In the present paper we report comprehensive analysis of the thermodynamic properties of novel hydrogen dominant potassium hydride superconductor (KH$_{6}$). Our computations are conducted within the Eliashberg theory which yields quantitative estimations of the most important thermodynamic properties of superconducting phase. In particular, we observe, that together with the increasing pressure all the thermodynamic properties decrease, e.g. $T_{C} \in \left< 72.91 , 55.50 \right>$ K for $p \in \left< 166 , 300 \right>$ GPa. It is predicted that such decreasing behavior corresponds to the decreasing hydrogen lattice molecularization with increasing pressure value. Futhermore, by calculating the dimensionless thermodynamic ratios, familiar in the Bardeen-Cooper-Schrieffer theory (BCS), it is proved that KH$_{6}$ material is a strong-coupling superconductor and cannot be quantitatively described within the BCS theory.
We have systematically searched for the ground state structures of bismuth hydrides based on evolutionary algorithm method and particle swarm optimization algorithm method. Given only rich-hydrogen region, except for BiH$_{3}$, other hydrides (BiH, BiH$_{2}$, BiH$_{4}$, BiH$_{5}$, BiH$_{6}$) have been predicted to be stable with pressurization. With the increase of hydrogen content, hydrogen exists in bismuth hydrides with the different forms and presents the characteristics of ionicity. Under high pressure, the remarkable structural feature is the emergence of H$_{2}$ units in BiH$_{2}$, BiH$…
The high critical superconducting temperatures ($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…
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 report the observation of conventional superconductivity at the highest temperature yet attained without mechanical compression, around 54 kelvin in palladium-hydride and 60 kelvin in palladium-deuteride. The remarkable increase in Tc compared to the previously known value was achieved by rapidly cooling the hydride and deuteride after loading with hydrogen or deuterium at elevated temperature. Our results encourage hope that conventional superconductivity under ambient conditions will be discovered in materials with very high hydrogen density, as predicted more than a decade ago.
Following the idea that hydrogen-rich compounds might be high-T$_c$ superconductors at high pressures, and the very recent breakthrough in predicting and synthesizing hydrogen sulfide with record-high T$_c$ = 203 K, ab initio evolutionary algorithm for crystal structure prediction was employed to find stable germanium hydrides. In addition to the earlier structure of germane with space group Ama2, we propose a new C2/m structure, which is energetically more favorable at pressures above 278 GPa (with inclusion of zero point energy). Our calculations indicate metallicity of the new C2/m phase of…
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…
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…
Hydrogen-rich superhydrides are promising high-Tc superconductors, with superconductivity experimentally observed near room temperature, as shown in recently discovered lanthanide superhydrides at very high pressures, e.g. LaH10 at 170 GPa and CeH9 at 150 GPa. Superconductivity is believed to be closely related with the high vibrational modes of the bound hydrogen ions. Here we study the limit of extreme pressures (above 200 GPa) where lanthanide hydrides with large hydrogen content have been reported. We focus on LaH16 and CeH16, two prototype candidates for achieving a large electronic contr…
Niobium provides the basis for all superconducting radio frequency (SRF) cavities in use, however, hydrogen is readily absorbed by niobium during cavity fabrication and subsequent niobium hydride precipitation when cooled to cryogenic temperatures degrades its superconducting properties. In the last few years the addition of dopant elements such as nitrogen has been experimentally shown to significantly improve the quality factor of niobium SRF cavities. One of the contributors to Q degradation can be presence of hydrides, however, the underlying mechanisms associated with the kinetics of hydr…
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…
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…
Hydrogen-based superconductors provide a route to the long-sought goal of room-temperature superconductivity, but the high pressures required to metallize these materials limit their immediate application. For example, carbonaceous sulfur hydride, the first room-temperature superconductor, can reach a critical temperature (Tc) of 288 K only at the extreme pressure of 267 GPa. The next recognized challenge is the realization of room-temperature superconductivity at significantly lower pressures. Here, we propose a strategy for the rational design of high-temperature superconductors at low press…
The method of mathematical modeling was used for the calculation of the structural, electronic, phonon, and other characteristics of various normal phases of phosphorus hydrides with stoichiometry PHk. It was shown that the di-phosphine may form 2D lattice of the metallic hydrogen, stabilized by phosphorus atoms under high hydrostatic compressive pressure. The resulting structure with the elements of H-P-H is a locally stable one in the phonon spectrum (or metastable). The properties of di-phosphine with the properties of similar structures such as thesulphur hydrides were compared.
The recent reports on high-temperature superconductivity above 190 K in hydrogen sulfide at 200 GPa pressure, exceeding all previously discovered superconductors, has greatly invigorated the interest in dense hydrogen-rich solids. In this paper, we investigate a possible way to optimize the critical temperature in these compounds using first-principles linear-response calculations. We construct hypothetical alchemical atoms to smoothly interpolate between elements of the chalcogen group and study their bonding and superconducting properties. Our results show that the already remarkable critica…
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）
A recent experimental work on palladium hydrides suggested that metastable structures with hydrogen atoms occupying tetrahedral sites could lead to superconductivity above 50 K, a huge increase compared to the 10 K critical temperature of the stable structure with all hydrogen atoms occupying octahedral sites. By generating many structures with hydrogen atoms randomly occupying the octahedral and tetrahedral sites of the face-centered cubic lattice and calculating their energy at different theoretical levels from first principles, we determine that metastable structures with partial or full oc…
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…
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.
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）
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…
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…
Ambient and Pressure Dependent Superconductivity with Hydrogen Storage Potential in Quaternary Hydride LiMgZr2H12: A Comprehensive First-principles Insights
Department of Physics, University of Rajshahi（拉杰沙希大学物理系）
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…
In the last five years a large number of new high-temperature superconductors have been predicted and experimentally discovered among hydrogen-rich crystals, at pressures which are way too high to meet any practical application. In this work, we report the computational prediction of a hydride superconductor, LaBH$_{8}$, with a T$_{c}$ of 126 K at a pressure of 50 GPa, thermodynamically stable above 100 GPa, and dynamically stable down to 40 GPa, an unprecedentedly low pressure for high-T$_{c}$ hydrides. LaBH$_{8}$ can be seen as a ternary sodalite-like hydride, in which a metallic hydrogen su…
Stability of numerous unexpected actinium hydrides was predicted via evolutionary algorithm USPEX. Electron-phonon interaction was investigated for the hydrogen-richest and most symmetric phases: R$\overline{3}$m-$AcH_{10}$, I4/mmm-$AcH_{12}$ and P$\overline{6}$m2-$AcH_{16}$. Predicted structures of actinium hydrides are consistent with all previously studied Ac-H phases and demonstrate phonon-mediated high-temperature superconductivity with Tc in the range 204-251 K for R$\overline{3}$m-$AcH_{10}$ at 200 GPa and 199-241 K for P$\overline{6}$m2-$AcH_{16}$ at 150 GPa which was estimated by dire…
The recent discovery of room temperature superconductivity (283 K) in carbonaceous sulfur hydride (C-S-H) has attracted lots of interests in ternary hydrogen rich materials. In this report, ternary hydride P-S-H has been synthesized through photochemical reaction from elemental sulfur (S), phosphorus (P) and molecular hydrogen (H2) at high pressures and room temperature. The Raman spectroscopy under pressure shows that H2S and PH3 compounds are synthesized after laser heating at 0.9 GPa and a ternary van der Waals compound P-S-H is synthesized with a further compression to 4.6 GPa. The P-S-H c…
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…
Observation of high-temperature superconductivity in sulfur hydrides at megabar pressures has generated an irresistible wave on searching for new superconductors in other compressed hydrogen-rich compounds. An immediate effort is towards exploration of the relevant candidate of tellurium hydrides, where tellurium is isoelectronic to sulfur but it has a heavier atomic mass and much weaker electronegativity. The hitherto unknown phase diagram of tellurium hydrides at high pressures was investigated by a first-principles swarm structure search. Four energetically stable and metallic stoichiometri…
Due to its low atomic mass hydrogen is the most promising element to search for high-temperature phononic superconductors. However, metallic phases of hydrogen are only expected at extreme pressures (400 GPa or higher). The measurement of a record superconducting critical temperature of 190 K in a hydrogen-sulfur compound at 200 GPa of pressure[1], shows that metallization of hydrogen can be reached at significantly lower pressure by inserting it in the matrix of other elements. In this work we re-investigate the phase diagram and the superconducting properties of the H-S system by means of mi…
The rapid variation of the superconductivity Tc in hydride sulfur (H3S) under high pressure [A. Drozdov et al, Nature 525, 73 (2015); M. Einaga et al, arXiv: 1509.03156] in a vicinity Pcr=123GPa is interpreted in terms of the 1st order transition, possibly related to a CDW-instability with a non-zero structural vector. The superconductivity mechanisms in high-Tc phase be discussed, ordinary methods of calculating Tc are shown not applicable in H3S because, beside the acoustic branches, its phonon spectrum contains hydrogen modes with much higher frequency. A modified approach provides realisti…
Origin of Flat Bands and Role of Electron Correlation in Lutetium Hydrides
University of Illinois Chicago（伊利诺伊大学芝加哥分校）；The University of Texas at Austin（德克萨斯大学奥斯汀分校）；Argonne National Laboratory（阿贡国家实验室）；Oak Ridge National Laboratory（橡树岭国家实验室）
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…
The computational and experimental exploration of the phase diagrams of binary hydrides under high pressure has uncovered phases with novel stoichiometries and structures, some which are superconducting at quite high temperatures. Herein we review the plethora of studies that have been undertaken in the last decade on the main group and transition metal hydrides, as well as a few of the rare earth hydrides at pressures attainable in diamond anvil cells. The aggregate of data shows that the propensity for superconductivity is dependent upon the species used to "dope" hydrogen, with some of the …
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-…
Hydrogen-rich superhydrides are believed to be very promising high-T$_c$ superconductors, with experimentally observed critical temperatures near room temperature, as shown in recently discovered lanthanide superhydrides at very high pressures, e.g. LaH$_{10}$ at 170 GPa and CeH$_9$ at 150 GPa. With the motivation of discovering new hydrogen-rich high-T$_c$ superconductors at lowest possible pressure, quantitative theoretical predictions are needed. In these promising compounds, superconductivity is mediated by the highly energetic lattice vibrations associated with hydrogen and their interpla…
We investigated the preferred electron dopants at the oxygen sites of 1111-type SmFeAsO by changing the atmospheres around the precursor with the composition of Sm:Fe:As:O = 1:1:1:1-x in high-pressure synthesis. Under H2O and H2 atmospheres, hydrogens derived from H2O or H2 molecules were introduced into the oxygen sites as a hydride ion, and SmFeAsO1-xHx was obtained. However, when the H2O and H2 sources were removed from the synthetic process, nearly stoichiometric SmFeAsO was obtained and the maximum amount of oxygen vacancies introduced remained x = 0.05(4). Density functional theory calcu…
Disentangling electronic and phononic contributions to high-temperature superconductivity in X2MH6 hydrides
Jimei University（集美大学）；Xiamen University（厦门大学）；Iowa State University（爱荷华州立大学）；Guangdong University of Technology（广东工业大学）；Ames National Laboratory, U.S. Department of Energy（美国能源部艾姆斯国家实验室）
