arXiv subjects

Ján Minár

Publications and source records attributed to Ján Minár.

At least 37 records · Page 2Linked to original sources

arXiv2025

Reexamining Circular Dichroism in Photoemission From a Topological Insulator

The orbital angular momentum (OAM) of electron states is an essential ingredient for topological and quantum geometric quantities in solids. For example, Dirac surface states with helical spin- and orbital-angular momenta are a hallmark of a 3D topological insulator. Angle-resolved photoemission spectroscopy (ARPES) with variable circular light polarization, known as circular dichroism (CD), has been assumed to be a direct probe of OAM and, by proxy, of the Berry curvature of electronic bands in energy- and momentum-space. Indeed, topological surface states have been shown to exhibit angle-dependent CD (CDAD), and more broadly, CD is often interpreted as evidence of spin-orbit coupling. Meanwhile, it is well-established that CD originates from the photoemission matrix elements, which can have extrinsic contributions related to the experimental geometry and the inherently broken inversion symmetry at the sample surface. Therefore, it is important to broadly examine CD-ARPES to determine the scenarios in which it provides a robust probe of intrinsic material physics. We performed CD-ARPES on the canonical topological insulator $\mathrm{Bi}_2\mathrm{Se}_3$ over a wide range of incident photon energies. Not only do we observe angle-dependent CD in the surface states, as expected, but we also find CD of a similar magnitude in virtually all bulk bands. Since OAM is forbidden by inversion symmetry in the bulk, we conclude this originates from symmetry-breaking in the photoemission process. Comparison with theoretical calculations supports this view and suggests that $\textit{hidden}$ OAM - localized to atomic sites within each unit cell - contributes significantly. Additional effects, including inter-atomic interference and final-state resonances, are responsible for the rapid variation of the CDAD signal with photon energy.

cond-mat.mtrl-sci↗

arXiv2024

Artical Functionalization of structural and electronic properties of multiferroic SrTiO3 thin films

This study examines the application of transition metal-doped SrTiO3 in photovoltaic technologies, such as photocatalysis. The core objective is to evaluate how different dopants influence the structural and electronic characteristics of the well-known perovskite, SrTiO3 (STO). By incorporating dopants, particularly transition metals, the material's physical properties can be enhanced by addressing limitations such as the large gap in the valence band. This study aims to determine the impact of these metals on factors like crystallite size, internal stress levels, electron-hole pair distribution in the valence band, and the shift in the electromagnetic spectrum toward the visible range. The primary focus is on assessing nickel's (Ni) influence on these properties, with additional investigation into the effects of yttrium (Y) and iron (Fe). Several experimental methods were employed to analyze the structural and electronic properties of SrTiO3. The same procedures were applied consistently across all samples, with the sole exception being the high-temperature XRD experiments, as described in the text. The techniques used include magnetron pulse deposition for sample preparation, followed by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). After conducting the experiments, the collected data were evaluated and used to guide subsequent steps. Finally, all data were consolidated and analyzed comprehensively.

cond-mat.mtrl-sci↗

arXiv2024

Persistent flat band splitting and strong selective band renormalization in a kagome magnet thin film

Magnetic kagome materials provide a fascinating playground for exploring the interplay of magnetism, correlation and topology. Many magnetic kagome systems have been reported including the binary FemXn (X=Sn, Ge; m:n = 3:1, 3:2, 1:1) family and the rare earth RMn6Sn6 (R = rare earth) family, where their kagome flat bands are calculated to be near the Fermi level in the paramagnetic phase. While partially filling a kagome flat band is predicted to give rise to a Stoner-type ferromagnetism, experimental visualization of the magnetic splitting across the ordering temperature has not been reported for any of these systems due to the high ordering temperatures, hence leaving the nature of magnetism in kagome magnets an open question. Here, we probe the electronic structure with angle-resolved photoemission spectroscopy in a kagome magnet thin film FeSn synthesized using molecular beam epitaxy. We identify the exchange-split kagome flat bands, whose splitting persists above the magnetic ordering temperature, indicative of a local moment picture. Such local moments in the presence of the topological flat band are consistent with the compact molecular orbitals predicted in theory. We further observe a large spin-orbital selective band renormalization in the Fe d_xy+d_(x^2-y^2 ) spin majority channel reminiscent of the orbital selective correlation effects in the iron-based superconductors. Our discovery of the coexistence of local moments with topological flat bands in a kagome system echoes similar findings in magic-angle twisted bilayer graphene, and provides a basis for theoretical effort towards modeling correlation effects in magnetic flat band systems.

cond-mat.mes-hall↗

arXiv2024

Anionic disorder and its impact on the surface electronic structure of oxynitride photoactive semiconductors

The conversion of solar energy into chemical energy, stored in the form of hydrogen, bears enormous potential as a sustainable fuel for powering emerging technologies. Photoactive oxynitrides are promising materials for splitting water into molecular oxygen and hydrogen. However, one of the issues limiting widespread commercial use of oxynitrides is the degradation during operation. While recent studies have shown the loss of nitrogen, its relation to the reduced efficiency has not been directly and systematically addressed with experiments. In this study, we demonstrate the impact of the anionic stoichiometry of BaTaO$_x$N$_y$ on its electronic structure and functional properties. Through experimental ion scattering, electron microscopy, and photoelectron spectroscopy investigations, we determine the anionic composition ranging from the bulk towards the surface of BaTaO$_x$N$_y$ thin films. This further serves as input for band structure computations modeling the substitutional disorder of the anion sites. Combining our experimental and computational approaches, we reveal the depth-dependent elemental composition of oxynitride films, resulting in downward band bending and the loss of semiconducting character towards the surface. Extending beyond idealized systems, we demonstrate the relation between the electronic properties of real oxynitride photoanodes and their performance, providing guidelines for engineering highly efficient photoelectrodes and photocatalysts for clean hydrogen production.

cond-mat.mtrl-sci↗

arXiv2024

Crystallite size and microstrain in the structure of SrTiO3 formed by magnetron deposition with and without O2 flow through the deposition chambre

Transition metal oxides, which have a perovskite structure, have received much attention in recent decades. This is because of the very suitable properties that can be used in various industries. One of the fields where the properties of Perovskites can and have already been applied is in the power industry. Experimental studies of the last few years have reliably demonstrated that, using simple modifications, perovskite oxides can be used in applications utilizing direct sunlight and photocatalytic applications. The original strontium titanate oxide SrTiO3 (abbreviated STO) is only able to use the UV component of incident radiation and is inactive in visible light, remaining transparent to visible light. The reason for this state is its wide band gap, which at room temperature has a value of 3.2 - 3.25 eV. Studies have shown that doping the structure of these oxides with transition metals (TM) can cause a shift in the valence and/or conduction band. This is because of the 3d dopant bands that create new energy levels in the mentioned band gap, effectively reducing it. One of the elements of the TM group is nickel. According to some studies, it is likely that the Ni ion occurs in the cubic structure of STO in the form of Ni2+ and substitutes the Ti4+ sites. In the case of using this dopant, the shift of the absorption edge of STO:Nix relative to NiO was found to be 1.1 eV. The present work investigates the differences in the structure of STO:Nix prepared by magnetron deposition method with different dopant amount settings. At the same time, the difference in structure was observed when prepared under vacuum with Ar working gas and when O2 was flowed through the deposition chamber. The premise of this experiment was to verify the formation of oxygen vacancies in the STO structure on which the photocatalytic phenomenon could occur.

cond-mat.mtrl-sci↗

arXiv2024

A topological material in the III-V family: heteroepitaxial InBi on InAs

InBi(001) is formed epitaxially on InAs(111)-A by depositing Bi on to an In-rich surface. Angle-resolved photoemission measurements reveal topological electronic surface states, close to the M bar high symmetry point. This demonstrates a heteroepitaxial system entirely in the III-V family with topological electronic properties. InBi shows coexistence of Bi and In surface terminations, in contradiction with other III-V materials. For the Bi termination, the study gives a consistent physical picture of the topological surface electronic structure of InBi(001) terminated by a Bi bilayer rather than a surface formed by splitting to a Bi monolayer termination. Theoretical calculations based on relativistic density functional theory and the one-step model of photoemission clarify the relationship between the InBi(001) surface termination and the topological surface states, supporting a predominant role of the Bi bilayer termination. Furthermore, a tight-binding model based on this Bi bilayer termination with only Bi-Bi hopping terms, and no Bi-In interaction, gives a deeper insight into the spin texture.

cond-mat.str-el↗

arXiv2024

Identification of asbestos fibres from soil sediments in the Pilsen region of the Czech Republic and the impact of these minerals on the health of the local population

Asbestos is the term for silicate minerals with a typical fibrous structure and crystallise as separable fibres that can be released into the environment as a result of natural processes and anthropogenic activities. There is a need to intensify geo-environmental monitoring of the occurrence of natural asbestos on a global scale. The study of this material is important to clarify the impact of asbestos on public health and to have an accurate knowledge of the requirements for asbestos replacement materials. At present, the technical as well as the ecological reasons for switching to these fibres are difficult, as asbestos replacement materials are subject to considerable technological and economic demands, as well as demands for their biological safety. The aim of this paper is to summarize the current knowledge regarding the vast issue of asbestos occurrence, to seek to establish an appropriate methodology for detecting the presence of asbestos in soil sediments, to identify the exact types of asbestos from a series of samples, to highlight its impact on public health, and to emphasize the need for deliberate mapping of the natural occurrence of asbestos and its inclusion in the laws and decrees of the Ministry of the Environment. Samples were analysed by electron microscopy and X-ray diffraction and compared with standards or available literature. The measurements demonstrated the presence of asbestos in the site sediments and identified specific types of asbestos. The conclusion of this work is the confirmation of the presence of asbestos in all samples, including its most dangerous types, which can cause very serious diseases. In this context, the mechanism of asbestos-related diseases will be further addressed, which is linked to the size and shape of the individual fibres, the chemical composition of the asbestos types and the links between their basic structural units.

cond-mat.mtrl-sci↗

arXiv2023

Nature of the metallic and in-gap states in Ni-doped SrTiO$_3$

Epitaxial thin films of SrTiO$_3$(100) doped with 6% and 12% Ni are studied with resonant angle-resolved photoelectron spectroscopy (ARPES) at the Ti and Ni L2,3-edges. We find that the Ni doping shifts the valence band (VB) of pristine SrTiO$_3$ towards the Fermi level (p-doping) and reduces its band gap. This is accompanied by an upward energy shift of the Ti t2g-derived mobile electron system (MES). Thereby, the in-plane dxy-derived bands reduce the embedded electron density, as evidenced by progressive reduction of their Fermi momentum with the Ni concentration, and the out-of-plane dxz/yz-derived bands depopulate, making the MES purely two-dimensional. Furthermore, the Ti and Ni L2,3-edge resonant photoemission is used to identify the Ni 3d impurity state in the vicinity of the valence-band maximum, and decipher the full spectrum of the VO-induced in-gap states originating from the Ni atoms, Ti atoms, and from their hybridized orbitals. Our experimental information about the dependence of the valence bands, MES and in-gap states in Ni-doped SrTiO$_3$ may help development of this material towards its device applications associated with the reduced optical band gap.

cond-mat.mtrl-sci↗

arXiv2023

k-dependent proximity-induced modulation of spin-orbit interaction in MoSe2 interfaced with amorphous Pb

The ability to modulate the spin-orbit (SO) interaction is crucial for engineering a wide range of spintronics-based quantum devices, extending from state-of-the-art data storage to materials for quantum computing. The use of proximity-induced effects for this purpose may become the mainstream approach, whereas their experimental verification using angle-resolved photoelectron spectroscopy (ARPES) has so far been elusive. Here, using the advantages of soft-X-ray ARPES on its probing depth and intrinsic resolution in three-dimensional momentum k, we identify a distinct modulation of the SO interaction in a van der Waals semiconductor (MoSe2) proximitized to a high-Z metal (Pb), and measure its variation through the k-space. The strong SO field from Pb boosts the SO splitting by up to 30% at the H-point of the bulk Brillouin zone, the spin-orbit hotspot of MoSe2. Tunability of the splitting via the Pb thickness allows its tailoring to particular applications in emerging quantum devices.

cond-mat.mtrl-sci↗

arXiv2023

Persistence of structural distortion and bulk band Rashba splitting in SnTe above its ferroelectric critical temperature

The ferroelectric semiconductor $α$-SnTe has been regarded as a topological crystalline insulator and the dispersion of its surface states has been intensively measured with angle-resolved photoemission spectroscopy (ARPES) over the last decade. However, much less attention has been given to the impact of the ferroelectric transition on its electronic structure, and in particular on its bulk states. Here, we investigate the low-energy electronic structure of $α$-SnTe with ARPES and follow the evolution of the bulk-state Rashba splitting as a function of temperature, across its ferroelectric critical temperature of about $T_c\sim 110$ K. Unexpectedly, we observe a persistent band splitting up to room temperature, which is consistent with an order-disorder contribution to the phase transition that requires the presence of fluctuating local dipoles above $T_c$. We conclude that no topological surface state can occur at the (111) surface of SnTe, at odds with recent literature.

cond-mat.mtrl-sci↗

arXiv2023

Ultrafast Hidden Spin Polarization Dynamics of Bright and Dark Excitons in 2H-WSe$_2$

We performed spin-, time- and angle-resolved extreme ultraviolet photoemission spectroscopy (STARPES) of excitons prepared by photoexcitation of inversion-symmetric 2H-WSe$_2$ with circularly polarized light. The very short probing depth of XUV photoemission permits selective measurement of photoelectrons originating from the top-most WSe$_2$ layer, allowing for direct measurement of hidden spin polarization of bright and momentum-forbidden dark excitons. Our results reveal efficient chiroptical control of bright excitons' hidden spin polarization. Following optical photoexcitation, intervalley scattering between nonequivalent K-K' valleys leads to a decay of bright excitons' hidden spin polarization. Conversely, the ultrafast formation of momentum-forbidden dark excitons acts as a local spin polarization reservoir, which could be used for spin injection in van der Waals heterostructures involving multilayer transition metal dichalcogenides.

cond-mat.mes-hall↗

arXiv2022

Dependence of the electronic structure of $β$-Si$_{6-z}$Al$_{z}$O$_{z}$N$_{8-z}$ on the (Al,O) concentration $z$ and on the temperature

SiAlON is a prominent example of systems suitable as hosts for creating materials for light-emitting diodes (LEDs). In this work, the electronic structure of a series of semiordered and disordered SiAlON systems is investigated by means of ab initio calculations, using the FLAPW and Green function KKR methods. Finite temperature effects are included by averaging over thermodynamic configurations within the alloy analogy model. We found that the dependence of the electronic structure on the (Al,O) concentration $z$ is similar for semiordered and disordered structures. The electronic band gap decreases with increasing $z$ by about 1.5 eV when going from $z$=0 to $z$=2. States at the top of the valence band are mostly associated with N atoms whereas the states at the bottom of the conduction band are mostly derived from O~atoms. Increasing the temperature leads to a shift of the bottom of the conduction band to lower energies. The amount of this shift increases with increasing $z$.

cond-mat.mtrl-sci↗

arXiv2021

Unveiling the Orbital Texture of 1T-TiTe$_2$ using Intrinsic Linear Dichroism in Multidimensional Photoemission Spectroscopy

The momentum-dependent orbital character in crystalline solids, referred to as orbital texture, is of capital importance in the emergence of symmetry-broken collective phases such as charge density waves as well as superconducting and topological states of matter. By performing extreme ultraviolet multidimensional angle-resolved photoemission spectroscopy for two different crystal orientations linked to each other by mirror symmetry, we isolate and identify the role of orbital texture in photoemission from the transition metal dichalcogenide 1T-TiTe$_2$. By comparing our experimental results with theoretical calculations based on both a quantitative one-step model of photoemission and an intuitive tight-binding model, we unambiguously demonstrate the link between the momentum-dependent orbital orientation and the emergence of strong intrinsic linear dichroism in the photoelectron angular distributions. Our results represent an important step towards going beyond band structure (eigenvalues) mapping and learn about electronic wavefunction and orbital texture of solids by exploiting matrix element effects in photoemission spectroscopy.

cond-mat.mtrl-sci↗

arXiv2021

Surface band characters of Weyl semimetal candidate material MoTe$_2$ revealed by one-step ARPES theory

The layered 2D-material MoTe$_2$ in the T$_d$ crystal phase is a semimetal which has theoretically been predicted to possess topologically non-trivial bands corresponding to Weyl fermions. Clear experimental evidence by angle-resolved photoemission spectroscopy (ARPES) is, however, lacking, which calls for a careful examination of the relation between ground state band structure calculations and ARPES intensity plots. Here we report a study of the near Fermi-energy band structure of MoTe$_2$(T$_d$) by means of ARPES measurements, density functional theory, and one-step-model ARPES calculations. Good agreement between theory and experiment is obtained. We analyze the orbital character of the surface bands and its relation to the ARPES polarization dependence. We find that light polarization has a major efect on which bands can be observed by ARPES. For s-polarized light, the ARPES intensity is dominated by subsurface Mo d orbitals, while p-polarized light reveals the bands composed mainly derived from Te p orbitals. Suitable light polarization for observing either electron or hole pocket are determined

cond-mat.mtrl-sci↗

arXiv2020

Bulk and surface electronic states in the dosed semimetallic HfTe$\boldsymbol{_2}$

The dosing of layered materials with alkali metals has become a commonly used strategy in ARPES experiments. However, precisely what occurs under such conditions, both structurally and electronically, has remained a matter of debate. Here we perform a systematic study of 1T-HfTe$_2$, a prototypical semimetal of the transition metal dichalcogenide family. By utilizing photon energy-dependent angle-resolved photoemission spectroscopy (ARPES), we have investigated the electronic structure of this material as a function of Potassium (K) deposition. From the k$_z$ maps, we observe the appearance of 2D dispersive bands after electron dosing, with an increasing sharpness of the bands, consistent with the wavefunction confinement at the topmost layer. In our highest-dosing cases, a monolayer-like electronic structure emerges, presumably as a result of intercalation of the alkali metal. Here, by bringing the topmost valence band below $E_F$, we can directly measure a band overlap of $\sim$ 0.2 eV. However, 3D bulk-like states still contribute to the spectra even after considerable dosing. Our work provides a reference point for the increasingly popular studies of the alkali metal dosing of semimetals using ARPES.

cond-mat.mtrl-sci↗

arXiv2019

Magneto-optic and transverse transport properties of non-collinear antiferromagnets

Previous studies on the anomalous Hall effect in coplanar non-collinear antiferromagnets are revisited and extended to magneto-optic properties, namely magneto-optic Kerr effect (MOKE) and X-ray magnetic dichroism (XMCD). Starting from group-theoretical considerations the shape of the frequency-dependent conductivity tensor for various actual and hypothetical spin configurations in cubic and hexagonal Mn$_3X$ compounds is determined. Calculated MOKE and X-ray dichroism spectra are used to confirm these findings and to give estimates of the size of the effects. For Mn$_3$IrPt and Mn$_3$PtRh alloys the concentration dependence of the anomalous and spin Hall conductivity is studied in addition.

cond-mat.mtrl-sci↗

arXiv2018

Element- and momentum-resolved electronic structure of the dilute magnetic semiconductor manganese doped gallium arsenide

The dilute magnetic semiconductors have promise in spin-based electronics applications due to their potential for ferromagnetic order at room temperature, and various unique switching and spin-dependent conductivity properties. However, the precise mechanism by which the transition-metal doping produces ferromagnetism has been controversial. Here we have studied a dilute magnetic semiconductor (5% manganese-doped gallium arsenide) with Bragg-reflection standing-wave hard X-ray angle-resolved photoemission spectroscopy, and resolved its electronic structure into element- and momentum- resolved components. The measured valence band intensities have been projected into element-resolved components using analogous energy scans of Ga 3d, Mn 2p, and As 3d core levels, with results in excellent agreement with element-projected Bloch spectral functions and clarification of the electronic structure of this prototypical material. This technique should be broadly applicable to other multi-element materials.

cond-mat.mtrl-sci↗

arXiv2017

Fermi surface and effective masses in photoemission response of the (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ superconductor

The angle-resolved photoemission spectra of the superconductor (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ have been investigated both experimentally and theoretically. Our results explain the previously obscured origins of all salient features of the ARPES response of this paradigm pnictide compound and reveal the origin of the Lifshitz transition. Comparison of calculated ARPES spectra with the underlying DMFT band structure shows an important impact of final state effects, which results for three-dimensional states in a deviation of the ARPES spectra from the true spectral function. In particular, the apparent effective mass enhancement seen in the ARPES response is not an entirely intrinsic property of the quasiparticle valence bands but may have a significant extrinsic contribution from the photoemission process and thus differ from its true value. Because this effect is more pronounced for low photoexcitation energies, soft-X-ray ARPES delivers more accurate values of the mass enhancement due to a sharp definition of the 3D electron momentum.

cond-mat.supr-con↗