Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session V57: 2D Materials: Metals, Superconductors, and Correlated Materials - 4Live
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Sponsoring Units: DMP Chair: Kin Fai Mak, Cornell University |
Thursday, March 18, 2021 3:00PM - 3:36PM Live |
V57.00001: Magnetotransport in a strain superlattice of graphene Invited Speaker: Nadya Mason Three-dimensional (3D) deformation of two-dimensional materials offers a route toward band structure engineering. In the case of graphene, a spatially nonuniform deformation and strain are known to generate an effective magnetic field, i.e., a pseudomagnetic field, although experimental realization of this effect in electronic devices has been challenging. We demonstrate how controllable, device-compatible strain patterns in graphene can be engineered by depositing graphene onto closely packed nanospheres. The 3D deformation profile creates a strain superlattice, for which we study the magnetotransport behavior both experimentally and via quantum transport simulations. We observe a weakening of superlattice features as we increase the magnetic field, which we find to be consistent with competing interactions between the external magnetic field and the strain-induced pseudomagnetic field. Our results demonstrate that strain superlattices are promising platforms to modulate the band structure and engineer the electronic transport. |
Thursday, March 18, 2021 3:36PM - 3:48PM Live |
V57.00002: Dynamical behavior and comprehensive vortex phase diagram in a 2D superconductor Yoshihiro Iwasa, Yu Saito, Yuki Itahashi, Tsutomu Nojima Two-dimensional (2D) superconductors exhibit novel aspects that are distinct from those of the 3D counterparts. Recent advances have developed methods to produce ideal 2D electron systems, which enable us to investigate the intrinsic properties of clean 2D superconductors [1]. In this talk, we focus on vortex matter in gate-induced 2D superconductors. In addition to the magnetic-field induced quantum metallic state in an equilibrium state, which has been never seen in 3D superconductors where the ground state is vortex solid, we found a unique dynamical behavior of 2D vortices under large current in gated MoS2 [2]. We constructed a comprehensive vortex phase diagram combined with previous results, which we believe captures the intrinsic nature of vortex states in clean 2D superconductors. |
Thursday, March 18, 2021 3:48PM - 4:00PM Live |
V57.00003: Hierarchy among the crystal lattice, charge density wave and superconducting orders in transition metal dichalcogenides Valeri Petkov Layered TMDs exhibit a variety of lattice structures and collective electronic states, including charge density waves (CDW)s and superconductivity (SC). Studies have indicated that, generally, the CDW and SC orders in TMDs compete unless mutually exclude each other but lack details. We studied exemplary Ta(Te/Se)2 solid solutions extending between the stable polymorphs of their end members, hexagonal 2H-TaSe2 and monoclinic 1T’-TaTe2, using high energy x-ray diffraction and large scale structure modeling. We will show that the emergence of CDWs and SC in TMDs does not indeed depend critically on the type of the underlying crystal lattice but depends strongly on its perfection. In particular, TMDs exhibiting strong lattice distortions, such as buckling of TM planes, do not exhibit CDWs and SC. The presence of a perfect lattice order in 2D is a prerequisite to the emergence of CDWs but insufficient to achieve a SC ordered state. For the SC order to emerge, the TM-sublattice should also appear periodic in 3D. We will also present evidence that distortions of TM-chalcogenide coordination polyhedra degrade the SC order to a certain extent, and this may well explain the observed irregular evolution of the SC critical temperature, Tc, with the degree of chalcogenide substitution. |
Thursday, March 18, 2021 4:00PM - 4:12PM Live |
V57.00004: Superconducting properties of a polar superconductor 3R-Ta1+xSe2 Maki Musashi, Yuki Tanaka, Hideki Matsuoka, Yoshihiro Iwasa, Masaki Nakano 2D materials research is one of the hot topics in condensed-matter science, but most studies are performed on nanometer-thick crystals fabricated either by mechanical exfoliation or by CVD. On the other hand, the bottom-up approach by MBE is very much limited despite its many advantages from materials science viewpoint, for example, availability of a large-area sample, potential creation of novel quantum 2D materials even including hardly-cleavable and thermodynamically-metastable compounds, as well as scalability to a variety of heterostructures. In this presentation, we will report MBE growth of 3R-Ta1+xSe2, a thermodynamically-metastable polar superconductor, and its superconducting properties, which turned out to be very much different from those of thermodynamically-stable 2H-TaSe2 in terms of Tc and superconducting anisotropy [1]. [1] Y. Tanaka et al., Nano Lett. 20, 1725 (2020). |
Thursday, March 18, 2021 4:12PM - 4:24PM Live |
V57.00005: Spin-Orbit-Parity-Coupled Superconductivity in Topological Monolayer WTe2 Yingming Xie, Benjamin T. Zhou, Kam Tuen Law Recent experiments reported gate-induced superconductivity in the monolayer 1T'-WTe2 which is a two-dimensional topological insulator in its normal state. The in-plane upper critical field Bc2 is found to exceed the conventional Pauli paramagnetic limit Bp by one to three times. The enhancement cannot be explained by the conventional spin-orbit coupling which vanishes due to inversion symmetry. In this Letter, we unveil some distinctive superconducting properties of centrosymmetric 1T'-WTe2 which arise from the coupling of spin, momentum and band parity degrees of freedom. As a result of this spin-orbit-parity coupling (SOPC): (i) there is a first-order superconductor-metal transition at Bc2 that is much higher than the Pauli paramagnetic limit Bp, (ii) spin-susceptibility is anisotropic with respect to in-plane directions and can result in possible anisotropic Bc2, and (iii) the Bc2 exhibits a strong gate dependence as the spin-orbit parity coupling is significant only near the topological band crossing points. The importance of SOPC on the topologically nontrivial inter-orbital pairing phase is also discussed. Our theory generally applies to centrosymmetric materials with topological band inversions. |
Thursday, March 18, 2021 4:24PM - 4:36PM Live |
V57.00006: Evidence of ideal excitonic insulator in bulk MoS2 under pressure Samaneh Ataei, Daniele Varsano, Elisa Molinari, Massimo Rontani Spontaneous condensation of excitons is a long sought phenomenon analogous to the condensation of Cooper pairs in a superconductor. It is expected to occur in a semiconductor at thermodynamic equilibrium if the binding energy of the excitons overcomes the band gap, giving rise to a new phase: the 'excitonic insulator' (EI). Transition metal dichalcogenides are excellent candidates for the EI realization because of reduced Coulomb screening, and indeed a structural phase transition was observed in few-layer systems. However, previous work could not disentangle to which extent the origin of the transition was in the formation of bound excitons or in the softening of a phonon. Here we focus on bulk MoS2 and predict from first principles that at high pressure it is prone to the condensation of genuine excitons of finite momentum, whereas the phonon dispersion remains regular [arXiv:2011.02380]. We show that the self-consistent electronic charge density of the EI sustains an out-of-plane permanent electric dipole moment with an in-plane antiferroelectric texture: At the onset of the EI phase, those optical phonons that share the exciton momentum provide a Raman fingerprint for the EI formation, which was observed experimentally. |
Thursday, March 18, 2021 4:36PM - 4:48PM Live |
V57.00007: Flat-Bands-Enabled Triplet Excitonic Insulator in a Di-atomic (Yin-Yang) Kagome Lattice Gurjyot Sethi, Linghan Zhu, Li Yang, Feng Liu The excitonic insulator (EI) state is a strongly correlated many-body ground state, arising from an instability in the band structure of narrow-gap semiconductors towards exciton formation. Here we show that the flat valence and conduction bands of a Yin-Yang Kagome lattice, as exemplified in a superatomic graphene lattice, conspire to enable an interesting state of triplet EI, based on first-principles calculations combined with many-body GW and Bethe-Salpeter equation. As a natural manifestation of flat bands, highly localized electron and hole wavefunctions significantly reduce the screening and enhance the exchange interaction. This leads to an unusually high triplet exciton binding energy (~1.2 eV) exceeding the GW band gap by ~0.2 eV and a huge singlet-triplet splitting of ~0.4 eV. The singlet exciton binding energy is still very large with a long intrinsic lifetime. The flat-bands-enabled triplet EI state also points to the possibility of complete population inversion between the Yin-Yang topological flat bands for the realization of excited quantum spin Hall effect. |
Thursday, March 18, 2021 4:48PM - 5:00PM Live |
V57.00008: Doping dependence of the electronic structure of potassium-doped fullerenes studied by ARPES Ping Ai, Drew W Latzke, Claudia Ojeda-Aristizabal, Ryo Mori, Alex K Zettl, Jonathan Denlinger, Alessandra Lanzara Quantum many-body interactions trigger new and unexpected emergent phenomena, such as superconductivity and magnetism, even in weak correlation materials, as recently exploited in twisted bilayer graphene. One classical example is superconductivity in Alkali-doped fullerides (e.g. K3C60) where conventional BCS theory and unconventional Mott physics meet. Despite a large number of works, the origin and the detailed mechanism of such a state remain still elusive. Here, by using angle-resolved photoemission spectroscopy on fullerene films grown on the Bi2Se3 substrate, we study the insulator-metal transition derived by doping. With a controllable potassium doping mechanism, doping dependence of the electronic band structures is investigated. |
Thursday, March 18, 2021 5:00PM - 5:12PM Live |
V57.00009: An efficient fluctuation exchange approach to low-temperature spin fluctuations and superconductivity: from the Hubbard model to NaxCoO2 * yH2O Niklas Witt, Erik van Loon, Takuya Nomoto, Ryotaro Arita, Tim Wehling Superconductivity arises mostly at energy and temperature scales that are much smaller than the typical bare electronic energies. Since the computational effort of diagrammatic many-body techniques increases with the number of required Matsubara frequencies and thus with the inverse temperature, phase transitions that occur at low temperatures are typically hard to adress numerically. In this work, we implement a fluctuation exchange (FLEX) approach to spin fluctuations and superconductivity using the "intermediate representation basis" (IR) [Shinaoka et al., PRB 96, 2017] for Matsubara Green functions. This FLEX+IR approach is numerically very efficient and enables us to reach temperatures on the order of 10-4 in units of the electronic band width in multi-orbital systems. After benchmarking the method in the doped repulsive Hubbard model on the square lattice, we study the possibility of spin fluctuation mediated superconductivity in the hydrated sodium cobalte material NaxCoO2 * yH2O reaching the scale of the experimental transition temperature Tc = 4.5 K and below. |
Thursday, March 18, 2021 5:12PM - 5:24PM Live |
V57.00010: Interpreting angle-dependent magnetoresistance (ADMR) measurements in pseudogapped cuprates Seth Musser, Debanjan Chowdhury, Brad Ramshaw, Patrick A Lee, Senthil Todadri We provide a model for thinking about ADMR in the small magnetic field limit and discuss what recent measurements of ADMR near the pseudogap critical point of Nd-LSCO are actually able to deduce about the Fermi surface. We further include the quasiparticle residue in these calculations and use our model to understand why its inclusion doesn't qualitatively alter the predicted ADMR for the spin density wave reconstructed Fermi surface. We attempt to propose a Fermi surface where the inclusion of the quasiparticle residue will alter the qualitative form of ADMR. |
Thursday, March 18, 2021 5:24PM - 5:36PM Live |
V57.00011: Superconductivity in bilayer Td-MoTe2 Apoorv Jindal, Daniel A Rhodes, Takashi Taniguchi, Kenji Watanabe, Cory Dean, James Hone, Abhay Narayan Encapsulated heterostructures of exfoliated van der Waals superconductors offer us a platform to observe 2D superconductivity in the clean limit. Low carrier density superconductors are of even more interest since their electronic properties can be tuned with an electric field or electrostatic doping. In this talk, we present our results on Td-MoTe2. We show that bilayer Td-MoTe2 is a superconductor with a Tc of ~ 2.5 K which can be tuned by electrostatic doping. Together with magnetotransport experiments and analysis, we conclude that superconductivity here requires the presence of both carrier types, electrons and holes. Doping into a region of one dominant carrier turns the material normal leading to a dome-shaped superconducting phase diagram as a function of electrostatic doping and temperature. We discuss possible interband pairing mechanisms that can give rise to such behavior. |
Thursday, March 18, 2021 5:36PM - 5:48PM Live |
V57.00012: First-principles study on electronic structure of Cr1/3TaS2 Minsung Kim, Kristjan Haule, David Vanderbilt MrM’X2 (with M = V, Cr, Mn, Fe, Co, Ni; M’ = Nb, Ta; X = S, Se; r = 1/3, 1/4) are layered materials in which M atoms are intercalated in the van der Waals gaps between M’X2 layers, exhibiting interesting structural and magnetic properties. Here we investigate the electronic structure of Cr1/3TaS2 using first-principles methods based on a combination of density functional theory and embedded dynamical mean field theory, partly motivated by recent experimental observations of unusual optical responses. We find that the system has localized orbitals in Cr 3d states as well as itinerant ones in Cr 3d and Ta 5d, where the metallic states with small scattering rates indicate a conventional Fermi liquid. The possibility of inducing an orbital-selective Mott transition is also explored. We further investigate the optical conductivity to examine the origin of the experimentally observed optical changes. Our study is important for the theoretical understanding of the electronic structure and the optical properties of Cr1/3TaS2. |
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