Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S14: Topological Materials - TransportFocus Session
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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 Goldhaber-Gordon The quantum anomalous Hall effect in thin film magnetic topological insulators (MTIs) is characterized by chiral, one-dimensional conduction along the film edges when the sample is uniformly magnetized. This has been experimentally confirmed by measurements of quantized Hall resistance and near-vanishing longitudinal resistivity in magnetically doped (Bi,Sb)2Te3. 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 co-propagating 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 Goldhaber-Gordon 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)2Te3[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, Qi-Kun 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 multi-terminal 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 Goldhaber-Gordon, 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)2Te3 thin films, we investigate the effects of temperature, current, and chemical potential to provide new clues about non-ideal 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 variable-range hopping conduction, and the nature of the current-induced breakdown [1,3] of the QAHE. |
Thursday, March 8, 2018 12:27PM - 12:39PM |
S14.00005: Disorder-induced decoupled surface transprot channels in thin films of doped topological insulator Hanbum Park, kwangsik Jeong, Mann-Ho Cho Non-ideal topological insulator film that the bulk states are not insulating due to unintentional doping reveals strong surface-bulk coupling. Furthermore, the surface-bulk coupling can induce the inter-surface coupling which affects the quantum interference effect on electrical conductivity known as weak anti-localization. Therefore, interpretation and control of the inter-surface coupling are important to utilize the TI-based quantum devices. In this report, from the transport studies of doped Bi2Se3 films under the perpendicular and parallel magnetic field, we observe the crossover between coupled and decoupled surface channels through intentional disorder controlled by a post-annealing process. The intentional disorder makes that the carriers of surface state rapidly lose their quantum phase and coherence. Consequently, the more disordered Bi2Se3 film reveals shorter penetration depth of the surface state into the bulk states and weaker inter-surface coupling even though it is expected stronger surface-bulk coupling. |
Thursday, March 8, 2018 12:39PM - 12:51PM |
S14.00006: Quasi-ballistic transport of spin-helical 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 quasi-ballistic 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 quasi-1D surface modes, rather than from the topological nature of a single perfectly-transmitted mode. The weak coupling of spin-helical modes can be revealed by the non-universal behavior of conductance fluctuations [3], and the spin and energy-dependence 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 nodal-line 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 nodal-line semimetal represents one type of topological quantum state which displays Dirac cones along a one-dimensional line, in contrast with the topological nodal point Dirac or Weyl semimetals. Here we report the quantum oscillation studies on the nodal-line semimetals ZrHM (H=Si, Ge, Sn; M=S, Se, Te). We have revealed the signatures of nodal-line states in these materials and studied their evolution with dimensionality and spin-orbit coupling, which varies with the selections of H and M. These properties, plus the availability of atomic thin two-dimensional 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: Angle-dependent Magnetotransport in Multilayer Magnetically-doped Topological Insulators near the Zero-Hall-Plateau Quantum Anomalous Hall State Michelle Tomczyk, Di Xiao, Cui-Zu Chang, Anthony Richardella, Chao-Xing Liu, Nitin Samarth Topological insulators (TIs) are the focus of a rapidly-growing 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, magnetically-doped TIs have exhibited the theoretically-predicted 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 non-trivial 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 magnetically-doped TI layers. Transport measurements are performed near the zero-Hall-plateau QAH insulator regime- a candidate regime for axionic behavior. Angle-resolved magnetic field dependence provides insight into the behavior of the magnetic domains and edge states, and the transition to the zero-Hall-plateau regime. |
Thursday, March 8, 2018 1:15PM - 1:27PM |
S14.00009: Abstract Withdrawn
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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 spin-momentum-locked edge states, which are protected by time-reversal symmetry. However, most known topological insulator materials naturally contain spinful nuclei, and their hyperfine coupling to helical edge states intrinsically breaks time-reversal symmetry, removing the topological protection and enabling the buildup of dynamic nuclear spin polarization through hyperfine-assisted 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 PdSn4: a comprehensive study. Na Hyun Jo, Yun Wu, Lin-Lin 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 PdSn4. We observe that PdSn4 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 PtSn4. By comparing similar compounds, PdSn4 and PtSn4, 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 Magnetic-Field-Free 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 ρs without subjecting the sample to magnetic field or putting leads [1]. The method is based on the London equation J=-ρsA, 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 ρs. 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 La1.875Sr0.125CuO4 rings: one with the current running only in the CuO2 planes, and another where the current must cross between planes. We find different Tc 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 time-reversal symmetry. However, several materials considered as typical candidates for topological insulators (such as Bi2Se3 and Sb2Te3) contain spinful nuclei or other types of magnetic impurities that break the time-reversal 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 Aharonov-Bohm effect to link the tunable parameters that control the band structure to the magnetic flux threading the ring. We further analyze the electron-impurity entanglement entropy of the system, which reveals maximal entanglement at the band edges. |
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