1,311 research outputs found
Multi-site mean-field theory for cold bosonic atoms in optical lattices
We present a detailed derivation of a multi-site mean-field theory (MSMFT)
used to describe the Mott-insulator to superfluid transition of bosonic atoms
in optical lattices. The approach is based on partitioning the lattice into
small clusters which are decoupled by means of a mean field approximation. This
approximation invokes local superfluid order parameters defined for each of the
boundary sites of the cluster. The resulting MSMFT grand potential has a
non-trivial topology as a function of the various order parameters. An
understanding of this topology provides two different criteria for the
determination of the Mott insulator superfluid phase boundaries. We apply this
formalism to -dimensional hypercubic lattices in one, two and three
dimensions, and demonstrate the improvement in the estimation of the phase
boundaries when MSMFT is utilized for increasingly larger clusters, with the
best quantitative agreement found for . The MSMFT is then used to examine
a linear dimer chain in which the on-site energies within the dimer have an
energy separation of . This system has a complicated phase diagram
within the parameter space of the model, with many distinct Mott phases
separated by superfluid regions.Comment: 30 pages, 23 figures, accepted for publication in Phys. Rev.
Spin and Charge Texture around In-Plane Charge Centers in the CuO_2 planes
Recent experiments on La_2Cu_{1-x}Li_xO_4 show that although the doped holes
remain localized near the substitutional Li impurities, magnetic order is
rapidly suppressed. An examination of the spin texture around a bound hole in a
CuO_2 plane shows that the formation of a skyrmion is favored in a wide range
of parameters, as was previously proposed in the context of Sr doping. The spin
texture may be observable by elastic diffuse neutron scattering, and may also
have a considerable effect on NMR lineshapes.Comment: 4 pages, postscript file, hardcopy available upon request, to appear
in PR
The role of pride in women with anorexia nervosa : a grounded theory study
Objective: Theory and clinical literature suggest that pride may play an important role in the maintenance of restrictive eating disorders. A grounded theory study explored experiences of, and reflections on, pride among women with a current or past diagnosis of anorexia nervosa.
Design: This is a qualitative study using grounded theory.
Method: Semi-structured interviews were conducted with 21 women recruited from an eating disorders unit in England, and from a UK self-help organization. Grounded theory from a constructivist lens was used. Analysis involved coding, constant comparison and memo-writing. Results: Pride evolves over the course of anorexia. Two overarching conceptual categories were identified: ‘Pride becoming intertwined with anorexia’ and ‘Pride during the journey towards recovery’. These categories encompassed different forms of pride: ‘alluring pride’, ‘toxic pride’, ‘pathological pride’, ‘anorexia pride’, ‘shameful pride’, ‘recovery pride’ and ‘resilient pride’. Initially, pride contributed to self-enhancement and buffered negative emotions. As the condition progressed, pride became a challenge to health and interfered with motivation to change. During recovery, perceptions of pride altered as a healthy approach to living ensued.
Conclusions: The evolving nature of pride plays a central role in development, maintenance, and treatment of anorexia. Understanding of pride and its role in psychotherapeutic work with this client group may increase motivation to change and promote recovery. Future work should investigate if tackling pride in eating disorders increases treatment efficacy and reduces the risk of relapsing
Satellite navigation for meteorological purposes: Inverse referencing for NOAA-N and ERS-1 imagers with a 1 km nadir pixel size
Iterative methods for inverse referencing from mean orbital elements or osculating position and velocity, accounting for all necessary orbital perturbations with respect to given nadir pixel size, are described. [Inverse referencing means that the geodetic coordinates of a point on the surface are given and the corresponding image coordinates (scan line number and pixel number) are obtained from satellite orbital elements or coordinates.] The idea is to treat a pixel like a satellite tracking station on the ground. This permits the use of existing software for the computation of satellite ephemerides and orbit determination. The time of culmination of a satellite over the pixel and the off-nadir angle at that moment have been computed. Two variants for such a computation have been tested. Numerical results for the NOAA-N meteorological satellites and ERS-1 are presented. The present state of our software for inverse referencing should fulfil ordinary requirements posed by meteorologists. For NOAA-N satellites, the accuracy achieved roughly the nadir pixel size. The main obstacle to an increase in accuracy is the low quality of the mean orbital elements usually available. For ERS-1, the accuracy may achieve a level of 100 m. A software package, containing versions of the FORTRAN 77 programs PIXPO 3, PIXPO 4 and PIXPOSC, for various data types, including US-2 line or TBUS mean elements or a state vector, is available for scientific exchange. © 1994
Chiral Plaquette Polaron Theory of Cuprate Superconductivity
Ab-initio density functional calculations on explicitly doped
La(2-x)Sr(x)CuO4 find doping creates localized holes in out-of-plane orbitals.
A model for superconductivity is developed based on the assumption that doping
leads to the formation of holes on a four-site Cu plaquette composed of the
out-of-plane A1 orbitals apical O pz, planar Cu dz2, and planar O psigma. This
is in contrast to the assumption of hole doping into planar Cu dx2-y2 and O
psigma orbitals as in the t-J model. Interaction of holes with the d9 spin
background leads to chiral polarons with either a clockwise or anti-clockwise
charge current. When the polaron plaquettes percolate through the crystal at
x~0.05 for LaSrCuO, a Cu dx2-y2 and planar O psigma band is formed. Spin
exchange Coulomb repulsion with chiral polarons leads to D-wave
superconductivity. The equivalent of the Debye energy in phonon
superconductivity is the maximum energy separation between a chiral polaron and
its time-reversed partner. An additive skew-scattering contribution to the Hall
effect is induced by chiral polarons and leads to a temperature dependent Hall
effect that fits the measured values for LaSrCuO. The integrated imaginary
susceptibility satisfies omega/T scaling due to chirality and spin-flip
scattering of polarons along with a uniform distribution of polaron energy
splittings. The derived functional form is compatible with experiments. The
static spin structure factor is computed and is incommensurate with a
separation distance from (pi,pi) given by ~(2pi)x. Coulomb scattering of the
x2-y2 band with polarons leads to linear resistivity. Coupling of the x2-y2
band to the undoped Cu d9 spins leads to the ARPES pseudogap and its doping and
temperature dependence.Comment: 32 pages, 17 figure
A reliable Pade analytical continuation method based on a high accuracy symbolic computation algorithm
We critique a Pade analytic continuation method whereby a rational polynomial
function is fit to a set of input points by means of a single matrix inversion.
This procedure is accomplished to an extremely high accuracy using a novel
symbolic computation algorithm. As an example of this method in action we apply
it to the problem of determining the spectral function of a one-particle
thermal Green's function known only at a finite number of Matsubara frequencies
with two example self energies drawn from the T-matrix theory of the Hubbard
model. We present a systematic analysis of the effects of error in the input
points on the analytic continuation, and this leads us to propose a procedure
to test quantitatively the reliability of the resulting continuation, thus
eliminating the black magic label frequently attached to this procedure.Comment: 11 pages, 8 eps figs, revtex format; revised version includes
reference to anonymous ftp site containing example codes (MapleVr5.1
worksheets) displaying the implementation of the algorithm, including the
padematinv.m library packag
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