DOCSLIB.ORG

Better Living Through Quantum Mechanics Seth Lloyd

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Better Living Through Quantum Mechanics Seth Lloyd Quantum Biology: Better Living Through Quantum Mechanics Seth Lloyd A quantum computer is a serious piece of hardware. My colleagues and I build quantum computers from superconducting systems, quantum dots, lasers operating on nonlinear crystals, and the like. Although the part of a quantum computer that actually performs the calculation is too small to be seen even under a microscope, the apparatus used to address and control the quantum computer typically takes up an entire laboratory full of equipment. In order to keep their sensitive components shielded from the environment, many quantum computers have to operate at very low temperatures, sometimes a few thousandths of a degree above absolute zero. So in the spring of 2007 when the New York Times reported that green sulphur-breathing bacteria were performing quantum computations during photosynthesis, my colleagues and I laughed. We thought it was the most crackpot idea we had heard in a long time. Closer examination of the paper, published in Nature, however, showed that something decidedly non-crackpot was going on. It is not easy being quantum. Photosynthesis converts light from the Sun into chemically useful energy inside cells. In photosynthesis, particles of light called photons are absorbed by light-sensitive molecules called chromophores (“light carriers” in ancient Greek), which are arranged in a tightly bound structure called an antenna photocomplex. When a photon is absorbed, a quantum particle of energy called an exciton is generated. (An exciton isn’t a particle in the traditional sense, but it acts enough like a particle that physicists find it useful to treat it as one. Such mathematical likenesses are called “quasi-particles.”) The exciton hops from chromophore to chromophore inside the photocomplex until it arrives at the reaction center, an agglomeration of molecules that take in the exciton and transform its energy into a form that the living system can put to use to perform cellular metabolism, grow, and reproduce. The great majority of the energy used by living systems once came from photosynthesis: Every calorie that you consume came originally from excitons that hopped through the antenna photocomplex of a photosynthetic organism. By zapping complexes of photosynthetic molecules with lasers, the authors of the paper were able to show that the excitons use quantum mechanics to make their journey through the photocomplex more efficient. The experimental evidence was strong and compelling. The authors also speculated that the excitons were performing a particular quantum computation algorithm called a quantum search, in which the wave-like nature of propagation allows the excitons to zero in on their target. As it turns out, the excitons were performing a different kind of quantum algorithm called a quantum walk, but the “crackpot” fact remained: Quantum computation was helping the bacteria move energy from point A to point B. How could tiny bacteria be performing the kind of sophisticated quantum manipulations that it takes human beings a room full of equipment to perform? Natural selection is a powerful force. Photosynthetic bacteria have been around for more than a billion years, and during that time, if a little quantum hanky panky allowed some bacteria to process energy and reproduce more efficiently than other bacteria, then quantum hanky panky stuck around for the next generation. Nature is also the great nanotechnologist. Living systems operate on the basis of molecular mechanisms, where atoms and energy are channeled systematically through molecular complexes within the cell. The molecules in turn are assembled using the laws of quantum mechanics—quantum weirdness is always lurking just around the chemical corner. These quantum changes can either help or hinder energy transport. Natural selection ensures that the role of quantum weirdness in cellular energy transport is a beneficial one. How can quantum weirdness assist in energy transport? The answer lies in a phenomenon called wave-particle duality. Wave-particle duality means that waves—light and sound—are at bottom composed of particles—photons and phonons. Conversely, things that we think of as particles, such as electrons, atoms, or for that matter soccer balls, have waves associated with them. The quantum wave of a soccer ball is about the same size as the ball itself, and doesn’t extend halfway down the soccer field (although the ball can sometimes seem to be on Lionel Messi’s left and right foot at the same time). But the wave corresponding to a particle can be much larger than the particle itself. While a single exciton consists of an excited electron within a chromophore, the wave corresponding to a propagating exciton can extend over many chromophores. Does that make sense? Of course not! Quantum mechanics is fundamentally strange and counterintuitive: Quantum particles don’t behave like soccer balls. To see how the wavelike nature of excitons can assist in their propagation, first visualize a classical kind of exciton dynamics. Imagine that each chromophore is a lilypad, and the exciton is a frog hopping randomly between neighboring lilypads. The frog starts at the edge of the circular lilypond. How long does it take to get to the pond’s center? Because the frog is hopping from lilypad to lilypad at random, it sometimes moves towards the center of the pond, but it is equally likely to move to the left or right, or even backward. The frog will land on a substantial fraction of all the lilypads in the pond on its way to the center. The number of hops the frog has to take to get to the center is proportional to the number of lilypads in the pond. Now consider a quantum frog. The frog’s initial wave is circularly symmetric at the edge of the lilypond and propagates inward, like a backward version of the wave created when you drop a stone in the center of a pond. The time it takes for the wave to travel from the shore to the center of the pond is proportional to the radius of the lilypond. But the radius of the lilypond goes as the square root of the number of lilypads in the pond, because the number of lilypads is proportional to the area of the pond, i.e., the radius squared. The wavelike nature of propagation in quantum mechanics—the “quantum hop”—has the potential to get the frog to the center of the pond much more quickly than the “classical hop.” So, for example, if the frog hops once a minute, and a classical frog takes 100 minutes to get to the center, then the quantum frog takes only 10 minutes. In green sulphur-breathing bacteria, the antenna photocomplex through which the excitons propagate is like the lilypond for the quantum frog: The waves corresponding to the excitons are spread out over many chromophores, and wavelike propagation allows excitons to move more quickly from chromophore to chromophore than classical hopping would allow. Together with Alan Aspuru-Guzik and Patrick Rebentrost at Harvard, my MIT colleague Masoud Mohseni and I constructed a general theory of how quantum walks in photosynthesis can use the wavelike nature of quantum mechanics to attain maximum efficiency. It turns out that wavelike transport is not always the best strategy. To understand why, suppose that the lilypond is full of rocks sticking up out of the water. As the wave moves through the pond, it scatters off the rocks. As a result, the wave never reaches the middle of the pond, which remains calm and protected. This is a phenomenon called destructive interference. Although the wave can propagate a short distance, eventually the random waves scattered off the rocks interfere with the overall wave’s propagation, effectively stopping it in its tracks. The quantum frog becomes completely stuck: A classical hopping strategy would have been more efficient. In the antenna photocomplex, the “rocks” are microscopic irregularities and molecular disorder that scatter the quantum wave as it tries to pass through. By constructing detailed quantum mechanical models, my collaborators and I were able to identify the optimal strategy for the interplay between wavelike propagation and classical hopping in photosynthesis. Over short distances, the wavelike propagation is more effective than random hopping. The exciton travels like a wave right up to the distance at which destructive interference causes it to get stuck. At this point, the fact that living systems are hot, wet environments comes into play: The environment effectively gives the exciton a whack that gets it unstuck and makes it perform a classical hop, which frees up the exciton to propagate again. (The technical term for this whack is “decoherence.”) Then the process repeats. The wave propagates until it gets stuck; the environment gives it a whack; the exciton hops. Eventually, the exciton reaches the reaction center in the minimum possible time. Expressed in terms of our quantum frog, the rule is simple: Wave until you get stuck, then hop. The birds, the bees, and the fruit flies Photosynthetic plants and bacteria are masters of the minutiae of quantum mechanics, manipulating quantum coherence and decoherence to attain almost 100% energy transport efficiency. If quantum hanky panky is so effective, are there other living beings that take advantage of quantum effects to live better and have more offspring? Only in the case of photosynthesis have scientists actually found the smoking gun (or maybe the smoking photon). However, there are several other organisms in which quantum mechanisms apparently play an important role. European robins are sensitive to the Earth’s magnetic field, which helps them during migration. Do they have a tiny compass in their heads, a piece of magnetite that swivels back and forth to point out magnetic north? Apparently not. Instead the evidence suggests another light-activated quantum mechanism.
Load more

Recommended publications
  • UNIVERSITY of CALIFORNIA, SAN DIEGO Exciton Transport
    UNIVERSITY OF CALIFORNIA, SAN DIEGO Exciton Transport Phenomena in GaAs Coupled Quantum Wells A dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy in Physics by Jason R. Leonard Committee in charge: Professor Leonid V. Butov, Chair Professor John M. Goodkind Professor Shayan Mookherjea Professor Charles W. Tu Professor Congjun Wu 2016 Copyright Jason R. Leonard, 2016 All rights reserved. The dissertation of Jason R. Leonard is approved, and it is acceptable in quality and form for publication on microfilm and electronically: Chair University of California, San Diego 2016 iii TABLE OF CONTENTS Signature Page . iii Table of Contents . iv List of Figures . vi Acknowledgements . viii Vita........................................ x Abstract of the Dissertation . xii Chapter 1 Introduction . 1 1.1 Semiconductor introduction . 2 1.1.1 Bulk GaAs . 3 1.1.2 Single Quantum Well . 3 1.1.3 Coupled-Quantum Wells . 5 1.2 Transport Physics . 7 1.3 Spin Physics . 8 1.3.1 D'yakanov and Perel' spin relaxation . 10 1.3.2 Dresselhaus Interaction . 10 1.3.3 Electron-Hole Exchange Interaction . 11 1.4 Dissertation Overview . 11 Chapter 2 Controlled exciton transport via a ramp . 13 2.1 Introduction . 13 2.2 Experimental Methods . 14 2.3 Qualitative Results . 14 2.4 Quantitative Results . 16 2.5 Theoretical Model . 17 2.6 Summary . 19 2.7 Acknowledgments . 19 Chapter 3 Controlled exciton transport via an optically controlled exciton transistor . 22 3.1 Introduction . 22 3.2 Realization . 22 3.3 Experimental Methods . 23 3.4 Results . 25 3.5 Theoretical Model .
    [Show full text]
  • Chapter 09584
    Author's personal copy Excitons in Magnetic Fields Kankan Cong, G Timothy Noe II, and Junichiro Kono, Rice University, Houston, TX, United States r 2018 Elsevier Ltd. All rights reserved. Introduction When a photon of energy greater than the band gap is absorbed by a semiconductor, a negatively charged electron is excited from the valence band into the conduction band, leaving behind a positively charged hole. The electron can be attracted to the hole via the Coulomb interaction, lowering the energy of the electron-hole (e-h) pair by a characteristic binding energy, Eb. The bound e-h pair is referred to as an exciton, and it is analogous to the hydrogen atom, but with a larger Bohr radius and a smaller binding energy, ranging from 1 to 100 meV, due to the small reduced mass of the exciton and screening of the Coulomb interaction by the dielectric environment. Like the hydrogen atom, there exists a series of excitonic bound states, which modify the near-band-edge optical response of semiconductors, especially when the binding energy is greater than the thermal energy and any relevant scattering rates. When the e-h pair has an energy greater than the binding energy, the electron and hole are no longer bound to one another (ionized), although they are still correlated. The nature of the optical transitions for both excitons and unbound e-h pairs depends on the dimensionality of the e-h system. Furthermore, an exciton is a composite boson having integer spin that obeys Bose-Einstein statistics rather than fermions that obey Fermi-Dirac statistics as in the case of either the electrons or holes by themselves.
    [Show full text]
  • Amplitude/Higgs Modes in Condensed Matter Physics
    Amplitude / Higgs Modes in Condensed Matter Physics David Pekker1 and C. M. Varma2 1Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15217 and 2Department of Physics and Astronomy, University of California, Riverside, CA 92521 (Dated: February 10, 2015) Abstract The order parameter and its variations in space and time in many different states in condensed matter physics at low temperatures are described by the complex function Ψ(r; t). These states include superfluids, superconductors, and a subclass of antiferromagnets and charge-density waves. The collective fluctuations in the ordered state may then be categorized as oscillations of phase and amplitude of Ψ(r; t). The phase oscillations are the Goldstone modes of the broken continuous symmetry. The amplitude modes, even at long wavelengths, are well defined and decoupled from the phase oscillations only near particle-hole symmetry, where the equations of motion have an effective Lorentz symmetry as in particle physics, and if there are no significant avenues for decay into other excitations. They bear close correspondence with the so-called Higgs modes in particle physics, whose prediction and discovery is very important for the standard model of particle physics. In this review, we discuss the theory and the possible observation of the amplitude or Higgs modes in condensed matter physics – in superconductors, cold-atoms in periodic lattices, and in uniaxial antiferromagnets. We discuss the necessity for at least approximate particle-hole symmetry as well as the special conditions required to couple to such modes because, being scalars, they do not couple linearly to the usual condensed matter probes.
    [Show full text]
  • Exciton Binding Energy in Small Organic Conjugated Molecule
    1 Exciton Binding Energy in small organic conjugated molecule. Pabitra K. Nayak* Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot, 76100, Israel Email: [email protected] Abstract: For small organic conjugated molecules the exciton binding energy can be calculated treating molecules as conductor, and is given by a simple relation BE ≈ 2 e /(4πε0εR), where ε is the dielectric constant and R is the equivalent radius of the molecule. However, if the molecule deviates from spherical shape, a minor correction factor should be added. 1. Introduction An understanding of the energy levels in organic semiconductors is important for designing electronic devices and for understanding their function and performance. The absolute hole transport level and electron transport level can be obtained from theoretical calculations and also from photoemission experiments [1][2][3]. The difference between the two transport levels is referred to as the transport gap (Et). The transport gap is different from the optical gap (Eopt) in organic semiconductors. This is because optical excitation gives rise to excitons rather than free carriers. These excitons are Frenkel excitons and localized to the molecule, hence an extra amount of energy, termed as exciton binding energy (Eb) is needed to produce free charge carriers. What is the magnitude of Eb in organic semiconductors? Answer to this question is more relevant in emerging field of Organic Photovoltaic cells (OPV), due to their impact in determining the output voltage of the solar cells. A lot of efforts are also going on to develop new absorbing material for OPV. It is also important to guess the magnitude of exciton binding energy before the actual synthesis of materials.
    [Show full text]
  • Pion-Induced Transport of Π Mesons in Nuclei
    Central Washington University ScholarWorks@CWU All Faculty Scholarship for the College of the Sciences College of the Sciences 2-8-2000 Pion-induced transport of π mesons in nuclei S. G. Mashnik R. J. Peterson A. J. Sierk Michael R. Braunstein Follow this and additional works at: https://digitalcommons.cwu.edu/cotsfac Part of the Atomic, Molecular and Optical Physics Commons, and the Nuclear Commons PHYSICAL REVIEW C, VOLUME 61, 034601 Pion-induced transport of ␲ mesons in nuclei S. G. Mashnik,1,* R. J. Peterson,2 A. J. Sierk,1 and M. R. Braunstein3 1T-2, Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 2Nuclear Physics Laboratory, University of Colorado, Boulder, Colorado 80309 3Physics Department, Central Washington University, Ellensburg, Washington 98926 ͑Received 28 May 1999; published 8 February 2000͒ A large body of data for pion-induced neutral pion continuum spectra spanning outgoing energies near 180 MeV shows no dip there that might be ascribed to internal strong absorption processes involving the formation of ⌬’s. This is the same observation previously made for the charged pion continuum spectra. Calculations in an intranuclear cascade model or a cascade exciton model with free-space parameters predict such a dip for both neutral and charged pions. We explore several medium modifications to the interactions of pions with internal nucleons that are able to reproduce the data for nuclei from 7Li through Bi. PACS number͑s͒: 25.80.Hp, 25.80.Gn, 25.80.Ls, 21.60.Ka I. INTRODUCTION the NCX data at angles forward of 90° by a factor of 2 ͓3͔.
    [Show full text]
  • Phonon-Exciton Interactions in Wse2 Under a Quantizing Magnetic Field
    ARTICLE https://doi.org/10.1038/s41467-020-16934-x OPEN Phonon-exciton Interactions in WSe2 under a quantizing magnetic field Zhipeng Li1,10, Tianmeng Wang 1,10, Shengnan Miao1,10, Yunmei Li2,10, Zhenguang Lu3,4, Chenhao Jin 5, Zhen Lian1, Yuze Meng1, Mark Blei6, Takashi Taniguchi7, Kenji Watanabe 7, Sefaattin Tongay6, Wang Yao 8, ✉ Dmitry Smirnov 3, Chuanwei Zhang2 & Su-Fei Shi 1,9 Strong many-body interaction in two-dimensional transitional metal dichalcogenides provides 1234567890():,; a unique platform to study the interplay between different quasiparticles, such as prominent phonon replica emission and modified valley-selection rules. A large out-of-plane magnetic field is expected to modify the exciton-phonon interactions by quantizing excitons into dis- crete Landau levels, which is largely unexplored. Here, we observe the Landau levels origi- nating from phonon-exciton complexes and directly probe exciton-phonon interaction under a quantizing magnetic field. Phonon-exciton interaction lifts the inter-Landau-level transition selection rules for dark trions, manifested by a distinctively different Landau fan pattern compared to bright trions. This allows us to experimentally extract the effective mass of both holes and electrons. The onset of Landau quantization coincides with a significant increase of the valley-Zeeman shift, suggesting strong many-body effects on the phonon-exciton inter- action. Our work demonstrates monolayer WSe2 as an intriguing playground to study phonon-exciton interactions and their interplay with charge, spin, and valley. 1 Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA. 2 Department of Physics, The University of Texas at Dallas, Richardson, TX 75080, USA.
    [Show full text]
  • Chapter 10 Dynamic Condensation of Exciton-Polaritons
    Chapter 10 Dynamic condensation of exciton-polaritons 1 REVIEWS OF MODERN PHYSICS, VOLUME 82, APRIL–JUNE 2010 Exciton-polariton Bose-Einstein condensation Hui Deng Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA Hartmut Haug Institut für Theoretische Physik, Goethe Universität Frankfurt, Max-von-Laue-Street 1, D-60438 Frankfurt am Main, Germany Yoshihisa Yamamoto Edward L. Ginzton Laboratory, Stanford University, Stanford, California 94305, USA; National Institute of Informatics, Hitotsubashi, Chiyoda-ku, Tokyo 101-8430, Japan; and NTT Basic Research Laboratories, NTT Corporation, Atsugi, Kanagawa 243-0198, Japan ͑Published 12 May 2010͒ In the past decade, a two-dimensional matter-light system called the microcavity exciton-polariton has emerged as a new promising candidate of Bose-Einstein condensation ͑BEC͒ in solids. Many pieces of important evidence of polariton BEC have been established recently in GaAs and CdTe microcavities at the liquid helium temperature, opening a door to rich many-body physics inaccessible in experiments before. Technological progress also made polariton BEC at room temperatures promising. In parallel with experimental progresses, theoretical frameworks and numerical simulations are developed, and our understanding of the system has greatly advanced. In this article, recent experiments and corresponding theoretical pictures based on the Gross-Pitaevskii equations and the Boltzmann kinetic simulations for a finite-size BEC of polaritons are reviewed. DOI: 10.1103/RevModPhys.82.1489 PACS number͑s͒: 71.35.Lk, 71.36.ϩc, 42.50.Ϫp, 78.67.Ϫn CONTENTS A. Polariton-phonon scattering 1500 B. Polariton-polariton scattering 1500 1. Nonlinear polariton interaction coefficients 1500 I. Introduction 1490 2. Polariton-polariton scattering rates 1502 II.
    [Show full text]
  • Amplitude Modes in Cold Atoms
    T01 Amplitude modes in cold atoms S.D. Huber, Institute for Theoretical Physics, ETH Zurich E-mail address: [email protected] The Bose Hubbard model exhibits a quantum phase transition between an insulating Mott state and a superfluid. We aim at the determination of the excitation spectrum of the superfluid phase, in particular its evolution from the vicinity of the Mott phase towards the weakly interacting regime. Furthermore, we discuss a method allowing to observe our findings in an experiment. We make the link between our microscopic findings and the notion of a “Higgs” mode. Specifically, we highlight the role of emergent symmetries. Recent developments in the description of amplitude modes in exotic chiral Mott insulators will be mentioned. [1] S.D. Huber, E. Altman, H.P. Buchler,¨ and G. Blatter, Phys. Rev. B, 75 085106 (2007) [2] S.D. Huber, B. Theiler, E. Altman, G. Blatter, Phys. Rev. Lett., 100 050404 (2008) [3] S.D. Huber and N.H. Lindner, Proc. Natl. Acad. Sci. USA, 108 19925 (2011) T02 Higgs mode and universal dynamics near quantum criticality D. Podolsky Department of Physics, Technion – Israel Institute of Technology, Haifa 32000, Israel E-mail address: [email protected] The Higgs mode is a ubiquitous collective excitation in condensed matter systems with broken continuous symmetry. Its detection is a valuable test of the corresponding field theory, and its mass gap measures the proximity to a quantum critical point. However, since the Higgs mode can decay into low energy Goldstone modes, its experimental visibility has been questioned. In this talk, I will show that the visibility of the Higgs mode depends on the symmetry of the measured susceptibility.
    [Show full text]
  • Arxiv:2006.01140V3 [Cond-Mat.Str-El] 28 Jul 2020
    arXiv:2006.01140 Deconfined criticality and ghost Fermi surfaces at the onset of antiferromagnetism in a metal Ya-Hui Zhang and Subir Sachdev Department of Physics, Harvard University, Cambridge, MA 02138, USA (Dated: July 29, 2020) Abstract We propose a general theoretical framework, using two layers of ancilla qubits, for deconfined criticality between a Fermi liquid with a large Fermi surface, and a pseudogap metal with a small Fermi surface of electron-like quasiparticles. The pseudogap metal can be a magnetically ordered metal, or a fractionalized Fermi liquid (FL*) without magnetic order. A critical `ghost' Fermi surface emerges (alongside the large electron Fermi surface) at the transition, with the ghost fermions carrying neither spin nor charge, but minimally coupled to (U(1) × U(1))=Z2 or (SU(2) × U(1))=Z2 gauge fields. The (U(1) × U(1))=Z2 case describes simultaneous Kondo breakdown and onset of magnetic order: the two gauge fields induce nearly equal attractive and repulsive interactions between ghost Fermi surface excitations, and this competition controls the quantum criticality. Away from the transition on the pseudogap side, the ghost Fermi surface absorbs part of the large electron Fermi surface, and leads to a jump in the Hall co-efficient. We also find an example of an \unnecessary quantum critical point" between a metal with spin density wave order, and a metal with local moment magnetic order. The ghost fermions contribute an enhanced specific heat near the transition, and could also be detected in other thermal probes. We relate our results to the phases of correlated electron compounds.
    [Show full text]
  • Exploring Exciton and Polaron Dominated Photophysical Phenomena in Ruddlesden–Popper Phases of Ban+1Zrns3n+1 (N = 1–3) From
    pubs.acs.org/JPCL Letter Exploring Exciton and Polaron Dominated Photophysical − Phenomena in Ruddlesden Popper Phases of Ban+1ZrnS3n+1 (n =1−3) from Many Body Perturbation Theory Deepika Gill,* Arunima Singh, Manjari Jain, and Saswata Bhattacharya* Cite This: J. Phys. Chem. Lett. 2021, 12, 6698−6706 Read Online ACCESS Metrics & More Article Recommendations *sı Supporting Information − ABSTRACT: Ruddlesden Popper (RP) phases of Ban+1ZrnS3n+1 are an evolving class of chalcogenide perovskites in the field of optoelectronics, especially in solar cells. However, detailed studies regarding its optical, excitonic, polaronic, and transport properties are hitherto unknown. Here, we have explored the excitonic and polaronic effect using several first- principles based methodologies under the framework of Many Body Perturbation Theory. Unlike its bulk counterpart, the optical and excitonic anisotropy are observed in Ban+1ZrnS3n+1 (n =1−3) RP phases. As per the Wannier−Mott approach, the ionic contribution to the dielectric constant is important, but it gets decreased on increasing n in Ban+1ZrnS3n+1. The exciton binding energy is found to be dependent on the presence of large electron−phonon coupling. We further observed maximum charge carrier mobility in the Ba2ZrS4 phase. As per our analysis, the optical phonon modes are observed to dominate the acoustic phonon modes, − leading to a decrease in polaron mobility on increasing n in Ban+1ZrnS3n+1 (n =1 3). − erovskites with the general chemical formula ABX3 have research toward its new phases named
    [Show full text]
  • Exciton-Polarons in Doped Semiconductors in a Strong Magnetic Field
    PHYSICAL REVIEW B 97, 235432 (2018) Exciton-polarons in doped semiconductors in a strong magnetic field Dmitry K. Efimkin and Allan H. MacDonald Center for Complex Quantum Systems, University of Texas at Austin, Austin, Texas 78712, USA (Received 19 July 2017; revised manuscript received 9 April 2018; published 21 June 2018) In previous work, we have argued that the optical properties of moderately doped two-dimensional semi- conductors can be described in terms of excitons dressed by their interactions with a degenerate Fermi sea of additional charge carriers. These interactions split the bare exciton into attractive and repulsive exciton-polaron branches. The collective excitations of the coupled system are many-body generalizations of the bound trion and unbound states of a single electron interacting with an exciton. In this paper, we consider exciton-polarons in the presence of an external magnetic field that quantizes the kinetic energy of the electrons in the Fermi sea. Our theoretical approach is based on a transformation to a new basis that respects the underlaying symmetry of magnetic translations. We find that the attractive exciton-polaron branch is only weakly influenced by the magnetic field, whereas the repulsive branch exhibits magnetic oscillations and splits into discrete peaks that reflect combined exciton-cyclotron resonance. DOI: 10.1103/PhysRevB.97.235432 I. INTRODUCTION dress excitons into exciton-polarons. Exciton-polarons have attractive and repulsive spectral branches that evolve from The monolayer transition metal dicholagenides (TMDCs), and generalize to degenerate carrier densities, the separate MoS , MoSe ,WS, and WSe , are widely studied 2 2 2 2 absorption processes associated with bound and unbound trion in beyond-graphene [1–3] two-dimensional semiconductors states.
    [Show full text]
  • Optical Physics of Quantum Wells
    Optical Physics of Quantum Wells David A. B. Miller Rm. 4B-401, AT&T Bell Laboratories Holmdel, NJ07733-3030 USA 1 Introduction Quantum wells are thin layered semiconductor structures in which we can observe and control many quantum mechanical effects. They derive most of their special properties from the quantum confinement of charge carriers (electrons and "holes") in thin layers (e.g 40 atomic layers thick) of one semiconductor "well" material sandwiched between other semiconductor "barrier" layers. They can be made to a high degree of precision by modern epitaxial crystal growth techniques. Many of the physical effects in quantum well structures can be seen at room temperature and can be exploited in real devices. From a scientific point of view, they are also an interesting "laboratory" in which we can explore various quantum mechanical effects, many of which cannot easily be investigated in the usual laboratory setting. For example, we can work with "excitons" as a close quantum mechanical analog for atoms, confining them in distances smaller than their natural size, and applying effectively gigantic electric fields to them, both classes of experiments that are difficult to perform on atoms themselves. We can also carefully tailor "coupled" quantum wells to show quantum mechanical beating phenomena that we can measure and control to a degree that is difficult with molecules. In this article, we will introduce quantum wells, and will concentrate on some of the physical effects that are seen in optical experiments. Quantum wells also have many interesting properties for electrical transport, though we will not discuss those here.
    [Show full text]