日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

Annual review of condensed matter physics

１ Ｂａｓｉｃ ｎｅｐｈｒｏｌｏｇｙ（生理；免疫・病理；分子生物学；検査・診断） ２ Ｃｌｉｎｉｃａｌ ｎｅｐｈｒｏｌｏｇｙ（糸球体障害；尿細管・間質障害；全身性疾患と腎障害；水電解質異常；腎不全）

Annual review 腎臓

Quantum anomalous Hall goes intrinsic Quantum anomalous Hall effect—the appearance of quantized Hall conductance at zero magnetic field—has been observed in thin films of the topological insulator Bi2Se3 doped with magnetic atoms. The doping, however, introduces inhomogeneity, reducing the temperature at which the effect occurs. Two groups have now observed quantum anomalous Hall effect in intrinsically magnetic materials (see the Perspective by Wakefield and Checkelsky). Serlin et al. did so in twisted bilayer graphene aligned to hexagonal boron nitride, where the effect enabled the switching of magnetization with tiny currents. In a complementary work, Deng et al. observed quantum anomalous Hall effect in the antiferromagnetic layered topological insulator MnBi2Te4. Science, this issue p. 900, p. 895; see also p. 848 Transport measurements indicate quantized Hall conductance without a magnetic field. 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.

Intrinsic Quantized Anomalous Hall Effect in a Moiré Heterostructure, Part III: Scanning Probe Magnetometry

Experiments showing that a single layer of WTe2 can conduct electricity along its edges while insulating in the interior suggests that this material is a two-dimensional topological insulator. A two-dimensional topological insulator (2DTI) is guaranteed to have a helical one-dimensional edge mode1,2,3,4,5,6,7,8,9,10,11 in which spin is locked to momentum, producing the quantum spin Hall effect and prohibiting elastic backscattering at zero magnetic field. No monolayer material has yet been shown to be a 2DTI, but recently the Weyl semimetal WTe2 was predicted12 to become a 2DTI in monolayer form if a bulk gap opens. Here, we report that, at temperatures below about 100 K, monolayer WTe2 does become insulating in its interior, while the edges still conduct. The edge conduction is strongly suppressed by an in-plane magnetic field and is independent of gate voltage, save for mesoscopic fluctuations that grow on cooling due to a zero-bias anomaly, which reduces the linear-response conductance. Bilayer WTe2 also becomes insulating at low temperatures but does not show edge conduction. Many of these observations are consistent with monolayer WTe2 being a 2DTI. However, the low-temperature edge conductance, for contacts spacings down to 150 nm, never reaches values higher than ∼20 μS, about half the predicted value of e2/h, suggesting significant elastic scattering in the edge.

/pdf/edge-conduction-in-monolayer-wte-2-4qrynsopuf.pdf

Edge conduction in monolayer $WTe_{2}$

We uncover topological features of neutral particle-hole pair excitations of correlated quantum anomalous Hall (QAH) insulators whose approximately flat conduction and valence bands have equal and opposite nonzero Chern number. Using an exactly solvable model we show that the underlying band topology affects both the center-of-mass and relative motion of particle-hole bound states. This leads to the formation of topological exciton bands whose features are robust to nonuniformity of both the dispersion and the Berry curvature. We apply these ideas to recently reported broken-symmetry spontaneous QAH insulators in substrate aligned magic-angle twisted bilayer graphene.

/pdf/exciton-band-topology-in-spontaneous-quantum-anomalous-hall-i44250xloh.pdf

Exciton Band Topology in Spontaneous Quantum Anomalous Hall Insulators: Applications to Twisted Bilayer Graphene.

The Luttinger liquid (LL) model of one-dimensional (1D) electronic systems provides a powerful tool for understanding strongly correlated physics including phenomena such as spin-charge separation. Substantial theoretical efforts have attempted to extend the LL phenomenology to two dimensions (2D), especially in models of closely packed perfect arrays of 1D quantum wires, each being described as a LL. For instance, such coupled-wire models have been successfully used to construct 2D anisotropic non-Fermi liquids, various quantum Hall states, topological phases, and quantum spin liquids. Despite these exciting theoretical developments, an experimental demonstration of high-quality arrays of 1D LLs suitable for realizing these models remains absent. Here we report the experimental realization of 2D arrays of 1D LLs with crystalline quality in a moire superlattice made of twisted bilayer tungsten ditelluride (tWTe$_{2}$). Originating from the anisotropic lattice of the monolayer, the moire pattern of tWTe$_{2}$ hosts identical, parallel 1D electronic channels, separated by a fixed nanoscale distance, which is tunable by the twist angle between layers. At a twist angle of ~ 5 degrees, we find that hole-doped tWTe$_{2}$ exhibits exceptionally large transport anisotropy with a resistance ratio of ~ 1000 between two orthogonal in-plane directions, suggesting the formation of 1D channels. The conductance measurement reveals a power-law scaling behavior, consistent with the formation of a 2D anisotropic phase that resembles an array of LLs. Our results open the door for realizing a variety of 2D correlated and topological quantum phases based on coupled-wire models and LL physics.

One-Dimensional Luttinger Liquids in a Two-Dimensional Moir\'e Lattice

We study the effect of an in-plane magnetic field on the noninteracting dispersion of twisted bilayer graphene. Our analysis is rooted in the chirally symmetric continuum model, whose zero-field band structure hosts exactly flat bands and large energy gaps at the magic angles. At the first magic angle, the central bands respond to a parallel field by forming a quadratic band crossing point (QBCP) at the moir\'e Brillouin zone center. Over a large range of fields, the dispersion is invariant with an overall scale set by the magnetic field strength. For deviations from the magic angle and for realistic interlayer couplings, the motion and merging of the Dirac points lying near charge neutrality are discussed in the context of the symmetries, and we show that small magnetic fields are able to induce a qualitative change in the energy spectrum. We conclude with a discussion on the possible ramifications of our study to the interacting ground states of twisted bilayer graphene systems.

Twisted bilayer graphene in a parallel magnetic field

Nodal semimetals are a unique platform to explore topological signatures of the unusual band structure that can manifest by accumulating a nontrivial phase in quantum oscillations. Here we report a study of the de Haas–van Alphen oscillations of the candidate topological nodal line semimetal CaAgAs using torque measurements in magnetic fields up to 45 T. Our results are compared with calculations for a toroidal Fermi surface originating from the nodal ring. We find evidence of a nontrivial π phase shift only in one of the oscillatory frequencies. We interpret this as a Berry phase arising from the semiclassical electronic Landau orbit which links with the nodal ring when the magnetic field lies in the mirror (ab) plane. Furthermore, additional Berry phase accumulates while rotating the magnetic field for the second orbit in the same orientation which does not link with the nodal ring. These effects are expected in CaAgAs due to the lack of inversion symmetry. Our study experimentally demonstrates that CaAgAs is an ideal platform for exploring the physics of nodal line semimetals and our approach can be extended to other materials in which trivial and nontrivial oscillations are present.

Quantum oscillations probe the Fermi surface topology of the nodal-line semimetal CaAgAs

We consider magic-angle twisted bilayer graphene (TBG) at filling $\nu=+3$,
where experiments have observed a robust quantized anomalous Hall effect. This
has been attributed to the formation of a valley- and spin-polarized Chern
insulating ground state that spontaneously breaks time-reversal symmetry, and
is stabilized by a hexagonal boron nitride (hBN) substrate. We identify three
different types of domain wall, and study their properties and energetic
selection mechanisms via theoretical arguments and Hartree-Fock calculations
adapted to deal with inhomogeneous moir\'e systems. We comment on the
implications of these results for transport and scanning probe experiments.

Yves H. Kwan

Papers

Exciton Band Topology in Spontaneous Quantum Anomalous Hall Insulators: Applications to Twisted Bilayer Graphene.

One-Dimensional Luttinger Liquids in a Two-Dimensional Moir\'e Lattice

Twisted bilayer graphene in a parallel magnetic field

Quantum oscillations probe the Fermi surface topology of the nodal-line semimetal CaAgAs

Domain wall competition in the Chern insulating regime of twisted bilayer graphene