arXiv subjects

J. Krempasky

Publications and source records attributed to J. Krempasky.

10 recordsLinked to original sources

arXiv2025

Floquet topological state induced by light-driven band inversion in SnTe

High intensity coherent light can dress matter, realizing new hybrid phases that are not accessible in equilibrium. This effect results from the coherent interaction between Bloch states inside the solid and the periodic field of impinging photons which produces hybrid light-matter states called Floquet-Bloch states that can alter properties of the solid. Optically inducing a topological state in a semiconductor using so-called Floquet engineering is an exciting prospect. However, it has not been realized, despite its theoretical prediction more than 10 years ago. Here we show that an ultrashort-lived topological state that is absent at equilibrium in the ground state of SnTe can be created with femtosecond light pulses. This occurs when the photoexcitation is similar in energy with the band gap of this polar semiconductor. We observe a concomitant renormalization of the band dispersions that reveals the generation of Floquet states connecting to the topological state. We therefore provide the first direct experimental observation of a Floquet topological state and propose that it is driven by a light-induced band inversion in SnTe. Our discovery opens the way for controlling optically on-demand the topological properties of semiconductors.

cond-mat.str-el↗

arXiv2024

Altermagnetism imaged and controlled down to the nanoscale

Nanoscale detection and control of the magnetic order underpins a broad spectrum of fundamental research and practical device applications. The key principle involved is the breaking of time-reversal ($\cal{T}$) symmetry, which in ferromagnets is generated by an internal magnetization. However, the presence of a net-magnetization also imposes severe limitations on compatibility with other prominent phases ranging from superconductors to topological insulators, as well as on spintronic device scalability. Recently, altermagnetism has been proposed as a solution to this restriction, since it shares the enabling $\cal{T}$-symmetry breaking characteristic of ferromagnetism, combined with the antiferromagnetic-like vanishing net-magnetization. To date, altermagnetic ordering has been inferred from spatially averaged probes. Here, we demonstrate nanoscale imaging and control of altermagnetic ordering ranging from nanoscale vortices to domain walls to microscale single-domain states in MnTe. We combine the $\cal{T}$-symmetry breaking sensitivity of X-ray magnetic circular dichroism with magnetic linear dichroism and photoemission electron microscopy, to achieve detailed imaging of the local altermagnetic ordering vector. A rich variety of spin configurations can be imposed using microstructure patterning or thermal cycling in magnetic fields. The demonstrated detection and control of altermagnetism paves the way for future research ranging from ultra-scalable digital and neuromorphic spintronic devices, to the interplay of altermagnetism with non-dissipative superconducting or topological phases.

cond-mat.mtrl-sci↗

arXiv2017

Spin-resolved electronic structure of ferroelectric α-GeTe and multiferroic Ge1-xMnxTe

Germanium telluride features special spin-electric effects originating from spin-orbit coupling and symmetry breaking by the ferroelectric lattice polarization, which opens up many prospectives for electrically tunable and switchable spin electronic devices. By Mn doping of the α-GeTe host lattice, the system becomes a multiferroic semiconductor possessing magnetoelectric properties in which the electric polarization, magnetization and spin texture are coupled to each other. Employing spin- and angle-resolved photoemission spectroscopy in bulk- and surface-sensitive energy ranges and by varying dipole transition matrix elements, we disentangle the bulk, surface and surface-resonance states of the electronic structure and determine the spin textures for selected parameters. From our results, we derive a comprehensive model of the α-GeTe surface electronic structure which fits experimental data and first principle theoretical predictions and we discuss the unconventional evolution of the Rashba-type spin splitting upon manipulation by external B- and E-fields.

cond-mat.mtrl-sci↗

arXiv2016

Entanglement and manipulation of the magnetic and spin-orbit order in multiferroic Rashba semiconductors

The interplay between electronic eigenstates, spin, and orbital degrees of freedom, combined with fundamental breaking of symmetries is currently one of the most exciting fields of research. Multiferroics such as (GeMn)Te fulfill these requirements providing unusual physical properties due to the coexistence and coupling between ferromagnetic and ferroelectric order in one and the same system. Here we show that multiferroic (GeMn)Te inherits from its parent ferroelectric α-GeTe compound a giant Rashba splitting of three-dimensional bulk states which competes with the Zeeman spin splitting induced by the magnetic exchange interactions. The collinear alignment of ferroelectric and ferromagnetic polarization leads to an opening of a tunable Zeeman gap of up to 100 meV around the Dirac point of the Rashba bands, coupled with a change in spin texture by entanglement of magnetic and spin-orbit order. Through applications of magnetic fields, we demonstrate manipulation of spin- texture by spin resolved photoemission experiments, which is also expected for electric fields based on the multiferroic coupling. The control of spin helicity of the bands and its locking to ferromagnetic and ferroelectric order opens fascinating new avenues for highly multifunctional multiferroic Rashba devices suited for reprogrammable logic and/or nonvolatile memory applications.

cond-mat.mtrl-sci↗

arXiv2014

Mixed Dimensionality of Confined Conducting Electrons in the Surface Region of SrTiO$_{3}$

Using angle-resolved photoemission spectroscopy, we show that the recently-discovered surface state on SrTiO$_{3}$ consists of non-degenerate $t_{2g}$ states with different dimensional characters. While the $d_{xy}$ bands have quasi-2D dispersions with weak $k_{z}$ dependence, the lifted $d_{xz}$/$d_{yz}$ bands show 3D dispersions that differ significantly from bulk expectations and signal that electrons associated with those orbitals permeate the near-surface region. Like their more 2D counterparts, the size and character of the $d_{xz}$/$d_{yz}$ Fermi surface components are essentially the same for different sample preparations. Irradiating SrTiO$_{3}$ in ultrahigh vacuum is one method observed so far to induce the "universal" surface metallic state. We reveal that during this process, changes in the oxygen valence band spectral weight that coincide with the emergence of surface conductivity are disproportionate to any change in the total intensity of the O $1s$ core level spectrum. This signifies that the formation of the metallic surface goes beyond a straightforward chemical doping scenario and occurs in conjunction with profound changes in the initial states and/or spatial distribution of near-$E_{F}$ electrons in the surface region.

cond-mat.mtrl-sci↗

arXiv2009

High-resolution soft-X-ray beamline ADRESS at Swiss Light Source for resonant inelastic X-ray scattering and angle-resolved photoelectron spectroscopies

We describe the concepts and technical realization of the high-resolution soft-X-ray beamline ADRESS operating in the energy range from 300 to 1600 eV and intended for Resonant Inelastic X-ray Scattering (RIXS) and Angle-Resolved Photoelectron Spectroscopy (ARPES). The photon source is an undulator of novel fixed-gap design where longitudinal movement of permanent magnetic arrays controls not only the light polarization (including circular and 0-180 deg rotatable linear polarizations) but also the energy without changing the gap. The beamline optics is based on the well-established scheme of plane grating monochromator (PGM) operating in collimated light. The ultimate resolving power E/dE is above 33000 at 1 keV photon energy. The choice of blazed vs lamellar gratings and optimization of their profile parameters is described. Due to glancing angles on the mirrors as well as optimized groove densities and profiles of the gratings, high photon flux is achieved up to 1.0e13 photons/s/0.01%BW at 1 keV. Ellipsoidal refocusing optics used for the RIXS endstation demagnifies the vertical spot size down to 4 um, which allows slitless operation and thus maximal transmission of the high-resolution RIXS spectrometer delivering E/dE better than 11000 at 1 keV photon energy. Apart from the beamline optics, we give an overview of the control system, describe diagnostics and software tools, and discuss strategies used for the optical alignment. An introduction to the concepts and instrumental realization of the ARPES and RIXS endstations is given.

physics.ins-det↗

arXiv2008

The electronic structure of La$_{1-x}$Sr$_{x}$MnO$_{3}$ thin films and its $T_c$ dependence as studied by angle-resolved photoemission

We present angle-resolved photoemission spectroscopy results for thin films of the three-dimensional manganese perovskite La$_{1-x}$Sr$_{x}$MnO$_{3}$. We show that the transition temperature ($T_c$) from the paramagnetic insulating to ferromagnetic metallic state is closely related to details of the electronic structure, particularly to the spectral weight at the ${\bf k}$-point, where the sharpest step at the Fermi level was observed. We found that this ${\bf k}$-point is the same for all the samples, despite their different $T_c$. The change of $T_c$ is discussed in terms of kinetic energy optimization. Our ARPES results suggest that the change of the electronic structure for the samples having different transition temperatures is different from the rigid band shift.

cond-mat.str-el↗

arXiv2005

Parity of the Pairing Bosons in a High-Temperature Superconductor

We report the observation of a novel effect in the bilayer Pb-Bi2212 high-TC superconductor by means of angle-resolved photoemission with circularly polarized excitation. Different scattering rates, determined as a function of energy separately for the bonding and antibonding copper-oxygen bands, strongly imply that the dominating scattering channel is odd with respect to layer exchange within a bilayer. This is inconsistent with a phonon-mediated scattering and favours the participation of the odd collective spin excitations in the scattering mechanism in near-nodal regions of the k-space, suggesting a magnetic nature of the pairing mediator.

cond-mat.supr-con↗

arXiv2005

Polarization-dependent ARPES study of La1-xSrxMnO3

We present angle-resolved photoemission spectroscopy results on thin films of the three-dimensional manganite perovskite La1-xSrxMnO3, with TC = 240 K. In the low-binding energy region, we found two energy bands, one dispersing along the (100) sample surface and the other along the normal to the surface. Through a detailed polarization-dependent analysis, we have determined the symmetries of the two bands. The initial states associated with the band that disperses along the surface are dominantly odd with respect to the (110) mirror plane and even with respect to the (010) mirror plane. Based upon this investigation, we have derived the expression of the spatial term for the wave functions describing these low-energy states.

cond-mat.str-el↗

arXiv2001

The SLS Beamlines Data Acquisition and Control System

On December 15th the Swiss Light Source (SLS) produced a stored beam for the first time. This important milestone was achieved in a very tight time schedule. The fact that all major systems are controlled by Epics made this challenge feasible. In the first phase there are four beamlines: two for the surface science community, one for powder and surface diffraction and computed micro-tomography, and the last one for protein crystallography. All of them are equipped with insertion devices, which users want to treat as active sub-systems like a monochromator or experimental station. The beamline control systems are based on the same hardware and software technology as is the machine. This implies extensive use of Personal Computers running Linux RedHat 6.2 and VME systems (PowerPC). The advantage of this choice is a staightforward implementation of the insertion devices into the beamline and experiment framework. Although the experiment Application Program Interfaces differ from beamline to beamline, the standard software technology for linking all sub-systems is based on the Epics toolkit and Cdev. The diagnostic tools provided by this toolkit are being extensively used during the beamline commissioning. Finallly we account on some examples of integrating dedicated 3rd party and commercial non Epics software products for experiment control into the beamline control system. Key elements in this domain are CORBA, Python and Portable Channel Access Server.

physics.acc-ph↗