21 research outputs found
Superconductivity in the Chalcogens up to Multimegabar Pressures
Highly sensitive magnetic susceptibility techniques were used to measure the
superconducting transition temperatures in S up to 231(5) GPa. S
transforms to a superconductor with T of 10 K and has a discontinuity in
T_c dependence at 160 GPa corresponding to bco to beta-Po phase transition.
Above this pressure T_c in S has a maximum reaching about 17.3(+/-0.5) K at 200
GPa and then slowly decreases with pressure to 15 K at 230 GPa.
This trend in the pressure dependence parallels the behavior of the heavier
members Se and Te. Superconductivity in Se was also observed from 15 to 25 GPa
with T_c changing from 4 to 6 K and above 150 GPa with T_c of 8 K.
Similiarities in the T_c dependences for S, Se, and Te, and the implications
for oxygen are discussed.Comment: 4 pages, 10 figure
Elemental Phosphorus: structural and superconducting phase diagram under pressure
Pressure-induced superconductivity and structural phase transitions in
phosphorous (P) are studied by resistivity measurements under pressures up to
170 GPa and fully crystal structure and superconductivity
calculations up to 350 GPa. Two distinct superconducting transition temperature
(T) vs. pressure () trends at low pressure have been reported more
than 30 years ago, and for the first time we are able to reproduce them and
devise a consistent explanation founded on thermodynamically metastable phases
of black-phosphorous. Our experimental and theoretical results form a single,
consistent picture which not only provides a clear understanding of elemental P
under pressure but also sheds light on the long-standing and unsolved
superconductivity trend. Moreover, at higher pressures we predict a
similar scenario of multiple metastable structures which coexist beyond their
thermodynamical stability range. Metastable phases of P experimentally
accessible at pressures above 240 GPa should exhibit T's as high as 15 K,
i.e. three times larger than the predicted value for the ground-state crystal
structure. We observe that all the metastable structures systematically exhibit
larger transition temperatures than the ground-state ones, indicating that the
exploration of metastable phases represents a promising route to design
materials with improved superconducting properties.Comment: 14 pages, 4 figure
Crystal Structure of 200 K-Superconducting Phase of Sulfur Hydride System
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-centered
cubic symmetry in the superconducting phase.Comment: 8 pages, 3 figure
A Perspective on Conventional High-Temperature Superconductors at High Pressure: Methods and Materials
Two hydrogen-rich materials, HS and LaH, synthesized at megabar
pressures, have revolutionized the field of condensed matter physics providing
the first glimpse to the solution of the hundred-year-old problem of room
temperature superconductivity. The mechanism underlying superconductivity in
these exceptional compounds is the conventional electron-phonon coupling. Here
we describe recent advances in experimental techniques, superconductivity
theory and first-principles computational methods which have made possible
these discoveries. This work aims to provide an up-to-date compendium of the
available results on superconducting hydrides and explain how the synergy of
different methodologies led to extraordinary discoveries in the field. Besides,
in an attempt to evidence empirical rules governing superconductivity in binary
hydrides under pressure, we discuss general trends in the electronic structure
and chemical bonding. The last part of the Review introduces possible
strategies to optimize pressure and transition temperatures in conventional
superconducting materials as well as future directions in theoretical,
computational and experimental research.Comment: 68 pages, 30 figures, Preprint submitted to Physics Report
Transparent dense sodium
Under pressure, metals exhibit increasingly shorter interatomic distances.
Intuitively, this response is expected to be accompanied by an increase in the
widths of the valence and conduction bands and hence a more pronounced
free-electron-like behaviour. But at the densities that can now be achieved
experimentally, compression can be so substantial that core electrons overlap.
This effect dramatically alters electronic properties from those typically
associated with simple free-electron metals such as lithium and sodium, leading
in turn to structurally complex phases and superconductivity with a high
critical temperature. But the most intriguing prediction - that the seemingly
simple metals Li and Na will transform under pressure into insulating states,
owing to pairing of alkali atoms - has yet to be experimentally confirmed. Here
we report experimental observations of a pressure-induced transformation of Na
into an optically transparent phase at 200 GPa (corresponding to 5.0-fold
compression). Experimental and computational data identify the new phase as a
wide bandgap dielectric with a six-coordinated, highly distorted
double-hexagonal close-packed structure. We attribute the emergence of this
dense insulating state not to atom pairing, but to p-d hybridizations of
valence electrons and their repulsion by core electrons into the lattice
interstices. We expect that such insulating states may also form in other
elements and compounds when compression is sufficiently strong that atomic
cores start to overlap strongly.Comment: Published in Nature 458, 182-185 (2009
The 2021 room-temperature superconductivity roadmap.
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
Superconducting hydrides under pressure
The measurement of superconductivity at above 200K in compressed samples of hydrogen sulfide and in lanthanum hydride at 250K is rein- vigorating the search for conventional high temperature superconduc- tors. At the same time it exposes a fascinating interplay between the- ory, computation and experiment. Conventional superconductivity is well understood, and theoretical tools are available for accurate predic- tions of the superconducting critical temperature. These predictions depend on knowing the microscopic structure of the material under consideration, and can now be provided through computational first principles structure predictions. The experiments at the megabar pres- sures required are extremely challenging, but, for some groups at least, permit the experimental exploration of materials space. We discuss the prospects for the search for new superconductors, ideally at lower pressures.CJP is supported by the Royal Society through a Royal Society Wolfson Research Merit award. IE has received funding from the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation programme (grant agreement No 802533), and the Spanish Ministry of Economy and Competitiveness (FIS2016-76617-P). MIE thanks the Max Planck community for invaluable support, and U. Po ̈schl for the constant encouragement
Conventional superconductivity at 203 kelvin at high pressures in the sulfur hydride system
A superconductor is a material that can conduct electricity without resistance below a superconducting transition temperature, Tc. The highest Tc that has been achieved to date is in the copper oxide system: 133 kelvin at ambient pressure and 164 kelvin at high pressures. As the nature of superconductivity in these materials is still not fully understood (they are not conventional superconductors), the prospects for achieving still higher transition temperatures by this route are not clear. In contrast, the Bardeen-Cooper-Schrieffer theory of conventional superconductivity gives a guide for achieving high Tc with no theoretical upper bound--all that is needed is a favourable combination of high-frequency phonons, strong electron-phonon coupling, and a high density of states. These conditions can in principle be fulfilled for metallic hydrogen and covalent compounds dominated by hydrogen, as hydrogen atoms provide the necessary high-frequency phonon modes as well as the strong electron-phonon coupling. Numerous calculations support this idea and have predicted transition temperatures in the range 50-235 kelvin for many hydrides, but only a moderate Tc of 17 kelvin has been observed experimentally. Here we investigate sulfur hydride, where a Tc of 80 kelvin has been predicted. We find that this system transforms to a metal at a pressure of approximately 90 gigapascals. On cooling, we see signatures of superconductivity: a sharp drop of the resistivity to zero and a decrease of the transition temperature with magnetic field, with magnetic susceptibility measurements confirming a Tc of 203 kelvin. Moreover, a pronounced isotope shift of Tc in sulfur deuteride is suggestive of an electron-phonon mechanism of superconductivity that is consistent with the Bardeen-Cooper-Schrieffer scenario. We argue that the phase responsible for high-Tc superconductivity in this system is likely to be H3S, formed from H2S by decomposition under pressure. These findings raise hope for the prospects for achieving room-temperature superconductivity in other hydrogen-based materials
High-resolution analytical electron microscopy of boron nitrides laser heated at high pressure
High-resolution transmission electron microscopy and electron energy loss spectroscopy have been carried out for cubic and hexagonal boron nitrides (BN) laser heated in argon or nitrogen media at pressures of 5-11 GPa in a diamond anvil cell. In particular, recrystallized products of irradiation from a fluid phase in the form of tiny flakes have been investigated. The observations revealed perfect crystallinity (either of cubic or hexagonal BN) in flakes recrystallized from the fluid and traces of melting in the bulk. Multishelled circular and polygonal BN nanotubes, which did not contain any additional inclusions, were found after laser heating of cubic and hexagonal BN in nitrogen. The nanotubes typically exhibited 3-10 shells, a characteristic inner dimension in cross-section of 2-6 nm and stoichiometry of B/N ~1. They were found to have grown either from a cubic BN matrix or from a mixture of amorphous + turbostratic + hexagonal BN, which had recrystalized on the specimens' surface from the fluid phase.</p