Domain Wall Motion in Thin-Film Magnets/ Topological Insulator Junctions

We derive the equations of motion of a Domain Wall in a thin-film magnet coupled to the surface states of a Topological Insulator in the presence of of both an electric field along the Domain Wall and a magnetic field perpendicular to the junction. We show how the electric field acts as a chirality stabilizer holding off the appearance of Walker breakdown and enhancing the terminal velocity. We also propose a mechanism to reverse the Domain Wall chirality in a controllable manner by tuning the chiral current flowing through the Wall. An input from a weak perpendicular magnetic field is required in order to break the reflection symmetry that protects the degeneracy of the chirality vacuum.Comment: Enlarged and revised version accepted in PR

Dirac electrons and domain walls: a realization in junctions of ferromagnets and topological insulators

We study a system of Dirac electrons with finite density of charge carriers coupled to an external electromagnetic field in two spatial dimensions, with a domain wall (DW) mass term. The interface between a thin-film ferromagnet and a three-dimensional topological insulator provides a condensed-matter realization of this model, when an out-of-plane domain wall magnetization is coupled to the TI surface states. We show how, for films with very weak intrinsic in-plane anisotropies, the torque generated by the edge electronic current flowing along the DW competes with an effective in-plane anisotropy energy, induced by quantum fluctuations of the chiral electrons bound to the wall, in a mission to drive the internal angle of the DW from a Bloch configuration towards a N\'eel configuration. Both the edge current and the induced anisotropy contribute to stabilize the internal angle, so that for weak intrinsic in-plane anisotropies DW motion is still possible without suffering from an extremely early Walker breakdown.Comment: 18 pages, 3 figure

Anomalous Nernst and Thermal Hall Effects in Tilted Weyl Semimetals

We study the anomalous Nernst and thermal Hall effects in a linearized low-energy model of a tilted Weyl semimetal, with two Weyl nodes separated in momentum space. For inversion symmetric tilt, we give analytic expressions in two opposite limits: for a small tilt, corresponding to a type-I Weyl semimetal, the Nernst conductivity is finite and independent of the Fermi level, while for a large tilt, corresponding to a type-II Weyl semimetal, it acquires a contribution depending logarithmically on the Fermi energy. This result is in a sharp contrast to the nontilted case, where the Nernst response is known to be zero in the linear model. The thermal Hall conductivity similarly acquires Fermi surface contributions, which add to the Fermi level independent, zero tilt result, and is suppressed as one over the tilt parameter at half filling in the Type-II phase. In the case of inversion breaking tilt, with the tilting vector of equal modulus in the two Weyl cones, all Fermi surface contributions to both anomalous responses cancel out, resulting in zero Nernst conductivity. We discuss two possible experimental setups, representing open and closed thermoelectric circuits

On chiral responses to geometric torsion

We show that geometric torsion does not lead to new chiral dissipationless transport effects. Instead apparent response to torsion can be viewed as a manifestation of the chiral vortical effect. We do however find a new dissipationless transport coefficient to curvature. In Einstein-Cartan spacetimes it can be computed by a two point function of currents to first order in derivatives. Its value is half that of the chiral magnetic conductivity

Emergent gauge fields and topological effects in dirac matter

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física de la Materia Condensada. Fecha de lectura: 16-09-2016In this thesis we deal with systems hosting Dirac quasiparticles. The main focus is put on the physics arising from the interplay of non trivial topology and the emergence of gauge elds, stemming from the interaction of electrons with either magnetic and elastic degrees of freedom. The investigation relies on tools from quantum eld theory, in particular the theory of quantum anomalies and topological eld theory. The physical systems involved are two dimensional Dirac materials, like graphene and single layer transition metal dichalcogenide semiconductors; three dimensional time reversal invariant topological insulators, from which we focused on the two dimensional physics occurring at the edge; and Weyl semimetals. Under certain conditions, all these materials can be expected to be in a topological phase of matter, denoted quantum Hall state. Most of the results derived in this thesis are consequences of the special properties of this state of matter.En esta tesis tratamos con sistemas que albergan quasipart culas de Dirac. La atenci on se centra en la f sica que surge de la interrelaci on entre la topolog a y los campos gauge emergentes en ciertos materiales, derivados de la interacci on de los electrones con los grados de libertad magn eticos y el asticos. La investigaci on se soporta en las herramientas de la teor a cu antica de campos, en particular en la teor a de anomal as cu anticas y la teor a de campos topol ogica. Los sistemas f sicos tratados son materiales de Dirac en dos dimensiones, como el grafeno y semiconductores dicalcogenuros de metales de transici on; aislantes topol ogicos tridimensionales, en los cuales nos centramos en la f sica bidimensional de su super cie; y semimetales de Weyl. Bajo ciertas condiciones, se puede esperar que estos materiales se encuentren en una fase topol ogica de la materia, llamada estado Hall cu antico. La mayor a de los resultados obtenidos en esta tesis son consecuencia de las propiedades especiales de este estado de la materia