269 research outputs found
The dimpling in the CuO_2 planes of YBa_2Cu_3O_x (x=6.806-6.984, T=20-300 K) measured by yttrium EXAFS
The dimpling of the CuO_2 planes (spacing between the O2,3 and Cu2 layers) in
YBa_2Cu_3O_x has been measured as a function of oxygen concentration and
temperature by yttrium x-ray extended-fine-structure spectroscopy (EXAFS). The
relative variations of the dimpling with doping (x=6.806-6.984) and temperature
(20-300 K) are weak (within 0.05 AA), and arise mainly from displacements of
the Cu2 atoms off the O2,3 plane towards Ba. The dimpling appears to be
connected with the transition from the underdoped to the overdoped regimes at
x=6.95, and with a characteristic temperature in the normal state, T*=150 K.Comment: 6 pages, 2 ps figs, LaTEX, Elsevier Elsart styl
Strain induced half-metal to semiconductor transition in GdN
We have investigated the electronic structure and magnetic properties of GdN
as a function of unit cell volume. Based on the first-principles calculations
of GdN, we observe that there is a transformation in conduction properties
associated with the volume increase: first from halfmetallic to semi-metallic,
then ultimately to semiconducting. We show that applying stress can alter the
carrier concentration as well as mobility of the holes and electrons in the
majority spin channel. In addition, we found that the exchange parameters
depend strongly on lattice constant, thus the Curie temperature of this system
can be enhanced by applying stress or doping impurities.Comment: 9 pages, 3 figure
Examining venous thromboembolic disease in postoperative neurosurgical and trauma patients in the ICU
Correlation between Zn vacancies and photoluminescence emission in ZnO films.
Photoluminescence and positron annihilation spectroscopy have been used to characterize and identify vacancy-type defects produced in ZnO films grown on sapphire by metal-organic chemical-vapor deposition. The photoluminescence of the samples in the near band edge region has been studied, paying particular attention to the emission at 370.5 nm (3.346 eV). This emission has been correlated to the concentration of Zn vacancies in the films, which has been determined by positron annihilation [email protected] [email protected]
Phonon dispersion and electron-phonon interaction for YBa_2Cu_3O_7 from first-principles calculations
We present a first principles investigation of the lattice dynamics and
electron-phonon coupling of the high-T_c superconductor YBa_2Cu_3O_7 within the
framework of density functional perturbation theory using a mixed-basis
pseudopotential method. The calculated phonon dispersion curves are in
excellent agreement with Raman, infrared and neutron data. Calculation of the
Eliashberg function alpha^2F leads to a small electron-phonon coupling
lambda=0.27 in disagreement with earlier approximate treatments. Our
calculations strongly support the view that conventional electron-phonon
coupling is not an important contribution to superconductivity in high-T_c
materials.Comment: 4 pages, 4 figure
Physical origin of the buckling in CuO: Electron-phonon coupling and Raman spectra
It is shown theoretically that the buckling of the CuO planes in
certain cuprate systems can be explained in terms of an electric field across
the planes which originates from different valences of atoms above and below
the plane. This field results also in a strong coupling of the Raman-active
out-of-phase vibration of the oxygen atoms ( mode) to the electronic
charge transfer between the two oxygens in the CuO plane. Consequently,
the electric field can be deduced from the Fano-type line shape of the
phonon. Using the electric field estimated from the electron-phonon coupling
the amplitude of the buckling is calculated and found to be in good agreement
with the structural data. Direct experimental support for the idea proposed is
obtained in studies of YBaCuO and
BiSr(CaY)CuO with different oxygen and
yttrium doping, respectively, including antiferromagnetic samples. In the
latter compound, symmetry breaking by replacing Ca partially by Y leads to an
enhancement of the electron-phonon coupling by an order of magnitude.Comment: 12 pages, 4 figures, and 1 tabl
Cu Nuclear Quadrupole Resonance Study of Site-Disorder and Chemical Pressure Effects on Y(Ba1-xSrx)2Cu4O8
We report a zero-field Cu nuclear quadrupole resonance (NQR) study on the
effects of nonmagnetic Sr substitution for high-Tc superconductors,
Y(Ba1-xSrx)2Cu4O8 (Tc=82-80 K for x=0-0.4), using a spin-echo technique. The
site-disordering and chemical pressure effects associated with doping Sr were
observed in the broadened, shifted Cu NQR spectra. Nevertheless, the site
disorder did not significantly affect the homogeneity of Cu electron spin
dynamics, in contrast to the in-plane impurity. The peak shift of Cu NQR
spectrum due to Sr was different between the chain- and the plane-Cu sites,
more remarkably than those under a hydrostatic physical pressure, suggesting
anisotropic or nonuniform local structural strains. The small decrease of Tc
due to Sr can be traced back to either a cancellation effect on Tc between the
disorder and the pressure, or an anisotropic or nonuniform chemical pressure
effect on Tc.Comment: 4 pages, 5 figure
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