Superconductivity at 250 K in lanthanum hydride under high pressures

Cryogenic C-band wavelength division multiplexing system using an AIM Photonics Foundry process design kit

Cryogenic environments make superconducting computing possible by reducing thermal noise, electrical resistance and heat dissipation. Heat generated by the electronics and thermal conductivity of electrical transmission lines to the outside world constitute two main sources of thermal load in such systems. As a result, higher data rates require additional transmission lines which come at an increasingly higher cooling power cost. Hybrid or monolithic integration of silicon photonics with the electronics can be the key to higher data rates and lower power costs in these systems. We present a 4-channel wavelength division multiplexing photonic integrated circuit (PIC) built from modulators in the AIM Photonics process development kit (PDK) that operate at 25 Gbps at room temperature and 10 Gbps at 40 K. We further demonstrate 2-channel operation for 20 Gbps aggregate data rate at 40 K using two different modulators/wavelengths, with the potential for higher aggregate bit rates by utilizing additional channels.

Hydrogen Pentagraphenelike Structure Stabilized by Hafnium: A High-Temperature Conventional Superconductor

The recent discovery of H_{3}S and LaH_{10} superconductors with record high superconducting transition temperatures T_{c} at high pressure has fueled the search for room-temperature superconductivity in the compressed superhydrides. Here we introduce a new class of high T_{c} hydrides with a novel structure and unusual properties. We predict the existence of an unprecedented hexagonal HfH_{10}, with remarkably high value of T_{c} (around 213-234 K) at 250 GPa. As concerns the novel structure, the H ions in HfH_{10} are arranged in clusters to form a planar "pentagraphenelike" sublattice. The layered arrangement of these planar units is entirely different from the covalent sixfold cubic structure in H_{3}S and clathratelike structure in LaH_{10}. The Hf atom acts as a precompressor and electron donor to the hydrogen sublattice. This pentagraphenelike H_{10} structure is also found in ZrH_{10}, ScH_{10}, and LuH_{10} at high pressure, each material showing a high T_{c} ranging from 134 to 220 K. Our study of dense superhydrides with pentagraphenelike layered structures opens the door to the exploration of a new class of high T_{c} superconductors.

A Metastable CaSH3 Phase Composed of HS Honeycomb Sheets that is Superconducting Under Pressure

Evolutionary searches have predicted a number of ternary Ca-S-H phases that could be synthesized at pressures of 100-300 GPa. P6₃/mmc CaSH₂, Pnma CaSH₂, Cmc2₁ CaSH₆, and I4̅ CaSH₂₀ were composed of a Ca-S lattice along with H₂ molecules coordinated in a "side-on" fashion to Ca. The H-H bond lengths in these semiconducting phases were elongated because of H₂ σ → Ca d donation, and Ca d → H₂ σ* back-donation, via a Kubas-like mechanism. P6̅m2 CaSH₃, consisting of two-dimensional HS and CaH₂ sheets, was metastable and metallic above 128 GPa. The presence of van Hove singularities increased its density of states at the Fermi level and concomitantly the superconducting critical temperature, which was estimated to be as high as ∼100 K at 128 GPa. This work will inspire the search for superconductivity in materials based upon honeycomb HX (X = S, Se, Te) and MH₂ (M = Mg, Ca, Sr, Ba) layers under pressure.

Na3FeH7 and Na3CoH6: Hydrogen-Rich First-Row Transition Metal Hydrides from High Pressure Synthesis

The formation of ternary hydrogen-rich hydrides involving the first-row transition metals TM = Fe and Co in high oxidation states is demonstrated from in situ synchrotron diffraction studies of reaction mixtures NaH–TM–H₂ at p ≈ 10 GPa. Na₃FeH₇ and Na₃CoH₆ feature pentagonal bipyramidal FeH₇^3– and octahedral CoH₆^3– 18-electron complexes, respectively. At high pressure, high temperature (300 < T ≤ 470 °C) conditions, metal atoms are arranged as in the face-centered cubic Heusler structure, and ab initio molecular dynamics simulations suggest that the complexes undergo reorientational dynamics. Upon cooling, subtle changes in the diffraction patterns evidence reversible and rapid phase transitions associated with ordering of the complexes. During decompression, Na₃FeH₇ and Na₃CoH₆ transform to tetragonal and orthorhombic low pressure forms, respectively, which can be retained at ambient pressure. The discovery of Na₃FeH₇ and Na₃CoH₆ establishes a consecutive series of homoleptic hydrogen-rich complexes for first-row transition metals from Cr to Ni.

In situ synchrotron diffraction studies of reaction mixtures NaH−TM−H₂ (TM = Fe, Co) at p ≈ 10 GPa revealed the formation of ternary hydrides Na₃FeH₇ and Na₃CoH₆ featuring pentagonal bipyramidal Fe(IV)H₇³⁻ and octahedral Co(III)H₆³⁻ complexes, respectively. The discovery of Na₃FeH₇ and Na₃CoH₆ establishes a consecutive series of homoleptic hydrogen-rich complexes for first-row transition metals from Cr to Ni.

Distinguishing the Structures of High-Pressure Hydrides with Nuclear Magnetic Resonance Spectroscopy

The structural characterization of high-pressure hydrides has encountered many difficulties mainly due to the weak X-ray scattering of hydrogen. Herein, we investigate the prospect of detecting the H₃S and LaH₁₀ structures with nuclear magnetic resonance (NMR) spectroscopy. Our calculations demonstrate that the different candidate structures of H₃S (or LaH₁₀) exhibit significant differences in the electric field gradient (EFG) tensor of the ³³S (or ¹³⁹La) sites, indicating that the NMR spectroscopy can well capture the structural differences, even the small changes in the atomic position, and hence can be used to effectively probe the structures and the phase transitions of H₃S and LaH₁₀. Our results clarify the relationship between the structures and the EFG tensor parameters and provide a potential means to detect the structures of high-pressure hydrides.

Route to high-[Formula: see text] superconductivity of [Formula: see text] via strong bonding of boron-carbon compound at high pressure

We have analyzed the compositions of boron-carbon system, in which the [Formula: see text] compound is identified as structural stability at high pressure. The first-principles calculation is used to identify the phase diagram, electronic structure, and superconductivity of [Formula: see text]. Our results have demonstrated that the [Formula: see text] is thermodynamically stable in the diamond-like [Formula: see text] structure at a pressure above 244 GPa, and under temperature also. Feature of chemical bonds between B and C atoms is presented using the electron localization function. The strong chemical bonds in diamond-like [Formula: see text] structure are covalent bonds, and it exhibits the s-p hybridization under the pressure compression. The Fermi surface shape displays the large sheet, indicating that the diamond-like [Formula: see text] phase can achieve a high superconducting transition temperature ([Formula: see text]). The outstanding property of [Formula: see text] at 250 GPa has manifested very high-[Formula: see text] of superconductivity as 164 K, indicating that the carbon-rich system can induce the high-[Formula: see text] value as well.

Combining Machine Learning Potential and Structure Prediction for Accelerated Materials Design and Discovery

The theoretical structure prediction method via quantum mechanical atomistic simulations such as density functional theory (DFT), based solely on chemical composition, has already become a routine tool to determine the structures of physical and chemical systems, e.g., solids and clusters. However, the application of DFT to more realistic simulations, to a large extent, is impeded because of the unfavorable scaling of the computational cost with respect to the system size. During recent years, the machine learning potential (MLP) method has been rapidly rising as an accurate and efficient tool for atomistic simulations. In this Perspective, we provide an introduction to the basic principles and advantages of the combination of structure prediction and MLP, as well as the challenges and opportunities associated with this promising approach.

Non-Bonded Radii of the Atoms Under Compression

We present quantum mechanical estimates for non-bonded, van der Waals-like, radii of 93 atoms in a pressure range from 0 to 300 gigapascal. Trends in radii are largely maintained under pressure, but atoms also change place in their relative size ordering. Multiple isobaric contractions of radii are predicted and are explained by pressure-induced changes to the electronic ground state configurations of the atoms. The presented radii are predictive of drastically different chemistry under high pressure and permit an extension of chemical thinking to different thermodynamic regimes. For example, they can aid in assignment of bonded and non-bonded contacts, for distinguishing molecular entities, and for estimating available space inside compressed materials. All data has been made available in an interactive web application.

Origin of enhanced chemical precompression in cerium hydride [Formula: see text]

The rare-earth metal hydrides with clathrate structures have been highly attractive because of their promising high-[Formula: see text] superconductivity at high pressure. Recently, cerium hydride [Formula: see text] composed of Ce-encapsulated clathrate H cages was synthesized at much lower pressures of 80-100 GPa, compared to other experimentally synthesized rare-earth hydrides such as [Formula: see text] and [Formula: see text]. Based on density-functional theory calculations, we find that the Ce 5p semicore and 4f/5d valence states strongly hybridize with the H 1s state, while a transfer of electrons occurs from Ce to H atoms. Further, we reveal that the delocalized nature of Ce 4f electrons plays an important role in the chemical precompression of clathrate H cages. Our findings not only suggest that the bonding nature between the Ce atoms and H cages is characterized as a mixture of ionic and covalent, but also have important implications for understanding the origin of enhanced chemical precompression that results in the lower pressures required for the synthesis of [Formula: see text].

Mechanical C-C Bond Formation by Laser Driven Shock Wave

Mechanically induced C-C bond formation was demonstrated by the laser driven shock wave generated in liquid normal alkanes at room temperature. Gas chromatography mass spectrometry analysis revealed the dehydrogenation condensation between two alkane molecules, for seven normal alkanes from pentane to undecane. Major products were identified to be linear and branched alkane molecules with double the number of carbons, and exactly coincided with the molecules predicted by supposing that a C-C bond was formed between two starting molecules. The production of the alkane molecules showed that the C-C bond formation occurred almost evenly at all the carbon positions. The dependence of the production on the laser pulse energy clearly indicated that the process was attributed to the shock wave. The C-C bond formation observed was not a conventional passive chemical reaction but an unprecedented active reaction.

Pressure-Suppressed Carrier Trapping Leads to Enhanced Emission in Two-Dimensional Perovskite (HA)2 (GA)Pb2 I7

A remarkable PL enhancement by 12 fold is achieved using pressure to modulate the structure of a recently developed 2D perovskite (HA)₂ (GA)Pb₂ I₇ (HA=n-hexylammonium, GA=guanidinium). This structure features a previously unattainable, extremely large cage. In situ structural, spectroscopic, and theoretical analyses reveal that lattice compression under a mild pressure within 1.6 GPa considerably suppresses the carrier trapping, leading to significantly enhanced emission. Further pressurization induces a non-luminescent amorphous yellow phase, which is retained and exhibits a continuously increasing band gap during decompression. When the pressure is released to 1.5 GPa, emission can be triggered by above-band gap laser irradiation, accompanied by a color change from yellow to orange. The obtained orange phase could be retained at ambient conditions and exhibits two-fold higher PL emission compared with the pristine (HA)₂ (GA)Pb₂ I₇ .

Synthesis of Superconducting Cobalt Trihydride

The Co-H system has been investigated through high-pressure, high-temperature X-ray diffraction experiments combined with first-principles calculations. On compression of elemental cobalt in a hydrogen medium, we observe face-centered cubic cobalt hydride (CoH) and cobalt dihydride (CoH₂) above 33 GPa. Laser heating CoH₂ in a hydrogen matrix at 75 GPa to temperatures in excess of ∼800 K produces cobalt trihydride (CoH₃) which adopts a primitive structure. Density functional theory calculations support the stability of CoH₃. This phase is predicted to be thermodynamically stable at pressures above 18 GPa and to be a superconductor below 23 K. Theory predicts that this phase remains dynamically stable upon decompression above 11 GPa where it has a maximum T_c of 30 K.

Searching new structures of beryllium-doped in small-sized magnesium clusters: Be2 Mgn Q (Q = 0, -1; n = 1-11) clusters DFT study

Using CALYPSO method to search new structures of neutral and anionic beryllium-doped magnesium clusters followed by density functional theory (DFT) calculations, an extensive study of the structures, electronic and spectral properties of Be₂ Mg_n ^Q (Q = 0, -1; n = 2-11) clusters is performed. Based on the structural optimization, it is found that the Be₂ Mg_n ^Q (Q = 0, -1) clusters are shown by tetrahedral-based geometries at n = 2-6 and tower-like-based geometries at n = 7-11. The calculations of stability indicate that Be₂ Mg₅ ^Q=0 , Be₂ Mg₅ ^Q=-1 , and Be₂ Mg₈ ^Q=-1 clusters are "magic" clusters with high stability. The NCP shows that the charges are transferred from Mg atoms to Be atoms. The s- and p-orbitals interactions of Mg and Be atoms are main responsible for their NEC. In particular, chemical bond analysis including molecular orbitals (MOs) and chemical bonding composition for magic clusters to further study their stability. The results confirmed that the high stability of these clusters is due to the interactions between the Be atom and the Mg₅ or Mg₈ host. Finally, theoretical calculations of infrared and Raman spectra of the ground state of Be₂ Mg_n ^Q (Q = 0, -1; n = 1-11) clusters were performed, which will be absolutely useful for future experiments to identify these clusters.

Crystal structure prediction of magnetic materials

We present a methodology to predict magnetic systems using ab initio methods. By employing crystal structure method and spin-polarized calculations, we explore the relation between crystalline structures and their magnetic properties. In this work, testbed cases of transition metal alloys (FeCr, FeMn, FeCo and FeNi) are study in the ferromagnetic case. We find soft-magnetic properties for FeCr, FeMn while for FeCo and FeNi hard-magnetic are predicted. In particular, for the family of FeNi, a candidate structure with energy lower than the tetrataenite was found. The structure has a saturation magnetization (M _s) of 1.2 MA m^-1, magnetic anisotropy energy (MAE) above 1200 kJ m^-3 and hardness value close to 1. Theoretically, this system made of abundant elements could be the right candidate for permanent magnet applications. Comparing with the state-of-the-art (Nd₂Fe₁₄B) hard-magnet, (M _s of 1.28 MA m^-1 and MAE of 4900 kJ m^-3) is appealing to explore this low energy polymorph of FeNi further. Considering the relatively limited number of magnets, predicting a new system may open routes for free rare-earth magnets. Furthermore, the use of the computational algorithm as the one presented in this work, hold promises in this field for which in near future improvements will allow to study numerous complex systems, larger simulations cells and tackled long-range antiferromagnetic cases.

The periodic table and the physics that drives it

Mendeleev's introduction of the periodic table of elements is one of the most important milestones in the history of chemistry, as it brought order into the known chemical and physical behaviour of the elements. The periodic table can be seen as parallel to the Standard Model in particle physics, in which the elementary particles known today can be ordered according to their intrinsic properties. The underlying fundamental theory to describe the interactions between particles comes from quantum theory or, more specifically, from quantum field theory and its inherent symmetries. In the periodic table, the elements are placed into a certain period and group based on electronic configurations that originate from the Pauli and Aufbau principles for the electrons surrounding a positively charged nucleus. This order enables us to approximately predict the chemical and physical properties of elements. Apparent anomalies can arise from relativistic effects, partial-screening phenomena (of type lanthanide contraction) and the compact size of the first shell of every l-value. Further, ambiguities in electron configurations and the breakdown of assigning a dominant configuration, owing to configuration mixing and dense spectra for the heaviest elements in the periodic table. For the short-lived transactinides, the nuclear stability becomes an important factor in chemical studies. Nuclear stability, decay rates, spectra and reaction cross sections are also important for predicting the astrophysical origin of the elements, including the production of the heavy elements beyond iron in supernova explosions or neutron-star mergers. In this Perspective, we critically analyse the periodic table of elements and the current status of theoretical predictions and origins for the heaviest elements, which combine both quantum chemistry and physics.

Superconductivity enhancement in FeSe/SrTiO3: a review from the perspective of electron-phonon coupling

Single-layer FeSe films grown on SrTiO3, with the highest superconducting transition temperature (TC) among all the iron-based superconductors, serves as an ideal platform for studying the microscopic mechanisms of high-TCsuperconductivity. The significant role of interfacial coupling has been widely recognized, while the precise nature of theTCenhancement remains open. In this review, we focus on the investigations of the interfacial coupling in FeSe/SrTiO3from the perspective of electron-phonon coupling (EPC). The main content will include an overview of the experimental measurements associated with different theoretical models and arguments about the EPC. Especially, besides the discussions of EPC based on the measurements of electronic states, we will emphasize the analyses based on phonon measurements. A uniform picture about the nature of the EPC and its relation to theTCenhancement in FeSe/SrTiO3has still not achieved, which should be the key for further studies aiming to the in-depth understanding of high-TCsuperconductivity and the discovery of new superconductors with even enhancedTC.

Complex Hydrogen Substructure in Semimetallic RuH4

When compressed in a matrix of solid hydrogen, many metals form compounds with increasingly high hydrogen contents. At high density, hydrogenic sublattices can emerge, which may act as low-dimensional analogues of atomic hydrogen. We show that at high pressures and temperatures, ruthenium forms polyhydride species that exhibit intriguing hydrogen substructures with counterintuitive electronic properties. Ru₃H₈ is synthesized from RuH in H₂ at 50 GPa and at temperatures in excess of 1000 K, adopting a cubic structure with short H-H distances. When synthesis pressures are increased above 85 GPa, we observe RuH₄ which crystallizes in a remarkable structure containing corner-sharing H₆ octahedra. Calculations indicate this phase is semimetallic at 100 GPa.

Unravelling the Mystery of Solid Solutions: A Case Study of 89 Y Solid-State NMR Spectroscopy

Incorporating heteroatoms in functional materials is an invaluable approach to modulate their properties, assuming a solid solution is formed. However, thorough understanding of key structural information on the resulting solid solution, such as the local environment of cations and vacancies, remains a challenge. Solid-state NMR (SSNMR) spectroscopy is a powerful structural characterization tool, very sensitive to the local environment. Due to the difficulty in signal acquisition and spectral interpretation, SSNMR spectroscopy is relatively less known to chemists and materials scientists. Herein, we present an introductory review to demonstrate how to use ⁸⁹ Y SS NMR spectroscopy to unravel the mystery of solid solutions. In general, ⁸⁹ Y chemical shift varies with different cation/vacancy arrangements in Y coordination spheres, providing ultrafine structural information in the atomic scale. As a case study and an extreme condition, the approach demonstrated in this review can be extended to other systems.

Prediction and Synthesis of Dysprosium Hydride Phases at High Pressure

Crystal structure prediction (CSP) methods recently proposed a series of new rare-earth (RE) hydrides at high pressures with novel crystal structures, unusual stoichiometries, and intriguing features such as high-T_c superconductivity. RE trihydrides (REH₃) generally undergo a phase transition from ambient P6₃/mmc or P3̅c1 to Fm3̅m at high pressure. This cubic REH₃ (Fm3̅m) was considered to be a precursor to further synthesize RE polyhydrides such as YH₄, YH₆, YH₉, and CeH₉ with higher hydrogen contents at higher pressures. However, the structural stability and equation of state (EOS) of any of the REH₃ have not been fully investigated at sufficiently high pressures. This work presents high-pressure X-ray diffraction (XRD) measurements in a laser-heated diamond anvil cell up to 100 GPa and ab initio evolutionary CSP of stable phases of DyH₃ up to 220 GPa. Experiments observed the Fm3̅m phase of DyH₃ to be stable at pressures from 17 to 100 GPa and temperatures up to ∼2000 K. After complete decompression, the P3̅c1 and Fm3̅m phases of DyH₃ recovered under ambient conditions. Our calculations predicted a series of phases for DyH₃ at high pressures with the structural phase transition sequence P3̅c1 → Imm2 → Fm3̅m → Pnma → P6₃/mmc at 11, 35, 135, and 194 GPa, respectively. The predicted P3̅c1 and Fm3̅m phases are consistent with experimental observations. Furthermore, electronic band structure calculations were carried out for the predicted phases of DyH₃, including the 4f states, within the DFT+U approach. The inclusion of 4f states shows significant changes in electronic properties, as more Dy d states cross the Fermi level and overlap with H 1s states. The structural phase transition from P3̅c1 to Fm3̅m observed in DyH₃ is systematically compared with other REH₃ compounds at high pressures. The phase transition pressure in REH₃ shows an inverse relation with the ionic radius of RE atoms.

Intense Reactivity in Sulfur-Hydrogen Mixtures at High Pressure under X-ray Irradiation

Superconductivity near room temperature in the sulfur-hydrogen system arises from a sequence of reactions at high pressures, with X-ray diffraction experiments playing a central role in understanding these chemical-structural transformations and the corresponding S:H stoichiometry. Here we document X-ray irradiation acting as both a probe and as a driver of chemical reaction in this dense hydride system. We observe a reaction between molecular hydrogen (H₂) and elemental sulfur (S₈) under high pressure, induced directly by X-ray illumination, at photon energies of 12 keV using a free electron laser. The rapid synthesis of hydrogen sulfide (H₂S) at 0.3 GPa was confirmed by optical observations, spectroscopic measurements, and microstructural changes detected by X-ray diffraction. These results document X-ray induced chemical synthesis of superconductor-forming dense hydrides, revealing an alternative production strategy and confirming the disruptive nature of X-ray exposure in studies on high-pressure hydrogen chalcogenides, from water to high-temperature superconductors.

High-pressure developments for resonant X-ray scattering experiments at I16

An experimental setup to perform high-pressure resonant X-ray scattering (RXS) experiments at low temperature on I16 at Diamond Light Source is presented. The setup consists of a membrane-driven diamond anvil cell, a panoramic dome and an optical system that allows pressure to be measured in situ using the ruby fluorescence method. The membrane cell, inspired by the Merrill-Bassett design, presents an asymmetric layout in order to operate in a back-scattering geometry, with a panoramic aperture of 100° in the top and a bottom half dedicated to the regulation and measurement of pressure. It is specially designed to be mounted on the cold finger of a 4 K closed-cycle cryostat and actuated at low-temperature by pumping helium into the gas membrane. The main parts of the body are machined from a CuBe alloy (BERYLCO 25) and, when assembled, it presents an approximate height of 20-21 mm and fits into a 57 mm diameter. This system allows different materials to be probed using RXS in a range of temperatures between 30 and 300 K and has been tested up to 20 GPa using anvils with a culet diameter of 500 µm under quasi-cryogenic conditions. Detailed descriptions of different parts of the setup, operation and the developed methodology are provided here, along with some preliminary experimental results.

Inelastic neutron scattering evidence for anomalous H-H distances in metal hydrides

Hydrogen-containing materials are of fundamental as well as technological interest. An outstanding question for both is the amount of hydrogen that can be incorporated in such materials, because that determines dramatically their physical properties such as electronic and crystalline structure. The number of hydrogen atoms in a metal is controlled by the interaction of hydrogens with the metal and by the hydrogen-hydrogen interactions. It is well established that the minimal possible hydrogen-hydrogen distances in conventional metal hydrides are around 2.1 Å under ambient conditions, although closer H-H distances are possible for materials under high pressure. We present inelastic neutron scattering measurements on hydrogen in [Formula: see text] showing nonexpected scattering at low-energy transfer. The analysis of the spectra reveals that these spectral features in part originate from hydrogen vibrations confined by neighboring hydrogen at distances as short as 1.6 Å. These distances are much smaller than those found in related hydrides, thereby violating the so-called Switendick criterion. The results have implications for the design and creation of hydrides with additional properties and applications.

Superconducting Zirconium Polyhydrides at Moderate Pressures

Highly compressed hydrides have been at the forefront of the search for high-T_c superconductivity. The recent discovery of record-high T_c's in H₃S and LaH_10±x under high pressure fuels the enthusiasm for finding good superconductors in similar hydride groups. Guided by first-principles structure prediction, we successfully synthesized ZrH₃ and Zr₄H₁₅ at modest pressures (30-50 GPa) in diamond anvil cells by two different reaction routes: ZrH₂ + H₂ at room temperature and Zr + H₂ at ∼1500 K by laser heating. From the synchrotron X-ray diffraction patterns, ZrH₃ is found to have a Pm3̅n structure corresponding to the familiar A15 structure, and Zr₄H₁₅ has an I4̅3d structure isostructural to Th₄H₁₅. Electrical resistance measurement and the dependence of T_c on the applied magnetic field of the sample showed the emergence of two superconducting transitions at 6.4 and 4.0 K at 40 GPa, which correspond to the two T_c's for ZrH₃ and Zr₄H₁₅.

Chemically Tuning Stability and Superconductivity of P-H Compounds

Experimental evidence has revealed superconductivity with a critical temperature, T_c, around 100 K in compressed solid phosphine, but theoretical studies have hitherto found no stable structure in any binary P-H system, leaving the characterization of the new superconductor unsettled. Here we present the findings of an advanced structure search and first-principles calculations unveiling the effect of Li as an electron donor that stabilizes the crystal structure and produces robust phonon-mediated superconductivity in the resulting Li-P-H compounds in wide ranges of stoichiometry and pressure. We showcase a trigonal LiP₂H₁₄ phase that reaches T_c of 169 K at 230 GPa and then decreases with rising pressure, which can be remedied by substituting Li with Be or Na, which considerably enhances T_c. These findings highlight the intricate and effective chemical tuning of stabilizing the crystal structure and enhancing the superconductivity in a distinct class of ternary hydrides, opening new avenues for designing and optimizing new high-T_c hydride superconductors.

Quantum crystal structure in the 250-kelvin superconducting lanthanum hydride

The discovery of superconductivity at 200 kelvin in the hydrogen sulfide system at high pressures¹ demonstrated the potential of hydrogen-rich materials as high-temperature superconductors. Recent theoretical predictions of rare-earth hydrides with hydrogen cages^2,3 and the subsequent synthesis of LaH₁₀ with a superconducting critical temperature (T_c) of 250 kelvin^4,5 have placed these materials on the verge of achieving the long-standing goal of room-temperature superconductivity. Electrical and X-ray diffraction measurements have revealed a weakly pressure-dependent T_c for LaH₁₀ between 137 and 218 gigapascals in a structure that has a face-centred cubic arrangement of lanthanum atoms⁵. Here we show that quantum atomic fluctuations stabilize a highly symmetrical [Formula: see text] crystal structure over this pressure range. The structure is consistent with experimental findings and has a very large electron-phonon coupling constant of 3.5. Although ab initio classical calculations predict that this [Formula: see text] structure undergoes distortion at pressures below 230 gigapascals^2,3, yielding a complex energy landscape, the inclusion of quantum effects suggests that it is the true ground-state structure. The agreement between the calculated and experimental T_c values further indicates that this phase is responsible for the superconductivity observed at 250 kelvin. The relevance of quantum fluctuations calls into question many of the crystal structure predictions that have been made for hydrides within a classical approach and that currently guide the experimental quest for room-temperature superconductivity^6-8. Furthermore, we find that quantum effects are crucial for the stabilization of solids with high electron-phonon coupling constants that could otherwise be destabilized by the large electron-phonon interaction⁹, thus reducing the pressures required for their synthesis.

Superconducting praseodymium superhydrides

Superhydrides have complex hydrogenic sublattices and are important prototypes for studying metallic hydrogen and high-temperature superconductors. Previous results for LaH10 suggest that the Pr-H system may be especially worth studying because of the magnetism and valence-band f-electrons in the element Pr. Here, we successfully synthesized praseodymium superhydrides (PrH9) in laser-heated diamond anvil cells. Synchrotron x-ray diffraction analysis demonstrated the presence of previously predicted F4 ¯ 3m-PrH9 and unexpected P63/mmc-PrH9 phases. Experimental studies of electrical resistance in the PrH9 sample showed the emergence of a possible superconducting transition (T c) below 9 K and T c dependent on the applied magnetic field. Theoretical calculations indicate that magnetic order and likely superconductivity coexist in a narrow range of pressures in the PrH9 sample, which may contribute to its low superconducting temperature. Our results highlight the intimate connections between hydrogenic sublattices, density of states, magnetism, and superconductivity in Pr-based superhydrides.

From LaH10 to room-temperature superconductors

Thermodynamic parameters of the LaH10 superconductor were an object of our interest. LaH10 is characterised by the highest experimentally observed value of the critical temperature: [Formula: see text] K (pa = 150 GPa) and [Formula: see text] K (pb = 190 GPa). It belongs to the group of superconductors with a strong electron-phonon coupling (λa ~ 2.2 and λb ~ 2.8). We calculated the thermodynamic parameters of this superconductor and found that the values of the order parameter, the thermodynamic critical field, and the specific heat differ significantly from the values predicted by the conventional BCS theory. Due to the specific structure of the Eliashberg function for the hydrogenated compounds, the qualitative analysis suggests that the superconductors of the LaδX1-δH10-type (LaXH-type) structure, where X ∈ {Sc, Y}, would exhibit significantly higher critical temperature than TC obtained for LaH10. In the case of LaScH we came to the following assessments: [Formula: see text] K and [Formula: see text] K, while the results for LaYH were: [Formula: see text] K and [Formula: see text] K.

Relation between Crystal Structure and Transition Temperature of Superconducting Metals and Alloys

Using the Roeser–Huber equation, which was originally developed for high temperature superconductors (HTSc) (H. Roeser et al., Acta Astronautica 62 (2008) 733), we present a calculation of the superconducting transition temperatures, T c , of some elements with fcc unit cells (Pb, Al), some elements with bcc unit cells (Nb, V), Sn with a tetragonal unit cell and several simple metallic alloys (NbN, NbTi, the A15 compounds and MgB 2 ). All calculations used only the crystallographic information and available data of the electronic configuration of the constituents. The model itself is based on viewing superconductivity as a resonance effect, and the superconducting charge carriers moving through the crystal interact with a typical crystal distance, x. It is found that all calculated T c -data fall within a narrow error margin on a straight line when plotting ( 2 x ) 2 vs. 1 / T c like in the case for HTSc. Furthermore, we discuss the problems when obtaining data for T c from the literature or from experiments, which are needed for comparison with the calculated data. The T c -data presented here agree reasonably well with the literature data.

Prediction of a novel robust superconducting state in TaS2 under high pressure

A novel superconducting I4/mmm phase has been predicted in TaS₂ under high pressure, illustrating an unusual superconductor–metal–superconductor transition.

High pressure: a feasible tool for the synthesis of unprecedented inorganic compounds

After a simple classification of inorganic materials synthesized at high-temperature and high-pressure, this tutorial reviews the important research results in the field of high-temperature and high-pressure inorganic synthesis in the past 5 years.

“Flat/steep band model” for superconductors containing Bi square nets

The crystal structures of a new family of superconductors containing a Bi square net and their electronic structures around the Fermi level have been reviewed. The structures of these compounds can be viewed as stacked layers denoted by [Bi][(RE)(M 2Bi2)(RE)] RE = rare earth or alkaline earth metal, M = transition metal. Flat/steep band features are shown to exist in all these new superconductors, though the pairing mechanisms may be very different. The Dirac Fermion behavior is reviewed and its implications are discussed.

Metallization and superconductivity in methane doped by beryllium at low pressure

As one of the simplest hydrocarbons, methane (CH₄) has great potential in the research of superconductors. However, the metallization of CH₄ has been an issue for a long time. Here, we report the structure, metallization, and superconductivity of CH₄ doped by Be at low pressures, based on first-principles calculations. The result shows that the thermodynamically stable BeCH₄ with P1[combining macron] space-group can transform into a metal at ambient pressure. This ternary hydride BeCH₄ exhibits a superconductivity of ∼6 K below 25.6 GPa. Interestingly, the superconducting critical temperature of BeCH₄ can reach ∼30 K at 80 GPa in the form of an a-P1 space-group phase. The charge transfer from Be to CH₄ molecules plays an important role in the superconductivity. Our results present a novel way to realize the metallization of methane at relative pressures and indicate that the doped methane is a potential candidate for seeking high temperature and low pressure superconductivity.

Measuring magnetic field texture in correlated electron systems under extreme conditions

Pressure is a clean, continuous, and systematic tuning parameter among the competing ground states in strongly correlated electron systems such as superconductivity and magnetism. However, owing to the restricted access to samples enclosed in high-pressure devices, compatible magnetic field sensors with sufficient sensitivity are rare. We used nitrogen vacancy centers in diamond as a spatially resolved vector field sensor for material research under pressure at cryogenic temperatures. Using a single crystal of BaFe2(As0.59P0.41)2 as a benchmark, we extracted the superconducting transition temperature, the local magnetic field profile in the Meissner state, and the critical fields. The method developed in this work offers a distinct tool for probing and understanding a range of quantum many-body systems.

Magnetic measurements on micrometer-sized samples under high pressure using designed NV centers

Pressure can be used to tune the interplay among structural, electronic, and magnetic interactions in materials. High pressures are usually applied in the diamond anvil cell, making it difficult to study the magnetic properties of a micrometer-sized sample. We report a method for spatially resolved optical magnetometry based on imaging a layer of nitrogen-vacancy (NV) centers created at the surface of a diamond anvil. We illustrate the method using two sets of measurements realized at room temperature and low temperature, respectively: the pressure evolution of the magnetization of an iron bead up to 30 gigapascals showing the iron ferromagnetic collapse and the detection of the superconducting transition of magnesium dibromide at 7 gigapascals.

Epitaxial Growth of Single-Phase Magnesium Dihydride Thin Films

We describe the epitaxial growth process of single-phase magnesium dihydride (MgH₂) thin films on MgO(100) substrates, achieved by reactive magnetron sputtering. We find that direct growth at substrate temperatures higher than 100 °C leads to partial MgH₂ decomposition to Mg, hindering single-phase epitaxy of MgH₂. To improve the crystallinity and suppress the decomposition of Mg, we optimize MgH₂ growth using a two-step process, consisting of (1) precursor growth at room temperature and (2) postdeposition annealing at 380 °C, under a pressure of 1.0 × 10⁵ Pa with H₂ (4%)/Ar (96%) premixed gases. Using this two-step process, we obtain single-phase MgH₂ epitaxial films with high crystallinity, transparency, and resistivity. Further, the application of this method to grow MgH₂ thin films on different MgF₂ and Al₂O₃ substrates enables us to use the epitaxial effects to control the growth orientation of MgH₂ thin films; we show that MgH₂(100) and MgH₂(001) epitaxial thin films can be grown on Al₂O₃(001) and MgF₂(001) substrates, respectively.

Synthesis of clathrate cerium superhydride CeH9 at 80-100 GPa with atomic hydrogen sublattice

Hydrogen-rich superhydrides are believed to be very promising high-T_c superconductors. Recent experiments discovered superhydrides at very high pressures, e.g. FeH₅ at 130 GPa and LaH₁₀ at 170 GPa. With the motivation of discovering new hydrogen-rich high-T_c superconductors at lowest possible pressure, here we report the prediction and experimental synthesis of cerium superhydride CeH₉ at 80–100 GPa in the laser-heated diamond anvil cell coupled with synchrotron X-ray diffraction. Ab initio calculations were carried out to evaluate the detailed chemistry of the Ce-H system and to understand the structure, stability and superconductivity of CeH₉. CeH₉ crystallizes in a P6₃/mmc clathrate structure with a very dense 3-dimensional atomic hydrogen sublattice at 100 GPa. These findings shed a significant light on the search for superhydrides in close similarity with atomic hydrogen within a feasible pressure range. Discovery of superhydride CeH₉ provides a practical platform to further investigate and understand conventional superconductivity in hydrogen rich superhydrides.

Hydrogen-rich superhydrides are promising high-temperature superconductors which have been observed only at pressures above 170 GPa. Here the authors show that CeH₉ can be synthesized at 80-100 GPa with laser heating, and is characterized by a clathrate structure with a dense 3-dimensional atomic hydrogen sublattice.

Classifying Induced Superconductivity in Atomically Thin Dirac-Cone Materials

Recently, Kayyalha et al. (Phys. Rev. Lett., 2019, 122, 047003) reported on the anomalous enhancement of the self-field critical currents (Ic (sf, T)) at low temperatures in Nb/BiSbTeSe2-nanoribbon/Nb Josephson junctions. The enhancement was attributed to the low-energy Andreev-bound states arising from the winding of the electronic wave function around the circumference of the topological insulator BiSbTeSe2 nanoribbon. It should be noted that identical enhancement in Ic (sf, T) and in the upper critical field (Bc2 (T)) in approximately the same reduced temperatures, were reported by several research groups in atomically thin junctions based on a variety of Dirac-cone materials (DCM) earlier. The analysis shows that in all these S/DCM/S systems, the enhancement is due to a new superconducting band opening. Taking into account that several intrinsic superconductors also exhibit the effect of new superconducting band(s) opening when sample thickness becomes thinner than the out-of-plane coherence length (c (0)), we reaffirm our previous proposal that there is a new phenomenon of additional superconducting band(s) opening in atomically thin films.