Cu2IrO3 has attracted recent interest due to its proximity to the Kitaev quantum spin liquid state and the complex structural response observed at high pressures. We use x-ray spectroscopy and scattering as well as electrical transport techniques to unveil the electronic structure of Cu2IrO3 at ambient and high pressures. Despite featuring a Ir4+ Jeff = 1/2 state at ambient pressure, Ir L3-edge resonant inelastic x-ray scattering reveals broadened electronic excitations that point to the importance of Ir 5d-Cu 3d interaction. High pressure first drives an Ir-Ir dimer state with collapsed (L S) and (Lz)/(Sz), signaling the formation of 5d molecular orbitals. A novel Cu -* Ir charge transfer is observed above 30 GPa at low temperatures, leading to an approximate Ir3+ and Cu1.5+ valence, with persistent insulating electrical transport seemingly driven by charge segregation of Cu1+/Cu2+ ions into distinct sites. Concomitant x-ray spectroscopy and diffraction measurements through different thermodynamic paths demonstrate a strong electron-lattice coupling, with Jeff = 1/2 and Ir3+/Cu1.5+ electronic states occurring only in phases 1 and 5, respectively. Remarkably, the charge-transfer state can only be reached if Cu2IrO3 is pressurized at low temperature, suggesting that phonons play an important role in the inhibiting this phase. These results point to the choice of thermodynamic path across interplanar collapse transition as a key parameter to access novel states in intercalated iridates.

Recent investigations into MoB$_{2}$ have unveiled a direct connection between a pressure-induced structural transition to a P6/mmm space group structure and the emergence of superconductivity, producing critical temperatures up to 32 K at 100 GPa. This pressure-induced superconducting state underscores the potential of doped MoB$_{2}$ as a possible candidate for metastable superconductivity at ambient pressure. In this work, we demonstrate that doping by Zr, Hf, or Ta stabilizes the P6/mmm structure at ambient pressure and results in the realization of a superconducting state with critical temperatures ranging from 2.4 up to 8.5 K depending on the specific doping. We estimate the electron-phonon coupling $\lambda$ and the density of states based on resistivity and specific heat data, finding that $\lambda$ ranges from 0.4 - 0.6 for these compounds. Finally, to investigate the role of possible metastable defect structures on the critical temperature, we analyze MoB$_{2}$, MoB$_{2.5}$, and Nb/Zr-doped MoB$_{2}$ using rapid cooling techniques. Notably, splat-quenching produces samples with higher critical temperatures and even retains superconductivity in MoB$_{2}$ at ambient pressure, achieving a critical temperature of 4.5 K.

The superconducting and structural properties of elemental strontium metal were investigated under pressures up to 60 GPa while maintaining cryogenic conditions during pressure application. Applying pressure at low temperatures reveals differences in superconducting and structural phases compared to previous reports obtained at room temperatures. Notably, the superconducting critical temperature exhibits a twofold increase under compression after cryogenic cooling within the pressure range of 35-42 GPa, compared to cryogenic cooling after room-temperature compression. Subsequently, the transition width becomes significantly sharper above 42 GPa. Low-temperature X-ray diffraction measurements under pressure reveal that this change corresponds to the Sr-III to Sr-IV transition, with no evidence of any metastable structure. Furthermore, the monoclinic Sr-IV structure was observed to remain stable to much higher pressures - at least up to 60 GPa, without the appearance of the incommensurate Sr-V phase present at room temperature. This implies that thermal activation energy plays an important role in overcoming the presence of a kinetic barrier to the Sr-V phase at room temperature.

The recent surprising discovery of superconductivity with critical temperature T_c = 32 K in MoB_2 above 70 GPa has led to the search for related materials that may superconduct at similarly high T_c values and lower pressures. We have studied the superconducting and structural properties of Re_0.10Mo_0.90B_2 to 170 GPa. A structural phase transition from R3m to P6/mmm commences at 48 GPa, with the first signatures of superconductivity appearing above 44 GPa. The critical temperature is observed to increase with pressure. A complete resistive transition is observed only above 150 GPa, where the highest onset T_c of 30 K is also achieved. Upon releasing pressure, the high pressure superconducting phase is found to be metastable. During unloading, a complete resistive superconducting transition is observed all the way down to 20 GPa (with onset T_c ∼ 20 K). Our results suggest that the P6/mmm structure is responsible for the observed superconductivity.

Single-crystalline FeSi samples with a conducting surface state were studied under high pressure and magnetic field by means of electrical resistance measurements to explore how the bulk semiconducting state and the surface state are tuned by the application of pressure. We found that the energy gap associated with the semiconducting bulk phase begins to close abruptly at a critical pressure of similar to 10 GPa and the bulk material becomes metallic with no obvious sign of any emergent phases or non-Fermi liquid behavior in temperature dependent electrical resistance in the neighborhood of the critical pressure above 3 K. Moreover, the metallic phase appears to remain at near-ambient pressure upon release of the pressure. Interestingly, the hysteresis in the electrical resistance vs magnetic field curve associated with the magnetically ordered conducting surface state decreases with pressure and vanishes at the critical pressure, while the slope of the electrical resistance vs magnetic field curve, which has a negative value for pressure below the critical pressure, decreases in magnitude with pressure and changes sign at the critical pressure. Thus the conducting surface state and the corresponding two-dimensional magnetic order collapse at the critical pressure where the energy gap of the bulk material starts to close abruptly, revealing the connection between the conducting surface state and the semiconducting bulk state in FeSi.

A recent work has demonstrated that MoB$_2$, transforming to the same structure as MgB$_2$ ($P6/mmm$), superconducts at temperatures above 30 K near 100 GPa [C. Pei $et$ $al$. Natl. Sci. Rev., nwad034 (2023)], and Nb-substitution in MoB$_2$ stabilizes the $P6/mmm$ structure down to ambient pressure [A. C. Hire $et$ $al$. Phys. Rev. B 106, 174515 (2022)]. The current work explores the high pressure superconducting behavior of Nb-substituted MoB$_2$ (Nb$_{0.25}$Mo$_{0.75}$B$_2$). High pressure x-ray diffraction measurements show that the sample remains in the ambient pressure $P6/mmm$ structure to at least 160 GPa. Electrical resistivity measurements demonstrate that from an ambient pressure $T_c$ of 8 K (confirmed by specific heat to be a bulk effect), the critical temperature is suppressed to 4 K at 50 GPa, before gradually rising to 5.5 K at 170 GPa. The critical temperature at high pressure is thus significantly lower than that found in MoB$_2$ under pressure (30 K), revealing that Nb-substitution results in a strong suppression of the superconducting critical temperature. Our calculations indeed find a reduced electron-phonon coupling in Nb$_{0.25}$Mo$_{0.75}$B$_2$, but do not account fully for the observed suppression, which may also arise from inhomogeneity and enhanced spin fluctuations.

A recent work has demonstrated that MoB$_2$, transforming to the same structure as MgB$_2$ ($P6/mmm$), superconducts at temperatures above 30 K near 100 GPa [C. Pei $et$ $al$. Natl. Sci. Rev., nwad034 (2023)], and Nb-substitution in MoB$_2$ stabilizes the $P6/mmm$ structure down to ambient pressure [A. C. Hire $et$ $al$. Phys. Rev. B 106, 174515 (2022)]. The current work explores the high pressure superconducting behavior of Nb-substituted MoB$_2$ (Nb$_{0.25}$Mo$_{0.75}$B$_2$). High pressure x-ray diffraction measurements show that the sample remains in the ambient pressure $P6/mmm$ structure to at least 160 GPa. Electrical resistivity measurements demonstrate that from an ambient pressure $T_c$ of 8 K (confirmed by specific heat to be a bulk effect), the critical temperature is suppressed to 4 K at 50 GPa, before gradually rising to 5.5 K at 170 GPa. The critical temperature at high pressure is thus significantly lower than that found in MoB$_2$ under pressure (30 K), revealing that Nb-substitution results in a strong suppression of the superconducting critical temperature. Our calculations indeed find a reduced electron-phonon coupling in Nb$_{0.25}$Mo$_{0.75}$B$_2$, but do not account fully for the observed suppression, which may also arise from inhomogeneity and enhanced spin fluctuations.

Recently it was discovered that, under elevated pressures, MoB2 exhibits superconductivity at a critical temperature Tc as high as 32 K. The superconductivity appears to develop following a pressure-induced structural transition from the ambient pressure R3 over bar m structure to an MgB2-like P6/mmm structure. This suggests that remarkably high Tc values among diborides are not restricted to MgB2 as previously appeared to be the case, and that similarly high Tc values may occur in other diborides if they can be coerced into the MgB2 structure. In this paper, we show that density functional theory calculations indicate that phonon free energy stabilizes the P6/mmm structure over the R3 over bar m at high temperatures across the Nb1-xMoxB2 series. X-ray diffraction confirms that the synthesized Nb-substituted MoB2 adopts the MgB2 crystal structure. High magnetic field electrical resistivity measurements and specific heat measurements demonstrate that NbxMo1-xB2 exhibits superconductivity with Tc as high as 8 K and critical fields approaching 6 T.

The Eliashberg theory of superconductivity accounts for the fundamental physics of conventional superconductors, including the retardation of the interaction and the Coulomb pseudopotential, to predict the critical temperature Tc. McMillan, Allen, and Dynes derived approximate closed-form expressions for the critical temperature within this theory, which depends on the electron–phonon spectral function α2F(ω). Here we show that modern machine-learning techniques can substantially improve these formulae, accounting for more general shapes of the α2F function. Using symbolic regression and the SISSO framework, together with a database of artificially generated α2F functions and numerical solutions of the Eliashberg equations, we derive a formula for Tc that performs as well as Allen–Dynes for low-Tc superconductors and substantially better for higher-Tc ones. This corrects the systematic underestimation of Tc while reproducing the physical constraints originally outlined by Allen and Dynes. This equation should replace the Allen–Dynes formula for the prediction of higher-temperature superconductors.

High-pressure electrical resistivity measurements reveal that the mechanical deformation of ultra-hard WB2 during compression induces superconductivity above 50 GPa with a maximum superconducting critical temperature, Tc of 17 K at 90 GPa. Upon further compression up to 190 GPa, the Tc gradually decreases. Theoretical calculations show that electron-phonon mediated superconductivity originates from the formation of metastable stacking faults and twin boundaries that exhibit a local structure resembling MgB2} (hP3, space group 191, prototype AlB2). Synchrotron x-ray diffraction measurements up to 145 GPa} show that the ambient pressure hP12 structure (space group 194, prototype WB2) continues to persist to this pressure, consistent with the formation of the planar defects above 50 GPa. The abrupt appearance of superconductivity under pressure does not coincide with a structural transition but instead with the formation and percolation of mechanically-induced stacking faults and twin boundaries. The results identify an alternate route for designing superconducting materials.

With $T_c \sim 9.6~\mathrm{K}$, Be$_{22}$Re exhibits one of the highest critical temperatures among Be-rich compounds. We have carried out a series of high-pressure electrical resistivity measurements on this compound to 30 GPa. The data show that the critical temperature $T_c$ is suppressed gradually at a rate of $dT_c/dP = -0.05~\mathrm{K/GPa}$. Using density functional theory (DFT) calculations of the electronic and phonon density of states (DOS) and the measured critical temperature, we estimate that the rapid increase in lattice stiffening in Be$_{22}$Re overwhelms a moderate increase in the electron-ion interaction with pressure, resulting in the decrease in $T_c$. High pressure x-ray diffraction measurements show that the ambient pressure crystal structure of Be$_{22}$Re persists to at least 154 GPa. We discuss the relationship between low-Z Be-rich superconductors and the high-$T_c$ superhydrides.

J. Lim, A. C. Hire, 3 Y. Quan, 2, 3 J. S. Kim, S. R. Xie, 3 R. S. Kumar, D. Popov, C. Park, R. J. Hemley, 6 J. J. Hamlin, R. G. Hennig, 3 P. J. Hirschfeld, and G. R. Stewart Department of Physics, University of Florida, Gainesville, Florida 32611, USA Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, USA Quantum Theory Project, University of Florida, Gainesville, Florida 32611, USA Department of Physics, University of Illinois Chicago, Chicago, Illinois 60607, USA HPCAT, X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA Department of Chemistry, University of Illinois Chicago, Chicago, Illinois 60607, USA (Dated: September 24, 2021)

A15 Nb 3 Si is, until now, the only ‘high’ temperature superconductor produced at high pressure (∼110 GPa) that has been successfully brought back to room pressure conditions in a metastable condition. Based on the current great interest in trying to create metastable-at-room-pressure high temperature superconductors produced at high pressure, we have restudied explosively compressed A15 Nb 3 Si and its production from tetragonal Nb 3 Si. First, diamond anvil cell pressure measurements up to 88 GPa were performed on explosively compressed A15 Nb 3 Si material to trace T c as a function of pressure. T c is suppressed to ∼5.2 K at 88 GPa. Then, using these T c ( P ) data for A15 Nb 3 Si, pressures up to 92 GPa were applied at room temperature (which increased to 120 GPa at 5 K) on tetragonal Nb 3 Si. Measurements of the resistivity gave no indication of any A15 structure production, i.e. no indications of the superconductivity characteristic of A15 Nb 3 Si. This is in contrast to the explosive compression (up to P ∼ 110 GPa) of tetragonal Nb 3 Si, which produced 50%–70% A15 material, T c = 18 K at ambient pressure, in a 1981 Los Alamos National Laboratory experiment. This implies that the accompanying high temperature (1000 °C) caused by explosive compression is necessary to successfully drive the reaction kinetics of the tetragonal → A15 Nb 3 Si structural transformation. Our theoretical calculations show that A15 Nb 3 Si has an enthalpy vs the tetragonal structure that is 70 meV atom −1 smaller at 100 GPa, while at ambient pressure the tetragonal phase enthalpy is lower than that of the A15 phase by 90 meV atom −1 . The fact that ‘annealing’ the A15 explosively compressed material at room temperature for 39 years has no effect shows that slow kinetics can stabilize high pressure metastable phases at ambient conditions over long times even for large driving forces of 90 meV atom −1 .

Ice VII and ice X are the two most dominant phases, stable over a large pressure range between 2 and 150 GPa and made of fundamentally different chemical bonding. Yet, the two ice phases share a similar bcc-based crystal structure and lattice constants, resulting in a challenge to discern the crystal structure of ice VII and ice X. Here, we present well-resolved X-ray diffraction data of H2O in quasi-hydrostatic H2 and He pressure media, clearly resolving the two ice phases to 130 GPa and the dissociative nature of ice VII to X transition occurring at 20-50 GPa in H2O-H2 and 60-70 GPa in H2O-He. The present diffraction data permits, for the first time, the accurate determination of the bulk moduli B0 of 225 (or 228) GPa for ice X and 6.2 (or 4.5) GPa for ice VII, in H2O-H2 (or H2O-He), which can provide new constraints for Giant planetary models.

S. R. Xie, 2 Y. Quan, 2, 3 A. C. Hire, 2 B. Deng, J. M. DeStefano, I. Salinas, U. S. Shah, L. Fanfarillo, 4 J. Lim, J. Kim, G. R. Stewart, J. J. Hamlin, P. J. Hirschfeld, and R. G. Hennig 2, ∗ Department of Materials Science and Engineering, University of Florida, Gainesville FL 32611, USA Quantum Theory Project, University of Florida, Gainesville FL 32611, USA Department of Physics, University of Florida, Gainesville, Florida 32611, USA Scuola Internazionale Superiore di Studi Avanzati (SISSA), Via Bonomea 265, 34136 Trieste, Italy (Dated: July 13, 2021)

We have studied the compression behaviors of Ne, Ar, and Kr in comparison with the isoelectronic counterparts of NaF, KCl, and RbBr to 120 GPa, using in situ angle-resolved x-ray diffraction. Interestingly, the pressure-volume compression data indicate the emergence between the isoelectronic pairs of NaF/Ne >60 GPa, KCl/Ar >30 GPa, and RbBr/Kr >25 GPa-analogous to that previously observed in CsI/Xe >80 GPa. The interatomic distances of the isoelectronic pairs also become similar as pressure increases, within the difference of similar to 3%, underscoring the pressure-induced electron polarization in novel gas solids, stronger in Xe and getting weaker in Kr, Ar, and Ne. In contrast, the compression curves and interatomic distances of He and LiF remain distinctive to 200 GPa.

We report measurements and calculations on the properties of the intermetallic compound Be_5Pt. High-quality polycrystalline samples show a nearly constant temperature dependence of the electrical resistivity over a wide temperature range. On the other hand, relativistic electronic structure calculations indicate the existence of a narrow pseudogap in the density of states arising from accidental approximate Dirac cones extremely close to the Fermi level. A small true gap of order 3 meV is present at the Fermi level, yet the measured resistivity is nearly constant from low to room temperature. We argue that this unexpected behavior can be understood by a cancellation of the energy dependence of density of states and relaxation time due to disorder, and discuss a model for electronic transport. With applied pressure, the resistivity becomes semiconducting, consistent with theoretical calculations that show that the band gap increases with applied pressure. We further discuss the role of Be inclusions in the samples.

We have studied the compression behavior of H-2-He mixtures in comparison with pure H-2 and He using powder synchrotron x-ray diffraction, and we present the pressure-volume (PV) compression data of H-2-He mixtures to 160 GPa. The results indicate that both H-2 and He in H-2-He mixtures remain in hcp to the maximum pressure studied, yet they develop a substantial level of lattice distortion in the (100) plane, most profound in He-rich solids and below 66 GPa. The measured PV data also indicate the softening of an He (or H-2)-rich lattice upon increasing the level of the guest H-2 (or He) concentration. We suggest that the observed softening and lattice distortion are due to a substitutional incorporation of H-2 (guest) molecules into the basal plane of the hcp-He (host) lattice, and thereby reflect the miscibility between H-2 and He in H-2-He mixtures. Interestingly, solid He exhibits a lesser degree of preferred orientation in H-2-He mixtures than in pure He, likely due to the presence of solid H-2 disturbing the crystalline ordering of He-rich solids. Finally, the present PV compression data of H-2-rich and He-rich solids to 160 GPa deviate from those of pure H-2 and pure He above similar to 70 and 45 GPa respectively, providing new constraints for the development of the equation of state for H-2-He mixtures for planetary models.