601 research outputs found
Metal-insulator transition in PrRuP and SmRuP investigated by optical spectroscopy
Electronic structures of the filled-skutterudite compounds PrRuP
and SmRuP, which undergo a metal-insulator transition (MIT) at
= 60 K and 16 K, respectively, have been studied by means of
optical spectroscopy. Their optical conductivity spectra develop an energy gap
of 10 meV below . The observed characteristics of the energy
gap are qualitatively different from those of the Kondo semiconductors. In
addition, optical phonon peaks in the spectra show anomalies upon the MIT,
including broadening and shifts at and an appearance of new peaks
below . These results are discussed in terms of density waves or
orbital ordering previously predicted for these compounds.Comment: 4pages, 4figures, submitted to Physical Review
Definitive experimental evidence for two-band superconductivity in MgB2
The superconducting gap of MgB2 has been studied by high-resolution
angle-resolved photoemission spectroscopy (ARPES). The momentum(k)-resolving
capability of ARPES enables us to identify the s- and p-orbital derived bands
predicted from band structure calculations and to successfully measure the
superconducting gap on each band. The results show that superconducting gaps
with values of 5.5 meV and 2.2 meV open on the s-band and the p-band,
respectively, but both the gaps close at the bulk transition temperature,
providing a definitive experimental evidence for the two-band superconductivity
in MgB2. The experiments validate the role of k-dependent electron-phonon
coupling as the origin of multiple-gap superconductivity in MgB2.Comment: PDF file onl
Metallic mean-field stripes, incommensurability and chemical potential in cuprates
We perform a systematic slave-boson mean-field analysis of the three-band
model for cuprates with first-principle parameters. Contrary to widespread
believe based on earlier mean-field computations low doping stripes have a
linear density close to 1/2 added hole per lattice constant. We find a
dimensional crossover from 1D to 2D at doping followed by a breaking
of particle-hole symmetry around doping 1/8 as doping increases. Our results
explain in a simple way the behavior of the chemical potential, the magnetic
incommensurability, and transport experiments as a function of doping. Bond
centered and site-centered stripes become degenerate for small overdoping.Comment: submitted to PR
A theory of new type of heavy-electron superconductivity in PrOs_4Sb_12: quadrupolar-fluctuation mediated odd-parity pairings
It is shown that unconventional nature of superconducting state of
PrOs_4Sb_12, a Pr-based heavy electron compound with the filled-Skutterudite
structure, can be explained in a unified way by taking into account the
structure of the crystalline-electric-field (CEF) level, the shape of the Fermi
surface determined by the band structure calculation, and a picture of the
quasiparticles in f-configuration with magnetically singlet CEF ground
state. Possible types of pairing are narrowed down by consulting recent
experimental results. In particular, the chiral "p"-wave states such as
p_x+ip_y is favoured under the magnetic field due to the orbital Zeeman effect,
while the "p"-wave states with two-fold symmetery such as p_x can be stabilized
by a feedback effect without the magnetic field. It is also discussed that the
double superconducting transition without the magnetic field is possible due to
the spin-orbit coupling of the "triplet" Cooper pairs in the chiral state.Comment: 12 pages, 2 figures, submitted to J. Phys.: Condens. Matter Lette
Evidence for ferromagnetic spin-pairing superconductivity in UGe: A Ge-NQR study under pressure
We report that a novel type of superconducting order parameter has been
realized in the ferromagnetic states in UGe via Ge
nuclear-quadrupole-resonance (NQR) experiments performed under pressure ().
Measurements of the nuclear spin-lattice relaxation rate have
revealed an unconventional nature of superconductivity such that the up-spin
band is gapped with line nodes, but the down-spin band remains gapless at the
Fermi level. This result is consistent with that of a ferromagnetic
spin-pairing model in which Cooper pairs are formed among ferromagnetically
polarized electrons. The present experiment has shed new light on a possible
origin of ferromagnetic superconductivity, which is mediated by ferromagnetic
spin-density fluctuations relevant to the first-order transition inside the
ferromagnetic states.Comment: 5 pages, 5 figure
Damping of the de Haas-van Alphen oscillations in the superconducting state of MgB_2
The de Haas-van Alphen (dHvA) signal arising from orbits on the Fermi
surface sheet of the two-gap superconductor MgB has been observed in the
vortex state below . An extra attenuation of the dHvA signal, beyond
those effects described in the conventional Lifshitz-Kosevich expression, is
seen due to the opening of the superconducting gap. Our data show that the
band gap is still present up to . The data are compared to
current theories of dHvA oscillations in the superconducting state which allow
us to extract estimates for the evolution of the band gap with magnetic
field. Contrary to results for other materials, we find that the most recent
theories dramatically underestimate the damping in MgB.Comment: 10 pages with figures. Submitted to Phys. Rev. B. PDF version with
higher quality figures can be found at
http://www.phy.bris.ac.uk/research/cond_matt/PdfPubs/mgb2RSdhva.pd
Anomalous Metal-Insulator Transition in Filled Skutterudite CeOsSb
Anomalous metal-insulator transition observed in filled skutterudite
CeOsSb is investigated by constructing the effective tight-binding
model with the Coulomb repulsion between f electrons. By using the mean field
approximation, magnetic susceptibilities are calculated and the phase diagram
is obtained. When the band structure has a semimetallic character with small
electron and hole pockets at and H points, a spin density wave
transition with the ordering vector occurs due to the
nesting property of the Fermi surfaces. Magnetic field enhances this phase in
accord with the experiments.Comment: 4 pages, 4 figure
Catastrophic Fermi surface reconstruction in the shape-memory alloy AuZn
AuZn undergoes a shape-memory transition at 67 K. The de Haas van Alphen
effect persists to 100 K enabling the observation of a change in the quantum
oscillation spectrum indicative of a catastrophic Fermi surface reconstruction
at the transition. Coexistence of both Fermi surfaces at low temperatures is
suggestive of an intrinsic phase separation in the bulk of the material. In
addition, a Dingle analysis reveals a sharp change in the scattering mechanism
at a threshold cyclotron radius, which we suggest to be related to the
underlying microstructure that drives the shape-memory effect.Comment: 4 pages, 4 figure
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