60 research outputs found
Effect of controlled disorder on quasiparticle thermal transport in BiSrCaCuO
Low temperature thermal conductivity, , of optimally-doped Bi2212 was
studied before and after the introduction of point defects by electron
irradiation. The amplitude of the linear component of remains
unchanged, confirming the universal nature of heat transport by zero-energy
quasiparticles. The induced decrease in the absolute value of at
finite temperatures allows us to resolve a nonuniversal term in due to
conduction by finite-energy quasiparticles. The magnitude of this term provides
an estimate of the quasiparticle lifetime at subkelvin temperatures.Comment: 5 pages including 2 .eps figuer
C-axis electronic Raman scattering in Bi_2Sr_2CaCu_2O_{8+\delta}
We report a c-axis-polarized electronic Raman scattering study of
Bi_2Sr_2CaCu_2O_{8+\delta} single crystals. In the normal state, a resonant
electronic continuum extends to 1.5 eV and gains significant intensity as the
incoming photon energy increases. In the superconducting state, a coherence
2\Delta peak appears around 50 meV, with a suppression of the scattering
intensity at frequencies below the peak position. The peak energy, which is
higher than that seen with in-plane polarizations, signifies distinctly
different dynamics of quasiparticle excitations created with out-of-plane
polarization.Comment: 12 pages, REVTEX, 3 postscript figure
C-axis Raman spectra of a normal plane-chain bilayer cuprate and the pseudogap
We investigate the Raman spectra in the geometry where both incident and
scattered photon polarizations are parallel to the -direction, for a
plane-chain bilayer coupled via a single-particle tunneling . The
Raman vertex is derived in the tight-binding limit and in the absence of
Coulomb screening, the Raman intensity can be separated into intraband
() and interband () transitions. In the
small- limit, the interband part dominates and a pseudogap will appear
as it does in the conductivity. Coulomb interactions bring in a two-particle
coupling and result in the breakdown of intra- and interband separation.
Nevertheless, when is small, the Coulomb screening () has little effect on the intensity to which the unscreened
interband transitions contribute most. In general, the total Raman spectra are
strongly dependent on the magnitude of .Comment: 23 pages, 6 figures, submitted to Phys. Rev.
Anomalous Self-Energy Effects of the B_1g Phonon in Y_{1-x}(Pr,Ca)_xBa_2Cu_3O_7 Films
In Raman spectra of cuprate superconductors the gap shows up both directly,
via a redistribution of the electronic background, the so-called "2Delta
peaks", and indirectly, e.g. via the renormalization of phononic excitations.
We use a model that allows us to study the redistribution and the related
phonon self-energy effects simultaneously. We apply this model to the B_1g
phonon of Y_{1-x}(Pr,Ca)_xBa_2Cu_3O_7 films, where Pr or Ca substitution
enables us to investigate under- and overdoped samples. While various
self-energy effects can be explained by the strength and energy of the 2\Delta
peaks, anomalies remain. We discuss possible origins of these anomalies.Comment: 6 pages including 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
C-axis lattice dynamics in Bi-based cuprate superconductors
We present results of a systematic study of the c axis lattice dynamics in
single layer Bi2Sr2CuO6 (Bi2201), bilayer Bi2Sr2CaCu2O8 (Bi2212) and trilayer
Bi2Sr2Ca2Cu3O10 (Bi2223) cuprate superconductors. Our study is based on both
experimental data obtained by spectral ellipsometry on single crystals and
theoretical calculations. The calculations are carried out within the framework
of a classical shell model, which includes long-range Coulomb interactions and
short-range interactions of the Buckingham form in a system of polarizable
ions. Using the same set of the shell model parameters for Bi2201, Bi2212 and
Bi2223, we calculate the frequencies of the Brillouin-zone center phonon modes
of A2u symmetry and suggest the phonon mode eigenvector patterns. We achieve
good agreement between the calculated A2u eigenfrequencies and the experimental
values of the c axis TO phonon frequencies which allows us to make a reliable
phonon mode assignment for all three Bi-based cuprate superconductors. We also
present the results of our shell model calculations for the Gamma-point A1g
symmetry modes in Bi2201, Bi2212 and Bi2223 and suggest an assignment that is
based on the published experimental Raman spectra. The
superconductivity-induced phonon anomalies recently observed in the c axis
infrared and resonant Raman scattering spectra in trilayer Bi2223 are
consistently explained with the suggested assignment.Comment: 29 pages, 13 figure
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