Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S14: Topological Materials  TransportFocus

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Sponsoring Units: DMP Chair: Lian Li, West Virginia Univ Room: LACC 304B 
Thursday, March 8, 2018 11:15AM  11:51AM 
S14.00001: Chiral 1D transport in magnetic topological insulators: precise quantization and manipulation Invited Speaker: David GoldhaberGordon , Eli Fox , Ilan Rosen , Yanfei Yang , George Jones , Randolf Elmquist , Xufeng Kou , Lei Pan , Kang Wang The quantum anomalous Hall effect in thin film magnetic topological insulators (MTIs) is characterized by chiral, onedimensional conduction along the film edges when the sample is uniformly magnetized. This has been experimentally confirmed by measurements of quantized Hall resistance and nearvanishing longitudinal resistivity in magnetically doped (Bi,Sb)_{2}Te_{3}. I will describe two recent advances: 1. We have measured quantized Hall resistance in absence of an external magnetic field to precision and accuracy better than one part per million, and longitudinal resistivity below 10 mΩ, using techniques developed for quantum Hall metrology. We have also achieved some insight into the nature of residual dissipation. 2. Chiral conduction is expected not only along film edges but also along magnetic domain walls. Clear detection of these modes in MTIs has until recently proved challenging. We have intentionally created magnetic domain walls in an MTI, and study electrical transport along those domain walls. In agreement with theoretical predictions, we observe chiral transport along domain walls. I will also describe evidence that two modes equilibrate while copropagating along the length of the domain wall. 
Thursday, March 8, 2018 11:51AM  12:03PM 
S14.00002: Observation of Chiral Transport along Magnetic Domain Walls in a Quantum Anomalous Hall Insulator Ilan Rosen , Eli Fox , Lei Pan , Xufeng Kou , Kang Wang , David GoldhaberGordon The quantum anomalous Hall (QAH) effect, which has been realized in thin films of ferromagnetic topological insulators, features a single chiral edge mode that circles the boundary of the film, similarly to the ν=1 quantum Hall (QH) system. Unlike QH, the chirality of the QAH edge mode is determined by the film's magnetization, not by the external magnetic field. Magnetic domain walls in QAH insulators therefore form adjacent QAH systems of opposite chirality; dissipationless chiral conduction is expected along such magnetic domain walls[1,2]. Using Meissner screening to locally modulate the applied magnetic field, we intentionally form a magnetic domain wall in Cr(Bi,Sb)_{2}Te_{3}[3]. We then use transport measurements to verify that conduction along magnetic domain walls is chiral and nearly dissipationless. 
Thursday, March 8, 2018 12:03PM  12:15PM 
S14.00003: Realization of helical edge transport in a magnetic topological insulator bilayer GAOYUAN JIANG , Yang FENG , WEIXIONG WU , Shaorui Li , Ke He , XUCUN MA , QiKun Xue , Yayu Wang Various quantum Hall systems are ideal platforms for investigating exotic transport phenomena. Here we construct a bilayer structure consisting of two magnetic topological insulator thin films with different coercivity. When the edge states of the two layers are tuned by varying the magnetic field, a synthetic quantum spin Hall phase with helical edge states can be realized. The robustness of this phase can be confirmed by multiterminal transport measurement. 
Thursday, March 8, 2018 12:15PM  12:27PM 
S14.00004: Probing the Nature of Residual Dissipation in the Quantum Anomalous Hall Effect Eli Fox , Ilan Rosen , David GoldhaberGordon , Lei Pan , Xufeng Kou , Kang Wang Recently, quantization of the Hall resistance in the quantum anomalous Hall effect (QAHE) in magnetic topological insulators has been demonstrated with record precision [1,2] at dilution refrigerator temperatures and currents under 100 nA. To further improve quantization for using the QAHE in resistance metrology and other technological applications where performance at higher currents and temperatures is required, it will be important to understand the sources of dissipation in this system. Using transport measurements of Cr(Bi,Sb)_{2}Te_{3} thin films, we investigate the effects of temperature, current, and chemical potential to provide new clues about nonideal behavior in the QAHE. In particular, we discuss whether dissipative transport occurs at the sample edges or in the 2D bulk, the interplay of thermally activated and variablerange hopping conduction, and the nature of the currentinduced breakdown [1,3] of the QAHE. 
Thursday, March 8, 2018 12:27PM  12:39PM 
S14.00005: Disorderinduced decoupled surface transprot channels in thin films of doped topological insulator Hanbum Park , kwangsik Jeong , MannHo Cho Nonideal topological insulator film that the bulk states are not insulating due to unintentional doping reveals strong surfacebulk coupling. Furthermore, the surfacebulk coupling can induce the intersurface coupling which affects the quantum interference effect on electrical conductivity known as weak antilocalization. Therefore, interpretation and control of the intersurface coupling are important to utilize the TIbased quantum devices. In this report, from the transport studies of doped Bi_{2}Se_{3} films under the perpendicular and parallel magnetic field, we observe the crossover between coupled and decoupled surface channels through intentional disorder controlled by a postannealing process. The intentional disorder makes that the carriers of surface state rapidly lose their quantum phase and coherence. Consequently, the more disordered Bi_{2}Se_{3} film reveals shorter penetration depth of the surface state into the bulk states and weaker intersurface coupling even though it is expected stronger surfacebulk coupling. 
Thursday, March 8, 2018 12:39PM  12:51PM 
S14.00006: Quasiballistic transport of spinhelical Dirac fermions in 3D topological insulator quantum wires Romain Giraud , Joseph Dufouleur , Louis Veyrat , Emmanouil Xypakis , Jens Bardarson , Silke Hampel , Bernd Buechner Despite strong disorder, the transport of surface Dirac fermions remains quasiballistic in narrow nanowires of a 3D topological insulator, as evidenced in Bi2Se3 or Bi2Te3 quantum wires [1,2]. We demonstrate that such a unique behavior for a mesoscopic conductor results from the spin helicity of all quasi1D surface modes, rather than from the topological nature of a single perfectlytransmitted mode. The weak coupling of spinhelical modes can be revealed by the nonuniversal behavior of conductance fluctuations [3], and the spin and energydependence of transmissions is well captured by both analytical and numerical models. Under appropriate conditions, such 3DTI quantum wires could be used not only for ballistic spin transport but also as spin filters. 
Thursday, March 8, 2018 12:51PM  1:03PM 
S14.00007: Quantum oscillation studies on topological nodalline semimetal ZrHM (H = Si, Ge, Sn; M = S, Se, Te) Jin Hu , Yanglin Zhu , David Graf , Zhiqiang Mao The breakthrough in the discovery of topological semimetals provides opportunities to explore the exotic properties of relativistic fermions in condensed matter. Among those materials, the Dirac nodalline semimetal represents one type of topological quantum state which displays Dirac cones along a onedimensional line, in contrast with the topological nodal point Dirac or Weyl semimetals. Here we report the quantum oscillation studies on the nodalline semimetals ZrHM (H=Si, Ge, Sn; M=S, Se, Te). We have revealed the signatures of nodalline states in these materials and studied their evolution with dimensionality and spinorbit coupling, which varies with the selections of H and M. These properties, plus the availability of atomic thin twodimensional flakes and varied strength of spin splitting, make ZrHM a tunable platform for investigating topological fermion physics and for further exploration of electronic and spintronic science and applications. 
Thursday, March 8, 2018 1:03PM  1:15PM 
S14.00008: Angledependent Magnetotransport in Multilayer Magneticallydoped Topological Insulators near the ZeroHallPlateau Quantum Anomalous Hall State Michelle Tomczyk , Di Xiao , CuiZu Chang , Anthony Richardella , ChaoXing Liu , Nitin Samarth Topological insulators (TIs) are the focus of a rapidlygrowing body of research due to the many interesting properties of their metallic surface states. The interaction of the surface states with other phenomenon such as magnetism or superconductivity results in novel and exotic behavior. In particular, magneticallydoped TIs have exhibited the theoreticallypredicted quantum anomalous Hall (QAH) effect, in which the surface states are gapped while leaving quantized edge states even in no external magnetic field. A theoretical extension of the QAHE is the axion insulator, in which all surface states become gapped in a nontrivial fashion. While there are multiple reports claiming an axionic state, there is still much discussion of the experimental signatures expected. Here, we study a heterostructure consisting of an undoped TI layer sandwiched between magneticallydoped TI layers. Transport measurements are performed near the zeroHallplateau QAH insulator regime a candidate regime for axionic behavior. Angleresolved magnetic field dependence provides insight into the behavior of the magnetic domains and edge states, and the transition to the zeroHallplateau regime. 
Thursday, March 8, 2018 1:15PM  1:27PM 
S14.00009: Transport Properties of Topological Semimetal AuTe_{2}Br and AuTe_{2}Cl Zeji Wang , Shuyu Cheng , Xin Gui , Huibing Zhou , Xitong Xu , Suyang Xu , Hsin Lin , TayRong Chang , Weiwei Xie , Shuang Jia We report crystal structure and electrical transport properties of layered, ternary compounds AuTe_{2}Br and AuTe_{2}Cl. As Zintl compounds, they consist of halogen ions and Au^{3+}(TeTe)^{2} layers. Our Calculations reveal that they are Dirac semimetals with “clean” band structure near the Fermi level. As prepared by hydrothermal synthesis, their single crystals show large magnetoresistance in the order of 10^{3} at base temperature. We also observed strong Shubnikov–de Haas and de Haas–van Alphen oscillations which stem from highly two dimensional electron pockets. 
Thursday, March 8, 2018 1:27PM  1:39PM 
S14.00010: Dynamic Nuclear Polarization from Topological Insulator Helical Edge States Antonio Russo , Edwin Barnes , Sophia Economou Topological insulators are promising for spintronics and related technologies due to their spinmomentumlocked edge states, which are protected by timereversal symmetry. However, most known topological insulator materials naturally contain spinful nuclei, and their hyperfine coupling to helical edge states intrinsically breaks timereversal symmetry, removing the topological protection and enabling the buildup of dynamic nuclear spin polarization through hyperfineassisted backscattering. Here, we calculate scattering probabilities and nuclear polarization for edge channels containing up to $34$ nuclear spins using a numerically exact analysis that exploits the symmetries of the problem to drastically reduce the computational complexity. We then show the emergence of universal scaling properties that allow us to extrapolate our findings to vastly larger and experimentally relevant system sizes. We find that significant nuclear polarization can result from relatively weak helical edge currents, suggesting that it may be an important factor affecting spin transport in topological insulator devices. 
Thursday, March 8, 2018 1:39PM  1:51PM 
S14.00011: Extremely large magnetoresistance and Kohler's rule in PdSn_{4}: a comprehensive study. Na Hyun Jo , Yun Wu , LinLin Wang , Peter Orth , Savannah Downing , Soham Manni , Daixiang Mou , Duane Johnson , Adam Kaminski , Sergey Bud'ko , Paul Canfield Systematic measurements of temperature and magnetic field dependent resistivity and magnetization, and electronic band structure properties obtained from angle resolved photoemission spectroscopy (ARPES) were conducted on high quality single crystals of PdSn_{4}. We observe that PdSn_{4} has anisotropic properties with extremely large magnetoresistance (XMR). ARPES shows interesting surface features near Z and X points, and extensive electronic structure calculations agree well with ARPES. The X point feature corresponds to a gapped Dirac node arc, like observed in PtSn_{4}. By comparing similar compounds, PdSn_{4} and PtSn_{4}, we address the origin of the XMR in these compounds; neither carrier compensation nor the Dirac node arc surface state appear to be primary reason for the XMR. Additionally, perhaps remarkably, we find that Kohler's rule scaling of the magnetoresistance is obeyed over the full range of temperatures and field strengths that we explore. 
Thursday, March 8, 2018 1:51PM  2:03PM 
S14.00012: The Stiffnessometer  a MagneticFieldFree Superconducting Stiffness Meter, Reveals Two Critical Temperatures in LSCO. Itzik Kapon , Amit Keren , Zaher Salman We have developed a new method to measure superconducting stiffness ρ_{s} without subjecting the sample to magnetic field or putting leads [1]. The method is based on the London equation J=ρ_{s}A, where J is the current density and A is the vector potential. Using rotor free A and measuring J via the magnetic moment of a superconducting ring, we determine ρ_{s}. The technique is sensitive to very small stiffness, which translates to penetration depth on the order of 1 mm. Naturally, the method does not suffer from demagnetization factor complications or the presence of vortices. Therefore, the absolute value of the stiffness is obtained. We apply this method to two different La_{1.875}Sr_{0.125}CuO_{4} rings: one with the current running only in the CuO_{2} planes, and another where the current must cross between planes. We find different T_{c} for the two rings. The stiffnessometer results are compared with the Low Energy μSR (LEM) measurements on the same sample. We show that the stiffnessometer can measure stiffness where LEM, and in fact all other techniques, fail. This leads to new conclusions regarding cuprates phase transition. 
Thursday, March 8, 2018 2:03PM  2:15PM 
S14.00013: Topological insulator ring with magnetic impurity Arian Vezvaee , Antonio Russo , Sophia Economou , Edwin Barnes Topological insulator edges contain gapless states that are topologically protected by timereversal symmetry. However, several materials considered as typical candidates for topological insulators (such as Bi_{2}Se_{3 }and Sb_{2}Te_{3}) contain spinful nuclei or other types of magnetic impurities that break the timereversal symmetry. We model a topological insulator quantum ring and address the problem of spin coupling of the edge states to a magnetic impurity on the ring. We present an analytical solution for scattering states and band structure. Furthermore, we discuss the consequences of placing the ring inside a magnetic field and use the AharonovBohm effect to link the tunable parameters that control the band structure to the magnetic flux threading the ring. We further analyze the electronimpurity entanglement entropy of the system, which reveals maximal entanglement at the band edges. 
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