arXiv subjects

J. Sánchez-Barriga

Publications and source records attributed to J. Sánchez-Barriga.

At least 19 recordsLinked to original sources

arXiv2025

Nodeless Hybridization as Proof of Trivial Topology in Samarium Hexaboride

Calculations unanimously predict samarium hexaboride to be a topological Kondo insulator, the first topological insulator driven by strong electron correlation. As of today it appears also experimentally established as the only representative of this material class. Here, we investigate the three-dimensional band structure of SmB$_6$ and show that it is incompatible with a topological Kondo insulator which must have hybridization nodes at high-symmetry points. In addition we clarify the remaining questions concerning the nature of the surface states with new data. We address consequences for the search for correlated topological insulators.

cond-mat.str-el↗

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↗

arXiv2024

Ising superconductivity in the bulk incommensurate layered material (PbS)$_{1.13}$(TaS$_2$)

Exploiting the spin-valley degree of freedom of electrons in materials is a promising avenue for energy-efficient information storage and quantum computing. A key challenge in utilizing spin-valley polarization is the realization of spin-valley locking in bulk systems. Here, we report a comprehensive study of the noncentrosymmetric bulk misfit compound (PbS)$_{1.13}$(TaS$_2$), showing a strong spin-valley locking. Our investigation reveals Ising superconductivity with a transition temperature of 3.14 K, closely matching that of a monolayer of TaS$_2$. Notably, the absence of charge density wave (CDW) signatures in transport measurements suggests that the PbS layers primarily act as spacers between the dichalcogenide monolayers. This is further supported by angle-resolved photoemission spectroscopy (ARPES), which shows negligible interlayer coupling, a lack of dispersion along the $k_{\perp}$ direction and significant charge transfer from the PbS to the TaS$_2$ layers. Spin resolved ARPES shows strong spin-valley locking of the electronic bands. Muon spin rotation experiments conducted in the vortex phase reveal an isotropic superconducting gap. However, the temperature dependence of the upper critical field and low-temperature specific heat measurements suggest the possibility of multigap superconductivity. These findings underscore the potential of misfit compounds as robust platforms for both realizing and utilizing spin-valley locking in bulk materials, as well as exploring proximity effects in two-dimensional structures.

cond-mat.supr-con↗

arXiv2021

Mn-rich MnSb2Te4: A topological insulator with magnetic gap closing at high Curie temperatures of 45-50 K

Ferromagnetic topological insulators exhibit the quantum anomalous Hall effect that might be used for high precision metrology and edge channel spintronics. In conjunction with superconductors, they could host chiral Majorana zero modes which are among the contenders for the realization of topological qubits. Recently, it was discovered that the stable 2+ state of Mn enables the formation of intrinsic magnetic topological insulators with A1B2C4 stoichiometry. However, the first representative, MnBi2Te4, is antiferromagnetic with 25 K Néel temperature and strongly n-doped. Here, we show that p-type MnSb2Te4, previously considered topologically trivial, is a ferromagnetic topological insulator in the case of a few percent of Mn excess. It shows (i) a ferromagnetic hysteresis with record high Curie temperature of 45-50 K, (ii) out-of-plane magnetic anisotropy and (iii) a two-dimensional Dirac cone with the Dirac point close to the Fermi level which features (iv) out-of-plane spin polarization as revealed by photoelectron spectroscopy and (v) a magnetically induced band gap that closes at the Curie temperature as demonstrated by scanning tunneling spectroscopy. Moreover, it displays (vi) a critical exponent of magnetization beta~1, indicating the vicinity of a quantum critical point. Ab initio band structure calculations reveal that the slight excess of Mn that substitutionally replaces Sb atoms provides the ferromagnetic interlayer coupling. Remaining deviations from the ferromagnetic order, likely related to this substitution, open the inverted bulk band gap and render MnSb2Te4 a robust topological insulator and new benchmark for magnetic topological insulators.

cond-mat.mtrl-sci↗

arXiv2020

Magnetization relaxation and search for the magnetic gap in bulk-insulating V-doped (Bi, Sb)$_2$Te$_3$

V-doped (Bi,Sb)$_2$Te$_3$ has a ten times higher magnetic coercivity than its Cr-doped counterpart and therefore is believed to be a superior system for the quantum anomalous Hall effect (QAHE). The QAHE requires the opening of a magnetic band gap at the Dirac point. We do not find this gap by angle-resolved photoelectron spectroscopy down to 1 K. By x-ray magnetic circular dichroism (XMCD) we directly probe the magnetism at the V site and in zerofield. Hysteresis curves of the XMCD signal show a strong dependence of the coercivity on the ramping velocity of the magnetic field. The XMCD signal decays on a time scale of minutes which we conclude contributes to the absence of a detectable magnetic gap at the Dirac point.

cond-mat.mtrl-sci↗

arXiv2018

A consistent view of the samarium hexaboride terminations to resolve the nature of its surface states

The research effort prompted by the prediction that SmB$_6$ could be the first topological Kondo insulator has produced a wealth of new results, though not all of these seem compatible. A major discrepancy exists between scanning tunneling microscopy / spectroscopy (STM/S) and angle-resolved photoemission spectroscopy (ARPES), because the two experimental methods suggest a very different number of terminations of the (100) surface with different properties. Here we tackle this issue in a combined STM/S and ARPES study. We find that two of the well-ordered topographies reported in earlier STM studies can be associated with the crystal terminations identified using photoemission. We further observe a reversal of the STM contrast with bias voltage for one of the topographies. We ascribe this result to a different energy dependence of Sm and B-derived states, and show that it can be used to obtain element specific images of SmB$_6$ and identify which topography belongs to which termination. We finally find STS results to support a modification of the low-energy electronic structure at the surface that has been proposed as the trivial origin of surface metallicity in this material.

cond-mat.str-el↗

arXiv2018

Large magnetic gap at the Dirac point in a Mn-induced Bi$_2$Te$_3$ heterostructure

Magnetically doped topological insulators enable the quantum anomalous Hall effect (QAHE) which provides quantized edge states for lossless charge transport applications. The edge states are hosted by a magnetic energy gap at the Dirac point but all attempts to observe it directly have been unsuccessful. The gap size is considered crucial to overcoming the present limitations of the QAHE, which so far occurs only at temperatures one to two orders of magnitude below its principle limit set by the ferromagnetic Curie temperature $T_C$. Here, we use low temperature photoelectron spectroscopy to unambiguously reveal the magnetic gap of Mn-doped Bi$_2$Te$_3$ films, which is present only below $T_C$. Surprisingly, the gap turns out to be $\sim$90 meV wide, which not only exceeds $k_BT$ at room temperature but is also 5 times larger than predicted by density functional theory. By an exhaustive multiscale structure characterization we show that this enhancement is due to a remarkable structure modification induced by Mn doping. Instead of a disordered impurity system, it forms an alternating sequence of septuple and quintuple layer blocks, where Mn is predominantly incorporated in the septuple layers. This self-organized heterostructure substantially enhances the wave-function overlap and the size of the magnetic gap at the Dirac point, as recently predicted. Mn-doped Bi$_2$Se$_3$ forms a similar heterostructure, however, only a large, nonmagnetic gap is formed. We explain both differences based on the higher spin-orbit interaction in Bi$_2$Te$_3$ with the most important consequence of a magnetic anisotropy perpendicular to the films, whereas for Bi$_2$Se$_3$ the spin-orbit interaction it is too weak to overcome the dipole-dipole interaction. Our findings provide crucial insights for pushing the lossless transport properties of topological insulators towards room-temperature applications.

cond-mat.mtrl-sci↗

arXiv2018

Impact of ultrafast transport on the high-energy states of a photoexcited topological insulator

Ultrafast dynamics in three-dimensional topological insulators (TIs) opens new routes for increasing the speed of information transport up to frequencies thousand times faster than in modern electronics. However, up to date, disentangling the exact contributions from bulk and surface transport to the subpicosecond dynamics of these materials remains a difficult challenge. Here, using time- and angle-resolved photoemission, we demonstrate that driving a TI from the bulk-conducting into the bulk-insulating transport regime allows to selectively switch on and off the emergent channels of ultrafast transport between the surface and the bulk. We thus establish that ultrafast transport is one of the main driving mechanisms responsible for the decay of excited electrons in prototypical TIs following laser excitation. We further show how ultrafast transport strongly affects the thermalization and scattering dynamics of the excited states up to high energies above the Fermi level. In particular, we observe how inhibiting the transport channels leads to a thermalization bottleneck that substantially slows down electron-hole recombination via electron-electron scatterings. Our results pave the way for exploiting ultrafast transport to control thermalization time scales in TI-based optoelectronic applications, and expand the capabilities of TIs as intrinsic solar cells.

cond-mat.mes-hall↗

arXiv2018

Band renormalization of blue phosphorus on Au(111)

Most recently, theoretical calculations predicted the stability of a novel two-dimensional phosphorus honeycomb lattice named blue phosphorus. Here, we report on the growth of blue phosphorus on Au(111) and unravel its structural details using diffraction, microscopy and theoretical calculations. Most importantly, by utilizing angle-resolved photoemission spectroscopy we identify its momentum-resolved electronic structure. We find that Au(111) breaks the sublattice symmetry of blue phosphorus leading to an orbital-dependent band renormalization upon the formation of a (4x4) superstructure. Notably, the semiconducting two-dimensional phosphorus realizes its valence band maximum at 0.9 eV binding energy, however, shifted in momentum space due to the substrate-induced band renormalization.

cond-mat.mes-hall↗

arXiv2018

Anomalous behavior of the electronic structure of (Bi$_{1-x}$In$_x$)$_2$Se$_3$ across the quantum-phase transition from topological to trivial insulator

Using spin- and angle-resolved spectroscopy and relativistic many-body calculations, we investigate the evolution of the electronic structure of (Bi$_{1-x}$In$_x$)$_2$Se$_3$ bulk single crystals around the critical point of the trivial to topological insulator quantum-phase transition. By increasing $x$, we observe how a surface gap opens at the Dirac point of the initially gapless topological surface state of Bi$_2$Se$_3$, leading to the existence of massive fermions. The surface gap monotonically increases for a wide range of $x$ values across the topological and trivial sides of the quantum-phase transition. By means of photon-energy dependent measurements, we demonstrate that the gapped surface state survives the inversion of the bulk bands which occurs at a critical point near $x=0.055$. The surface state exhibits a non-zero in-plane spin polarization which decays exponentially with increasing $x$, and that persists on both the topological and trivial insulator phases. Its out-of-plane spin polarization remains zero demonstrating the absence of a hedgehog spin texture expected from broken time-reversal symmetry. Our calculations reveal qualitative agreement with the experimental results all across the quantum-phase transition upon the systematic variation of the spin-orbit coupling strength. A non-time reversal symmetry breaking mechanism of bulk-mediated scattering processes that increase with decreasing spin-orbit coupling strength is proposed as explanation.

cond-mat.mes-hall↗

arXiv2018

Strong spin dependence of correlation effects in Ni due to Stoner excitations

Using high-resolution angle-resolved photoemission, we observe a strong spin-dependent renormalization and lifetime broadening of the quasiparticle excitations in the electronic band structure of Ni(111) in an energy window of $\sim$0.3 eV below the Fermi level. We derive a quantitative result for the spin-dependent lifetime broadening by comparing the scattering rates of majority and minority $d$ states, and further show that spin-dependent electron correlations are instead negligible for $sp$ states. From our analysis we experimentally determine the effective on-site Coulomb interaction $U$ caused by Stoner-like interband transitions between majority and minority $d$ states. The present results unambiguously demonstrate the remarkable impact of spin-dependent electron correlation effects originating from single-particle excitations in a prototypical 3$d$ transition metal, paving the way for further refinement of current many-body theoretical approaches.

cond-mat.str-el↗

arXiv2017

Negative longitudinal magnetoresistance from anomalous N=0 Landau level in topological materials

Negative longitudinal magnetoresistance (NLMR) is shown to occur in topological materials in the extreme quantum limit, when a magnetic field is applied parallel to the excitation current. We perform pulsed and DC field measurements on Pb1-xSnxSe epilayers where the topological state can be chemically tuned. The NLMR is observed in the topological state, but is suppressed and becomes positive when the system becomes trivial. In a topological material, the lowest N=0 conduction Landau level disperses down in energy as a function of increasing magnetic field, while the N=0 valence Landau level disperses upwards. This anomalous behavior is shown to be responsible for the observed NLMR. Our work provides an explanation of the outstanding question of NLMR in topological insulators and establishes this effect as a possible hallmark of bulk conduction in topological matter.

cond-mat.mes-hall↗

arXiv2017

Suppression of electron scattering resonances in graphene by quantum dots

Transmission of low-energetic electrons through two-dimensional materials leads to unique scattering resonances. These resonances contribute to photoemission from occupied bands where they appear as strongly dispersive features of suppressed photoelectron intensity. Using angle-resolved photoemission we have systematically studied scattering resonances in epitaxial graphene grown on the chemically differing substrates Ir(111), Bi/Ir, Ni(111) as well as in graphene/Ir(111) nanopatterned with a superlattice of uniform Ir quantum dots. While the strength of the chemical interaction with the substrate has almost no effect on the dispersion of the scattering resonances, their energy can be controlled by the magnitude of charge transfer from/to graphene. At the same time, a superlattice of small quantum dots deposited on graphene eliminates the resonances completely. We ascribe this effect to a nanodot-induced buckling of graphene and its local rehybridization from sp$^{2}$ to sp$^{3}$ towards a three-dimensional structure. Our results suggest nanopatterning as a prospective tool for tuning optoelectronic properties of two-dimensional materials with graphene-like structure.

cond-mat.mtrl-sci↗

arXiv2017

Laser-induced persistent photovoltage on the surface of a ternary topological insulator at room temperature

Using time- and angle-resolved photoemission, we investigate the ultrafast response of excited electrons in the ternary topological insulator (Bi$_{1 x}$Sb$_{x}$)$_2$Te$_3$ to fs-infrared pulses. We demonstrate that at the critical concentration $x$=0.55, where the system becomes bulk insulating, a surface voltage can be driven at room temperature through the topological surface state solely by optical means. We further show that such a photovoltage persists over a time scale that exceeds $\sim$6 $μ$s, i.e, much longer than the characteristic relaxation times of bulk states. We attribute the origin of the photovoltage to a laser-induced band-bending effect which emerges near the surface region on ultrafast time scales. The photovoltage is also accompanied by a remarkable increase in the relaxation times of excited states as compared to undoped topological insulators. Our findings are relevant in the context of applications of topological surface states in future optical devices.

cond-mat.mes-hall↗

arXiv2017

Sub-picosecond spin dynamics of excited states in the topological insulator Bi$_2$Te$_3$

Using time-, spin- and angle-resolved photoemission, we investigate the ultrafast spin dynamics of hot electrons on the surface of the topological insulator Bi$_2$Te$_3$ following optical excitation by fs-infrared pulses. We observe two surface-resonance states above the Fermi level coexisting with a transient population of Dirac fermions that relax in about $\sim$2 ps. One state is located below $\sim$0.4 eV just above the bulk continuum, the other one at $\sim$0.8 eV inside a projected bulk band gap. At the onset of the excitation, both states exhibit a reversed spin texture with respect to that of the transient Dirac bands, in agreement with our one-step photoemission calculations. Our data reveal that the high-energy state undergoes spin relaxation within $\sim$0.5 ps, a process that triggers the subsequent spin dynamics of both the Dirac cone and the low-energy state, which behave as two dynamically-locked electron populations. We discuss the origin of this behavior by comparing the relaxation times observed for electrons with opposite spins to the ones obtained from a microscopic Boltzmann model of ultrafast band cooling introduced into the photoemission calculations. Our results demonstrate that the nonequilibrium surface dynamics is governed by electron-electron rather than electron-phonon scattering, with a characteristic time scale unambiguously determined by the complex spin texture of excited states above the Fermi level. Our findings reveal the critical importance of detecting momentum and energy-resolved spin textures with fs resolution to fully understand the sub-ps dynamics of transient electrons on the surface of topological insulators.

cond-mat.mes-hall↗

arXiv2016

Observation of antiphase coherent phonons in the warped Dirac cone of Bi$_2$Te$_3$

In this Rapid Communication we investigate the coupling between excited electrons and phonons in the highly anisotropic electronic structure of the prototypical topological insulator Bi$_2$Te$_3$. Using time- and angle-resolved photoemission spectroscopy we are able to identify the emergence and ultrafast temporal evolution of the longitudinal-optical A$_{1g}$ coherent-phonon mode in Bi$_2$Te$_3$. We observe an antiphase behavior in the onset of the coherent-phonon oscillations between the $\overline{ΓK}$ and the $\overline{ΓM}$ high-symmetry directions that is consistent with warping. The qualitative agreement between our density-functional theory calculations and the experimental results reveals the critical role of the anisotropic coupling between Dirac fermions and phonon modes in the topological insulator Bi$_2$Te$_3$.

cond-mat.mes-hall↗

arXiv2016

Surface Fermi arc connectivity in the type-II Weyl semimetal candidate WTe$_{2}$

We perform ultrahigh resolution angle-resolved photoemission experiments at a temperature T=0.8 K on the type-II Weyl semimetal candidate WTe$_{2}$. We find a surface Fermi arc connecting the bulk electron and hole pockets on the (001) surface. Our results show that the surface Fermi arc connectivity to the bulk bands is strongly mediated by distinct surface resonances dispersing near the border of the surface-projected bulk band gap. By comparing the experimental results to first-principles calculations we argue that the coupling to these surface resonances, which are topologically trivial, is compatible with the classification of WTe$_{2}$ as a type-II Weyl semimetal hosting topological Fermi arcs. We further support our conclusion by a systematic characterization of the bulk and surface character of the different bands and discuss the similarity of our findings to the case of topological insulators.

cond-mat.mtrl-sci↗

arXiv2016

Nonmagnetic band gap at the Dirac point of the magnetic topological insulator (Bi$_{1-x}$Mn$_x)_2$Se$_3$

Magnetic doping is expected to open a band gap at the Dirac point of topological insulators by breaking time-reversal symmetry and to enable novel topological phases. Epitaxial (Bi$_{1-x}$Mn$_{x}$)$_{2}$Se$_{3}$ is a prototypical magnetic topological insulator with a pronounced surface band gap of $\sim100$ meV. We show that this gap is neither due to ferromagnetic order in the bulk or at the surface nor to the local magnetic moment of the Mn, making the system unsuitable for realizing the novel phases. We further show that Mn doping does not affect the inverted bulk band gap and the system remains topologically nontrivial. We suggest that strong resonant scattering processes cause the gap at the Dirac point and support this by the observation of in-gap states using resonant photoemission. Our findings establish a novel mechanism for gap opening in topological surface states which challenges the currently known conditions for topological protection.

cond-mat.mes-hall↗