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 (MHx, with x > 6 and M selected rare earth elements) – with superconducting critical temperatures (Tc) 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 x-ray 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‘s at and above room temperature.
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X-ray Diffraction and Equation of State of the C-S-H Room-Temperature Superconductor
X-ray diffraction indicates that the structure of the recently discovered carbonaceous sulfur hydride (C-S-H) room temperature superconductor is derived from previously established van der Waals compounds found in the H2S-H2 and CH4-H2 systems. Crystals of the superconducting phase were produced by a photochemical synthesis technique leading to the superconducting critical temperature Tc of 288 K at 267 GPa. X-ray diffraction patterns measured from 124 to 178 GPa, within the pressure range of the superconducting phase, are consistent with an orthorhombic structure derived from the Al2Cu-type determined for (H2S)2H2 and (CH4)2H2 that differs from those predicted and observed for the S-H system to these pressures. The formation and stability of the C-S-H compound can be understood in terms of the close similarity in effective volumes of the H2S and CH4 components, and denser carbon-bearing S-H phases may form at higher pressures. The results are crucial for understanding the very high superconducting Tc found in the C-S-H
system at megabar pressures.
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- Award ID(s):
- 1933622
- NSF-PAR ID:
- 10295407
- Date Published:
- Journal Name:
- Journal of chemical physics
- ISSN:
- 1089-7690
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract It is well known that elasticity is a key physical property in the determination of the structure and composition of the Earth and provides critical information for the interpretation of seismic data. This study investigates the stress‐induced variation in elastic wave velocities, known as the acoustoelastic effect, in San Carlos olivine. A recently developed experimental ultrasonic acoustic system, the Directly Integrated Acoustic System Combined with Pressure Experiments (DIASCoPE), was used with the D‐DIA multi‐anvil apparatus to transmit ultrasonic sound waves and collect the reflections. We use the DIASCoPE to obtain longitudinal (
P ) and shear (S ) elastic wave velocities from San Carlos olivine at pressures ranging from 3.2–10.5 GPa and temperatures from 450–950°C which we compare to the stress state in the D‐DIA derived from synchrotron X‐ray diffraction. We use elastic‐plastic self‐consistent (EPSC) numerical modeling to forward model X‐ray diffraction data collected in D‐DIA experiments to obtain the macroscopic stress on our sample. We can observe the relationship between the relative elastic wave velocity change (ΔV /V ) and macroscopic stress to determine the acoustoelastic constants, and interpret our observations using the linearized first‐order equation based on the model proposed by Hughes and Kelly (1953),https://doi.org/10.1103/physrev.92.1145 . This work supports the presence of the acoustoelastic effect in San Carlos olivine, which can be measured as a function of pressure and temperature. This study will aid in our understanding of the acoustoelastic effect and provide a new experimental technique to measure the stress state in elastically deformed geologic materials at high pressure conditions. -
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Sub-Neptunes are common among the discovered exoplanets. However, lack of knowledge on the state of matter in
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Abstract Since the discovery of superconductivity at ~ 200 K in H3S [1], similar or higher transition temperatures,
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