9,020 research outputs found
Hydrodynamics of compressible superfluids in confined geometries
We present a study of the hydrodynamics of compressible superfluids in
confined geometries. We use a perturbative procedure in terms of the
dimensionless expansion parameter where is the typical speed of
the flow and the speed of sound. A zero value of this parameter
corresponds to the incompressible limit. We apply the procedure to two specific
problems: the case of a trapped superfluid with a gaussian profile of the local
density, and that of a superfluid confined in a rotating obstructed cylinder.
We find that the corrections due to finite compressibility which are, as
expected, negligible for liquid He, are important but amenable to the
perturbative treatment for typical ultracold atomic systems.Comment: 17 pages, including 7 figures. To appear in Journ. Phys.
Superfluid Field response to Edge dislocation motion
We study the dynamic response of a superfluid field to a moving edge
dislocation line to which the field is minimally coupled. We use a dissipative
Gross-Pitaevskii equation, and determine the initial conditions by solving the
equilibrium version of the model. We consider the subsequent time evolution of
the field for both glide and climb dislocation motion and analyze the results
for a range of values of the constant speed of the moving dislocation. We
find that the type of motion of the dislocation line is very important in
determining the time evolution of the superfluid field distribution associated
with it. Climb motion of the dislocation line induces increasing asymmetry, as
function of time, in the field profile, with part of the probability being, as
it were, left behind. On the other hand, glide motion has no effect on the
symmetry properties of the superfluid field distribution. Damping of the
superfluid field due to excitations associated with the moving dislocation line
occurs in both cases.Comment: 10 pages 7 figures. To appear in Phys. Rev
Dislocation Mobility and Anomalous Shear Modulus Effect in He Crystals
We calculate the dislocation glide mobility in solid He within a model
that assumes the existence of a superfluid field associated with dislocation
lines. Prompted by the results of this mobility calculation, we study within
this model the role that such a superfluid field may play in the motion of the
dislocation line when a stress is applied to the crystal. To do this, we relate
the damping of dislocation motion, calculated in the presence of the assumed
superfluid field, to the shear modulus of the crystal. As the temperature
increases, we find that a sharp drop in the shear modulus will occur at the
temperature where the superfluid field disappears. We compare the drop in shear
modulus of the crystal arising from the temperature dependence of the damping
contribution due to the superfluid field, to the experimental observation of
the same phenomena in solid He and find quantitative agreement. Our results
indicate that such a superfluid field plays an important role in dislocation
pinning in a clean solid He at low temperatures and in this regime may
provide an alternative source for the unusual elastic phenomena observed in
solid He.Comment: 17 pages, 2 figures. To appear in JLT
Ising Model on a random network with annealed or quenched disorder
We study the equilibrium properties of an Ising model on a disordered random
network where the disorder can be quenched or annealed. The network consists of
four-fold coordinated sites connected via variable length one-dimensional
chains. Our emphasis is on nonuniversal properties and we consider the
transition temperature and other equilibrium thermodynamic properties,
including those associated with one dimensional fluctuations arising from the
chains. We use analytic methods in the annealed case, and a Monte Carlo
simulation for the quenched disorder. Our objective is to study the difference
between quenched and annealed results with a broad random distribution of
interaction parameters. The former represents a situation where the time scale
associated with the randomness is very long and the corresponding degrees of
freedom can be viewed as frozen, while the annealed case models the situation
where this is not so. We find that the transition temperature and the entropy
associated with one dimensional fluctuations are always higher for quenched
disorder than in the annealed case. These differences increase with the
strength of the disorder up to a saturating value. We discuss our results in
connection to physical systems where a broad distribution of interaction
strengths is present.Comment: 11 pages including 9 figures. To appear in Phys. Rev.
Multidimensional persistence behaviour in an Ising system
We consider a periodic Ising chain with nearest-neighbour and -th
neighbour interaction and quench it from infinite temperature to zero
temperature. The persistence probability , measured as the probability
that a spin remains unflipped upto time , is studied by computer simulation
for suitable values of . We observe that as time progresses, first
decays as (-the {\em first} regime), then the curve has a
small slope (in log-log scale) for some time (-the {\em second} regime) and at
last it decays nearly as (-the {\em third} regime). We argue that in
the first regime, the persistence behaviour is the usual one for a
two-dimensional system, in the second regime it is like that of a
non-interacting (`zero-dimensional') system and in the third regime the
persistence behaviour is like that of a one dimensional Ising model. We also
provide explanations for such behaviour.Comment: 6 pages, 12 figure
Heterogeneity and Vulnerability in the Urban Informal Economy
The focus of this paper is on the low-value urban informal economy in sub-Saharan Africa. The paper presents a case study from Uganda, with the focus on secondary cities where much of the urban population growth is expected in the next decade. There is also good evidence that informality will persist within both dynamic and in slow urban economies in sub-Saharan Africa. Analysis of the results from fieldwork is presented in support of the basic argument that, as the informal economy is likely to persist there is need for a more conducive policy approach to ensure its positive contribution to the urban economic development; seen as a mainstay of Africa’s future economic development. Currently, the informal economy is considered ephemeral, with little institutional engagement, with the focus on restrictive
measures.
The paper explores the range of theoretical perspectives on the urban informal economy leading to its re-conceptualizing as part of a continuum of activities in which both formal and informal elements are interlinked. Given the need to improve productivity of activities of different constituent groups along the continuum, the aim of this paper is to develop a conceptual framework that would enable the systematic analyses and identification of the internal diversities within the informal economy. It could be used in future policy research to identify diagnostic groups for entry points. The findings from the fieldwork carried out in the cities of Mbale and Mbarara, show that there are several viable options for sustainable livelihoods, particularly in a dynamic urban center. However, while women dominate in numbers, men remain the main actors. This calls for an inclusive, gender-mainstreamed, pro-poor development policy to complement an enterprise-based approach
Magnetic properties of Mn-doped Ge46 and Ba8Ge46 clathrates
We present a detailed study of the magnetic properties of unique cluster
assembled solids namely Mn doped Ge46 and Ba8Ge46 clathrates using density
functional theory. We find that ferromagnetic (FM) ground states may be
realized in both the compounds when doped with Mn. In Mn2Ge44, ferromagnetism
is driven by hybridization induced negative exchange splitting, a generic
mechanism operating in many diluted magnetic semiconductors. However, for
Mn-doped Ba8Ge46 clathrates incorporation of conduction electrons via Ba
encapsulation results in RKKY-like magnetic interactions between the Mn ions.
We show that our results are consistent with the major experimental
observations for this system.Comment: 6 pages, 4 figure
Validity of the linear coupling approximation in heavy-ion fusion reactions at sub barrier energies
The role of higher order coupling of surface vibrations to the relative
motion in heavy-ion fusion reactions at near-barrier energies is investigated.
The coupled channels equations are solved to all orders, and also in the linear
and the quadratic coupling approximations. Taking Ni + Zr
reactions as examples, it is shown that all order couplings lead to
considerably improved agreement with the experimentally measured fusion cross
sections and average angular momenta of the compound nucleus for such heavy
nearly symmetric systems. The importance of higher order coupling is also
examined for asymmetric systems like O + Cd, Sm, for
which previous calculations of the fusion cross section seemed to indicate that
the linear coupling approximation was adequate. It is shown that the shape of
the barrier distributions and the energy dependence of the average angular
momentum can change significantly when the higher order couplings are included,
even for systems where measured fusion cross sections may seem to be well
reproduced by the linear coupling approximation.Comment: Latex file, 15 pages, 6 figure
Persistence in nonequilibrium surface growth
Persistence probabilities of the interface height in (1+1)- and
(2+1)-dimensional atomistic, solid-on-solid, stochastic models of surface
growth are studied using kinetic Monte Carlo simulations, with emphasis on
models that belong to the molecular beam epitaxy (MBE) universality class. Both
the initial transient and the long-time steady-state regimes are investigated.
We show that for growth models in the MBE universality class, the nonlinearity
of the underlying dynamical equation is clearly reflected in the difference
between the measured values of the positive and negative persistence exponents
in both transient and steady-state regimes. For the MBE universality class, the
positive and negative persistence exponents in the steady-state are found to be
and ,
respectively, in (1+1) dimensions, and and
, respectively, in (2+1) dimensions. The noise
reduction technique is applied on some of the (1+1)-dimensional models in order
to obtain accurate values of the persistence exponents. We show analytically
that a relation between the steady-state persistence exponent and the dynamic
growth exponent, found earlier to be valid for linear models, should be
satisfied by the smaller of the two steady-state persistence exponents in the
nonlinear models. Our numerical results for the persistence exponents are
consistent with this prediction. We also find that the steady-state persistence
exponents can be obtained from simulations over times that are much shorter
than that required for the interface to reach the steady state. The dependence
of the persistence probability on the system size and the sampling time is
shown to be described by a simple scaling form.Comment: 28 pages, 16 figure
Dimension-adaptive bounds on compressive FLD Classification
Efficient dimensionality reduction by random projections (RP) gains popularity, hence the learning guarantees achievable in RP spaces are of great interest. In finite dimensional setting, it has been shown for the compressive Fisher Linear Discriminant (FLD) classifier that forgood generalisation the required target dimension grows only as the log of the number of classes and is not adversely affected by the number of projected data points. However these bounds depend on the dimensionality d of the original data space. In this paper we give further guarantees that remove d from the bounds under certain conditions of regularity on the data density structure. In particular, if the data density does not fill the ambient space then the error of compressive FLD is independent of the ambient dimension and depends only on a notion of ‘intrinsic dimension'
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