5,513 research outputs found
Low Temperature AC Conductivity in BSCCO (2212)
We report measurements of anamolously large dissipative conductivities in
BiSrCaCuO(2212) at low temperatures. We have measured the complex conductivity
of BSCCO thin films at 100-600 GHz as a function of doping from the underdoped
to the overdoped state. At low temperatures there exists a residual dissipative
conductivity which scales with the T=0 superfluid density as the doping is
varied. This residual dissipative conductivity is larger than the possible
contribution from a thermal population of quasiparticles at the d-wave gap
nodes.Comment: Submitted to the Proceedings of the 22nd International Low
Temperature Physics Conference. To be published in Physica B
(http://www.elsevier.nl/locate/physb); 2 Pages with 2 Figure
Optical control of competing exchange interactions and coherent spin-charge coupling in two-orbital Mott insulators
In order to have a better understanding of ultrafast electrical control of
exchange interactions in multi-orbital systems, we study a two-orbital Hubbard
model at half filling under the action of a time-periodic electric field. Using
suitable projection operators and a generalized time-dependent canonical
transformation, we derive an effective Hamiltonian which describes two
different regimes. First, for a wide range of non-resonant frequencies, we find
a change of the bilinear Heisenberg exchange that is
analogous to the single-orbital case. Moreover we demonstrate that also the
additional biquadratic exchange interaction can be enhanced,
reduced and even change sign depending on the electric field. Second, for
special driving frequencies, we demonstrate a novel spin-charge coupling
phenomenon enabling coherent transfer between spin and charge degrees of
freedom of doubly ionized states. These results are confirmed by an exact
time-evolution of the full two-orbital Mott-Hubbard Hamiltonian.Comment: 3 pages, 6 figure
Reduced leakage current in Josephson tunnel junctions with codeposited barriers
Josephson junctions were fabricated using two different methods of barrier
formation. The trilayers employed were Nb/Al-AlOx/Nb on sapphire, where the
first two layers were epitaxial. The oxide barrier was formed either by
exposing the Al surface to O2 or by codepositing Al in an O2 background. The
codeposition process yielded junctions that showed the theoretically predicted
subgap current and no measurable shunt conductance. In contrast, devices with
barriers formed by thermal oxidation showed a small shunt conductance in
addition to the predicted subgap current.Comment: 3 pages, 4 figure
Interface hole-doping in cuprate-titanate superlattices
The electronic structure of interfaces between YBaCuO and
SrTiO is studied using local spin density approximation (LSDA) with
intra-atomic Coulomb repulsion (LSDA+U). We find a metallic state in
cuprate/titanate heterostructures with the hole carriers concentrated
substantially in the CuO-layers and in the first interface TiO and SrO
planes. This effective interface doping appears due to the polarity of
interfaces, caused by the first incomplete copper oxide unit cell.
Interface-induced high pre-doping of CuO-layers is a key mechanism
controlling the superconducting properties in engineered field-effect devices
realized on the basis of cuprate/titanate superlattices.Comment: 5 pages, 5 figure
Transmission Electron Study of Heteroepitaxial Growth in the BiSrCaCuO System
Films of BiSrCaCuO and BiSrCuO have been grown using Atomic-Layer-by-Layer Molecular Beam
Epitaxy (ALL-MBE) on lattice-matched substrates. These materials have been
combined with layers of closely-related metastable compounds like BiSrCaCuO (2278) and rare-earth-doped
compounds like BiSrDyCaCuO
(Dy:2212) to form heterostructures with unique superconducting properties,
including superconductor/insulator multilayers and tunnel junctions.
Transmission electron microscopy (TEM) has been used to study the morphology
and microstructure of these heterostructures. These TEM studies shed light on
the physical properties of the films, and give insight into the growth mode of
highly anisotropic solids like BiSrCaCuO.Comment: 17 pages, submitted to J. Materials Research. Email to
[email protected] if you want to receive copies of the figure
On smoothness of Black Saturns
We prove smoothness of the domain of outer communications (d.o.c.) of the
Black Saturn solutions of Elvang and Figueras. We show that the metric on the
d.o.c. extends smoothly across two disjoint event horizons with topology R x
S^3 and R x S^1 x S^2. We establish stable causality of the d.o.c. when the
Komar angular momentum of the spherical component of the horizon vanishes, and
present numerical evidence for stable causality in general.Comment: 47 pages, 5 figure
Photoemission Evidence for a Remnant Fermi Surface and d-Wave-Like Dispersion in Insulating Ca2CuO2Cl2
An angle resolved photoemission study on Ca2CuO2Cl2, a parent compound of
high Tc superconductors is reported. Analysis of the electron occupation
probability, n(k) from the spectra shows a steep drop in spectral intensity
across a contour that is close to the Fermi surface predicted by the band
calculation. This analysis reveals a Fermi surface remnant even though
Ca2CuO2Cl2 is a Mott insulator. The lowest energy peak exhibits a dispersion
with approximately the |cos(kxa)-cos(kya)| form along this remnant Fermi
surface. Together with the data from Dy doped Bi2Sr2CaCu2O(8 + delta) these
results suggest that this d-wave like dispersion of the insulator is the
underlying reason for the pseudo gap in the underdoped regime.Comment: 9 pages, including 7 figures. Published in Science, one figure
correcte
Vanishing of phase coherence in underdoped Bi_2Sr_2CaCu_2O_8+d
Coherent time-domain spectroscopy is used to measure the screening and
dissipation of high-frequency electromagnetic fields in a set of underdoped
Bi_2Sr_2CaCu_2O_8+d thin films. The measurements provide direct evidence for a
phase-fluctuation driven transition from the superconductor to normal state,
with dynamics described well by the Berezinskii-Kosterlitz-Thouless theory of
vortex-pair unbinding.Comment: Nature, Vol. 398, 18 March 1999, pg. 221 4 pages with 4 included
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