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Most metals adopt simple structures such as body-centered cubic (BCC), face-centered cubic (FCC), and hexagonal close-packed (HCP) structures in specific groupings across the periodic table, and many undergo transitions to surprisingly complex structures on compression, not expected from conventional free-electron-based theories of metals. First-principles calculations have been able to reproduce many observed structures and transitions, but a unified, predictive theory that underlies this behavior is not yet in hand. Discovered by analyzing the electronic properties of metals in various lattices over a broad range of sizes and geometries, a remarkably simple theory shows that the stability of metal structures is governed by electrons occupying local interstitial orbitals and their strong chemical interactions. The theory provides a basis for understanding and predicting structures in solid compounds and alloys over a broad range of conditions.

The recent report of superconductivity in nitrogen-doped lutetium hydride at near-ambient pressures and temperatures has attracted great attention but also continuing controversy. Several experimental groups have reported no observation of superconductivity at these conditions in Lu-N-H samples they have prepared. To address this issue, we have carried out a series of studies of phases in the Lu-N-H system using a variety of techniques. We report here electrical resistance measurements on a Lu-N-H sample that are in very good agreement with both previously reported Tc and its pressure dependence in the vicinity of room temperature for nitrogen-doped lutetium hydride., Comment: 11 pages, 7 figures

We report P-V-T equation of state measurements of B4C to 50 GPa and approximately 2500 K in a laser-heated diamond anvil cell. We obtain an ambient temperature, third-order Birch–Murnaghan fit to the P-V data that yields a bulk modulus K0, 221(2) GPa and derivative, (dK/dP)0 of 3.3(1). These were used in both the Mie–Grüneissen–Debye fit and a temperature dependent, Birch–Murnaghan equation of state that includes thermal pressure estimated by thermal expansion (α) and a temperature-dependent bulk modulus (dK0/dT). The ambient pressure thermal expansion coefficient (α0 + α1T), Grüneissen γ(V) = γ0(V/V0)q and a volume-dependent Debye temperature, were used as input parameters for these fits and found to be sufficient to describe the data in the whole P-T range of this study.This article is part of the theme issue ‘Exploring the length scales, timescales and chemistry of challenging materials (Part 1)’.

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 susceptibility technique adapted for a diamond anvil cell confirms a Tc of 260 kelvin at 133 GPa in carbonaceous sulfur hydride. Furthermore, direct synchrotron infrared reflectivity measurements on the same sample under the same conditions reveal a superconducting gap of ~85 meV at 100 K in close agreement to the expected value from Bardeen-Cooper-Schrieffer (BCS) theory13-18. Additionally, x-ray diffraction in tandem with AC magnetic susceptibility measurements above and below the superconducting transition temperature, and as a function of pressure at 107-133 GPa, reveal the Pnma structure of the material is responsible for the close to room-temperature superconductivity at these pressures.

The strongly correlated material FeSi exhibits several unusual thermal, magnetic, and structural properties under varying pressure-temperature (P-T) conditions. It is a potential thermoelectric alloy and a materials of several geological implications as a possible constituent at the Earth's core mantle boundary (CMB). The phase transition behavior and lattice dynamics of FeSi under different P-T conditions remain elusive. A previous theoretical work predicted a pressure induced B20-B2 transition at ambient temperature, yet the transition is only observed at high P-T conditions in the experiments. Furthermore, the closing of the electronic gap due to a dramatic renormalization of the electronic structure and phonon anomalies has been reported based on density function calculations. In this study we have performed high pressure powder diffraction and Nuclear Resonant Inelastic X-ray Scattering (NRIXS) measurements up to 120 GPa to understand the phase stability and the lattice dynamics. Our study shows evidence for a nonhydrostatic stress induced B20-B2 transition in FeSi around 36 GPa for the first time. The Fe partial phonon density of states (PDOS) and thermal parameters were derived from NRIXS up to 120 GPa with the density function theoretical (DFT) calculations. These calculations further predict and are consistent with pressure-induced metallization and band gap closing around 12 GPa., Comment: 8 pages, 6 figures and supplementary information

First-principles density functional theory (DFT) calculations of Lu-H-N compounds reveal low-energy configurations of Fm$\overline{3}$m Lu$_{8}$H$_{23-x}$N structures that exhibit novel electronic properties such as flat bands, sharply peaked densities of states (van Hove singularities, vHs), and intersecting Dirac cones near the Fermi energy (E$_F$). These N-doped LuH$_3$-based structures also exhibit an interconnected metallic hydrogen network, which is a common feature of high-T$_c$ hydride superconductors. Electronic property systematics give estimates of T$_c$ for optimally ordered structures that are well above the critical temperatures predicted for structures considered previously. The vHs and flat bands near E$_F$ are enhanced in DFT+U calculations, implying strong correlation physics should also be considered for first-principles studies of these materials. These results provide a basis for understanding the novel electronic properties observed for nitrogen-doped lutetium hydride., Comment: Submitted to PRL. 4 main figures, 15 SI Figures, 1 table. 5 pages (13 pgs SI)

Designing materials with advanced functionalities is the main focus of contemporary solid-state physics and chemistry. Research efforts worldwide are funneled into a few high-end goals, one of the oldest, and most fascinating of which is the search for an ambient temperature superconductor (A-SC). The reason is clear: superconductivity at ambient conditions implies being able to handle, measure and access a single, coherent, macroscopic quantum mechanical state without the limitations associated with cryogenics and pressurization. This would not only open exciting avenues for fundamental research, but also pave the road for a wide range of technological applications, affecting strategic areas such as energy conservation and climate change. In this roadmap we have collected contributions from many of the main actors working on superconductivity, and asked them to share their personal viewpoint on the field. The hope is that this article will serve not only as an instantaneous picture of the status of research, but also as a true roadmap defining the main long-term theoretical and experimental challenges that lie ahead. Interestingly, although the current research in superconductor design is dominated by conventional (phonon-mediated) superconductors, there seems to be a widespread consensus that achieving A-SC may require different pairing mechanisms. In memoriam, to Neil Ashcroft, who inspired us all. © 2022 The Author(s). Published by IOP Publishing Ltd. Yundi Quan has provided many useful discussions on this topic, and assistance with preparation of the figures in this paper. Giustino’s review [12] contains a wealth of references on this topic. This work was supported by US National Science Foundation Grant DMR 1607139. 3. We acknowledge Ashkan Salamat, Elliot Snider, Nathan Dasenbrock-Gammon, Raymond McBride, Mathew Debessai for useful discussions. This was supported by NSF, Grant No. DMR-1809649, and by the DOE Stockpile Stewardship Academic Alliance Program, Grant No. DE-NA0003898.5. CH acknowledges support from the Austrian Science Fund (FWF) Project No. P 32144-N36. 6. This work was supported principally by the US National Science Foundation (DMR-1933622), and by the US Department of Energy (DE-SC0020340 and DE-NA0003975). 7. I would like to thank all my collaborators and friends that contributed to this work: D M Ceperley, M Holzmann, V Gorelov, G Rillo, Y Yubo, M A Morales. This work was supported by ANR-France under the program ‘Accueil de Chercheurs de Haut Niveau 2015’ project: HyLightExtreme. 8. MA acknowledges support from the Swiss National Science Foundation (Project P4P4P2-180669). 9. This work was supported by the National Natural Science Foundation of China (Grant Nos. 52090024 and 12074138), the Science Challenge Project (Grant No. TZ2016001), the Fundamental Research Funds for the Central Universities (Jilin University, JLU), the Program for JLU Science and Technology Innovative Research Team (JLUSTIRT), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB33000000) 10. ANK acknowledges the support through the NSF Award No. DMR-1821815. 11. We thank the Russian Science Foundation (Grant 19-72-30043) for support. 12. I thank Alice Shipley and Michael Hutcheon for their careful reading of the manuscript and insightful comments. This work has been funded by the EPSRC over many years (Projects EP/G007489/1 and EP/P022596/1), and through a Royal Society Wolfson Research Merit Award. 13. We acknowledge the National Science Foundation (DMR-1827815) for financial support. 14. The author acknowledges the financial support by JSPS KAK-ENHI Grant No. 19H05825. 15. This research was supported by the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Program (Grant Agreement No. 802533). 16. We acknowledge financial support by the European Research Council Advanced Grant FACT (ERC-2017-AdG-788890). AS acknowledges hospitality of the Physics Department of La Sapienza, under the program ‘Professori Visitatori 2020’. 17. ERM acknowledges support from the National Science Foundation (Award No. OAC-1740263). 18. The work was supported by the US Department of Energy Basic Energy Sciences under Contract No. DE-SC-0020385. 19. We thank G Profeta and G Lamura for fruitful scientific discussions. We acknowledge funding from the MIUR PRIN-2017 program (Grant No. 2017Z8TS5B—‘Tuning and understanding Quantum phases in 2D materials—Quantum2D’). 20. We thank G Profeta and G Lamura for fruitful scientific discussions. We acknowledge funding from the MIUR PRIN-2017program (Grant No. 2017Z8TS5B—‘Tuning and understanding Quantum phases in 2D materials—Quantum2D’). 21. We acknowledge financial support by the Deutsche Forschungs-Gemeinschaft through Grant (DFG) for funding through TRR 288–422213477 (B05). This research was supported in part by the National Science Foundation under Grant No. NSF PHY-1748958.

Pressure is a unique tuning parameter for probing the properties of materials and has been particularly useful for studies of electronic materials such as high-temperature cuprate superconductors. Here we report the effects of quasi-hydrostatic compression produced by a neon pressure-medium on the structures of bismuth-based high $\mathit{T_c}$ cuprate superconductors with the nominal composition $Bi_2Sr_2Ca_{n-1}Cu_nO_{2n+4+δ}$ (n=1,2,3) up to 155 GPa. The structures of all three compositions obtained by synchrotron X-ray diffraction can be described as pseudo-tetragonal over the entire pressure range studied. We show that previously reported pressure-induced distortions and structural changes arise from the large strains that can be induced in these layered materials by non-hydrostatic stresses. The pressure-volume equations of state (EOS) measured under these quasi-hydrostatic conditions cannot be fit to single phenomenological formulation over the pressure ranges studied, starting below 20 GPa. This intrinsic anomalous compression as well as the sensitivity of $Bi_2Sr_2Ca_{n-1}Cu_nO_{2n+4+δ}$ to deviatoric stresses provides explanations for the numerous inconsistencies in reported EOS parameters for these materials. We conclude that the anomalous compressional behavior of all three compositions is a manifestation of the changes in electronic properties that are also responsible for the remarkable non-monotonic dependence of $\mathit{T_c}$ with pressure, including the increase in $\mathit{T_c}$ at the highest pressures studied so far for each. Transport and spectroscopic measurements up to megabar pressures are needed to fully characterize and explore still higher possible critical temperatures in these materials.

Recently, superhydrides have been computationally identified and subsequently synthesized with a variety of metals at very high pressures. In this work, we evaluate the possibility of synthesizing superhydrides by uniquely combining electrochemistry and applied pressure. We perform computational searches for palladium superhydrides using density functional theory and particle swarm optimization calculations over a broad range of pressures and electrode potentials. We incorporate exchange-correlation functional uncertainty using the Bayesian error estimation formalism to quantify the uncertainty associated with the identified stable phases. Based on a thermodynamic analysis, we construct pressure-potential phase diagrams and provide an alternate synthesis concept, $\mathcal{P}^2$ (pressure-potential), to accessing novel phases having high hydrogen content. Palladium-hydrogen is a widely-studied material system with the highest hydride phase being Pd$_3$H$_4$. Most strikingly for this system, at potentials above hydrogen evolution and $\sim$300 MPa pressure, we find the possibility to make palladium superhydrides (e.g., PdH$_{10}$). We demonstrate the generalizability of this approach for La-H, Y-H and Mg-H with 10-100 fold reduction in required pressure for stabilizing phases. In addition, the $\mathcal{P}^2$ strategy allows stabilizing new phases that cannot be done purely by either pressure or potential and is a general approach that is likely to work for synthesizing other superhydrides at modest pressures., Comment: 30 pages, 7 figures, 13 pages of Supplementary Material

The use of high pressure to realize superconductivity in the vicinity of room temperature has a long history, much of it focused on achieving this in hydrogen rich materials. This paper provides a brief overview of the work presented at this May 2018 conference, together with background on motivation and techniques, the theoretical predictions of superconductivity in lanthanum hydride, and the subsequent experimental confirmation. Theoretical calculations using density functional based structure search methods combined with BCS type models predicted a new class of dense, hydrogen rich materials superhydrides with superconducting critical temperatures in the vicinity of room temperature at and above 200 GPa pressures. The existence of a series of these phases in the La H system was subsequently confirmed experimentally, and techniques were developed for their syntheses and characterization, including measurements of structural and transport properties, at megabar pressures. Four probe electrical transport measurements of a cubic phase identified as LaH10 display signatures of superconductivity at temperatures above 260 K near 200 GPa. The results are supported by pseudo four probe conductivity measurements, critical current determinations, low-temperature xray diffraction, and magnetic susceptibility measurements. The measured high Tc is in excellent agreement with the original calculations. The experiments also reveal additional superconducting phases with Tc between 150 K and above 260 K. This effort highlights the novel physics in hydrogen-rich materials at high densities, the success of materials by design in the discovery and creation of new materials, and the possibility of new classes of superconductors Tc at and above room temperature.

Compressed hydrogen passes through a series of layered structures in which the layers can be viewed as distorted graphene sheets. The electronic structures of these layered structures can be understood by studying simple model systems- an ideal single hydrogen graphene sheet and three-dimensional model lattices consisting of such sheets. The energetically stable structures result from structural distortions of model graphene-based systems due to electronic instabilities towards Peierls or other distortions associated with the opening of a band gap. Two factors play crucial roles in the metallization of compressed hydrogen: (i) crossing of conduction and valence bands in hexagonal or graphene-like layers due to topology and (ii) formation of bonding states with $2p_z$ $\pi$ character., Comment: 8 pages, 8 figures

We present high pressure diamond anvil cell synchrotron X-ray, resistivity, and ac-susceptibility measurements on electron-doped cuprate Pr$_{2-x}$Ce$_{x}$CuO$_{4}$ to much higher pressures than previously reported. At 2.72 GPa between 88 and 98$%$ of the superconducting T$^\prime$ phase \cite{Tprime} of the optimally doped Pr$_{1.85}$Ce$_{0.15}$CuO$_{4}$ transforms into the insulating phase T. The T$_{c}$ of the remaining 2-12$%$ T$^\prime$ phase is suppressed continuously from 22 K to 18.5 K at about 14 GPa. Remarkably, the T$_{c}$ of the over doped Pr$_{1.83}$Ce$_{0.17}$CuO$_{4}$ remains practically unchanged even at 32 GPa. This behavior of the electron-doped cuprate contrasts with that of the hole-doped cuprate for which T$_{c}$ is first substantially enhanced with applied pressure., Comment: 6 pages, 6 figures

The crystal structure of a high-pressure phase of silane (SiH4), observed between 10 and 25GPa, is solved using powder synchrotron x-ray diffraction and shown to be of the SnBr4 type. The phase is an insulating molecular solid with a monoclinic unit cell containing four tetrahedrally bonded molecules, space group P21∕c. Ab initio calculations show the SnBr4-type structure to be favored in this pressure range relative to other recently proposed structures. The fit of the pressure dependence of volume to the Birch-Murnagham equation of state gives the following parameters at ambient pressure if K′0 is fixed to 4: V0=250(9)Å3 and K0=7.8(9)GPa for the experimental data, and V0=255(2)Å3 and K0=6.1(2)GPa for the data obtained from ab initio calculations., This work was supported by NSF (DMR, DOE Grant No. DEFG02-02ER4595, and DOE - CDAC). The x-ray measurements were performed at HPCAT Sector 16, Advanced Photon Source APS, Argonne National Laboratory. The HPCAT facility is supported by DOE-BES, DOENNSA - CDAC, NSF, DOD-TACOM, and the W.M. Keck Foundation. The use of the APS was supported by DOE-BES under Contract No. W-31-109-ENG-38. M.M.C. would like to thank the Spanish Ministry of Education and Science for providing a grant. M.M.C. and A.B. acknowledge partial support from the EC sixth Framework Network of Excellence NANOQUANTA NMP4-CT-2004-500198.

We have investigated the pressure dependence of the superconducting transition temperature $T_{c}$ up to 18 GPa of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+��}$ single crystals ranging from the highly underdoped through the nearly optimally doped to the highly overdoped level. For all three samples studied, $T_{c}$ is found to increase initially and then saturate at a critical pressure $P_{c}$ but decrease modestly with further compression. Oxygen doping tends to reduce the increase in $T_{c}$ and $P_{c}$. A new high-pressure phase diagram between the saturated $T_{c}$ and $P_{c}$ is then obtained. Theoretical interpretation is given by using the competition between the hole carrier density and pairing interaction strength based on the high-pressure transport data of the resistivity and Hall coefficient in this system., 8 pages, 7 figures, to appear in Physical Review B

We review recent development in Raman techniques and diamond anvil cell technology, that allow measurements of Raman of metals in the megabar pressure range. The Raman technique includes holographic transmission optics and single-stage spectrographs with CCD array detectors. Use of synthetic high-purity diamonds and anglular excitation geometry reduces spurious background scattering substantially, which makes observable very weak Raman peaks in metals. We give several examples of studies including the hexagonal close packed (hcp) $\epsilon$-Fe, Fe$_{(1-x)}$Ni${_x}$, and Re and the recently discovered high-temperature superconductor MgB$_2$. The shift of phonon modes with pressure is used for determination of the mode-Gr\"uneisen parameters, estimation of shear elastic moduli and provide information on structural and electronic transitions., Comment: 18 pages, 10 figures, 2 tables, Proceedings of the International School of Physics "Enrico Fermi", Volume CXLVII, "High-Pressure Phenomena", edited by R. J. Hemley, M. Bernasconi, L. Ulivi and G. Chiarotti, in press

The pressure dependence of electrical resistance of single-crystal magnetite (Fe3O4) was measured under quasi-hydrostatic conditions to 100 GPa using low-temperature, megabar diamond-anvil cell techniques in order to gain insight into the anomalous behavior of this material that has been reported over the years in different high-pressure experiments. The measurements under nearly hydrostatic pressure conditions allowed us to detect the clear Verwey transition and the high-pressure structural phase. The appearance of a metallic ground state after the suppression of the Verwey transition around 20 GPa and the concomitant enhancement of the electrical resistance caused by the structural transformation to the high-pressure phase form reentrant semiconducting-metallic-semiconducting behavior, although the appearance of the metallic phase is highly sensitive to stress conditions and details of the measurement technique. (C) 2016 AIP Publishing LLC.