Magnetic competition in topological kagome magnets

Magnetic competition in topological kagome magnets is studied by incorporating the spin-orbit coupling, anisotropic Hund coupling and spin exchange into a tight-binding electron dynamics in the kagome lattice. Using the Bogoliubov variational principle we find the stable phases at zero and finite temperatures. At zero temperature and in the strong Ising-Hund coupling regime, a magnetic tunability from the out-of-plane ferromagnetism to the in-plane antiferromagnetism is achieved through a universal property of the critical in-plane Hund coupling. At low temperature the out-of-plane ferromagnetism is stable until a finite crossing temperature. Above the crossing temperature the in-plane antiferromagnetism is stable, but the magnetization of the out-of-plane ferromagnetism still survives. This suggests a metastable coexistence of these magnetic phases in a finite temperature range. A large anomalous Hall conductance is observed in the Ising-Hund coupling limit.

A Majorana perspective on understanding and identifying axion insulators

An axion insulator is theoretically introduced to harbor unique surface states with half-integer Chern number $${{{{{{{\mathcal{C}}}}}}}}$$ C . Recently, experimental progress has been made in different candidate systems, while a unique Hall response to directly reflect the half-integer Chern number is still lacking to distinguish an axion state from other possible insulators. Here we show that the $${{{{{{{\mathcal{C}}}}}}}}=\frac{1}{2}$$ C = 1 2 axion state corresponds to a topological state with Chern number $${{{{{{{\mathcal{N}}}}}}}}=1$$ N = 1 in the Majorana basis. In proximity to an s − wave superconductor, a topological phase transition to an $${{{{{{{\mathcal{N}}}}}}}}=0$$ N = 0 phase takes place at critical superconducting pairing strength. Our theoretical analysis shows that a chiral Majorana hinge mode emerges at the boundary of $${{{{{{{\mathcal{N}}}}}}}}=1$$ N = 1 and $${{{{{{{\mathcal{N}}}}}}}}=0$$ N = 0 regions on the surface of an axion insulator. Furthermore, we propose a half-integer quantized thermal Hall conductance via a thermal transport measurement, which is a signature of the gapless chiral Majorana mode and thus confirms the $${{{{{{{\mathcal{C}}}}}}}}=\frac{1}{2}$$ C = 1 2 ($${{{{{{{\mathcal{N}}}}}}}}=1$$ N = 1 ) topological nature of an axion state. Our proposals help to theoretically comprehend and experimentally identify the axion insulator and may benefit the research of topological quantum computation.

Estimating Precipitating Energy Flux, Average Energy, and Hall Auroral Conductance From THEMIS All-Sky-Imagers With Focus on Mesoscales

Recent attention has been given to mesoscale phenomena across geospace (∼10 s km to 500 km in the ionosphere or ∼0.5 RE to several RE in the magnetosphere), as their contributions to the system global response are important yet remain uncharacterized mostly due to limitations in data resolution and coverage as well as in computational power. As data and models improve, it becomes increasingly valuable to advance understanding of the role of mesoscale phenomena contributions—specifically, in magnetosphere-ionosphere coupling. This paper describes a new method that utilizes the 2D array of Time History of Events and Macroscale Interactions during Substorms (THEMIS) white-light all-sky-imagers (ASI), in conjunction with meridian scanning photometers, to estimate the auroral scale sizes of intense precipitating energy fluxes and the associated Hall conductances. As an example of the technique, we investigated the role of precipitated energy flux and average energy on mesoscales as contrasted to large-scales for two back-to-back substorms, finding that mesoscale aurora contributes up to ∼80% (∼60%) of the total energy flux immediately after onset during the early expansion phase of the first (second) substorm, and continues to contribute ∼30–55% throughout the remainder of the substorm. The average energy estimated from the ASI mosaic field of view also peaked during the initial expansion phase. Using the measured energy flux and tables produced from the Boltzmann Three Constituent (B3C) auroral transport code (Strickland et al., 1976; 1993), we also estimated the 2D Hall conductance and compared it to Poker Flat Incoherent Scatter Radar conductance values, finding good agreement for both discrete and diffuse aurora.

Half-integer quantized anomalous thermal Hall effect in the Kitaev material candidate α-RuCl3

Half-integer thermal quantum Hall conductance has recently been reported for the two-dimensional honeycomb material α-RuCl3. We found that the half-integer thermal Hall plateau appears even for a magnetic field with no out-of-plane components. The measured field-angular variation of the quantized thermal Hall conductance has the same sign structure as the topological Chern number of the pure Kitaev spin liquid. This observation suggests that the non-Abelian topological order associated with fractionalization of the local magnetic moments persists even in the presence of non-Kitaev interactions in α-RuCl3.

Correlations of quantum curvature and variance of Chern numbers

We analyse the correlation function of the quantum curvature in complex quantum systems, using a random matrix model to provide an exemplar of a universal correlation function. We show that the correlation function diverges as the inverse of the distance at small separations. We also define and analyse a correlation function of mixed states, showing that it is finite but singular at small separations. A scaling hypothesis on a universal form for both types of correlations is supported by Monte-Carlo simulations. We relate the correlation function of the curvature to the variance of Chern integers which can describe quantised Hall conductance.

Parameters controlling the substorm onset location

<p>Substorm onset location varies over a range of magnetic local time (MLT) and magnetic latitudes (MLat). It is well known that about 5% of the variation in onset MLT can be explained by variations in interplanetary magnetic field orientation and dipole tilt angle. Both parameters introduce an azimuthal component in the magnetic field in the magnetosphere such that the projection of the onset MLT in the ionosphere is shifted. The MLT of the onset near the magnetopsheric equatorial plane is even less predictable. Recent studies have suggested that gradients in the ionospheric Hall conductance lead to a duskward shift of tail dynamics, which could also influence the location of substorm onset. Our goal is to test these ideas by quantifying the dependence of the spatial variation of the onset location on external and internal conditions. We focus on the correlation between the substorm onset location with conditions prior to the onset, such as the interplanetary magnetic field By component, dipole tilt angle, and estimates of the Hall conductance. Linear regression analysis is used to determine the substorm onset location dependence on the proposed variables.</p>