9,211 research outputs found
Landau theory of phase separation in cuprates
I discuss the problem of phase separation in cuprates from the point of view
of the Landau theory of Fermi liquids. I calculate the rate of growth of
unstable regions for the hydrodymanics and collisionless limit and, in presence
of long range Coulomb interactions, the size of these regions. These are
analytic results valid for any strength of the Landau parameters.Comment: RevteX, preprint ITP (1994
Measurement of the Hyperfine Structure and Isotope Shifts of the 3s23p2 3P2 to 3s3p3 3Do3 Transition in Silicon
The hyperfine structure and isotope shifts of the 3s23p2 3P2 to 3s3p3 3Do3
transition in silicon have been measured. The transition at 221.7 nm was
studied by laser induced fluorescence in an atomic Si beam. For 29Si, the
hyperfine A constant for the 3s23p2 3P2 level was determined to be -160.1+-1.3
MHz (1 sigma error), and the A constant for the 3s3p3 3Do3 level is -532.9+-0.6
MHz. This is the first time that these constants were measured. The isotope
shifts (relative to the abundant isotope 28Si) of the transition were
determined to be 1753.3+-1.1 MHz for 29Si and 3359.9+-0.6 MHz for 30Si. This is
an improvement by about two orders of magnitude over a previous measurement.
From these results we are able to predict the hyperfine structure and isotope
shift of the radioactive 31Si atom, which is of interest in building a scalable
quantum computer
Phase Coexistence Near a Morphotropic Phase Boundary in Sm-doped BiFeO3 Films
We have investigated heteroepitaxial films of Sm-doped BiFeO3 with a
Sm-concentration near a morphotropic phase boundary. Our high-resolution
synchrotron X-ray diffraction, carried out in a temperature range of 25C to
700C, reveals substantial phase coexistence as one changes temperature to
crossover from a low-temperature PbZrO3-like phase to a high-temperature
orthorhombic phase. We also examine changes due to strain for films greater or
less than the critical thickness for misfit dislocation formation.
Particularly, we note that thicker films exhibit a substantial volume collapse
associated with the structural transition that is suppressed in strained thin
films
Theory of Magnetic Field Induced Spin Density Wave in High Temperature Superconductors
The induction of spin density wave (SDW) and charge density wave (CDW)
orderings in the mixed state of high superconductors (HTS) is
investigated by using the self-consistent Bogoliubov-de Gennes equations based
upon an effective model Hamiltonian with competing SDW and d-wave
superconductivity interactions. For optimized doping sample, the modulation of
the induced SDW and its associated CDW is determined by the vortex lattice and
their patterns obey the four-fold symmetry. By deceasing doping level, both SDW
and CDW show quasi-one dimensional like behavior, and the CDW has a period just
half that of the SDW along one direction. From the calculation of the local
density of states (LDOS), we found that the majority of the quasi-particles
inside the vortex core are localized. All these results are consistent with
several recent experiments on HTS
Charge ordering in extended Hubbard models: Variational cluster approach
We present a generalization of the recently proposed variational cluster
perturbation theory to extended Hubbard models at half filling with repulsive
nearest neighbor interaction. The method takes into account short-range
correlations correctly by the exact diagonalisation of clusters of finite size,
whereas long-range order beyond the size of the clusters is treated on a
mean-field level. For one dimension, we show that quantum Monte Carlo and
density-matrix renormalization-group results can be reproduced with very good
accuracy. Moreover we apply the method to the two-dimensional extended Hubbard
model on a square lattice. In contrast to the one-dimensional case, a first
order phase transition between spin density wave phase and charge density wave
phase is found as function of the nearest-neighbor interaction at onsite
interactions U>=3t. The single-particle spectral function is calculated for
both the one-dimensional and the two-dimensional system.Comment: 15 pages, 12 figure
Classical Phase Fluctuations in High Temperature Superconductors
Phase fluctuations of the superconducting order parameter play a larger role
in the cuprates than in conventional BCS superconductors because of the low
superfluid density of a doped insulator. In this paper, we analyze an XY model
of classical phase fluctuations in the high temperature superconductors using a
low-temperature expansion and Monte Carlo simulations. In agreement with
experiment, the value of the superfluid density at temperature T=0 is a quite
robust predictor of Tc, and the evolution of the superfluid density with T,
including its T-linear behavior at low temperature, is insensitive to
microscopic details.Comment: 4 pages, 1 figur
Superconductivity in the Cuo Hubbard Model with Long-Range Coulomb Repulsion
A multiband CuO Hubbard model is studied which incorporates long-range (LR)
repulsive Coulomb interactions. In the atomic limit, it is shown that a
charge-transfer from copper to oxygen ions occurs as the strength of the LR
interaction is increased. The regime of phase separation becomes unstable, and
is replaced by a uniform state with doubly occupied oxygens. As the holes
become mobile a superfluid condensate is formed, as suggested by a numerical
analysis of pairing correlation functions and flux quantization. Although most
of the calculations are carried out on one dimensional chains, it isComment: LATEX, 14 pages, 4 figures available as postcript files or hard copy,
preprint ORNL-CCIP/93/1
The Whole Heliosphere Interval in the Context of a Long and Structured Solar Minimum: An Overview from Sun to Earth
Throughout months of extremely low solar activity during the recent extended solar-cycle minimum, structural evolution continued to be observed from the Sun through the solar wind and to the Earth. In 2008, the presence of long-lived and large low-latitude coronal holes meant that geospace was periodically impacted by high-speed streams, even though solar irradiance, activity, and interplanetary magnetic fields had reached levels as low as, or lower than, observed in past minima. This time period, which includes the first Whole Heliosphere Interval (WHI 1: Carrington Rotation (CR) 2068), illustrates the effects of fast solar-wind streams on the Earth in an otherwise quiet heliosphere. By the end of 2008, sunspots and solar irradiance had reached their lowest levels for this minimum (e.g., WHI 2: CR 2078), and continued solar magnetic-flux evolution had led to a flattening of the heliospheric current sheet and the decay of the low-latitude coronal holes and associated Earth-intersecting high-speed solar-wind streams. As the new solar cycle slowly began, solar-wind and geospace observables stayed low or continued to decline, reaching very low levels by June – July 2009. At this point (e.g., WHI 3: CR 2085) the Sun–Earth system, taken as a whole, was at its quietest. In this article we present an overview of observations that span the period 2008 – 2009, with highlighted discussion of CRs 2068, 2078, and 2085. We show side-by-side observables from the Sun’s interior through its surface and atmosphere, through the solar wind and heliosphere and to the Earth’s space environment and upper atmosphere, and reference detailed studies of these various regimes within this topical issue and elsewhere
Anomalous Proximity Effect in Underdoped YBaCuO Josephson Junctions
Josephson junctions were photogenerated in underdoped thin films of the
YBaCuO family using a near-field scanning optical microscope.
The observation of the Josephson effect for separations as large as 100 nm
between two wires indicates the existence of an anomalously large proximity
effect and show that the underdoped insulating material in the gap of the
junction is readily perturbed into the superconducting state. The critical
current of the junctions was found to be consistent with the conventional
Josephson relationship. This result constrains the applicability of SO(5)
theory to explain the phase diagram of high critical temperature
superconductors.Comment: 11 pages, 4 figure
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