505 research outputs found
Characteristics of oxygen isotope substitutions in the quasiparticle spectrum of BiSrCaCuO
There is an ongoing debate about the nature of the bosonic excitations
responsible for the quasiparticle self energy in high temperature
superconductors -- are they phonons or spin fluctuations? We present a careful
analysis of the bosonic excitations as revealed by the `kink' feature at 70 meV
in angle resolved photoemission data using Eliashberg theory for a d-wave
superconductor. Starting from the assumption that nodal quasiparticles are not
coupled to the magnetic resonance, the sharp structure at meV
can be assigned to phonons. We find that not only can we account for the shifts
of the kink energy seen on oxygen isotope substitution but also get a
quantitative estimate of the fraction of the area under the electron-boson
spectral density which is due to phonons. We conclude that for optimally doped
BiSrCaCuO phonons contribute % and
non-phononic excitations %.Comment: 6 pages, 3 figure
LiBC by polarized Raman spectroscopy: Evidence for lower crystal symmetry ?
The paper presents polarized Raman scattering study on a few-micron-size
crystallite of LiBC with natural faces. The experiment on as grown sample has
revealed a four lattice modes with frequencies at 1276 cm^-1, 830 cm^-1, 546
cm^-1 and 170 cm^-1, respectively. The number of observed Raman lines and their
selection rules are incompatible with the assumed D6h symmetry. The modes at
1276 cm^-1 and 170 cm^-1 correspond to the expected Raman active modes. In
contrast with the superconducting compound MgB2, the B-C bond stretching mode
(at 1276 cm^-1) has rather small damping. The two "forbidden" modes (at 830
cm^-1 and 546 cm^-1) disappeared after subsequent thermal treatment.Comment: 4 pages, LaTeX, complementary experimental resul
Precise Tight-binding Description of the Band Structure of MgB2
We present a careful recasting of first-principles band structure
calculations for MgB2 in a non-orthogonal sp-tight-binding (TB) basis. Our TB
results almost exactly reproduce our full potential linearized augmented plane
wave results for the energy bands, the densities of states and the total
energies. Our procedure generates transferable Slater-Koster parameters which
should be useful for other studies of this important material.Comment: REVTEX, 2 Encapsulated PostScript Figure
Electronic and optical properties of LiBC
LiBC, a semiconducting ternary borocarbide constituted of the lightest
elements only, has been synthesized and characterized by x-ray powder
diffraction, dielectric spectroscopy, and conductivity measurements. Utilizing
an infrared microscope the phonon spectrum has been investigated in single
crystals. The in-plane B-C stretching mode has been detected at 150 meV,
noticeably higher than in AlB2, a non-superconducting isostructural analog of
MgB2. It is this stretching mode, which reveals a strong electron-phonon
coupling in MgB2, driving it into a superconducting state below 40 K, and is
believed to mediate predicted high-temperature superconductivity in hole-doped
LiBC [H. Rosner, A. Kitaigorodsky, and W. E. Pickett, Phys. Rev. Lett. 88,
127001 (2002)].Comment: 4 pages, 4 figure
Heat Capacity and Magnetic Phase Diagram of the Low-Dimensional Antiferromagnet YBaCuO
A study by specific heat of a polycrystalline sample of the low-dimensional
magnetic system YBaCuO is presented. Magnetic fields up to 14 T are
applied and permit to extract the (,) phase diagram. Below
T, the N\'eel temperature, associated with a
three-dimensional antiferromagnetic long-range ordering, is constant and equals
K. Above , increases linearly with and a
field-induced increase of the entropy at is related to the presence of an
isosbestic point at K, where all the specific heat curves cross.
A comparison is made between YBaCuO and the quasi-two-dimensional
magnetic systems BaNiVO, SrCuOCl, and
PrCuO, for which very similar phase diagrams have been reported. An
effective field-induced magnetic anisotropy is proposed to explain these phase
diagrams.Comment: 14 pages, 7 figure
Size Dependence In The Disordered Kondo Problem
We study here the role randomly-placed non-magnetic scatterers play on the
Kondo effect. We show that spin relaxation effects (with time )in the
vertex corrections to the Kondo self-energy lead to an exact cancellation of
the singular temperature dependence arising from the diffusion poles. For a
thin film of thickness and a mean-free path , disorder provides a
correction to the Kondo resistivity of the form
that explains both the disorder and sample-size depression of the Kondo effect
observed by Blachly and Giordano (PRB {\bf 51}, 12537 (1995)).Comment: 11 pages, LaTeX, 2 Postscript figure
Cost-utility of exercise therapy added to general practitioners’ care versus general practitioners’ care alone in patients with hip osteoarthritis
Giant infrared intensity of the Peierls mode at the neutral-ionic phase transition
We present exact diagonalization results on a modified Peierls-Hubbard model
for the neutral-ionic phase transition. The ground state potential energy
surface and the infrared intensity of the Peierls mode point to a strong,
non-linear electron-phonon coupling, with effects that are dominated by the
proximity to the electronic instability rather than by electronic correlations.
The huge infrared intensity of the Peierls mode at the ferroelectric
transition is related to the temperature dependence of the dielectric constant
of mixed-stack organic crystals.Comment: 4 pages, 4 figure
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