arXiv subjects

Martin Zahradník

Publications and source records attributed to Martin Zahradník.

3 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$_{2-δ}$ 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↗

arXiv2022

Femtosecond pump-probe absorption edge spectroscopy of cubic GaN

Time-dependent femtosecond pump-probe spectroscopic ellipsometry studies on zincblende gallium-nitride (zb-GaN) are performed and analyzed between 2.9-3.7eV. An ultra-fast change of the absorption onset (3.23eV for zb-GaN) is observed by investigating the imaginary part of the dielectric function. The 266nm (4.66eV) pump pulses induce a large free-carrier concentration up to $4\times 10^{20}$cm$^{-3}$, influencing the transition energy between conduction and valence bands due to many-body effects, like band filling and band gap renormalization, up to $\approx$500meV. Additionally, the absorption of the pump-beam creates a free-carrier profile within the 605nm zb-GaN layer. This leads to varying optical properties from sample surface to substrate, which are taken into account by grading analysis for an accurate description of the experimental data. A temporal resolution of 100fs allows in-depth investigations of occurring ultra-fast relaxation and recombination processes. We provide a quantitative description of the free-carrier concentration and absorption onset at the sample surface as a function of relaxation, recombination, and diffusion yielding a characteristic relaxation time of 0.19ps and a recombination time of 26.1ps.

cond-mat.mtrl-sci↗

arXiv2019

Magnetic domain wall motion in SrRuO$_3$ thin films

Influence of substrate miscut on magnetization dynamics in SrRuO$_3$ (SRO) thin films was studied. Two films were grown on SrTiO$_3$ substrates with high ($\sim1^{\circ}$) and low ($\sim0.1^{\circ}$) miscut angles, respectively. As expected, high miscut angle leads to suppression of multi-variant growth. By means of SQUID magnetometry, comparable relaxation effects were observed in both the multi-variant and the nearly single-variant sample. Differences in the magnetization reversal process were revealed by magnetic force microscopy. It showed that the multi-variant growth leads to higher density of defects acting as pinning or nucleation sites for magnetic domains, which consequently results in deterioration of magnetic properties. It was demonstrated that the use of high miscut substrate is important for fabrication of high quality SRO thin films with low density of crystallographic defects and excellent magnetic properties.

cond-mat.mtrl-sci↗