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The thermal Hall effect, defined as a heat current response transversal to an applied temperature gradient, is a central experimental probe of exotic electrically insulating phases of matter. A key question is how the interplay between magnetic and structural degrees of freedom gives rise to a nonzero thermal Hall conductivity (THC). Here, we present evidence for an intrinsic thermal Hall effect in the Heisenberg-Kitaev antiferromagnet and spin-liquid candidate Na$_2$Co$_2$TeO$_6$ brought about by the quantum-geometric Berry curvature of so-called magnon polarons, resulting from magnon-phonon hybridization. At low temperatures, our field- and temperature-dependent measurements show a negative THC for magnetic fields below 10 T and a sign change to positive THC above. Theoretically, the sign and the order of magnitude of the THC cannot be solely explained with magnetic excitations. We demonstrate that, by incorporating spin-lattice coupling into our theoretical calculations, the Berry curvature of magnon polarons counteracts the purely magnonic contribution, reverses the overall sign of the THC, and increases its magnitude, which significantly improves agreement with experimental data. Our work highlights the crucial role of spin-lattice coupling in the thermal Hall effect., Comment: 19 pages, 4 figures

We calculate the strength of the effective onsite Coulomb interaction (Hubbard $U$) in two-dimensional (2D) transition-metal (TM) dihalides MX$_2$ and trihalides MX$_3$ (M=Ti, V, Cr, Mn, Fe, Co, Ni; X=Cl, Br, I) from first principles using the constrained random-phase approximation. The correlated subspaces are formed from $t_{2g}$ or $e_g$ bands at the Fermi energy. Elimination of the efficient screening taking place in these narrow bands gives rise to sizable interaction parameters U between the localized $t_{2g}$ ($e_g$) electrons. Due to this large Coulomb interaction, we find $U/W >1$ (with the band width $W$) in most TM halides, making them strongly correlated materials. Among the metallic TM halides in paramagnetic state, the correlation strength $U/W$ reaches a maximum in NiX$_2$ and CrX$_3$ with values much larger than the corresponding values in elementary TMs and other TM compounds. Based on the Stoner model and the calculated $U$ and $J$ values, we discuss the tendency of the electron spins to order ferromagnetically.

We calculate the strength of the effective on-site Coulomb interaction (Hubbard U) in two-dimensional transition-metal (TM) dihalides MX2 and trihalides MX3 (M=Ti, V, Cr, Mn, Fe, Co, Ni; X=Cl, Br, I) from first principles using the constrained random-phase approximation. The correlated subspaces are formed from t2g or eg bands at the Fermi energy. Elimination of the efficient screening taking place in these narrow bands gives rise to sizable interaction parameters U between the localized t2g (eg) electrons. Due to this large Coulomb interaction, we find U/W>1 (with the bandwidth W) in most TM halides, making them strongly correlated materials. Among the metallic TM halides in the paramagnetic state, the correlation strength U/W reaches a maximum in NiX2 and CrX3 with values much larger than the corresponding values in elementary TMs and other TM compounds. Based on the Stoner model and the calculated U and J values, we discuss the tendency of the electron spins to order ferromagnetically.

Spin caloric transport refers to the coupling of heat with spin transport. Its applications primarily concern the generation of spin currents and control of magnetisation by temperature gradients for information technology, known by the synonym spin caloritronics. Within the framework of ab initio theory, new tools are being developed to provide an additional understanding of these phenomena in realistic materials, accounting for the complexity of the electronic structure without adjustable parameters. Here, we review this progress, summarising the principles of the density-functional-based approaches in the field and presenting a number of application highlights. Our discussion includes the three most frequently employed approaches to the problem, namely the Kubo, Boltzmann, and Landauer-Büttiker methods. These are showcased in specific examples that span, on the one hand, a wide range of materials, such as bulk metallic alloys, nano-structured metallic and tunnel junctions, or magnetic overlayers on heavy metals, and, on the other hand, a wide range of effects, such as the spin-Seebeck, magneto-Seebeck, and spin-Nernst effects, spin disorder, and the thermal spin-transfer and thermal spin-orbit torques. © 2018 IOP Publishing Ltd.

The scope of this review is to present the recent progress in the understanding of the microscopic origin of thermoelectric transport in semiconducting heterostructures and to identify and elucidate mechanisms which could lead to enhanced thermoelectric conversion efficiency. Based on first-principles calculations a consistent and convenient method is presented to fully describe the thermoelectric properties in the diffusive limit of transport for bulk systems and their associated heterostructures. While fundamentals of the functionality of phonon-blocking and electron-transmitting superlattices could be unveiled, we provide also distinct analysis and ideas for thermoelectric enhancement for two archetypical thermoelectric heterostructures based on inline image and Si/Ge. A focus was on the influence of bulk and interfacial strain, varying charge carrier concentration, temperature, and superlattice periods on the thermoelectric transport properties.

We studied structural, electronic and magnetic properties of a cubic perovskite BaFeO$_{3-��}$ ($0 \le ��\le 0.5$) within the density functional theory using a generalized gradient approximation and a GGA+U method. According to our calculations, BaFeO$_3$ in its stoichiometric cubic structure should be half-metallic and strongly ferromagnetic, with extremely high Curie temperature ($T_C$) of 700 - 900 K. However, a such estimate of $T_C$ disagrees with all available experiments, which report that $T_C$ of the BaFeO$_3$ and undoped BaFeO$_{3-��}$ films varies between 111 K and 235 K or, alternatively, that no ferromagnetic order was detected there. Fitting the calculated x-ray magnetic circular dichroism spectra to the experimental features seen for BaFeO$_3$, we concluded that the presence of oxygen vacancies in our model enables a good agreement. Thus, the relatively low $T_C$ measured in BaFeO$_3$ can be explained by oxygen vacancies intrinsically presented in the material. Since iron species near the O vacancy change their oxidation state from $4+$ to $3+$, the interaction between Fe$^{4+}$ and Fe$^{3+}$, which is antiferromagnetic, weakens the effective magnetic interaction in the system, which is predominantly ferromagnetic. With increasing $��$ in BaFeO$_{3-��}$, its $T_C$ decreases down to the critical value when the magnetic order becomes antiferromagnetic. Our calculations of the electronic structure of BaFeO$_{3-��}$ illustrate how the ferromagnetism originates and also how one can keep this cubic perovskite robustly ferromagnetic far above the room temperature.

We present ab initio electronic structure calculations based on density functional theory for the thermoelectric properties of Bi$_2$Te$_3$ films. Conductivity and thermopower are computed in the diffusive limit of transport based on the Boltzmann equation. Bulk and surface contribution to the transport coefficients are separated by a special projection technique. As a result we show clear signatures of the topological surface state in the thermoelectric properties., 6 pages, 4 figures

The thermoelectric transport properties of $\text{Bi}_2\text{Te}_3/\text{Sb}_2\text{Te}_3$superlattices are analyzed on the basis of first-principles calculations and semi-classical Boltzmann theory. The anisotropy of the thermoelectric transport under electron and hole-doping was studied in detail for different superlattice periods at changing temperature and charge carrier concentrations. A clear preference for thermoelectric transport under hole-doping, as well as for the in-plane transport direction was found for all superlattice periods. At hole-doping the electrical transport anisotropies remain bulk-like for all investigated systems, while under electron-doping quantum confinement leads to strong suppression of the cross-plane thermoelectric transport at several superlattice periods. In addition, insights on the Lorenz function, the electronic contribution to the thermal conductivity and the resulting figure of merit are given.

thermoelectric properties of Bi2Te3/Sb2Te3 superlattices are discussed

Trabajo presentado al CECAM Workshop on "Topological Insulators and Non-Perturbative Spin-Orbit Coupling" celebrado en Lausanne (Suiza) del 9 al 11 de Enero de 2012., Topological insulators are characterized by the presence of spin-momentum-locked surface states with Dirac points that span the fundamental bulk band gap. We show by first-principles calculations that the surface state of the insulator Bi2Te3 survives upon moderate Mn doping of the surface layers. The spin texture of both undoped and Mn-doped Bi2Te3 is much more complicated than commonly believed, showing layer-dependent spin reversal and spin vortices. The surface state in Mn-doped samples looses its topological character as a function of magnetization direction. The dispersion depends considerably on the direction of the Mn magnetization: For perpendicular magnetization, a gap of 16meV opens up at the Dirac point and strongly modifies the topological character; for in-plane magnetization, the Dirac point is conserved but shifted off the Brillouin zone center. The magnetic order is ferromagnetic with a critical temperature of 12 K. The results provide a path towards a magnetic control of the topological character of the surface state and its consequences to spin-dependent transport properties.

The thermoelectric transport properties of Bi2Te3/Sb2Te3 superlattices are analyzed on the basis of first-principles calculations and semi-classical Boltzmann theory. The anisotropy of the thermoelectric transport under electron and hole-doping was studied in detail for different superlattice periods at changing temperature and charge carrier concentrations. A clear preference for thermoelectric transport under hole-doping, as well as for the in-plane transport direction was found for all superlattice periods. At hole-doping the electrical transport anisotropies remain bulk-like for all investigated systems, while under electron-doping quantum confinement leads to strong suppression of the cross-plane thermoelectric transport at several superlattice periods. In addition, insights on the Lorenz function, the electronic contribution to the thermal conductivity and the resulting figure of merit are given.