238 research outputs found
Defect generation and deconfinement on corrugated topographies
We investigate topography-driven generation of defects in liquid crystals
films coating frozen surfaces of spatially varying Gaussian curvature whose
topology does not automatically require defects in the ground state. We study
in particular disclination-unbinding transitions with increasing aspect ratio
for a surface shaped as a Gaussian bump with an hexatic phase draped over it.
The instability of a smooth ground state texture to the generation of a single
defect is also discussed. Free boundary conditions for a single bump are
considered as well as periodic arrays of bumps. Finally, we argue that defects
on a bump encircled by an aligning wall undergo sharp deconfinement transitions
as the aspect ratio of the surface is lowered.Comment: 24 page
Exact Resummations in the Theory of Hydrodynamic Turbulence: I The Ball of Locality and Normal Scaling
This paper is the first in a series of three papers that aim at understanding
the scaling behaviour of hydrodynamic turbulence. We present in this paper a
perturbative theory for the structure functions and the response functions of
the hydrodynamic velocity field in real space and time. Starting from the
Navier-Stokes equations (at high Reynolds number Re) we show that the standard
perturbative expansions that suffer from infra-red divergences can be exactly
resummed using the Belinicher-L'vov transformation. After this exact (partial)
resummation it is proven that the resulting perturbation theory is free of
divergences, both in large and in small spatial separations. The hydrodynamic
response and the correlations have contributions that arise from mediated
interactions which take place at some space- time coordinates. It is shown that
the main contribution arises when these coordinates lie within a shell of a
"ball of locality" that is defined and discussed. We argue that the real
space-time formalism developed here offers a clear and intuitive understanding
of every diagram in the theory, and of every element in the diagrams. One major
consequence of this theory is that none of the familiar perturbative mechanisms
may ruin the classical Kolmogorov (K41) scaling solution for the structure
functions. Accordingly, corrections to the K41 solutions should be sought in
nonperturbative effects. These effects are the subjects of papers II and III in
this series, that will propose a mechanism for anomalous scaling in turbulence,
which in particular allows multiscaling of the structure functions.Comment: PRE in press, 18 pages + 6 figures, REVTeX. The Eps files of figures
will be FTPed by request to [email protected]
Circulation Statistics in Three-Dimensional Turbulent Flows
We study the large limit of the loop-dependent characteristic
functional , related
to the probability density function (PDF) of the circulation around a closed
contour . The analysis is carried out in the framework of the
Martin-Siggia-Rose field theory formulation of the turbulence problem, by means
of the saddle-point technique. Axisymmetric instantons, labelled by the
component of the strain field -- a partially annealed variable in
our formalism -- are obtained for a circular loop in the plane, with
radius defined in the inertial range. Fluctuations of the velocity field around
the saddle-point solutions are relevant, leading to the lorentzian asymptotic
behavior . The
subleading correction and the asymmetry between right and left PDF tails due to
parity breaking mechanisms are also investigated.Comment: Computations are discussed in a more detailed way; accepted for
publication in Physical Review
Numerical study of the spherically-symmetric Gross-Pitaevskii equation in two space dimensions
We present a numerical study of the time-dependent and time-independent
Gross-Pitaevskii (GP) equation in two space dimensions, which describes the
Bose-Einstein condensate of trapped bosons at ultralow temperature with both
attractive and repulsive interatomic interactions. Both time-dependent and
time-independent GP equations are used to study the stationary problems. In
addition the time-dependent approach is used to study some evolution problems
of the condensate. Specifically, we study the evolution problem where the trap
energy is suddenly changed in a stable preformed condensate. In this case the
system oscillates with increasing amplitude and does not remain limited between
two stable configurations. Good convergence is obtained in all cases studied.Comment: 9 latex pages, 7 postscript figures, To appear in Phys. Rev.
The molecular epidemiology of human immunodeficiency virus type 1 in six cities in Britain and Ireland
The authors sequenced the p17 coding regions of the gag gene from 211 patients infected either through injecting drug use (IDU) or by sexual intercourse between men from six cities in Scotland, N. England, N. Ireland, and the Republic of Ireland. All sequences were of subtype 5. Phylogenetic analysis revealed substantial heterogeneity in the sequences from homosexual men. In contrast, sequence from over 80% of IDUs formed a relatively tight cluster, distinct both from those of published isolates and of the gay men. There was no large-scale clustering of sequences by city in either risk group, although a number of close associations between pairs of individuals were observed. From the known date of the HIV-1 epidemic among IDUs in Edinburgh, the rate of sequence divergence at synonymous sites is estimated to be about 0.8%. On this basis it has been estimated that the date of divergence of the sequences among homosexual men to be about 1975, which may correspond to the origin of the B subtype epidemic
Holographic mesons in various dimensions
We calculate the spectrum of fluctuations of a probe Dk-brane in the
background of N Dp-branes, for k=p,p+2,p+4 and p< 5. The result corresponds to
the mesonic spectrum of a (p+1)-dimensional super-Yang-Mills (SYM) theory
coupled to `dynamical quarks', i.e., fields in the fundamental representation
-- the latter are confined to a defect for k=p and p+2. We find a universal
behaviour where the spectrum is discrete and the mesons are deeply bound. The
mass gap and spectrum are set by the scale M ~ m_q/g_{eff}(m_q), where m_q is
the mass of the fundamental fields and g_{eff}(m_q) is the effective coupling
evaluated at the quark mass, i.e. g_{eff}^2(m_q)=g_{ym}^2 N m_q^{p-3}. We
consider the evolution of the meson spectra into the far infrared of
three-dimensional SYM, where the gravity dual lifts to M-theory. We also argue
that the mass scale appearing in the meson spectra is dictated by holography.Comment: 44 pages, 2 figures; v2: typos corrected, references adde
Information needs and decision-making preferences of older women offered a choice between surgery and primary endocrine therapy for early breast cancer
Objectives: To establish older women's (≥75 years) information preferences regarding 2 breast cancer treatment options: surgery plus adjuvant endocrine therapy versus primary endocrine therapy. To quantify women's preferences for the mode of information presentation and decision-making (DM) style.
Methods: This was a UK multicentre survey of women, ≥75 years, who had been offered a choice between PET and surgery at diagnosis of breast cancer. A questionnaire was developed including 2 validated scales of decision regret and DM preferences.
Results: Questionnaires were sent to 247 women, and 101 were returned (response rate 41%). The median age of participants was 82 (range 75 to 99), with 58 having had surgery and 37 having PET. Practical details about the impact, safety, and efficacy of treatment were of most interest to participants. Of least interest were cosmetic outcomes after surgery. Information provided verbally by doctors and nurses, supported by booklets, was preferred. There was little interest in technology-based sources of information. There was equal preference for a patient- or doctor-centred DM style and lower preference for a shared DM style. The majority (74%) experienced their preferred DM style. Levels of decision regret were low (15.73, scale 0-100).
Conclusions: Women strongly preferred face to face information. Written formats were also helpful but not computer-based resources. Information that was found helpful to women in the DM process was identified. The study demonstrates many women achieved their preferred DM style, with a preference for involvement, and expressed low levels of decision regret
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