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 timereversalinvariant superconductors. The results show that the multipole response is governed by crystal symmetry, and that a onetoone 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 magneticdipole response, but those on superconductors with spin3/2 electrons may display a magneticoctupole response in leading order, which uniquely characterizes highspin superconductors. Detection of such an octupole response provides direct evidence of highspin superconductivity, such as in halfHeusler superconductors. 
Wednesday, March 4, 2020 11:51AM  12:03PM 
M60.00002: Anomalous normal fluid response in helical spintriplet superconductor UTe2 Seokjin Bae, Hyunsoo Kim, Sheng Ran, Yun Suk Eo, ILin Liu, Wesley T Fuhrman, Johnpierre Paglione, Nicholas Butch, Steven Anlage We report evidence for a helical spintriplet pairing state of UTe_{2} with significant surface normal fluid response. The microwave surface impedance of UTe_{2} 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 spintriplet state. The temperature dependence of the superfluid conductivity also reveals a low impurity scattering rate and low frequencytoenergygap 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 spintriplet superconductor, are discussed. 
Wednesday, March 4, 2020 12:03PM  12:15PM 
M60.00003: Microscopic evidence for chiral superconductivity in a heavy fermion superconductor UTe_{2} Lin Jiao, Sean Howard, Sheng Ran, Zhenyu Wang, Jorge Olivares Rodriguez, Manfred W Sigrist, Ziqiang Wang, Nicholas Butch, Vidya Madhavan Spintriplet superconductivity is a condensate of electron pairs with spin1 and an odd parity pair wavefunction. A particularly interesting manifestation of triplet pairing is a chiral pwave state which is topologically nontrivial 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 felectron elements that can harbor both strong correlations and magnetism are considered ideal candidate spintriplet superconductors. In this talk I will present scanning tunneling microscopy (STM) studies of the newly discovered heavy fermion superconductor, UTe_{2} with a T_{SC} of 1.6 K. We find signatures of coexisting Kondo effect and superconductivity which show competing spatial modulations within one unitcell. Most strikingly, STM spectroscopy at step edges show signatures of chiral edge states, indicating UTe_{2} is a 3D chiral superconductor. 
Wednesday, March 4, 2020 12:15PM  12:27PM 
M60.00004: Effects of uniaxial pressure in Uraniumbased superconductors Sean Thomas, Frederico Benedetto Santos, Tomoya Asaba, Eric Bauer, Joe D Thompson, Priscila Rosa, Filip Ronning Uraniumbased superconductors have been proposed to host a number of exotic superconducting states. In some p and dwave 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 Uraniumbased superconductors using insitu 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 spinorbit 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 inversionsymmetry indicators for topological superconductors Anastasiia Skurativska, Titus Neupert, Mark H Fischer Symmetry indicators have proven to be extremely helpful in identifying topologically nontrivial crystalline insulators using symmetrygroup representations of their Bloch states. An extension of this approach to superconducting systems requires defining an appropriate atomic limit for BogoliubovdeGennes Hamiltonians. Here, we introduce such a notion of atomic limit and derive a Z_{2d} valued symmetry indicator for inversion symmetric superconductors of d dimensions. This indicator allows for a refined topological classification including higherorder phases for systems in the superconducting symmetry classes D and DIII. We further elucidate the bulkboundary correspondence of these phases using Dirac surface theories. Requiring only the normalstate band structure and the superconducting orderparameter symmetry as an input, this indicator is well suited for a search of topological superconductors using firstprinciples calculations. 
Wednesday, March 4, 2020 12:51PM  1:03PM 
M60.00007: Unconventional Superconductivity in spin3/2 fermions Julia Link, Igor Boettcher, Igor Herbut We study unconventional superconductivity in spinorbitcoupled threedimensional 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 shortrange interaction can lead to a dwave superconductivity which is described by a complex tensor order parameter. We investigate the structure of the corresponding GinzburgLandau 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 timereversal 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 Au_{2}Pb 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 Au_{2}Bi 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 Au_{2}Pb associated with the suppression of structural transitions. 
Wednesday, March 4, 2020 1:15PM  1:27PM 
M60.00009: Observation of halfquantum flux in topological superconductor βBi_{2}Pd Yufan Li, Xiaoying Xu, M.H. Lee, M.W Chu, ChiaLing Chien Magnetic flux quantization is one of the defining characteristics of a superconductor. We report the observation of halfinteger magnetic flux quantization in mesoscopic rings of superconducting βBi_{2}Pd thin films [1]. The halfquantum fluxoid manifests itself as a πphase 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 βBi_{2}Pd having a spintriplet pairing symmetry, which may be expected from βBi_{2}Pd 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: Spintriplet Superconductivity Evidenced by HalfInteger Quantum Flux in a Noncentrosymmetric Superconductor Xiaoying Xu, Yufan Li, ChiaLing Chien Spintriplet 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 spintriplet Cooper pairs. Unique features of halfquantum fluxoid in flux quantization is one of the few methods that can identify spintriplet superconductors [1]. We fabricated thin films of noncentrosymmetric aBiPd by sputtering and patterned them into subµmsized rings. Measurements of the LittleParks oscillations in magnetoresistance show halfquantum fluxoid (n + ½)Φ_{o}, a key signature of spintriplet pairing, where flux quantum Φ_{o} = hc/2e and n is an integer. Our extensive measurements support mixing pairing states of spinsinglet and spintriplet Cooper pairs in αBiPd. 
Wednesday, March 4, 2020 1:39PM  2:15PM 
M60.00011: Floquet higherorder topological insulators: principles and path towards realizations Invited Speaker: Gil Refael The coexistence of spatial and nonspatial symmetries together with appropriate commutation/anticommutation relations between them can give rise to static higherorder topological phases, which host gapless boundary modes of codimension higher than one. Alternatively, spacetime symmetries in a Floquet system can also lead to anomalous Floquet boundary modes of higher codimensions, with different commutation/anticommutation relations with respect to nonspatial 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 nontrivial Floquet higherorder topological phases. 
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