Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session W60: SuperconductivityFocus Recordings Available
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Chair: Christopher Gutierrez, UCLA Room: Hyatt Regency Hotel -DuSable C |
Thursday, March 17, 2022 3:00PM - 3:36PM |
W60.00001: Elucidating the structure of the charge density wave in the topological kagome metal CsV3Sb5 Invited Speaker: John W Harter The recent discovery of the AV3Sb5 (A = K, Rb, Cs) material family offers an exciting opportunity to investigate the interplay of correlations, topology, and superconductivity in kagome metals. Emerging from a topologically nontrivial band structure, an unusual charge density wave phase dominates the low energy physics of these materials. The observation of a giant anomalous Hall effect and chiral charge order suggest that this charge density wave may spontaneously break time reversal symmetry, even while there is no evidence of local moment magnetism. A combination of ultrafast coherent phonon spectroscopy and first-principles density functional theory calculations is used to examine the structure of the charge density wave order in CsV3Sb5. It is found that the charge density wave results from a simultaneous condensation of three optical phonon modes and can be described as tri-hexagonal ordering with an interlayer transverse shift. This distortion breaks C6 rotational symmetry of the crystal and may offer a natural explanation for reports of uniaxial order in this material family. These results highlight the important role of characterization and modeling in deciphering the exotic properties of topological kagome metals. |
Thursday, March 17, 2022 3:36PM - 3:48PM |
W60.00002: Acoustic-phonon-mediated superconductivity in Bernal bilayer graphene Yang-Zhi Chou, Fengcheng Wu, Jay D Sau We present a systematic theory of acoustic-phonon-mediated superconductivity, which incorporates Coulomb repulsion, explaining the recent experiment in Bernal bilayer graphene under a large displacement field. The acoustic-phonon mechanism predicts that $s$-wave spin-singlet and $f$-wave spin-triplet pairings are degenerate and dominant. Assuming a spin-polarized valley-unpolarized normal state, we obtain $f$-wave spin-triplet superconductivity with a $T_c\sim 20$ mK near $n_e=-0.6\times 10^{12}$ cm$^{-2}$ for hole doping, in approximate agreement with the experiment. We further predict the existence of superconductivity for larger doping in both electron-doped and hole-doped regimes. Our results indicate that the observed spin-triplet superconductivity in Bernal bilayer graphene arises from acoustic phonons. |
Thursday, March 17, 2022 3:48PM - 4:00PM |
W60.00003: Electron-phonon coupling and superconductivity in rhombohedral trilayer graphene Jingwei Jiang, Zhenglu Li, Steven G Louie A recent experiment shows the rhombohedral phase of trilayer graphene manifesting superconducting behavior at low temperature. With electric field and doping introduced, the rhombohedral trilayer graphene sample shows vanishing resistivity under sub-kelvin temperature. However, a good understanding of the mechanism behind this phenomenon is still lacking. In this work, we first perform ab initio DFT calculations to address the role of electron-phonon coupling in the superconductivity of rhombohedral trilayer graphene using density functional perturbation theory (DFPT). The electron-phonon coupling strength may be used to estimate the superconducting transition temperature. To include the effects of many-body interactions, we next perform GW perturbation theory (GWPT) calculations to obtain electron-phonon coupling matrix elements that include the GW self-energy. Any change in the matrix elements would affect the electron-phonon coupling strength, which has been shown to be quite large in some correlated systems, and thus would modify the superconducting transition temperature. |
Thursday, March 17, 2022 4:00PM - 4:12PM |
W60.00004: Nonlinear Hall effect under time reversal symmetric condition in trigonal superconductor PbTaSe2 Yuki Itahashi, Toshiya Ideue, Shintaro Hoshino, Chihiro Goto, Hiromasa Namiki, Takao Sasagawa, Yoshihiro Iwasa Symmetry breaking in solids is one of the central issues of condensed matter physics. To date, many unique physical properties and novel functionalities have been explored in noncentrosymmetric crystals. Among them, nonlinear anomalous Hall effect, which is the directional dependent spontaneous Hall effect under time reversal symmetry, is an emerging nonlinear quantum transport [1]. So far, it has been investigated in low-symmetric materials such as WTe2 and TaIrTe4 with only one mirror plane and resultant Berry curvature dipole [2-4]. In principle, however, noncentrosymmetric crystals with higher symmetry (e.g., trigonal crystals) can also host nonlinear anomalous Hall effect [5] despite the lack of Berry curvature dipole. Moreover, the search for anomalous Hall effect in exotic quantum phases including superconductivity has been missing and important challenge. |
Thursday, March 17, 2022 4:12PM - 4:24PM |
W60.00005: Tunneling resonances in superconducting single-layer NbSe2 as evidence for competing pairing channels Fernando De Juan, Wen Wan, Paul L Dreher, Daniel Muñoz-Segovia, Rishav Harsh, Francisco Guinea, Miguel M Ugeda While bulk 2H-NbSe2 is generally accepted to be a conventional superconductor, several unconventional features of the superconducting state have been reported in the monolayer limit, including the breaking of threefold symmetry in magnetotransport and anomalously large in-plane critical fields. In this talk, I will first present another unconventional feature: the existence of satellite peaks in the STM spectra of NbSe2 monolayers which exist only in the superconducting state. After discussing potential candidate explanations, I will propose a scenario of competing pairing between s-wave and subleading f-wave triplet channels to address the different experimental observations. |
Thursday, March 17, 2022 4:24PM - 4:36PM |
W60.00006: Acoustic-Phonon-Mediated Superconductivity in Rhombohedral Trilayer Graphene Fengcheng Wu, Yang-Zhi Chou, Jay D Sau Motivated by the observation of two distinct superconducting phases in the moiréless ABC-stacked rhombohedral trilayer graphene, we investigate the electron-acoustic-phonon coupling as a possible pairing mechanism. We predict the existence of superconductivity with the highest Tc∼3 K near the Van Hove singularity. Away from the Van Hove singularity, Tc remains finite in a wide range of doping. In our model, the s-wave spin-singlet and f-wave spin-triplet pairings yield the same Tc, while other pairing states have negligible Tc. Our theory provides a simple explanation for the two distinct superconducting phases in the experiment and suggests that superconductivity and other interaction-driven phases (e.g., ferromagnetism) can have different origins. |
Thursday, March 17, 2022 4:36PM - 4:48PM |
W60.00007: Current-phase relation in bilayer graphene/WSe2 Josephson junction Prasanna K Rout, Nikos Papadopoulos, Mate Kedves, Kenji Watanabe, Takashi Taniguchi, Peter Makk, Srijit Goswami The proximity coupling of WSe2 to graphene can create a large spin-orbit interaction (SOI) in graphene. In particular, for bilayer graphene (BLG) encapsulated in WSe2 (WSe2/BLG/WSe2) it was shown that displacement fields can drive a topological phase transition at zero density due to Ising SOI [1]. Here we study the current-phase relation (CPR) of Josephson junctions created in such van der Waals heterostructures. At zero (in-plane) magnetic field we find that the CPR is largely independent of carrier density and displacement field. However at finite magnetic fields (~ few hundred mT) we observe anomalous CPRs displaying large gate-dependent phase shifts. While phase shifts are in general expected in Josephson junctions with Rashba-type SOI, the effect we observe is significantly larger and cannot be accounted for by existing models. Furthermore, we find that this effect is absent for WSe2/SLG/WSe2, BLG/WSe2 and trivial BLG JJs, suggesting that it is possibly related to the specific band structure of WSe2/BLG/WSe2. |
Thursday, March 17, 2022 4:48PM - 5:00PM |
W60.00008: Tunable anisotropic superconductivity induced at the interface between ultrathin lead films and black phosphorus Malte Roesner, Anand Kamlapure, Manuel Simonato, Emil Sierda, Manuel Steinbrecher, Umut Kamber, Elze J Knol, Peter Krogstrup, Mikhail Katsnelson, Alexander A Khajetoorians Epitaxial semiconductor-superconductor heterostructures are promising as a platform for gate-tunable superconducting electronics. Here, we demonstrate that the hybrid electronic structure derived at the interface between semiconducting black phosphorus and atomically thin films of lead can drastically modify the superconducting properties of the thin metallic film. Using ultra-low temperature scanning tunneling microscopy and spectroscopy, we ascertain the moiré structure driven by the interface, and observe a strongly anisotropic renormalization of the superconducting gap and vortex structure of the lead film. Based on density functional theory, we attribute the renormalization of the superconductivity to weak hybridization at the interface where the anisotropic characteristics of the semiconductor band structure is imprinted on the Fermi surface of the superconductor. Based on a hybrid two-band model, we link this hybridization-driven renormalization to a weighting of the superconducting order parameter that quantitatively reproduces the measured spectra. These results illustrate the effect of interfacial hybridization at superconductor-semiconductor heterostructures, and pathways for engineering quantum technologies based on gate-tunable superconducting electronics. |
Thursday, March 17, 2022 5:00PM - 5:12PM |
W60.00009: Superconducting collective Leggett modes in single-layer 1H-NbSe2 Daniel Muñoz-Segovia, Wen Wan, Paul L Dreher, Rishav Harsh, Francisco Guinea, Fernando De Juan, Miguel M Ugeda The superconducting state of single-layer 1H-NbSe2 has recently been shown to display several surprising features, including our report of the existence of a bosonic mode intrinsic to the superconducting state seen as a set of resonances in tunneling spectra, which show a clear anticorrelation with the superconducting gap. We have argued that these resonances can be interpreted as a Leggett-like superconducting collective mode associated to a subleading f-wave triplet channel. In this talk, I will present a calculation of the Leggett mode energy using a simplified continuum model for NbSe2. I will discuss how our model compares with the experimental data, discuss the origin of the anticorrelation, and argue that it provides support to the competing pairing scenario, with a sizable attraction in the subleading triplet channel. |
Thursday, March 17, 2022 5:12PM - 5:24PM |
W60.00010: Theory of superconductivity in doped quantum paraelectrics Suk Bum Chung, Yue YU, Srinivas Raghu, Harold Y Hwang Recent experiments on Nb-doped SrTiO3 have shown that the superconducting energy gap to the transition temperature ratio maintains the Bardeen-Cooper-Schrieffer (BCS) value throughout its superconducting dome. Motivated by these and related studies, we show that the Cooper pairing mediated by a single soft transverse-optical phonon is the most natural mechanism for such a superconducting dome given experimental constraints, and present the microscopic theory for this pairing mechanism. Furthermore, we show that this mechanism is consistent with the T2 resistivity in the normal state. Lastly, we discuss what physical insights SrTiO3 provides for superconductivity in other quantum paraelectrics such as KTaO3. |
Thursday, March 17, 2022 5:24PM - 5:36PM |
W60.00011: p-wave superconductivity induced from valley symmetry breaking in twisted trilayer graphene Jose Gonzalez, Tobias Stauber We show that the e-e interaction induces a strong breakdown of valley symmetry in twisted trilayer graphene, just before the superconducting instability develops in the hole-doped material. We analyze this effect by means of an atomistic self-consistent Hartree-Fock approximation, which is a sensible approach as the Fock part becomes crucial to capture the breakdown of symmetry. This effect allows us to reproduce the experimental observation of the Hall density, including the reset at 2-hole doping. Moreover, the breakdown of valley symmetry has important consequences for the superconductivity, as it implies a reduction of symmetry down to the C3 group. We observe that the second valence band has a three-fold van Hove singularity, which is pinned to the Fermi level at the experimental optimal doping for superconductivity. We further find that the C3 configuration of the saddle points leads to a version of Kohn-Luttinger superconductivity where the dominant pairing amplitude has p-wave symmetry[1]. We stress that the breakdown of symmetry down to C3 may be shared by other materials with valley symmetry breaking, so that it may be an essential ingredient to capture there the right order parameter of the superconductivity. |
Thursday, March 17, 2022 5:36PM - 5:48PM |
W60.00012: Effects of longer-range Coulomb interactions on the superconducting state of the Kagome system AV3Sb5 Shinibali Bhattacharyya, Astrid T Rømer, Morten Holm Christensen, Andreas Kreisel, Brian M Andersen, Roser Valenti, Paul Wunderlich With the recent discovery of superconductivity in the Kagome material AV3Sb5, the nature of the superconducting gap symmetry has been under debate. In this work, we investigate spin- and charge-fluctuation mediated superconducting pairing phenomena in AV3Sb5, taking into account both local and longer-range Coulomb interactions at different fillings. In the presence of longer-range Coulomb interaction, charge fluctuations are enhanced which substantially modifies the pairing symmetries and hence, the leading order instabilities. Our results shed light on the role of longer-range Coulomb repulsion in deciding the nature of superconductivity in the Kagome system, which is already known to display a variety of phenomena arising out of sublattice interference due to its underlying lattice structure. We analyze the properties of these states and compare them with available experimental data. |
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