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Ichinokura S, Tokuda K, Toyoda M, Tanaka K, Saito S, Hirahara T. Van Hove Singularity and Enhanced Superconductivity in Ca-Intercalated Bilayer Graphene Induced by Confinement Epitaxy. ACS NANO 2024; 18:13738-13744. [PMID: 38741024 DOI: 10.1021/acsnano.4c01757] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/16/2024]
Abstract
We demonstrate the impact of high-density calcium introduction into Ca-intercalated bilayer graphene on a SiC substrate, wherein a metallic layer of Ca has been identified at the interface. We have discerned that the additional Ca layer engenders a free-electron-like band, which subsequently hybridizes with a Dirac band, leading to the emergence of a van Hove singularity. Coinciding with this, there is an increase in the critical temperature for superconductivity. These findings allude to the manifestation of Ca-driven confinement epitaxy, augmenting superconductivity through the enhancement of attractive interactions in a pair of electron and hole bands with flat dispersion around the Fermi level.
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Affiliation(s)
- Satoru Ichinokura
- Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan
| | - Kei Tokuda
- Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan
| | - Masayuki Toyoda
- Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan
| | - Kiyohisa Tanaka
- UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
| | - Susumu Saito
- Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan
| | - Toru Hirahara
- Department of Physics, Tokyo Institute of Technology, Tokyo 152-8551, Japan
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2
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Zeng J, Li Q, Yang X, Xu DH, Wang R. Chiral-Flux-Phase-Based Topological Superconductivity in Kagome Systems with Mixed Edge Chiralities. PHYSICAL REVIEW LETTERS 2023; 131:086601. [PMID: 37683163 DOI: 10.1103/physrevlett.131.086601] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/30/2023] [Revised: 06/25/2023] [Accepted: 08/04/2023] [Indexed: 09/10/2023]
Abstract
Recent studies have attracted intense attention on the quasi-2D kagome superconductors AV_{3}Sb_{5} (A=K, Rb, and Cs) where the unexpected chiral flux phase (CFP) associates with the spontaneous time-reversal symmetry breaking in charge density wave states. Here, commencing from the 2-by-2 charge density wave phases, we bridge the gap between topological superconductivity and time-reversal asymmetric CFP in kagome systems. Several chiral topological superconductor (TSC) states featuring distinct Chern numbers emerge for an s-wave or a d-wave superconducting pairing symmetry. Importantly, these CFP-based TSC phases possess unique gapless edge modes with mixed chiralities (i.e., both positive and negative chiralities), but with the net chiralities consistent with the Bogoliubov-de Gennes Chern numbers. We further study the transport properties of a two-terminal junction, using Chern insulator or normal metal leads via atomic Green's function method with Landauer-Büttiker formalism. In both cases, the normal electron tunneling and the crossed Andreev reflection oscillate as the chemical potential changes, but together contribute to plateau transmissions (1 and 3/2, respectively) that exhibit robustness against disorder. These behaviors can be regarded as the signature of a TSC hosting edge states with mixed chiralities.
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Affiliation(s)
- Junjie Zeng
- Institute for Structure and Function and Department of Physics and Chongqing Key Laboratory for Strongly Coupled Physics, Chongqing University, Chongqing 400044, People's Republic of China
| | - Qingming Li
- Department of Physics, Shijiazhuang University, Shijiazhuang, Hebei 050035, People's Republic of China
| | - Xun Yang
- Institute for Structure and Function and Department of Physics and Chongqing Key Laboratory for Strongly Coupled Physics, Chongqing University, Chongqing 400044, People's Republic of China
| | - Dong-Hui Xu
- Institute for Structure and Function and Department of Physics and Chongqing Key Laboratory for Strongly Coupled Physics, Chongqing University, Chongqing 400044, People's Republic of China
| | - Rui Wang
- Institute for Structure and Function and Department of Physics and Chongqing Key Laboratory for Strongly Coupled Physics, Chongqing University, Chongqing 400044, People's Republic of China
- Center of Quantum materials and devices, Chongqing University, Chongqing 400044, People's Republic of China
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3
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Jois S, Lado JL, Gu G, Li Q, Lee JU. Andreev Reflection and Klein Tunneling in High-Temperature Superconductor-Graphene Junctions. PHYSICAL REVIEW LETTERS 2023; 130:156201. [PMID: 37115873 DOI: 10.1103/physrevlett.130.156201] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/31/2022] [Accepted: 03/15/2023] [Indexed: 06/19/2023]
Abstract
Scattering processes in quantum materials emerge as resonances in electronic transport, including confined modes, Andreev states, and Yu-Shiba-Rusinov states. However, in most instances, these resonances are driven by a single scattering mechanism. Here, we show the appearance of resonances due to the combination of two simultaneous scattering mechanisms, one from superconductivity and the other from graphene p-n junctions. These resonances stem from Andreev reflection and Klein tunneling that occur at two different interfaces of a hole-doped region of graphene formed at the boundary with superconducting graphene due to proximity effects from Bi_{2}Sr_{2}Ca_{1}Cu_{2}O_{8+δ}. The resonances persist with gating from p^{+}-p and p-n configurations. The suppression of the oscillation amplitude above the bias energy which is comparable to the induced superconducting gap indicates the contribution from Andreev reflection. Our experimental measurements are supported by quantum transport calculations in such interfaces, leading to analogous resonances. Our results put forward a hybrid scattering mechanism in graphene-high-temperature superconductor heterojunctions of potential impact for graphene-based Josephson junctions.
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Affiliation(s)
- Sharadh Jois
- College of Nanoscale Science and Engineering, SUNY Polytechnic Institute, Albany, New York 12203, USA
| | - Jose L Lado
- Department of Applied Physics, Aalto University, 00076 Aalto, Espoo, Finland
| | - Genda Gu
- Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA
| | - Qiang Li
- Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA
- Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794, USA
| | - Ji Ung Lee
- College of Nanoscale Science and Engineering, SUNY Polytechnic Institute, Albany, New York 12203, USA
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Wu YM, Wu Z, Yao H. Pair-Density-Wave and Chiral Superconductivity in Twisted Bilayer Transition Metal Dichalcogenides. PHYSICAL REVIEW LETTERS 2023; 130:126001. [PMID: 37027848 DOI: 10.1103/physrevlett.130.126001] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/18/2022] [Accepted: 02/24/2023] [Indexed: 06/19/2023]
Abstract
We theoretically explore possible orders induced by weak repulsive interactions in twisted bilayer transition metal dichalcogenides (e.g., WSe_{2}) in the presence of an out-of-plane electric field. Using renormalization group analysis, we show that superconductivity survives even with the conventional van Hove singularities. We find that topological chiral superconducting states with Chern number N=1, 2, 4 (namely, p+ip, d+id, and g+ig) appear over a large parameter region with a moiré filling factor around n=1. At some special values of applied electric field and in the presence of a weak out-of-plane Zeeman field, spin-polarized pair-density-wave (PDW) superconductivity can emerge. This spin-polarized PDW state can be probed by experiments such as spin-polarized STM measuring spin-resolved pairing gap and quasiparticle interference. Moreover, the spin-polarized PDW could lead to a spin-polarized superconducting diode effect.
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Affiliation(s)
- Yi-Ming Wu
- Institute for Advanced Study, Tsinghua University, Beijing 100084, China
- Stanford Institute for Theoretical Physics, Stanford University, Stanford, California 94305, USA
| | - Zhengzhi Wu
- Institute for Advanced Study, Tsinghua University, Beijing 100084, China
| | - Hong Yao
- Institute for Advanced Study, Tsinghua University, Beijing 100084, China
- State Key Laboratory of Low Dimensional Quantum Physics, Tsinghua University, Beijing 100084, China
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5
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Inter-valley coherent order and isospin fluctuation mediated superconductivity in rhombohedral trilayer graphene. Nat Commun 2022; 13:6013. [PMID: 36224167 PMCID: PMC9556532 DOI: 10.1038/s41467-022-33561-w] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/21/2021] [Accepted: 09/22/2022] [Indexed: 12/02/2022] Open
Abstract
Superconductivity was recently discovered in rhombohedral trilayer graphene (RTG) in the absence of a moiré potential. Superconductivity is observed proximate to a metallic state with reduced isospin symmetry, but it remains unknown whether this is a coincidence or a key ingredient for superconductivity. Using a Hartree-Fock analysis and constraints from experiments, we argue that the symmetry breaking is inter-valley coherent (IVC) in nature. We evaluate IVC fluctuations as a possible pairing glue, and find that they lead to chiral unconventional superconductivity when the fluctuations are strong. We further elucidate how the inter-valley Hund’s coupling determines the spin-structure of the IVC ground state and breaks the degeneracy between spin-singlet and triplet superconductivity. Remarkably, if the normal state is spin-unpolarized, we find that a ferromagnetic Hund’s coupling favors spin-singlet superconductivity, in agreement with experiments. Instead, if the normal state is spin-polarized, then IVC fluctuations lead to spin-triplet pairing. Trilayer graphene was recently shown to exhibit superconductivity without a Moire pattern that had proved important in tuning superconductivity in bilayer graphene. Here, the authors explore correlated metallic phases and the pairing mechanism of superconductivity in trilayer graphene, and show that intervalley coherent fluctuations can act as a pairing glue, giving rise to chiral unconventional superconductivity.
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Wolf S, Di Sante D, Schwemmer T, Thomale R, Rachel S. Triplet Superconductivity from Nonlocal Coulomb Repulsion in an Atomic Sn Layer Deposited onto a Si(111) Substrate. PHYSICAL REVIEW LETTERS 2022; 128:167002. [PMID: 35522509 DOI: 10.1103/physrevlett.128.167002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/14/2021] [Accepted: 03/29/2022] [Indexed: 06/14/2023]
Abstract
Atomic layers deposited on semiconductor substrates introduce a platform for the realization of the extended electronic Hubbard model, where the consideration of electronic repulsion beyond the on-site term is paramount. Recently, the onset of superconductivity at 4.7 K has been reported in the hole-doped triangular lattice of tin atoms on a silicon substrate. Through renormalization group methods designed for weak and intermediate coupling, we investigate the nature of the superconducting instability in hole-doped Sn/Si(111). We find that the extended Hubbard nature of interactions is crucial to yield triplet pairing, which is f-wave (p-wave) for moderate (higher) hole doping. In light of persisting challenges to tailor triplet pairing in an electronic material, our finding promises to pave unprecedented ways for engineering unconventional triplet superconductivity.
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Affiliation(s)
- Sebastian Wolf
- School of Physics, University of Melbourne, Parkville, Victoria 3010, Australia
| | - Domenico Di Sante
- Department of Physics and Astronomy, University of Bologna, 40127 Bologna, Italy
- Center for Computational Quantum Physics, Flatiron Institute, New York, New York 10010, USA
| | - Tilman Schwemmer
- Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Am Hubland Campus Süd, Würzburg 97074, Germany
| | - Ronny Thomale
- Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Am Hubland Campus Süd, Würzburg 97074, Germany
| | - Stephan Rachel
- School of Physics, University of Melbourne, Parkville, Victoria 3010, Australia
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Toyama H, Akiyama R, Ichinokura S, Hashizume M, Iimori T, Endo Y, Hobara R, Matsui T, Horii K, Sato S, Hirahara T, Komori F, Hasegawa S. Two-Dimensional Superconductivity of Ca-Intercalated Graphene on SiC: Vital Role of the Interface between Monolayer Graphene and the Substrate. ACS NANO 2022; 16:3582-3592. [PMID: 35209713 DOI: 10.1021/acsnano.1c11161] [Citation(s) in RCA: 9] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/14/2023]
Abstract
Ca-intercalation has enabled superconductivity in graphene on SiC. However, the atomic and electronic structures that are critical for superconductivity are still under discussion. We find an essential role of the interface between monolayer graphene and the SiC substrate for superconductivity. In the Ca-intercalation process, at the interface a carbon layer terminating SiC changes to graphene by Ca-termination of SiC (monolayer graphene becomes a bilayer), inducing more electrons than a free-standing model. Then, Ca is intercalated in between the graphene layers, which shows superconductivity with the updated critical temperature (TC) of up to 5.7 K. In addition, the relation between TC and the normal-state conductivity is unusual, "dome-shaped". These findings are beyond the simple C6CaC6 model in which s-wave BCS superconductivity is theoretically predicted. This work proposes a general picture of the intercalation-induced superconductivity in graphene on SiC and indicates the potential for superconductivity induced by other intercalants.
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Affiliation(s)
- Haruko Toyama
- Department of Physics, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
| | - Ryota Akiyama
- Department of Physics, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
| | - Satoru Ichinokura
- Department of Physics, Tokyo Institution of Technology, Meguro, Tokyo 152-8551, Japan
| | - Mizuki Hashizume
- Department of Physics, Tokyo Institution of Technology, Meguro, Tokyo 152-8551, Japan
| | - Takushi Iimori
- The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
| | - Yukihiro Endo
- Department of Physics, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
| | - Rei Hobara
- Department of Physics, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
| | - Tomohiro Matsui
- Department of Physics, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
| | - Kentaro Horii
- Department of Physics, Tokyo Institution of Technology, Meguro, Tokyo 152-8551, Japan
| | - Shunsuke Sato
- Department of Physics, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
| | - Toru Hirahara
- Department of Physics, Tokyo Institution of Technology, Meguro, Tokyo 152-8551, Japan
| | - Fumio Komori
- The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
| | - Shuji Hasegawa
- Department of Physics, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
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8
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Alsharari AM, Ulloa SE. Inducing chiral superconductivity on honeycomb lattice systems. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2022; 34:205403. [PMID: 35235911 DOI: 10.1088/1361-648x/ac5a03] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/09/2021] [Accepted: 03/02/2022] [Indexed: 06/14/2023]
Abstract
Superconductivity in graphene-based systems has recently attracted much attention, as either intrinsic behavior or induced by proximity to a superconductor may lead to interesting topological phases and symmetries of the pairing function. A prominent system considers the pairing to have chiral symmetry. The question arises as to the effect of possible spin-orbit coupling on the resulting superconducting quasiparticle (QP) spectrum. Utilizing a Bogolyubov-de Gennes (BdG) Hamiltonian, we explore the interplay of different interaction terms in the system, and their role in generating complex Berry curvatures in the QP spectrum, as well as non-trivial topological behavior. We demonstrate that the topology of the BdG Hamiltonian in these systems may result in the appearance of edge states along the zigzag edges of nanoribbons in the appropriate regime. For suitable chemical potential and superconducting pairing strength, we find the appearance of robust midgap states at zigzag edges, well protected by large excitation gaps and momentum transfer.
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Affiliation(s)
| | - Sergio E Ulloa
- Department of Physics and Astronomy, and Nanoscale and Quantum Phenomena Institute, Ohio University, Athens, OH 45701, United States of America
- Center for Quantum Devices, Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark
- Center for Nanostructured Graphene, DTU Physics, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark
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9
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Ghazaryan A, Holder T, Serbyn M, Berg E. Unconventional Superconductivity in Systems with Annular Fermi Surfaces: Application to Rhombohedral Trilayer Graphene. PHYSICAL REVIEW LETTERS 2021; 127:247001. [PMID: 34951779 DOI: 10.1103/physrevlett.127.247001] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/07/2021] [Accepted: 11/12/2021] [Indexed: 06/14/2023]
Abstract
We show that in a two-dimensional electron gas with an annular Fermi surface, long-range Coulomb interactions can lead to unconventional superconductivity by the Kohn-Luttinger mechanism. Superconductivity is strongly enhanced when the inner and outer Fermi surfaces are close to each other. The most prevalent state has chiral p-wave symmetry, but d-wave and extended s-wave pairing are also possible. We discuss these results in the context of rhombohedral trilayer graphene, where superconductivity was recently discovered in regimes where the normal state has an annular Fermi surface. Using realistic parameters, our mechanism can account for the order of magnitude of T_{c}, as well as its trends as a function of electron density and perpendicular displacement field. Moreover, it naturally explains some of the outstanding puzzles in this material, that include the weak temperature dependence of the resistivity above T_{c}, and the proximity of spin singlet superconductivity to the ferromagnetic phase.
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Affiliation(s)
| | - Tobias Holder
- Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel
| | - Maksym Serbyn
- IST Austria, Am Campus 1, 3400 Klosterneuburg, Austria
| | - Erez Berg
- Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot 76100, Israel
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10
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Yu T, Kennes DM, Rubio A, Sentef MA. Nematicity Arising from a Chiral Superconducting Ground State in Magic-Angle Twisted Bilayer Graphene under In-Plane Magnetic Fields. PHYSICAL REVIEW LETTERS 2021; 127:127001. [PMID: 34597086 DOI: 10.1103/physrevlett.127.127001] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/06/2021] [Accepted: 08/16/2021] [Indexed: 06/13/2023]
Abstract
Recent measurements of the resistivity in magic-angle twisted bilayer graphene near the superconducting transition temperature show twofold anisotropy, or nematicity, when changing the direction of an in-plane magnetic field [Cao et al., Science 372, 264 (2021)SCIEAS0036-807510.1126/science.abc2836]. This was interpreted as strong evidence for exotic nematic superconductivity instead of the widely proposed chiral superconductivity. Counterintuitively, we demonstrate that in two-dimensional chiral superconductors the in-plane magnetic field can hybridize the two chiral superconducting order parameters to induce a phase that shows nematicity in the transport response. Its paraconductivity is modulated as cos(2θ_{B}), with θ_{B} being the direction of the in-plane magnetic field, consistent with experiment in twisted bilayer graphene. We therefore suggest that the nematic response reported by Cao et al. does not rule out a chiral superconducting ground state.
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Affiliation(s)
- Tao Yu
- Max Planck Institute for the Structure and Dynamics of Matter, Luruper Chaussee 149, 22761 Hamburg, Germany
| | - Dante M Kennes
- Max Planck Institute for the Structure and Dynamics of Matter, Luruper Chaussee 149, 22761 Hamburg, Germany
- Institut für Theorie der Statistischen Physik, RWTH Aachen University and JARA-Fundamentals of Future Information Technology, 52056 Aachen, Germany
| | - Angel Rubio
- Max Planck Institute for the Structure and Dynamics of Matter, Luruper Chaussee 149, 22761 Hamburg, Germany
- Center for Computational Quantum Physics (CCQ), The Flatiron Institute, 162 Fifth Avenue, New York, New York 10010, USA
- Nano-Bio Spectroscopy Group, Departamento de Física de Materiales, Universidad del País Vasco, 20018 San Sebastian, Spain
| | - Michael A Sentef
- Max Planck Institute for the Structure and Dynamics of Matter, Luruper Chaussee 149, 22761 Hamburg, Germany
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11
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Tchougréeff A, Plekhanov E, Dronskowski R. Solid-state quantum chemistry with ΘΦ (ThetaPhi): Spin-liquids, superconductors, and magnetic superstructures made computationally available. J Comput Chem 2021; 42:1498-1513. [PMID: 34018219 DOI: 10.1002/jcc.26561] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/29/2021] [Revised: 05/02/2021] [Accepted: 05/04/2021] [Indexed: 01/17/2023]
Abstract
We present a standalone ΘΦ (ThetaPhi) package capable to read the results of ab initio DFT/PAW quantum-chemical solid-state calculations processed through various tools projecting them to the atomic basis states as an input and to perform on top of this an analysis of so derived electronic structure which includes (among other options) the possibility to obtain a superconducting (Bardeen-Cooper-Schrieffer, BCS), spin-liquid (resonating valence bond, RVB) states/phases as solutions of the electronic structure problem along with the magnetically ordered phases with an arbitrary pitch (magnetic superstructure) vector. Remarkably, different solutions of electronic-structure problems come out as temperature-dependent (exemplified by various superconducting and spin-liquid phases) which feature is as well implemented. All that is exemplified by model calculations on 1D chain, 2D square lattice as well as on more realistic superconducting doped graphene, magnetic phases of iron, and spin-liquid and magnetically ordered states of a simplest nitrogen-based copper pseudo-oxide, CuNCN, resembling socalled metal-oxide framework (MOF) phases by the atomic interlinkage.
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Affiliation(s)
- Andrei Tchougréeff
- A.N. Frumkin Institute of Physical Chemistry and Electrochemistry of Russian Academy of Science, Moscow, Russia
| | - Evgeny Plekhanov
- King's College London, Theory and Simulation of Condensed Matter (TSCM), The Strand, London WC2R 2LS, United Kingdom and A.N. Frumkin Institute of Physical Chemistry and Electrochemistry of Russian Academy of Science, Moscow, Russia
| | - Richard Dronskowski
- Institut für anorganische Chemie, RWTH Aachen University Aachen, Germany, Shenzhen, China.,Jülich-Aachen Research Alliance, JARA-HPC, RWTH Aachen University, Aachen, Germany.,Hoffmann Institute of Advanced Materials, Shenzhen Polytechnic, Shenzhen, China
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12
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Wang Z, Dong L, Xiao C, Niu Q. Berry Curvature Effects on Quasiparticle Dynamics in Superconductors. PHYSICAL REVIEW LETTERS 2021; 126:187001. [PMID: 34018795 DOI: 10.1103/physrevlett.126.187001] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/18/2020] [Revised: 03/23/2021] [Accepted: 04/13/2021] [Indexed: 06/12/2023]
Abstract
We construct a theory for the semiclassical dynamics of superconducting quasiparticles by following their wave packet motion and reveal rich contents of Berry curvature effects in the phase space spanned by position and momentum. These Berry curvatures are traced back to the characteristics of superconductivity, including the nontrivial momentum-space geometry of superconducting pairing, the real-space supercurrent, and the charge dipole of quasiparticles. The Berry-curvature effects strongly influence the spectroscopic and transport properties of superconductors, such as the local density of states and the thermal Hall conductivity. As a model illustration, we apply the theory to study the twisted bilayer graphene with a d_{x^{2}+y^{2}}+id_{xy} superconducting gap function and demonstrate Berry-curvature induced effects.
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Affiliation(s)
- Zhi Wang
- School of Physics, Sun Yat-sen University, Guangzhou 510275, China
| | - Liang Dong
- Department of Physics, The University of Texas at Austin, Austin, Texas 78712, USA
| | - Cong Xiao
- Department of Physics, The University of Texas at Austin, Austin, Texas 78712, USA
| | - Qian Niu
- Department of Physics, The University of Texas at Austin, Austin, Texas 78712, USA
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13
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Rosenzweig P, Karakachian H, Marchenko D, Küster K, Starke U. Overdoping Graphene beyond the van Hove Singularity. PHYSICAL REVIEW LETTERS 2020; 125:176403. [PMID: 33156643 DOI: 10.1103/physrevlett.125.176403] [Citation(s) in RCA: 20] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/14/2020] [Accepted: 09/09/2020] [Indexed: 06/11/2023]
Abstract
At very high doping levels the van Hove singularity in the π^{*} band of graphene becomes occupied and exotic ground states possibly emerge, driven by many-body interactions. Employing a combination of ytterbium intercalation and potassium adsorption, we n dope epitaxial graphene on silicon carbide past the π^{*} van Hove singularity, up to a charge carrier density of 5.5×10^{14} cm^{-2}. This regime marks the unambiguous completion of a Lifshitz transition in which the Fermi surface topology has evolved from two electron pockets into a giant hole pocket. Angle-resolved photoelectron spectroscopy confirms these changes to be driven by electronic structure renormalizations rather than a rigid band shift. Our results open up the previously unreachable beyond-van-Hove regime in the phase diagram of epitaxial graphene, thereby accessing an unexplored landscape of potential exotic phases in this prototype two-dimensional material.
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Affiliation(s)
- Philipp Rosenzweig
- Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, 70569 Stuttgart, Germany
| | - Hrag Karakachian
- Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, 70569 Stuttgart, Germany
| | - Dmitry Marchenko
- Helmholtz-Zentrum Berlin für Materialien und Energie, Elektronenspeicherring BESSY II, Albert-Einstein-Straße 15, 12489 Berlin, Germany
| | - Kathrin Küster
- Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, 70569 Stuttgart, Germany
| | - Ulrich Starke
- Max-Planck-Institut für Festkörperforschung, Heisenbergstraße 1, 70569 Stuttgart, Germany
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14
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Jiang QD, Hansson TH, Wilczek F. Geometric Induction in Chiral Superconductors. PHYSICAL REVIEW LETTERS 2020; 124:197001. [PMID: 32469552 DOI: 10.1103/physrevlett.124.197001] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/10/2020] [Accepted: 04/17/2020] [Indexed: 06/11/2023]
Abstract
We consider a number of effects due to the interplay of superconductivity, electromagnetism, and elasticity, which are unique for thin membranes of layered chiral superconductors. Some of them should be within the reach of present technology, and could be useful for characterizing materials. More speculatively, the enriched control of Josephson junctions they afford might find useful applications.
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Affiliation(s)
- Qing-Dong Jiang
- Department of Physics, Stockholm University, Stockholm SE-106 91, Sweden
| | - T H Hansson
- Department of Physics, Stockholm University, Stockholm SE-106 91, Sweden
| | - Frank Wilczek
- Department of Physics, Stockholm University, Stockholm SE-106 91, Sweden
- Center for Theoretical Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
- Wilczek Quantum Center, Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China
- Department of Physics, Arizona State University, Tempe Arizona 25287, USA
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15
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Zhang YX, Chiu WT, Costa NC, Batrouni GG, Scalettar RT. Charge Order in the Holstein Model on a Honeycomb Lattice. PHYSICAL REVIEW LETTERS 2019; 122:077602. [PMID: 30848616 DOI: 10.1103/physrevlett.122.077602] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/01/2018] [Indexed: 06/09/2023]
Abstract
The effect of electron-electron interactions on Dirac fermions, and the possibility of an intervening spin-liquid phase between the semimetal and antiferromagnetic (AF) regimes, has been a focus of intense quantum simulation effort over the last five years. We use determinant quantum Monte Carlo simulations to study the Holstein model on a honeycomb lattice and explore the role of electron- phonon interactions on Dirac fermions. We show that they give rise to charge-density-wave (CDW) order and present evidence that this occurs only above a finite critical interaction strength. We evaluate the temperature for the transition into the CDW which, unlike the AF transition, can occur at finite values owing to the discrete nature of the broken symmetry.
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Affiliation(s)
- Y-X Zhang
- Department of Physics, University of California, Davis, California 95616, USA
| | - W-T Chiu
- Department of Physics, University of California, Davis, California 95616, USA
| | - N C Costa
- Instituto de Física, Universidade Federal do Rio de Janeiro Cx.P. 68.528, 21941-972 Rio de Janeiro RJ, Brazil
| | - G G Batrouni
- Université Côte d'Azur, INPHYNI, CNRS, 0600 Nice, France
- MajuLab, CNRS-UCA-SU-NUS-NTU International Joint Research Unit, 117542 Singapore
- Centre for Quantum Technologies, National University of Singapore, 2 Science Drive 3, 117542 Singapore
- Department of Physics, National University of Singapore, 2 Science Drive 3, 117542 Singapore
- Beijing Computational Science Research Center, Beijing 100193, China
| | - R T Scalettar
- Department of Physics, University of California, Davis, California 95616, USA
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16
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Kvorning T, Hansson TH, Quelle A, Smith CM. Proposed Spontaneous Generation of Magnetic Fields by Curved Layers of a Chiral Superconductor. PHYSICAL REVIEW LETTERS 2018; 120:217002. [PMID: 29883131 DOI: 10.1103/physrevlett.120.217002] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/20/2017] [Indexed: 06/08/2023]
Abstract
We demonstrate that two-dimensional chiral superconductors on curved surfaces spontaneously develop magnetic flux. This geometric Meissner effect provides an unequivocal signature of chiral superconductivity, which could be observed in layered materials under stress. We also employ the effect to explain some puzzling questions related to the location of zero-energy Majorana modes.
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Affiliation(s)
- T Kvorning
- Department of Physics, Stockholm University, AlbaNova University Center, SE-106 91 Stockholm, Sweden
| | - T H Hansson
- Department of Physics, Stockholm University, AlbaNova University Center, SE-106 91 Stockholm, Sweden
- Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, SE-106 91 Stockholm, Sweden
| | - A Quelle
- Institute for Theoretical Physics, Center for Extreme Matter and Emergent Phenomena, Utrecht University, Princetonplein 5, 3584CC Utrecht, Netherlands
| | - C Morais Smith
- Institute for Theoretical Physics, Center for Extreme Matter and Emergent Phenomena, Utrecht University, Princetonplein 5, 3584CC Utrecht, Netherlands
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17
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Agterberg DF, Brydon PMR, Timm C. Bogoliubov Fermi Surfaces in Superconductors with Broken Time-Reversal Symmetry. PHYSICAL REVIEW LETTERS 2017; 118:127001. [PMID: 28388193 DOI: 10.1103/physrevlett.118.127001] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/05/2016] [Indexed: 06/07/2023]
Abstract
It is commonly believed that, in the absence of disorder or an external magnetic field, there are three possible types of superconducting excitation gaps: The gap is nodeless, it has point nodes, or it has line nodes. Here, we show that, for an even-parity nodal superconducting state which spontaneously breaks time-reversal symmetry, the low-energy excitation spectrum generally does not belong to any of these categories; instead, it has extended Bogoliubov Fermi surfaces. These Fermi surfaces can be visualized as two-dimensional surfaces generated by "inflating" point or line nodes into spheroids or tori, respectively. These inflated nodes are topologically protected from being gapped by a Z_{2} invariant, which we give in terms of a Pfaffian. We also show that superconducting states possessing these Fermi surfaces can be energetically stable. A crucial ingredient in our theory is that more than one band is involved in the pairing; since all candidate materials for even-parity superconductivity with broken time-reversal symmetry are multiband systems, we expect these Z_{2}-protected Fermi surfaces to be ubiquitous.
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Affiliation(s)
- D F Agterberg
- Department of Physics, University of Wisconsin, Milwaukee, Wisconsin 53201, USA
| | - P M R Brydon
- Department of Physics, University of Otago, P.O. Box 56, Dunedin 9054, New Zealand
| | - C Timm
- Institute of Theoretical Physics, Technische Universität Dresden, 01062 Dresden, Germany
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18
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Sun SJ, Chung CH, Chang YY, Tsai WF, Zhang FC. Helical Majorana fermions in d(x2-y2) + id(xy)-wave topological superconductivity of doped correlated quantum spin Hall insulators. Sci Rep 2016; 6:24102. [PMID: 27064108 PMCID: PMC4827071 DOI: 10.1038/srep24102] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/13/2015] [Accepted: 03/16/2016] [Indexed: 11/09/2022] Open
Abstract
There has been growing interest in searching for exotic self-conjugate, charge-neutral low-energy fermionic quasi-particles, known as Majorana fermions (MFs) in solid state systems. Their signatures have been proposed and potentially observed at edges of topological superconcuctors with non-trivial topological invariant in the bulk electronic band structure. Much effort have been focused on realizing MFs in odd-parity superconductors made of strong spin-orbit coupled materials in proximity to conventional superconductors. In this paper, we propose a novel mechanism for realizing MFs in 2D spin-singlet topological superconducting state induced by doping a correlated quantum spin Hall (Kane-Mele) insulator. Via a renormalized mean-field approach, the system is found to exhibits time-reversal symmetry (TRS) breaking d(x2-y2) + id(xy)-wave (chiral d-wave) superconductivity near half-filling in the limit of large on-site repulsion. Surprisingly, however, at large spin-orbit coupling, the system undergoes a topological phase transition and enter into a new topological phase protected by a pseudo-spin Chern number, which can be viewed as a persistent extension of the quantum spin Hall phase upon doping. From bulk-edge correspondence, this phase is featured by the presence of two pairs of counter-propagating helical Majorana modes per edge, instead of two chiral propagating edge modes in the d + id' superconductors.
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Affiliation(s)
- Shih-Jye Sun
- Department of Applied Physics, National University of Kaohsiung, Kaohsiung, Taiwan, R.O.C.
| | - Chung-Hou Chung
- Electrophysics Department, National Chiao-Tung University, HsinChu, Taiwan, 300, R.O.C.
- Physics Division, National Center for Theoretical Sciences, HsinChu, Taiwan, 300 R.O.C.
| | - Yung-Yeh Chang
- Electrophysics Department, National Chiao-Tung University, HsinChu, Taiwan, 300, R.O.C.
| | - Wei-Feng Tsai
- Department of Physics, National Sun Yat-Sen University, Kaohsiung, Taiwan, R.O.C.
| | - Fu-Chun Zhang
- Department of Physics, Zhejiang University, Hangzhou, China
- Collaborative Innovation Center of Advanced Microstructures, Nanjing, China
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19
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Accessing topological superconductivity via a combined STM and renormalization group analysis. Nat Commun 2015; 6:8232. [PMID: 26348438 DOI: 10.1038/ncomms9232] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/15/2014] [Accepted: 07/31/2015] [Indexed: 11/08/2022] Open
Abstract
The search for topological superconductors has recently become a key issue in condensed matter physics, because of their possible relevance to provide a platform for Majorana bound states, non-Abelian statistics, and quantum computing. Here we propose a new scheme which links as directly as possible the experimental search to a material-based microscopic theory for topological superconductivity. For this, the analysis of scanning tunnelling microscopy, which typically uses a phenomenological ansatz for the superconductor gap functions, is elevated to a theory, where a multi-orbital functional renormalization group analysis allows for an unbiased microscopic determination of the material-dependent pairing potentials. The combined approach is highlighted for paradigmatic hexagonal systems, such as doped graphene and water-intercalated sodium cobaltates, where lattice symmetry and electronic correlations yield a propensity for a chiral singlet topological superconductor. We demonstrate that our microscopic material-oriented procedure is necessary to uniquely resolve a topological superconductor state.
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