56 research outputs found
Reentrant valence transition in EuO at high pressures: beyond the bond-valence model
The pressure-dependent relation between Eu valence and lattice structure in
model compound EuO is studied with synchrotron-based x-ray spectroscopic and
diffraction techniques. Contrary to expectation, a 7% volume collapse at
45 GPa is accompanied by a reentrant Eu valence transition into a
\emph{lower} valence state. In addition to highlighting the need for probing
both structure and electronic states directly when valence information is
sought in mixed-valent systems, the results also show that widely used
bond-valence methods fail to quantitatively describe the complex electronic
valence behavior of EuO under pressure.Comment: 5 pages, 4 figure
Pressure induced effects on the Fermi surface of superconducting 2H-NbSe
The pressure dependence of the critical temperature and upper critical
field has been measured up to 19 GPa in the layered superconducting
material 2H-NbSe. Relating the behavior of to Fermi surface
parameters, we find that the electron phonon coupling of the 2D Nb 4d derived
bands shows a peak at 5 GPa when the charge density wave (CDW) order is
suppressed. On the other hand, shows a bell shaped curve with a
maximum at 10.5 GPa, well above the pressure for the suppression of the CDW
order. Changes in the band structure produce this shift in the maximum of
, demonstrating that 2H-NbSe shows important differences with
respect to other compounds where has a maximum in the temperature-density
phase diagram shaped by the suppression of another, non-superconducting, ground
state.Comment: 5 pages, 4 figures. Small changes in discussion. Typos correcte
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
Strong enhancement of superconductivity at high pressures within the charge-density-wave states of 2H-TaS 2 and 2H-TaSe 2
We present measurements of the superconducting and charge density wave
critical temperatures (Tc and TCDW) as a function of pressure in the transition
metal dichalchogenides 2H-TaSe2 and 2H-TaS2. Resistance and susceptibility
measurements show that Tc increases from temperatures below 1 K up to 8.5 K at
9.5 GPa in 2H-TaS2 and 8.2 K at 23 GPa in 2H-TaSe2. We observe a kink in the
pressure dependence of TCDW at about 4 GPa that we attribute to the lock-in
transition from incommensurate CDW to commensurate CDW. Above this pressure,
the commensurate TCDW slowly decreases coexisting with superconductivity within
our full pressure range.Comment: Published in Phys. Rev B 93, 184512 (2016
Pressure dependence of the upper critical field of MgB2 and of YNi2B2C
We present measurements of H under pressure in MgB and in
YNiBC. The changes in the shape of H are interpreted within
current models and show the evolution of the main Fermi surface velocities
and electron-phonon coupling parameters with pressure. In
MgB the electron-phonon coupling strength of the nearly two dimensional
band, responsible for the high critical temperature, is more affected
by pressure than the band coupling, and the hole doping of the
band decreases. In YNiBC, the peculiar positive curvature of
H is weakened by pressure.Comment: 5 pages, 5 figure
Coexistence of superconductivity and charge-density waves in a two-dimensional Holstein model at half-filling
In one dimension the coupling of electrons to phonons leads to a transition
from a metallic to a Peierls distorted insulated state if the coupling exceeds
a critical value. On the other hand, in two dimensions the electron-phonon
interaction may also lead to the formation of Cooper pairs. This competition of
superconductivity and charge order (in conjunction with a lattice distortion)
is studied in this letter by means of the projector-based renormalization
method (PRM). Increasing the electron-phonon interaction, we find a crossover
behavior between a purely superconducting state and a charge-density wave where
a well-defined parameter range of coexistence of superconductivity and lattice
distortion exists.Comment: 11 pages, 2 figure
Pressure Effects and Large Polarons in Layered MgB_2 Superconductor
We consider the dependence of the MgB_2 superconducting critical temperature
on the pressure. Our model exploits the influence of the large polarons on the
band structure of the layered MgB_2 superconductor. Namely, the hole
Pekar-Froehlich polarons form quasi two-dimensional potential wells in the
boron plane which shift the positions of the sigma- and pi-bands. This energy
shift depends on the pressure and the Cooper pairing of the correlated
sigma-electrons happens inside polaron wells. The results obtained are as
follows: dT_c/dp = -\alpha (5.2 \pm 0.9) K/GPa or dT_c/dp = -\alpha (6.9\pm
1.1) K/GPa for a different choice of the Grueneisen parameter. Being compared
with known experimental data they give us a resonable interval for the value of
the Froehlich electron-phonon coupling constant: \alpha = 0.15 - 0.45.Comment: 6 pages, 1 fig, LaTeX, subm. to Phys. Rev.
Strain effect on electronic transport and ferromagnetic transition temperature in LaSrMnO thin films
We report on a systematic study of strain effects on the transport properties
and the ferromagnetic transition temperature of high-quality
LaSrMnO thin films epitaxially grown on (100) SrTiO
substrates. Both the magnetization and the resistivity are critically dependent
on the film thickness. is enhanced with decreasing the film thickness
due to the compressive stain produced by lattice mismatch. The resistivity
above 165 K of the films with various thicknesses is consistent with small
polaronic hopping conductivity. The polaronic formation energy is
reduced with the decrease of film thickness. We found that the strain
dependence of mainly results from the strain-induced electron-phonon
coupling. The strain effect on is in good agreement with the
theoretical predictions.Comment: 6 pages and 5 figures, accepted for publication in Phys. Rev.
Assembling the puzzle of superconducting elements: A Review
Superconductivity in the simple elements is of both technological relevance
and fundamental scientific interest in the investigation of superconductivity
phenomena. Recent advances in the instrumentation of physics under pressure
have enabled the observation of superconductivity in many elements not
previously known to superconduct, and at steadily increasing temperatures. This
article offers a review of the state of the art in the superconductivity of
elements, highlighting underlying correlations and general trends.Comment: Review, 10 pages, 11 figures, 97 references; to appear in Superc.
Sci. Techno
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