arXiv subjects

Juan A. Cañas

Publications and source records attributed to Juan A. Cañas.

10 recordsLinked to original sources

arXiv2026

Non-Hermitian impurity scattering in graphene: Boltzmann transport and thermoelectric response

We investigate charge and thermoelectric transport in monolayer graphene containing a dilute distribution of finite-range non-Hermitian scattering centers. The impurities are modeled as circular complex potentials, whose imaginary component describes local carrier loss or gain. By solving the Dirac scattering problem exactly within a partial-wave approach, we obtain the nonunitary scattering matrix and derive the transport and absorption cross sections, which separately characterize momentum relaxation and net carrier exchange with the environment. To connect the microscopic scattering problem with stationary transport, we formulate a semiclassical Boltzmann description in which an external reservoir compensates the equilibrium particle loss or gain. This leads to an effective relaxation time governed by both elastic momentum scattering and non-Hermitian flux exchange. Using the full energy-dependent relaxation time, we evaluate the Onsager coefficients and the resulting electrical conductivity, electronic thermal conductivity, Seebeck coefficient, Lorenz ratio, and electronic thermoelectric figure of merit. We find that weak gain increases the effective carrier lifetime and enhances both charge and heat conductivities, while absorption produces the opposite behavior. More importantly, gain enhances the magnitude of the thermopower and the electronic figure of merit, whereas loss suppresses them. The Lorenz ratio remains close to the Sommerfeld value, with non-Hermiticity mainly modifying its finite-temperature corrections. Our results show that non-Hermitian scattering provides an additional mechanism for controlling the energy dependence of carrier relaxation and, consequently, the thermoelectric response of graphene.

cond-mat.mes-hall↗

arXiv2026

Disorder-induced modulation of the nonlinear Hall effect in Weyl semimetals

We study the effects of impurity scattering on the nonlinear Hall response of Weyl semimetals within the semiclassical Boltzmann approach. We derive the rectified and second-harmonic conductivity tensors for a general momentum-dependent transport relaxation time and evaluate this quantity microscopically for short-range, Gaussian, screened Coulomb, and magnetic impurities. For scalar disorder, the relaxation time is isotropic and chirality independent. The nonlinear Hall response is then determined by the anomalous velocity associated with the Berry curvature, and a finite net response requires energetically inequivalent Weyl nodes to avoid cancellation between opposite chiralities. Polarized magnetic impurities lead to a qualitatively different behavior. We find that interference between the first- and second-order Born amplitudes generates a helicity-dependent anisotropic correction to the relaxation time. This anisotropy changes the tensor structure of the nonlinear Hall conductivity and gives rise to a finite second-harmonic current for suitable orientations of the electric field relative to the impurity polarization. For representative parameters, however, the anisotropic contribution is several orders of magnitude smaller than the dominant isotropic response. These results establish how the microscopic form of impurity scattering enters the nonlinear Hall response of Weyl semimetals through the transport relaxation time.

cond-mat.mes-hall↗

arXiv2026

Spontaneous persistent currents and time-reversal symmetry breaking in thick-walled Weyl semimetal cylinders

We theoretically investigate the Aharonov-Bohm effect in a thick-walled Weyl semimetal (WSM) cylinder subject to an external axial magnetic field. By employing a low-energy effective Hamiltonian, we analytically solve the eigenvalue problem for both infinite and finite-length cylindrical geometries. We apply infinite-mass boundary conditions at the radial walls and MIT bag boundary conditions at the cylinder caps to properly account for intra-node confinement and inter-valley scattering, respectively. Our numerical results demonstrate that the spatial separation of the Weyl nodes acts as an internal chiral gauge field. This geometric field intrinsically breaks time-reversal (TR) symmetry, lifting the chiral degeneracy even at zero external flux. This symmetry breaking manifests as spontaneous persistent currents and the unfolding of conductance channels. Furthermore, longitudinal confinement induces propagation-direction-dependent energy splitting, altering the partial density of states and causing spatio-chiral current imbalances.

cond-mat.mes-hall↗

arXiv2026

Non-Hermitian thermoelectric transport in graphene: Tunable anomalous transmission through complex barriers

We investigate thermoelectric transport in monolayer graphene across a finite complex barrier within a Landauer scattering framework. Solving the Dirac-Weyl problem exactly, we show that the imaginary part of the barrier renders the scattering matrix nonunitary and replaces the usual Hermitian flux conservation by a generalized flux-balance relation determined by the net gain or loss inside the barrier. In the Hermitian limit, the standard graphene $n$-$p$-$n$ barrier behavior is recovered, including perfect transmission at normal incidence and Fabry-Perot-type resonances. For a finite imaginary part, however, the same resonant channels are selectively attenuated or amplified, which significantly modifies both the angular response and the conductance profile. We further show that the lead-resolved conductances become dependent on the bias partition, providing a direct signature of the breakdown of gauge invariance in the effective two-terminal response. At finite temperature, the exact linear-response coefficients reveal a clear trade-off controlled by the imaginary part of the barrier: gain enhances both the electrical and thermal conductances, whereas loss suppresses the thermal conductance more efficiently and yields the largest thermoelectric figure of merit within the parameter range considered. These results demonstrate that complex barriers extend the range of transport behaviors accessible in graphene beyond the usual Hermitian $n$-$p$-$n$ junction. They also suggest a practical interpretation of the imaginary potential as an effective reduced description of unresolved source-sink channels or additional probes coupled to the device, particularly when a fully microscopic model of the environment is not available.

cond-mat.mes-hall↗

arXiv2026

Exact classical emergence from high-energy quantum superpositions

We examine the correspondence principle for an equiprobable superposition of high-energy eigenstates of the infinite square well using a fully analytical Fourier-based approach. We derive a closed-form asymptotic expression for the interference terms $ρ_α^{\text{a}}(x)$ by expanding them into a geometric series of quantum Fourier coefficients. We show these terms act as functional envelopes that do not vanish individually but become asymptotically equivalent in the large-$n$ limit. Furthermore, we prove the total probability density for a superposition of $2Δ+1$ states converges exactly to the uniform classical distribution as $Δ\to \infty$. Dynamically, the expectation value of position reproduces the classical triangular trajectory asymptotically. Residual quantum deviations remain confined to boundary layers whose relative width vanishes under macroscopic resolution. These results establish a rigorous asymptotic realization of the classical limit for isolated bound systems in both static and dynamical contexts.

quant-ph↗

arXiv2026

Charge and energy transport in graphene with smooth finite-range disorder

We investigate charge and energy transport in monolayer graphene with smooth finite-range disorder, modeled by soft impurity potentials. Using a continuum Dirac model, we go beyond the Born approximation by computing the exact scattering matrix for individual impurities. This captures the full nonperturbative physics of smooth disorder. From the exact scattering data, we evaluate transport coefficients by solving the Boltzmann equation with energy-resolved phase shifts. We analyze electrical and electronic thermal conductivities versus carrier density and temperature, including deviations from the Wiedemann-Franz law. Our results reveal that finite-range disorder nontrivially modifies charge and heat currents, especially at low energies where perturbative methods fail. These findings provide a more accurate transport characterization for disordered Dirac materials and clarify how smooth disorder governs energy flow in graphene.

cond-mat.mes-hall↗

arXiv2025

Thermoelectric transport in graphene under strain fields modeled by Dirac oscillators

Graphene has emerged as a paradigmatic material in condensed matter physics due to its exceptional electronic, mechanical, and thermal properties. A deep understanding of its thermoelectric transport behavior is crucial for the development of novel nanoelectronic and energy-harvesting devices. In this work, we investigate the thermoelectric transport properties of monolayer graphene subjected to randomly distributed localized strain fields, which locally induce impurity-like perturbations. These strain-induced impurities are modeled via 2D Dirac oscillators, capturing the coupling between pseudorelativistic charge carriers and localized distortions in the lattice. Employing the semiclassical Boltzmann transport formalism, we compute the relaxation time using a scattering approach tailored to the Dirac oscillator potential. From this framework, we derive analytical expressions for the electrical conductivity, Seebeck coefficient, and thermal conductivity. The temperature dependence of the scattering centers density is also investigated. Our results reveal how strain modulates transport coefficients, highlighting the interplay between mechanical deformations and thermoelectric performance in graphene. This study provides a theoretical foundation for strain engineering in thermoelectric graphene-based devices.

cond-mat.mes-hall↗

arXiv2024

Emergent universality of free fall from quantum mechanics

Classical and quantum mechanical descriptions of motion are fundamentally different. The universality of free fall (UFF) is a distinguishing feature of the classical motion (which has been verified with astonishing precision), while quantum theory tell us only about probabilities and uncertainties thus breaking the UFF. There are strong reasons to believe that the classical description must emerge, under plausible hypothesis, from quantum mechanics. In this Essay we show that the UFF is an emergent phenomenon: the coarse-grained quantum distribution for high energy levels leads to the classical distribution as the lowest order plus quantum corrections. We estimate the size of these corrections on the Eötvös parameter and discuss the physical implications.

quant-ph↗

arXiv2022

Exact classical limit of the quantum bouncer

In this paper we develop a systematic approach to determine the classical limit of periodic quantum systems and applied it successfully to the problem of the quantum bouncer. It is well known that, for periodic systems, the classical probability density does not follow the quantum probability density. Instead, it follows the local average in the limit of large quantum numbers. Guided by this fact, and expressing both the classical and quantum probability densities as Fourier expansions, here we show that local averaging implies that the Fourier coefficients approach each other in the limit of large quantum numbers. The leading term in the quantum Fourier coefficient yields the exact classical limit, but subdominant terms also emerge, which we may interpret as quantum corrections at the macroscopic level. We apply this theory to the problem of a particle bouncing under the gravity field and show that the classical probability density is exactly recovered from the quantum distribution. We show that for realistic systems, the quantum corrections are strongly suppressed (by a factor of $\sim 10^{-10}$) with respect to the classical result.

quant-ph↗

arXiv2022

Testing the equivalence principle with time-diffracted free-falling quantum particles

The equivalence principle of gravity is examined at the quantum level using the diffraction in time of matter waves in two ways. First, we consider a quasi-monochromatic beam of particles incident on a shutter which is removed at time $t = 0$ and fall due to the gravitational field. The probability density exhibits a set of mass-dependent oscillations which are genuinely quantum in nature, thereby reflecting quantum violations to the weak equivalence principle, although the strong equivalence principle remains valid. We estimate the degree of violation in terms of the width of the diffraction-in-time effect. Second, motivated by the recent advances in the manipulation of ultracold atoms and neutrons as well as the experimental observation of quantum states of ultracold neutrons in the gravitational field above a flat mirror, we study the diffraction in time of a suddenly released beam of particles initially prepared in gravitational quantum bound states. In this case, we quantify the degree of violation by comparing the time of flight from the mean position of the initial wave packet versus the time of flight as measured from the mirror. We show that, in this case both the weak and strong versions of the equivalence principle are violated. We demonstrate that compatibility between equivalence principle and quantum mechanics is recovered in the macroscopic (large-mass) limit. Possible realizations with ultracold neutrons, cesium atoms and large molecules are discussed.

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