arXiv subjects

Kirill S. Pervakov

Publications and source records attributed to Kirill S. Pervakov.

9 recordsLinked to original sources

arXiv2026

Yttrium Superhydrides Revisited: Advanced Experimental and Theoretical Studies of YH$_6$, YH$_9$ and YH$_{10}$

Yttrium polyhydrides are benchmark materials in high-pressure superconductivity, yet several key properties of the Y-H system remain insufficiently characterized. Here we combine contact transport, contactless radio-frequency measurements, pulsed-field experiments, and first-principles calculations to reinvestigate YH$_6$, YH$_9$, and YH$_{10}$ in the pressure range 140-213 GPa. Yttrium hydrides YH$_6$ ($\textit{$T_c$}$ = 218-221 K) and YH$_9$ ($\textit{$T_c$}$ = 235-237 K) demonstrate narrow superconducting transitions ($\textit{$Δ$T$_c$}$ = 2-5 K), approaching the limit imposed by thermal fluctuations. Pulsed-field measurements on YH$_6$ up to 60 T establish an extended superconducting phase diagram with a linear slope $\textit{dB$_{c2}$/dT}$ = -0.52 T/K, pronounced transition broadening above 30 T, and negligible normal-state magnetoresistance. We report the radio-frequency AC susceptibility study of YH$_6$, providing evidence for superconductivity via high-frequency field screening in a contactless geometry. Experiments involving Pd incorporation, Pd thin-film sputtering, and Al alloying show strong suppression of high-temperature superconductivity, with no transitions detected above 78-120 K. Finally, using density-functional theory with the stochastic self-consistent harmonic approximation, superconducting density-functional theory, and full-bandwidth Migdal-Eliashberg calculations, we show that anharmonic effects substantially reduce the predicted $\textit{$T_c$}$ of cubic YH$_{10}$ to approximately 260-270 K. These results strongly disfavor room-temperature superconductivity in binary yttrium superhydrides.

cond-mat.supr-con↗

arXiv2026

Anomalous and Topological Hall Effects in Antiferromagnetic EuSn2As2 Nanostructures

We investigate magnetotransport in exfoliated nanostructures of the candidate magnetic 3D topological insulator $\mathrm{EuSn_{2}As_{2}}$. Similar to macroscopic single crystals, the negative magnetoresistance observed below the Néel temperature ($T_N$ = 24 K) is related to the canted antiferromagnetic state (CAF) of an easy-plane antiferromagnet (AFM), with an increase of the saturation field when tilting the applied magnetic field away from the (ab) plane ($μ_{0} H^{c}_{s}$ = 4.9 T, $μ_{0} H^{ab}_{s}$ = 3.6 T). Higher-accuracy measurements in nanostructures up to 14 T further evidence a non-linear normal Hall response due to several electronic bands. Interestingly, the transverse resistance due to magnetism reveals an anomalous Hall effect in the CAF state, but also a topological Hall effect due to chiral spin textures, as found in AFM or helical magnets. The presence of real-space chiral spin texture, already reported in another magnetic topological insulator, $\mathrm{MnBi_{2}Te_{4}}$, could be a characteristic generally appearing in magnetic 3D topological insulators.

cond-mat.mes-hall↗

arXiv2026

Superhydrides on the way to ambient pressure: weak localization and persistent X-ray photoconductivity in BaSiH$_{8}$

Reducing the stabilization pressure of superhydrides represents one of the most important challenges in hydrogen-saturated compound chemistry. Moving in this direction, we studied the Ba-Si-H system at 0-142 GPa using transport measurements, 1H nuclear magnetic resonance, single-crystal and powder X-ray diffraction in the temperature range of 4-317 K. We synthesized the previously predicted cubic BaSiH$_{8}$ at pressures of 18-31 GPa. Remarkably, we demonstrate that BaSiH$_8$ remains stable upon decompression to ambient conditions and can be recovered from the diamond anvil cell. Obtained Ba-Si polyhydrides exhibit metallic and superconducting properties ($\textit{T$_c$}$ = 9 K, $\textit{B$_{c2}$}$(0)=13-16 T) at 142 GPa. However, at pressures below 50 GPa, these hydrides behave as degenerate semiconductors (bandgap < 0.4 meV) or poor metals with weak electron localization, negative magnetoresistance, photovoltaic effect, and persistent photoconductivity in the X-ray and visible range. Our work demonstrates the high-pressure synthesis of Ba-Si polyhydrides that remain stable upon decompression to ambient conditions, overcoming a critical bottleneck in superhydride chemistry and establishing a foundation for practical applications in hydrogen storage.

cond-mat.supr-con↗

arXiv2025

Ternary superhydrides under pressure of Anderson's theorem: Near-record superconductivity in (La,Sc)H$_{12}$

Lanthanum-hydrogen system and its derivatives remain among the most promising for achieving room-temperature superconductivity. In this study, we examined the formation of ternary lanthanum-scandium superhydrides at pressures up to 220 GPa. The primary product of the LaSc alloy's reaction with hydrogen is newly discovered cubic (La,Sc)H$_{12}$, demonstrating clear superconducting transition in all six channels of the van der Pauw scheme below 244-248 K. In this compound with an unusually large unit cell volume, virtually no magnetoresistance was observed in fields up to 68 Tesla. Synthesized samples of (La,Sc)H$_{12}$ demonstrate pronounced superconducting diode and SQUID-like effects at a record high temperature of 233 K. Furthermore, our analysis revealed the formation of lower hexagonal polyhydrides (La,Sc)H$_{6-7}$, which could potentially account for the anomaly in electrical resistance observed near 274 K. This anomaly between 265-290 K also appears in the radio-frequency transmission measurements and may be of a superconducting nature.

cond-mat.supr-con↗

arXiv2023

Anatomy of the band structure of the newest apparent near-ambient superconductor LuH$_{3-x}$N$_x$

Recently it was claimed that nitrogen-doped lutetium hydride exhibited a near-ambient superconducting transition with a temperature of 294 K at a pressure of only 10 kbar, this pressure being several orders of magnitude lower than previously demonstrated for hydrides under pressure. In this paper, we investigate within DFT+U the electronic structure of both parent lutetium hydride LuH$_3$ and nitrogen doped lutetium hydride LuH$_{2.75}$N$_{0.25}$. We calculated corresponding bands, density of states and Fermi surfaces. It is shown that in the stoichiometric system the Lu-5d states cross the Fermi level while the H-1s states make almost no contribution at the Fermi level. However, with nitrogen doping, the N-2p states enter the Fermi level in large quantities and bring together a significant contribution from the H-1s states. The presence of N-2p and H-1s states at the Fermi level in a doped compound can facilitate the emergence of superconductivity. For instance, nitrogen doping almost doubles the value of DOS at the Fermi level. Simple BCS analysis shows that the nitrogen doping of LuH$_3$ can provide T$_c$ more than 100K and even increase it with further hole doping.

cond-mat.supr-con↗

arXiv2022

Anomalous metallic oxygen band in the potential superconductor KCa$_2$Fe$_4$As$_4$O$_2$

The electronic structure, magnetism and Fermi surface of a hole self-doped potential iron-based superconductor KCa$_2$Fe$_4$As$_4$O$_2$ (12442) were studied by a first-principes DFT/GGA approach. The projection onto Wannier functions basis is proposed to calculate directly the self-doping value. In particular for KCa$_2$Fe$_4$As$_4$O$_2$ it is 0.75 hole on Fe-3d states. Surprisingly, for the KCa$_2$Fe$_4$As$_4$O$_2$ it was found that except the Fe-3d bands the O-2p bands also cross the Fermi level, which is quite anomalous behavior for O-2p states. In general, the O-2p states are usually fully occupied in transition metal oxides. Here it is not the case due to the fact that CaO layer gives some of the electrons to the FeAs layer, thus leaving O-2p states partially occupied. The O-2p states form additional Fermi surface sheet with propeller-like shape around $Γ$-point and modify the Fe-3d Fermi surface sheets due to the hybridization. To prove the presence of anomalous metallic O-2p band in KCa$_2$Fe$_4$As$_4$O$_2$ the comparative study with the same family systems KCa$_2$Fe$_4$As$_4$F$_2$ and RbGd$_2$Fe$_4$As$_4$O$_2$ (where it is not the case) was performed. The magnetic ground state is found to be antiferromagnetic with checkerboard ordering. Therefore, we expect that the KCa$_2$Fe$_4$As$_4$O$_2$ might be a potential superconductor with unusual properties.

cond-mat.supr-con↗

arXiv2022

Effect of paramagnetic impurities on superconductivity in polyhydrides: $\textit{s}$-wave order parameter in Nd-doped LaH$_{10}$

Polyhydrides are a novel class of superconducting materials with extremely high critical parameters, which is very promising for applications. On the other hand, complete experimental study of the magnetic phase diagram for the best so far known superconductor, lanthanum decahydride LaH$_{10}$, encounters a serious complication because of the large upper critical magnetic field $\textit{H}$$_{C2}$(0), exceeding 120-160 T. Partial replacement of La atoms by magnetic Nd atoms results in a decrease of the upper critical field, which makes it attainable for existing pulse magnets. We found that addition of neodymium leads to significant suppression of superconductivity in LaH$_{10}$: each atomic % of Nd causes decrease in $\textit{T}$$_{C}$ by 10-11 K. Using strong pulsed magnetic fields up to 68 T, we constructed the magnetic phase diagram of the ternary (La,Nd)H$_{10}$ superhydride, which appears to be surprisingly linear with $\textit{H}$$_{C2}$ $\propto$ |$\textit{T}$ - $\textit{T}$$_C$|. The pronounced suppression of superconductivity in LaH$_{10}$ by magnetic Nd atoms and the robustness of $\textit{T}$$_C$ with respect to nonmagnetic impurities (e.g., Y, Al, C) under Anderson's theorem indicate the isotropic ($\textit{s}$-wave) character of conventional electron-phonon pairing in the synthesized superhydrides.

cond-mat.supr-con↗

arXiv2020

Superconductivity at 253 K in lanthanum-yttrium ternary hydrides

Polyhydrides offer intriguing perspectives as high-temperature superconductors. Here we report the high-pressure synthesis of a series of lanthanum-yttrium ternary hydrides: cubic hexahydride $(La,Y)H_{6}$ with a critical temperature $T_{C}$ = 237 +/- 5 K and decahydrides $(La,Y)H_{10}$ with a maximum $T_{C}$ ~${253 K}$ and an extrapolated upper critical magnetic field $B_{C2(0)}$ up to ${135 T}$ at 183 GPa. This is one of the first examples of ternary high-$T_{C}$ superconducting hydrides. Our experiments show that a part of the atoms in the structures of recently discovered ${Im3m}$-$YH_{6}$ and ${Fm3m}$-$LaH_{10}$ can be replaced with lanthanum (~70 %) and yttrium (~25 %), respectively, with a formation of unique ternary superhydrides containing incorporated $La@H_{24}$ and $Y@H_{32}$ which are specific for ${Im3m}$-$LaH_{6}$ and ${Fm3m}$-$YH_{10}$. Ternary La-Y hydrides were obtained at pressures of 170-196 GPa via the laser heating of $P6_{3}$${/mmc}$ lanthanum-yttrium alloys in the ammonia borane medium at temperatures above 2000 K. A novel tetragonal $(La,Y)H_{4}$ was discovered as an impurity phase in synthesized cubic $(La,Y)H_{6}$. The current-voltage measurements show that the critical current density $J_{C}$ in $(La,Y)H_{10}$ may exceed $2500 A/mm^{2}$ at 4.2 K, which is comparable with that for commercial superconducting wires such as ${NbTi}$, $Nb_{3}$${Sn}$. Hydrides that are unstable in a pure form may nevertheless be stabilized at relatively low pressures in solid solutions with superhydrides having the same structure.

cond-mat.supr-con↗

arXiv2020

Bulk cyclotron resonance in the topological insulator Bi2Te3

We investigated magneto-optical response of undoped Bi2Te3 films in the terahertz frequency range (0.3 - 5.1 THz, 10 - 170 cm-1) in magnetic fields up to 10 T. The optical transmission, measured in the Faraday geometry, is dominated by a broad Lorentzian-shaped mode, whose central frequency linearly increases with applied field. In zero field, the Lorentzian is centered at zero frequency, representing hence the free-carrier Drude response. We interpret the mode as a cyclotron resonance (CR) of free carriers in Bi2Te3. Because the mode's frequency position follows a linear magnetic-field dependence and because undoped Bi2Te3 is known to possess an appreciable number of bulk carriers, we associate the mode with a bulk CR. In addition, the cyclotron mass obtained from our measurements fits well the literature data on the bulk effective mass in Bi2Te3. Interestingly, the width of the CR mode demonstrates a behavior non-monotonous in field. We propose that the CR width is defined by two competing factors: impurity scattering, which rate decreases in increasing field, and electron-phonon scattering, which rate exhibits the opposite behavior.

cond-mat.mes-hall↗