Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session S45: Topological Insulators: Magnetism and Magnetic ResponseLive
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Sponsoring Units: DCMP Chair: Wang Kong Tse, Univ of Alabama - Tuscaloosa |
Thursday, March 18, 2021 11:30AM - 11:42AM Live |
S45.00001: Cooperative orbital moments and edge magnetoresistance in monolayer WTe2 Arpit Arora, Likun Shi, Justin Song Quantum Spin Hall (QSH) edge states exhibit dissipationless transport that is insensitive to the crystal structure and disorder of the sample device as long as time-reversal symmetry is preserved. Here we argue that when a magnetic field is applied, the edge states of QSH insulators become highly sensitive to bulk crystalline symmetries. In particular, we find that the QSH insulator WTe2 can display an anisotropic edge magnetoresistance that depend, and can be controlled by the Rashba and Ising spin orbit couplings introduced due to low symmetry of the crystal structure. Strikingly, maximum edge magnetoresistance occurs when the magnetic field is applied at a canted angle θ oblique to the plane, which is in stark contrast to that found in HgCdTe QSH insulators (where magnetoresistance is maximum for out-of-plane magnetic fields) or ideal QSH insulators (where maximum magnetoresistance is expected to arise for in-plane magnetic fields). We find this behavior proceeds from an unconventional orbital contribution to the edge magnetic moment that arises from “cooperative orbital moments” due to the unique low-symmetry crystal structure of WTe2. |
Thursday, March 18, 2021 11:42AM - 11:54AM Live |
S45.00002: Magnetoresistance Measurements of Polycrystalline and Amorphous Bi2Te3 Thin Films Alexander Nguyen, Golrokh Akhgar, David Cortie, Michael S Fuhrer, Mark T Edmonds, Julie Karel Crystalline topological insulators have been on the forefront of condensed matter research for the past decade. However, disordered topological insulators have not been examined in much detail since current models of topological insulators rely heavily on the materials’ electronic band structure. Without the translational symmetries necessary to create a well-defined reciprocal space disordered materials have long been presumed to not exhibit topological properties. However, very recent theoretical models have predicted the existence of topologically protected states in disordered materials. This talk will discuss temperature dependent magnetoresistance, Hall effect, and temperature dependent resistivity measurements of polycrystalline and amorphous Bi2Te3 thin films. |
Thursday, March 18, 2021 11:54AM - 12:06PM Live |
S45.00003: Fabry-Pérot resonant vortexes and magneto conductance in topological insulator constrictions with magnetic barriers Renan Maciel, Augusto de Lelis Araujo, Caio H Lewenkopf, Gerson J. Ferreira The physics of topological systems has received great attention since its electronic transport properties show prominent applications in many fields (e.g.spintronics). The key ingredient is the band inversion, usually due to strong spin-orbit couplings, and its main feature is the robustness of the edge states against non-magnetic impurities. An interesting approach to investigate these properties is via a geometric manipulation of the system. Here, we study the transport in one-dimensional (1D, ribbon) constrictions of two-dimensional (2D) systems, which leads to interesting phenomena, such as quantum interference between edge states. In particular, in our analysis, this process drives the constriction region of the system to show Fabry-Pérot resonance peaks in the conductance. Such effect rises due to the velocity mismatch between the edge states in the constriction and the perfect leads. We show that magnetic barriers inside that constriction allow us to manipulate these peaks and obtain significant changes in the system spin-resolved magnetoconductance. |
Thursday, March 18, 2021 12:06PM - 12:18PM Live |
S45.00004: Quantum Anomalous Hall Effect and Exchange Interaction between Magnetic Topological Insulator and Antiferromagnetic Insulator Peng Zhang, Lei Pan, Alexander Grutter, Tomohiro Nozaki, Scott Chambers, Jing Xia, Kang-Lung Wang Integration of a quantum anomalous Hall (QAH) insulator with a magnetically ordered material provides an additional degree of freedom to manipulate the magnetic orders and exotic quantum states. Here, we report molecular beam epitaxy (MBE) growth of high quality magnetically doped topological insulator (MTI) and the observation of QAH effect on an antiferromagnetic insulator Cr2O3. The exchange coupling between the two materials is investigated using field-cooling-dependent magnetometry and polarized neutron reflectometry. Exchange bias is observed when the sample is field-cooled under an out-of-plane magnetic field, and polarized neutron reflectometry reveals an exchange spring-like magnetic depth profile when the system is magnetized within the film plane. Both techniques show strong interfacial interaction between the antiferromagnetic order of the Cr2O3 and the magnetic topological insulator. The exchange interaction between MTI and adjacent antiferromagnetic insulators enables the effective manipulation of the magnetic and topological order in the topological insulator films. |
Thursday, March 18, 2021 12:18PM - 12:30PM Live |
S45.00005: Unconventional isotropic in-plane Hall and MR response in Bi2Se3/EuS bilayer system Satyaki Sasmal, Dhavala Suri, Karthik Raman Proximity effects on topological insulator/magnetic insulator (TI/MI) bilayers intrigue the scientific community due to exotic ground states that emerge by locally lifting the ground state degeneracy [1]. In this talk, we present in-plane magnetic field study on a Bi2Se3 (TI)/EuS (MI) bilayer device where magnetic exchange coupling is one-sided and acting only on the top surface state [2,3]. Our magneto-transport study reveals longitudinal magnetoresistance (LMR) and planar Hall resistance (PHR) which exhibit a sharp switch in resistance at magnetic field < coercive field and symmetric about B=0 suggesting it's nontrivial topological origin. This effect is isotropic w.r.t azimuthal angle rotation of in-plane field and the effect persists in temperature regime T < 4K. A plausible mechanism that includes symmetry breaking by proximity exchange interaction on the surface states will be presented. |
Thursday, March 18, 2021 12:30PM - 12:42PM Live |
S45.00006: Hybridization and Electron-Hole Symmetry Breaking Effects on the Magento-Optical Conductivity of Sb2Te3 Thin-Films Gaurav Gupta, Mahmoud Asmar, Wang Kong Tse The surface states residing at opposite surfaces of topological insulators (TIs) such as Bi2Se3 and Sb2Te3 hybridize if the TI thickness becomes comparable to their decay lengths, leading to a spectral gap. The TI Sb2Te3 has not received much attention as it is assumed that its properties are similar to Bi2Se3, as they belong to the same topological class. Although the same k.p Hamiltonian can describe these two TIs, their k.p parameters lead to crucial differences. These differences are prominent in their thin-films since finite-size effects and electron-hole (e-h) asymmetry play a more prominent role in the hybridization gap of Sb2Te3 due to longer decay lengths in this material. To discern the crucial difference between these TIs, Bi2Se3, and Sb2Te3, we study their thin-films' magneto-optical response through a self-consistent consideration of their bulk Hamiltonians, boundary conditions, and their magneto-optical conductivity. We discuss the consequences of e-h asymmetry, bulk contributions, and the Zeeman coupling on the real and imaginary parts of the optical conductivity of Sb2Te3 thin-films and contrast them to their Bi2Se3 counterpart. |
Thursday, March 18, 2021 12:42PM - 12:54PM Live |
S45.00007: Anomalous Hall Effect induced by extremely low field in ZrTe5 Joshua Mutch, Paul Malinowski, Qianni Jiang, Chong Wang, Di Xiao, Jiun-Haw Chu ZrTe5 has gathered interest in recent years due to its non-trivial topology. In monolayer form, it is predicted to be a quantum spin hall insulator. In bulk, it is predicted to reside extremely close to a phase transition between a strong and weak topological insulator. We report detailed measurements of the anomalous hall effect (AHE) in ZrTe5. We find the hall resistance saturates at an extremely low magnetic field ( B << 1T ), and remains at the saturation value for fields up to 32T. This AHE is present despite no evidence of magnetism in ZrTe5. Furthermore, in exfoliated devices in a quasi-bulk regime (dozens of layers thick) we find that the low-field orbital hall effect is suppressed and the AHE is clearly seen. We investigate the AHE effect in ZrTe5 as a function of temperature, magnetic field angle, and doping, and discuss the potential origins of this effect related to a non-vanishing berry curvature. |
Thursday, March 18, 2021 12:54PM - 1:06PM Live |
S45.00008: Low Temperature Electronic Measurements in Novel Topological Insulator-Antiferromagnetic Insulator Thin Film Heterostructures. Ryan Van Haren, David Lederman Topological insulators (TIs) are of great interest for their unique combination of insulating bulk and metallic edge states. In 3D TIs, these edge states are spin polarized 2D conducting surface states protected from backscattering by time reversal symmetry. These states have great applicational potential in spintronic and quantum computing devices. The Dirac cone that forms at the surface of a 3D TI is robust to non-magnetic perturbations, but a gap can be opened through proximity to an ordered magnetic material. The insulating antiferromagnet nickel fluoride NiF2 makes an interesting candidate for this proximity effect because of its weak ferromagnetic moment resulting from a spontaneous canting, in addition to terahertz frequency magnon fluctuations arising from the antiferromagnetic ordering. Our work shows how a NiF2-3D TI interface can be epitaxially grown via molecular beam epitaxy and presents low temperature charge carrier measurements performed in these heterostructures. These experiments demonstrate these structures can be fabricated into thin film devices and presents a path forward for further study and manipulation of these topologically protected surface states. |
Thursday, March 18, 2021 1:06PM - 1:18PM Live |
S45.00009: Nanosecond dynamics in intrinsic topological insulator Bi2-xSbxSe3 revealed by time-resolved optical reflectivity Adam Gross, Yasen Hou, Antonio Rossi, Dong Yu, Inna Vishik Bi2Se3 is an ideal three-dimensional topological insulator in which the chemical potential can be brought into the bulk band gap with antimony doping. Here, we utilize ultrafast time-resolved transient reflectivity to characterize the photoexcited carrier decay in Bi2-xSbxSe3 nanoplatelets. We report a substantial slowing of the bulk carrier decay rate in bulk-insulating Bi2-xSbxSe3 as compared to n-type bulk-metallic Bi2Se3 at low temperatures, which approaches 0.30 ns-1 in the zero pump fluence limit. This long-lived decay is correlated across different fluences and antimony concentrations, revealing unique decay dynamics not present in n-type Bi2Se3, namely the slow bimolecular recombination of bulk carriers. |
Thursday, March 18, 2021 1:18PM - 1:30PM Live |
S45.00010: Evidence for a conducting surface state in FeSi and comparison with SmB6 Yuhang Deng, Alexander Breindel, Camilla Moir, Yuankan Fang, Hongbo Lou, Shubin Li, Qiaoshi Zeng, Lei Shu, Christian T Wolowiec, Ivan Schuller, Priscila Rosa, Zachary Fisk, John Singleton, M Brian Maple The recent discovery of a conducting surface state in FeSi suggests the possibility of topological behavior [1]. We further explore this state using magnetoresistance M(H,T) measurements in high magnetic fields H to 60 T, electrical resistivity ρ(T) measurements at high pressures P to 7.6 GPa, and magnetic field modulated microwave spectroscopy (MFMMS). The two energy gaps Δ1 and Δ2 of FeSi from the T-dependence of ρ increased with P up to 7 GPa, followed by a drop coinciding with a sharp suppression on the conducting surface transition temperature. The closing of Δ1 and Δ2 with P was also seen in the suspected topological Kondo insulator SmB6 [2]. The behavior of M(H,T) shows additional evidence for the conducting surface state by comparing the behavior perpendicular and parallel to H. Comparisons of M(H,T) of FeSi and SmB6 show similar behavior. However, the MFMMS results suggests differences in the underlying physics of the surface state in these two compounds. |
Thursday, March 18, 2021 1:30PM - 1:42PM Live |
S45.00011: Controlled incorporation of Mn into Sb2Te3 using molecular beam epitaxy to grow highly ordered crystals Ido Levy, Haiming Deng, Candice Forrester, Martha R McCartney, David J Smith, Lia Krusin-Elbaum, Maria C Tamargo Intrinsic magnetic topological materials in the MB2T4-family (M= V or Mn, B= Bi or Sb, T= Te) were predicted to show intrinsic axion insulator behavior and quantum anomalous Hall (QAH) effect. Incorporation of M in the B2T3 crystal structure changes it from a quintuple layer (QL) structure to septuple layers (SL) of the form T-B-T-M-T-B-T. Even though each SL is ferromagnetic, they couple antiferromagnetically when stacked into the bulk, making it difficult to reach zero field QAH conductance. In addition, studies have shown that separating SLs with QLs turns the resulting structure into a ferromagnet (FM). Here we report the growth by molecular beam epitaxy of Mn doped Sb2Te3 where the incorporation of Mn in the lattice can be accurately controlled. High crystalline quality materials with varying effective Mn atomic % from 1% to 32%, where pure MnSb2Te4 corresponds to 14%, are achieved by adjusting the Mn to Sb flux ratios during growth, while keeping other variables constant. FM behavior with high Tc values was obtained for all the samples with more than 6% Mn, as witnessed by large anomalous Hall signal with coercive field ~0.1 T at 2 K. Relationship between the structural properties and the magnetic behavior will be discussed. |
Thursday, March 18, 2021 1:42PM - 1:54PM Live |
S45.00012: Unconventional Proximity Driven Exchange Coupling Effects in Topological Insulator/ Ferromagnetic Insulator Bilayers Dhavala Suri, Archit Bhardwaj, Andrew K Saydjari, Jagadeesh S Moodera, Karthik Raman Experimental observations of emergent electronic states as a result of breaking time reversal symmetry in topological insulators (TIs)/ferromagnetic insulator (FI) bilayers are crucial to understand proximity driven effects in these systems. Often, the interface magnetic behavior is more robust than that of the adjoining FI as demonstrated in Bi2Se3/EuS [Nature 533, 516 (2016)]. We report unusual magneto-transport behavior in Bi2Te3/EuS bilayer; the exchange coupled system displays anisotropic effects through planar Hall effect (PHE) and anisotropic magnetoresistance (AMR) observations. The behavior of the charge carriers at respective magnetic field and temperature regimes reflect the above two effects, each dominating at different regimes. At magnetic field > 1 kOe we find that the PHE dominates and persists up to 200 K, while the AMR effect is only below 4 K and magnetic field < 250 Oe. The representative hysteresis loops are unconventional with symmetric hysteresis loops on either side of B=0 and exhibit topological features; which might be an interplay of spin-orbit coupling and exchange coupling effects; canted moments at the interface with respect to tilt of the Dirac cone. |
Thursday, March 18, 2021 1:54PM - 2:06PM Live |
S45.00013: Mechanical dissipation via image potential states on Bi2Te3 surface Dilek Yildiz, Marcin Kisiel, Urs Gysin, Oguzhan Gurlu, Ernst Meyer We studied the frictional response of the Bi2Te3 (0001) crystal by means of combined pendulum atomic force microscopy (AFM) and scanning tunneling microscopy (STM). While Joule dissipation is suppressed due to a topologically protected surface state, another dissipation mechanism due to single electron tunneling into image potential states (IPS), which are slightly above the Bi2Te3 surface is observed. Dissipation peaks are localized at relatively large voltages as expected for IPS and observed to be shifted to even higher voltages as tip-sample distance increases. We observed that the application of a magnetic field leads to the breakdown of the topological protection of the surface states and restores the expected Joule dissipation process. [1] – D. Yildiz, et al., Nat. Mater. 18, 1201-1206, (2019). |
Thursday, March 18, 2021 2:06PM - 2:18PM On Demand |
S45.00014: Large Magnetoresistance and Fermi Surface Topology of SnxPb1-xTe Single Crystals Keshav Shrestha, Duncan Miertschin, Raman Sankar, Bernd Lorenz, Ching W Chu We have investigated the magnetotransport properties of a topological crystalline insulator, SnxPb1-xTe. Resistance shows a large positive magnetoresistance that reaches a value up to 310% at H = 13 T without showing any sign of saturation. At higher fields, both longitudinal and Hall resistance show clear Shubnikov de Haas (SdH) oscillations. There exists a single oscillation frequency, f = 59 T, however, additional frequencies show up at higher tilt angles of applied field with the normal to the sample surface. We have explored the Fermi surface topology by carrying out a detailed angle dependent measurement of quantum oscillations and Berry phase calculations. |
Thursday, March 18, 2021 2:18PM - 2:30PM On Demand |
S45.00015: Large Magnetoresistance and Fermi Surface Studies of Sb2Te2-xSex Single Crystals Thinh Nguyen, Narayan Poudel, Duncan Miertschin, Mitch Chou, Hung-Duen Yang, Krzysztof Gofryk, Keshav Shrestha
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