274 research outputs found
Proximity to a Nearly Superconducting Quantum Critical Liquid
The coupling between superconductors and a quantum critical liquid that is
nearly superconducting provides natural interpretation for the Josephson effect
over unexpectedly long junctions, and the remarkable stripe-spacing dependence
of the critical temperature in LSCO and YBCO superconductors.Comment: four two-column pages, no figure
Pinned Balseiro-Falicov Model of Tunneling and Photoemission in the Cuprates
The smooth evolution of the tunneling gap of Bi_2Sr_2CaCu_2O_8 with doping
from a pseudogap state in the underdoped cuprates to a superconducting state at
optimal and overdoping, has been interpreted as evidence that the pseudogap
must be due to precursor pairing. We suggest an alternative explanation, that
the smoothness reflects a hidden SO(N) symmetry near the (pi,0) points of the
Brillouin zone (with N = 3, 4, 5, or 6). Because of this symmetry, the
pseudogap could actually be due to any of a number of nesting instabilities,
including charge or spin density waves or more exotic phases. We present a
detailed analysis of this competition for one particular model: the pinned
Balseiro-Falicov model of competing charge density wave and (s-wave)
superconductivity. We show that most of the anomalous features of both
tunneling and photoemission follow naturally from the model, including the
smooth crossover, the general shape of the pseudogap phase diagram, the
shrinking Fermi surface of the pseudogap phase, and the asymmetry of the
tunneling gap away from optimal doping. Below T_c, the sharp peak at Delta_1
and the dip seen in the tunneling and photoemission near 2Delta_1 cannot be
described in detail by this model, but we suggest a simple generalization to
account for inhomogeneity, which does provide an adequate description. We show
that it should be possible, with a combination of photoemission and tunneling,
to demonstrate the extent of pinning of the Fermi level to the Van Hove
singularity. A preliminary analysis of the data suggests pinning in the
underdoped, but not in the overdoped regime.Comment: 18 pages LaTeX, 26 ps. figure
Dispersion of Ordered Stripe Phases in the Cuprates
A phase separation model is presented for the stripe phase of the cuprates,
which allows the doping dependence of the photoemission spectra to be
calculated. The idealized limit of a well-ordered array of magnetic and charged
stripes is analyzed, including effects of long-range Coulomb repulsion.
Remarkably, down to the limit of two-cell wide stripes, the dispersion can be
interpreted as essentially a superposition of the two end-phase dispersions,
with superposed minigaps associated with the lattice periodicity. The largest
minigap falls near the Fermi level; it can be enhanced by proximity to a (bulk)
Van Hove singularity. The calculated spectra are dominated by two features --
this charge stripe minigap plus the magnetic stripe Hubbard gap. There is a
strong correlation between these two features and the experimental
photoemission results of a two-peak dispersion in LaSrCuO, and
the peak-dip-hump spectra in BiSrCaCuO. The
differences are suggestive of the role of increasing stripe fluctuations. The
1/8 anomaly is associated with a quantum critical point, here expressed as a
percolation-like crossover. A model is proposed for the limiting minority
magnetic phase as an isolated two-leg ladder.Comment: 24 pages, 26 PS figure
Quantum vortex fluctuations in cuprate superconductors
We study the effects of quantum vortex fluctuations in two-dimensional
superconductors using a dual theory of vortices, and investigate the relevance
to underdoped cuprates where the superconductor-insulator transition (SIT) is
possibly driven by quantum vortex proliferation. We find that a broad enough
phase fluctuation regime may exist for experimental observation of the quantum
vortex fluctuations near SIT in underdoped cuprates. We propose that this
scenario can be tested via pair-tunneling experiments which measure the
characteristic resonances in the zero-temperature pair-field susceptibility in
the vortex-proliferated insulating phase.Comment: RevTex 5 pages, 2 eps figures; expanded; to appear in Phys. Rev.
Van Hove Excitons and High-T Superconductivity: VIIIC Dynamic Jahn-Teller Effects vs Spin-Orbit Coupling in the LTO Phase of LaSrCuO
The possible role of the van Hove singularity (vHs) in stabilizing the
low-temperature orthorhombic (LTO) phase transition in
La\-Sr\-CuO (LSCO) is discussed. It is found that the vHs can
drive a structural distortion in two different ways, either due to spin-orbit
coupling or to dynamic Jahn-Teller (JT) effects. This paper discusses the
latter effect in some detail. It is shown that a model Hamiltonian introduced
earlier to describe the coupled electron -- octahedral tilt motions (`cageons')
has a series of phase transitions, from a high-temperature disordered JT phase
(similar to the high-temperature tetragonal phase of LSCO) to an intermediate
temperature dynamic JT phase, of average orthorhombic symmetry (the LTO phase)
to a low temperature static JT phase (the low temperature tetragonal phase).
For some parameter values, the static JT phase is absent.Comment: 28 pages plain TeX, 14 figures available upon request,
NU-MARKIEWIC-93-0
Enhancement of long-range magnetic order by magnetic field in superconducting La2CuO(4+y)
We report a detailed study, using neutron scattering, transport and
magnetization measurements, of the interplay between superconducting (SC) and
spin density wave (SDW) order in La2CuO(4+y). Both kinds of order set in below
the same critical temperature. However, the SDW order grows with applied
magnetic field, whereas SC order is suppressed. Most importantly, the field
dependence of the SDW Bragg peak intensity has a cusp at zero field, as
predicted by a recent theory of competing SDW and SC order. This leads us to
conclude that there is a repulsive coupling between the two order parameters.
The question of whether the two kinds of order coexist or microscopically phase
separate is discussed.Comment: Version accepted for publication in Phys. Rev. B. Improved discussion
in connection with the muSR result
Muon spin rotation study of the intercalated graphite superconductor CaC6 at low temperatures
Muon spin rotation (muSR) experiments were performed on the intercalated
graphite CaC6 in the normal and superconducting state down to 20 mK. In
addition, AC magnetization measurements were carried out resulting in an
anisotropic upper critical field Hc2, from which the coherence lengths
xi_ab(0)=36.3(1.5) nm and xi_c(0)=4.3(7) nm were estimated. The anisotropy
parameter gamma_H= H_c2_ab/H_c2_c increases monotonically with decreasing
temperature. A single isotropic s-wave description of superconductivity cannot
account for this behaviour. From magnetic field dependent muSR experiments the
absolute value of the in-plane magnetic penetretion depth lambda_ab=78(3) nm
was determined. The temperature dependence of the superfluid density rho_s(T)
is slightly better described by a two-gap than a single-gap model
Interplay of structural and electronic phase separation in single crystalline La(2)CuO(4.05) studied by neutron and Raman scattering
We report a neutron and Raman scattering study of a single-crystal of
La(2)CuO(4.05) prepared by high temperature electrochemical oxidation. Elastic
neutron scattering measurements show the presence of two phases, corresponding
to the two edges of the first miscibility gap, all the way up to 300 K. An
additional oxygen redistribution, driven by electronic energies, is identified
at 250 K in Raman scattering (RS) experiments by the simultaneous onset of
two-phonon and two-magnon scattering, which are fingerprints of the insulating
phase. Elastic neutron scattering measurements show directly an
antiferromagnetic ordering below a N\'eel temperature of T_N =210K. The opening
of the superconducting gap manifests itself as a redistribution of electronic
Raman scattering below the superconducting transition temperature, T_c = 24K. A
pronounced temperature-dependent suppression of the intensity of the (100)
magnetic Bragg peak has been detected below T_c. We ascribe this phenomenon to
a change of relative volume fraction of superconducting and antiferromagnetic
phases with decreasing temperature caused by a form of a superconducting
proximity effect.Comment: 9 pages, including 9 eps figures, submitted to PR
Proximity induced metal/insulator transition in superlattices
The far-infrared dielectric response of superlattices (SL) composed of
superconducting YBaCuO (YBCO) and ferromagnetic La%
CaMnO (LCMO) has been investigated by ellipsometry. A drastic
decrease of the free carrier response is observed which involves an unusually
large length scale of d20 nm in YBCO and d10
nm in LCMO. A corresponding suppression of metallicity is not observed in SLs
where LCMO is replaced by the paramagnetic metal LaNiO. Our data suggest
that either a long range charge transfer from the YBCO to the LCMO layers or
alternatively a strong coupling of the charge carriers to the different and
competitive kind of magnetic correlations in the LCMO and YBCO layers are at
the heart of the observed metal/insulator transition. The low free carrier
response observed in the far-infrared dielectric response of the magnetic
superconductor RuSrGdCuO is possibly related to this effect
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