308 research outputs found
Competition between superconductivity and the pseudogap phase in the t-J model
The t-J model in the large N limit (N denotes the number of spin components)
yields a pseudogap phase in the underdoped region which is related to a d-wave
charge density wave (d-CDW). We present results for the doping dependence of
the superconducting and d-CDW order parameters as well as for collective
excitations in the presence of these two order parameters. We argue that the
electronic Raman spectrum with B symmetry probes the amplitude
fluctuations of the d-CDW at zero momentum.Comment: 4 pages, 4 figures, will appear in Proc. of "Physics of Magnetism'02
Theory of the hourglass dispersion of magnetic excitations in high-T cuprates
A theory for the dispersion of collective magnetic excitations in
superconducting cuprates is presented with the aim to cover both high and low
doping regimes. Besides of spin fluctuations describable in the random phase
approximation (RPA) we allow for local spin rotations within a mode-coupling
theory. At low temperatures and moderately large correlation lengths we obtain
two branches of excitations which disperse up- and downwards exhibiting the
hourglass behavior observed experimentally at intermediate dopings. At large
and small dopings our theory essentially reduces to the RPA and spin wave
theory, respectively.Comment: 4 pages, 5 Figure
Raman scattering from a superconductivity-induced bound state in
It is shown that the sharp peak in the Raman spectrum of
superconducting is due to a bound state caused by the electron-phonon
coupling. Our theory explains why this peak appears only in the spectra with
symmetry and only in the but not bands. The properties
of the bound state and the Raman spectrum are investigated, also in the
presence of impurity scattering.Comment: 4 pages, 4 figures, will appear in PR
Electronic correlations, electron-phonon interaction, and isotope effect in high-Tc cuprates
Using a large-N expansion we present and solve the linearized equation for
the superconducting gap for a generalized t-J model which also contains phonons
within a Holstein model. The leading Tc has d-wave symmetry with phonons giving
a positive contribution to Tc. The corresponding isotope coefficient is very
small at optimal doping and increases towards the classical value 1/2 with
increasing dopings similar as in many cuprates.Comment: 14 pages, 7 figure
C-Axis Tunneling Spectra in High-T Superconductors in the Presence of a d Charge-Density Wave
The optimally doped and underdoped region of the model at large N (N is
the number of spin components) is governed by the competition of d-wave
superconductivity (SC) and a d Charge-Density Wave (d-CDW).The partial
destruction of the Fermi surface by the d-CDW and the resulting density of
states are discussed. Furthermore, c-axis conductances for incoherent and
coherent tunneling are calculated, considering both an isotropic and an
anisotropic in-plane momentum dependence of the hopping matrix element between
the planes. The influence of self-energy effects on the conductances is also
considered using a model where the electrons interact with a dispersionless,
low-lying branch of bosons. We show that available tunneling spectra from
break-junctions are best explained by assuming that they result from incoherent
tunneling with a strongly anisotropic hopping matrix element of the form
suggested by band structure calculations. The conductance spectra are then
characterized by one single peak which evolves continuously from the
superconducting to the d-CDW state with decreasing doping. The intrinsic c-axis
tunneling spectra are, on the other hand, best explained by coherent tunneling.
Calculated spectra show at low temperatures two peaks due to SC and d-CDW. With
increasing temperature the BCS-like peak moves to zero voltage and vanishes at
T,exactly as in experiment.Our results thus can explain why break junction
and intrinsic tunneling spectra are different from each other. Moreover, they
support a scenario of two competing order parameters in the underdoped region
of high-T superconductors.Comment: 12 pages, 16 figure
Self-energy effects in electronic Raman spectra of doped cuprates due to magnetic fluctuations
We present results for magnetic excitations in doped copper oxides using the
random phase approximation and itinerant electrons. In the [1,0] direction the
observed excitations resemble dispersive quasi-particles both in the normal and
superconducting state similar as in recent resonant inelastic X-ray scattering
(RIXS) experiments. In the [1,1] direction the excitations form, except for the
critical region near the antiferromagnetic wave vector ,
only very broad continua. Using the obtained spin propagators we calculate
electron self-energies and their effects on electronic Raman spectra. We show
that the recently observed additional peak at about twice the pair breaking in
B symmetry below T in HgBaCuO can be explained as a
self-energy effect where a broken Cooper pair and a magnetic excitation appear
as final states. The absence of this peak in B symmetry, which probes
mainly electrons near the nodal direction, is explained by their small
self-energies compared to those in the antinodal direction.Comment: 5 pages, 5 figure
Superconductivity, d Charge-Density Wave and Electronic Raman Scattering in High-T Superconductors
The competition of superconductivity and a d charge-density wave (CDW) is
studied in the t-J model as a function of temperature at large N where N is the
number of spin components. Applying the theory to electronic Raman scattering
the temperature dependence of the and the spectra are
discussed for a slightly underdoped case.Comment: 2 pages, 3 figures, Proc. M2S-HTSC-VII, to appear in Physica
Isotope effect on the superconducting critical temperature of cuprates in the presence of charge order
Using the large- limit of the - model and allowing also for phonons
and the electron-phonon interaction we study the isotope effect for
coupling constants appropriate for YBCO. We find that has a minimum at
optimal doping and increases strongly (slightly) towards the underdoped
(overdoped) region. Using values for the electron phonon interaction from the
local density approximation we get good agreement for as a function of
and doping with recent experimental data in YBCO. Our results
strongly suggest that the large increase of in the underdoped region
is (a) caused by the shift of electronic spectral density from low to high
energies associated with a competing phase (in our case a charge density wave)
and the formation of a gap, and (b) compatible with the small electron phonon
coupling constants obtained from the local density approximation. We propose a
similar explanation for the anomalous behavior of in Sr doped
LaCuO near the doping 1/8.Comment: 14 pages, 6 figure
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