arXiv subjects

Martin Veis

Publications and source records attributed to Martin Veis.

At least 19 recordsLinked to original sources

arXiv2026

Optical and magneto-optical interactions in Co-doped CeO$_2$ thin films prepared by pulsed laser deposition

Magnetically doped CeO$_2$ is a dilute magnetic semiconductor, promising for various applications in photonics, but the origin of its ferromagnetic properties is not fully understood. Here, thin films of Ce$_{1-x}$Co$_x$O$_{2-\delta}$ prepared by pulsed laser deposition on MgO ($x=0.05$ and $0.10$) and oxidized Si ($x=0.20$) substrates were systematically studied by spectroscopic ellipsometry and magneto-optical spectroscopy. Both diagonal and off-diagonal permittivity-tensor elements were obtained. Diagonal spectra revealed two optical transitions between oxygen and cerium states. Off-diagonal spectra revealed two paramagnetic transitions involving cobalt ions, from which an essential influence of cobalt doping on resulting ferromagnetic properties of CeO$_2$ was inferred. The full permittivity-tensor spectra are provided for further use in prospective modelling of magneto-optical device concepts.

cond-mat.mtrl-sci↗

arXiv2026

Terahertz spin-current transparency through rough interfaces

Spin transport across interfaces is critical for spintronic devices, yet remains difficult to probe on ultrafast timescales. We use terahertz emission spectroscopy on Co|Pt heterostructures whose interface roughness is tuned through the thickness of an underlying Au buffer layer, while leaving other growth parameters unchanged. From the measured THz electric field, we extract the interface spin-current transparency ts after correcting for the changes in sample impedance and optical absorption of the stack. Surprisingly, we find that ts decreases by only approximately 30% as the interface root-mean-square roughness and the lateral grain size both increase by a factor of three, with no measurable change in the THz spectrum. These results demonstrate that interfacial spin transport is relatively robust against morphological variations on ultrafast timescales, establishing terahertz emission spectroscopy as a reliable probe of spin dynamics across imperfect interfaces.

cond-mat.mes-hall↗

arXiv2026

Room-temperature antiferromagnetic resonance in NaMnAs

We report on antiferromagnetic resonance experiments in bulk tetragonal NaMnAs -- a room-temperature antiferromagnetic semiconductor. Our results corroborate previous ab initio studies, which propose that NaMnAs is an easy-axis antiferromagnet with the N\'eel vector oriented along the tetragonal axis. At $ B = 0 $, we find a single antiferromagnetic resonance line at 7 meV and associate it with a doubly degenerate ($ k = 0 $) magnon mode. Its energy softens considerably with increasing $ T $, but remains clearly visible in the data up to room temperature. From the experimental data, we estimate the single-ion anisotropy of the Mn ions in NaMnAs in the range 0.1-0.2 meV, a value that is relatively large compared to other manganese-based antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2025

Structure of Antiphase boundaries in Ni-M-Ga: multiscale modelling

Antiphase boundaries (APBs) are ubiquitous in ordered Heusler alloys and strongly influence magnetic coercivity in Ni-Mn-Ga, yet the link between their atomic-scale exchange interactions and micrometer-scale magnetic contrast measured by magnetic force microscopy (MFM) remains unclear. We combine density functional theory (DFT) and finite-element magnetostatics to bridge these scales in Ni-Mn-Ga. DFT calculations on supercells containing planar APBs show that the lowest-energy configuration comprises a pair of parallel APBs enclosing a nanoscale region - only three Mn-Ga atomic layers thick - whose magnetization is antiparallel to the surrounding matrix due to strong antiferromagnetic exchange across each APB (in contrast to ferromagnetic coupling in bulk martensite). According to our magnetostatic finite element model, this thin region with antiparallel magnetization generates the characteristic MFM contrast extending approx. 100 nm from the APB pair. When the APBs are further apart than 50 nm, dipole-dipole penalties outweigh exchange gains, preventing formation of an extended antiparallel domain, in agreement with experimental evidence. These results identify APB pairs as the origin of the observed MFM contrast and offer an interpretation of the modest strengths of domain-wall pinning by APBs, informing the design of magnetic shape-memory alloys with tailored coercivity.

cond-mat.mtrl-sci↗

arXiv2025

TeO2-BaO-Bi2O3 tellurite optical glasses II. -- Linear and non-linear optical and magneto-optical properties

The present study investigates the linear and non-linear optical and magneto-optical properties of TeO$_2$-BaO-Bi$_2$O$_3$ (TeBaBi) glasses prepared by the conventional melt-quenching technique at 900 {\deg}C. Prepared glass composition ranges across the whole glass-forming-ability (GFA) region focusing on mutual substitution trends of constituent oxides, where TeO$_2$: 55-85 mol.%, BaO: 10-35 mol.%, Bi$_2$O$_3$: 5-15 mol.%. Studied glasses exhibit high values of linear ($n_{632} \approx$ 1.922-2.084) and non-linear refractive index ($n_2\approx$1.63-3.45$\times10^{-11}$ esu), Verdet constant ($V_{632} \approx$ 26.7-45.3 radT$^{-1}$m$^{-1}$) and optical band gap energy ($E_g \approx$ 3.1-3.6 eV). The introduction of TeO$_2$ and Bi$_2$O$_3$ results in increase of both linear/non-linear refractive index and Verdet constant, with a more pronounced influence of Bi$_2$O$_3$. Measured spectral dispersion of refractive index and Verdet constant were used for estimation of magneto-optic anomaly parameter ($\gamma \approx$ 0.71-0.92), which may be used for theoretical modelling of magneto-optic response in diamagnetic TeBaBi glasses. Additionally, the properties of the prepared TeBaBi glasses were directly compared to those of the TeO$_2$-ZnO-BaO glass system, which was prepared and characterized under similar experimental conditions. The compositional dependence of the refractive index in both glass systems was described using multilinear regression analysis, demonstrating high correlation and uniformity of estimation across the entire GFA region. This makes them highly promising for precise dispersion engineering and construction of optical devices operating from visible to mid-infrared spectral region.

cond-mat.mtrl-sci↗

arXiv2025

Methodology for Topological Interface Engineering in 2D Photonic Crystals

Topological photonics provides a robust and flexible platform for controlling light, enabling functionalities such as backscattering-immune edge transport and slow-light propagation. In this work, we design and characterize photonic topological interfaces in two-dimensional photonic crystals. We introduce an iterative band connection algorithm that preserves mode symmetry and present a general framework for band symmetry recognition, essential for identifying Z2 topological phases. Design strategies for unit cell geometries are developed to achieve targeted band inversions, overlapping bandgaps, and tailored dispersions. Furthermore, the approach can be readily adapted to specific material platforms and operating wavelengths, including the telecommunication range, by appropriately scaling the lattice parameter as long as absorption remains low. We investigate the trade-off between bandgap size and band flatness, identifying exceptions governed by lattice geometry. Additionally, we demonstrate how photonic crystal periodicity influences the stability of topological modes and enhances unidirectional energy transport.

physics.optics↗

arXiv2025

Twinning in ferromagnetic Heusler Rh2MnSb epitaxial thin films

Epitaxially grown full Heusler alloy of Rh2MnSb thin films were prepared for the first time using DC magnetron sputtering. The films were deposited on MgO [001] substrates with a deposition temperature of 600{\deg}C, 700{\deg}C, and 800{\deg}C. We report the structural, morphological, optical, magneto-optical, and magnetic properties of the films with a 200 nm nominal thickness. The grown-at-600{\deg}C film was close to stoichiometric and exhibited L21 ordering typical for Heusler alloys. The single-phase Rh2MnSb film had a tetragonal structure with lattice parameters close to the bulk material. X-ray photoelectron spectroscopy revealed the metallic character of the film free from contamination. The tetragonal films exhibited discernible regular twinning with the majority of twin domains with the c-axis perpendicular to the surface due to a substrate constraint. The twin formation was studied by atomic force and transmission electron microscopy and by X-ray diffraction. Magnetic measurements showed TC of about 220-275 K and saturation magnetization of about 55 emu/g, close to the bulk material. Magneto-optical Kerr effect measurements of the film prepared at 600 {\deg}C affirmed paramagnetic behavior at room temperature and suggested the half-metallic behavior. The observed properties highlight the potential for further investigations of Rh2MnSb's thin films, focusing on compositional and structural control.

cond-mat.mtrl-sci↗

arXiv2025

Obstructed Atomic Limit Topological Protection in C4-Symmetric Photonic Crystals for Optical Communications

Recent developments in photonic topological phases have revealed that protected edge modes can emerge not only from global topological invariants, but also from symmetry-enforced polarization mismatches between distinct bulk phases. In this work, we investigate the capabilities and limitations of a square-lattice ($C_4$-symmetric) photonic crystal composed of a single dielectric material that supports interface-localized modes at the boundary between regions characterized by distinct obstructed atomic limits (OALs). These modes are confined to a common band gap and exhibit high transmission, even in the presence of structural perturbations. Our analysis reveals that the interface modes are stabilized by a mismatch in the position of Wannier centers between the two adjoining crystals. We demonstrate nearly lossless transmission around sharp turns and through localized defects, though the robustness depends asymmetrically on the side of perturbation, reflecting the partial nature of the protection. We also show that increasing the number of photonic crystal periods surrounding the interface enhances both modal confinement and spectral stability. These findings establish polarization mismatch between OALs as a practical and fabrication-compatible mechanism for engineering robust photonic transport in $C_4$-symmetric systems.

physics.optics↗

arXiv2024

Anomalous Spectroscopical Effects in an Antiferromagnetic Semiconductor

Following the recent observation of anomalous Hall effect in antiferromagnetic hexagonal MnTe thin films, related phenomena at finite frequencies have come into focus. Magnetic circular dichroism (MCD) is the key material property here. In the x-ray range, the XMCD has already been demonstrated and used to visualise domains via photoemission electron microscopy (PEEM). Here we report on MCD in optical range and discuss its microscopic mechanism.

cond-mat.mtrl-sci↗

arXiv2023

Magneto-optical properties of textured La$_{2/3}$Sr$_{1/3}$MnO$_3$ thin films integrated on silicon via a Ca$_2$Nb$_3$O$_{10}$ nanosheet layer

We demonstrate the possibility of growing textured La$_{2/3}$Sr$_{1/3}$MnO$_3$ (LSMO) thin films on silicon substrates with magneto-optical and optical properties comparable to high-quality epitaxial layers grown on bulk SrTiO$_3$ (STO). The pulsed laser deposition growth of LSMO is achieved by a two-dimensional nanosheet (NS) seed layer of Ca$_2$Nb$_3$O$_{10}$ (CNO) inducing epitaxial stabilization of LSMO films. The resulting layers possess a higher Curie temperature and a lower overall magnetization than samples of LSMO on STO. Spectra of the full permittivity tensor were calculated from optical and magneto-optical measurements. Spectral dependencies of both the diagonal and off-diagonal elements share many similarities between the LSMO/NS/Si and LSMO/STO samples. These similarities indicate comparable electronic structures of the layers and demonstrate comparable optical quality of textured LSMO on NS/Si and epitaxial LSMO on STO.

cond-mat.mtrl-sci↗

arXiv2023

Magnetic properties of cobalt ultrathin film structures controlled by buffer layer roughness

Growth optimization of multilayers is a topic of interest due to their unique physical properties. Systems containing magnetic materials, such as platinum-cobalt, have been studied because of their potential for technological applications, e.g. spintronics, magnetic storage and magnetic sensors. Since the magnetic properties of thin layers are strongly related to the growth parameters, the fine tuning of these parameters is necessary to produce multilayers with specific properties required in various applications. Here, an efficient approach to tune the coercive field of Co ultrathin films in the multilayer by varying the underlayer thickness is demonstrated. Using magnetron sputtering, we prepared multilayer systems of Au(x)/Pt(5nm)/Co(0.7nm)/Au(5nm) with various thicknesses of Au underlayer. The surface morphology of Au(x)/Pt(5nm) stack on which Co layer was deposited was studied by atomic force microscopy. We show the possibility to control the interfacial roughness by changing the Au underlayer thickness due to its island-like growth mechanism (Volmer-Weber mode). As the nominal thickness of Au increases, the islands grow in larger lateral size, resulting in a higher overall roughness of the layer surface. Magnetization measurements indicate a direct influence of the underlayer roughness on the coercivity of the multilayers by promoting additional magnetic anisotropy. With thickness of the Au layer up to 20 nm, we can change the coercive field in the range from ~200 Oe to ~1100 Oe, while remaining a nearly constant saturation magnetization. The use of Cu replacing Au underlayer in the same multilayers was also investigated, demonstrating the possibility of coercivity adjustment using different materials. The results are important for applications where the magnetic properties of multilayer structures based on Co thin films could be adjusted via buffer layer roughness engineering.

cond-mat.mtrl-sci↗

arXiv2023

Theoretical studies of enhanced anomalous Nernst effect in Fe$_3$Ga

The anomalous Nernst effect (ANE) is a member of the extensive family of topological effects in solid state physics. It converts a heat current into electric voltage and originates from the Berry curvature of electronic bands near the Fermi level. Recent results established the Fe$_3$Ga alloy as one of the most promising candidates for applications, due to its flat band structure consisting of rich web of nodal lines. In this theoretical work, we study the effect of deformation of Fe$_3$Ga on the anomalous Nernst effect, which naturally occurs in thin films. Furthermore, we demonstrate that doping, which effectively shifts the position of the Fermi level, can also significantly modify the strength of the effect. Lastly, we provide detailed analysis of the origin of ANE in the electronic structure of Fe$_3$Ga which yields a deeper insight into the generating mechanisms, understanding of which can lead to substantial enhancement of the effect in the future.

cond-mat.mtrl-sci↗

arXiv2022

High-angular momentum excitations in collinear antiferromagnet FePS$_3$

We report on magneto-optical studies of the quasi-two-dimensional van der Waals antiferromagnet FePS$_3$. Our measurements reveal an excitation that closely resembles the antiferromagnetic resonance mode typical of easy-axis antiferromagnets, nevertheless, it displays an unusual, four-times larger Zeeman splitting in an applied magnetic field. We identify this excitation with an $|S_z|=4$ multipolar magnon -- a single-ion 4-magnon bound state -- that corresponds to a full reversal of a single magnetic moment of the Fe$^{2+}$ ion. We argue that condensation of multipolar magnons in large-spin materials with a strong magnetic anisotropy can produce new exotic states.

cond-mat.str-el↗

arXiv2022

High pressure tuning of magnon-polarons in the layered antiferromagnet FePS$_3$

Magnetic layered materials have emerged recently as promising systems to introduce magnetism in structures based on two-dimensional (2D) materials and to investigate exotic magnetic ground states in the 2D limit. In this work, we apply high hydrostatic pressures up to P = 8.7 GPa to the bulk layered antiferromagnet FePS$_3$ to tune the collective lattice excitations (phonons) in resonance with magnetic excitations (magnons). Close to P = 4 GPa, the magnon-phonon resonance is achieved and the strong coupling between these collective modes leads to the formation of new quasi-particles, the magnon-polarons, evidenced in our low temperature Raman scattering experiments by a particular avoided crossing behavior between the phonon and the doubly degenerate antiferromagnetic magnon. At the pressure-induced magnon-phonon resonance, three distinct coupled modes emerge. As it is mainly defined by intralayer properties, we show that the energy of the magnon is nearly pressure independent. We additionally apply high magnetic fields up to B = 30 T to fully identify and characterize the magnon excitations, and to explore the different magnon-polaron regimes for which the phonon has an energy lower-, equal to-, or higher- than the magnon energy. The description of our experimental data requires introducing a phonon-phonon coupling not taken into account in actual calculations.

cond-mat.mes-hall↗

arXiv2021

Circular displacement current induced anomalous magneto-optical effects in high index Mie resonators

Dielectric Mie nanoresonators showing strong light-matter interaction at the nanoscale may enable new functionality in photonic devices. Recently, strong magneto-optical effects have been observed in magneto-optical nanophotonic devices due to the electromagnetic field localization. However, most reports so far have been focused on the enhancement of conventional magneto-optical effects. Here, we report the observation of circular displacement current induced anomalous magneto-optical effects in high-index-contrast Si/Ce:YIG/YIG/SiO2 Mie resonators. In particular, giant modulation of light intensity in transverse magnetic configuration up to 6.4 % under s-polarized incidence appears, which is non-existent in planar magneto-optical thin films. Apart from that, we observe a large rotation of transmitted light polarization in the longitudinal magnetic configuration under near normal incidence conditions, which is two orders of magnitude higher than for a planar magneto-optical thin film. These phenomena are essentially originated from the unique circular displacement current when exciting the magnetic resonance modes in the Mie resonators, which changes the incident electric field direction locally. Our work indicates an uncharted territory of light polarization control based on the complex modal profiles in all-dielectric magneto-optical Mie resonators and metasurfaces, which opens the door for versatile control of light propagation by magnetization for a variety of applications in vectoral magnetic field and biosensing, free space non-reciprocal photonic devices, magneto-optical imaging and optomagnetic memories.

physics.optics↗

arXiv2021

Landau level spectroscopy of PbSnSe topological crystalline insulator

We report on an infrared magneto-spectroscopy study of Pb$_{1-x}$Sn$_x$Se, a topological crystalline insulator. We have examined a set of samples, all in the inverted regime of electronic bands, with the tin composition varying from $x=0.2$ to $0.33$. Our analysis shows that the observed response, composed of a series of interband inter-Landau level excitations, can be interpreted and modelled using the relativistic-like Hamiltonian for three-dimensional massive Dirac electrons, expanded to include diagonal quadratic terms that impose band inversion. In our data, we have not found any clear signature of massless electron states that are present on the surface of Pb$_{1-x}$Sn$_x$Se crystals in the inverted regime. Reasons for this unexpected result are discussed.

cond-mat.mes-hall↗

arXiv2020

Skyrmion Lattice Phases in Thin Film Multilayer

Phases of matter are ubiquitous with everyday examples including solids and liquids. In reduced dimensions, particular phases, such as the two-dimensional (2D) hexatic phase and corresponding phase transitions occur. A particularly exciting example of 2D ordered systems are skyrmion lattices, where in contrast to previously studied 2D colloid systems, the skyrmion size and density can be tuned by temperature and magnetic field. This allows us to drive the system from a liquid phase to a hexatic phase as deduced from the analysis of the hexagonal order. Using coarse-grained molecular dynamics simulations of soft disks, we determine the skyrmion interaction potentials and we find that the simulations are able to reproduce the full two-dimensional phase behavior. This shows that not only the static behavior of skyrmions is qualitatively well described in terms of a simple two-dimensional model system but skyrmion lattices are versatile and tunable two-dimensional model systems that allow for studying phases and phase transitions in reduced dimensions.

cond-mat.mtrl-sci↗

arXiv2020

Scaling of quadratic and linear magnetooptic Kerr effect spectra with L2$_1$ ordering of Co$_2$MnSi Heusler compound

The Heusler compound Co$_2$MnSi provides a crystallographic transition from B2 to L2$_1$ structure with increasing annealing temperature $T_a$, being a model system for investigating the influence of crystallographic ordering on structural, magnetic, optic, and magnetooptic (MO) properties. Here, we present quadratic magnetooptic Kerr effect (QMOKE) spectra depending on $M^2$ in addition to the linear magnetooptic Kerr effect (LinMOKE) spectra being proportional to $M$, both in the extended visible spectral range of light from 0.8\,eV to 5.5\,eV. We investigated a set of Co$_2$MnSi thin films deposited on MgO(001) substrates and annealed from 300$^\circ$C to 500$^\circ$C. The amplitude of LinMOKE and QMOKE spectra scales linearly with $T_a$, and this effect is well pronounced at the resonant peaks below 2.0\,eV of the QMOKE spectra. Furthermore, the spectra of the MO parameters, which fully describe the MO response of Co$_2$MnSi up to the second order in $M$, are obtained dependend on $T_a$. Finally, the spectra are compared to ab-initio calculations of a purely L2$_1$ ordered Co$_2$MnSi Heusler compound.

physics.app-ph↗