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Zhou H, Holleis L, Saito Y, Cohen L, Huynh W, Patterson CL, Yang F, Taniguchi T, Watanabe K, Young AF. Isospin magnetism and spin-polarized superconductivity in Bernal bilayer graphene. Science 2022; 375:774-778. [PMID: 35025604 DOI: 10.1126/science.abm8386] [Citation(s) in RCA: 41] [Impact Index Per Article: 20.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
In conventional superconductors, Cooper pairing occurs between electrons of opposite spin. We observe spin-polarized superconductivity in Bernal bilayer graphene when doped to a saddle-point van Hove singularity generated by a large applied perpendicular electric field. We observe a cascade of electrostatic gate-tuned transitions between electronic phases distinguished by their polarization within the isospin space defined by the combination of the spin and momentum-space valley degrees of freedom. Although all of these phases are metallic at zero magnetic field, we observe a transition to a superconducting state at finite magnetic field B‖ ≈ 150 milliteslas applied parallel to the two-dimensional sheet. Superconductivity occurs near a symmetry-breaking transition and exists exclusively above the B‖ limit expected of a paramagnetic superconductor with the observed transition critical temperature TC ≈ 30 millikelvins, consistent with a spin-triplet order parameter.
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Affiliation(s)
- Haoxin Zhou
- Department of Physics, University of California at Santa Barbara, Santa Barbara, CA 93106, USA
| | - Ludwig Holleis
- Department of Physics, University of California at Santa Barbara, Santa Barbara, CA 93106, USA
| | - Yu Saito
- Department of Physics, University of California at Santa Barbara, Santa Barbara, CA 93106, USA
| | - Liam Cohen
- Department of Physics, University of California at Santa Barbara, Santa Barbara, CA 93106, USA
| | - William Huynh
- Department of Physics, University of California at Santa Barbara, Santa Barbara, CA 93106, USA
| | - Caitlin L Patterson
- Department of Physics, University of California at Santa Barbara, Santa Barbara, CA 93106, USA
| | - Fangyuan Yang
- Department of Physics, University of California at Santa Barbara, Santa Barbara, CA 93106, USA
| | - Takashi Taniguchi
- International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
| | - Kenji Watanabe
- Research Center for Functional Materials, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
| | - Andrea F Young
- Department of Physics, University of California at Santa Barbara, Santa Barbara, CA 93106, USA
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Groshev AG, Arzhnikov AK. Thermal fluctuations in superconducting phases with chiral d+ i dand ssymmetry on a triangular lattice. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2021; 33:215604. [PMID: 33592591 DOI: 10.1088/1361-648x/abe6e0] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/19/2020] [Accepted: 02/16/2021] [Indexed: 06/12/2023]
Abstract
The behavior of thermal fluctuations of a superconducting order parameter with extendedsand chirald+ idsymmetry is investigated. The study is carried out on a triangular lattice within the framework of the quasi-two-dimensional single-band model with attraction between electrons at neighboring sites. The method of consistent consideration of the order parameter fluctuations and the charge carrier scattering by fluctuations of coupled electron pairs, based on the theory of functional integration is used. The distribution functions of the phase fluctuation probabilities depending on temperature and charge carrier concentration are obtained. The temperature dependences of the amplitudes of the averaged superconducting order parameter are calculated. A phase diagram of superconducting states is constructed for the entire range of variation in the charge carrier concentration 0 <n< 2. Near the boundaries of this range, topologically trivial superconducting states with extendedssymmetry are realized, while a superconducting state with topologically nontrivial chirald+ idsymmetry is realized between them. The calculated anomalous self-energies are compared with the experimental ones obtained using machine learning techniques.
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Affiliation(s)
- A G Groshev
- Udmurt Federal Research Center of the Ural Branch of the Russian Academy of Sciences, T. Baramzinoy st. 34, Izhevsk 426067, Russia
| | - A K Arzhnikov
- Udmurt Federal Research Center of the Ural Branch of the Russian Academy of Sciences, T. Baramzinoy st. 34, Izhevsk 426067, Russia
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Identifying the chiral d-wave superconductivity by Josephson φ 0-states. Sci Rep 2017; 7:43899. [PMID: 28266582 PMCID: PMC5339699 DOI: 10.1038/srep43899] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/02/2016] [Accepted: 01/30/2017] [Indexed: 11/17/2022] Open
Abstract
We propose the Josephson junctions linked by a normal metal between a d + id superconductor and another d + id superconductor, a d-wave superconductor, or a s-wave superconductor for identifying the chiral d + id superconductivity. The time-reversal breaking in the chiral d-wave superconducting state is shown to result in a Josephson φ0-junction state where the current-phase relation is shifted by a phase φ0 from the sinusoidal relation, other than 0 and π. The ground-state phase difference φ0 and the critical current can be used to definitely confirm and read the information about the d + id superconductivity. A smooth evolution from conventional 0-π transitions to tunable φ0-states can be observed by changing the relative magnitude of two types of d-wave components in the d + id pairing. On the other hand, the Josephson junction involving the d + id superconductor is also the simplest model to realize a φ0- junction, which is useful in superconducting electronics and superconducting quantum computation.
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Black-Schaffer AM, Honerkamp C. Chiral d-wave superconductivity in doped graphene. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2014; 26:423201. [PMID: 25238054 DOI: 10.1088/0953-8984/26/42/423201] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
A highly unconventional superconducting state with a spin-singlet dx2-y2+/-idxy-wave, or chiral d-wave symmetry has recently been suggested to emerge from electron-electron interactions in doped graphene. It has been argued that graphene doped to the van Hove singularity at 1/4 doping, where the density of states diverge, is particularly likely to be a chiral d-wave superconductor. In this review we summarize the currently mounting theoretical evidence for the existence of a chiral d-wave superconducting state in graphene, obtained with methods ranging from mean-field studies of effective Hamiltonians to angle-resolved renormalization group calculations. We further discuss the multiple distinctive properties of the chiral d-wave superconducting state in graphene, as well as its stability in the presence of disorder. We also review the means of enhancing the chiral d-wave state using proximity-induced superconductivity. The appearance of chiral d-wave superconductivity is intimately linked to the hexagonal crystal lattice and we also offer a brief overview of other materials which have also been proposed to be chiral d-wave superconductors.
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