss, L. Shi, X. Kong, M. Selke, M.E. Jung, and S. Weiss, J. Am. Chem. Soc. 129, 6865-6871 (2007).

CEE2D | PISA IT | 2015 39

INDEX

PROGRAM

POSTER SESSION

ABSTRACTS OF TALKS

ABSTRACTS OF POSTERS

40 www.web.nano.cnr.it/CEE2D ROOM-TEMPERATURE RECTIFICATION IN GRAPHENE/LAO/STO HETEROSTRUCTURES

I. Aliaj1, E. di Gennaro2, V. Miseikis3, I. Torre1, A. Gamucci4,3, C. Coletti3,4, M. Polini1, F. M. Granozio2, V. Pellegrini4, F. Beltram1, and S. Roddaro1

1NEST, Istituto Nanoscienze - CNR and Scuola Normale Superiore, I-56126 Pisa, Italy 2CNR-SPIN and Dipartimento di Fisica, Complesso Universitario di Monte S.Angelo, Via Cintia, 80126 Naples, Italy 3Center for Nanotechnology Innovation @NEST, Istituto Italiano di Tecnologia, Piazza San Silvestro 12, 56127 Pisa, Italy 4Istituto Italiano di Tecnologia, Graphene labs, Via Morego 30, I-16163 Genova, Italy

Two-dimensional electron systems (2DES) embedded in solid state materials are central to modern electronics and to fundamental as well as applied research in condensed matter physics. Two recent prominent examples are the chiral Dirac fermions in graphene [1] and the emerging 2DES at the interface between the bulk insulating oxides LaAlO3/SrTiO3 (LAO/STO) [2], exhibiting multiple electric field-tunable properties, such as superconductivity [3], magnetism [4] and spin-orbit coupling [5]. Vertical heterostructures combining the two materials, in analogy to graphene/GaAs heterostructures [6], are expected to display a strong interlayer Coulomb coupling that can drive novel collective phases, and promise interesting device applications [7]. In this poster, I will illustrate our results on the room-temperature electrical characterization of high-quality CVD graphene monocrystals transferred on a LAO/STO substrate. We demonstrate the capability of the interfacial 2DES in LAO/STO to tune the graphene chemical potential across the Dirac point, and show that the effect is dominated by graphene’s quantum capacitance. Furthermore, in contrast with Metal/LAO/STO systems [8], we find a pronounced rectifying behaviour in the room temperature electrical transport between graphene and the LAO/STO 2DES. Our results can shed light on the fundamental physics of these novel material systems and open up possible future applications.

References [1] A. H. Castro Neto et al., Rev. Mod. Phys. 81, 109 (2009). [2] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004). [3] N. Reyren et al., Science 317, 1196 (2007). [4] A. Brinkman et al., Nat. Mater. 6, 493 (2007). [5] A. D. Caviglia et al., Phys. Rev. Lett. 104, 126803 (2010); M. Ben Shalom et al., Phys. Rev. Lett. 104 (12), 126802 (2010). [6] A. Gamucci et. al., Nature Commun. 5, 5824 (2014). [7] M. Huang et al., APL Materials 3, 062502 (2015). [8] G. Singh-Bhalla et al., Nature Phys. 7, 80 (2010).

CEE2D | PISA IT | 2015 41 MACROSCOPIC OCCUPATION OF DARK EXCITONIC STATES IN A TRAP

Mussie Beian1, Mathieu Alloing2, Romain Anankine2, Edmond Cambril3, Carmen Gomez Carbonell3, Aristide Lemaître3, and François Dubin1,2

1ICFO- The Institute of Photonic Sciences, Av. Carl Friedrich Gauss, num. 3, 08860 Castelldefels (Barcelona), Spain 2Institut des Nanosciences de Paris, UPMC Paris 06, CNRS UMR 7588, 4 pl. Jussieu, 75005 Paris, France 3Laboratoire de Photonique et Nanostructures, LPN/CNRS, Route de Nozay, 91460 Marcoussis, France

We present recent studies of Bose-Einstein condensation of spatially indirect excitons confined in a microscopic 10 µm wide electrostatic trap [1]. We show that trapped excitons have a photoluminescence which drops suddenly below 2 K as the indirect excitons temperature is lowered. Our observations realized at a fixed density are then interpreted as a manifestation for the condensation of excitons in the lowest energy optically inactive (dark) states. We create spatially indirect excitons in a double quantum well where valence holes and conduction electrons are spatially separated using an external electric field tilting the energy bands. Indirect excitons are then characterized by their giant electric dipole moment while electrons and holes have their wavefunctions which overlap weakly. The recombination between them is then damped compared to direct excitons such that indirect excitons exhibit a radiative life-time increased up to 70 ns. While cooling indirect excitons from 3.5 to 0.33 K, we show that the photoluminescence integrated intensity decreases long after the laser pulse, i.e. when the trapped gas is well thermalized. At the same time, we observe that the decay time of the photoluminescence decreases thus showing that the excitons temperature is lowered [2]. Our experiments being realized at a fixed density in the trap, these combined observations lead us to conclude that bright indirect excitons are strongly depleted below a critical temperature ~2 K [3], inducing the theoretically predicted macroscopic occupation of the lowest energy dark states [4].

References [1] M. Beian et al., Europhys. Lett. 110, 270001 (2015). [2] A. Ivanov et al., Phys. Rev. B 59, 5032 (1999). [3] M. Beian et al., arXiv:1506.08020 (2015). [4] M. Combescot, O. Betbedet-Matibet, and R. Combescot, Phys. Rev. Lett. 99, 176403 (2007).

42 www.web.nano.cnr.it/CEE2D LATERAL TRANSPORT OF INDIRECT EXCITON SPINS IN DOUBLE QUANTUM WELL STRUCTURES

S. Büyükköse1, C. Hubert1, A. Violante1, and P. V. Santos1

1Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany

Indirect excitons (IXs) in double quantum well (DQW) structures are promising particles for information storage and processes due to their long, electrically controlled lifetimes together with the strong interaction with photons [1]. Furthermore, their spin dynamics can be controlled by applying external fields [2]. Here, we report on transport of IX spins in GaAs DQWs using spatially and polarization resolved photoluminescence (PL). A voltage-tunable in-plane potential gradient was used to transport excitons by exploiting the quantum confined Stark effect in combination with a lithographically defined resistive top gate. Transport direction was chosen as [-110] on (001) GaAs sample surface to minimize the symmetry effects on the spin-orbit interaction [3]. Sample was excited with a focused and circularly polarized laser beam and resulting photoluminescence was investigated by using spatially and polarization resolved photoluminescence spectroscopy. Effect of different bias voltage and magnetic field conditions were investigated.

References [1] K. Sivalertporn, L. Mouchliadis, A. L. Ivanov, R. Philp, and E. A. Muljarov, Phys. Rev. B 85, 045207 (2012). [2] A. Violante, R. Hey, and P. V. Santos, Phys. Rev. B 91, 125302 (2014). [3] A. Hernández-Mínguez, K. Biermann, R. Hey and P. V. Santos, Phys. Status Solidi B 251, 1736 (2014).

CEE2D | PISA IT | 2015 43 OXIDE HETEROSTRUCTURES AS A POSSIBLE INNOVATIVE ROAD FOR STUDYING INDIRECT EXCITONS

L. Maritato1, A. Galdi1, C. Sacco2, and D.G. Schlom3

1Dipartimento di Ingegneria dell'Informazione, Ingegneria Elettrica e Matematica Applicata- DIEM, Università di Salerno e CNR-SPIN, UOS Salerno, Via Giovanni Paolo II 132, 84084, Fisciano, Salerno 2Dipartimento di Ingegneria Industriale-DIIN, Università di Salerno e CNR-SPIN, UOS Salerno, Via Giovanni Paolo II 132, 84084, Fisciano, Salerno 3Cornell University, Kavli Institute for Nanoscale Science, Ithaca, NY 14853 USA

Recent advances in the molecular beam epitaxy growth of oxide-based heterostructures have allowed atomic-scale thickness control and abrupt interfaces with the possibility to change the chemical composition over a distance of a single unit cell [1]. In view of their exploitation to the studies on indirect excitons, oxide-heterostructures have the potential advantage of allowing higher n and p carrier densities and shorter separation distances when compared to semiconductor-based double quantum wells [2]. We present preliminary studies on manganite and cuprate based heterostructures of interest for the study of indirect excitons.

References [1] See for example, J. Mannhart and D.G. Schlom, Science 327, 1607-1611 (2012). [2] A. J. Millis and D. G. Schlom, Physical Review B 82, 073101 (2010).

44 www.web.nano.cnr.it/CEE2D CAVITY POLARITONS UNDER THE INFLUENCE OF THE LANDAU QUANTIZATION, RASHBA SPIN-ORBIT COUPLING, ZEEMAN SPLITTING AND GYROTROPY EFFECTS

S.A. Moskalenko1, I.V. Podlesny1, E.V. Dumanov1, and M.A. Liberman2

1Institute of Applied Physics of the Academy of Sciences of Moldova, Academiei str. 5, Chisinau MD–2028, Republic of Moldova 2Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, 10691 Stockholm, Sweden

The energy spectrum of the two-dimensional (2D) cavity polaritons under the influence of a strong magnetic and electric fields perpendicular to the surface of the GaAs-type quantum wells (QWs) with p-type valence band embedded into the resonators is considered. As the first step in this direction the Landau quantization of the electrons and heavy-holes (hh) was investigated taking into account the Rashba spin-orbit coupling with third-order chirality terms for hh and with non- parabolicity terms in their dispersion law including as well the Zeeman splitting effects. The non- parabolicity term proportional to the strength of the electric field was introduced to avoid the collapse of the semiconductor energy gap under the influence of the third order chirality terms. The exact solutions for the eigenfunctions and eigenenergies were obtained using the Rashba method [1]. On the second step we derive in the second quantization representation the Hamiltonians describing the Coulomb electron-electron and the electron-radiation interactions. This allows us to determine the magnetoexciton energy branches and to deduce the Hamiltonian of the magnetoexciton-photon interaction. On the third step the fifth order dispersion equation describing the energy spectrum of the cavity magnetoexciton-polariton is investigated. It takes into account the interaction of the cavity photons with two dipole-active and with two quadrupole-active 2D magnetoexciton energy branches. It was shown that the Rabi frequency ΩR of the polariton branches and the magnetoexciton oscillator strength fosc increase in dependence on the magnetic field strength B as ΩR~ B, and fosc~B. The optical gyrotropy effects may be revealed if changing the sign of the photon circular polarization at a given sign of the wave vector longitudinal projection kz or equivalently changing the sign of the longitudinal projection kz at the same selected light circular polarization. I.V.P. and E.V.D. thanks the Foundation for Young Scientists of the Academy of Sciences of Moldova for financial support (14.819.02.18F).

Reference [1] E.I. Rashba, Fiz. Tverd. Tela (Leningrad) 2, 1224 (1960) [Sov. Phys. Solid State 2, 1109 (1960)].

CEE2D | PISA IT | 2015 45 MAGNETOEXCITON IN NANORING OF NON-UNIFORM THICKNESS

L. C. Porras and I. D. Mikhailov

Universidad Industrial de Santander - UIS, cl 9 cr 27, Bucaramanga, Colombia, A.A. 678

To study the magneto-optical properties of a neutral exciton confined in a quantum ring of non- uniform thickness in the presence of magnetic field applied along the ring’s symmetry axis, we consider a model of a nanostructure in a form of a volcano with a circular ridge of variable height. We show that in the structural adiabatic limit, when the width of the pattern of the particles pathways within the ring is much smaller than the ring’s radius, the wave function of the exciton confined in such structure can be found in a form of a double Fourier series expansion [1]. We calculate the density of the states, oscillator strength and the photoluminescence spectrum for different ring radii and scales of the non-uniformity. The dependencies of the magneto-optical properties of the exciton on the magnetic field strength are discussed. Our results show a substantial change of the amplitudes of the Aharonov–Bohm oscillations of energy levels, density of states and photoluminescence spectrum of neutral excitons induced by the non-uniformity.

Reference [1] L. C. Porras and I. D. Mikhailov, Physica E 53, 41 (2013); L. F. Garcia, S. Yu. Revinova, and I. D. Mikhailov, Physica E 71, 101 (2015).

46 www.web.nano.cnr.it/CEE2D BIASED CONTROLLED DIPOLE ORIENTED POLARITON BISTABILITY

Pramod K Sharma1,2, Simeon I. Tsintzos2, Gabriel Christmann2, Zacharias Hatzopoulos2, Jeremy J Baumberg3, and Pavlos G Savvidis1,2

1Department of Materials Science and Technology, University of Crete, 71300 Heraklion, Greece 2Microelectronics Research Group, IESL-FORTH, PO Box 1385, 71110 Heraklion, Greece 3Dept. of Physics, Cavendish Laboratory, University of Cambridge, Cambridge, CB3 0HE, UK

Strong light-matter coupling regime in asymmetric double quantum well (QW) based microcavities has been shown to give rise to dipole oriented polariton states [1]. These structures are expected to exhibit remarkable nonlinearities owing to unique possibility these systems offer to engineer polariton-polariton interactions. Strong nonlinearities refer to the optical bistability which is the primary element for the optical computing where the information stored as energy and switching a device from logic 0 level to logic 1 level required a definite switching energy [2]. Optical bistability has a great impact of intense research in quantum electronics because it has potential in field of, all optical logic and quantum computing system [3]. Our recent experimental demonstration is mainly emphasized on biased controlled Dipole-oriented Polariton Bistability (DPB) which shows the bistable behaviour in a system where the relationship of output to input characterized by hysteresis loop. This bistable characteristic is crucial for the basis of binary switching element it will be responsible for the optical logic gate circuits. The p-i-n microcavity diode structure consists of two DBRs forming a cavity and 4 sets of asymmetric double InGaAs Quantum wells (QWs) positioned inside the cavity. To map out polariton dispersions transmission measurements using white light are performed at 160K. Figure (a) shows clear anticrossing behaviour between DX, IX and cavity modes when tuning the bias. When in resonance the spatially direct and indirect excitons become coupled sharing oscillator strength and provide strong dipole moment in growth direction. Bistability measurements are performed when pumping at 1.3923eV with near resonant laser under application of electric bias.

Applied Bias ~ - 0.2V

Applied Bias ~ 0.2V 250

200

150

100 '0.2mW' '0.4mW' OutputIntensity (µW) 50 '0.6mW' '0.8mW' 0 '1.0mW' 2 4 6 8 10 Input Intensity (mW)

Fig: (a) Polariton branches under application of bias close to the resonance in transmission configuration for a mesa with 4nm barrier layer showing clear anticrossing at 160K. (b) Bias controlled bistability different power shown in figure respectively.

References [1] Cristofolini et al., Science 336, 704 (2012). [2] SD Smith et al., Nature 316 (1985). [3] Y R Shen et al., Nature 299 (1982).

CEE2D | PISA IT | 2015 47 EXCITON DYNAMICS IN DISK-LIKE QUANTUM DOTS WITH A MAGNETIC IMPURITY

V. Moldoveanu1, I. V. Dinu1, R. Dragomir1, and B. Tanatar2

1National Institute of Materials Physics, PO Box MG-7, Bucharest-Magurele, Romania 2Department of Physics, Bilkent University, Bilkent, 06800 Ankara, Turkey

In the recent years semiconductor quantum dots doped with a single magnetic impurity turned into promising candidates for optical spin manipulation [1,2]. The coherent precession of a Mn atom embedded in a CdTe quantum dot has been recently probed [3] . Trojnar et al. [4] also investigated the formation of biexcitons in self-assembled QDs in the presence of a Mn atom.

In this work we study theoretically the exciton dynamics under optical pulses in the presence of both electron-Mn and hole-Mn exchange interactions. The latter coupling does not affect heavy- hole (HH) spin but flips the spin of the light-holes (LH). We use the Master equation approach [5] to investigate the optical properties of Mn-doped CdTe quantum dots by taking into account the specific disk-like geometry of the system along with the corresponding optical selection rules. We find that the systems exhibiting heavy-hole light-hole mixing present a more complicated dynamics of both excitons and Mn spin. The numerical calculations include charge relaxation processes as they are crucial for the dynamics of the p-shell excitons.

References [1] Paul M. Koenraad and Michael E. Flatte, Nat. Mater. 10, 91-100 (2011). [2] J. Kobak, T. Smolenski, M. Goryca, M. Papaj, K. Gietka, A. Bogucki, M. Koperski, J.-G. Rousset, J. Suffczynski, E. Janik, M. Nawrocki, A. Golnik, P. Kossacki, and W. Pacuski, Nat. Commun. 5, 1-8 (2014). [3] M. Goryca, M. Koperski, P. Wojnar, T. Smoleski, T. Kazimierczuk, A. Golnik, and P. Kossacki, Phys. Rev. Lett. 113, 227202 (2014). [4] A. H. Trojnar, M. Korkusinski, U. C. Mendes, M. Goryca, M. Koperski, T. Smolenski, P. Kossacki, P. Wojnar, and P. Hawrylak, Phys. Rev. B 87, 205311 (2013). [5] V. Moldoveanu, I. V. Dinu, and R. Dragomir, Phys. Rev. B 89, 245415 (2014).

48 www.web.nano.cnr.it/CEE2D QUANTUM RINGS AND QUANTUM DOTS: OPTICAL PROPERTIES AND ANNEALING PROCESS

M. Triki1, D. Elmaghraoui1, and S. Jaziri1,2

1Laboratoire de Physique de la Matière Condensée, Faculté des Sciences de Tunis, Université Tunis El Manar, Tunisia 2Laboratoire de Physique et des Matériaux, Département de Physique, Bizerte, Université de Carthage, Tunisia

Semiconductor nanostructures are of major interest for technological applications as well as being a very interesting arena for basic physics. Thus, a detailed computation and understanding of their properties is much needed. Among various topics in which are interested many experimental and theoretical researches, this works, has focused on two subjects reported in two separated parts. In first part, we discuss the effects of interdiffusion on self-assembled InAs/GaAs quantum dot (QD) under thermal process. In fact, although better device performance had been expected from self-assembled QDs, it has been found that such growth method usually resulted in ensembles of dots that vary in size and shape, resulting in inhomogeneous broadening of the photoluminescence emission of the dots which negates their advantages and constitutes a technical barrier for the development of optoelectronic devices. Therefore, the need to use a narrowing technique of the photoluminescence emission is presently a major goal among the researchers. In other hand, several published works [1–3] have evidenced that blue shifts and narrowing of optical emission of QDs can be induced by a rapid thermal annealing (RTA) procedure. In this context, we study the modification of the well depths and the effective masses profiles induced by the interdiffusion of the QD’s constituent atoms across its interfaces under high temperatures. Then, we calculate the electron and the hole spectrums in these intermixed QDs. A good agreement is shown when calculated energy transitions are compared with some previously published experimental data. We found also, that such QDs retain their zero-dimensional density of states even after the diffusion of the potential[4]. The second part of the work is devoted to optical properties of InAs/GaAs quantum ring. Such nanostructures are obtained when a pause is introduced after cupping the dots with a GaAs layer thinner than the InAs island height [5] . Nanostructure with a ring like shape is of particularly interest due to its peculiar optical emission. Otherwise, external electric fields applied on Quantum rings can provides two different behaviors for the electronic structure which depend on the direction of the electric field; this is due to the hole in the middle of the QR. While an axial electric field changes the positions of the energy levels under the quantum confined Stark effect, a lateral electric field decreases the symmetry of problem and mixes the energy levels and wave functions of the QR. The main idea of this work is to investigate theoretically the optical emission and the electronic structure of InGaAs/GaAs QRs subjected to electric field. By using a numerical method and a model of QR very close to real one, our results clearly show that the one-electron ground state presents an absolute minimum when studied as a function of the ring radius. For large sizes, the electron levels become close to each other: quasibidimensional limit. Comparing our results to experimental ones, we found that the calculated spectra well reproduce the experiment [6]. A lateral electric-field applied to the QR gives rise to anticrossings between the energy levels.

References [1] A. Babinski and J. Jasinski, Thin Solid Films 412, 84 (2002). [2] E. A. Zibik, W. H. Ng, L. R. Wilson, M. S. Skolnick, and J. W. Cockburn, Appl. Phys. Lett. 90, 163107 (2007). [3] J. J. Dubowski, C. Nì Allen, and S. Fafard, Appl. Phys. Lett. 77, 3584 (2000). [4] M. Triki, S. Jaziri and R. Bennaceur J. Appl. Phys. 111, 104304 (2012).

CEE2D | PISA IT | 2015 49 [5] J. M. Garcia, G. Medrios. Ribeiro, K. Schmidt, T. Ngo, J. L. Fing, A. Lorke, J. P. Kotthaus, and P. M. Petroff, Appl. Phys. Lett. 71, 2014 (1997). [6] M. Triki, D. Elmaghraoui, S. Jaziri and R. Bennaceur, Solid State Communication 150, 2197 (2010).

50 www.web.nano.cnr.it/CEE2D

This booklet was edited by Luisa Neri, Cnr Nano S3, Italy. September 20. 2015