arXiv subjects

Andrey V. Sadakov

Publications and source records attributed to Andrey V. Sadakov.

12 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 ($\textit{${\Delta}$T$_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↗

arXiv2025

Universal negative magnetoresistance in antiferromagnetic metals caused by symmetry breaking of electron wave functions

Layered van der Waals crystals of topologically non-trivial and trivial semimetals with antiferromagnetic (AFM) ordering of magnetic sublattice are known to exhibit a negative magnetoresistance that is well correlated with AFM magnetization changes in a magnetic field. This effect is reported in several experimental studies with EuFe2As2, EuSn2As2, EuSn2P2, etc., where the resistance decreases quadratically with field by about 5% up to the spin-polarization field. Although this effect is well documented experimentally, its theoretical explanation is missing up to date. Here, we propose a theoretical mechanism describing the observed magnetoresistance that is inherent in AFM metals and is based on violation the binary T2 symmetry. It is almost isotropic to the field and current directions, contrary to the known mechanisms such as giant magnetoresistance and chiral anomaly. The proposed intrinsic mechanism of magnetoresistance is strong in a wide class of the layered AFM-ordered semimetals. The theoretically calculated magnetoresistance is qualitatively consistent with experimental data for crystals of various composition.

cond-mat.str-el↗

arXiv2025

Stability and Superconductivity of Ternary Polyhydrides

We review five years of experimental and theoretical attempts (2020-2025) to enhance the superconducting critical temperature ($\textit{T$_c$}$) of hydrogen-rich compounds by alloying binary superhydrides with additional elements. Despite predictions of higher $\textit{T$_c$}$ in ternary systems such as La-Y-H, La-Ce-H, and Ca-Mg-H, experiments consistently show that the maximum $\textit{T$_c$}$ in disordered ternary superhydrides does not exceed that of the best binary parent hydrides within experimental uncertainty. Instead, alloying primarily stabilizes high-symmetry polyhydride phases at lower pressures, enabling $\textit{T$_c$}$ = 200 K near 110-120 GPa, while also improving vortex pinning and upper critical fields. Magnetic dopants suppress $\textit{T$_c$}$, whereas nonmagnetic additives leave it nearly unchanged, reminiscent of Anderson's theorem. These findings indicate that alloying is unlikely to raise $\textit{T$_c$}$, but can reduce the pressures required to stabilize high-$\textit{T$_c$}$ phases. We propose that fully ordered ternary hydrides, synthesized via controlled hydrogenation of intermetallic precursors, offer a promising route toward this goal. One of the most promising compounds of this kind is the recently discovered LaSc$_2$H$_{24}$.

cond-mat.supr-con↗

arXiv2025

Vortex matter and strong pinning in underdoped PrFeAs(O,F) with atomic-sized defects

We present a comprehensive investigation of the field-dependent critical current density and pinning force, combined with a detailed analysis of the nanostructural defect landscape in single crystal of underdoped PrFeAs(O,F) superconductor. Our study demonstrates that for both in-plane and out-of-plane magnetic field orientations critical current density exhibits a strong pinning regime in intermediate fields across the entire temperature range. The dominant contribution to pinning originates from oxygen-to-fluorine substitutional defects, oxygen vacancies, which all act as point defects via a quasiparticle mean free path fluctuation mechanism. Scanning transmission electron microscope studies did not reveal any volume or surface defect types within the lattice.

cond-mat.supr-con↗

arXiv2025

Superconducting gap in covalent bismuth dihydride BiH$_2$ under extreme conditions

Hydride superconductors at megabar pressures provide a promising platform for exploring room-temperature superconductivity. However, their superconducting gaps remain largely inaccessible to conventional spectroscopic due to diamond anvil cell constraints and minute sample dimensions. Here we develop a pulsed current method and apply it to covalent BiH$_2$ synthesized at 157--176 GPa. BiH$_2$ exhibits superconductivity at 58--70 K and upper critical fields of 11--17 T, substantially lower than those of many clathrate superhydrides, corresponding to a relatively long coherence length and an experimentally accessible critical current density. Short rectangular pulses minimize sustained Joule heating and enable currents up to 160 mA, allowing $J_c(T)$ to be measured in the low-temperature regime down to 2 K at 176 GPa. The normalized critical-current response remains reproducible between two measurement runs and is better described by a two-scale $s$-wave model than by single-gap $s$- or $d$-wave models, yielding effective energy scales of approximately 6.9 and 1.5 meV. Fully anisotropic Migdal--Eliashberg calculations yield a single highly anisotropic gap, suggesting that the two-gap behavior observed experimentally originates from gap anisotropy rather than two independent gaps. These results establish pulsed critical-current measurements as a practical gap-sensitive transport probe under extreme pressure and, with further increases in peak-current capability, provide a route toward investigating room-temperature hydrides such as La--Sc--H.

cond-mat.supr-con↗

arXiv2024

Progress, problems and prospects of room-temperature superconductivity

Discovery of superconductivity at megabar (MB) pressures in hydrogen sulfide H3S, then in metal polyhydrides, starting with binary, LaH10, etc., and ending with ternary ones, including (La, Y)H10, revolutionized the field of condensed matter physics. These discoveries strengthen hopes for solution of the century-old problem of creating materials that are superconducting at room temperature. In experiments performed over the past 5 years at MB pressures, in addition to the synthesis of hydrides itself, their physical properties were studied using optical, X-ray and Mossbauer spectroscopy, as well as galvanomagnetic measurement techniques. This paper presents the major results of galvanomagnetic studies, including measurements in high static (up to 21T) and pulsed (up to 70T) magnetic fields. Measurements of resistance drops to vanishingly small level at temperatures below the critical Tc value, a decrease in the critical temperature Tc with increasing magnetic field, as well as diamagnetic screening, indicate the superconducting state of the polyhydrides. The results of measurements of the isotope effect, together with the effect of magnetic impurities on Tc, indicate the electron-phonon mechanism of electron pairing. However, electron-electron correlations in polyhydrides are by no means small, both in the superconducting and normal states. It is possible that this is precisely what accounts for the unusual properties of polyhydrides that have not yet received a satisfactory explanation, such as a linear temperature dependence of the second critical field Hc2(T), a linear dependence of resistance R(T), and a linear magnetoresistance, very similar to that discovered by P. L. Kapitza in 1929.

cond-mat.supr-con↗

arXiv2023

Quasi Two-dimensional Vortex Matter in ThH$_{10}$ Superhydride

A comprehensive study of the vortex phases and vortex dynamics is presented for a recently discovered high-temperature superconductor ThH$_{10}$ with $\textit{T}$$_C$ = 153 K at 170 GPa. The obtained results strongly suggest a quasi two-dimensional (2D) character of the vortex glass phase transition in ThH$_{10}$. The activation energy yields a logarithmic dependence $\textit{U}$$_0$ $\propto$ ln($\textit{H}$) on magnetic field in a low field region and a power law dependence $\textit{U}$$_0$ ~ $\textit{H}$$^{-1}$ in a high field region, signaling a crossover from 2D regime to 3D collective pinning regime, respectively. Additionally, a pinning force field dependence showcases dominance of surface-type pinning in the vicinity of $\textit{T}$$_C$. Thermal activation energy ($\textit{U}$$_0$), derived within thermally activated flux flow (TAFF) theory, takes very high values above 2$\times$10$^5$ K together with the Ginzburg number $\textit{Gi}$ = 0.039 - 0.085, which is lower only than those of BiSrCaCuO cuprates and 10-3-8 family of iron based superconductor. This indicates the enormous role of thermal fluctuations in the dynamics of the vortex lattice of superhydrides, the physics of which is similar to the physics of unconventional high-temperature superconductors.

cond-mat.supr-con↗

arXiv2023

Non-Fermi-Liquid Behavior of Superconducting SnH$_4$

We studied chemical interaction of Sn with H$_2$ by X-ray diffraction methods at pressures of 180-210 GPa. A previously unknown tetrahydride SnH$_4$ with a cubic structure (${fcc}$) exhibiting superconducting properties below ${T}$$_C$ = 72 K was obtained; the formation of a high molecular ${C2/m}$-SnH$_{14}$ superhydride and several lower hydrides, ${fcc}$ SnH$_2$ and ${C2}$-Sn$_{12}$H$_{18}$, was also detected. The temperature dependence of critical current density ${J}$$_C$(T) in SnH$_4$ yields the superconducting gap 2$Δ$(0) = 23 meV at 180 GPa. SnH$_4$ has unusual behavior in strong magnetic fields: ${B,T}$-linear dependences of magnetoresistance and the upper critical magnetic field ${B}$$_{C2}$(T) $\propto$ (${T}$$_C$ - ${T}$). The latter contradicts the Wertheimer-Helfand-Hohenberg model developed for conventional superconductors. Along with this, the temperature dependence of electrical resistance of ${fcc}$ SnH$_4$ in non-superconducting state exhibits a deviation from what is expected for phonon-mediated scattering described by the Bloch-Grüneisen model, and is beyond the framework of the Fermi liquid theory. Such anomalies occur for many superhydrides, making them much closer to cuprates than previously believed.

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

Anomalous high-temperature superconductivity in YH$_6$

Pressure-stabilized hydrides are a new rapidly growing class of high-temperature superconductors which is believed to be described within the conventional phonon-mediated mechanism of coupling. Here we report the synthesis of yttrium hexahydride Im3m-YH$_6$ that demonstrates the superconducting transition with T$_c$ = 224 K at 166 GPa, much lower than the theoretically predicted (>270 K). The measured upper critical magnetic field B$_c$$_2$(0) of YH$_6$ was found to be 116-158 T, which is 2-2.5 times larger than the calculated value. A pronounced shift of T$_c$ in yttrium deuteride YD$_6$ with the isotope coefficient 0.4 supports the phonon-assisted superconductivity. Current-voltage measurements showed that the critical current I$_c$ and its density J$_c$ may exceed 1.75 A and 3500 A/mm$^2$ at 0 K, respectively, which is comparable with the parameters of commercial superconductors, such as NbTi and YBCO. The superconducting density functional theory (SCDFT) and anharmonic calculations suggest unusually large impact of the Coulomb repulsion in this compound. The results indicate notable departures of the superconducting properties of the discovered YH$_6$ from the conventional Migdal-Eliashberg and Bardeen-Cooper-Schrieffer theories.

cond-mat.supr-con↗

arXiv2019

Superconductivity at 161 K in Thorium Hydride $ThH_{10}$: Synthesis and Properties

Here we report targeted high-pressure synthesis of two novel high-$T_C$ hydride superconductors, $P6_3/mmc$-$ThH_9$ and $Fm\bar{3}m$-$ThH_{10}$, with the experimental critical temperatures ($T_C$) of 146 K and 159-161 K and upper critical magnetic fields ($μ$$H_C$) 38 and 45 Tesla at pressures 170-175 Gigapascals, respectively. Superconductivity was evidenced by the observation of zero resistance and a decrease of $T_C$ under external magnetic field up to 16 Tesla. This is one of the highest critical temperatures that has been achieved experimentally in any compounds, along with such materials as $LaH_{10}$, $H_3S$ and $HgBa_2Ca_xCu_2O_{6+z}$. Our experiments show that $fcc$-$ThH_{10}$ has stabilization pressure of 85 GPa, making this material unique among all known high-$T_C$ metal polyhydrides. Two recently predicted Th-H compounds, $I4/mmm$-$ThH_4$ (> 86 GPa) and $Cmc2_1$-$ThH_6$ (86-104 GPa), were also synthesized. Equations of state of obtained thorium polyhydrides were measured and found to perfectly agree with the theoretical calculations. New phases were examined theoretically and their electronic, phonon, and superconducting properties were calculated.

cond-mat.mtrl-sci↗