Searching for superconducting hydrides has so far largely focused on finding materials exhibiting the highest possible critical temperatures ($T_c$). This has led to a bias towards materials stabilised at very high pressures, which introduces a number of technical difficulties in experiment. Here we apply machine learning methods in an effort to identify superconducting hydrides which can operate closer to ambient conditions. The output of these models informs structure searches, from which we identify and screen stable candidates before performing electron-phonon calculations to obtain $T_c$. Hydrides of alkali and alkaline earth metals are identified as particularly promising; a $T_c$ of up to 115 K is calculated for RbH$_{12}$ at 50 GPa and a $T_c$ of up to 90 K is calculated for CsH$_7$ at 100 GPa.
The discovery of a record high superconducting transition temperature (Tc) of 288 K in a pressurized hydride inspires new hope to realize ambient condition superconductivity. Here, we give a perspective on the theoretical and experimental studies of hydride superconductivity. Predictions based on the BCS-Eliashberg-Midgal theory with the aid of density functional theory have been playing a leading role in the research and guiding the experimental realizations. To date, about twenty hydrides experiments have been reported to exhibit high-Tc superconductivity and their Tc agree well with the pre…
Motivated by the ambiguous experimental data for the superconducting phase in silane (SiH$_{4}$), which may originate from the platinum hydride (PtH), we provide a theoretical study of the superconducting state in the latter alloy. The quantitative estimates of the thermodynamics of PtH at 100 GPa are given for a wide range of the Coulomb pseudopotential values ($μ^{*}$) within the Eliashberg formalism. The obtained critical temperature value ($T_C\in\left<12.94, 20.01\right>$ for $μ^{*}\in\left<0.05,0.15\right>$) agrees well with the experimental $T_{C}$ for SiH$_{4}$ which may be ascribed to…
Palladium hydrides display the largest isotope effect anomaly known in literature. Replacement of hydrogen with the heavier isotopes leads to higher superconducting temperatures, a behavior inconsistent with harmonic theory. Solving the self-consistent harmonic approximation by a stochastic approach, we obtain the anharmonic free energy, the thermal expansion and the superconducting properties fully ab initio. We find that the phonon spectra are strongly renormalized by anharmonicity far beyond the perturbative regime. Superconductivity is phonon mediated, but the harmonic approximation largel…
Reaching superconductivity at ambient conditions is one of the biggest scientific dreams. The discoveries in the last few years at high pressures place hydrogen-based compounds as the best candidates for making it true. As the recent history shows, first-principles calculations are expected to continue guiding the experimental quest in the right track in the coming years. Considering that ionic quantum fluctuations largely affect the structural and vibrational properties of superconducting hydrides, in many cases making them stable at much lower pressures than expected, it will be crucial to i…
Room-temperature superconductivity was recently discovered in carbonaceous sulfur hydride (C-S-H) close to 3\,Mbar. We report significant differences in the superconducting response of C-S-H, with a maximum $T_{C}$ of 191(1)\,K, below a 1\,Mbar. Variations in intensity of the C-H Raman modes reveal carbon content can vary between crystals synthesized with the same photo-induced method. Synchrotron single crystal x-ray diffraction identifies polymorphism with increasing degrees of covalency. These unique metastable states are highly sensitive to thermodynamic pathways.
High-temperature conventional superconductivity in hydrogen-rich materials under high pressure has been reportedly found in twelve different compounds in recent years. However, the experimental evidence on which these claims are based has recently been called into question. Here we discuss the measurement of trapped magnetic flux, that should establish definitively that these materials are indeed high-temperature superconductors. Its absence should confirm claims to the contrary.
At present, metal hydrides are considered highly promising materials for phonon-mediated superconductors, that exhibit high values of the critical temperature. In the present study, the superconducting properties of the compressed selenium hydride in its simplest form (HSe) are analyzed, toward quantitative characterization of this phase. By using the state-of-art Migdal-Eliashberg formalism, it is shown that the critical temperature in this material is relatively high ($T_{c}$=42.65 K) and surpass the level of magnesium diboride superconductor, assuming that the Coulomb pseudopotential takes …
Due to the value of the isotope shift in sulfur hydrides, a phonon-mediated pairing scenario of superconductivity is generally accepted for these high-temperature superconductors which is consistent with the Bardeen-Cooper-Schrieffer (BCS) framework. Knowing that a large electronic density of states enhances $T_c$, generalized Fermi surface topologies are used to increase it. A multi-band model within the BCS framework is proposed in this work for the description of sulfur hydride superconductors. This model is used to describe some properties of the $H_3S$ superconductor. Strong coupling effe…
We theoretically give an infinite number of metastable crystal structures for the superconducting sulfur hydride H$_{x}$S under pressure. It has been thought that theoretically predicted structures of H$_{2}$S and H$_{3}$S exhibit low and high $T_{\rm c}$ in the experiment, respectively. The newly found structures are long-period modulated crystals where slab-like H$_{2}$S and H$_{3}$S regions intergrow in a microscopic scale. The extremely small formation enthalpy for the H$_{2}$S--H$_{3}$S boundary indicated with the first-principles calculations suggests possible alloying of these phases th…
A diamond anvil cell (DAC) which can generate extremely high pressure of multi-megabar is promising tool to develop a further physics such a high-transition temperature superconductivity. However, electrical transport measurements, which is one of the most important properties of such functional materials, using the DAC is quite difficult because the sample space is very small and a deformation of electrodes under extreme condition. In this study, we fabricated a boron-doped diamond micro-electrode and an undoped diamond insulation on a beveled culet surface of the diamond anvil. By using the …
The high critical superconducting temperatures ($T_c$s) of metal hydride phases with clathrate-like hydrogen networks have generated great interest. Herein, we employ the Density Functional Theory-Chemical Pressure (DFT-CP) method to explain why certain electropositive elements adopt these structure types, whereas others distort the hydrogenic lattice, thereby decreasing the $T_c$. The progressive opening of the H$_{24}$ polyhedra in MH$_6$ phases is shown to arise from internal pressures exerted by large metal atoms, some of which may favor an even higher hydrogen content that loosens the met…
It is demonstrated that resistive transition at 203 K observed in metallic sulfur hydride system at high pressure can be magnetic (rather than superconducting (SC)) in nature. The onset temperature of genuine superconducting transition in these compounds appears to be essentially lower on temperature. The normal-state magnetic (AF SDW) phase transition preceding a superconducting one (Tc < Tm) is characteristic for HTSC cuprates, pnictides (selenides) and organic superconductors. The resistive drop is provided by disappearing of magnetic (AF spin fluctuation) scattering of conduction electrons…
The recent report of superconductivity under high pressure at the record transition temperature of Tc=203K in sulfur hydride has been identified as conventional in view of the observation of an isotope effect upon deuteration. Here it is demonstrated that conventional theories of superconductivity in the sense of BCS or Eliashberg formalisms can neither account for the observed values of Tc nor the pressure dependence of the isotope coefficient. The only way out of the dilemma is a multi-band approach of superconductivity where already small interband coupling suffices to achieve the high valu…
The thermodynamic parameters of the superconducting state in PtH at the pressure 76 GPa have been examined. The calculations have been carried out in the framework of the Eliashberg formalism. It has been found that the critical temperature changes in the range from 30.6 K to 16.8 K, depending on the assumed value of the Coulomb pseudopotential (0.1, 0.2 and 0.3). The other thermodynamic quantities differ significantly from the predictions of the classical BCS theory. In particular: (i) The parameter which corresponds to the superconducting band gap reaches the values from 4.25 to 3.98. (ii) T…
Design of a 60.8 K superconducting hydride LiMgZr2H12 at ambient pressure via Lithium doping
School of Physics, Xidian University（西安电子科技大学物理学院）；College of Physics and Optoelectronic Technology, Baoji University of Arts and Sciences（宝鸡文理学院物理与光电技术学院）
Many modern and proposed future particle accelerators rely on superconducting radio frequency cavities made of bulk niobium as primary particle accelerating structures. Such cavities suffer from the anomalous field dependence of their quality factors Q0. High field degradation - so-called high field Q-slope - is yet unexplained even though an empirical cure is known. Here we propose a mechanism based on the presence of proximity-coupled niobium hydrides, which can explain this effect. Furthermore, the same mechanism can be present in any surface-sensitive experiments or superconducting devices…
Binary hydrides formed by the pnictogens of phosphorus, arsenic and antimony are studied at high pressures using first principles methods. Stable structures are predicted and their electronic, vibrational and superconducting properties are investigated. We predict that SbH$_{4}$ and AsH$_{8}$ will be high-temperature superconductors at megabar pressures, with critical temperatures in excess of 100 K. The highly symmetric hexagonal SbH$_{4}$ phase is predicted to be stabilized above about 150 GPa, which is readily achievable in diamond anvil cell experiments. We find that all phosphorus hydride…
At present, hydrogen-based compounds constitute one of the most promising classes of materials for applications as a phonon-mediated high-temperature superconductors. Herein, the behavior of the superconducting phase in tellurium hydride (HTe) at high pressure ($p=300$ GPa) is analyzed in details, by using the isotropic Migdal-Eliashberg equations. The chosen pressure conditions are considered here as a case study which corresponds to the highest critical temperature value ($T_{c}$) in the analyzed material, as determined within recent density functional theory simulations. It is found that th…
This article reports the experimentally clarified crystal structure of a recently discovered sulfur hydride in high temperature superconducting phase which has the highest critical temperature Tc over 200 K which has been ever reported. For understanding the mechanism of the high superconductivity, the information of its crystal structure is very essential. Herein we have carried out the simultaneous measurements electrical resistance and synchrotron x-ray diffraction under high pressure, and clearly revealed that the hydrogen sulfide, H2S, decomposes to H3S and its crystal structure has body-…
Capitani and coworkers [1] reported that infrared optical reflectance measurements provided evidence for a superconducting transition in sulfur hydride [2] under 150 GPa pressure, and that the transition is driven by the electron-phonon interaction. Here we argue that the measured data did not provide evidence that the system undergoes a transition to a superconducting state, nor do the data support any role of phonons in driving a transition. Rather, the data are consistent with the system remaining in the normal state down to temperature 50K, the lowest temperature measured in the experiment…
We present a new technique for measuring the critical temperature Tc in the high pressure, high Tc electron-phonon-driven superconducting hydrides. This technique does not require connecting leads to the sample. In the multiphonon region of the absorption spectrum, the reflectance mirrors the temperature variation of the superconducting order parameter. For an appropriately chosen value of photon energy of order twice the gap plus 1.5 times the maximum phonon energy, the temperature dependence of the reflectance varies much more rapidly below T=Tc than above. It increases with increasing tempe…
We calculate superconducting transition temperatures ($T_{\rm c}$) in sulfur hydrides H$_{2}$S and H$_{3}$S from first principles using the density functional theory for superconductors. At pressures of $\lesssim$150 GPa, the high values of $T_{\rm c}$ ($\gtrsim$130 K) observed in the recent experiment [A. P. Drozdov, M. I. Eremets, and I. A. Troyan, arXiv:1412.0460] are accurately reproduced by assuming that H$_{2}$S decomposes into $R3m$-H$_{3}$S and S. For the higher pressures, the calculated $T_{\rm c}$s for $Im3m$-H$_{3}$S are systematically higher than those for $R3m$-H$_{3}$S and the ex…
We report the ac magnetic susceptibility, electrical resistance, and X-ray diffraction measurements of platinum hydride (PtHx) in diamond anvil cells, which reveal its superconducting transition. At 32 GPa, when PtHx is in a P63/mmc structure, PtHx exhibits superconducting transition at 6.7 K and superconducting transition temperature (Tc) decreases with pressure to 4.8 K at 36 GPa. The observed T c is higher than that of powdered Pt by more than three orders of magnitude. It is suggested that hydrides of noble metals have higher Tc than the elements.
Superconducting transitions in the absence of magnetic field should be non-hysteretic. Here we address the fact that the drops in electrical resistance that have been interpreted as evidence of superconductivity in several hydrides under high pressure (so-called "superhydrides") show hysteresis. We argue that the experimental evidence shows that the observed hysteresis cannot be attributed to experimental artifacts but is intrinsic to the samples. Assuming that the drops in resistance signal a thermodynamic phase transition, we argue that the presence of intrinsic thermal hysteresis indicates …
Recently, the discovery of room-temperature superconductivity (SC) was experimentally realized in the fcc phase of LaH$_{10}$ under megabar pressure. Specifically, the isotope effect of $T_{\rm c}$ was measured by the replacement of hydrogen (H) with deuterium (D), demonstrating a driving role of phonons in the observed room-temperature SC. Herein, based on the first-principles calculations within the harmonic approximation, we reveal that (i) the identical electron-phonon coupling constants of fcc LaH$_{10}$ and LaD$_{10}$ decrease monotonously with increasing pressure and (ii) the isotope ef…
Noble metals adopt close-packed structures at ambient pressure and rarely undergo structural transformation at high pressures. Platinum (Pt), in particular, is normally considered to be unreactive and is therefore not expected to form hydrides under pressure. We predict that platinum hydride (PtH) has a lower enthalpy than its constituents solid Pt and molecular hydrogen at pressures above 21.5 GPa. We have calculated structural phase transitions from tetragonal to hexagonal close-packed or face-centered cubic (fcc) PtH between 70 and 80 GPa. Linear response calculations indicate that PtH is a…
We report ab-initio calculations of the superconducting properties of two high-Tc sodalite-like clathrate yttrium hydrides, YH6 and YH10, within the fully anisotropic ME theory, including Coulomb corrections. For both compounds we find almost isotropic superconducting gaps, resulting from a uniform distribution of the electron-phonon coupling over phonon modes and electronic states of mixed Y and H character. The Coulomb screening is rather weak, resulting in a Morel-Anderson pseudopotential mu*= 0:11, at odds with claims of unusually large Tc in lanthanum hydrides. The corresponding critical …
The maximum critical temperature for superconductivity in pressurized hydrides appears at the top of superconducting domes in Tc versus pressure curves at a particular pressure, which is not predicted by standard superconductivity theories. The a high-order anisotropic van Hove singularity near the Fermi level observed in band structure calculations of pressurized sulfur hydride, typical of a supermetal, has been associated with the array of metallic hydrogen wires modules forming a nanoscale heterostructure at atomic limit called superstripes phase. Here we propose that pressurized sulfur hyd…
The discovery of superconductivity at 203K in SH$_3$ is an important step toward higher values of $T_c$. Predictions based on state-of-the-art DFT for the electronic structure, including one preceding experimental confirmation, showed the mechanism to be the electron-phonon interaction. This was confirmed in optical spectroscopy measurements. For photon energies between $\sim 450$ and 600 meV in SH$_3$, the reflectance in the superconducting state is below that in its normal state. This difference decreases as $T$ approaches $T_c$. Decreasing absorption with increasing $T$ is opposite to what …
The measurement of superconductivity at above 200K in compressed samples of hydrogen sulfide and lanthanum hydride at 250K is reinvigorating the search for conventional high temperature superconductors. At the same time it exposes a fascinating interplay between theory, computation and experiment. Conventional superconductivity is well understood, and theoretical tools are available for accurate predictions of the superconducting critical temperature. These predictions depend on knowing the microscopic structure of the material under consideration, and can now be provided through computational…
Search for thermodynamically stable ambient-pressure superconducting hydrides in GNoME database
Max-Planck-Institut für Mikrostrukturphysik（马克斯·普朗克微结构物理研究所）；Martin-Luther-Universität Halle-Wittenberg（马丁·路德·哈勒-维滕贝格大学）；University of Coimbra（科英布拉大学）；Periodic Labs（周期实验室）；University of the Basque Country (UPV/EHU)（巴斯克大学）；Centro de Física de Materiales (CFM-MPC), CSIC-UPV/EHU（材料物理中心，西班牙国家研究委员会-巴斯克大学）；Donostia International Physics Center (DIPC)（圣塞巴斯蒂安国际物理中心）
Recently, compressed H$_2$S has been shown to become superconducting at 203 K under a pressure of 155 GPa. One might expect fluctuations to dominate at such temperatures. Using the magnetisation critical current, we determine the ground-state London penetration depth, $λ_0$=189 nm, and the superconducting energy gap, $Δ_0$=27.8 meV, and find these parameters are similar to those of cuprate superconductors. We also determine the fluctuation temperature scale, $T_{\textrm{fluc}}=1470$ K, which shows that, unlike the cuprates, $T_c$ of the hydride is not limited by fluctuations. This is due to it…
Lanthanide hydrogen-rich materials have long been considered as one of the candidates with high-temperature superconducting properties in condensed matter physics, and have attracted great interest. Attempts to investigate the effects of different compositions of lanthanide hydrogen-rich materials are ongoing, with predictions and experimental studies in recent years having shown that substances such as LaH 10 , CeH 9 , and LaH 16 exhibit extremely high superconducting temperatures between 150-250 GPa. In particular, researchers have noted that in those materials an increase in the f character…
The phenomenon of high temperature superconductivity, approaching room temperature, has been realized in a number of hydrogen-dominant alloy systems under high pressure conditions1-12. A significant discovery in reaching room temperature superconductivity is the photo-induced reaction of sulfur, hydrogen, and carbon that initially forms of van der Waals solids at sub-megabar pressures. Carbonaceous sulfur hydride has been demonstrated to be tunable with respect to carbon content, leading to different superconducting final states with different structural symmetries. A modulated AC susceptibili…
We synthesized lanthanum hydride (LaHx) by laser heating of lanthanum in hydrogen atmosphere at pressure P = 170 GPa. The sample shows a superconducting step at 209 K and 170 GPa and temperature dependence of resistance. By releasing the pressure to 150 GPa, the superconducting transition temperature Tc increases to 215 K - the record Tc. This finding supports a way of achieving Tc higher than the one in H3S (203 K) in hydrides with sodalite-like structures, first proposed for CaH6 (Tc=245 K) and later for yttrium and lanthanum hydrides where higher, room temperature superconductivity is expec…
It has been a hot issue to search for hydrides at high pressure with high superconducting transition temperature (Tc). In this work, ternary hydrogen-rich compounds KYHn (n = 1-16) were studied and calculated using the density function theory (DFT) method and he results showed that the KYH8 demonstrates good stability and superconductivity. And keep thermodynamically stable in the pressure range of 50-300 GPa. The research found that the Tc of KYH8-C2/m was predicted to be 111.3K, 122.0K, and 142.3K at 200, 250 and 300 GPa, respectively. Our current study provides a possibility for searching n…
We introduce below the concept of defecton, and describe briefly the electron polaron effect, the clustering of defectons, and also the defecton mechanism of superconductivity. It is shown that in the case of high-temperature superconducting metal hydrides, which acquire superconducting properties under pressure of hundreds of gigapascals, this mechanism can make a significant contribution to the Cooper pairing.
Physical properties of transition metal hydride superconductors Mg2TmH6 (Tm = Rh, Pd, Ir, Pt) by first-principles calculations
Mawlana Bhashani Science and Technology University（毛拉纳·巴沙尼科技大学）；University of Rajshahi（拉杰沙希大学）；Rajshahi University of Engineering and Technology（拉杰沙希工程与技术大学）
Recently YH6, compound from a group of theoretically predicted stable compressed MH6 hydrides (M/ Ca, Mg, Y, Sc) with bcc Im-3m crystal structure, was successfully experimentally realized. Superconductivity of pressurized YH6 was confirmed experimentally, Tc / 224 K at 166 GPa, but with critical temperature considerably lower than value predicted by Migdal-Eliashberg (ME) theory. Here we present theoretical reinvestigation of superconductivity in MH6 hydrides. Our results confirm that YH6 and ScH6 with Im-3m structure at corresponding GPa pressures are superconductors but with an antiadiabatic…
Evolutionary algorithms and the particle swarm optimization method have been used to predict stable and metastable high hydrides of iron between 150-300 GPa that have not been discussed in previous studies. Cmca FeH5, Pmma FeH6 and P2/c FeH6 contain hydrogenic lattices that result from slight distortions of the previously predicted I4/mmm FeH5 and Cmmm FeH6 structures. Density functional theory calculations show that neither the I4/mmm nor the Cmca symmetry FeH5 phases are superconducting. A P1 symmetry FeH7 phase, which is found to be dynamically stable at 200 and 300 GPa, adds another member…
