217 research outputs found
Understanding high-Tc cuprates based on the phase string theory of doped antiferromagnet
We present a self-consistent RVB theory which unifies the metallic
(superconducting) phase with the half-filling antiferromagnetic (AF) phase. Two
crucial factors in this theory include the RVB condensation which controls
short-range AF spin correlations and the phase string effect introduced by hole
hopping as a key doping effect. We discuss both the uniform and non-uniform
mean-field solutions and show the unique features of the characteristic spin
energy scale, superconducting transition temperature, and the phase diagram,
which are all consistent with the experimental measurements of high-
cuprates.Comment: 4 pages, 4 embeded eps figures, minor typos are corrected, to appear
in the proceedings of M2S-HTSC-VI conferenc
Moyal Representation of the String Field Star Product in the Presence of a B-background
In this paper we show that in the presence of an anti-symmetric tensor
-background, Witten's star algebra for open string fields persists to
possess the structure of a direct product of commuting Moyal pairs. The
interplay between the noncommutativity due to three-string overlap and that due
to the -background is our main concern. In each pair of noncommutative
directions parallel to the -background, the Moyal pairs mix string modes in
the two directions and are labeled, in addition to a continuous parameter, by
{\it two} discrete values as well. However, the Moyal parameters are
-dependent only for discrete pairs. We have also demonstrated the large-
contraction of the star algebra, with one of the discrete Moyal pairs dropping
out while the other giving rise to the center-of-mass noncommutative function
algebra.Comment: minor notation chang
Electron-electron interactions and two-dimensional - two-dimensional tunneling
We derive and evaluate expressions for the dc tunneling conductance between
interacting two-dimensional electron systems at non-zero temperature. The
possibility of using the dependence of the tunneling conductance on voltage and
temperature to determine the temperature-dependent electron-electron scattering
rate at the Fermi energy is discussed. The finite electronic lifetime produced
by electron-electron interactions is calculated as a function of temperature
for quasiparticles near the Fermi circle. Vertex corrections to the random
phase approximation substantially increase the electronic scattering rate. Our
results are in an excellent quantitative agreement with experiment.Comment: Revtex style, 21 pages and 8 postscript figures in a separate file;
Phys. Rev. B (in press
Dynamic modelling and simulation of eutectic freeze crystallization process for recovery of ammonium sulphate from aqueous solutions
The advancement of industrial sectors generates large amount of wastewater containing ammonium sulphate
(AS) ions, yet the current AS recovery technologies are energy-extensive and produce excessive sludge. In this
study, a dynamic model for a batch crystallizer was developed to investigate the eutectic freeze crystallization
process of AS in aqueous solutions using the method of moment (MoM). The variables affecting the crystal size
distribution of AS and ice were described in terms of zeroth moment (ÎĽ0), first moment (ÎĽ1), second moment (ÎĽ2)
and third moment (ÎĽ3). The ÎĽ0 of ice crystals increased significantly until 2 s and slowed down thereafter while
ÎĽ0 of AS crystal increased steadily and reached a constant value of 1/m3
s after 1 s. The ÎĽ1, ÎĽ2, and ÎĽ3 of the AS
and ice crystals depicted a similar increasing trend. Through process optimization using a 3-D plot on the initial
concentration of the system and initial temperature of cooling fluid which had been identified as the significant
parameters via iterative plot using MATLAB software, the AS and ice reached maximum crystallization at 255 K
which was close to the eutectic point of AS-water system at 254 K. The dynamic model which comprised of
population, mass and energy balance equations established in this study demonstrated effectiveness in the
prediction of crystallization of AS in aqueous solutions
Polariton propagation in weak confinement quantum wells
Exciton-polariton propagation in a quantum well, under centre-of-mass
quantization, is computed by a variational self-consistent microscopic theory.
The Wannier exciton envelope functions basis set is given by the simple
analytical model of ref. [1], based on pure states of the centre-of-mass wave
vector, free from fitting parameters and "ad hoc" (the so called additional
boundary conditions-ABCs) assumptions. In the present paper, the former
analytical model is implemented in order to reproduce the centre-of-mass
quantization in a large range of quantum well thicknesses (5a_B < L < inf.).
The role of the dynamical transition layer at the well/barrier interfaces is
discussed at variance of the classical Pekar's dead-layer and ABCs. The Wannier
exciton eigenstates are computed, and compared with various theoretical models
with different degrees of accuracy. Exciton-polariton transmission spectra in
large quantum wells (L>> a_B) are computed and compared with experimental
results of Schneider et al.\cite{Schneider} in high quality GaAs samples. The
sound agreement between theory and experiment allows to unambiguously assign
the exciton-polariton dips of the transmission spectrum to the pure states of
the Wannier exciton center-of-mass quantization.Comment: 15 pages, 15 figures; will appear in Phys.Rev.
Emergence of quasi-metallic state in disordered 2D electron gas due to strong interactions
The interrelation between disorder and interactions in two dimensional
electron liquid is studied beyond weak coupling perturbation theory. Strong
repulsion significantly reduces the electronic density of states on the Fermi
level. This makes the electron liquid more rigid and strongly suppresses
elastic scattering off impurities. As a result the weak localization, although
ultimately present at zero temperature and infinite sample size, is
unobservable at experimentally accessible temperature at high enough densities.
Therefore practically there exists a well defined metallic state. We study
diffusion of electrons in this state and find that the diffusion pole is
significantly modified due to "mixture" with static photons similar to the
Anderson - Higgs mechanism in superconductivity. As a result several effects
stemming from the long range nature of diffusion like the Aronov - Altshuler
logarithmic corrections to conductivity are less pronounced.Comment: to appear in Phys. Rev.
Salerno's model of DNA reanalysed: could solitons have biological significance?
We investigate the sequence-dependent behaviour of localised excitations in a
toy, nonlinear model of DNA base-pair opening originally proposed by Salerno.
Specifically we ask whether ``breather'' solitons could play a role in the
facilitated location of promoters by RNA polymerase. In an effective potential
formalism, we find excellent correlation between potential minima and {\em
Escherichia coli} promoter recognition sites in the T7 bacteriophage genome.
Evidence for a similar relationship between phage promoters and downstream
coding regions is found and alternative reasons for links between AT richness
and transcriptionally-significant sites are discussed. Consideration of the
soliton energy of translocation provides a novel dynamical picture of sliding:
steep potential gradients correspond to deterministic motion, while ``flat''
regions, corresponding to homogeneous AT or GC content, are governed by random,
thermal motion. Finally we demonstrate an interesting equivalence between
planar, breather solitons and the helical motion of a sliding protein
``particle'' about a bent DNA axis.Comment: Latex file 20 pages, 5 figures. Manuscript of paper to appear in J.
Biol. Phys., accepted 02/09/0
Stripes, Pseudogaps, and Van Hove Nesting in the Three-band tJ Model
Slave boson calculations have been carried out in the three-band tJ model for
the high-T_c cuprates, with the inclusion of coupling to oxygen breathing mode
phonons. Phonon-induced Van Hove nesting leads to a phase separation between a
hole-doped domain and a (magnetic) domain near half filling, with long-range
Coulomb forces limiting the separation to a nanoscopic scale. Strong
correlation effects pin the Fermi level close to, but not precisely at the Van
Hove singularity (VHS), which can enhance the tendency to phase separation. The
resulting dispersions have been calculated, both in the uniform phases and in
the phase separated regime. In the latter case, distinctly different
dispersions are found for large, random domains and for regular (static)
striped arrays, and a hypothetical form is presented for dynamic striped
arrays. The doping dependence of the latter is found to provide an excellent
description of photoemission and thermodynamic experiments on pseudogap
formation in underdoped cuprates. In particular, the multiplicity of observed
gaps is explained as a combination of flux phase plus charge density wave (CDW)
gaps along with a superconducting gap. The largest gap is associated with VHS
nesting. The apparent smooth evolution of this gap with doping masks a
crossover from CDW-like effects near optimal doping to magnetic effects (flux
phase) near half filling. A crossover from large Fermi surface to hole pockets
with increased underdoping is found. In the weakly overdoped regime, the CDW
undergoes a quantum phase transition (), which could be obscured
by phase separation.Comment: 15 pages, Latex, 18 PS figures Corrects a sign error: major changes,
esp. in Sect. 3, Figs 1-4,6 replace
Theory of Coexistence of Superconductivity and Ferroelectricity : A Dynamical Symmetry Model
We propose and investigate a model for the coexistence of Superconductivity
(SC) and Ferroelectricity (FE) based on the dynamical symmetries for
the pseudo-spin SC sector, for the displaced oscillator FE sector, and
for the composite system. We assume a minimal
symmetry-allowed coupling, and simplify the hamiltonian using a double mean
field approximation (DMFA). A variational coherent state (VCS) trial
wave-function is used for the ground state: the energy, and the relevant order
parameters for SC and FE are obtained. For positive sign of the SC-FE coupling
coefficient, a non-zero value of either order parameter can suppress the other
(FE polarization suppresses SC and vice versa). This gives some support to
"Matthias' Conjecture" [1964], that SC and FE tend to be mutually exclusive.
For such a Ferroelectric Superconductor we predict: a) the SC gap
(and ) will increase with increasing applied pressure when pressure
quenches FE as in many ferroelectrics, and b) the FE polarization will increase
with increaesing magnetic field up to . The last result is equivalent to
the prediction of a new type of Magneto-Electric Effect in a coexistent SC-FE
material. Some discussion will be given of the relation of these results to the
cuprate superconductors.Comment: 46 page
Analysis of stomatal and convective resistances to transpirational flow
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47839/1/484_2005_Article_BF01554062.pd
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