arXiv subjects

Elena Díaz

Publications and source records attributed to Elena Díaz.

11 recordsLinked to original sources

arXiv2025

Superballistic conduction in hydrodynamic antidot graphene superlattices

Viscous electron flow exhibits exotic signatures such as superballistic conduction. In order to observe hydrodynamics effects, a 2D device where the current flow is as inhomogeneous as possible is desirable. To this end, we build three antidot graphene superlattices with different hole diameters. We measure their electrical properties at various temperatures and under the effect of a perpendicular magnetic field. We find an enhanced superballistic effect, suggesting the effectiveness of the geometry at bending the electron flow. In addition, superballistic conduction, which is related to a transition from a non-collective to a collective regime of transport, behaves non-monotonically with the magnetic field. We also analyze the device resistance as a function of the size of the antidot superlattice to find characteristic scaling laws describing the different transport regimes. We prove that the antidot superlattice is a convenient geometry for realizing hydrodynamic flow and provide valuable explanations for the technologically relevant effects of superballistic conduction and scaling laws.

cond-mat.mes-hall↗

arXiv2025

Superballistic paradox in electron fluids: Evidence of tomographic transport

Electron hydrodynamics encompasses the exotic fluid-like behavior of electrons in two-dimensional materials such as graphene. It accounts for superballistic conduction, also known as the Gurzhi effect, where increasing temperature reduces the electrical resistance. In analogy with conventional fluids, the Gurzhi effect is only expected in the hydrodynamic regime, with the decrease in the resistance occurring at intermediate temperatures. Nonetheless, experiments on electron fluids consistently show that superballistic conduction starts at close-to-zero temperatures. To address this paradox, we study hydrodynamic flow, and we find that replacing the classical dynamics with tomographic dynamics, where only head-on collisions are allowed between electrons, solves the dilemma. The latter strengthens superballistic conduction, with potential applications in low-dissipation devices, and explains its differences with the Molenkamp effect and conventional fluids dynamics. Our study reveals that the superballistic paradox is resolved by considering the electrons not as classical particles but as fermions.

cond-mat.mes-hall↗

arXiv2024

Alternative routes to electron hydrodynamics

Viscous flow of interacting electrons in two dimensional materials features a bunch of exotic effects. A model resembling the Navier-Stokes equation for classical fluids accounts for them in the so called hydrodynamic regime. We performed a detailed analysis of the physical conditions to achieve electron hydrodynamic transport and found three new alternative routes: favouring frequent inelastic collisions, the application of a magnetic field or a high-frequency electric field. As a mayor conclusion, we show that the conventional requirement of frequent electron-electron collisions is too restrictive and, as a consequence, materials and phenomena to be described using hydrodynamics are widened. In view of our results, we discuss recent experimental evidence on viscous flow and point out alternative avenues to reduce electric dissipation in optimized devices.

cond-mat.mes-hall↗

arXiv2021

Fast response photogating in monolayer MoS2 phototransistors

Two-dimensional transition metal dichalcogenide (TMD) phototransistors have been object of intensive research during the last years due to their potential for photodetection. Photoresponse in these devices is typically caused by a combination of two physical mechanisms: photoconductive effect (PCE) and photogating effect (PGE). In earlier literature for monolayer (1L) MoS2 phototransistors PGE is generally attributed to charge trapping by polar molecules adsorbed to the semiconductor channel, giving rise to a very slow photoresponse. Thus, the photoresponse of 1L-MoS2 phototransistors at high-frequency light modulation is assigned to PCE alone. Here we investigate the photoresponse of a fully h-BN encapsulated monolayer (1L) MoS2 phototransistor. In contrast with previous understanding, we identify a rapidly responding PGE mechanism that becomes the dominant contribution to photoresponse under high-frequency light modulation. Using a Hornbeck-Haynes model for the photocarrier dynamics, we fit the illumination power dependence of this PGE and estimate the energy level of the involved traps. The resulting energies are compatible with shallow traps in MoS2 caused by the presence of sulfur vacancies.

cond-mat.mes-hall↗

arXiv2020

Many-impurity scattering on the surface of a topological insulator

We theoretically address the impact of a random distribution of non-magnetic impurities on the surface states formed at the interface between a trivial and a topological insulator. The interaction of electrons with the impurities is accounted for by a separable pseudo-potential method that allows us to obtain closed expressions for the density of states. Spectral properties of surface states are assessed by means of the Green's function averaged over disorder realizations. For comparison purposes, the configurationally averaged Green's function is calculated by means of two different self-consistent methods, namely the self-consistent Born approximation (SCBA) and the coherent potential approximation (CPA). The latter is often regarded as the best single-site theory for the study of the spectral properties of disordered systems. However, although a large number of works employ the SCBA for the analysis of many-impurity scattering on the surface of a topological insulator, CPA studies of the same problem are scarce in the literature. In this work we find that the SCBA overestimates the impact of the random distribution of impurities on the spectral properties of surface states compared to the CPA predictions. The difference is more pronounced when increasing the magnitude of the disorder.

cond-mat.mes-hall↗

arXiv2020

Excitons, trions and Rydberg states in monolayer MoS2 revealed by low temperature photocurrent spectroscopy

We investigate excitonic transitions in a h-BN encapsulated monolayer $\textrm{MoS}_2$ phototransistor by photocurrent spectroscopy at cryogenic temperature (T = 5 K). The spectra presents excitonic peaks with linewidths as low as 8 meV, one order of magnitude lower than in earlier photocurrent spectroscopy measurements. We observe four spectral features corresponding to the ground states of neutral excitons ($\textrm{X}_{\textrm{1s}}^\textrm{A}$ and $\textrm{X}_{\textrm{1s}}^\textrm{B}$) and charged trions ($\textrm{T}^\textrm{A}$ and $\textrm{T}^\textrm{B}$) as well as up to eight additional spectral lines at energies above the $\textrm{X}_{\textrm{1s}}^\textrm{B}$ transition, which we attribute to the Rydberg series of excited states of $\textrm{X}^\textrm{A}$ and $\textrm{X}^\textrm{B}$. The relative intensities of the different spectral features can be tuned by the applied gate and drain-source voltages, with trions and Rydberg excited states becoming more prominent at large gate voltages. Using an effective-mass theory for excitons in two-dimensional transition-metal dichalcogenides we are able to accurately fit the measured spectral lines and unambiguously associate them with their corresponding Rydberg states. The fit also allows us to determine the quasiparticle bandgap and spin-orbit splitting of monolayer $\textrm{MoS}_2$, as well as the exciton binding energies of $\textrm{X}^\textrm{A}$ and $\textrm{X}^\textrm{B}$.

cond-mat.mtrl-sci↗

arXiv2020

Understanding the UV luminescence of zinc germanate: The role of native defects

Achieving efficient and stable ultraviolet emission is a challenging goal in optoelectronic devices. Herein, we investigate the UV luminescence of zinc germanate Zn2GeO4 microwires by means of photoluminescence measurements as a function of temperature and excitation conditions. The emitted UV light is composed of two bands (a broad one and a narrow one) associated with the native defects structure. In addition, with the aid of density functional theory (DFT) calculations, the energy positions of the electronic levels related to native defects in Zn2GeO4 have been calculated. In particular, our results support that zinc interstitials are the responsible for the narrow UV band, which is, in turn, split into two components with different temperature dependence behaviour. The origin of the two components is explained on the basis of the particular location of Zn_i in the lattice and agrees with DFT calculations. Furthermore, a kinetic luminescence model is proposed to ascertain the temperature evolution of this UV emission. These results pave the way to exploit defect engineering in achieving functional optoelectronic devices to operate in the UV region.

cond-mat.mtrl-sci↗

arXiv2019

Electric field manipulation of surface states in topological semimetals

We investigate the consequences of applying electric fields perpendicularly to thin films of topological semimetals. In particular, we consider Weyl and Dirac semimetals in a configuration such that their surface Fermi arcs lie on opposite edges of the films. We develop an analytical approach based on perturbation theory and a single-surface approximation and we compare our analytical results with numerical calculations. The effect of the electric field on the dispersion is twofold: it shifts the dispersion relation and renormalizes the Fermi velocity, which would, in turn, have direct effects on quantum transport measurements. Additionally, it modifies the spatial decay properties of surface states which will impact the connection of the Fermi arcs in opposite sides of a narrow thin film.

cond-mat.mes-hall↗

arXiv2018

Low-field electromagnet for a high-resolution MRI system

This paper presents the design and experimental characterization of a 1 T electromagnet tailored to meet the demands of a Magnetic Resonance Imaging (MRI) system conceived for spatial resolutions at the level of tens of microns. For high image quality, MRI requires an homogeneous magnetic field over the Field of View (FoV) where the sample is imaged. We measure the relative inhomogeneity of the field generated by our magnet to be well below 100 parts per million over a spherical FoV of 20 mm diameter, while strongly constraining fringe field lines to avoid interference with other devices. The magnet performance closely follows our expectations from numerical simulations in all the experimental tests carried out. Additionally, we present the solutions adopted for thermal management and the design of a mechanical structure to distribute the weight and integrate the platform to move the sample in and out of the magnet.

physics.ins-det↗

arXiv2013

Super Bloch oscillations with modulated interaction

We study super Bloch oscillations of ultracold atoms in a shaken lattice potential, subjected to a harmonically modulated mean-field interaction. Usually, any interaction leads to the decay of the wave packet and its super Bloch oscillation. Here, we use the phases of interaction and shaking with respect to the free Bloch oscillation as control parameters. We find two types of long-living cases: (i) suppression of the immediate broadening of the wave packet, and (ii) dynamical stability of all degrees of freedom. The latter relies on the rather robust symmetry argument of cyclic time [Gaul et al., Phys. Rev. A 84, 053627 (2011)].

cond-mat.quant-gas↗

arXiv2011

Stability and decay of Bloch oscillations in presence of time-dependent nonlinearity

We consider Bloch oscillations of Bose-Einstein condensates in presence of a time-modulated s-wave scattering length. Generically, interaction leads to dephasing and decay of the wave packet. Based on a cyclic-time argument, we find---additionally to the linear Bloch oscillation and a rigid soliton solution---an infinite family of modulations that lead to a periodic time evolution of the wave packet. In order to quantitatively describe the dynamics of Bloch oscillations in presence of time-modulated interactions, we employ two complementary methods: collective-coordinates and the linear stability analysis of an extended wave packet. We provide instructive examples and address the question of robustness against external perturbations.

cond-mat.quant-gas↗