143 research outputs found
Symmetry dependence of phonon lineshapes in superconductors with anisotropic gaps
The temperature dependence below of the lineshape of optical phonons
of different symmetry as seen in Raman scattering is investigated for
superconductors with anisotropic energy gaps. It is shown that the symmetry of
the electron-phonon vertex produces non-trivial couplings to an anisotropic
energy gap which leads to unique changes in the phonon lineshape for phonons of
different symmetry. The phonon lineshape is calculated in detail for
and phonons in a superconductor with pairing
symmetry. The role of satellite peaks generated by the electron-phonon coupling
are also addressed. The theory accounts for the substantial phonon narrowing of
the phonon, while narrowing of the phonon which is
indistinguishable from the normal state is shown, in agreement with recent
measurements on BSCCO.Comment: 15 pages (3 Figures available upon request), Revtex, 1
Hole concentration and phonon renormalization in Ca-doped YBa_2Cu_3O_y (6.76 < y < 7.00)
In order to access the overdoped regime of the YBa_2Cu_3O_y phase diagram, 2%
Ca is substituted for Y in YBa_2Cu_3O_y (y = 7.00,6.93,6.88,6.76). Raman
scattering studies have been carried out on these four single crystals.
Measurements of the superconductivity-induced renormalization in frequency
(Delta \omega) and linewidth (\Delta 2\gamma) of the 340 cm^{-1} B_{1g} phonon
demonstrate that the magnitude of the renormalization is directly related to
the hole concentration (p), and not simply the oxygen content. The changes in
\Delta \omega with p imply that the superconducting gap (\Delta_{max})
decreases monotonically with increasing hole concentration in the overdoped
regime, and \Delta \omega falls to zero in the underdoped regime. The linewidth
renormalization \Delta 2\gamma is negative in the underdoped regime, crossing
over at optimal doping to a positive value in the overdoped state.Comment: 18 pages; 5 figures; submitted to Phys. Rev. B Oct. 24, 2002 (BX8292
Nernst Effect and Superconducting Fluctuations in Zn-doped YBaCuO
We report the measurements of in-plane resistivity, Hall effect, and Nernst
effect in Zn doped YBaCuO epitaxial thin films grown
by pulsed laser deposition technique. The pseudogap temperature, ,
determined from the temperature dependence of resistivity, does not change
significantly with Zn doping. Meanwhile the onset temperature () of
anomalous Nernst signal above , which is interpreted as evidence for
vortex-like excitations, decreases sharply as the superconducting transition
temperature does. A significant decrease in the maximum of vortex
Nernst signal in mixed state is also observed, which is consistent with the
scenario that Zn impurities cause a decrease in the superfluid density and
therefore suppress the superconductivity. The phase diagram of ,
, and versus Zn content is presented and discussed.Comment: 6 pages, 5 figures, Latex; v2: to be published in PR
Epitaxial and layer-by-layer growth of EuO thin films on yttria-stabilized cubic zirconia (001) using MBE distillation
We have succeeded in growing epitaxial and highly stoichiometric films of EuO
on yttria-stabilized cubic zirconia (YSZ) (001). The use of the Eu-distillation
process during the molecular beam epitaxy assisted growth enables the
consistent achievement of stoichiometry. We have also succeeded in growing the
films in a layer-by-layer fashion by fine tuning the Eu vs. oxygen deposition
rates. The initial stages of growth involve the limited supply of oxygen from
the YSZ substrate, but the EuO stoichiometry can still be well maintained. The
films grown were sufficiently smooth so that the capping with a thin layer of
aluminum was leak tight and enabled ex situ experiments free from trivalent Eu
species. The findings were used to obtain recipes for better epitaxial growth
of EuO on MgO (001).Comment: 10 pages, 15 figure
Epitaxy, stoichiometry, and magnetic properties of Gd-doped EuO films on YSZ (001)
We have succeeded in preparing high-quality Gd-doped single-crystalline EuO
films. Using Eu-distillation-assisted molecular beam epitaxy and a systematic
variation in the Gd and oxygen deposition rates, we have been able to observe
sustained layer-by-layer epitaxial growth on yttria-stabilized cubic zirconia
(001). The presence of Gd helps to stabilize the layer-by-layer growth mode. We
used soft x-ray absorption spectroscopy at the Eu and Gd M4,5 edges to confirm
the absence of Eu3+ contaminants and to determine the actual Gd concentration.
The distillation process ensures the absence of oxygen vacancies in the films.
From magnetization measurements we found the Curie temperature to increase
smoothly as a function of doping from 70 K up to a maximum of 125 K. A
threshold behavior was not observed for concentrations as low as 0.2%.Comment: 8 pages, 9 figure
Superconductivity-Induced Anomalies in the Spin Excitation Spectra of Underdoped YBa_2 Cu_3 O_{6+x}
Polarized and unpolarized neutron scattering has been used to determine the
effect of superconductivity on the magnetic excitation spectra of YBa_2 Cu_3
O_{6.5} (T_c = 52K) and YBa_2 Cu_3 O_{6.7} (T_c = 67K). Pronounced enhancements
of the spectral weight centered around 25 meV and 33 meV, respectively, are
observed below T_c in both crystals, compensated predominantly by a loss of
spectral weight at higher energies. The data provide important clues to the
origin of the 40 meV magnetic resonance peak in YBa_2 Cu_3 O_7.Comment: LaTex, 4 pages, 4 ps figures. to appear in Phys. Rev. Let
Strain dependent transport properties of the quasi two-dimensional correlated metal, LaNiO
We explore the electrical transport and magneto-conductance in quasi
two-dimensional strongly correlated ultrathin films of LaNiO (LNO) to
investigate the effect of hetero-epitaxial strain on electron-electron and
electron-lattice interactions from the low to intermediate temperature range
(2K170K). The fully epitaxial 10 unit cell thick films spanning tensile
strain up to are used to investigate effects of enhanced carrier
localization driven by a combination of weak localization and electron-electron
interactions at low temperatures. The magneto-conductance data shows the
importance of the increased contribution of weak localization to low
temperature quantum corrections. The obtained results demonstrate that with
increasing tensile strain and reduced temperature the quantum confined LNO
system gradually evolves from the Mott into the Mott-Anderson regime
Neutron Scattering and the B_{1g} Phonon in the Cuprates
The momentum dependent lineshape of the out-of-phase oxygen vibration as
measured in recent neutron scattering measurements is investigated. Starting
from a microscopic coupling of the phonon vibration to a local crystal field,
the phonon lineshift and broadening is calculated as a function of transfered
momentum in the superconducting state of YBaCuO. It is shown
that the anisotropy of the density of states, superconducting energy gap, and
the electron-phonon coupling are all crucial in order to explain these
experiments.Comment: new figures and discussio
Influence of the pseudogap on the superconductivity-induced phonon renormalization in high-T superconductors
We investigate the influence of a d-density wave (DDW) gap on the
superconductivity-induced renormalization of phonon frequency and linewidth.
The results are discussed with respect to Raman and inelastic neutron
scattering experiments. It turns out that the DDW gap can enhance the range of
frequencies for phonon softening depending on the underlying band
structure. Moreover we show that an anisotropic 'd-wave' pseudogap can also
contribute to the q-dependent linewidth broadening of the 340cm phonon
in YBaCuO.Comment: 4 page
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