Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session M60: Symmetry and Topological SuperconductivityFocus
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Sponsoring Units: DMP Chair: Shinsei Ryu, University of Chicago Room: Mile High Ballroom 4A |
Wednesday, March 4, 2020 11:15AM - 11:51AM |
M60.00001: Majorana multipole response of topological superconductors Invited Speaker: Masatoshi Sato Crystalline symmetry enriches topological phases of matter. For instance, Dirac semimetals with crystalline symmetry are one of the promising ways to realize a variety of topological superconductivity. In contrast to elementary Majorana particles, emergent Majorana fermions (MFs) in topological crystalline supercondutors may have electromagnetic multipoles. We developed a general theory of magnetic multipoles for surface helical MFs on time-reversal-invariant superconductors. The results show that the multipole response is governed by crystal symmetry, and that a one-to-one correspondence exists between the symmetry of Cooper pairs and the representation of magnetic multipoles under crystal symmetry. The latter property provides a way to identify nonconventional pairing symmetry via the magnetic response of surface MFs. We also find that most helical MFs exhibit a magnetic-dipole response, but those on superconductors with spin-3/2 electrons may display a magnetic-octupole response in leading order, which uniquely characterizes high-spin superconductors. Detection of such an octupole response provides direct evidence of high-spin superconductivity, such as in half-Heusler superconductors. |
Wednesday, March 4, 2020 11:51AM - 12:03PM |
M60.00002: Anomalous normal fluid response in helical spin-triplet superconductor UTe2 Seokjin Bae, Hyunsoo Kim, Sheng Ran, Yun Suk Eo, I-Lin Liu, Wesley T Fuhrman, Johnpierre Paglione, Nicholas Butch, Steven Anlage We report evidence for a helical spin-triplet pairing state of UTe2 with significant surface normal fluid response. The microwave surface impedance of UTe2 crystals was measured and converted to complex conductivity. The anomalous residual normal fluid conductivity (real part) in the zero temperature limit supports the presence of a significant normal fluid response in the ground state. The superfluid conductivity (imaginary part) follows the low temperature behavior predicted for the helical spin-triplet state. The temperature dependence of the superfluid conductivity also reveals a low impurity scattering rate and low frequency-to-energy-gap ratio, implying that the observed normal fluid response is not due to an extrinsic origin. Candidate mechanisms such as a surface Majorana normal fluid, which are predicted for the helical spin-triplet superconductor, are discussed. |
Wednesday, March 4, 2020 12:03PM - 12:15PM |
M60.00003: Microscopic evidence for chiral superconductivity in a heavy fermion superconductor UTe2 Lin Jiao, Sean Howard, Sheng Ran, Zhenyu Wang, Jorge Olivares Rodriguez, Manfred W Sigrist, Ziqiang Wang, Nicholas Butch, Vidya Madhavan Spin-triplet superconductivity is a condensate of electron pairs with spin-1 and an odd parity pair wavefunction. A particularly interesting manifestation of triplet pairing is a chiral p-wave state which is topologically non-trivial and a natural platform for realizing Majorana edge modes. Triplet pairing is however rare in solid state systems and so far, no unambiguous identification has been made in any bulk compound. Since pairing is most naturally mediated by ferromagnetic spin fluctuations, uranium based heavy fermion systems containing f-electron elements that can harbor both strong correlations and magnetism are considered ideal candidate spin-triplet superconductors. In this talk I will present scanning tunneling microscopy (STM) studies of the newly discovered heavy fermion superconductor, UTe2 with a TSC of 1.6 K. We find signatures of coexisting Kondo effect and superconductivity which show competing spatial modulations within one unit-cell. Most strikingly, STM spectroscopy at step edges show signatures of chiral edge states, indicating UTe2 is a 3D chiral superconductor. |
Wednesday, March 4, 2020 12:15PM - 12:27PM |
M60.00004: Effects of uniaxial pressure in Uranium-based superconductors Sean Thomas, Frederico Benedetto Santos, Tomoya Asaba, Eric Bauer, Joe D Thompson, Priscila Rosa, Filip Ronning Uranium-based superconductors have been proposed to host a number of exotic superconducting states. In some p- and d-wave superconductors, uniaxial stress is expected to split the superconducting transition temperature between the two components of the order parameter. The anisotropic dependence of this splitting can provide hints about the underlying nature of the superconductivity in these materials. Here, we probe several Uranium-based superconductors using in-situ tuning of uniaxial stress and discuss the implications of the results on the allowed pairing mechanisms. |
Wednesday, March 4, 2020 12:27PM - 12:39PM |
M60.00005: Majorana fermions in narrow zigzag honeycomb nanoribbons Renan Bento Ribeiro Campos, Jorge Huamani Correa, Antonio Carlos Seridonio, Marcos Figueira In this work, we study the generation of Majorana fermions in narrow zigzag honeycomb nanoribbons (ZHNR) deposited over an 's' superconductor. We study the system using an effective minimal model that consists of the first nearest neighbor hopping, Rashba spin-orbit interaction, triplet superconducting pairing between the magnetic moments of the ZHNR edges and an external exchange magnetic field [1]. |
Wednesday, March 4, 2020 12:39PM - 12:51PM |
M60.00006: Atomic limit and inversion-symmetry indicators for topological superconductors Anastasiia Skurativska, Titus Neupert, Mark H Fischer Symmetry indicators have proven to be extremely helpful in identifying topologically non-trivial crystalline insulators using symmetry-group representations of their Bloch states. An extension of this approach to superconducting systems requires defining an appropriate atomic limit for Bogoliubov-de-Gennes Hamiltonians. Here, we introduce such a notion of atomic limit and derive a Z2d -valued symmetry indicator for inversion- symmetric superconductors of d dimensions. This indicator allows for a refined topological classification including higher-order phases for systems in the superconducting symmetry classes D and DIII. We further elucidate the bulk-boundary correspondence of these phases using Dirac surface theories. Requiring only the normal-state band structure and the superconducting order-parameter symmetry as an input, this indicator is well suited for a search of topological superconductors using first-principles calculations. |
Wednesday, March 4, 2020 12:51PM - 1:03PM |
M60.00007: Unconventional Superconductivity in spin-3/2 fermions Julia Link, Igor Boettcher, Igor Herbut We study unconventional superconductivity in spin-orbit-coupled three-dimensional electronic systems without inversion symmetry, featuring gapless fermions with the Hamiltonian given by H=p\cdot J, where J are the generators of total angular momentum of 3/2. These systems have a Fermi points with a fixed ratio of the Fermi velocities. A favorable short-range interaction can lead to a d-wave superconductivity which is described by a complex tensor order parameter. We investigate the structure of the corresponding Ginzburg-Landau free energy and demonstrate that already at the quartic level the superconducting state of the system is uniquely determined. For a chemical potential right at the Fermi point, the ground state of the system is given by an uniaxial nematic state. In the case of a finite chemical potential, we find that the cyclic state is favored as a ground state. The cyclic state breaks time-reversal symmetry maximally, has no average magnetization, and exhibits robust small Bogoliubov Fermi surfaces in the excitation spectrum of the quasiparticles. |
Wednesday, March 4, 2020 1:03PM - 1:15PM |
M60.00008: Chemical doping effect on a topological superconductor candidate Kaya Kobayashi, Takeshi Takahashi, Harald Jeschke, Jun Akimitsu Au2Pb crystallizing in a cubic Laves phase exhibits a bulk Dirac cone in the vicinity of the Gamma point. The material also shows superconductivity, but not in the cubic phase because the material goes through several structural transitions at low temperatures. On the other hand, one of the oldest alloy superconductors Au2Bi possesses the same crystal structure as the cubic Laves phase. There is no detailed report for the latter compound apart from the superconducting transition temperature (Tc) at 1.8 K. We will report the enhancement of Tc by Bi substitution in Au2Pb associated with the suppression of structural transitions. |
Wednesday, March 4, 2020 1:15PM - 1:27PM |
M60.00009: Observation of half-quantum flux in topological superconductor β-Bi2Pd Yufan Li, Xiaoying Xu, M.-H. Lee, M.-W Chu, Chia-Ling Chien Magnetic flux quantization is one of the defining characteristics of a superconductor. We report the observation of half-integer magnetic flux quantization in mesoscopic rings of superconducting β-Bi2Pd thin films [1]. The half-quantum fluxoid manifests itself as a π-phase shift in the quantum oscillations of the superconducting critical temperature. The superconducting ring energetically prefers half flux quanta with fractional quantum numbers of 1/2, 3/2, 5/2, etc., instead of usual integer numbers of 0, 1, 2, etc. This result is consistent with β-Bi2Pd having a spin-triplet pairing symmetry, which may be expected from β-Bi2Pd as a topological supercondcutor [2,3]. Our findings usher in new venues for studying topological superconductivity, and new designs of flux qubit for quantum computing, which may operate without external magnetic fields. |
Wednesday, March 4, 2020 1:27PM - 1:39PM |
M60.00010: Spin-triplet Superconductivity Evidenced by Half-Integer Quantum Flux in a Non-centrosymmetric Superconductor Xiaoying Xu, Yufan Li, Chia-Ling Chien Spin-triplet superconductors, rare and challenging to identify, play central roles in Majorana fermions and quantum computing. It has been proposed that superconductors with broken inversion symmetry may host spin-triplet Cooper pairs. Unique features of half-quantum fluxoid in flux quantization is one of the few methods that can identify spin-triplet superconductors [1]. We fabricated thin films of non-centrosymmetric a-BiPd by sputtering and patterned them into sub-µm-sized rings. Measurements of the Little-Parks oscillations in magnetoresistance show half-quantum fluxoid (n + ½)Φo, a key signature of spin-triplet pairing, where flux quantum Φo = hc/2e and n is an integer. Our extensive measurements support mixing pairing states of spin-singlet and spin-triplet Cooper pairs in α-BiPd. |
Wednesday, March 4, 2020 1:39PM - 2:15PM |
M60.00011: Floquet higher-order topological insulators: principles and path towards realizations Invited Speaker: Gil Refael The co-existence of spatial and non-spatial symmetries together with appropriate commutation/anticommutation relations between them can give rise to static higher-order topological phases, which host gapless boundary modes of co-dimension higher than one. Alternatively, space-time symmetries in a Floquet system can also lead to anomalous Floquet boundary modes of higher co-dimensions, with different commutation/anticommutation relations with respect to non-spatial symmetries. In my talk I will review how these dynamical analogs of the static HOTI's emerge, and also show how a coherently excited phonon mode can be used to support non-trivial Floquet higher-order topological phases. |
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