11,835 research outputs found
A renormalized Gross-Pitaevskii Theory and vortices in a strongly interacting Bose gas
We consider a strongly interacting Bose-Einstein condensate in a spherical
harmonic trap. The system is treated by applying a slave-boson representation
for hard-core bosons. A renormalized Gross-Pitaevskii theory is derived for the
condensate wave function that describes the dilute regime (like the
conventional Gross-Pitaevskii theory) as well as the dense regime. We calculate
the condensate density of a rotating condensate for both the vortex-free
condensate and the condensate with a single vortex and determine the critical
angular velocity for the formation of a stable vortex in a rotating trap.Comment: 13 pages, 5 figures; revision and extension, figure 2 adde
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Stochastic modelling of the effects of interdependencies between critical infrastructure
An approach to Quantitative Interdependency Analysis, in the context of Large Complex Critical Infrastructures, is presented in this paper. A Discrete state–space, Continuous–time, Stochastic Process models the operation of critical infrastructure, taking interdependencies into account. Of primary interest are the implications of both model detail (that is, level of model abstraction) and model parameterisation for the study of dependencies. Both of these factors are observed to affect the distribution of cascade–sizes within and across infrastructure
Thermodynamic Properties of Kagome Lattice in ZnCu_3(OH)_6Cl_2 Herbertsmithite
Strongly correlated Fermi systems are among the most intriguing and
fundamental systems in physics, whose realization in some compounds is still to
be discovered. We show that herbertsmithite ZnCu_3(OH)_6Cl_2 can be viewed as a
strongly correlated Fermi system whose low temperature thermodynamic in
magnetic fields is defined by a quantum critical spin liquid. Our calculations
of its thermodynamic properties are in good agreement with recent experimental
facts and allow us to reveal their scaling behavior which strongly resembles
that observed in HF metals and 2D 3He.Comment: 4 pages, 6 figure
Normal and Anomalous Averages for Systems with Bose-Einstein Condensate
The comparative behaviour of normal and anomalous averages as functions of
momentum or energy, at different temperatures, is analysed for systems with
Bose-Einstein condensate. Three qualitatively distinct temperature regions are
revealed: The critical region, where the absolute value of the anomalous
average, for the main energy range, is much smaller than the normal average.
The region of intermediate temperatures, where the absolute values of the
anomalous and normal averages are of the same order. And the region of low
temperatures, where the absolute value of the anomalous average, for
practically all energies, becomes much larger than the normal average. This
shows the importance of the anomalous averages for the intermediate and,
especially, for low temperatures, where these anomalous averages cannot be
neglected.Comment: Latex file, 17 pages, 6 figure
Multidimensional Worldline Instantons
We extend the worldline instanton technique to compute the vacuum pair
production rate for spatially inhomogeneous electric background fields, with
the spatial inhomogeneity being genuinely two or three dimensional, both for
the magnitude and direction of the electric field. Other techniques, such as
WKB, have not been applied to such higher dimensional problems. Our method
exploits the instanton dominance of the worldline path integral expression for
the effective action.Comment: 22 pages, 13 figure
The Stokes Phenomenon and Schwinger Vacuum Pair Production in Time-Dependent Laser Pulses
Particle production due to external fields (electric, chromo-electric or
gravitational) requires evolving an initial state through an interaction with a
time-dependent background, with the rate being computed from a Bogoliubov
transformation between the in and out vacua. When the background fields have
temporal profiles with sub-structure, a semiclassical analysis of this problem
confronts the full subtlety of the Stokes phenomenon: WKB solutions are only
local, while the production rate requires global information. Incorporating the
Stokes phenomenon, we give a simple quantitative explanation of the recently
computed [Phys. Rev. Lett. 102, 150404 (2009)] oscillatory momentum spectrum of
e+e- pairs produced from vacuum subjected to a time-dependent electric field
with sub-cycle laser pulse structure. This approach also explains naturally why
for spinor and scalar QED these oscillations are out of phase.Comment: 5 pages, 4 figs.; v2 sign typo corrected, version to appear in PR
Observation of a 2D Bose-gas: from thermal to quasi-condensate to superfluid
We present experimental results on a Bose gas in a quasi-2D geometry near the
Berezinskii, Kosterlitz and Thouless (BKT) transition temperature. By measuring
the density profile, \textit{in situ} and after time of flight, and the
coherence length, we identify different states of the gas. In particular, we
observe that the gas develops a bimodal distribution without long range order.
In this state, the gas presents a longer coherence length than the thermal
cloud; it is quasi-condensed but is not superfluid. Experimental evidence
indicates that we observe the superfluid transition (BKT transition).Comment: 5 pages, 6 figure
Non-Abelian Vortices, Super-Yang-Mills Theory and Spin(7)-Instantons
We consider a complex vector bundle E endowed with a connection A over the
eight-dimensional manifold R^2 x G/H, where G/H = SU(3)/U(1)xU(1) is a
homogeneous space provided with a never integrable almost complex structure and
a family of SU(3)-structures. We establish an equivalence between G-invariant
solutions A of the Spin(7)-instanton equations on R^2 x G/H and general
solutions of non-Abelian coupled vortex equations on R^2. These vortices are
BPS solitons in a d=4 gauge theory obtained from N=1 supersymmetric Yang-Mills
theory in ten dimensions compactified on the coset space G/H with an
SU(3)-structure. The novelty of the obtained vortex equations lies in the fact
that Higgs fields, defining morphisms of vector bundles over R^2, are not
holomorphic in the generic case. Finally, we introduce BPS vortex equations in
N=4 super Yang-Mills theory and show that they have the same feature.Comment: 14 pages; v2: typos fixed, published versio
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