354 research outputs found
A Vertex Correction in the Gap Equation for the High Temperature Superconductors
We show that the Migdal theorem is obviously violated in the high Tc cuprates
and the vertex correction should be included, in particular, in the gap
equation, in order to be consistent with the anomalously strong inelastic
scattering in the ``hot spots'', which is observed from the various normal
state experiments. The vertex correction is obtained by utilizing the
generalized Ward identity, which is shown to hold in the important scattering
channel for the pairing interaction in the high Tc cuprates. As a result, we
find a strong enhancement of Tc from the vertex correction despite of the
strong pair breaking effect due to the inelastic scattering.Comment: 5 pages, 2 figure
Strong enhancement of spin fluctuations in the low-temperature-tetragonal phase of antiferromagnetically ordered La_{2-x-y}Eu_ySr_xCuO_4
Measurements of the static magnetization, susceptibility and ESR of Gd spin
probes have been performed to study the properties of antiferromagnetically
ordered La_{2-x-y}Eu_ySr_xCuO_4 (x less or equal 0.02) with the low temperature
tetragonal structure. According to the static magnetic measurements the CuO_2
planes are magnetically decoupled in this structural phase. The ESR study
reveals strong magnetic fluctuations at the ESR frequency which are not present
in the orthorhombic phase. It is argued that this drastic enhancement of the
spin fluctuations is due to a considerable weakening of the interlayer exchange
and a pronounced influence of hole motion on the antiferromagnetic properties
of lightly hole doped La_2CuO_4. No evidence for the stripe phase formation at
small hole doping is obtained in the present study.Comment: 10 pages, LaTeX, 3 EPS figures; to be published in Journal of
Physics: Condensed Matte
A Theory of the Longitudinal and Hall Conductivities of the Cuprate Superconductors
We establish the applicability to transport phenomena in the cuprate
superconductors of a nearly antiferromagnetic Fermi liquid (NAFL) description
of the magnetic interaction between planar quasiparticles by using it to obtain
the temperature dependent resistivity and Hall conductivity seen experimentally
in the normal state. Following a perturbative calculation of the anisotropic
(as one goes around the Fermi surface) quasiparticle lifetimes which are the
hallmark of a NAFL, we obtain simple approximate expressions for the
longitudinal, , and Hall, , conductivities which
reflect the magnetic crossovers seen experimentally as one varies the doping
level and temperature. We present a simple phenomenological model for the
variation in mean free path around the Fermi surface, and use this to extract
from experiments on and quasiparticle lifetimes in
the hot (strongly coupled quasiparticle) and cold (weakly coupled
quasiparticle) regions of the Fermi surface which are consistent with the
perturbation theory estimates. We improve upon the latter by carrying out
direct numerical (non-variational) solutions of the Boltzmann equation for
representative members of the YBaCuO and
LaSrCuO systems, with results for transport properties in
quantitative agreement with experiment. Using the same numerical approach we
study the influence of CuO chains on the a-b plane anisotropy and find results
in agreement with experimental findings in YBaCuO.Comment: 49 pages + 24 PostScript figure
Non-Universal Power Law of the "Hall Scattering Rate" in a Single-Layer Cuprate Bi_{2}Sr_{2-x}La_{x}CuO_{6}
In-plane resistivity \rho_{ab}, Hall coefficient, and magnetoresistance (MR)
are measured in a series of high-quality Bi_{2}Sr_{2-x}La_{x}CuO_{6} crystals
with various carrier concentrations, from underdope to overdope. Our crystals
show the highest T_c (33 K) and the smallest residual resistivity ever reported
for Bi-2201 at optimum doping. It is found that the temperature dependence of
the Hall angle obeys a power law T^n with n systematically decreasing with
increasing doping, which questions the universality of the Fermi-liquid-like
T^2 dependence of the "Hall scattering rate". In particular, the Hall angle of
the optimally-doped sample changes as T^{1.7}, not as T^2, while \rho_{ab}
shows a good T-linear behavior. The systematics of the MR indicates an
increasing role of spin scattering in underdoped samples.Comment: 4 pages, 5 figure
Transport Anomalies and the Role of Pseudogap in the "60-K Phase" of YBa_{2}Cu_{3}O_{7-\delta}
We report the result of our accurate measurements of the a- and b-axis
resistivity, Hall coefficient, and the a-axis thermopower in untwinned
YBa_{2}Cu_{3}O_{y} single crystals in a wide range of doping. It is found that
both the a-axis resistivity and the Hall conductivity show anomalous
dependences on the oxygen content y in the "60-K phase" below the pseudogap
temperature T^*. The complete data set enables us to narrow down the possible
pictures of the 60-K phase, with which we discuss a peculiar role of the
pseudogap in the charge transport.Comment: 4 pages, 4 figures, accepted for publication in PR
Statistics of charged solitons and formation of stripes
The 2-fold degeneracy of the ground state of a quasi-one-dimensional system
allows it to support topological excitations such as solitons. We study the
combined effects of Coulomb interactions and confinement due to interchain
coupling on the statistics of such defects. We concentrate on a 2D case which
may correspond to monolayers of polyacetylene or other charge density waves.
The theory is developped by a mapping to the 2D Ising model with long-range
4-spin interactions. The phase diagram exhibits deconfined phases for liquids
and Wigner crystals of kinks and confined ones for bikinks. Also we find
aggregated phases with either infinite domain walls of kinks or finite rods of
bikinks. Roughening effects due to both temperature and Coulomb repulsion are
observed. Applications may concern the melting of stripes in doped correlated
materials.Comment: 16 pages, 7 figure
Magnetic Coherence as a Universal Feature of Cuprate Superconductors
Recent inelastic neutron scattering (INS) experiments on
LaSrCuO have established the existence of a {\it magnetic
coherence effect}, i.e., strong frequency and momentum dependent changes of the
spin susceptibility, , in the superconducting phase. We show, using the
spin-fermion model for incommensurate antiferromagnetic spin fluctuations, that
the magnetic coherence effect establishes the ability of INS experiments to
probe the electronic spectrum of the cuprates, in that the effect arises from
the interplay of an incommensurate magnetic response, the form of the
underlying Fermi surface, and the opening of the d-wave gap in the fermionic
spectrum. In particular, we find that the magnetic coherence effect observed in
INS experiments on LaSrCuO requires that the Fermi surface be
closed around up to optimal doping. We present several predictions
for the form of the magnetic coherence effect in YBaCuO in
which an incommensurate magnetic response has been observed in the
superconducting state.Comment: 9 pages, 12 figures; extended version of Phys. Rev B, R6483 (2000
Effective Lorentz Force due to Small-angle Impurity Scattering: Magnetotransport in High-Tc Superconductors
We show that a scattering rate which varies with angle around the Fermi
surface has the same effect as a periodic Lorentz force on magnetotransport
coefficients. This effect, together with the marginal Fermi liquid inelastic
scattering rate gives a quantitative explanation of the temperature dependence
and the magnitude of the observed Hall effect and magnetoresistance with just
the measured zero-field resistivity as input.Comment: 4 pages, latex, one epsf figure included in text. Several revisions
and corrections are included. Major conclusions are the sam
NMR and Neutron Scattering Experiments on the Cuprate Superconductors: A Critical Re-Examination
We show that it is possible to reconcile NMR and neutron scattering
experiments on both LSCO and YBCO, by making use of the Millis-Monien-Pines
mean field phenomenological expression for the dynamic spin-spin response
function, and reexamining the standard Shastry-Mila-Rice hyperfine Hamiltonian
for NMR experiments. The recent neutron scattering results of Aeppli et al on
LSCO (x=14%) are shown to agree quantitatively with the NMR measurements of
and the magnetic scaling behavior proposed by Barzykin and Pines.
The reconciliation of the relaxation rates with the degree of
incommensuration in the spin fluctuation spectrum seen in neutron experiments
is achieved by introducing a new transferred hyperfine coupling between
oxygen nuclei and their next nearest neighbor spins; this leads to a
near-perfect cancellation of the influence of the incommensurate spin
fluctuation peaks on the oxygen relaxation rates of LSCO. The inclusion of the
new term also leads to a natural explanation, within the one-component
model, the different temperature dependence of the anisotropic oxygen
relaxation rates for different field orientations, recently observed by
Martindale . The measured significant decrease with doping of the
anisotropy ratio, in LSCO system, from
for to for LSCO (x=15%) is made compatible with the
doping dependence of the shift in the incommensurate spin fluctuation peaks
measured in neutron experiments, by suitable choices of the direct and
transferred hyperfine coupling constants and B.Comment: 24 pages in RevTex, 9 figures include
Singularities in the optical response of cuprates
We argue that the detailed analysis of the optical response in cuprate
superconductors allows one to verify the magnetic scenario of superconductivity
in cuprates, as for strong coupling charge carriers to antiferromagnetic spin
fluctuations, the second derivative of optical conductivity should contain
detectable singularities at , , and
, where is the amplitude of the
superconducting gap, and is the resonance energy of spin
fluctuations measured in neutron scattering. We argue that there is a good
chance that these singularities have already been detected in the experiments
on optimally doped .Comment: 6 pages, 4 figure
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