11,905 research outputs found
Superfluid-Insulator and Roughening Transitions in Domain Walls
We have performed quantum Monte Carlo simulations to investigate the
superfluid behavior of one- and two-dimensional interfaces separating
checkerboard solid domains. The system is described by the hard-core
Bose-Hubbard Hamiltonian with nearest-neighbor interaction. In accordance with
Ref.1, we find that (i) the interface remains superfluid in a wide range of
interaction strength before it undergoes a superfluid-insulator transition;
(ii) in one dimension, the transition is of the Kosterlitz-Thouless type and is
accompanied by the roughening transition, driven by proliferation of charge 1/2
quasiparticles; (iii) in two dimensions, the transition belongs to the 3D U(1)
universality class and the interface remains smooth. Similar phenomena are
expected for domain walls in quantum antiferromagnets.Comment: 6 pages, 7 figures; references added, typo corrected in fig
A variational problem on Stiefel manifolds
In their paper on discrete analogues of some classical systems such as the
rigid body and the geodesic flow on an ellipsoid, Moser and Veselov introduced
their analysis in the general context of flows on Stiefel manifolds. We
consider here a general class of continuous time, quadratic cost, optimal
control problems on Stiefel manifolds, which in the extreme dimensions again
yield these classical physical geodesic flows. We have already shown that this
optimal control setting gives a new symmetric representation of the rigid body
flow and in this paper we extend this representation to the geodesic flow on
the ellipsoid and the more general Stiefel manifold case. The metric we choose
on the Stiefel manifolds is the same as that used in the symmetric
representation of the rigid body flow and that used by Moser and Veselov. In
the extreme cases of the ellipsoid and the rigid body, the geodesic flows are
known to be integrable. We obtain the extremal flows using both variational and
optimal control approaches and elucidate the structure of the flows on general
Stiefel manifolds.Comment: 30 page
A Few Aspects of Heavy Quark Expansion
Two topics in heavy quark expansion are discussed. The heavy quark potential
in perturbation theory is reviewed in connection to the problem of the heavy
quark mass. The nontrivial reason behind the failure of the "potential
subtracted" mass in higher orders is elucidated. The heavy quark sum rules are
the second subject. The physics behind the new exact sum rules is described and
a simple quantum mechanical derivation is given. The question of saturation of
sum rules is discussed. A comment on the nonstandard possibility which would
affect analysis of BR_sl(B) vs. n_c is made.Comment: 21 pages, LaTeX, 7 eps figures. To appear in the Proceedings of the
UK Phenomenology Workshop on Heavy Flavour and CP Violation, Durham, UK,
17-22 September 200
Topological excitations in 2D spin system with high spin
We construct a class of topological excitations of a mean field in a
two-dimensional spin system represented by a quantum Heisenberg model with high
powers of exchange interaction. The quantum model is associated with a
classical one (the continuous classical analogue) that is based on a
Landau-Lifshitz like equation, and describes large-scale fluctuations of the
mean field. On the other hand, the classical model is a Hamiltonian system on a
coadjoint orbit of the unitary group SU() in the case of spin . We
have found a class of mean field configurations that can be interpreted as
topological excitations, because they have fixed topological charges. Such
excitations change their shapes and grow preserving an energy.Comment: 10 pages, 1 figur
Discrete Nonholonomic LL Systems on Lie Groups
This paper applies the recently developed theory of discrete nonholonomic
mechanics to the study of discrete nonholonomic left-invariant dynamics on Lie
groups. The theory is illustrated with the discrete versions of two classical
nonholonomic systems, the Suslov top and the Chaplygin sleigh. The preservation
of the reduced energy by the discrete flow is observed and the discrete
momentum conservation is discussed.Comment: 32 pages, 13 figure
Three natural mechanical systems on Stiefel varieties
We consider integrable generalizations of the spherical pendulum system to
the Stiefel variety for a certain metric. For the case
of V(n,2) an alternative integrable model of the pendulum is presented.
We also describe a system on the Stiefel variety with a four-degree
potential. The latter has invariant relations on which provide the
complete integrability of the flow reduced on the oriented Grassmannian variety
.Comment: 14 page
Unstable and stable regimes of polariton condensation
Modulational instabilities play a key role in a wide range of nonlinear
optical phenomena, leading e.g. to the formation of spatial and temporal
solitons, rogue waves and chaotic dynamics. Here we experimentally demonstrate
the existence of a modulational instability in condensates of cavity
polaritons, arising from the strong coupling of cavity photons with quantum
well excitons. For this purpose we investigate the spatiotemporal coherence
properties of polariton condensates in GaAs-based microcavities under
continuous-wave pumping. The chaotic behavior of the instability results in a
strongly reduced spatial and temporal coherence and a significantly
inhomogeneous density. Additionally we show how the instability can be tamed by
introducing a periodic potential so that condensation occurs into negative mass
states, leading to largely improved coherence and homogeneity. These results
pave the way to the exploration of long-range order in dissipative quantum
fluids of light within a controlled platform.Comment: 7 pages, 5 figure
Infrared safety of impact factors for colourless particle interactions
We demonstrate, to next-to-leading order accuracy, the cancellation of the
infrared singularities in the impact factors which arise in the QCD description
of high energy processes A + B -> A' + B' of colourless particles. We study the
example where A is a virtual photon in detail, but show that the result is true
in general.Comment: 31 pages latex including 10 figure
Spin Susceptibility of an Ultra-Low Density Two Dimensional Electron System
We determine the spin susceptibility in a two dimensional electron system in
GaAs/AlGaAs over a wide range of low densities from 2cm to
4cm. Our data can be fitted to an equation that describes
the density dependence as well as the polarization dependence of the spin
susceptibility. It can account for the anomalous g-factors reported recently in
GaAs electron and hole systems. The paramagnetic spin susceptibility increases
with decreasing density as expected from theoretical calculations.Comment: 5 pages, 2 eps figures, to appear in PR
Bloch oscillations in an aperiodic one-dimensional potential
We study the dynamics of an electron subjected to a static uniform electric
field within a one-dimensional tight-binding model with a slowly varying
aperiodic potential. The unbiased model is known to support phases of localized
and extended one-electron states separated by two mobility edges. We show that
the electric field promotes sustained Bloch oscillations of an initial Gaussian
wave packet whose amplitude reflects the band width of extended states. The
frequency of these oscillations exhibit unique features, such as a sensitivity
to the initial wave packet position and a multimode structure for weak fields,
originating from the characteristics of the underlying aperiodic potential.Comment: 6 pages, 7 figure
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