978 research outputs found
Critical exponents of the driven elastic string in a disordered medium
We analyze the harmonic elastic string driven through a continuous random
potential above the depinning threshold. The velocity exponent beta = 0.33(2)
is calculated. We observe a crossover in the roughness exponent zeta from the
critical value 1.26 to the asymptotic (large force) value of 0.5. We calculate
directly the velocity correlation function and the corresponding correlation
length exponent nu = 1.29(5), which obeys the scaling relation nu = 1/(2-zeta),
and agrees with the finite-size-scaling exponent of fluctuations in the
critical force. The velocity correlation function is non-universal at short
distances.Comment: 4 pages, 3 figures. corrected references and typo
Knowledge and practices on the prevention and management of diarrhea in children under-2 years among women dwelling in urban slums of Karachi, Pakistan
Diarrhea is the second leading cause of death especially among children. The age-proportionate mortality of diarrheal disease in infants under 2 years is 72%, among children under 5 years of age. Children living in urban slums are more prone to develop diarrhea. Although the disease can be prevented by many simple cost-effective interventions, i.e. proper sanitation and hygiene, appropriate feeding, and timely vaccination, poverty and lack of basic life amenities often potentiate diarrhea mortality. Gadap town is the largest town of Karachi with a deprived health system. This study aims to assess pediatric diarrhea prevalence and related knowledge-practice gaps in the slums of Gadap Town, Karachi, Pakistan
Quantum Dimer Model on the triangular lattice: Semiclassical and variational approaches to vison dispersion and condensation
After reviewing the concept of vison excitations in Z_2 dimer liquids, we
study the liquid-crystal transition of the Quantum Dimer Model on the
triangular lattice by means of a semiclassical spin-wave approximation to the
dispersion of visons in the context of a "soft-dimer" version of the model.
This approach captures some important qualitative features of the transition:
continuous nature of the transition, linear dispersion at the critical point,
and \sqrt{12}x\sqrt{12} symmetry-breaking pattern. In a second part, we present
a variational calculation of the vison dispersion relation at the RK point
which reproduces the qualitative shape of the dispersion relation and the order
of magnitude of the gap. This approach provides a simple but reliable
approximation of the vison wave functions at the RK point.Comment: 12 pages, 10 figures. v2: minor changes, to appear in Phys. Rev.
Universal Scaling of the Conductivity at the Superfluid-Insulator Phase Transition
The scaling of the conductivity at the superfluid-insulator quantum phase
transition in two dimensions is studied by numerical simulations of the
Bose-Hubbard model. In contrast to previous studies, we focus on properties of
this model in the experimentally relevant thermodynamic limit at finite
temperature T. We find clear evidence for deviations from w_k-scaling of the
conductivity towards w_k/T-scaling at low Matsubara frequencies w_k. By careful
analytic continuation using Pade approximants we show that this behavior
carries over to the real frequency axis where the conductivity scales with w/T
at small frequencies and low temperatures. We estimate the universal dc
conductivity to be 0.45(5)Q^2/h, distinct from previous estimates in the T=0,
w/T >> 1 limit.Comment: Accepted for publication in PR
Adding a Myers Term to the IIB Matrix Model
We show that Yang-Mills matrix integrals remain convergent when a Myers term
is added, and stay in the same topological class as the original model. It is
possible to add a supersymmetric Myers term and this leaves the partition
function invariant.Comment: 8 pages, v2 2 refs adde
Polyakov Lines in Yang-Mills Matrix Models
We study the Polyakov line in Yang-Mills matrix models, which include the
IKKT model of IIB string theory. For the gauge group SU(2) we give the exact
formulae in the form of integral representations which are convenient for
finding the asymptotic behaviour. For the SU(N) bosonic models we prove upper
bounds which decay as a power law at large momentum p. We argue that these
capture the full asymptotic behaviour. We also indicate how to extend the
results to some correlation functions of Polyakov lines.Comment: 19 pages, v2 typos corrected, v3 ref adde
The Generic, Incommensurate Transition in the two-dimensional Boson Hubbard Model
The generic transition in the boson Hubbard model, occurring at an
incommensurate chemical potential, is studied in the link-current
representation using the recently developed directed geometrical worm
algorithm. We find clear evidence for a multi-peak structure in the energy
distribution for finite lattices, usually indicative of a first order phase
transition. However, this multi-peak structure is shown to disappear in the
thermodynamic limit revealing that the true phase transition is second order.
These findings cast doubts over the conclusion drawn in a number of previous
works considering the relevance of disorder at this transition.Comment: 13 pages, 10 figure
Bosons Confined in Optical Lattices: the Numerical Renormalization Group revisited
A Bose-Hubbard model, describing bosons in a harmonic trap with a
superimposed optical lattice, is studied using a fast and accurate variational
technique (MF+NRG): the Gutzwiller mean-field (MF) ansatz is combined with a
Numerical Renormalization Group (NRG) procedure in order to improve on both.
Results are presented for one, two and three dimensions, with particular
attention to the experimentally accessible momentum distribution and possible
satellite peaks in this distribution. In one dimension, a comparison is made
with exact results obtained using Stochastich Series Expansion.Comment: 10 pages, 15 figure
Monte Carlo study of fermionic trions in a square lattice with harmonic confinement
We investigate the strong-coupling limit of a three-component Fermi mixture
in an optical lattice with attractive interactions. In this limit bound states
(trions) of the three components are formed. We derive an effective Hamiltonian
for these composite fermions and show that it is asymptotically equivalent to
an antiferromagnetic Ising model. By using Monte-Carlo simulations, we
investigate the spatial arrangement of the trions and the formation of a
trionic density wave (CDW), both in a homogeneous lattice and in the presence
of an additional harmonic confinement. Depending on the strength of the
confinement and on the temperature, we found several scenarios for the trionic
distribution, including coexistence of disordered trions with CDW and band
insulator phases. Our results show that, due to a proximity effect, staggered
density modulations are induced in regions of the trap where they would not
otherwise be present according to the local density approximation.Comment: 10 pages, 8 figure
Ultracold Bosonic Atoms in Disordered Optical Superlattices
The influence of disorder on ultracold atomic Bose gases in quasiperiodic
optical lattices is discussed in the framework of the one-dimensional
Bose-Hubbard model. It is shown that simple periodic modulations of the well
depths generate a rich phase diagram consisting of superfluid, Mott insulator,
Bose-glass and Anderson localized phases. The detailed evolution of mean
occupation numbers and number fluctuations as function of modulation amplitude
and interaction strength is discussed. Finally, the signatures of the different
phases, especially of the Bose-glass phase, in matter-wave interference
experiments are investigated.Comment: 4 pages, 4 figures, using REVTEX
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