386 research outputs found
Semimetallic molecular hydrogen at pressure above 350 GPa
According to the theoretical predictions, insulating molecular hydrogen
dissociates and transforms to an atomic metal at pressures P~370-500 GPa. In
another scenario, the metallization first occurs in the 250-500 GPa pressure
range in molecular hydrogen through overlapping of electronic bands. The
calculations are not accurate enough to predict which option is realized. Here
we show that at a pressure of ~360 GPa and temperatures <200 K the hydrogen
starts to conduct, and that temperature dependence of the electrical
conductivity is typical of a semimetal. The conductivity, measured up to 440
GPa, increases strongly with pressure. Raman spectra, measured up to 480 GPa,
indicate that hydrogen remains a molecular solid at pressures up to 440 GPa,
while at higher pressures the Raman signal vanishes, likely indicating further
transformation to a good molecular metal or to an atomic state
Structure and bonding of dense liquid oxygen from first principles simulations
Using first principles simulations we have investigated the structural and
bonding properties of dense fluid oxygen up to 180 GPa. We have found that band
gap closure occurs in the molecular liquid, with a "slow" transition from a
semi-conducting to a poor metallic state occurring over a wide pressure range.
At approximately 80 GPa, molecular dissociation is observed in the metallic
fluid. Spin fluctuations play a key role in determining the electronic
structure of the low pressure fluid, while they are suppressed at high
pressure.Comment: 4 figure
Magnetic measurements at pressures above 10 GPa in a miniature ceramic anvil cell for a superconducting quantum interference device magnetometer
A miniature ceramic anvil high pressure cell (mCAC) was earlier designed by
us for magnetic measurements at pressures up to 7.6 GPa in a commercial
superconducting quantum interference (SQUID) magnetometer [N. Tateiwa et al.,
Rev. Sci. Instrum. 82, 053906 (2011)]. Here, we describe methods to generate
pressures above 10 GPa in the mCAC. The efficiency of the pressure generation
is sharply improved when the Cu-Be gasket is sufficiently preindented. The
maximum pressure for the 0.6 mm culet anvils is 12.6 GPa when the Cu-Be gasket
is preindented from the initial thickness of 0.30 to 0.06 mm. The 0.5 mm culet
anvils were also tested with a rhenium gasket. The maximum pressure attainable
in the mCAC is about 13 GPa. The present cell was used to study YbCu2Si2 which
shows a pressure induced transition from the non-magnetic to magnetic phases at
8 GPa. We confirm a ferromagnetic transition from the dc magnetization
measurement at high pressure. The mCAC can detect the ferromagnetic ordered
state whose spontaneous magnetic moment is smaller than 1 mB per unit cell. The
high sensitivity for magnetic measurements in the mCAC may result from the the
simplicity of cell structure. The present study shows the availability of the
mCAC for precise magnetic measurements at pressures above 10 GPa
Equation of state of cubic boron nitride at high pressures and temperatures
We report accurate measurements of the equation of state (EOS) of cubic boron
nitride by x-ray diffraction up to 160 GPa at 295 K and 80 GPa in the range
500-900 K. Experiments were performed on single-crystals embedded in a
quasi-hydrostatic pressure medium (helium or neon). Comparison between the
present EOS data at 295 K and literature allows us to critically review the
recent calibrations of the ruby standard. The full P-V-T data set can be
represented by a Mie-Gr\"{u}neisen model, which enables us to extract all
relevant thermodynamic parameters: bulk modulus and its first
pressure-derivative, thermal expansion coefficient, thermal Gr\"{u}neisen
parameter and its volume dependence. This equation of state is used to
determine the isothermal Gr\"{u}neisen mode parameter of the Raman TO band. A
new formulation of the pressure scale based on this Raman mode, using
physically-constrained parameters, is deduced.Comment: 8 pages, 7 figure
Low temperature phase diagram of hydrogen at pressures up to 380 GPa. A possible metallic phase at 360 GPa and 200 K
Two new phases of hydrogen have been discovered at room temperature in Ref.1:
phase IV above 220 GPa and phase V above ~270 GPa. In the present work we have
found a new phase VI at P~360 GPa and T<200 K. This phase is likely metallic as
follows from the featureless Raman spectra, a strong drop in resistance, and
absence of a photoconductive response. We studied hydrogen at low temperatures
with the aid of Raman, infrared absorption, and electrical measurements at
pressures up to 380 GPa, and have built a new phase diagram of hydrogen.Comment: 9 pages, 12 figure
Spectroscopy of HS: evidence of a new energy scale for superconductivity
The discovery of a superconducting phase in sulfur hydride under high
pressure with a critical temperature above 200 K has provided a new impetus to
the search for even higher . Theory predicted and experiment confirmed
that the phase involved is HS with Im-3m crystal structure. The observation
of a sharp drop in resistance to zero at , its downward shift with
magnetic field and a Meissner effect confirm superconductivity but the
mechanism involved remains to be determined. Here, we provide a first optical
spectroscopy study of this new superconductor. Experimental results for the
optical reflectivity of HS, under high pressure of 150 GPa, for several
temperatures and over the range 60 to 600 meV of photon energies, are compared
with theoretical calculations based on Eliashberg theory using DFT results for
the electron-phonon spectral density F(). Two significant
features stand out: some remarkably strong infrared active phonons at
160 meV and a band with a depressed reflectance in the superconducting state in
the region from 450 meV to 600 meV. In this energy range, as predicted by
theory, HS is found to become a better reflector with increasing
temperature. This temperature evolution is traced to superconductivity
originating from the electron-phonon interaction. The shape, magnitude, and
energy dependence of this band at 150 K agrees with our calculations. This
provides strong evidence of a conventional mechanism. However, the unusually
strong optical phonon suggests a contribution of electronic degrees of freedom.Comment: 10 pages, 8 figures. Main manuscript and supplementary informatio
Pressure-induced Superconductivity in a Ferromagnet UGe -- Resistivity Measurements in Magnetic Field --
The electrical resistivity measurements in the magnetic field are carried out
on the pressure-induced superconductor UGe. The superconductivity is
observed from 1.06 to 1.44 GPa. The upper critical field of is
anisotropic where exhibits positive curvature for and
-axis. The characteristic enhancement of is reconfirmed for
-axis. In the temperature and field dependence of resistivity at where the ferromagnetic ordering disappears, it is observed that the
application of the external field along the {\it a}-axis increases the
coefficient of Fermi liquid behavior correspondingly to the
metamagnetic transition.Comment: To be published in the proceeding of the International Conference on
High Pressure Science and Technology(AIRAPT-18),Beijing,China,23-27 July 200
Superconductivity at 17 K in Yttrium Metal under Nearly Hydrostatic Pressures to 89 GPa
In an experiment in a diamond anvil cell utilizing helium pressure medium,
yttrium metal displays a superconducting transition temperature which increases
monotonically from Tc ? 3.5 K at 30 GPa to 17 K at 89.3 GPa, one of the highest
transition temperatures for any elemental superconductor. The pressure
dependence of Tc differs substantially from that observed in previous studies
under quasihydrostatic pressure to 30 GPa. Remarkably, the dependence of Tc on
relative volume V/Vo is linear over the entire pressure range above 33 GPa,
implying that higher values of Tc are likely at higher pressures. For the
trivalent metals Sc, Y, La, Lu there appears to be some correlation between Tc
and the ratio of the Wigner-Seitz radius to the ion core radius.Comment: submitted for publicatio
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