571 research outputs found
Relaxor ferroelectricity induced by electron correlations in a molecular dimer Mott insulator
We have investigated the dielectric response in an antiferromagnetic
dimer-Mott insulator beta'-(BEDT-TTF)2ICl2 with square lattice, compared to a
spin liquid candidate kappa-(BEDT-TTF)2Cu2(CN)3. Temperature dependence of the
dielectric constant shows a peak structure obeying Curie-Weiss law with strong
frequency dependence. We found an anisotropic ferroelectricity by pyrocurrent
measurements, which suggests the charge disproportionation in a dimer. The
ferroelectric actual charge freezing temperature is related to the
antiferromagnetic interaction, which is expected to the charge-spin coupled
degrees of freedom in the system.Comment: 5 pages, 4 figures, to be published in Phys. Rev.
Thermal Conductivity near H_c2 for spin-triplet superconducting States with line nodes in Sr_2RuO_4
We calculate the thermal conductivity kappa in magnetic fields near H_c2 for
spin-triplet superconducting states with line nodes vertical and horizontal
relative to the RuO_2-planes. The method for calculating the Green's functions
takes into account the spatial variation of the order parameter and
superconducting flow for the Abrikosov vortex lattice. For in-plane magnetic
field we obtain variations of the in-plane kappa with two-fold symmetry as a
function of rotation angle where the minima and maxima occur for field
directions parallel and perpendicular to the heat flow. The amplitude of the
variation decreases with increasing impurity scattering and temperature. At
higher temperatures the minima and maxima of the variation are interchanged.
Since the results for vertical and horizontal line nodes are almost the same we
cannot say which of the two pairing models is more compatible with recent
measurements of kappa in Sr_2RuO_4. The observed four-fold modulation of kappa
in YBa_2Cu_3O_(7-\delta) is obtained for d-wave pairing by taking into account
the particular shape of the Fermi surface and the finite temperature effect.
The results for kappa for the f-wave pairing state with horizontal line nodes
disagree in some respects with the measurements on UPt_3.Comment: 8 pages, 6 figures. To be published in Phys. Rev.
Two-band superconductivity featuring different anisotropies in the ternary iron silicide LuFeSi
We report detailed studies of the upper critical field and low-temperature
specific heat in the two-gap superconductor LuFeSi. The
anisotropy of the upper critical field suggests that the active band is
quasi-one-dimensional. Low-temperature specific heat in magnetic fields reveals
that the virtual in the passive band is almost isotropic. These
results strongly indicate that the two bands have two different anisotropies,
similar to the typical two-gap superconductor MgB, and their interplay
may be essential to the two-gap superconductivity in LuFeSi.Comment: 5 pages, 5 figure
Double Giant Dipole Resonance in ^{208}Pb
Double-dipole excitations in ^{208}Pb are analyzed within a microscopic model
explicitly treating 2p2h-excitations. Collective states built from such
2p2h-excitations are shown to appear at about twice the energy of the isovector
giant dipole resonance, in agreement with the experimental findings. The
calculated cross section for Coulomb excitation at relativistic energies cannot
explain simultaneously the measured single-dipole and double-dipole cross
sections, however.Comment: 7 pages, Latex, 5 postscript figure
Dynamics vs electronic states of vortex core of high-T_c superconductors investigated by high-frequency impedance measurement
Dynamics of vortices reflects the electronic states of quasiparticles in the
core. To understand this, we investigated the following three issues. (1) We
investigated the complex surface impedance, Zs, of YBa2Cu3Oy as a function of
magnetic field, H. The total features were well expressed by the Coffey-Clem
model. From the data, we estimated the viscosity and pinning frequency, which
were found to be independent of frequency. In particular, the obtained
viscosity definitely shows that the core of vortex of YBa2Cu3Oy is moderately
clean. This result suggests that new physics will show up, for the physics of
quantum moderately clean vortex core is unknown at all. (2) An anomaly found in
the surface reactance at the first order transition (FOT) of vortex lattice was
investigated in Bi2Sr2CaCu2Oy with various doping levels. As a result, the
anomaly was found only in the samples exhibiting the FOT. On the other hand, we
did not observe the anomaly in YBa2Cu3Oy. These suggest that the anomaly is due
to the change in the electronic states of the vortices characteristic of
materials with very strong anisotropy. (3) We measured H dependence of both the
thermal conductivity \kappa(H) and Zs(H) in exactly the same pieces of crystal.
We could not find any anomaly in Zs(H) even at the onset of the plateau. This
result suggests that the origin of the plateau in \kappa(H) is not a drastic
phase transition but is rather gradual crossover.Comment: 6 pages, 5 figures, Proceedings of Plasma2000(Sendai), to be
published in Physica
Mechanism of spin-triplet superconductivity in Sr2RuO4
The unique Fermi surfaces and their nesting properties of Sr2RuO4 are
considered. The existence of unconventional superconductivity is shown
microscopically, for the first time, from the magnetic interactions (due to
nesting) and the phonon-mediated interactions. The odd-parity superconductivity
is favored in the and sheets of the Fermi surface, and the
various superconductivities are possible in the sheet. There are a
number of possible odd-parity gaps, which include the gaps with nodes, the
breaking of time-reversal symmetry and .Comment: 4 pages, 3 figure
Thermo-magnetic hysteretic properties resembling superconductivity in the normal state of La1.85Sr0.15CuO4
We have performed detailed magnetic and thermal hysteresis experiments in the
normal-state magnetization of La1.85Sr0.15CuO4 single crystal. Using a
combination of in-field and in-zero-magnetic-field measurements at different
stages of thermal history of the sample, we identified subtle effects
associated with the presence of magnetic signatures which resemble those below
the superconducting transition temperature (Tc=36 K) but survive up to 250 K.Comment: One file including text and figure
Bulk and surface low-energy excitations in YBa2Cu3O7-d studied by high-resolution angle-resolved photoemission spectroscopy
We have performed high-resolution angle-resolved photoemission spectroscopy
on YBa2Cu3O7-delta (Y123; delta = 0.06; Tc = 92 K). By accurately determining
the Fermi surface and energy band dispersion, we solve long-standing
controversial issues as to the anomalous electronic states of Y-based high-Tc
cuprates. We unambiguously identified surface-bilayer-derived bonding and
antibonding bands, together with their bulk counterparts. The surface bands are
highly overdoped (hole concentration x = 0.29), showing no evidence for the gap
opening or the dispersion anomaly in the antinodal region, while the bulk bands
show a clear dx2-y2-wave superconducting gap and the Bogoliubov
quasiparticle-like behavior with a characteristic energy scale of 50-60 meV
indicative of a strong electron-boson coupling in the superconducting state.
All these results suggest that the metallic and superconducting states coexist
at the adjacent bilayer of Y123 surface.Comment: Accepted for publication in Phys. Rev.
Variational Calculation for the Equation of State of Nuclear Matter at Finite Temperatures
An equation of state (EOS) for uniform nuclear matter is constructed at zero
and finite temperatures with the variational method starting from the realistic
nuclear Hamiltonian composed of the Argonne V18 and UIX potentials. The energy
is evaluated in the two-body cluster approximation with the three-body-force
contribution treated phenomenologically so as to reproduce the empirical
saturation conditions. The obtained energies for symmetric nuclear matter and
neutron matter at zero temperature are in fair agreement with those by Akmal,
Pandharipande and Ravenhall, and the maximum mass of the neutron star is 2.2
Msolar. At finite temperatures, a variational method by Schmidt and
Pandharipande is employed to evaluate the free energy, which is used to derive
various thermodynamic quantities of nuclear matter necessary for supernova
simulations. The result of this variational method at finite temperatures is
found to be self-consistent.Comment: Revised Versio
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