Bulletin of the American Physical Society
APS March Meeting 2021
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session F52: Topological Superconductivity: Symmetry Breaking and SpinOrbit CouplingFocus Live

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Sponsoring Units: DMP Chair: Daniel Agterberg, University of Wisconsin  Milwaukee 
Tuesday, March 16, 2021 11:30AM  12:06PM Live 
F52.00001: Multipole theory of Majorana fermions in topological superconductors Invited Speaker: Masatoshi Sato The emergence of Majorana fermions is a salient feature of topological superconductors. Whereas the emergent Majorana fermions are often discussed as possible qubits of topological quantum computers, they also provide important information to identify paring symmetry of underlying unconventional superconductors. We develop here a theory of electric and magnetic responses of Majorana fermions in superconductors and clarify the relation between the electromagnetic responses and Cooper pair symmetry [1,2]. We point out that Majorana fermions belong to short representations of particlehole or chiral symmetries, which is a group theoretical manifestation of their selfantiparticle nature. We reveal magnetic octupole responses of Majorana fermions that characterize spin 3/2 superconductors or nonsymmorphic superconductors. We also apply our theory to spin 3/2 halfHeusler superconductors and other unconventional superconductors. 
Tuesday, March 16, 2021 12:06PM  12:18PM Live 
F52.00002: Observation of halfquantum flux in topological superconductor βBi_{2}Pd Yufan Li, Xiaoying Xu, M.H. Lee, M.W. Chu, C. L. 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 (HQF) manifests itself as a 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 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 a new paradigm for identifying spintriplet pairing, and new venues for studying topological superconductivity. 
Tuesday, March 16, 2021 12:18PM  12:30PM Live 
F52.00003: Control of the nematic superconductivity of Sr_{x}Bi_{2}Se_{3} and its domains by uniaxial strain Shingo Yonezawa, Ivan Kostylev, Zhiwei Wang, Yoichi Ando, Yoshiteru Maeno Recently, nematic superconductivity, exhibiting spontaneous rotational symmetry breaking in bulk superconducting quantities, has been discovered in A_{x}Bi_{2}Se_{3} (A = Cu, Sr, Nb) [1]. In an analogy to the nematic liquid crystals accompanying high controllability of the orderparameter directions, similar controllability can be expected for nematic superconductivity. Indeed, we succeeded in controlling nematic superconductivity in Sr_{x}Bi_{2}Se_{3} via external uniaxial strain [2]. By applying uniaxial strain in situ along the a axis, we reversibly controlled the superconducting nematic domain structure, from the multidomain state under zero strain into a nearly singledomain state under 1% uniaxial compression. This is the first achievement of domain engineering using nematic superconductors. 
Tuesday, March 16, 2021 12:30PM  12:42PM Live 
F52.00004: Chiral topological superconducting phase with Chern number C=2 in Pb_{3}Bi/Ge(111) system Shuwen Sun, Wei Qin, Leiqiang Li, Zhenyu Zhang Experimentally realizable chiral topological superconductors are actively pursued since they are expected to host Majorana quasiparticles. Here we develop an effective tightbinding description of Pb_{3}Bi/Ge(111), a candidate system for realizing chiral pwave topological superconductor [Nat. Phys. 15, 796 (2019)]. The model is demonstrated to be able to capture the two central characters of the electronic bands found in firstprinciple calculations, namely, the giant Rashba spinorbit coupling and typeII van Hove singularity. By introducing local superconducting pairing and perpendicular Zeeman field, we show the system can transform into a chiral topological superconducting phase with Chern number C=2. The nontrivial topology is further confirmed by edge state calculations and manifested as two pairs of chiral Majorana edge modes propagating along the same direction. We also discuss the experimental conditions for observing such a topological state by taking account of realistic material parameters, including the Landé gfactor, upper critical magnetic field, and realspace length scale of the Majorana edge modes. This study provides a guide for realizing and detecting Majorana quasiparticles in the Pb_{3}Bi/Ge(111) system. 
Tuesday, March 16, 2021 12:42PM  12:54PM Live 
F52.00005: Spintriplet Superconductivity Evidenced by Half Quantum Flux in a Noncentrosymmetric Superconductor Xiaoying Xu, Yufan Li, C. L. 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 αBiPd 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 an admixture of pairing states of spinsinglet and spintriplet Cooper pairs in αBiPd [2]. 
Tuesday, March 16, 2021 12:54PM  1:06PM Live 
F52.00006: Superconducting properties in the topological superconductor candidate PbTaSe_{2} family RIFFAT MUNIR, K A M Hasan Siddiquee, Charuni Dissanayake, Xinzhe Hu, Yasumasa Takano, Eun Sang Choi, Yasuyuki Nakajima Nodal line semimetals are novel quantum states of matter, hosting topological nodal rings in the bulk band structures. Among the nodal line semimetals, PbTaSe_{2} family without the inversion symmetry in the crystal structure has been reported to be superconducting, and can be a promising platform for the study of topological superconductivity [1,2]. Here we present the superconducting properties of single crystals of PbTaSe_{2} family. Our resistivity measurements under high magnetic fields reveal the unusual temperature dependence and magnitude of the upper critical field in this family, strikingly distinct from those for conventional BCStype superconductors. Compering theoretical models, we will discuss the possible superconducting pairing symmetry realized in this family. 
Tuesday, March 16, 2021 1:06PM  1:18PM Live 
F52.00007: Potential topological superconductivity in doped IVVI semiconductors Zhe Li We theoretically study potential superconductivity in doped IVVI semiconductors in the presence of the interaction restricted to the next nearest neighbors. We show that pointgroupsymmetrybreaking Cooper pairing, for example in the E_{u}, the T_{1u} or the T_{2u}channel, appears in different regions of the phase diagram that is plotted with respect to model parameters such as effective mass anisotropy. The topological properties of these superconductors are also predicted and discussed. 
Tuesday, March 16, 2021 1:18PM  1:30PM Live 
F52.00008: Majorana Fermion in Topological Superconducting Element Hcp Thallium Motoaki Hirayama, Takuya Nomoto, Ryotaro Arita The Majorana fermion is an exotic particle that is its own antiparticle. Chiral Majorana fermion can arise in a onedimensional edge of topological materials, and especially that in a topological superconductor can be exploited in nonAbelian quantum computation. While the chiral Majorana mode (CMM) remains elusive, a promising situation is realized when superconductivity coexists with a topologically nontrivial surface state. Here, we perform fully nonempirical calculation for the CMM considering superconductivity and surface relaxation, and show that hexagonal closepacked thallium (Tl) has an ideal electronic state that harbors the CMM [1]. The k_{z} = 0 plane corresponds to the TCI with mirror Chern number N_{M}=2. One of the Dirac points on the (0110) surface is located almost at the Fermi level. Tl is a textbooklike swave superconductor and the gap function has no significant wavenumber dependence. Only one of the two Dirac points is relevant for the gap opening due to the superconducting transition, and the CMM appears at the hinge under the Zeeman field. Our calculation indicates that Tl will provide a new platform of the Majorana fermion and quantum computation. 
Tuesday, March 16, 2021 1:30PM  1:42PM Live 
F52.00009: Superconductivity in the nonsymmorphic linenodal compound CaSb_{2} Atsutoshi Ikeda, Mayo Kawaguchi, Shun Koibuchi, Hidemitsu Takahashi, Shunsaku Kitagawa, Kenji Ishida, Tatsuki Hashimoto, Takuto Kawakami, Shingo Yonezawa, Masatoshi Sato, Yoshiteru Maeno We found superconductivity in CaSb_{2} with the transition onset of 1.8 K by means of electricalresistivity, magneticsusceptibility, and specificheat measurements [1]. Zero resistance was observed below 1.2 K, and the superconducting volume fraction at 0 K was estimated to be 65% from specific heat. This material crystallizes in a nonsymmorphic structure and is predicted to have multiple Dirac nodal lines in the bulk electronic band structure protected by crystalline symmetry even in the presence of spinorbit coupling. In this presentation, we will briefly introduce the band structure of CaSb_{2} and then show experimental results evidencing its bulk superconductivity. 
Tuesday, March 16, 2021 1:42PM  1:54PM Live 
F52.00010: Multicomponent Chiral Topological Superconductors in 2D Superlattices Luiz Santos, Lakshmi Iyer, Jian Wang, Daniel Shaffer The advent of superlattices in recent years has made possible the realization of twodimensional lattices with large magnetic fluxes per unit cell when in the presence of an external magnetic field. The interplay of lattice effects and magnetic field creates a scenario of multiple Fermi surfaces related by magnetic translation symmetry. I will discuss the onset of timereversal symmetry breaking superconductivity in such systems, and demonstrate the existence of meanfield superconductors characterized by multicomponent triplet order parameters, which are greatly constrained by the action of the magnetic translation group. An outstanding property of these large flux superconductors is that Cooper pairs have finite momentum, and bulk state is characterized by higher Chern numbers, signaling the presence of many chiral Majorana edge modes. I will discuss their bulk properties as well as multistage phase transitions that change the bulk topology and the nature of the chiral edge Majorana fermions. 
Tuesday, March 16, 2021 1:54PM  2:06PM On Demand 
F52.00011: Order Parameter and Edge Currents of a 2D Topological Superconductor under Consecutive Symmetry Breakings Maximilian Holst, Manfred W Sigrist, Mark Fischer Twodimensional topological superconductors host topologically protected edge states guaranteed by the bulkboundary correspondence. However, the associated edge currents are not quantized as the superconducting state breaks the electron’s U(1) symmetry. Indeed, here we show how the spin and charge edge currents can change continuously even when going through a topological phase transition. 
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