278 research outputs found
Why is d-wave pairing in HTS robust in the presence of impurities?
In the recent theory of strong correlations by Kulic and Zeyher it has been
shown that by lowering doping concentration a forward peak in the charge
scattering channel is developed. Accordingly, near the optimal doping the
nonmagnetic scattering is pronounced in the d-channel and its effect on d-wave
pairing is reduced. As a consequence, d-wave pairing is robust against defects
and impurities, the order parameter keeps its d-wave shape for any scattering
rate and the density of states becomes finite at the Fermi surface. For large
doping scattering anisotropy parameter is small and d-wave loses its
robustness. The theory is generally formulated for the bi-layer model by
including: 1) intra- and inter-plane pairing; 2) intra- and inter-plane
impurities.Comment: Complete revision, 4 pages with 2 PS figures, RevTeX, submitted to
Phys. Rev. Let
Resistivity saturation revisited: results from a dynamical mean field theory
We use the dynamical mean field method to study the high-temperature
resistivity of electrons strongly coupled to phonons. The results reproduce the
qualtiative behavior of the temperature and disorder dependence of the
resistivity of the 'A-15' materials, which is commonly described in terms of
saturation, but imply that the resistivity does not saturate. Rather, a change
in temperature dependence occurs when the scattering becomes strong enough to
cause a breakdown of the Migdal approximation.Comment: Minor revisions in response to referee report; latex error corrected
so paper prints properl
New insight into the physics of iron pnictides from optical and penetration depth data
We report theoretical values for the unscreened plasma frequencies Omega_p of
several Fe pnictides obtained from DFT based calculations within the LDA and
compare them with experimental plasma frequencies obtained from reflectivity
data. The sizable renormalization observed for all considered compounds points
to the presence of many-body effects beyond the LDA. From the large empirical
background dielectric constant of about 12-15, we estimate a large arsenic
polarizability of about 9.5 +- 1.2 Angstroem^3 where the details depend on the
polarizabilities of the remaining ions taken from the literature. This large
polarizability can significantly reduce the value of the Coulomb repulsion U_d
about 4 eV on iron known from iron oxides to a level of 2 eV or below. In
general, this result points to rather strong polaronic effects as suggested by
G.A. Sawatzky et al., in Refs. arXiv:0808.1390 and arXiv:0811.0214 (Berciu et
al.). Possible consequences for the conditions of a formation of bipolarons are
discussed, too. From the extrapolated muon spin rotation penetration depth data
at T= 0 and the experimental Omega_p we estimate the total coupling constant
lambda_tot for the el-boson interaction within the Eliashberg-theory adopting a
single band approximation. For LaFeAsO_0.9F_0.1 a weak to intermediately strong
coupling regime and a quasi-clean limit behaviour are found. For a pronounced
multiband case we obtain a constraint for various intraband coupling constants
which in principle allows for a sizable strong coupling in bands with either
slow electrons or holes.Comment: 34 pages, 10 figures, submitted to New Journal of Physics
(30.01.2009
Image of the Energy Gap Anisotropy in the Vibrational Spectum of a High Temperature Superconductor
We present a new method of determining the anisotropy of the gap function in
layered high-Tc superconductors. Careful inelastic neutron scattering
measurements at low temperature of the phonon dispersion curves in the (100)
direction in La_(1.85)Sr_(.15)CuO_4 would determine whether the gap is
predominately s-wave or d-wave. We also propose an experiment to determine the
gap at each point on a quasi-two-dimensional Fermi surface.Comment: 12 pages + 2 figures (included
Correlation between the residual resistance ratio and magnetoresistance in MgB2
The resistivity and magnetoresistance in the normal state for bulk and
thin-film MgB2 with different nominal compositions have been studied
systematically. These samples show different temperature dependences of normal
state resistivity and residual resistance ratios although their superconducting
transition temperatures are nearly the same, except for the thin-film sample.
The correlation between the residual resistance ratio (RRR) and the power law
dependence of the low temperature resistivity, rho vs. T^c, indicates that the
electron-phonon interaction is important. It is found that the
magnetoresistance (MR) in the normal state scales well with the RRR, a0(MR)
proportional to (RRR)^2.2 +/- 0.1 at 50 K. This accounts for the large
difference in magnetoresistance reported by various groups, due to different
defect scatterings in the samples.Comment: 10 pages, 3 figures, submitted to Phys. Rev. B (July 6, 2001; revised
September 27, 2001); discussion of the need for excess Mg in processing and
of the power law dependence of the low temperature resistivity added in
response to referee's comment
Many-body Effects in Angle-resolved Photoemission: Quasiparticle Energy and Lifetime of a Mo(110) Surface State
In a high-resolution photoemission study of a Mo(110) surface state various
contributions to the measured width and energy of the quasiparticle peak are
investigated. Electron-phonon coupling, electron-electron interactions and
scattering from defects are all identified mechanisms responsible for the
finite lifetime of a valence photo-hole. The electron-phonon induced mass
enhancement and rapid change of the photo-hole lifetime near the Fermi level
are observed for the first time.Comment: RevTEX, 4 pages, 4 figures, to be published in PR
Change of the Ground State upon Hole Doping Unveiled by Ni Impurity in High- Cuprates
The electronic ground state in high- cuprates where the
superconducting state is suppressed by Ni substitution has been investigated in
LaSrCuNiO from the specific heat and muon spin
relaxation measurements. It has been found that the ground state changes from a
magnetically ordered state with the strong hole-trapping by Ni to a metallic
state with the Kondo effect of Ni with increasing hole-concentration. Moreover,
the analysis of the results has revealed that a phase separation into the
magnetically ordered phase and the metallic phase occurs around the boundary of
two phases.Comment: 11pages, 4 figure
Spin-gap effect on resistivity in the t-J model
We calculate the spin-gap effect on dc resistivity in the t-J model of
high- cuprates by using the Ginzburg-Landau theory coupled with a
gauge field as its effective field theory to get , where is the spin-gap onset temperature. By taking the
compactness of massive gauge field into account, the exponent deviates from
its mean-field value 1/2 and becomes a nonuniversal -dependent quantity,
which improves the correspondence with the experiments.Comment: 4 pages, REVTeX format, 2 eps-figure
Marginal Fermi liquid analysis of 300 K reflectance of Bi2Sr2CaCu2O8+x
We use 300 K reflectance data to investigate the normal-state electrodynamics
of the high temperature superconductor BiSrCaCuO
over a wide range of doping levels. The data show that at this temperature the
free carriers are coupled to a continuous spectrum of fluctuations. Assuming
the Marginal Fermi Liquid (MFL) form as a first approximation for the
fluctuation spectrum, the doping-dependent coupling constant can
be estimated directly from the slope of the reflectance spectrum. We find that
decreases smoothly with the hole doping level, from underdoped
samples with ( K) where to overdoped
samples with , ( K) where . An analysis of
the intercept and curvature of the reflectance spectrum shows deviations from
the MFL spectrum symmetrically placed at the optimal doping point . The
Kubo formula for the conductivity gives a better fit to the experiments with
the MFL spectrum up to 2000 cm and with an additional Drude component or
an additional Lorentz component up to 7000 cm. By comparing three
different model fits we conclude that the MFL channel is necessary for a good
fit to the reflectance data. Finally, we note that the monotonic variation of
the reflectance slope with doping provides us with an independent measure of
the doping level for the Bi-2212 system.Comment: 11 pages, 11 figure
Saturation of electrical resistivity
Resistivity saturation is observed in many metallic systems with a large
resistivity, i.e., when the resistivity has reached a critical value, its
further increase with temperature is substantially reduced. This typically
happens when the apparent mean free path is comparable to the interatomic
separations - the Ioffe-Regel condition. Recently, several exceptions to this
rule have been found. Here, we review experimental results and early theories
of resistivity saturation. We then describe more recent theoretical work,
addressing cases both where the Ioffe-Regel condition is satisfied and where it
is violated. In particular we show how the (semiclassical) Ioffe-Regel
condition can be derived quantum-mechanically under certain assumptions about
the system and why these assumptions are violated for high-Tc cuprates and
alkali-doped fullerides.Comment: 16 pages, RevTeX, 15 eps figures, additional material available at
http://www.mpi-stuttgart.mpg.de/andersen/saturation
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