arXiv subjects

Pedro M. Echenique

Publications and source records attributed to Pedro M. Echenique.

10 recordsLinked to original sources

arXiv2026

Large differential attosecond delays in solid state photoemission

Time-resolved photoelectron spectroscopy provides access to the electronic structure and non-equilibrium electron dynamics in matter. At solid surfaces photoemission dynamics can be investigated on its natural time scale by measuring attosecond time delays of emitted electrons. Photoelectrons with final state energies of several tens of eV need tens to hundreds of attoseconds to be released into the vacuum. Competing effects determine the emission dynamics and, hence, the full picture of the process is still under debate. The rather large energy differences between the final states probed in commonly reported relative photoemission delays obscure their complex fine structure and hinders the interpretation of the measurements. Here we report differential attosecond delays $τ_{\mathrm{DAD}}$, i.e., relative photoemission delays for energetically close-lying spin-orbit split states. Differential attosecond delays on the order of 30 to 100 as for Bi 5d, Te 4d, and Se 3d core level photoemission from Bi$_2$Te$_3$ and Bi$_2$Se$_3$ can neither be attributed to intra-atomic delays, nor to ballistic transport and subsequent emission. Instead, calculations based on the one-step photoemission theory reveal that photoemission delays vary strongly on the energy scale of the spin-orbit splitting and quantitatively match experimental observations. This strong variation arises from multiple scattering at the surface leading to final states that involve both evanescent and propagating Bloch waves. Their relative amplitudes vary strongly affecting thereby the timing of the photoemission event since evanescent and propagating components exhibit inherently different dynamics.

physics.optics↗

arXiv2025

Prediction and observation of the first antiferromagnetic topological insulator

Magnetic topological insulators (MTIs) are narrow gap semiconductor materials that combine non-trivial band topology and magnetic order. Unlike their nonmagnetic counterparts, MTIs may have some of the surfaces gapped due to breaking the time-reversal symmetry, which enables a number of exotic phenomena having potential applications in spintronics. So far, MTIs have only been created by means of doping nonmagnetic TIs with 3d transition metal elements, however, such an approach leads to strongly inhomogeneous magnetic and electronic properties of these materials, restricting the observation of important effects to very low temperatures. Finding intrinsic MTI, i.e. a stoichiometric well-ordered magnetic compound, could be an ideal solution to these problems, but no such material was observed to date. Here, using density functional theory we predict and further confirm by means of structural, transport, magnetic, angle- and spin-resolved photoemission spectroscopy measurements the realization of the antiferromagnetic (AFM) TI phase, that is hosted by the van der Waals layered compound MnBi$_2$Te$_4$. An interlayer AFM ordering makes MnBi$_2$Te$_4$ invariant with respect to the combination of the time-reversal ($Θ$) and primitive-lattice translation ($T_{1/2}$) symmetries, $S = ΘT_{1/2}$, giving rise to the $Z_2$ topological classification of AFM insulators. We find $Z_2 = 1$ for MnBi$_2$Te$_4$, which confirms its topologically nontrivial nature. The $S$-breaking (0001) surface of MnBi$_2$Te$_4$ exhibits a giant bandgap in the topological surface state as evidenced by ab initio calculations and photoemission measurements. These results culminate almost a decade-long search of an AFMTI, predicted in 2010. Furthermore, MnBi$_2$Te$_4$ is the first intrinsic magnetic TI realized experimentally.

cond-mat.mtrl-sci↗

arXiv2018

Unique thickness-dependent properties of the van der Waals interlayer antiferromagnet $\mathrm{MnBi_2Te_4}$ films

Using density functional theory and Monte Carlo calculations, we study the thickness dependence of the magnetic and electronic properties of a van der Waals interlayer antiferromagnet in the two-dimensional limit. Considering $\mathrm{MnBi_2Te_4}$ as a model material, we find it to demonstrate a remarkable set of thickness-dependent magnetic and topological transitions. While a single septuple layer block of $\mathrm{MnBi_2Te_4}$ is a topologically trivial ferromagnet, the thicker films made of an odd (even) number of blocks are uncompensated (compensated) interlayer antiferromagnets, which show wide bandgap quantum anomalous Hall (zero plateau quantum anomalous Hall) states. Thus, $\mathrm{MnBi_2Te_4}$ is the first stoichiometric material predicted to realize the zero plateau quantum anomalous Hall state intrinsically. This state has been theoretically shown to host the exotic axion insulator phase.

cond-mat.mtrl-sci↗

arXiv2018

Highly-ordered wide bandgap materials for quantized anomalous Hall and magnetoelectric effects

An interplay of spin-orbit coupling and intrinsic magnetism is known to give rise to the quantum anomalous Hall and topological magnetoelectric effects under certain conditions. Their realization could open access to low power consumption electronics as well as many fundamental phenomena like image magnetic monopoles, Majorana fermions and others. Unfortunately, being realized very recently, these effects are only accessible at extremely low temperatures and the lack of appropriate materials that would enable the temperature increase is a most severe challenge. Here, we propose a novel material platform with unique combination of properties making it perfectly suitable for the realization of both effects at elevated temperatures. The key element of the computational material design is an extension of a topological insulator (TI) surface by a thin film of ferromagnetic insulator, which is both structurally and compositionally compatible with the TI. Following this proposal we suggest a variety of specific systems and discuss their numerous advantages, in particular wide band gaps with the Fermi level located in the gap.

cond-mat.mtrl-sci↗

arXiv2018

Quantum friction between oscillating crystal slabs: Graphene monolayers on dielectric substrates

We present a theoretical description of energy transfer processes between two noncontact quasi- twodimensional crystals separated by distance a, oscillating with frequency omega0 and amplitude rho0 , and compare it with the case of two quasi-twodimensional crystals in uniform parallel motion. We apply the theory to calculate van der Waals energy and dissipated energy in two oscillating slabs where each slab consists of a graphene monolayer deposited on SiO2 substrate. The graphene dielectric response is determined from first principles, and SiO2 surface response is described using empirical local dielectric function. We studied the modification of vdW attraction as function of the driving frequency and graphene doping. We propose the idea of controlling the sticking and unsticking of slabs by tuning the graphene dopings EF i and driving frequency omega0 . We found simple rho02 dependence of vdW and dissipated energy. As the Dirac plasmons are the dominant channels through which the energy between slabs can be transferred, the dissipated power in equally doped EF1 = EF2 = 0 graphenes shows strong omega0 = 2omegap peak. This peak is substantially reduceed when graphenes are deposited on SiO2 substrate. If only one graphene is pristine (EFi = 0) the 2omegap peak disappears. For larger separations a the phononic losses also become important and the doping causes shifts, appearance and disappearance of many peaks originating from resonant coupling between hybridized electronic-phononic excitations in graphene-substrate slabs.

cond-mat.mes-hall↗

arXiv2015

Resolubility of Image-Potential Resonances

A theory of image-potential states is presented for the general case where these surface electronic states are resonant with a bulk continuum. The theory extends the multiple scattering approach of Echenique and Pendry into the strong coupling regime while retaining independence from specific forms of surface and bulk potentials. The theory predicts the existence of a well-resolved series of resonances for arbitrary coupling strengths. Surprisingly, distinct image-potential resonances are thus expected to exist on almost any metal surface, even in the limiting case of jellium.

cond-mat.mes-hall↗

arXiv2014

Low coverage surface diffusion in complex energy landscapes: Analytical solution and application to intercalation in topological insulators

A general expression is introduced for the tracer diffusivity in complex periodic energy landscapes with more than one distinct hop rate in two- and three-dimensional diluted systems (low coverage, single-tracer limit). For diffusion in two dimensions, a number of formulas are presented for complex combinations of hop rates in systems with triangular, rectangular and square symmetry. The formulas provide values in excellent agreement with Kinetic Monte Carlo simulations, concluding that the diffusion coefficient can be directly determined from the proposed expressions without performing such simulations. Based on the diffusion barriers obtained from first principles calculations and a physically-meaningful estimate of the attempt frequencies, the proposed formulas are used to analyze the diffusion of Cu, Ag and Rb adatoms on the surface and within the van der Waals (vdW) gap of a model topological insulator, Bi$_{2}$Se$_{3}$. Considering the possibility for adsorbate intercalation from the terraces to the vdW gaps at morphological steps, we infer that, at low coverage and room temperature: (i) a majority of the Rb atoms bounce back at the steps and remain on the terraces, (ii) Cu atoms mostly intercalate into the vdW gap, the remaining fraction staying at the steps, and (iii) Ag atoms essentially accumulate at the steps and gradually intercalate into the vdW gap. These conclusions are in good qualitative agreement with previous experiments.

cond-mat.mtrl-sci↗

arXiv2012

Ideal two-dimensional electron systems with a giant Rashba-type spin splitting in real materials: surfaces of bismuth tellurohalides

Spintronics is aimed at active controlling and manipulating the spin degrees of freedom in semiconductor devices. A promising way to achieve this goal is to make use of the tunable Rashba effect that relies on the spin-orbit interaction (SOI) in a two-dimensional (2D) electron system immersed in an inversion-asymmetric environment. The SOI induced spin-splitting of the 2D-electron state provides a basis for many theoretically proposed spintronic devices. However, the lack of semiconductors with large Rashba effect hinders realization of these devices in actual practice. Here we report on a giant Rashba-type spin splitting in 2D electron systems which reside at tellurium-terminated surfaces of bismuth tellurohalides. Among these semiconductors, BiTeCl stands out for its isotropic metallic surface-state band with the Gamma-point energy lying deep inside the bulk band gap. The giant spin-splitting of this band ensures a substantial spin asymmetry of the inelastic mean free path of quasiparticles with different spin orientations.

cond-mat.mtrl-sci↗

arXiv2010

Image potential states as quantum probe of graphene interfaces

Image potential states (IPSs) are electronic states localized in front of a surface in a potential well formed by the surface projected bulk band gap on one side and the image potential barrier on the other. In the limit of a two-dimensional solid a double Rydberg series of IPSs has been predicted which is in contrast to a single series present in three-dimensional solids. Here, we confirm this prediction experimentally for mono- and bilayer graphene. The IPSs of epitaxial graphene on SiC are measured by scanning tunnelling spectroscopy and the results are compared to ab-initio band structure calculations. Despite the presence of the substrate, both calculations and experimental measurements show that the first pair of the double series of IPSs survives, and eventually evolves into a single series for graphite. Thus, IPSs provide an elegant quantum probe of the interfacial coupling in graphene systems.

cond-mat.mes-hall↗

arXiv2001

Anomalous Quasiparticle Lifetime in Graphite: Band Structure Effects

We report ab initio calculation of quasiparticle lifetimes in graphite, as determined from the imaginary part of the self-energy operator within the GW aproximation. The inverse lifetime in the energy range from 0.5 to 3.5 eV above the Fermi level presents significant deviations from the quadratic behavior naively expected from Fermi liquid theory. The deviations are explained in terms of the unique features of the band structure of this material. We also discuss the experimental results from different groups and make some predictions for future experiments.

cond-mat↗