arXiv subjects

Enrique Muñoz

Publications and source records attributed to Enrique Muñoz.

53 records · Page 3Linked to original sources

arXiv2019

Interaction Between Two Single Superconducting Vortices Inside A Superconducting Hollow Cylindrical domain

Inspired by the seminal, ground-breaking work of Abrikosov in 1957, we developed a new approximation to the interaction between two widely separated superconducting vortices. In contrast with Abrikosov's, we take into account the finite size of the vortices and their internal magnetic profile. We consider the vortices to be embedded within a superconducting, infinitely long hollow cylinder, in order to simplify the symmetry and boundary conditions for the mathematical analysis. We study this system in the context of a magnetic Ginzburg-Landau functional theory, by solving for the magnetic field profile inside each vortex, as well as in the superconducting region, subject to physical boundary conditions inspired by the classical analogue of two mutually inducting coils. Under isothermal conditions, the effective force between these vortices is given by the gradient of the Helmholtz free energy constructed from the Ginzburg-Landau functional. From our results, we explicitly show that, in agreement with well established theoretical arguments and experiments, the interaction between widely separated vortices is repulsive in this context, and their equilibrium positions are constrained by the fluxoid's conservation. Moreover, we find that the equilibrium positions of the vortices centers are stable due to the convexity of the Helmholtz free energy profile. Remarkably, the effect of the boundaries of the region over the effective interaction between the vortices is important in the chosen geometric configuration.

cond-mat.supr-con↗

arXiv2019

Optical Conductivity in an effective model for Graphene: Finite temperature corrections

In this article, we investigate the temperature and chemical potential dependence of the optical conductivity of graphene, within a field theoretical representation in the continuum approximation, arising from an underlying tight-binding atomistic model, that includes up to next-to-nearest neighbor coupling. Our calculations allow us to obtain the dependence of the optical conductivity on frequency, temperature and finite chemical potential, generalizing our previously reported calculations at zero temperature, and reproducing the universal and experimentally verified value at zero frequency.

cond-mat.mes-hall↗

arXiv2019

Thermo-magnetic corrections to $π$-$π$ Scattering Lengths in the Linear Sigma Model

In this article, we extend our previous study of the $π$-$π$ scattering lengths under the presence of an external magnetic field, including finite temperature effects. The novelty of this work is precisely the introduction of temperature into the discussion, and its interplay with the magnetic field. As in the previous article, we base our analysis in the linear sigma-model, and our calculations are exact within this context. Although the effects are comparatively small, it is interesting to remark that magnetic field and temperature display opposite effects over the scattering lengths.

hep-ph↗

arXiv2019

A hidden constraint on the Hamiltonian formulation of relativistic worldlines

Gauge theories with general covariance are particularly reluctant to quantization. We discuss the example of the Hamiltonian formulation of the relativistic point particle that, despite its apparent simplicity, is of crucial importance since a number of point particle systems can be cast into this form on a higher dimensional Rindler background, as recently pointed out by Hojman. It is shown that this system can be equipped with a hidden local, symmetry generating, constraint which on the one hand does not bother the classical evolution and on the other hand simplifies the realization of the path integral quantization. Even though the positive impact of the hidden symmetry is more evident in the Lagrangian version of the theory, it is still present through the suggested Hamiltonian constraint.

hep-th↗

arXiv2018

Thermoelectric transport in torsional strained Weyl semimetals

In a recent paper [R. Soto-Garrido and E. Muñoz, J. Phys. Condens. Matter 30, 195302 (2018)], we studied the electronic transport properties in Weyl semimetals (WSMs) submitted to the combined effects of torsional mechanical strain and magnetic field, showing that this configuration induces a node-polarization effect on the current that can be used to measure the torsion angle from transmission experiments. In this article, we extend our previous work to study thermoelectric transport in WSMs under torsional strain and an external magnetic field. Our analysis involves only the electronic contribution to the transport coefficients, and it is thus valid at low temperatures where the phonon contribution is negligible. We provide exact analytical expressions for the scattering cross section and the transmitted heat current, in order to calculate the thermal conductance and the Seebeck coefficient under this configuration. Our results suggest that thermoelectric transport coefficients in these materials can be engineered by appropriately tuning the magnitude of the magnetic field, torsional strain, and the applied bias or thermal gradient, leading to a potentially very high figure of merit.

cond-mat.mes-hall↗

arXiv2018

Electronic transport in torsional strained Weyl semimetals

In a recent paper (JPCM 29, 445302 (2017)) we have studied the effects of mechanical strain and magnetic field on the electronic transport properties in graphene. In this article we extended our work to Weyl semimetals (WSM). We show that although the WSM are 3D materials, most of the analysis done for graphene (2D material) can be carried out. In concrete, we studied the an electronic transport through a cylindrical region submitted to torsional strain and external magnetic field. We provide exact analytical expressions for the scattering cross section and the transmitted electronic current. In addition, we show the node-polarization effect on the current and propose a recipe to measure the torsion angle from transmission experiments.

cond-mat.mes-hall↗

arXiv2017

Optical Transparency in an effective model for Graphene

Motivated by experiments confirming that the optical transparency of graphene is defined through the fine structure constant and that it could be fully explained within the relativistic Dirac fermions in 2D picture, in this article we investigate how this property is affected by next-to-nearest neighbor coupling in the low-energy continuum description of graphene. A detailed calculation within the linear response regime allows us to conclude that, somewhat surprisingly, the zero-frequency limit of the optical conductivity that determines the transparency remains robust up to this correction.

cond-mat.str-el↗

arXiv2017

Symmetries of relativistic world-lines

Symmetries are essential for a consistent formulation of many quantum systems. In this paper we discuss a previously unnoticed symmetry, which is present for any Lagrangian term that involves $\dot{x}^2$. As a basic model that incorporates the fundamental symmetries of quantum gravity and string theory, we consider the Lagrangian action of the relativistic point particle. A path integral quantization for this seemingly simple system has for long presented notorious problems. Here we show that those problems are overcome by taking into account the newly discovered additional symmetry, leading directly to the exact Klein-Gordon propagator.

hep-th↗

arXiv2017

Analytic approach to magneto-strain tuning of electronic transport through a graphene nanobubble: Perspectives for a strain sensor

We consider the scattering of Dirac particles in graphene due to the superposition of an external magnetic field and mechanical strain. As a model for a graphene nanobubble, we find exact analytical solutions for single-particle states inside and outside a circular region submitted to the fields. Finally, we obtain analytical expressions for the scattering cross-section, as well as for the Landauer current through the circular region. Our results provide a fully-analytical treatment for electronic transport through a graphene nanobubble, showing that a combination of a physical magnetic field and strain leads to valley polarization and filtering of the electronic current. Moreover, our analytical model provides an explicit metrology principle to measure strain by performing conductance experiments under a controlled magnetic field imposed over the sample.

cond-mat.mes-hall↗

arXiv2016

The renormalized superperturbation theory (SPT) approach to the Anderson model in and out of equilibrium

The properties of current-carrying steady states of strongly correlated systems away from the linear-response regime are of topical interest. In this article, we review the renormalized perturbation theory, or renormalized SPT of reference [1] for the Anderson model. We present an extension to higher orders and compare the higher-order results with NRG calculations. Finally, we elucidate the role of Ward identities in calculating out-of-equilibrium properties and address claims made in the literature.

cond-mat.str-el↗

arXiv2015

Magneto-strain-driven quantum engine on a graphene flake

We propose a novel conceptual design for a graphene-based quantum engine, driven by a superposition of mechanical strain and an external magnetic field. Engineering of strain in a nanoscale graphene flake creates a gauge field with an associated uniform pseudo-magnetic field. The strain-induced pseudo-magnetic field can be combined with a real magnetic field, leading to the emergence of discrete relativistic Landau levels within the single-particle picture. The inter-level distance and hence their statistical population can be modulated by quasi-statically tuning the magnetic field along a sequence of reversible transformations that constitute a quantum mechanical analogue of the classical Otto cycle.

cond-mat.mes-hall↗

arXiv2014

Magnetically driven quantum heat engine

We studied the efficiency of two different schemes for a magnetically driven quantum heat engine, by considering as the working substance a single nonrelativistic particle trapped in a cylindrical potential well, in the presence of an external magnetic field. The first scheme is a cycle, composed of two adiabatic and two isoenergetic reversible trajectories in configuration space. The trajectories are driven by a quasistatic modulation of the external magnetic-field intensity. The second scheme is a variant of the former, where the isoenergetic trajectories are replaced by isothermal ones, along which the system is in contact with macroscopic thermostats. This second scheme constitutes a quantum analog of the classical Carnot cycle.

quant-ph↗

arXiv2013

Reply to 'Comment on Universal out-of-equilibrium transport in Kondo-correlated quantum dots'

A recent comment on our work (Phys. Rev. Lett., vol. 110, 016601 (2013)) by A.A.Aligia claims that we "made mistakes in the evaluation of the lesser quantities". It is further claimed that the distribution function of the single-particle selfenergy of the interacting region in the Fermi liquid regime, e.g. at small bias voltage, low temperature, and small frequency, is continuous. These claims are based on a comparison of the particle-hole symmetric case with results obtained from the approach of A.A.Aligia. We disagree with these claims and show that the discrepancies that the comment alludes to originate from a violation of Ward identities by the method employed in the comment. A comparison of our approach with the numerical renormalization group shows perfect agreement for the symmetric case.

cond-mat.str-el↗

arXiv2013

Transport Characterization of Kondo-Correlated Single Molecule Devices

A single molecule break junction device serves as a tunable model system for probing the many body Kondo state. The low-energy properties of this state are commonly described in terms of a Kondo model, where the response of the system to different perturbations is characterized by a single emergent energy scale, k_B*T_K. Comparisons between different experimental systems have shown issues with numerical consistency. With a new constrained analysis examining the dependence of conductance on temperature, bias, and magnetic field simultaneously, we show that these deviations can be resolved by properly accounting for background, non-Kondo contributions to the conductance that are often neglected. We clearly demonstrate the importance of these non-Kondo conduction channels by examining transport in devices with total conductances exceeding the theoretical maximum due to Kondo-assisted tunneling alone.

cond-mat.str-el↗

arXiv2013

Nonlinear thermoelectric response of quantum dots: renormalized dual fermions out of equilibrium

The thermoelectric transport properties of nanostructured devices continue to attract attention from theorists and experimentalist alike as the spatial confinement allows for a controlled approach to transport properties of correlated matter. Most of the existing work, however, focuses on thermoelectric transport in the linear regime despite the fact that the nonlinear conductance of correlated quantum dots has been studied in some detail throughout the last decade. Here, we review our recent work on the effect of particle-hole asymmetry on the nonlinear transport properties in the vicinity of the strong coupling limit of Kondo-correlated quantum dots and extend the underlying method, a renormalized superperturbation theory on the Keldysh contour, to the thermal conductance in the nonlinear regime. We determine the charge, energy, and heat current through the nanostructure and study the nonlinear transport coefficients, the entropy production, and the fate of the Wiedemann-Franz law in the non-thermal steady-state. Our approach is based on a renormalized perturbation theory in terms of dual fermions around the particle-hole symmetric strong-coupling limit.

cond-mat.str-el↗

arXiv2013

Universal out-of-equilibrium Transport in Kondo-correlated quantum dots: Renormalized dual Fermions on the Keldysh contour

The nonlinear conductance of semiconductor heterostructures and single molecule devices exhibiting Kondo physics has recently attracted attention. We address the observed sample dependence of the measured steady state transport coefficients by considering additional electronic contributions in the effective low-energy model underlying these experiments that are absent in particle-hole symmetric setups. A novel version of the superperturbation theory of Hafermann et al. in terms of dual fermions is developed, which correctly captures the low-temperature behavior. We compare our results with the measured transport coefficients.

cond-mat.mes-hall↗

arXiv2012

Phonon-limited transport coefficients in extrinsic graphene

The effect of electron-phonon scattering processes over the thermoelectric properties of extrinsic graphene was studied. Electrical and thermal resistivity, as well as the thermopower, were calculated within the Bloch theory approximations. Analytical expressions for the different transport coefficients were obtained from a variational solution of the Boltzmann equation. The phonon-limited electrical resistivity ρ_{e-ph} shows a linear dependence at high temperatures, and follows ρ_{e-ph} \sim T^{4} at low temperatures, in agreement with experiments and theory previously reported in the literature. The phonon-limited thermal resistivity at low temperatures exhibits a \sim T dependence, and achieves a nearly constant value at high temperatures. The predicted Seebeck coefficient at verylow temperatures is Q(T) \sim -π2 k_B T /(3 e E_F), which shows a n^{-1/2} dependence with the density of carriers, in agreement with experimental evidence. Our results suggest that thermoelectric properties can be controlled by adjusting the Bloch-Gruneisen temperature through its dependence on the extrinsic carrier density in graphene.

cond-mat.mes-hall↗