arXiv subjects

Jiří Hejtmánek

Publications and source records attributed to Jiří Hejtmánek.

5 recordsLinked to original sources

arXiv2026

Optimizing high-temperature electron mobility in single-crystal Bi$_2$O$_2$Se based on its unconventional dependence on concentration

Quasi-2D Bi$_2$O$_2$Se is part of an intensive materials research effort aimed at finding new semiconductors that outperform silicon-based electronics in terms of speed and power consumption. This material exhibits exceptionally high carrier mobility at low temperatures but mediocre mobility at 300 K. Its high mobility is generally associated with its high permittivity ($\varepsilon_r$~500), which is also associated with metallicity persisting down to very low carrier concentrations. This material exhibits a counterintuitive increase in carrier mobility as the concentration increases. The connection between low both carrier concentration and mobility in Se-rich conditions, and both high carrier concentration and mobility in Se-poor conditions suggests that the increase is related to native defects. We demonstrate that these defects can alter the effective mass of charge carriers. Specifically, substitutional Se(Bi) defects, which appear under Se-rich conditions, destroy the Bi$_2$O$_2$ channel and compromise charge transport properties. These defects increase the effective mass of charge carriers transforming the original semiconductor into a semimetal and introducing holes into charge transport. Additionally, we show that single crystals are generally inhomogeneous, particularly those grown under Se-rich conditions. Unlike Se-poor conditions, Se-rich conditions induce a higher concentration of dislocations and extraneous phases. These findings suggest that the perfection of the Bi$_2$O$_2$ channel is crucial for mobility, particularly at room temperature.

cond-mat.mtrl-sci↗

arXiv2026

Thermal conductivity of CdCr$_{2}$Se$_{4}$ ferromagnet at low temperatures: role of grain boundaries and porosity

It is unambiguously demonstrated that the low temperature magnon specific heat in a ferromagnet varies as T$^{3/2}$ and the magnon thermal conductivity, due to T$^{1/2}$ - dependent effective velocity of magnons, as T$^{2}$. The confirmation of these model comportments is based on the experimental study of chalcospinel CdCr$_{2}$Se$_{4}$, which represents relatively rare example of a ferromagnetic insulator (T$_{C}$ = 130 K) without undesirable masking contributions of the itinerant electron excitations and nuclear specific heat that both make impossible to conclusively unveil the role of magnons. The ratio of the magnon to lattice specific heat is found to reach 87:13 at 2 K and is in accordance with predictions based on the spin-wave stiffness D = 33.5 meVA$^{2}$ and Debye temperature $θ_{D}$ = 237 K. On the other hand, the ratio of the magnon to phonon thermal conductivity reaching 27:73 at 2 K is much lower than expected for standard model of the grain boundary limited transport. This suggests that mean free paths for long-wavelength magnon/phonon heat carriers are largely different - shorter than the grain size (of 1$μ$m) for magnons and longer than grain size for phonons. The phonon dominated low temperature thermal conductivity exhibits, moreover, a T$^{2.3}$ temperature dependence instead of the standard predicted model in T$^{3}$. The relevant scattering mechanisms, both the phonon frequency independent and dependent ones, are discussed in detail.

cond-mat.mtrl-sci↗

arXiv2026

Phonon properties and unconventional heat transfer in quasi-2D $Bi_2O_2Se$ crystal

Bi2O2Se belongs to a group of quasi-2D semiconductors that can replace silicon in future high-speed/low-power electronics. However, the correlation between crystal/band structure and other physical properties still eludes understanding: carrier mobility increases non-intuitively with carrier concentration; the observed $T^2$ temperature dependence of resistivity lacks explanation. Moreover, a very high relative out-of-plane permittivity of about 150 has been reported in the literature. A proper explanation for such a high permittivity is still lacking. We have performed infrared (IR) reflectivity and Raman scattering experiments on a large perfect single crystal with defined mosaicity, carrier concentration and mobility. Five of the eight phonons allowed by factor group theory have been observed and their symmetries determined. The IR spectra show that the permittivity measured in the tetragonal plane is as high as $ε_r{\approx}500$, and this high value is due to a strong polar phonon with a low frequency of ~34 $cm^{-1}$ (~1 THz). Such an unusually high permittivity allows the screening of charge defects, leading to the observation of high electron mobility at low temperatures. It also allows effective modulation doping providing a platform for high performance 2D electronics. DFT calculations suggest the existence of a very low frequency acoustic phonon ~14 $cm^{-1}$ (~0.4 THz). Both the low frequency phonons cause anomalous phonon DOS, which is reflected in the unconventional temperature dependence of the heat capacity, $c_M{\approx}T^{3.5}$. The temperature-dependent, two-component group velocity is proposed to explains the unusual temperature dependence of the thermal conductivity, $κ{\approx}T^{1.5}$

cond-mat.mtrl-sci↗

arXiv2026

Semiclassical model of magnons in double-layered antiferromagnets

The stability and magnonic properties of double-layered antiferromagnets are investigated using two model systems, a linear chain (LC) and a more complex chain of railroad trestle (RT) geometry, and the results are confronted with properties of the real material CrN. The spin-paired order ($\cdots{+}{+}{-}{-}\cdots$) in LC requires alternating ferromagnetic and antiferromagnetic (AFM) exchanges, whereas in RT, an analogous order remains stable even when all interactions are AFM within certain analytical constraints. The rock-salt structure of CrN evokes clear magnetic frustration since Cr atoms in a face-centered cubic lattice form links to twelve nearest neighbors (NNs) all equivalent and AFM. Nonetheless, the magnetostructural transition to an orthorhombically distorted phase below $T_\text{N} = 287~\text{K}$ leads to four different NN Cr-Cr distances and consequently, to a large diversification of the exchange strength, which suppresses the frustration and allows for stable double-layered AFM order of CrN. This behavior originates from a competition at each NN link between Cr-Cr direct exchange and 90$^\circ$ Cr-N-Cr superexchange, both exhibiting specific power-law dependences on the interatomic distance. Finally, based on the $\textit{ab initio}$ calculated exchange parameters, the magnon spectrum and temperature evolution of ordered magnetic moments are derived.

cond-mat.str-el↗

arXiv2025

The preparation and properties of polycrystalline Bi$_2$O$_2$Se -- pitfalls and difficulties with reproducibility and charge transport limiting parameters

Thermoelectric materials allow the direct conversion of waste heat into electricity, and novel materials are being investigated for this purpose. Recently, doped Bi$_2$O$_2$Se has shown high application potential. In this study, we discuss causes for large variation in reported transport properties of pure Bi$_2$O$_2$Se and present a preparation method that improves the reproducibility of undoped polycrystalline samples and improves their stability under thermal cycling. Key steps of this method include calcination of the Bi$_2$O$_3$ precursor, purification of the synthesized material in a temperature gradient, use of a coarse particle fraction and compaction of the powders in a Si3N4 die instead of a graphite die. The resulting polycrystalline material exhibits improved reproducibility and enhanced resistance to thermal cycling. It has room temperature electrical conductivity σRT ~ 500 S.m-1 and Seebeck coefficient S ~ -300 $μ$V.K$^{-1}$. These properties make it suitable as a reference material for future doping studies. The presented synthesis approach may provide a more reliable platform for investigating the intrinsic behavior and doping response of Bi$_2$O$_2$Se in thermoelectric applications.

cond-mat.mtrl-sci↗