20 research outputs found
Crystal structures and high-temperature superconductivity in molybdenum-hydrogen binary system under high pressure
Motivated by advances in hydrogen-rich superconductors in the past decades,
we conducted variable-composition structural searches in Mo-H binary system at
high pressure. A new composition-pressure phase diagram of thermodynamically
stable structures has been derived. Besides all previously discovered
superconducting molybdenum hydrides, we also identified series of
thermodynamically metastable superconducting structures, including
I4/mmm-MoH, I4cm-MoH, P4/nmm-MoH and P422-MoH,
with the superconducting transition temperatures from 55 to 126 K at 300 GPa.
In these superconducting molybdenum hydrides, vibrations of the Mo-atoms
contributes significantly to the electron-phonon coupling and the
superconducting transition temperature, in complementary to the contributions
by the vibrations of the H-atoms. Our works highlight the importance of
compounds with non-integer composition ratio and metastable states in material
searches, for example the potential high temperature superconductors
Leading components and pressure-induced color changes in N-doped lutetium hydride
Recent experimental study by Dias {\it et al.} claims to have discovered
room-temperature superconductivity in lutetium-nitrogen-hydrogen system at 1
GPa [Nature 615, 244 (2023)], which sheds light on the long-held dream of
ambient superconductivity. However, all follow-up experiments found no evidence
of superconductivity. The compositions and the crystal structures of the
lutetium-nitrogen-hydrogen system remain unknown. By employing the density
functional theory based structure prediction algorithm, we suggest that in
lutetium-nitrogen-hydrogen the major component is LuH (Fmm),
together with minor LuN (Fmm). The blue LuH at ambient pressure
will turn into purple and red color at higher pressures, possibly accompanied
by the formation of vacancies at hydrogen-sites. In LuH and LuN, the
density of states at the Fermi level is dominated by the Lu-5d orbitals, while
those from hydrogen and nitrogen are very small, leading to the absence of
superconductivity in these two compounds. Nitrogen-doping to LuH fails to
enhance the superconductivity as well. In this work, we identify the leading
components in N-doped lutetium hydride, explain its intriguing color changes
under pressure, and elucidate why superconductivity is absent in the follow-up
experiments
Collapse of critical nematic fluctuations in FeSe under pressure
We report the evolution of the electronic nematic susceptibility in FeSe via
Raman scattering as a function of hydrostatic pressure up to 5.8 GPa where the
superconducting transition temperature reaches its maximum. The
critical nematic fluctuations observed at low pressure vanish above 1.6 GPa,
indicating they play a marginal role in the four-fold enhancement of at
higher pressures. The collapse of nematic fluctuations appears to be linked to
a suppression of low energy electronic excitations which manifests itself by
optical phonon anomalies at around 2 GPa, in agreement with lattice dynamical
and electronic structure calculations using local density approximation
combined with dynamical mean field theory. Our results reveal two different
regimes of nematicity in the phase diagram of FeSe under pressure: a d-wave
Pomeranchuk instability of the Fermi surface at low pressure and a magnetic
driven orthorhombic distortion at higher pressure.Comment: 7 pages, 4 figures. Supplementary Material available upon reques
Remarkable low-energy properties of the pseudogapped semimetal Be5Pt
We report measurements and calculations on the properties of the intermetallic compound Be5Pt. 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 3c3 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 bandgap increases with applied pressure. We further discuss the role of Be inclusions in the samples