816 research outputs found
Free energies and critical exponents of the A_1^{(1)}, B_n^{(1)}, C_n^{(1)} and D_n^{(1)} face models
We obtain the free energies and critical exponents of models associated with
elliptic solutions of the star-triangle relation and reflection equation. The
models considered are related to the affine Lie algebras A_1^{(1)},
B_n^{(1)},C_n^{(1)} and D_n^{(1)}. The bulk and surface specific heat exponents
are seen to satisfy the scaling relation 2\alpha_s = \alpha_b + 2. It follows
from scaling relations that in regime III the correlation length exponent \nu
is given by \nu=(l+g)/2g, where l is the level and g is the dual Coxeter
number. In regime II we find \nu=(l+g)/2l.Comment: 9 pages, Latex, no figure
The transmission of nosocomial pathogens in an intensive care unit: a space–time clustering and structural equation modelling approach
We investigated the incidence of cases of nosocomial pathogens and risk factors in an intensive treatment unit ward to determine if the number of cases is dependent on location of patients and the colonization/infection history of the ward. A clustering approach method was developed to investigate the patterns of spread of cases through time for five microorganisms [methicillin-resistant Staphylococcus aureus (MRSA), Acinetobacter spp., Klebsiella spp., Candida spp., and Pseudomonas aeruginosa] using hospital microbiological monitoring data and ward records of patient-bed use. Cases of colonization/infection by MRSA, Candida and Pseudomonas were clustered in beds and through time while cases of Klebsiella and Acinetobacter were not. We used structural equation modelling to analyse interacting risk factors and the potential pathways of transmission in the ward. Prior nurse contact with colonized/infected patients, mediated by the number of patient-bed movements, were important predictors for all cases, except for those of Pseudomonas. General health and invasive surgery were significant predictors of cases of Candida and Klebsiella. We suggest that isolation and bed movement as a strategy to manage MRSA infections is likely to impact upon the incidence of cases of other opportunist pathogen
The Importance of being Odd
In this letter I consider mainly a finite XXZ spin chain with periodic
boundary conditions and \bf{odd} \rm number of sites. This system is described
by the Hamiltonian . As it turned out, its ground state
energy is exactly proportional to the number of sites for a special
value of the asymmetry parameter . The trigonometric polynomial
, zeroes of which being the parameters of the ground state Bethe
eigenvector is explicitly constructed. This polynomial of degree
satisfy the Baxter T-Q equation. Using the second independent solution of this
equation corresponding to the same eigenvalue of the transfer matrix, it is
possible to find a derivative of the ground state energy w.r.t. the asymmetry
parameter. This derivative is closely connected with the correlation function
. In its turn this correlation
function is related to an average number of spin strings for the ground state
of the system under consideration: . I would like
to stress once more that all these simple formulas are \bf wrong \rm in the
case of even number of sites. Exactly this case is usually considered.Comment: 9 pages, based on the talk given at NATO Advanced Research Workshop
"Dynamical Symmetries in Integrable Two-dimensional Quantum Field Theories
and Lattice Models", 25-30 September 2000, Kyiv, Ukraine. New references are
added plus some minor correction
Manifestation of a Ferroelectric Phase Transition
Temperature dependences of the dielectric properties of ultrathin polyvinylidene fluoride films prepared using the Langmuir-Blodgett method were studied by linear and nonlinear dielectric spectroscopy. It is shown that ultrathin Langmuir films of polyvinylidene fluoride exhibit a manifestation of a first-order ferroelectric phase transition, which can be assigned to the interaction between the spontaneous polarization and the surfaces bounding the film. As the film thickness increases, the effect of the surfaces decreases and the ferroelectric phase transition shifts to high temperatures to vanish altogether when the temperature region of the transition rises above the melting point
Auxiliary matrices on both sides of the equator
The spectra of previously constructed auxiliary matrices for the six-vertex
model at roots of unity are investigated for spin-chains of even and odd
length. The two cases show remarkable differences. In particular, it is shown
that for even roots of unity and an odd number of sites the eigenvalues contain
two linear independent solutions to Baxter's TQ-equation corresponding to the
Bethe ansatz equations above and below the equator. In contrast, one finds for
even spin-chains only one linear independent solution and complete strings. The
other main result is the proof of a previous conjecture on the degeneracies of
the six-vertex model at roots of unity. The proof rests on the derivation of a
functional equation for the auxiliary matrices which is closely related to a
functional equation for the eight-vertex model conjectured by Fabricius and
McCoy.Comment: 22 pages; 2nd version: one paragraph added in the conclusion and some
typos correcte
Modeling of micro- and nano-scale domain recording by high-voltage atomic force microscopy in ferroelectrics-semiconductors
The equilibrium sizes of micro- and nano-domains caused by electric field of
atomic force microscope tip in ferroelectric semiconductor crystals have been
calculated. The domain was considered as a prolate semi-ellipsoid with rather
thin domain walls. For the first time we modified the Landauer model allowing
for semiconductor properties of the sample and the surface energy of the domain
butt. The free carriers inside the crystal lead to the formation of the
screening layer around the domain, which partially shields its interior from
the depolarization field. We expressed the radius and length of the domain
though the crystal material parameters (screening radius, spontaneous
polarization value, dielectric permittivity tensor) and atomic force microscope
tip characteristics (charge, radius of curvature). The obtained dependence of
domain radius via applied voltage is in a good quantitative agreement with the
ones of submicron ferroelectric domains recorded by high-voltage atomic force
and scanning probe microscopy in LiNbO3 and LiTaO3 crystals.Comment: 21 pages, 5 figure
Finite size and intrinsic field effect on the polar-active properties of the ferroelectric-semiconductor heterostructures
Using Landau-Ginzburg-Devonshire approach we calculated the equilibrium
distributions of electric field, polarization and space charge in the
ferroelectric-semiconductor heterostructures containing proper or incipient
ferroelectric thin films. The role of the polarization gradient and intrinsic
surface energy, interface dipoles and free charges on polarization dynamics are
specifically explored. The intrinsic field effects, which originated at the
ferroelectric-semiconductor interface, lead to the surface band bending and
result into the formation of depletion space-charge layer near the
semiconductor surface. During the local polarization reversal (caused by the
inhomogeneous electric field induced by the nanosized tip of the Scanning Probe
Microscope (SPM) probe) the thickness and charge of the interface layer
drastically changes, it particular the sign of the screening carriers is
determined by the polarization direction. Obtained analytical solutions could
be extended to analyze polarization-mediated electronic transport.Comment: 35 pages, 12 figures, 1 table, 2 appendices, to be submitted to Phys.
Rev.
Optical second harmonic generation probe of two-dimensional ferroelectricity
Optical second harmonic generation (SHG) is used as a noninvasive probe of
two-dimensional (2D) ferroelectricity in Langmuir-Blodgett (LB) films of
copolymer vinylidene fluoride with trifluorethylene. The surface 2D
ferroelectric-paraelectric phase transition in the topmost layer of LB films
and a thickness independent (almost 2D) transition in the bulk of these films
are observed in temperature studies of SHG.Comment: 9 pages, 2 figures, Optics Letters, in prin
Analytic and geometric properties of photoinduced effects in noncentrosymmetric crystals: photovoltaic current and optical rectification
An original dispersion relation between the stationary coherent nonlinear
optical responses by current and polarisation is obtained. The dispersion
relation provides a new complimentary tool that can be employed to study
light-induced charge transport models and facilitate experimental data
analysis. It is shown that the origin of the coherent current and the
dc-polarisation induced in a noncentrosymmetric crystal under illumination is
related to the theory of the Berry phase and can be represented in terms of the
renormalised geometric potentials. This renormalisation originates from the
extra phase difference acquired by a carrier in the light field on the quantum
transition between the electronic bands. The gauge invariance of the
corresponding expressions for the current and the polarisation is demonstrated.Comment: 7 page
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