arXiv subjects

Juan José Palacios

Publications and source records attributed to Juan José Palacios.

9 recordsLinked to original sources

arXiv2026

Angular-resolved nonlinear optical response as a probe of Lorentz violation in noncentrosymmetric materials

We propose a methodology to detect weak Lorentz-violating (LV) backgrounds through the nonlinear shift photocurrent in noncentrosymmetric crystals. Using a spinful Rice--Mele model, we show that a LV background induces a momentum-odd correction to the Bloch Hamiltonian that reshapes the phase of the interband dipole matrix elements. As a result, the shift conductivity develops a robust $π$-periodic modulation as a function of the angle of a perpendicularly applied static electric field, in contrast to a weakly $2π$-periodic response of the Lorentz-symmetric case. This change in angular periodicity provides a signature of LV effects which can be directly identified through a photocurrent measurement. For realistic optical intensities, the predicted signal lies in the picoampere range, which can be enhanced in a matrix of weakly interacting chains, allowing sensitivity to LV coupling strengths of the order of $ξ\sim10^{-24}\,\mathrm{C\,m}$. These results establish nonlinear optical transport as a viable probe of emergent LV effects in solid-state systems.

cond-mat.mes-hall↗

arXiv2026

Conductance of atomic size contacts of Ag and Au at high magnetic fields

Electronic conduction at the atomic scale can be described by Landauer's formalism. In single atom point contacts of noble metals like Au and Ag, there is just one channel open between both electrodes and the conductance is very close to the quantum of conductance $G \approx G_0=\frac{2e^2}{h}$, with the factor of two coming from spin degeneracy. The magnetoconductivity of atomic size contacts has been studied for numerous systems, unveiling local Kondo screening, magnetic order and spin-polarized currents. However, these have been mostly performed in elements with multiple open conduction channels where $G$ differs from $G_0$. The realization of a magnetically active conductor with a single open channel remains difficult to achieve. Here we present measurements of the electronic conductance of single channel Au and Ag atomic-size contacts in magnetic fields up to 20 Tesla. We observe a decrease in $G$ which goes up to about 15% in many Au contacts at 20 T. We perform calculations and find that pure Ag and Au do not present a strong field dependence of $G$, in agreement with previous results at smaller magnetic fields. We also find, however, that residual O$_2$ molecules attached close to the contact produce an an induced spin-polarized current, which leads to a decrease in $G$. We discuss the role of the magnetic response of the electrodes in the jump to contact. Our results suggest that single channel atomic size conductors with a sizeable response to a magnetic field can be built by combining noble metals and magnetically active molecular systems.

cond-mat.mes-hall↗

arXiv2025

Enabling the bulk photovoltaic effect in centrosymmetric materials through an external electric field

We develop a practical approach to electrically tuning the nonlinear photoresponse of two-dimensional semiconductors by explicitly incorporating a static out-of-plane electric field into the electronic ground state prior to optical excitation, as a gate bias. The method is implemented by dressing a Wannier-interpolated Hamiltonian with the field through its position matrix elements, which allows the gate bias to modify orbital hybridization and band dispersion beyond perturbative treatments. Within the independent-particle approximation, the resulting second-order (shift) conductivity is evaluated for both centrosymmetric and non-centrosymmetric layered systems. Applied to MoS$_2$, the approach captures the emergence of a finite shift current in centrosymmetric bilayers and the tunability of intrinsic responses in polar structures. The shift conductivity rises linearly at small fields and saturates at higher intensities, reflecting the competition between the growing shift vector and the weakening interband coupling as resonant transitions move away from high-symmetry valleys. A Taylor expansion of the field-dressed conductivity connects this behavior to the third-order optical response, revealing a unified picture of field-induced nonlinearities. These results establish field dressing of Wannier Hamiltonians as a practical route to model and predict nonlinear photocurrents in layered materials.

cond-mat.mes-hall↗

arXiv2024

Topologically protected photovoltaics in Bi nanoribbons

Photovoltaic efficiency in solar cells is hindered by many unwanted effects. Radiative channels (emission of photons) sometimes mediated by non-radiative ones (emission of phonons) are principally responsible for the decrease in exciton population before charge separation can take place. Another unwanted effect is electron-hole recombination at surfaces where in-gap edge states serve as the non-radiative channels. Topological insulators (TIs), in particular the most common ones characterized by a Z2 invariant, are rarely explored from an optoelectronics standpoint, possibly because of their typically small gaps. Otherwise, they are not much different from small-gap semiconductors and excitons are also expected to be their simplest excitations. Here we show that one can take advantage of the non-radiative decay channel due to the surface states to generate a topologically protected photovoltaic current. Focusing on two-dimensional TIs, and specifically for illustration purposes on a Bi(111) monolayer, we show the potential of TI nanoribbons to generate edge charge accumulation and edge currents under illumination.

cond-mat.mes-hall↗

arXiv2023

Efficient computation of optical excitations in two-dimensional materials with the Xatu code

Here we describe an efficient numerical implementation of the Bethe-Salpeter equation to obtain the excitonic spectrum of semiconductors. This is done on the electronic structure calculated either at the simplest tight-binding level or through density funcional theory calculations based on local orbitals. We use a simplified model for the electron-electron interactions which considers atomic orbitals as point-like orbitals and a phenomenological screening. The optical conductivity can then be optionally computed within the Kubo formalism. Our results for paradigmatic two-dimensional materials such as hBN and MoS2, when compared with those of more sophisticated first-principles methods, are excellent and envision a practical use of our implementation beyond the computational limitations of such methods.

cond-mat.mtrl-sci↗

arXiv2022

A theoretical approach for electron dynamics and ultrafast spectroscopy

In this manuscript we present a theoretical framework and its numerical implementation to simulate the out-of-equilibrium electron dynamics induced by the interaction of ultrashort laser pulses in condensed-matter systems. Our approach is based on evolving in real-time the density matrix of the system in reciprocal space. It considers excitonic and non-perturbative light-matter interactions. We show some relevant examples that illustrate the efficiency and flexibility of the approach to describe realistic ultrafast spectroscopy experiments. Our approach is suitable for modeling the promising and emerging ultrafast studies at the attosecond time scale that aim at capturing the electron dynamics and the dynamical electron-electron correlations via X-ray absorption spectroscopy.

physics.optics↗

arXiv2022

Deep learning for disordered topological insulators through entanglement spectrum

Calculation of topological invariants for crystalline systems is well understood in reciprocal space, allowing for the topological classification of a wide spectrum of materials. In this work, we present a new technique based on the entanglement spectrum, which can be used to identify the hidden topology of systems without translational invariance. By training a neural network to distinguish between trivial and topological phases using the entanglement spectrum obtained from crystalline or weakly disordered phases, we can predict the topological phase diagram for generic disordered systems. This approach becomes particularly useful for gapless systems, while providing a computational speed-up compared to the commonly used Wilson loop technique for gapful situations. Our methodology is illustrated in two-dimensional models based on the Wilson-Dirac lattice Hamiltonian.

cond-mat.dis-nn↗

arXiv2017

Strain engineering of Schottky barriers in single- and few-layer MoS2 vertical devices

We study the effect of local strain in the electronic transport properties of vertical metal-atomically thin MoS2-metal structures. We use a conductive atomic force microscope tip to apply different load forces to monolayer and few-layer MoS2 crystals deposited onto a conductive indium tin oxide (ITO) substrate while measuring simultaneously the I-V characteristics of the vertical tip/MoS2/ITO structures. The structures show rectifying I-V characteristics, with rectification ratios strongly dependent on the applied load. To understand these results, we compare the experimental I-Vs with a double Schottky barrier model, which is in good agreement with our experimental results and allows us to extract quantitative information about the electronic properties of the tip/MoS2/ITO structures and their dependence on the applied load. Finally, we test the stability of the studied structures using them as mechanically tunable current rectifiers.

cond-mat.mes-hall↗

arXiv2011

Magnetoresistance and Magnetic Ordering Fingerprints in Hydrogenated Graphene

Spin-dependent features in the conductivity of graphene, chemically modified by a random distribution of hydrogen adatoms, are explored theoretically. The spin effects are taken into account using a mean-field self-consistent Hubbard model derived from first-principles calculations. A Kubo-Greenwood transport methodology is used to compute the spin-dependent transport fingerprints of weakly hydrogenated graphene-based systems with realistic sizes. Conductivity responses are obtained for paramagnetic, antiferromagnetic, or ferromagnetic macroscopic states, constructed from the mean-field solutions obtained for small graphene supercells. Magnetoresistance signals up to $\sim 7%$ are calculated for hydrogen densities around 0.25%. These theoretical results could serve as guidance for experimental observation of induced magnetism in graphene.

cond-mat.mes-hall↗