scholarly journals Large discrete jumps observed in the transition between Chern states in a ferromagnetic topological insulator

2016 ◽  
Vol 2 (7) ◽  
pp. e1600167 ◽  
Author(s):  
Minhao Liu ◽  
Wudi Wang ◽  
Anthony R. Richardella ◽  
Abhinav Kandala ◽  
Jian Li ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Metastable State ◽  
Large Fraction ◽  
Surface States ◽  
Anomalous Hall Effect ◽  
Escape Rate ◽  
Zero Magnetic Field ◽  
Hall Resistance ◽  
Edge States ◽  
Longitudinal Resistance

A striking prediction in topological insulators is the appearance of the quantized Hall resistance when the surface states are magnetized. The surface Dirac states become gapped everywhere on the surface, but chiral edge states remain on the edges. In an applied current, the edge states produce a quantized Hall resistance that equals the Chern numberC= ±1 (in natural units), even in zero magnetic field. This quantum anomalous Hall effect was observed by Changet al. With reversal of the magnetic field, the system is trapped in a metastable state because of magnetic anisotropy. We investigate how the system escapes the metastable state at low temperatures (10 to 200 mK). When the dissipation (measured by the longitudinal resistance) is ultralow, we find that the system escapes by making a few very rapid transitions, as detected by large jumps in the Hall and longitudinal resistances. Using the field at which the initial jump occurs to estimate the escape rate, we find that raising the temperature strongly suppresses the rate. From a detailed map of the resistance versus gate voltage and temperature, we show that dissipation strongly affects the escape rate. We compare the observations with dissipative quantum tunneling predictions. In the ultralow dissipation regime, two temperature scales (T1~ 70 mK andT2~ 145 mK) exist, between which jumps can be observed. The jumps display a spatial correlation that extends over a large fraction of the sample.

scholarly journals Intrinsic quantized anomalous Hall effect in a moiré heterostructure

Science ◽  
2019 ◽  
Vol 367 (6480) ◽  
pp. 900-903 ◽  
Author(s):  
M. Serlin ◽  
C. L. Tschirhart ◽  
H. Polshyn ◽  
Y. Zhang ◽  
J. Zhu ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Energy Gap ◽  
Hexagonal Boron Nitride ◽  
Magnetic Ordering ◽  
Anomalous Hall Effect ◽  
Zero Magnetic Field ◽  
Strong Interactions ◽  
Hall Resistance ◽  
Magnetic Memory ◽  
Electrical Currents

The quantum anomalous Hall (QAH) effect combines topology and magnetism to produce precisely quantized Hall resistance at zero magnetic field. We report the observation of a QAH effect in twisted bilayer graphene aligned to hexagonal boron nitride. The effect is driven by intrinsic strong interactions, which polarize the electrons into a single spin- and valley-resolved moiré miniband with Chern number C = 1. In contrast to magnetically doped systems, the measured transport energy gap is larger than the Curie temperature for magnetic ordering, and quantization to within 0.1% of the von Klitzing constant persists to temperatures of several kelvin at zero magnetic field. Electrical currents as small as 1 nanoampere controllably switch the magnetic order between states of opposite polarization, forming an electrically rewritable magnetic memory.

scholarly journals Quantum anomalous Hall effect from intertwined moiré bands

2021 ◽  
Author(s):  
Kin Fai Mak ◽  
Tingxin Li ◽  
Shengwei Jiang ◽  
Bowen Shen ◽  
Yang Zhang ◽  
...  
Keyword(s):  
Electric Field ◽  
Electron Correlation ◽  
Anomalous Hall Effect ◽  
Mott Insulator ◽  
Zero Magnetic Field ◽  
Hall Resistance ◽  
High Symmetry ◽  
Topological Phase ◽  
Wigner Crystals ◽  
And Topology

Abstract Electron correlation and topology are two central threads of modern condensed matter physics. Semiconductor moiré materials provide a highly tunable platform for studies of electron correlation. Correlation-driven phenomena, including the Mott insulator, generalized Wigner crystals, stripe phases and continuous Mott transition, have been demonstrated. However, nontrivial band topology has remained elusive. Here we report the observation of a quantum anomalous Hall (QAH) effect in AB-stacked MoTe2/WSe2 moiré heterobilayers. Unlike in the AA-stacked structures, an out-of-plane electric field controls not only the bandwidth but also the band topology by intertwining moiré bands centered at different high-symmetry stacking sites. At half band filling, corresponding to one particle per moiré unit cell, we observe quantized Hall resistance, h/e^2 (with h and e denoting the Planck’s constant and electron charge, respectively), and vanishing longitudinal resistance at zero magnetic field. The electric-field-induced topological phase transition from a Mott insulator to a QAH insulator precedes an insulator-to-metal transition; contrary to most known topological phase transitions, it is not accompanied by a bulk charge gap closure. Our study paves the path for discovery of a wealth of emergent phenomena arising from the combined influence of strong correlation and topology in semiconductor moiré materials.

scholarly journals HALL EFFECT STUDIES OF THE SUPERCONDUCTING FERROMAGNET UCoGe

2017 ◽  
Vol 35 (2) ◽  
Author(s):  
Danica Krstovska ◽  
Eden Steven ◽  
Andhika Kiswandhi ◽  
James S. Brooks
Keyword(s):  
Magnetic Field ◽  
Hall Effect ◽  
Anomalous Hall Effect ◽  
Simple Relation ◽  
Hall Resistance ◽  
Unusual Behavior ◽  
Perpendicular Component ◽  
Induced Changes ◽  
The Magnetic Field ◽  
Characteristic Field

We find that the Hall effect in a single crystal of UCoGe varies as a function of the angle  between the applied magnetic field and the easy magnetic axis up to fields of 18 T at 0.2 K, i.e. in the region where both superconductivity and ferromagnetic order coexist. Instead of following the conventional cos dependence the two components that com-prise the total Hall resistance, the anomalous and ordinary Hall effect, exhibit quite an unusual behavior with the field direction. The anomalous Hall effect is found to be determined by the parallel component of the magnetization. We sug-gest that the field induced changes in magnetization due to the field rotation play an important role in the observed unu-sual behavior. The ordinary Hall effect cannot be described by the simple relation to the perpendicular component of the magnetic field implying that this component of the Hall effect may be also affected by the variations in magnetization at the characteristic field (kink field). A field induced moment polarization is also observed in Hall effect as in magnetore-sistance, which advances previous findings in UCoGe. The Hall effect slope reverses sign at the kink field indicative of small but possible Fermi surface reconstruction around this field. Our findings show that in UCoGe multiple mecha-nisms contribute to the observed field induced moment polarization at the kink field.

scholarly journals Precision measurement of the quantized anomalous Hall resistance at zero magnetic field

2018 ◽  
Vol 112 (7) ◽  
pp. 072102 ◽  
Author(s):  
Martin Götz ◽  
Kajetan M. Fijalkowski ◽  
Eckart Pesel ◽  
Matthias Hartl ◽  
Steffen Schreyeck ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Precision Measurement ◽  
Zero Magnetic Field ◽  
Hall Resistance

scholarly journals Precise Quantization of the Anomalous Hall Effect near Zero Magnetic Field

2015 ◽  
Vol 114 (18) ◽  
Author(s):  
A. J. Bestwick ◽  
E. J. Fox ◽  
Xufeng Kou ◽  
Lei Pan ◽  
Kang L. Wang ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Hall Effect ◽  
Anomalous Hall Effect ◽  
Zero Magnetic Field

Anomalous Hall effect and magnetotransport effects in the organic superconductor (TMTSF ) 2 CIO 4

1985 ◽  
Vol 314 (1528) ◽  
pp. 97-103
Keyword(s):  
Magnetic Field ◽  
Hall Effect ◽  
Magnetic State ◽  
Quantum Hall ◽  
Anomalous Hall Effect ◽  
Organic Superconductor ◽  
Hall Resistance ◽  
Threshold Field ◽  
Organic Conductor ◽  
Lower Temperature

The organic conductor (TMTSF) 2 CIO 4 exhibits unusual magnetotransport effects below 30 K. The resistivity and thermopower have large, anisotropic changes in a magnetic field, whereas the thermal conductivity is hardly affected. At lower temperature ( T ≤ 5 K) a magnetic field applied along the c * direction causes a phase transition from a metallic, non-magnetic state to a semimetallic, magnetic state. This orbitally induced transition appears to be unique in nature. Above the threshold field for this transition steps in the Hall resistance are observed, suggestive of the quantum Hall effect. In this paper we review the magnetotransport experiment in these materials and discuss the possible origins of the unusual phenomena observed.

scholarly journals Chiral anomaly trapped in Weyl metals: Nonequilibrium valley polarization at zero magnetic field

2021 ◽  
Vol 11 (2) ◽  
Author(s):  
Pablo M. Perez-Piskunow ◽  
Nicandro Bovenzi ◽  
Anton Akhmerov ◽  
Maxim Breitkreiz
Keyword(s):  
Magnetic Field ◽  
Three Dimensional ◽  
Finite Size ◽  
Surface States ◽  
Direct Consequence ◽  
Field Application ◽  
Zero Magnetic Field ◽  
Chiral Anomaly ◽  
Valley Polarization ◽  
Disorder Potential

In Weyl semimetals, the application of parallel electric and magnetic fields leads to valley polarization---an occupation disbalance of valleys of opposite chirality---a direct consequence of the chiral anomaly. In this work, we present numerical tools to explore such nonequilibrium effects in spatially confined three-dimensional systems with a variable disorder potential, giving exact solutions to leading order in the disorder potential and the applied electric field. Application to a Weyl-metal slab shows that valley polarization also occurs without an external magnetic field as an effect of chiral anomaly ``trapping'': Spatial confinement produces chiral bulk states, which enable the valley polarization in a similar way as the chiral states induced by a magnetic field. Despite its finite-size origin, the valley polarization can persist up to macroscopic length scales if the disorder potential is sufficiently long ranged, so that direct inter-valley scattering is suppressed and the relaxation then goes via the Fermi-arc surface states.

scholarly journals Confinement Properties of the 3D Topological Insulators Bi₂Se₃, Bi₂Te₃, and Sb₂Te₃

2021 ◽  
Author(s):  
◽  
Markus Kotulla
Keyword(s):  
Magnetic Field ◽  
Topological Insulators ◽  
Surface States ◽  
Cold Atom ◽  
Zeeman Splitting ◽  
Dirac Fermion ◽  
Topological Phases ◽  
Edge States ◽  
Experimental Realization ◽  
Band Inversion

<p>Recent discoveries have spurred the theoretical prediction and experimental realization of novel materials that have topological properties arising from band inversion. Such topological insulators have conductive surface or edge states but are insulating in the bulk. How the signatures of topological behavior evolve when the system size is reduced is noteworthy from both a fundamental and an application-oriented point of view, as such understanding may form the basis for tailoring systems to be in specific topological phases. This thesis investigates the softly confined topological insulator family of Bi₂Se₃ and its properties when subjected to an in-plane magnetic field. The model system provides a useful platform for systematic study of the transition between the normal and the topological phases, including the development of band inversion and the formation of massless-Dirac-fermion surface states. The effects of bare size quantization, two-dimensional-subband mixing, and electron-hole asymmetry are disentangled and their corresponding physical consequences elucidated.  When a magnetic field is present, it is found that the Dirac cone which is formed in surface states, splits into two cones separated in momentum space and that these cones exhibit properties of Weyl fermions. The effective Zeeman splitting is much larger for the surface states than for the bulk states. Furthermore, the g-factor of the surface states depends on the size of the material. The mathematical model presented here may be realizable experimentally in the frame of optical lattices in ultra cold atom gases.</p>

scholarly journals Direct visualization of edge state in even-layer MnBi2Te4 at zero magnetic field

2021 ◽  
Author(s):  
Weiyan Lin ◽  
Yang Feng ◽  
Yongchao Wang ◽  
Zichen Lian ◽  
Hao Li ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Time Reversal ◽  
Edge State ◽  
Quantum Spin ◽  
Zero Magnetic Field ◽  
Zero Field ◽  
Topological Phases ◽  
Edge States ◽  
Direct Visualization ◽  
Translation Symmetry

Abstract Being the first intrinsic antiferromagnetic (AFM) topological insulator, MnBi2Te4 is argued to be a topological axion state in its even-layer form due to the antiparallel magnetization between the top and bottom layers. Here we combine both transport and scanning microwave impedance microscopy (sMIM) to investigate such axion state in atomically thin MnBi2Te4 with even-layer thickness at zero magnetic field. While transport measurements show a zero Hall plateau signaturing the axion state, sMIM uncovers an unexpected edge state raising questions regarding the nature of the “axion state”. Based on our model calculation, we propose that the even-layer MnBi2Te4 at zero field is in an AFM quantum spin Hall (QSH) state hosting a pair of helical edge states. Such novel AFM QSH is originated from the combination of half translation symmetry and time-reversal symmetry in MnBi2Te4. Our finding thus signifies the richness of topological phases in MnB2Te4 that has yet to be fully explored.

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