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 (T_c = 218-221 K) and YH_9 (T_c = 235-237 K) demonstrate narrow superconducting transitions (ΔT_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 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 T_c of cubic YH_10 to approximately 260-270 K. These results strongly disfavor room-temperature superconductivity in binary yttrium superhydrides.

We review five years of experimental and theoretical attempts (2020–2025) to enhance the superconducting critical temperature ( T c ) of hydrogen-rich compounds by alloying binary superhydrides with additional elements. Despite predictions of higher T c in ternary systems such as La–Y–H, La–Ce–H, and Ca–Mg–H, experiments consistently show that the maximum 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 T c ≈ 200 K near 100–110 GPa, while also strengthening vortex pinning and upper critical fields. Magnetic dopants suppress T c , whereas nonmagnetic additives leave it nearly unchanged, reminiscent of Anderson's theorem. These findings indicate that alloying is unlikely to raise T c , but can reduce the pressures required to stabilize high- 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 .

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 T̂_2 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. Layered van der Waals crystals with antiferromagnetic ordering can exhibit magnetoresistance, as observed experimentally in several semimetals. Here, a theoretical mechanism is proposed to describe magnetoresistance in antiferromagnetic metals.

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.

A new superconductor BaAg1.8Bi2 with a previously unknown variant of the monoclinically distorted CaBe2Ge2 structure (space group C2/m) has been crystallized in the form of single crystals from a bismuth flux. The temperature and magnetic field dependences of the magnetic susceptibility and resistance have shown that this compound transits to a superconducting state at the temperature Tc = 5.4 K. According to the found Ginzburg–Landau parameter κ = 27, this compound is a type-II superconductor with the first and second critical fields Hc1(0) = 53 Oe and Hc2(0) = 2.1 × 104 Oe and the critical current density reaching 4.4 kA/cm2 at 2.5 K. It can be assumed that, similar to some superconducting 112 type bismuthides, superconductivity in this compound is due to the planar square bismuth sublattice contained in the fluorite-like [BiAg0.8] layer rather than to the [AgBi] layers where Ag atoms are locally disordered. This could explain the unusually high Tc value for bismuthides belonging to the CaBe2Ge2 structural type and its derivatives.

The superconducting order parameter of the RbCa2Fe4As4F2 compound belonging to the new 12442 family of iron-based superconductors with a critical temperature of T c ~ 32 K has been studied. Two superconducting condensates with the order parameters ΔL ~ 6.3 meV and ΔS ~ 2.8 meV have been detected for the first time by multiple Andreev reflection spectroscopy. The temperature dependence of the superconducting critical current density J c(T) in the intrinsic field has been measured. The approximation of the dependence J c(T) demonstrates the correspondence of the experimental data to the double-gap model with the s-wave order parameter and the gaps of ΔL ~ 6 meV and ΔS ~ 2 meV. The superconducting order parameters obtained by two different methods are in good agreement with each other.

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.

The paper reports complementary studies of the order parameter of the newly discovered iron-based superconductor, KCa 2 Fe 4 As 4 F 2 ( T c = 34 K), that consists of alternating blocks of KFe 2 As 2 (122) phase and CaFeAsF (1111) phase. We performed high-resolution intrinsic multiple Andreev reflection spectroscopy supplemented with self-field critical current measurements. Andreev spectra obtained by means of a break-junction technique reveal two distinct superconducting order parameters Δ L and Δ S . Their temperature dependencies were measured from 1.5 K to T c . The shapes of Andreev reflections dips, corresponding to the two gaps are symmetrical, which is a signature of s -wave order parameter. Temperature dependence of the self-field critical current can also be described by two s -wave gaps model.

The behavior of the critical current density is studied in a self-field of a single-crystal NaFe1 – xCoxAs (Tc = 21.1 K) sample in the temperature range from 4 K to Tc. Within the Bardeen–Cooper–Schrieffer (BCS) model, the dependences of the superconducting carrier density ρs(T) are obtained under the assumption of the two-band case. It is shown in the case of two-gap uperconductivity, the experimental data are well described by the theory, and the superconducting gaps Δ(0) obtained by approximation are in agreement with the values determined by studying the Andreev reflection spectra.

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 Mössbauer spectroscopy, as well as galvanomagnetic measurement techniques.This paper presents the major results of galvanomagnetic studies, including measurements in high static (up to 21 T) and pulsed (up to 70 T) 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 ρ(T ), and a linear magnetoresistance, very similar to that discovered by P. L. Kapitza in 1929.

Layered topologically non-trivial and trivial semimetals with AFM-type 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 EuFe_2As_2, EuSn_2As_2, EuSn_2P_2, etc., where the resistance decreases quadratically with field by about δρ/ρ∼ 4-6% up to the spin-polarization field. Despite the fact that this effect is well documented experimentally, its theoretical explanation is missing up to date. In this paper we propose a novel theoretical mechanism describing the observed magnetoresistance that does not imply either topological origin of the materials, surface roughness, their potential defect structure, or electron-magnon scattering. We believe, the proposed intrinsic mechanism of magnetoresistance is applicable to a wide class of the layered AFM- ordered semimetals. The theoretically calculated magnetoresistance is qualitatively consistent with experimental data for crystals of various composition.

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.

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.

Superconducting memory is a promising technology for data storage because of its speed, high energy efficiency, non-volatility, and compatibility with quantum computing devices. However, the need for cryogenic temperatures renders superconducting memory an extremely expensive and specialized device. Ternary lanthanum polyhydrides, due to their high critical temperatures of 240-250 K, represent a convenient platform for studying effects associated with superconductivity in disordered granular systems. In this work, we investigate trapped magnetic flux and memory effects in recently discovered lanthanum-neodymium (La,Nd)H10 and lanthanum- scandium (La,Sc)H12 superhydrides at a pressure of 175-196 GPa. We use a steady magnetic field of a few Tesla (T) and strong pulsed fields up to 68 T to create the trapped flux state in the compressed superhydrides. We find a clockwise hysteresis of magnetoresistance in cerium CeH 9-10 and lanthanum-cerium (La,Ce)H10+x poly- hydrides, a characteristic feature of granular superconductors. A study of the current-voltage characteristics and voltage-temperature curves of the samples with trapped magnetic flux indicates a significant memory effect in La-Sc polyhydrides already at 225-230 K.

We report a synthesis of two members of recently discovered high-temperature superconductors of 12442 family, with formula MCa2Fe4As4F2 (M=Rb, K) and transition temperatures of 32.7 and 34.6 K, respectively. Quality of the samples was assessed using X-ray powder diffraction, superconducting transitions were identified through transport and magnetic experiments. The temperature dependence of the upper critical field and vortex activation energy was investigated under magnetic fields up to 19 T. Two distinct thermally activated flux flow regimes were observed in both systems. Field dependences of activation energy U0(H) indicate a change in the properties of vortex matter in these regimes and distinctly different dissipation mechanisms, reminiscent of cuprate HTSC.

The paper reports complementary studies of the order parameter of the newly discovered iron-based superconductor, KCa2Fe4As4F2 (Tc = 34 K), that consists of alternating blocks of KFe2As2 (122) phase and CaFeAsF (1111) phase. We performed high-resolution intrinsic multiple Andreev reflection spectroscopy supplemented with self-field critical current measurements. Andreev spectra, obtained by means of a break-junction technique reveal two distinct superconducting order parameters – ΔL and ΔS.T heir temperature dependencies were measured from 1.5 K to Tc. The shape of Andreev reflections dips, corresponding to the two gaps, are symmetrical, which is a signature of s-wave order parameter. Temperature dependence of the self-field critical current can also be described by two s-wave gaps model.

The transport properties of nematic aerogels, which consist of oriented mullite nanofibers coated with a graphene shell, were studied. It is shown that the magnetoresistance of this system is well approximated by two contributions - negative one, described by the formula for systems with weak localization , and positive contribution, linear in the field and unsaturated in large magnetic fields. The behavior of phase coherence length on temperature obtained from the analysis of the negative contribution indicates the main role of the electron-electron interaction in the destruction of phase coherence and, presumably, the transition at low temperatures from a two-dimensional weak localization regime to a one-dimensional one. The positive linear contribution to magnetoresistance is apparently due to the inhomogeneous distribution of the local carrier density in the conductive medium. It has also been established that the temperature dependence of the resistance for graphenized aerogels with a low carbon content, when the graphene coating is apparently incomplete, can be represented as the sum of two contributions, one of which is characteristic of weak localization, and the second is described by hopping mechanism corresponding to the Shklovskii-Efros law in the case of a granular conductive medium. For samples with a high carbon content, there is no second contribution.

Measurements of the magnetoresistance and Hall effect on transparent films of single-wall nanotubes before and after their doping with iodine, in the temperature range from 3-5 to 40 K and magnetic fields up to 16 T, are reported. The measured magnetoresistance is fairly well approximated by the sum of two contributions: negative, described in the framework of two-dimensional theory of weak localization (2D-WL), and linear in field positive, unsaturated in strong magnetic fields. The linear temperature dependence of L (phi) (- 2 )( T ) ( L-phi is the phase decoherence length) indicates electron-electron scattering as the main mechanism of dephasing, and the coincidence of these linear dependencies for the pristine and doped films suggests that the state of static defects and the nature of electron-electron scattering do not change when doping nanotubes. The values of the two-dimensional density of charge carriers determined from Hall measurements, being, apparently, highly overestimated, increase more than twice after iodine doping. This increase is clearly correlated with the magnetoresistance data processed in 2D-WL theory. The linear in field positive contribution to magnetoresistance is explained by the inhomogeneous distribution of the local density of charge carriers.