Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session G48: Superconductivity in Low Dimensional Systems II |
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Sponsoring Units: DCMP Chair: Ken Burch Room: Mile High Ballroom 1A |
Tuesday, March 3, 2020 11:15AM - 11:27AM |
G48.00001: Dimensionality of the Zeeman energy in conductors Richard Klemm, Aiying Zhao, Qiang Gu, Timothy Haugan The Dirac equation is extended for a relativistic electron or hole in an orthorhombically-anisotropic conduction band with effective masses mj for j=1,2,3 with geometric mean mg=(m1m2m3)1/3. Its covariance is established with general proper and improper Lorentz transformations. The non-relativistic Hamiltonian is evaluated to order 1/(mc2)4, where mc2 is it's Einstein rest energy. For the magnetic induction B in the crystallographic direction êj, the Zeeman g factor is 2m(mj/mg3)1/2. While propagating in a monolayer two-dimensional conduction band, g is much less than 2 for B parallel to the monolayer, as observed recently in superconducting monolayer NbSe2, Bc2,|| of which appears to violate the ``Pauli limit'' by a order of magnitude. In one-dimensional chain conductors of atomic thickness, g is small for all B directions.while the particle is in its conduction band. The precise form for the quantum spin Hall energy is also found for a particle in a two-dimensional metal. |
Tuesday, March 3, 2020 11:27AM - 11:39AM |
G48.00002: Controlling the charge dispersion of a nearly-open superconducting island Arno Bargerbos, Willemijn Uilhoorn, Bernard Van Heck, Chung-Kai Yang, Peter Krogstrup, Leo P Kouwenhoven, Gijs De Lange, Angela Kou Isolation from the environment determines the extent to which charge is confined on an island, which manifests experimentally through Coulomb oscillations such as charge dispersion. In superconducting circuits, the link to the environment has typically been formed from tunnel junctions. If instead a transparent ballistic junction forms the link between the superconducting island and the environment, Coulomb oscillations are predicted to suppress far more rapidly than for tunnel junctions due to imaginary-time Landau-Zener tunneling. Here we investigate the charge dispersion of a nanowire transmon hosting a quantum dot in the junction. We observe rapid suppression of the charge dispersion consistent with the scaling law resulting from diabatic transitions between Andreev bound states. We also observe greatly improved qubit coherence times at the point of highest charge dispersion suppression. Our observations further our fundamental understanding of charging effects in superconductors and suggests novel approaches for building charge-insensitive qubits. |
Tuesday, March 3, 2020 11:39AM - 11:51AM |
G48.00003: Calorimetric Observation of Magnetic-Field-Induced High-Field FFLO Superconducting Phase in λ-(BETS)2GaCl4 Nathanael Fortune, Andreas Rydh, Joyce Ellen Palmer-Fortune, Charles C Agosta, Akiko Kobayashi When an external magnetic field is applied parallel to the conducting layers in quasi-2D organic superconductors, a phase transition to a new, high-field, superconducting state with a spatially modulated superconducting order parameter can occur at the Clogston-Chandrasekhar paramagnetic limit Hp, as first predicted by Fulde, Ferrell, Larkin, and Ovchinnikov (FFLO). Candidate FFLO phases have been identified through NMR, RF penetration depth, magnetic-torque, and transport measurements in a number of organic superconductors; in the case of the archetypal 2D organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 , corroborating evidence has been provided by the calorimetric observation of a first order phase transition at Hp into a new high-field, higher-entropy superconducting phase. This evidence has been lacking, however, in other candidate FFLO systems. We report here low-temperature specific heat measurements in a second expected FFLO organic superconductor: λ-(BETS)2GaCl4 . For field-angles within 0.5° of plane-parallel, we observe an enhancement of the upper critical field superconducting phase boundary Hc2(T), a calorimetric signature of pancake vortex formation with decreasing field at Hp, and a field-induced phase transition between the upper and lower superconducting states. |
Tuesday, March 3, 2020 11:51AM - 12:03PM |
G48.00004: Anomalous Metallic Phase in Tunable Destructive Superconductors Saulius Vaitiekenas, Peter Krogstrup, Charles Marcus The Little-Parks effect---a flux-dependent modulation of the transition temperature in multiply connected superconductors---results from the quantization of fluxoid through holes in superconductors. In hollow superconducting cylinders with diameter smaller than the superconducting coherence length, flux-induced supercurrents can give rise to the destructive Little-Parks effect, characterized by repeated reentrant quantum phase transitions between superconducting and metallic phases. Here, we use axial and transverse magnetic fields to control the crossover between the conventional and destructive Little-Parks regimes in nanowires with an epitaxial Al shell fully surrounding InAs core. The observed dependence on flux, transverse field, temperature, and current bias is in excellent agreement with theory. Near the crossover between the conventional and destructive regimes, an anomalous metal phase is found. The anomalous metallic phase is characterized by a field-controllable, temperature-independent resistivity between adjacent superconducting lobes. |
Tuesday, March 3, 2020 12:03PM - 12:15PM |
G48.00005: Study of 2D superconductivity at oxide interfaces by microwave resonators Yildiz Saglam, Edouard Lesne, Daniel Bothner, Felix Schmidt, Marc Gabay, Andrea Caviglia, Gary Steele The emergent two-dimensional electron system (2DES) formed at the interface between LaAlO3 (LAO) and SrTiO3 (STO) insulating oxides has been a subject of great interest in condensed matter physics during the last decade. Recently, (111)-oriented LAO/STO interfaces have been shown to exhibit an electronic correlation driven reconstruction of its band structure and a two-dimensional superconducting (SC) ground state, both tunable by electrostatic field-effect. |
Tuesday, March 3, 2020 12:15PM - 12:27PM |
G48.00006: Ising superconductors: the interplay of magnetic field, triplet channels and disorder David Möckli, Maxim Khodas We study the superconducting instability in disordered non-centrosymmetric monolayers with intrinsic Ising spin-orbit coupling (SOC) subjected to an in-plane Zeeman magnetic field. The pairing interaction contains the channels allowed by crystal symmetry, such that in general, the pairing state is a mixture of singlet and triplet Cooper pairs. The joint action of SOC and Zeeman field selects a specific in-plane d-vector triplet component to couple with the singlets, which gains robustness against disorder through the coupling. The out-of-plane d-vector component, that in the clean case is immune to both the Zeeman field and SOC is obliterated by a small impurity scattering rate. We formulate the quasi-classical theory of Ising superconductors and solve the linearized Eilenberger equations to obtain the pair-breaking equations that determine the Zeeman field - temperature dependence of the continuous superconducting transition. Our discussion emphasizes how the Zeeman field, SOC and disorder affect the different superconducting order parameters, and we show how the spin-fields inevitably induce odd-frequency pairing correlations. |
Tuesday, March 3, 2020 12:27PM - 12:39PM |
G48.00007: Theory of Pseudogap Effects in Magic Angle Twisted Bilayer Graphene Gaurav Chaudhary, Zhiqiang Wang, Kathryn Levin In this talk we discuss the signatures of the pseudogap effects seen in transport studies of superconducting magic angle twisted bilayer graphene (MATBG). They should be viewed as rather natural; quite generally pseudogap phenomena are widely observed in very thin superconducting films of conventional materials such as Pb and TiN, high $T_c$ cuprate superconductors and in interfacial superconductors. The pseudogap regime is characterized by two distinct temperature scales corresponding to the onset of phase coherence, i.e. $T_{BKT}$ and the onset of pairing, i.e. $T^{\ast}$. Here we develop the pairing fluctuation theory of the Berezinskii-Kosterlitz-Thouless (BKT) transition for the MATBG, demonstrating how pseudogap effects are enhanced in two dimensional superconductors. Moreover, they do not require anomalously strong attractive pairing mechanisms as proposed for the 3D cuprates. Using our theory along with the transport estimates for the pairing onset temperature $T^{\ast}$, we quantify the expected strength of the pairing and fluctuations for the MATBG superconductors. Finally, we propose new tunneling based experiments from one two dimensional system to another, which can further probe the pseudogap phenomena. |
Tuesday, March 3, 2020 12:39PM - 12:51PM |
G48.00008: Quantum transport of atomically thin 1Td-MoTe2 Xirui Wang, Kenji Yasuda, Daniel A Rhodes, Takehito Suzuki, Kenji Watanabe, Takashi Taniguchi, Joseph G Checkelsky, James C Hone, Pablo Jarillo-Herrero There has been growing interest in the study of crystalline 2D superconductors, especially those with low carrier densities with gate tunability. Recently, it is reported that monolayer 1Td-MoTe2 exhibits superconductivity with transition temperature of 8 K [1], much enhanced from the bulk whose Tc = 0.1 K [2], and higher than the isostructural monolayer 1Td-WTe2 with its Tc = 0.7 K [3,4]. Whereas monolayer MoTe2 is inversion-symmetric, inversion-symmetry is broken in bilayer MoTe2 due to the stacking arrangement, which can change the superconducting properties. Here we will report the quantum transport properties of bilayer and monolayer MoTe2, for a better understanding of its superconductivity, and pursuing topologically non-trivial state in this platform. |
Tuesday, March 3, 2020 12:51PM - 1:03PM |
G48.00009: Strong pairing in two dimensions: Pseudogaps, domes, and other implications Kathryn Levin, Xiaoyu Wang, Qijin Chen The recent interest in superconductivity of strongly correlated 2D materials is driven by exciting discoveries of novel superconductors such as magic-angle twisted bilayer graphene (MATBG), FeSe monolayers and transition metal dichalcogenides. These are generally thought to be distinct from BCS-Eliashberg superconductors, and can be argued, from the Uemura plot, to be intermediate between BCS and Bose-Einstein condensation (BCS-BEC). In this talk we compute the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature TBKT and pairing onset T* as a function of pairing interaction strength g and of density using this crossover theory. Rather than solving directly for the phase stiffness parameter we follow the literature on atomic Bose systems (which is based on a Quantum Monte Carlo analysis) and which constrains the total phase space density at the transition. Our results for TBKT compare favorably with Fermi gas experiments and yield the expected BCS and BEC asymptotes for a film with lattice dispersion. Given the measured TBKT, we provide estimates for the pairing gap and for T* in concrete systems such as MATBG. |
Tuesday, March 3, 2020 1:03PM - 1:15PM |
G48.00010: In situ mutual inductance measurement of superconducting atomic films: Si(111)-√7 × √3-In and monolayer FeSe on SrTiO3 Ming-chao Duan, Yanfu Wu, Gang Yao, Canhua Liu, Jinfeng Jia Many of the novel 2D superconducting films with highly crystalline structure and atomic thickness, fabricated in ultrahigh vacuum chambers, are highly vulnerable to air exposure, making it difficult to measure intrinsic superconducting properties such as zero resistance and perfect diamagnetism with ex situ experimental techniques. To realize vacuum-based measurements, we developed a multifunctional scanning tunneling microscope. The conventional STM probe in the microscope can be switched, under vacuum conditions, to a two-coil probe that can detect whether a material exclude magnetic flux. By using this in situ mutual inductance technique, for the first time, we succeeded in detecting the intrinsic diamagnetic response in both two-atomic-layer indium film on a Si(111) surface and monolayer FeSe films on SrTiO3. The Si(111)-√7 × √3-In films is expected as a conventional superconductor according to previous STM study. Our diamagnetic measurements verify that the low-temperature variation of penetration depth in Si(111)-√7 × √3-In films follows the BCS theory in the dirty limit. On the other hand, the behavior of monolayer FeSe films on SrTiO3 in the external magnetic field indicates a vortex melting picture. |
Tuesday, March 3, 2020 1:15PM - 1:27PM |
G48.00011: Multi-carrier states in the Peierls electron-phonon model Alberto Nocera, John Sous, Mona Berciu Motivated by the discovery of light polarons and bipolarons in the Peierls electron-phonon model on a one dimensional lattice [1], we numerically study few-carrier states (up to six) using the density matrix renormalization group method. Our results show that a bipolaron liquid is a stable ground state in a wide range of phonon frequencies and electron-phonon interaction strengths. More interestingly, we provide numerical evidence that this homogeneous multi-carrier state for a sufficiently large electron-phonon coupling strength becomes unstable in favor of a multi-polaron complex suggestive of phase separation. We provide analytical arguments in support of our numerics based on an effective multi-body interaction |
Tuesday, March 3, 2020 1:27PM - 1:39PM |
G48.00012: Bi2Sr2CaCu2O8 single crystal exfoliation, characterization, and manipulation Pedro Mercado Lozano, Qiang Li, Young Jae Shin, Genda Gu Bi2Sr2CaCu2O8 (BSCCO) is a high Tc cuprate superconductor whose layered crystal structure allows the study of thickness dependence of its superconducting properties down to mono-layer. We developed a process to manipulate and analyze BSCCO single crystal and bi-crystal junctions at different film thickness. First, thickness characterization was performed through profile extraction with atomic force microscopy (AFM). A transfer stage was then used to isolate individual flakes of varying thickness in order to study their magnetic response on a SQUID magnetometer. Thin films of less than 100 nm were found to maintain their superconducting properties, which can be inferred from the observation of Meissner effect on the samples. Finally, by using the transfer stage, different layers of BSCCO were rotated around the c-axis and stacked on top of each other. These manipulations enable the study of superconducting properties of bi-crystal junction as a function of the twist angle and the layer thickness. |
Tuesday, March 3, 2020 1:39PM - 1:51PM |
G48.00013: An unexpected charge doping mechanism for quasi-one-dimensional ACr3As3 superconductors Keith Taddei, Liurukara D Sanjeewa, Bing-Hua Lei, Yuhao Fu, Qiang Zheng, David Singh, Athena S. Sefat, Clarina Reloj Dela Cruz In the race to find topological materials, superconductivity has found renewed interest as potential hosts of Majorana Fermions. However, realizing such states is non-trivial requiring for instance Cooper pairs with finite orbital-momentum or Dirac-like dispersions in the normal state. Recently, a new family of quasi-1D superconductors A1,2Cr3As3 (A= Alkali metal) was discovered which is interesting in these regards for realizing spin-triplet superconductivity and having Dirac-like crossings near EF. However, its study has been hampered due to extreme air sensitivity and an inability to charge dope. Here, we report results of diffraction studies and DFT work on KCr3As3 which solve both these problems. We show that the reported growth technique inherently intercalates H into the quasi-1D CrAs tubes and that the H acts as an electron donor. Furthermore, we reveal that the reported discrepancy in sample behavior (with some superconducting and others spin-glasses) is actually due to the amount of H. This work suggests a new stoichiometry KH1-xCr3As3, which is air stable and provides a charge doping mechanism. This allows for tuning between frustrated magnetism and superconductivity in a quasi-1D material as well as a potential route to reach the predicted Dirac points. |
Tuesday, March 3, 2020 1:51PM - 2:03PM |
G48.00014: Textured one-dimensional superconducting nanostructures Robert Lynn, Kristine Ung, Leo V Sementilli, Nina Markovic We have fabricated one-dimensional superconducting nanostructures with precisely controlled texture. The fabrication method that combines nanoscale shadow mask deposition with in situ controlled etching allows nanoscale features to be drawn directly onto the substrate. By controlling the speed with which the shadow mask is translated over the substrate, we can vary the thickness of the nanostructure as it is being deposited. The thickness modulation allows us to control the shape of the superconducting wavefunction in these nanostructured wires, which can be detected in electrical transport measurements at low temperatures and in magnetic field. |
Tuesday, March 3, 2020 2:03PM - 2:15PM |
G48.00015: New Method for Identifying the FFLO State in λ-(BETS)2GaCl4 Using Sample Rotation Brett Laramee, Raju Ghimire, Calvin Bales, William A Coniglio, Charles C Agosta, John A Schlueter, Akiko Kobayashi The FFLO state is a state of inhomogeneous superconductivity that exists at high magnetic fields. In specific crystals, the point where Cooper pairs begin to break due to paramagnetic effects, Hp, happens before reaching the ultimate critical field, Hc2. We will present rf penetration depth measurements on the quasi-2D layered organic superconductor λ-(BETS)2GaCl4 (BETS) using a tunnel diode oscillator. The quasi-2D structure of BETS makes features such as Hc2, the FFLO phase transition, and vortex effects like the lock-in effect highly sensitive to the angle that the conduction planes make with the external field. By rotating the sample in a constant external magnetic field, we can locate points on the FFLO phase line that are not easily identifiable via field sweeps. In addition, we will visualize the FFLO phase transitions in BETS by showing a Field−Angle phase diagram at very low temperature (60 mK). With the addition of this new technique, we have begun constructing a full 3D Field−Angle−Temperature phase diagram of the FFLO state. |
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