arXiv subjects

Y. Lytvynenko

Publications and source records attributed to Y. Lytvynenko.

8 recordsLinked to original sources

arXiv2025

Orbital magnetization in the Nb-substituted Kagome metal CsV$_3$Sb$_5$

This study uses angle-resolved photoemission spectroscopy to examine the low-temperature electronic structure of Cs(V$_{0.95}$Nb$_{0.05}$)$_3$Sb$_5$, demonstrating that partially substituting V atoms with isoelectronic Nb atoms results in \blue{an increase of the band width} and enhanced gap opening at the Dirac-like crossings due to the resulting chemical pressure. This increases the magnetic circular dichroism signal in the angular distribution (MCDAD) compared to CsV$_3$Sb$_5$, enabling detailed analysis of magnetic circular dichroism in several bands near the Fermi level. These results \blue{substantiate} the predicted coupling of orbital magnetic moments to three van Hove singularities near the Fermi level at M points. Previous studies have observed that Nb doping \blue{lowers the charge density transition temperature} and increases the critical temperature for superconductivity. This article demonstrates that Nb doping concomitantly increases the magnetic circular dichroism signal attributed to orbital moments.

cond-mat.mtrl-sci↗

arXiv2024

The electronic structure of EuPd$_2$Si$_2$ in the vicinity of the critical endpoint

Hard X-ray angle-resolved photoemission spectroscopy reveals significant alterations in the valence band states of EuPd$_2$Si$_2$ at a temperature $T_V$, where the Eu ions undergo a temperature-induced valence crossover from a magnetic Eu$^{2+}$ state to a low-temperature valence-fluctuating state. The introduction of small amounts of Au on Pd lattice sites and Ge on Si sites, respectively, results in a decrease in $T_V$ and the emergence of an antiferromagnetic state at low temperatures without valence fluctuations. It has been proposed that the boundary between AFM order and valence crossover represents a first-order phase transition associated with a specific type of second-order critical end point. In this scenario, strong coupling effects between fluctuating charge, spin, and lattice degrees of freedom are to be expected. In the case of EuPd$_2$(Si$_{1-x}$Ge$_x$)$_2$ with x=0.13, which is situated close to the critical end point, a splitting of conduction band states and the emergence of flat bands with one-dimensional character have been observed. A comparison with ab initio theory demonstrates a high degree of correlation with experimental findings, particularly in regard to the bands situated in proximity to the critical end point.

cond-mat.str-el↗

arXiv2024

Chirality in the Kagome Metal CsV$_3$Sb$_5$

Using x-ray photoelectron diffraction (XPD) and angle-resolved photoemission spectroscopy, we study photoemission intensity changes related to changes in the geometric and electronic structure in the kagome metal CsV$_3$Sb$_5$ upon transition to an unconventional charge density wave (CDW) state. The XPD patterns reveal the presence of a chiral atomic structure in the CDW phase. Furthermore, using circularly polarized x-rays, we have found a pronounced non-trivial circular dichroism in the angular distribution of the valence band photoemission in the CDW phase, indicating a chirality of the electronic structure. This observation is consistent with the proposed orbital loop current order. In view of a negligible spontaneous Kerr signal in recent magneto-optical studies, the results suggest an antiferromagnetic coupling of the orbital magnetic moments along the $c$-axis. While the inherent structural chirality may also induce circular dichroism, the observed asymmetry values seem to be too large in the case of the weak structural distortions caused by the CDW.

cond-mat.str-el↗

arXiv2024

Multimode Objective Lens for Momentum Microscopy and XPEEM: Experiments

A new type of objective lens has recently been proposed for use in X-ray photoemission electron microscopes (XPEEMs) and momentum microscopes. Adding a ring electrode concentric with the extractor allows the field in the gap between the sample and the extractor to be shaped. Forming a lens field in this gap reduces the field strength at the sample by up to an order of magnitude. This mitigates the risk of field emission, particularly for cleaved samples with sharp edges. A retarding field can redirect all slow electrons, thus eliminating the primary contribution to the space-charge interaction. Here we present the first experimental investigation of the new lens, examining its performance at photon energies ranging from the extreme ultraviolet produced by a high-harmonic generation (HHG)-based source to soft and hard X-rays at two synchrotron facilities. The gap lens in a region without electrodes enables large working distances up to 23 mm. Reduced aberrations allow for larger fields of view in both k-space and real-space imaging, with resolutions comparable to those of conventional cathode lenses. However, field strengths are an order of magnitude smaller. The zero-field mode enables the study of 3D structured objects and is therefore beneficial for small cleaved samples as well as for operando devices involving top electrodes. The repeller mode reduces space-charge effects, but results in a smaller k-field diameter. This reduction ranges from 10% at hard X-ray energies to 50% in the XUV range. The usable energy interval is also reduced by a factor of two. In time-of-flight XPEEM mode the raw data show a resolution of 250 nm, which can be improved to better than 100 nm through data processing.

cond-mat.mtrl-sci↗

arXiv2023

Doping-Induced Electronic and Structural Phase Transition in the Bulk Weyl Semimetal Mo1-xWxTe2

A comprehensive study of the electronic and structural phase transition from 1T` to Td in the bulk Weyl semimetal Mo1-xWxTe2 at different doping concentrations has been carried out using time-of-flight momentum microscopy (including circular and linear dichroism), X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD), X-ray diffraction (XRD), angle-resolved Raman spectroscopy, transport measurements, density functional theory (DFT) and Kikuchi pattern calculations. High-resolution angle-resolved photoemission spectroscopy (ARPES) at 20 K reveals surface electronic states, which are indicative of topological Fermi arcs. Their dispersion agrees with the position of Weyl points predicted by DFT calculations based on the experimental crystal structure of our samples determined by XRD. Raman spectroscopy confirms the inversion symmetry breaking for the Td -phase, which is a necessary condition for the emergence of topological states. Transport measurements show that increasing the doping concentration from 2 to 9% leads to an increase in the temperature of the phase transition from 1T` to Td from 230 K to 270 K. Magnetoresistance and longitudinal elastoresistance show significantly increased values in the Td -phase due to stimulated inter-pocket electron backscattering. The results demonstrate the close relationship between electronic properties and elastic deformations in MoTe2.

cond-mat.mtrl-sci↗

arXiv2023

Valence-transition-induced changes of the electronic structure in EuPd$_2$Si$_2$

We present results of hard X-ray angle-resolved photoemission spectroscopy and photoemission diffraction measurements performed on high-quality single crystals of the valence transition compound EuPd$_2$Si$_2$ for temperatures 25~K $\leq$ T $\leq$ 300~K. At low temperatures we observe a Eu $4f$ valence $v=2.5$, % occupation number $n=6.5$, which decreases to $v=2.1$ for temperatures above the valence transition around $T_V \approx 160$~K. The experimental valence numbers resulting from an evaluation of the Eu(III)/Eu(II) $3d$ core levels, are used for calculating band structures using density functional theory. The valence transition significantly changes the band structure as determined by angle-resolved photoemission spectroscopy. In particular, the Eu $5d$ valence bands are shifted to lower binding energies with increasing Eu $4f$ occupancy. To a lesser extent, bands derived from the Si $3p$ and Pd $4d$ orbitals are also affected. This observation suggests a partial charge transfer between Eu and Pd/Si sites. Comparison with {\it ab-initio} theory shows a good agreement with experiment, in particular concerning the unequal band shift with increasing Eu $4f$ occupancy.

cond-mat.str-el↗

arXiv2023

Control of the asymmetric band structure in Mn2Au by a ferromagnetic driver layer

Hard X-ray angle-resolved photoemission spectroscopy reveals the momentum-resolved band structure in an epitaxial Mn2Au(001) film capped by a 2 nm thick ferromagnetic Permalloy layer. By magnetizing the Permalloy capping layer, the exceptionally strong exchange bias aligns the Neel vector in the Mn2Au(001) film accordingly. Uncompensated interface Mn magnetic moments in Mn2Au were identified as the origin of the exchange bias using X-ray magnetic circular dichroism in combination with photoelectron emission microscopy. Using time-of-flight momentum microscopy, we measure the asymmetry of the band structure in Mn2Au resulting from the homogeneous orientation of the Neel vector. Comparison with theory shows that the Neel vector, determined by the magnetic moment of the top Mn layer, is antiparallel to the Permalloy magnetization. The experimental results demonstrate that hard X-ray photoemission spectroscopy can measure the band structure of epitaxial layers beneath a metallic capping layer and corroborate the asymmetric band structure in Mn2Au that was previously inferred only indirectly.

cond-mat.mtrl-sci↗

arXiv2023

Asymmetric Electrostatic Dodecapole: Compact Bandpass Filter with Low Aberrations for Momentum Microscopy

Imaging energy filters in photoelectron microscopes and momentum microscopes use spherical fields with deflection angles of 90{\deg}, 180{\deg}, and even 2 x 180{\deg}. These instruments are optimized for high energy resolution, but exhibit image aberrations when operated in high transmission mode at medium energy resolution. Here we present a new approach for bandpass-filtered imaging in real or reciprocal space using an electrostatic dodecapole with an asymmetric electrode array. In addition to energy-dispersive beam deflection, this multipole allows aberration correction up to the 3rd order. Here we describe its use as a bandpass prefilter in a time-of-flight momentum microscope at the hard X-ray beamline P22 of PETRA III. The entire instrument is housed in a straight vacuum tube because the deflection angle is only 4{\deg} and the beam displacement in the filter is only 8 mm. The multipole is framed by transfer lenses in the entrance and exit branches. Two sets of 16 different sized entrance and exit apertures on piezomotor driven mounts allow selection of the desired bandpass. For pass energies between 100 and 1400 eV and slit widths between 0.5 and 4 mm the transmitted kinetic energy intervals are between 10 eV and a few hundred eV (FWHM). The filter eliminates all higher or lower energy signals outside the selected bandpass, significantly improving the signal-to-background ratio in the ToF analyzer.

physics.ins-det↗