arXiv subjects

María A. H. Vozmediano

Publications and source records attributed to María A. H. Vozmediano.

At least 19 recordsLinked to original sources

arXiv2025

A new scale anomaly in Dirac matter

The dynamics of Dirac semimetals is modeled at low energies by the massless Dirac Hamiltonian with the Fermi velocity replacing the velocity of light. The classical action is scale invariant. In 3D materials, Coulomb interactions induce a conformal anomaly associated to the charge renormalization already known in quantum field theory. In this work, we describe a new conformal anomaly induced by the running of the Fermi velocity that applies to Dirac semimetals in two and three dimensions. The case of graphene is particularly interesting. We analyze the anomaly and explore its thermodynamic and hydrodynamic consequences. The anomaly modifies the propagation speed of hydrodynamic sound waves, alters the thermodynamic equation of state, and induces a non-vanishing bulk viscosity proportional to the beta function of the Fermi velocity.

hep-th↗

arXiv2024

Topological Anderson insulating phases in the interacting Haldane model

We analyze the influence of disorder and strong correlations on the topology in two dimensional Chern insulators. A mean field calculation in the half-filled Haldane model with extended Hubbard interactions and Anderson disorder shows that disorder favors topology in the interacting case and extends the topological phase to a larger region of the Hubbard parameters. In the absence of a staggered potential, we find a novel disorder-driven topological phase with Chern number C=1, with co-existence of topology with long range spin and charge orders. More conventional topological Anderson insulating phases are also found in the presence of a finite staggered potential.

cond-mat.str-el↗

arXiv2023

Unconventional thermoelectric transport in tilted Weyl semimetals

We analyze the effect of the tilt on the transverse thermoelectric coefficient of Weyl semimetals in the \emph{conformal} limit, i.e., zero temperature and zero chemical potential. Using the Kubo formalism, we find a nonmonotonic behavior of the thermoelectric conductivity as a function of the tilt perpendicular to the magnetic field, and a linear behavior when the tilt is aligned to the magnetic field. An ``axial Nernst" current (chiral currents counter propagating perpendicularly to the magnetic field and the temperature gradient) is generated in inversion symmetric Weyl materials when the tilt vector has a projection in the direction of the magnetic field. This analysis will help in the design and interpretation of thermoelectric transport experiments in recently discovered topological quantum materials.

cond-mat.mes-hall↗

arXiv2022

Strain-induced collapse of Landau Levels in real Weyl semimetals

The collapse of Landau levels under an electric field perpendicular to the magnetic field is one of the distinctive features of Dirac materials. So is the coupling of lattice deformations to the electronic degrees of freedom in the form of gauge fields which allows the formation of pseudo-Landau levels from strain. We analyze the collapse of Landau levels induced by strain on realistic Weyl semimetals hosting anisotropic, tilted Weyl cones in momentum space. We perform first-principles calculations, to establish the conditions on the external strain for the collapse of Landau levels in TaAs which can be experimentally accessed.

cond-mat.mes-hall↗

arXiv2021

Thermal transport, geometry, and anomalies

The relation between thermal transport and gravity was highlighted in the seminal work by Luttinger in 1964, and has been extensively developed to understand thermal transport, most notably the thermal Hall effect. Here we review the novel concepts that relate thermal transport, the geometry of space-time and quantum field theory anomalies. We give emphasis to the cross-pollination between emergent ideas in condensed matter, notably Weyl and Dirac semimetals, and the understanding of gravitational and scale anomalies stemming from high-energy physics. We finish by relating to recent experimental advances and presenting a perspective of several open problems.

cond-mat.mes-hall↗

arXiv2020

Thermoelectric relations in the conformal limit in Dirac and Weyl semimetals

In the Fermi liquid description of metals, electrical and thermoelectric transport coefficients are linked by robust relations which can be challenged by strong interactions or when the electron liquid enters a different regime. These relations have been very powerful in the characterisation of novel materials. We show that Dirac and Weyl semimetals at zero doping and zero temperature (the conformal limit) have a very singular behavior due to a quantum anomaly. Away from this point, a Mott relation can be established.

cond-mat.mes-hall↗

arXiv2020

Chiral sound waves in strained Weyl semimetals

We show that a strained wire of a Weyl semimetal supports a new type of gapless excitation, the chiral sound wave (CSW). It is a longitudinal charge density wave analog to the chiral magnetic wave predicted in the quark-gluon plasma but driven by an elastic axial pseudo-magnetic field. It involves the axial-axial-axial contribution to the chiral anomaly which couples the chiral charge density to the elastic axial gauge field. The chiral sound is unidirectional: it propagates along the elastic magnetic field and not in the opposite direction. The CSW may propagate for long distances as it does not couple directly to quickly dissipating electromagnetic plasmons, while its damping is controlled by the slow chirality flip rate. We propose an experimental setup to directly detect the chiral sound, which is excited by mechanical vibrations of the crystal lattice in the GHz frequency range. Our findings contribute to a new trend, the chiral acoustics, in strained Weyl semimetals.

cond-mat.mes-hall↗

arXiv2019

Robust band of critical states in T broken fermionic systems with lattice selective disorder

We analyze the localization properties of two dimensional systems based on partite lattices with a basis. Contrary to standard results, we find that a band of critical states emerges for systems in the unitary class A preserving spin symmetry when disorder is unevenly distributed over the basis atoms. The critical metal arises when the less disordered sublattice is connected and has time reversal symmetry broken. The unexpected robustness to disorder presented here is an appealing result which can be measured in optical lattices.

cond-mat.str-el↗

arXiv2019

Fingerprints of the conformal anomaly on the thermoelectric transport in Dirac and Weyl semimetals: Result from a Kubo formula

In a recent work, a contribution to the Nernst current of a Dirac or Weyl semimetal coming from the conformal anomaly was reported. Being originated from an anomaly - a vacuum contribution -, a non-zero transport coefficient was predicted at zero temperature and chemical potential. In this work we perform a Kubo formula calculation of the thermoelectrical coefficient and confirm that the result agrees with the quantum field theory estimation in the limit of zero temperature and chemical potential. For finite chemical potential μ around μ = 0 the transverse Seebeck coefficient shows a plateau indicating that only the zero conformal Landau levels contributes to this intrinsic effect. This result opens the way to the experimental observation of a geometric anomaly which is much harder to explore than the standard chiral anomaly.

cond-mat.mes-hall↗

arXiv2018

Anisotropic fixed points in Dirac and Weyl semimetals

The effective low energy description of interacting Dirac and Weyl semimetals is that of massless quantum electrodynamics with several Lorentz breaking material parameters. We perform a renormalization group analysis of Coulomb interaction in anisotropic Dirac and Weyl semimetals and show that the anisotropy persists in the material systems at the infrared fixed point. In addition, a tilt of the fermion cones breaking inversion symmetry induces a magnetoelectric term in the electrodynamics of the material whose magnitude runs to match that of the electronic tilt at the fixed point.

cond-mat.str-el↗

arXiv2018

Rotational strain in Weyl semimetals: A continuum approach

The coupling of lattice deformations to the low energy electronic excitations of Dirac matter involve novel types of electron--phonon couplings as the celebrated elastic gauge fields first analyzed in graphene. In the continuum low energy approach, lattice deformations coupling to the electronic degrees of freedom are characterized by the (symmetric) strain tensor defined in elasticity theory. We review these couplings in Weyl semimetals and examine the coupling of electronic excitations to the antisymmetric part of the deformation gradient tensor associated to rotational strain. The new couplings, absent in the two dimensional materials, have important physical implications: they give rise to new elastic gauge fields, contribute to the deformation potential, tilt the cones and generate new pseudo--Zeeman couplings.

cond-mat.mes-hall↗

arXiv2018

Elastic Gauge Fields and Hall Viscosity of Dirac Magnons

We analyze the coupling of elastic lattice deformations to the magnon degrees of freedom of magnon Dirac materials. For a Honeycomb ferromagnet we find that, as it happens in the case of graphene, elastic gauge fields appear coupled to the magnon pseudospinors. For deformations that induce constant pseudomagnetic fields, the spectrum around the Dirac nodes splits into pseudoLandau levels. We show that when a Dzyaloshinskii-Moriya interaction is considered, a topological gap opens in the system and a Chern-Simons effective action for the elastic degrees of freedom is generated. Such a term encodes a phonon Hall viscosity response, entirely generated by fluctuations of magnons living in the vicinity of the Dirac points. The magnon Hall viscosity vanishes at zero temperature, and grows as temperature is raised and the states around the Dirac points are increasingly populated.

cond-mat.mes-hall↗

arXiv2017

Collapse of Landau levels in Weyl semimetals

It is known that in two dimensional relativistic Dirac systems, the Landau levels can collapse in the presence of a critical in plane electric field. We extend this mechanism to the three dimensional Weyl semimetals and analyze the physical consequences for the cases of both, real and pseudo Landau levels arising form strain induced elastic magnetic fields.

cond-mat.mes-hall↗

arXiv2016

Raise and collapse of strain-induced pseudo-Landau levels in graphene

Lattice deformations couple to the low energy electronic excitations of graphene as vector fields similar to the electromagnetic potential \cite{SA02b,VKG10}. The suggestion that certain strain configurations would be able to induce pseudo landau levels in the spectrum of graphene \cite{GKG10,GGKN10}, and the subsequent experimental observation of these \cite{LBetal10} has been one of the most exciting events in an already fascinating field. It opened a new field of research "straintronics" linked to new applications, and had a strong influence on the physics of the new Dirac materials in two and three dimensions \cite{Amorim16}. The experimental observation of pseudo landau levels with scanning tunnel microscopy presents nevertheless some ambiguities. Similar strain patterns show different images sometimes difficult to interpret. In this work we strain the analogy of the pseudo versus real electromagnetic fields, as well as the fact that graphene has a relativistic behavior and show that, for some strain configurations, the deformation potential acts like a parallel electric field able to destabilize the Landau level structure via a mechanism identical to that described in \cite{LSB07,PC07} for real electromagnetic fields. The underlying physics is the combination of Maxwell electrodynamics and special relativity implying that different reference frames observe different electromagnetic fields \cite{LSB07}. The mechanism applies equally if the electric field has an external origin, which opens the door to an electric control of the giant pseudomagnetic fields in graphene.

cond-mat.mes-hall↗

arXiv2016

Mapping the effect of defect-induced strain disorder on the Dirac states of topological insulators

We provide a detailed microscopic characterization of the influence of defects-induced disorder onto the Dirac spectrum of three dimensional topological insulators. By spatially resolved Landau-levels spectroscopy measurements, we reveal the existence of nanoscale fluctuations of both the Dirac point energy as well as of the Dirac-fermions velocity which is found to spatially change in opposite direction for electrons and holes, respectively. These results evidence a scenario which goes beyond the existing picture based on chemical potential fluctuations. The findings are consistently explained by considering the microscopic effects of local stain introduced by defects, which our model calculations show to effectively couple to topological states, reshaping their Dirac-like dispersion over a large energy range. In particular, our results indicate that the presence of microscopic spatially varying stain, inevitably present in crystals because of the random distribution of defects, effectively couple to topological states and should be carefully considered for correctly describing the effects of disorder.

cond-mat.mes-hall↗

arXiv2016

Strain induced Chiral Magnetic Effect in Weyl semimetals

We argue that strain applied to a time-reversal and inversion breaking Weyl semi-metal in a magnetic field can induce an electric current via the chiral magnetic effect. A tight binding model is used to show that strain generically changes the locations in the Brillouin zone but also the energies of the band touching points (tips of the Weyl cones). Since axial charge in a Weyl semi-metal can relax via inter-valley scattering processes the induced current will decay with a timescale given by the lifetime of a chiral quasiparticle. We estimate the strength and lifetime of the current for typical material parameters and find that it should be experimentally observable.

cond-mat.mes-hall↗

arXiv2016

Anomalous Hall metals from strong disorder in class A systems on partite lattices

Topological matter is a trending topic in condensed matter: From a fundamental point of view it has introduced new phenomena and tools, and for technological applications, it holds the promise of basic stable quantum computing. Similarly, the physics of localization by disorder, an old paradigm of obvious technological importance in the field, continues revealing surprises when new properties of matter appear. This work deals with the localization behavior of electronic systems based on partite lattices with special attention to the role of topology. We find an unexpected result from the point of view of localization properties: A robust topological metallic state characterized by a non--quantized Hall conductivity arises from strong disorder in class A (time reversal symmetry broken) insulators. The key issue is the nature of the disorder realization: selective disorder in only one sublattice in systems based on bipartite lattices. The generality of the result is based on the partite nature of most recent 2D materials as graphene or transition metal dichalcogenides, and the possibility of the physical realization of the particular disorder demonstrated in ref. 1. An anomalous Hall metal arises also when the original clean insulator is topologically trivial.

cond-mat.dis-nn↗

arXiv2015

Anderson localization and topological transition in Chern insulators

We analyse the topological transition and localization evolution of disordered two dimensional systems with non trivial topology based on bipartite lattices. Chern insulators with broken time reversal symmetry show non standard behavior for disorder realizations selectively distributed on only one of the sublattices. The Chern number survives to a much stronger disorder strength (one order of magnitude higher) than in the equally distributed disordered case and the final state in the strongly disordered case is metallic.

cond-mat.mes-hall↗