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 .

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.

Lanthanum-hydrogen system and its derivatives hold significant promise for achieving room-temperature superconductivity. In this study, the formation of ternary lanthanum-scandium superhydrides is examined at pressures up to 220 GPa. The primary product of the LaSc alloy's reaction with hydrogen is a newly discovered cubic (La,Sc)H12, demonstrating a clear superconducting transition in all six channels of the van der Pauw-contact scheme at 244-248 K. In this compound with an unusually large unit-cell volume, virtually no magnetoresistance is observed in fields up to 68 Tesla. Synthesized samples of (La,Sc)H12 demonstrate pronounced superconducting diode and SQUID-like effects at a record high temperature of 233 K, which opens up prospects for the use of superhydrides in compact electronics. Furthermore, the analysis reveals the possible formation of a lower hexagonal polyhydride (La,Sc)H6-7, which can potentially account for the drop 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.

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.

An Erratum to this paper has been published: https://doi.org/10.1134/S0021364024010016

In the two-phase crystal EuRbFe4As4/EuFe2As2 (1144/122), a linear ordering of Abrikosov vortices, uncharacteristic for superconducting pnictides, has been found using the method of decorating with magnetic nanoparticles. The observed chains of vortices directed along crystallographic 〈110〉 axes of the orthorhombic EuFe2As2 phase are explained by pinning of vortices in the superconducting phase 1144 on linear defects associated with the twin boundaries of the non-superconducting 122 phase.

Here, by applying a comprehensive approach including magnetic, transport measurements, ARPES band structure measurements, DFT calculations, and analytical theory consideration, we unveil the puzzling origin of the negative isotropic magnetoresistance in the highly anisotropic semimetals, particularly, Eu$_2$Sn$_2$As$_2$ with AFM ordering of Eu atoms. The isotropic magnetoresistance developing along with the magnetization changes up to the complete spin polarization field was reported previously in several experimental studies, though its theoretical explanation was missing up to date. Recently, we proposed a novel theoretical mechanism to describe the observed magnetoresistance in layered AFM compounds by exchange splitting of the electron energy levels and by confining the electron wave functions with different spin projection in the vicinity of the respective magnetic layer. In this paper, we present more detailed experimental studies of the negative magnetoresistance with several samples of EuSn$_2$As$_2$ in order to identify its sample-independent features including temperature dependence. We also substantiate the proposed theory by comparing it with magnetotransport data, with ARPES measurements of the energy band structure, and DFT energy spectrum calculations.

Polyhydride superconductors have been shown to possess metallic properties with a Bardeen-Cooper-Schrieffer-type superconducting ground state. Here, we provide evidence for unconventional transport associated with a pseudogap phase in cubic cerium superhydride CeH_10 (T_C = 116 K) at pressure of 115-125 GPa. A large negative magnetoresistance in the non-superconducting state below 90 K, quasi T-linear electrical resistance, and a sign-change of its temperature dependence mark the emergence of this phase. We studied the magnetic phase diagrams and the upper critical fields B_C2(T) of CeH_10, CeH_9, and CeD_9 in pulsed fields up to 70 T. B_C2(T) of CeH_9 and CeD_9 exhibits pronounced saturation at low temperatures in accordance with the Werthamer-Helfand-Hohenberg model, whereas CeH_10 stands out in particular, as it does not obey this model. Our observations, therefore, reveal the unconventional nature of non-superconducting state of cerium superhydride CeH_10.

The paper describes results of the band structure measurements of SnAs single crystals by the ARPES technique. We performed detailed analysis of isoenergetic surfaces in the vicinity and below the Fermi energy. The ARPES experimental data are consistent with theoretically predicted shape of the SnAs Fermi surface. The determined type of the Fermi surface provides the basis for estimating the Ginzburg–Landau parameter, from which it follows that SnAs is a type I superconductor. In addition, our results of ARPES measurements confirm the presence of band splitting in the energy spectrum at the Γ̅ -point at electron binding energies in the range of 0.6–1.2 eV associated with spin-orbit interaction.

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 with and without spin-orbit coupling (SOC). It is shown that in the stoichiometric system the Lu-5 d 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. Surprisingly, SOC splits quite significantly (0.1–0.25 eV) nitrogen bands in LuH 2.75 N 0.25 just below the Fermi level. 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 100 K and even increase it with further hole doping.

Using electron spin resonance spectroscopy and SQUID-magnetometry we obtained direct evidence of the occurrence of magnetic domains in the antiferromagnetically ordered state of a EuFe2As2 single crystal. The resonance spectra of europium ions were measured in the temperature range from 4 to 200 K. Using an equa-tion for the resonance field in an antiferromagnet that takes into account the exchange and anisotropy fields, we have performed an analysis of the angular dependence of the spectrum at a temperature of 4.8 K, measured upon the crystal rotation around the c axis. Data analysis showed that EuFe2As2 is the antiferromagnet with easy anisotropy plane. Besides, we found in the ab-plane the second order axes of easy magnetization for each of the two types of magnetic domains, related to the structural transition and the formation of twins. Mag-netic anisotropy caused by the exchange interaction of europium ions with iron ions indicates the occurrence of nematic magnetic ordering in the basal ab plane. An estimate of the magnitude of the exchange field and the anisotropy field is obtained from the angular dependence of the resonance fields.

Abstract The chemical interaction of Sn with H2 by X‐ray diffraction methods at pressures of 180–210 GPa is studied. A previously unknown tetrahydride SnH4 with a cubic structure (fcc) exhibiting superconducting properties below TC = 72 K is obtained; the formation of a high molecular C2/m‐SnH14 superhydride and several lower hydrides, fcc SnH2, and C2‐Sn12H18, is also detected. The temperature dependence of critical current density JC(T) in SnH4 yields the superconducting gap 2Δ(0) = 21.6 meV at 180 GPa. SnH4 has unusual behavior in strong magnetic fields: B,T‐linear dependences of magnetoresistance and the upper critical magnetic field BC2(T) ∝ (TC – T). The latter contradicts the Wertheimer–Helfand–Hohenberg model developed for conventional superconductors. Along with this, the temperature dependence of electrical resistance of fcc SnH4 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.

A comprehensive study of vortex phases and vortex dynamics is presented for a recently discovered high-temperature superconductor YH6 with Tc(onset) of 215 K under a pressure of 200 GPa. The thermal activation energy (U0) is derived within the framework of the thermally activated flux flow (TAFF) theory. The activation energy yields a power law dependence U0 ∝ Hα on magnetic field with a possible crossover at a field around 8-10 T. Furthermore, we have depicted the vortex phase transition from the vortex-glass to vortex-liquid state according to the vortex-glass theory. Finally, vortex phase diagram is constructed for the first time for superhydrides. Very high estimated values of flux flow barriers U0(H) = (1.5-7) × 104 K together with high crossover fields make YH6 a rather outstanding superconductor as compared to most cuprates and iron-based systems. The Ginzburg number for YH6 Gi = (3-7) × 10-3 indicates that thermal fluctuations are not so strong and cannot broaden superconducting transitions in weak magnetic fields.

Magnetic state of the EuFe_2As_2 single crystal was investigated by electron spin resonance (ESR). The ESR spectra of europium ions were registered in the temperature range from 250 to 4 K. To study the anisotropy, the in-plane angular dependences were investigated at temperatures of 15 K upon rotating a crystal around the c -axis. The data analysis shows that EuFe_2As_2 is an antiferromagnet with an easy anisotropy plane. Moreover, the second-order easy magnetization axis is observed for each of the two types of magnetic domains related with structural twins arising from the tetragonal-to-orthorhombic crystal lattice transition.