Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K05: Topological Superconductivity: Febased MaterialsFocus

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Sponsoring Units: DMP Chair: Qimin Yan, Temple Univ Room: BCEC 108 
Wednesday, March 6, 2019 8:00AM  8:36AM 
K05.00001: Majorana bound state observed in Febased superconductors Invited Speaker: Lingyuan Kong Ironbase superconductor (FeSC) is nontrivial on both topological band structure and exotic quasiparticle excitations. Recently, Majorana bound state (MBS) was observed as nonsplit zerobias conductance peak across a vortex core of Fe(Te, Se). Due to a small Fermi energy, a large energy gap separates the MBS with other trivial bound states, which makes FeSC a promising highT_{c} platform for advance study of MBS and topological quantum qubit. In this talk, I will focus on the properties of MBS shown in our STM experiments. We found the observed MBS is most probably contributed by surface “Dirac fermions”. The “unavoidable” bulk of Fe(Te, Se) plays as quasiparticle bath, which destroys MBS in a lifting temperature. I will also discuss some new Majorana evidences of FeSC identified in recent tunneling experiments. 
Wednesday, March 6, 2019 8:36AM  8:48AM 
K05.00002: Probing the Superconducting State of FeTe_{0.55}Se_{0.55} with PointContact Spectroscopy Mason Gray, Ryan O'Connor, Samuel Jenkins, Josef Freudenstein, Ruidan Zhong, Genda Gu, Kenneth Burch The material FeTe_{1x}Se_{x} provides an easy platform to study the Ironbased superconductors due to its simple crystal structure and relatively high superconducting transition temperature. In addition, some recent ARPES and STM studies suggest that FeTe0.55Se0.45 may be topologically nontrivial. With this in mind, we use PointContact Differential Conductance studies to further investigate this material. We find a “Cusplike” Zero Bias Conductance Peak (ZBCP) emerges below T_{c}. The magnetic field dependence of this ZBCP is investigated as a function of field angle and temperature in order to determine its physical origin. 
Wednesday, March 6, 2019 8:48AM  9:00AM 
K05.00003: Fragile zeroenergy vortex bound state in the topological superconductor candidate FeTe_{0.6}Se_{0.4} Tadashi Machida, Yue Sun, Sunseng Pyon, Shun Takeda, Yuhki Kohsaka, Tetsuo Hanaguri, Takao Sasagawa, Tsuyoshi Tamegai Majorana fermions are predicted to emerge in vortex cores of topological superconductors. Although the presence of a zeroenergy bound state (ZEBS) is a hallmark of the Majorana fermions, it is still controversial due to the limited energy resolution and statistics [14]. Using a dilutionrefrigerator scanning tunneling microscope [5], we performed highenergyresolution tunneling spectroscopy on quantumlimit vortex cores in the topological superconductor candidate FeTe_{0.6}Se_{0.4}. We have found that some vortices host ZEBS and others do not. The proportion of the vortices with the ZEBS decreases with the field strength, from > 80 % at 1 T to < 10 % at 6 T. This apparent fragility of the ZEBS sheds light on the relation between the ZEBS and Majorana fermions. 
Wednesday, March 6, 2019 9:00AM  9:12AM 
K05.00004: Topological Vortex Phase Transitions in IronBased Superconductors Shengshan Qin, Lunhui Hu, Xianxin Wu, Xia Dai, Congcong Le, Chen Fang, FuChun Zhang, Jiangping Hu We study topological vortex phases in ironbased superconductors. Besides previously known Majorana zero mode(MZM) phase 
Wednesday, March 6, 2019 9:12AM  9:24AM 
K05.00005: Mixed phase of iron based Dirac superconductors Elio Koenig, Piers Coleman Recently, bulk 3D Dirac semimetallic touching points were observed via ARPES in Li(Fe_{1x}Co_{x})As.[1] Separately, zerobias vortexcore Majorana subgap states were reported using STM on the surface of the topological iron based superconductor FeTe_{0.55}Se_{0.45}.[2] Crucially, close to the topologically relevant ΓZ line, this second material has a very similar band structure as Li(Fe_{1x}Co_{x})As. The Majorana modes are attributed to emergent topological 2D Dirac surface states with proximity induced superconductivity, i.e. via a FuKane like mechanism which is independent from the 3D Dirac bulk states. Yet, we show that, in certain parameter regimes, the 3D Dirac semimetallic touching points also provide Majorana subgap states which disperse along the vortex tube. Here, we investigate experimental signatures of the latter and thereby demonstrate a route to unveil the rich topological structure of iron based superconductors. 
Wednesday, March 6, 2019 9:24AM  9:36AM 
K05.00006: Quantum anomalous vortex and Majorana zero mode in ironbased superconductor Fe(Te,Se) Ziqiang Wang, Kun Jiang, Xi Dai In conventional spinsinglet swave superconductors, a timereversal symmetry breaking magnetic impurity creates a vortexfree defect hosting the YuShibaRusinov (YSR) states inside the superconducting (SC) gap. We show that this folklore changes in swave superconductors with strong spinorbit coupling (SOC). In this case, topological defect excitations can nucleate through a quantized phase winding of the SC order parameter around the magnetic ion without applying an external magnetic field. The role of the magnetic field is played by the exchange field and SOC as in the anomalous Hall effect. Such vortices, dubbed quantum anomalous vortices (QAVs), support robust Majorana zeroenergy modes (MZMs) when superconductivity is induced in the topological surface states. We demonstrate that the zeroenergy bound states observed in Fe(Te,Se) superconductors are possible realizations of the MZMs localized at the QAVs produced by the interstitial magnetic Fe. The quantum anomalous vortex matter can provide an advantageous platform for manipulating MZMs in quantum computing. 
Wednesday, March 6, 2019 9:36AM  9:48AM 
K05.00007: Electronic Structure in the Antiferromagnetic State of Possible Topological Nidoped TlCo_{2}Se_{2} Yifei Fang, Rui Peng, Hai chao Xu, Donglai Feng In this work, we present angleresolved photoemission spectroscopy study for 122* type TlCo_{2x}Ni_{x}Se_{2} (x=0, 0.6, 1.2, 1.6, 1.9, 2.0) samples. The parent TlCo_{2}Se_{2} (x=0) sample has an incommensurate spiral spin structre. With the increase of Ni doping content, the Néel temperature increase firstly and then decrease above x_{c}, in conjunction with the variation of lattice parameters. The AFM ordering disappears at x~1.7 eventually after which superconductivity occurs with maximum T_{C}~4 K for x=2. The superconducting temperature becomes higher with the doping concentration increases. ARPES results show a Dirac cone band along ZM momentum path. It shifts down rigidly with doping which should be owing to the change of chemical potential. The photoemission circular dichroism suggests the chirality of the orbital angular momentum. Our results thus strongly suggest their possible topological origin. We also shown that the change of effective bandwidth, which is closely related to the iternerant and electron correlation. The study of Fermi surfaces evolution can clarify the carrierdopinginduced phase transition in this system. Our results faciliate to figure out the electronic structure for deep understanding on the complex phase diagram in the system. 
Wednesday, March 6, 2019 9:48AM  10:00AM 
K05.00008: Prediction of CoSb as a new layered superconductor Wenjun Ding, M. Usman Muzaffar, Wei Qin, Ping Cui, Zhenyu Zhang FeSebased superconductors have been actively studied in the field of high temperature superconductivity due to their relatively simple structures and exotic superconducting phenomena. Bulk FeSe possesses a superconducting transition temperature (T_{c}) of ~8 K, while substantially enhanced T_{c} of ~65 K has been reported in oneunitcell FeSe films on the SrTiO_{3}(001) substrate. To date, the underlying superconducting and enhancement mechanisms are still under active debate. Here we employ firstprinciples calculations to predict a new layered superconductor of CoSb, which is isovalent to, and shares the same planar crystal structure with the singlelayer FeSe. The electronic structures of the two systems are also very similar. In contrast, the magnetic properties of CoSb are distinctly different from that of FeSe, thereby offering new opportunities for exploiting highT_{c} superconductivity and the underlying pairing mechanisms. 
Wednesday, March 6, 2019 10:00AM  10:12AM 
K05.00009: Observation of concurrent mass enhancement and superconductivity in a topological state Nader Zaki, Jonathan David Rameau, Genda Gu, Peter Johnson Low energy, laserbased ARPES is used to examine the temperature dependence of a topological state previously reported^{1,2} for the Febased superconductors, FeSe_{x}Te_{1x}. In the present study of the same system at temperatures below the superconducting transition we observe the concurrent onset of mass enhancement at the Dirac point, suggestive of time reversal symmetry breaking, and the appearance of the superconducting gap at the Fermi level crossing. We discuss possible explanations for the coincident presence of these two gaps below T_{c}. At the same time, focusing on the Dirac state provides some evidence for increased robustness of superconductivity in the surface layer. 
Wednesday, March 6, 2019 10:12AM  10:24AM 
K05.00010: Topological Excitations in Threeband Anisotropic Superconductors Andrea Benfenati, Egor Babaev In the recent experimental paper [1] it was reported a disordereddriven transition from the s_{±} to s_{++} states in Ba(Fe_{1x} Rh_{x})_{2} As_{2}. 
Wednesday, March 6, 2019 10:24AM  10:36AM 
K05.00011: Nonlocal electrostatic gating in Majorana nanowires Jouri Bommer, Hao Zhang, Michiel De Moor, Di Xu, Sasa Gazibegovic, Roy L. M. Op het Veld, Erik P. A. M. Bakkers, Mihir Pendharkar, Joon Sue Lee, Chris Palmstrom, Leo P Kouwenhoven Ever since the predicted existence of Majorana ZeroModes (MZMs), the building blocks of topological quantum computation in hybrid semiconductorsuperconductor wires, experimental efforts to detect signatures of MZMs have mainly focused on tunneling experiments, culminating in the recent observation of quantized Majorana conductance. However, the local probes used in previous studies cannot unambiguously reveal the nonlocal separation of MZMs, the hallmark of topological superconductivity. Here we study nonlocal properties of MZMs in tunneling spectroscopy on InSb nanowires covered with epitaxial aluminium by using two separate gates underneath the superconductor, one to tune part of the wire into the topological phase and an additional remote gate to tune the potential at the far end of the topological section. By tuning the remote gate we observe zerobias peak splitting in the local tunneling measurement, presumably due to variation in the overlap between MZMs at the two ends. In contrast, trivial Andreev Bounds States, identified by instability with respect to the local gates, are completely insensitive to the remote gate. 
Wednesday, March 6, 2019 10:36AM  10:48AM 
K05.00012: Spectroscopic evidence of chiral Majorana modes in a quantum anomalous Hall insulator/superconductor heterostructure Jian Lyu Topological superconductors are considered the key to realize topological quantum computation. With the discovery of quantum anomalous Hall insulator (QAHI), it has been proposed that heterostructure of QAHI with conventional superconductor can be an ideal platform for obtaining Majorana fermion. By introducing superconductivity into dissipationless edge state of QAHI, two chiral Majorana edge modes with different Chern number N of 1 and 2, respectively, can be generated. Recently, He et al discovered the signature of integer and halfinteger quantized conductance plateaus in QAHI/SC heterostructure. However, few arguments have indicated that there may be other mechanism to explain the phenomenon. Here we show spectroscopic evidence to support the existance of superconducting QAHI state at the edge of QAHI/SC heterostructure by using nanopoint contact technique. Two different topological nontrivial phases are observed when the QAHI is gradually driven through its magnetization reversal. The N = ±1 phase with a conductance near 2e^{2}/h occurs in a narrow field regime just before the QAHI enters a trivial insulating state. Our results are consistent with theoretical predictions and will help to confirm the previous result of halfinteger quantization due to Majorana fermion. 
Wednesday, March 6, 2019 10:48AM  11:00AM 
K05.00013: Topological vortex chain realized in typeII superconductors Wei Qin, Zhenyu Zhang In typeII superconductors, the Carolide GennesMatricon (CdGM) states were predicted as the lowenergy quasiparticle excitations around a single Abrikosov vortex. For a periodic vertex lattice, the quantum tunneling effect between intersite CdGM states leads to the formation of Blochlike bands inside the superconducting gap. In contrast to normal solids, such type of Bloch band describes the behaviors of Bogoliubov quasiparticles with the particlehole symmetry naturally preserved. Here, we carry out theoretical studies on the potential topological properties harbored by these exotic Bloch bands. First, we investigate the quasiparticle excitations around an isolated magnetic vortex in the presence of Rashba spinorbit coupling (SOC), which is demonstrated to lift the doubly degenerate CdGM states. Next, we construct a onedimensional (1D) vortex chain and explore the SOCinduced band gap opening around the Fermi level. Based on symmetry analysis, the opened band gap possesses odd parity, indicating that the vortex chain essentially mimic a Kitaev chain, which is characterized by Majorana end modes. These findings can be naturally extended to 2D vortex lattice, thus providing generic appealing platforms for realizing and manipulating Majorana fermions. 
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