Intrinsic quantized anomalous Hall effect in a moiré heterostructure

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.

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

Science Advances ◽

10.1126/sciadv.1600167 ◽

2016 ◽

Vol 2 (7) ◽

pp. e1600167 ◽

Cited By ~ 36

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.

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Quantum anomalous Hall effect from intertwined moiré bands

10.21203/rs.3.rs-687478/v1 ◽

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.

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HALL EFFECT STUDIES OF THE SUPERCONDUCTING FERROMAGNET UCoGe

Contributions Section of Natural Mathematical and Biotechnical Sciences ◽

10.20903/csnmbs.masa.2014.35.2.56 ◽

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.

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Topological valley currents via ballistic edge modes in graphene superlattices near the primary Dirac point

Communications Physics ◽

10.1038/s42005-020-00495-y ◽

2020 ◽

Vol 3 (1) ◽

Author(s):

Yang Li ◽

Mario Amado ◽

Timo Hyart ◽

Grzegorz. P. Mazur ◽

Jason W. A. Robinson

Keyword(s):

Magnetic Field ◽

Boron Nitride ◽

Dirac Point ◽

Hexagonal Boron Nitride ◽

Zero Magnetic Field ◽

Decay Length ◽

Topological Phase ◽

Show Evidence ◽

Nonlocal Transport

AbstractGraphene on hexagonal boron nitride (hBN) can exhibit a topological phase via mutual crystallographic alignment. Recent measurements of nonlocal resistance (Rnl) near the secondary Dirac point (SDP) in ballistic graphene/hBN superlattices have been interpreted as arising due to the quantum valley Hall state. We report hBN/graphene/hBN superlattices in which Rnl at SDP is negligible, but below 60 K approaches the value of h/2e2 in zero magnetic field at the primary Dirac point with a characteristic decay length of 2 μm. Furthermore, nonlocal transport transmission probabilities based on the Landauer-Büttiker formalism show evidence for spin-degenerate ballistic valley-helical edge modes, which are key for the development of valleytronics.

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

Applied Physics Letters ◽

10.1063/1.5009718 ◽

2018 ◽

Vol 112 (7) ◽

pp. 072102 ◽

Cited By ~ 20

Author(s):

Martin Götz ◽

Kajetan M. Fijalkowski ◽

Eckart Pesel ◽

Matthias Hartl ◽

Steffen Schreyeck ◽

...

Keyword(s):

Magnetic Field ◽

Precision Measurement ◽

Zero Magnetic Field ◽

Hall Resistance

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Precise Quantization of the Anomalous Hall Effect near Zero Magnetic Field

Physical Review Letters ◽

10.1103/physrevlett.114.187201 ◽

2015 ◽

Vol 114 (18) ◽

Cited By ~ 154

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

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SPECIFIC HEAT OF Mg11B2 IN MAGNETIC FIELDS: TWO ENERGY GAPS IN THE SUPERCONDUCTING STATE

International Journal of Modern Physics B ◽

10.1142/s0217979202013870 ◽

2002 ◽

Vol 16 (20n22) ◽

pp. 3180-3183

Author(s):

R. A. FISHER ◽

F. BOUQUET ◽

N. E. PHILLIPS ◽

D. G. HINKS ◽

J. D. JORGENSEN

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Specific Heat ◽

Normal State ◽

Energy Gap ◽

Zero Magnetic Field ◽

Energy Gaps ◽

State Electron ◽

Electron Contribution ◽

Semi Empirical

We present specific-heat measurements on Mg 11 B 2 in magnetic fields to 9 T. The anomaly at Tc is rapidly broadened and attenuated in fields, as expected for an anisotropic, randomly oriented superconductor. At low temperature there is a strongly field-dependent feature that shows the existence of a second energy gap. The Sommerfeld constant, γ, increases rapidly and non-linearly with magnetic field, which cannot be accounted for by anisotropy. It approaches γn = 2.6 mJ K -2 mol -1, the coefficient of the normal-state electron contribution, asymptotically for fields greater than 5 T. In zero magnetic field the data can be fitted with a phenomenological two-gap model, a generalization of a semi-empirical model for single-gap superconductors. Both of the gaps close at the same Tc; one is larger and one smaller than the BCS weak coupling limit, in the ratio ~ 4:1, and each accounts for ~ 50% of the normal-state electron density of states. The parameters characterizing the fit agree well with those from theory and are in approximate agreement with some spectroscopic measurements.

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

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1985.0010 ◽

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.

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Quantum anomalous Hall edge channels survive up to the Curie temperature

Nature Communications ◽

10.1038/s41467-021-25912-w ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Kajetan M. Fijalkowski ◽

Nan Liu ◽

Pankaj Mandal ◽

Steffen Schreyeck ◽

Karl Brunner ◽

...

Keyword(s):

Magnetic Field ◽

Curie Temperature ◽

Careful Analysis ◽

Zero Magnetic Field ◽

Hall Resistance ◽

Thermally Activated ◽

Open Questions ◽

Channel Transport ◽

Potential Applications ◽

Non Local

AbstractAchieving metrological precision of quantum anomalous Hall resistance quantization at zero magnetic field so far remains limited to temperatures of the order of 20 mK, while the Curie temperature in the involved material is as high as 20 K. The reason for this discrepancy remains one of the biggest open questions surrounding the effect, and is the focus of this article. Here we show, through a careful analysis of the non-local voltages on a multi-terminal Corbino geometry, that the chiral edge channels continue to exist without applied magnetic field up to the Curie temperature of bulk ferromagnetism of the magnetic topological insulator, and that thermally activated bulk conductance is responsible for this quantization breakdown. Our results offer important insights on the nature of the topological protection of these edge channels, provide an encouraging sign for potential applications, and establish the multi-terminal Corbino geometry as a powerful tool for the study of edge channel transport in topological materials.

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