We study the electronic contribution to the thermal conductivity and the thermopower of Weyl and Dirac semimetals using a semiclassical Boltzmann approach. We investigate the effect of various relaxation processes including disorder and interactions on the thermoelectric properties, and also consider doping away from the Weyl or Dirac point. We find that the thermal conductivity and thermopower have an interesting dependence on the chemical potential that is characteristic of the linear electronic dispersion, and that the electron-electron interactions modify the Lorenz number. For the interacting system, we also use the Kubo formalism to obtain the transport coefficients. We find exact agreement between the Kubo and Boltzmann approaches at high temperatures. We also consider the effect of electric and magnetic fields on the thermal conductivity in various orientations with respect to the temperature gradient. Notably, when the temperature gradient and magnetic field are parallel, we find a large contribution to the longitudinal thermal conductivity that is quadratic in the magnetic field strength, similar to the magnetic field dependence of the longitudinal electrical conductivity due to the presence of the chiral anomaly when no thermal gradient is present.Comment: 17 pages, 2 Figure

We theoretically study magnetic and topological properties of antiferromagnetic kagome spin systems in the presence of both in-and out-of-plane Dzyaloshinskii-Moriya interactions. In materials such as the iron jarosites, the in-plane interactions stabilize a canted noncollinear "umbrella" magnetic configuration with finite scalar spin chirality. We derive expressions for the canting angle, and use the resulting order as a starting point for a spinwave analysis. We find topological magnon bands, characterized by non-zero Chern numbers. We calculate the magnon thermal Hall conductivity, and propose the iron jarosites as a promising candidate system for observing the magnon thermal Hall effect in a noncollinear spin configuration. We also show that the thermal conductivity can be tuned by varying an applied magnetic field, or the in-plane Dzyaloshinskii-Moriya strength. In contrast with previous studies of topological magnon bands, the effect is found to be large even in the limit of small canting. PACS numbers: 75.25.-j,75.30.Ds,75.47.-m arXiv:1804.09783v2 [cond-mat.str-el]

We present a theoretical method to generate a highly accurate time-independent Hamiltonian governing the finite-time behavior of a time-periodic system. The method exploits infinitesimal unitary transformation steps, from which renormalization group-like flow equations are derived to produce the effective Hamiltonian. Our tractable method has a range of validity reaching into frequencyand drive strength-regimes that are usually inaccessible via high frequency ω expansions in the parameter h/ω, where h is the upper limit for the strength of local interactions. We demonstrate exact properties of our approach on a simple toy-model, and test an approximate version of it on both interacting and non-interacting many-body Hamiltonians, where it offers an improvement over the more well-known Magnus expansion and other high frequency expansions. For the interacting models, we compare our approximate results to those found via exact diagonalization. While the approximation generally performs better globally than other high frequency approximations, the improvement is especially pronounced in the regime of lower frequencies and strong external driving. This regime is of special interest because of its proximity to the resonant regime where the effect of a periodic drive is the most dramatic. Our results open a new route towards identifying novel non-equilibrium regimes and behaviors in driven quantum many-particle systems.

What is the correct low-energy spin Hamiltonian description of α-RuCl 3 ? The material is a promising Kitaev spin liquid candidate, but is also known to order magnetically, the description of which necessitates additional interaction terms. The nature of these interactions, their magnitudes and even signs, remain an open question. In this work we systematically investigate dynamical and thermodynamic magnetic properties of proposed effective Hamiltonians. We calculate zero-temperature inelastic neutron scattering (INS) intensities using exact diagonalization, and magnetic specific heat using a thermal pure quantum states method. We find that no single current model satisfactorily explains all observed phenomena of α-RuCl 3 . In particular, we find that Hamiltonians derived from first principles can capture the experimentally observed high-temperature peak in the magnetic specific heat, while overestimating the magnon energy at the zone center. In contrast, other models reproduce important features of the INS data, but do not adequately describe the magnetic specific heat. 1, 4 2 3 5 6 7 8 9 10 11 14 15 16 17 (c) Spread of proposed models FIG. 1. α-RuCl 3 . (a) The zigzag magnetic order. (b) The honeycomb lattice and its different bonds. Solid, dotted and dashed lines represent nearest, second-nearest and third-nearest neighbor bonds, respectively. (c) The variability in two nearest neighbor (NN) parameters between various proposed spin Hamiltonians for α-RuCl 3 .The Hamiltonians marked by red, bold numbers (blue, roman) are discussed in the main text (Supplemental Information). Here K 1 and J 1 are the NN Kitaev and Heisenberg couplings, respectively, and Γ is an NN symmetric off-diagonal interaction. Models with ferromagnetic (antiferromagnetic) K 1 are marked with crosses (open circles). Bond averaged values were used for anisotropic models. magnetic specific heat [25, 33, 34], NMR [30, 35], microwave absorption [36], Raman scattering [37-39], and THz spec-arXiv:1906.07579v1 [cond-mat.str-el]