Superconductivity and Charge Density Waves in the Clean 2D Limit

A. W. Tsen University of Waterloo Understanding and Controlling Electronic Phases of Matter

Metal Charge Density Wave

Au Superconductor NbSe3 BSCCO

NdFeB Spin Density Wave Insulator Ferromagnet

Cr Understanding and Controlling Electronic Phases of Matter

Metal Charge Density Wave

Au Superconductor NbSe3 S-I transition BSCCO

M-I transition NdFeB Spin Density Wave Insulator Ferromagnet

Cr Dimensional Control of Electronic Structure

Graphite k Graphene

Geim, Nature 2013 Bulk MoS2 Monolayer MoS2

Can we create new electronic phases in 2D? Structure and control of charge density waves in two-dimensional 1T-TaS2 Adam W. Tsena, Robert Hovdenb, Dennis Wangc, Young Duck Kimd, Junichi Okamotoe, Katherine A. Spothb, Yu Liuf, Wenjian Luf, Yuping Sunf,g,h, James C. Honed, Lena F. Kourkoutisb,i, Philip Kima,j,1, and Abhay N. Pasupathya,1 Abhay Pasupathy Philip Kim aDepartment of Physics, Columbia University, New York, NY 10027; bSchool of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853; cDepartment of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027; dDepartment of Mechanical Engineering, Columbia University, New York, NY 10027; eDepartment of Physics, University of Hamburg, D-20355 Hamburg, Germany; fKey Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’sRepublicofChina;gHigh Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China; hCollaborative Innovation Centre of Advanced Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China; iKavli Institute at Cornell for Nanoscale Science, Ithaca, NY 14853; and jDepartment of Physics, Harvard University, Cambridge, MA 02138

Edited by N. Phuan Ong, Princeton University, Princeton, NJ, and approved October 29, 2015 (received for review June 19, 2015) The layered transition metal dichalcogenides host a rich collection crystal at low and room temperature (C phase, blue panel; NC of chargeDistinct density wave surface phases in whichand both bulk the conduction charge electrons density waves in ultrathin 1T-TaS phase, red panel). The bright peaks (connected by2 dashed lines) Robert Hovden Lena Kourkoutis and the atomic structure display translational symmetry breaking. correspond to Bragg scattering from a triangular lattice of Ta Manipulating these complex states by purely electronic methods atoms with lattice constant a = 3.36 Å. Additional weaker dif- has been a long-sought scientific and technological goal. Here, we fraction peaks appear from the periodic atomic displacements of 1* 2 1 1 1 3 Ruishow He how, thisJunichi can be achieved Okamoto in 1T-TaS, 2 inZhipeng the 2D limit. Ye We first, Gaihuathe CDW. Ye In the, Heidi low-temperature Anderson C phase,, TaXia atoms Dai displace, to demonstrate that3 the intrinsic properties4 of atomically5 thin flakes make5 Star-of-David clusters5,6,7 (blue inset, Fig. 1B). The outer8 12 Xianxinare preserved Wu by, Jiangping encapsulation Hu with, hexagonal Yu Liu boron, W nitrideenjian in Lu , Yuping Sun , Abhay N. Pasupathy , 9* atoms within each star displace slightly inward toward the atom andinert Adam atmosphere. W. Tsen We use this facile assembly method together at the center, giving rise to a commensurate superstructure with with transmission electron microscopy and transport measurements wavelength λC = p13a that is rotated ϕC = 13.9° with respect to to probe the nature of the 2D state and show that its conductance is http://arxiv.org/abs/1603.021101 the atomic lattice. The NC phase at room temperature also Departmentdominated by of discommensurations. Physics, University The discommensuration of Northern struc- Iowa,consists Cedar of Falls, such 13-atom Iowaffiffiffiffiffi distortions.50614, USA Scanning tunneling micro- ture can be precisely tuned in few-layer samples by an in-plane scope (STM) measurements have revealed, however, that such electric current, allowing continuous electrical control over the Rui He Junichi Okamoto 2 ordering is only preserved in quasi-hexagonal domains consisting Departmentdiscommensuration-melting of Physics, transition University in 2D. of Hamburg, D-20355 Hamburg, Germany of tens of stars (11, 12), with domain periodicity 60–90 Å depending on temperature (13, 14). The domains are separated by a dis- two-dimensional materials strongly correlated systems 3LETTERS | | commensuration network forming a Kagome lattice, inside of which Institutecharge density of Physics, waves Chinese Academy of Sciences, Beijing 100190, People’s Republic of China PUBLISHED ONLINE: 7 DECEMBER 2015 | DOI: 10.1038/NPHYS3579 the Ta displacements are substantially reduced (15). A schematic of this structure is shown in the red inset of Fig. 1B. 4 ayered 1T-TaS2 exhibits a number of unique structural and Department of Physics and Astronomy, Purdue University,When ultrathinWest Lafayette, 1T-TaS2 crystals Indiana (approximately 47907<5nmthickness) Lelectronic phases. At low temperature and ambient pressure, are exfoliated in an ambient air environment, the CDW structure is the ground state is a commensurate (C) charge density wave 5 not observed by the TEM electron diffraction. In the left panel of Key(CDW). Laboratory On heating, of it undergoes Materials a sequence Physics, of first-order Institute phase of Fig. Solid 1C,weshowaroomtemperatureelectrondiffractionpattern State Physics, Chinese Academy of transitions to a nearly commensurate (NC) CDW at 225 K, to an Nature of the quantum metaltaken in on a a few-layer two-dimensional flake. The presence of Bragg peaks without Sciences,incommensurate Hefei (IC)230031, CDW atPeople’s 355 K, and Republic finally to aof metallic China CDW scattering suggests that the 1T-TaS layers are in a phase that Ben Hunt Cory Dean phase at 545 K. Each transition involves both conduction elec- 2 crystalline6 tron and lattice degrees superconductor of freedom—large changes in electronic Hightransport Magnetic properties F occur,ield concomitant Laboratory, with structural Chinese changes AcademySignificance of Sciences, Hefei 230031, People’s Republicto the crystal. 1of China By either1 † chemical doping2 or applying3 high pres-3,4 5 2 6 1 A. W.sures, Tsen it is, B. possible Hunt to, suppressY. D. Kim the, CDWs Z. J. Yuan and induce, S. Jia super-, R. J.The Cava ability, J. to Hone electrically, P. control Kim , collective C. R. Dean electron* states is a – 1 central goal of materials research and may allow for the de- and7 conductivity A. N. Pasupathy (1 3). For device applications, it is desirable to Collaborativecontrol these phases Innovation* by electrical Centre means, but of this Advanced capability is Microstructures,velopment of novel Nanjing devices. 1T-TaS University,2 is an ideal Nanjing candidate for 210093,difficult People's to achieve Republic in bulk crystals of dueChina to the high conduction such devices due to the existence of various charge ordered Two-dimensionalelectron density. (2D) Recent materials efforts are to not produce expected thin to samples be metals by me-superconductorstates in its exfoliated phase diagram. from a layered, Although single various crystal, techniques can exist have at lowchanical temperature exfoliation owing provide to electron a new localization avenue for1. manipulating Consistent thein the regimebeen demonstrated of minimal disorder, to manipulate allowing charge new order insight in into 1T-TaS the2,a 8 fundamental understanding of the effects is still lacking, and withCDWs this, pioneering in 1T-TaS2 studies(4–8). These on thin studies films have reported demonstrated only thenature of the vortex state in two dimensions. In this work, we Department of Physics, Columbia University, New York,the methods New York used are 10027, incompatible USA with device fabrication. By superconductingsuppression of and CDW insulating phase transitions ground states, using with polar a electrolytes, direct realize such a system using a clean bilayer of NbSe2, a well- transition as well between as resistive the switching two as between a function the different of disorder phases. or As the using both high-resolution transmission electron microscopy19,20 9 knownand type-II electronic superconductor transport to with investigateTc 7 K atomically in bulk thin form 1T-TaS. magneticInstitutematerial field approaches2 for–6. However, Quantum the more 2D limit, recentComputing however, works have significant and revealed Department changes of Chemistry, University⇠ of Waterloo, 2 Uniquesamples, to thissample, we clarify the the normal microscopic state sheet nature resistance of the charge is two or- have been observed in the transport properties (4, 5, 8). How- /( )2 the presence of an intermediate quantum metallic state ordersdered of magnitude phases in below the 2Dh limit2e andand further insulating control behaviour them by is all- Waterloo,ever, the microscopic Ontario N2L nature 3G1, of the 2DCanada state remains 7–10 unclear. In occupying a substantial region of the phase diagram , whose never observed.electrical means. Instead, the intermediate metallic phase emerges nature this is work, intensely we use debated transmission11–17. Here, electron we microscopy observe such (TEM) a state togetherin an exceptionally large region of the magnetic field–temperature * with transport measurements to develop a systematic understanding in theCorrespondence disorder-free limit to: of awtsen@ a crystallineuwaterloo.ca 2D superconductor,, [email protected] diagram. contributions: Unlike A.W.T., the typical P.K., and exponentialA.N.P. designed research;behaviour A.W.T., associated R.H., D.W., Y.D.K., of the CDW phases and phase transitions in ultrathin 1T-TaS2.We 7,15 produced by mechanical co-lamination of NbSe2 in an inert withK.A.S., the quantum Y.L., W.L., Y.S., tunnelling J.C.H., and L.F.K. of performedfermionic research; quasiparticles A.W.T., J.O., P.K.,,we and A.N.P. find that charge ordering disappears in flakes with few atomic layers analyzed data; and A.W.T., P.K., and A.N.P. wrote the paper. atmosphere. Under a small perpendicular magnetic field, we observe a new power-law scaling as a function of field at low due to surface oxidation. When samples are instead environ- induceAbstract: a transition from superconductor to the quantum metal. temperatureThe authors that declare is consistent no conflict of with interest. the Bose-metal scenario of the mentally protected, the CDWs persist and their transitions can We find a unique power-law scaling with field in this phase, metallicThis phase article is11 a–14 PNAS. Direct Submission. be carefully tuned by electric currents. 1 which is consistent with the Bose-metal model where metallic To whom correspondence may be addresssed. Email: [email protected] or Both the atomic and CDW structure of 1T-TaS can be visualized Earlier studies on isolated NbSe2 flakes do not observe behaviour arisesWe employ from strong low phase-frequency fluctuations Raman caused2 byspectroscopy the [email protected]. to study the nearly commensurate (NC) in reciprocal space by TEM electron diffraction (9, 10). In Fig. 1Aa, superconducting transition to a zero-resistance state in the magnetic field11–14. This article contains supporting21,22 information online at www.pnas.org/lookup/suppl/doi:10. we show diffraction images taken from a bulk-like, 50-nm-thickatomically1073/pnas.1512092112/-/DCSupplemental thin limit . Recently, however,. it has been shown the Global superconductivity emerges in a sample when conduction surfaces of metallic materials may oxidize, altering the electronic to commensurate (C) charge density wave (CDW) transition in 1T-TaS2 ultrathin flakes electrons form Cooper pairs and condense into a macroscopic, properties of thin samples23. Exfoliation and encapsulation by a phase-coherent15054–15059 quantum| PNAS | state.December In 8, two 2015 dimensions,| vol. 112 | no. the 49 phase protective layer in an inert atmosphere www.pnas.org/cgi/doi/10.1073/pnas.1512092112 is thus crucial for preserving coherenceprotected can befrom disrupted oxidation. even at zero We temperature identify by new increasing modesthe that intrinsic originate properties from of theC phase 2D material CDW23,24. phonon We achieves on this disorder—either by degrading sample quality or by applying using a mechanical transfer set-up installed inside a nitrogen- magnetic fields to create vortices2. Granular or amorphous filled glove box (see Methods). In short, within the glove box, an superconductingthe surface thinlayers films,. for By which monitoring disorder levels an can individual be varied exfoliated mode with NbSe 2 temperature,flake is electrically we contacted find by that graphite surface (G), ands during growth, have thus provided an established platform for the entire device is protected by an insulating layer of hexagonal the study of quantum phase transitions in 2D superconductors. boron nitride (BN). The graphite leads are then contacted using Withinundergo the conventional a separate theoretical, low-hysteresis framework, increasing NC-C phase sample transitionan edge-metallization that is decoupled technique in thefrom ambient the environment transition25 ,26in.A disorder or magnetic field perpendicular to a strongly disordered schematic depicting the assembly and fabrication process is shown film at T 0 induces a direct transition to an insulating state as in Fig. 1a and optical images of the heterostructure are shown in the bulk= layers. This indicates the activation of a secondary phase nucleation process in the limit the normal state sheet resistance approaches the pair quantum Fig. 1b before (left) and after (right) device fabrication. resistance h/(2e)2 6.4 k (refs 2,4). As film quality improved In the main panel of Fig. 2a, we show four-terminal sheet = overof time,weak however, interlayer an intervening CDW metallicinteraction phase, with which resistance can beresistance understood as a function from energy of temperature considerations for a particular. NbSe2 much lower than the normal state resistance has been observed in device prepared using the method described above. The NbSe2 several systems with generally less disorder7–10. Its origin is not well thickness is 1.5 nm, as determined by an atomic force microscope understood, and the various theoretical treatments can be divided (AFM) outside the glovebox after BN/graphite transfer, suggesting between bosonic-based models, in which Cooper pairing persists in 1 it consists of only two atomic layers. The resistance in the normal 11–14 the metallic phase but phase coherence is lost , and models that state is RN 75 . This corresponds to a residual resistivity that = also incorporate other fermionic degrees of freedom15–17. is ten times larger than that of bulk crystals20, yet the sheet Recently, mechanical exfoliation has emerged as a technique resistance is still an order of magnitude less than that of the most to produce ultraclean, crystalline 2D materials, with graphene conductive amorphous superconducting films10. We observe a clear being a well-known example18. Like amorphous films, the thickness superconducting transition to a zero-resistance state measured to of these samples can be easily controlled down to the level of the limit of our instrument resolution. The critical temperature is individual atomic layers. In contrast to amorphous films, a 2D T 5.26 K, as defined by where the resistance is 90% of the normal c =

1Department of Physics, Columbia University, New York, New York 10027, USA. 2Department of Mechanical Engineering, Columbia University, New York, New York 10027, USA. 3International Center for Quantum Materials, Peking University, Beijing 100871, China. 4Collaborative Innovation Center of Quantum Matter, Beijing 100871, China. 5Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA. 6Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA. †Present address: Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA. *e-mail: [email protected]; [email protected]

CDW (1T-TaS2, SDW TaSe2, (FeTe) VSe2, TiSe2)

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