51,532 research outputs found
Green-Function-Based Monte Carlo Method for Classical Fields Coupled to Fermions
Microscopic models of classical degrees of freedom coupled to non-interacting
fermions occur in many different contexts. Prominent examples from solid state
physics are descriptions of colossal magnetoresistance manganites and diluted
magnetic semiconductors, or auxiliary field methods for correlated electron
systems. Monte Carlo simulations are vital for an understanding of such
systems, but notorious for requiring the solution of the fermion problem with
each change in the classical field configuration. We present an efficient,
truncation-free O(N) method on the basis of Chebyshev expanded local Green
functions, which allows us to simulate systems of unprecedented size N.Comment: 4 pages, 3 figure
Dynamo Effects Near The Transition from Solar to Anti-Solar Differential Rotation
Numerical MHD simulations play increasingly important role for understanding
mechanisms of stellar magnetism. We present simulations of convection and
dynamos in density-stratified rotating spherical fluid shells. We employ a new
3D simulation code for the solution of a physically consistent anelastic model
of the process with a minimum number of parameters. The reported dynamo
simulations extend into a "buoyancy-dominated" regime where the buoyancy
forcing is dominant while the Coriolis force is no longer balanced by pressure
gradients and strong anti-solar differential rotation develops as a result. We
find that the self-generated magnetic fields, despite being relatively weak,
are able to reverse the direction of differential rotation from anti-solar to
solar-like. We also find that convection flows in this regime are significantly
stronger in the polar regions than in the equatorial region, leading to
non-oscillatory dipole-dominated dynamo solutions, and to concentration of
magnetic field in the polar regions. We observe that convection has different
morphology in the inner and at the outer part of the convection zone
simultaneously such that organized geostrophic convection columns are hidden
below a near-surface layer of well-mixed highly-chaotic convection. While we
focus the attention on the buoyancy-dominated regime, we also demonstrate that
conical differential rotation profiles and persistent regular dynamo
oscillations can be obtained in the parameter space of the rotation-dominated
regime even within this minimal model.Comment: Published in the Astrophysical Journa
Aspect ratio analysis for ground states of bosons in anisotropic traps
Characteristics of the initial condensate in the recent experiment on
Bose-Einstein condensation (BEC) of Rb atoms in an anisotropic
magnetic trap is discussed. Given the aspect ratio , the quality of BEC is
estimated. A simple analytical Ansatz for the initial condensate wave function
is proposed as a function of the aspect ratio which, in contrast to the
Baym-Pethick trial wave function, reproduces both the weak and the strong
intaraction limits and which is in better agreement with numerical results than
the latter.Comment: 12 pages, latex, 3 figures added, minor corrections; to appear in J.
Res. Nat. Inst. of Standards and Technolog
The Consumption of Reference Resources
Under the operational restriction of the U(1)-superselection rule, states
that contain coherences between eigenstates of particle number constitute a
resource. Such resources can be used to facilitate operations upon systems that
otherwise cannot be performed. However, the process of doing this consumes
reference resources. We show this explicitly for an example of a unitary
operation that is forbidden by the U(1)-superselection rule.Comment: 4 pages 6x9 page format, 2 figure
Anisotropy induced Feshbach resonances in a quantum dipolar gas of magnetic atoms
We explore the anisotropic nature of Feshbach resonances in the collision
between ultracold magnetic submerged-shell dysprosium atoms, which can only
occur due to couplings to rotating bound states. This is in contrast to
well-studied alkali-metal atom collisions, where most Feshbach resonances are
hyperfine induced and due to rotation-less bound states. Our novel
first-principle coupled-channel calculation of the collisions between
open-4f-shell spin-polarized bosonic dysprosium reveals a striking correlation
between the anisotropy due to magnetic dipole-dipole and electrostatic
interactions and the Feshbach spectrum as a function of an external magnetic
field. Over a 20 mT magnetic field range we predict about a dozen Feshbach
resonances and show that the resonance locations are exquisitely sensitive to
the dysprosium isotope.Comment: 5 pages, 4 figure
Getting a kick out of the stellar disk(s) in the galactic center
Recent observations of the Galactic center revealed a nuclear disk of young
OB stars, in addition to many similar outlying stars with higher eccentricities
and/or high inclinations relative to the disk (some of them possibly belonging
to a second disk). Binaries in such nuclear disks, if they exist in
non-negligible fractions, could have a major role in the evolution of the disks
through binary heating of this stellar system. We suggest that interactions
with/in binaries may explain some (or all) of the observed outlying young stars
in the Galactic center. Such stars could have been formed in a disk, and later
on kicked out from it through binary related interactions, similar to ejection
of high velocity runaway OB stars in young clusters throughout the galaxy.Comment: 2 pages, 2 figs. To be published in the proceedings of the IAU 246
symposium on "Dynamical evolution of dense stellar systems
Pattern theorems, ratio limit theorems and Gumbel maximal clusters for random fields
We study occurrences of patterns on clusters of size n in random fields on
Z^d. We prove that for a given pattern, there is a constant a>0 such that the
probability that this pattern occurs at most an times on a cluster of size n is
exponentially small. Moreover, for random fields obeying a certain Markov
property, we show that the ratio between the numbers of occurrences of two
distinct patterns on a cluster is concentrated around a constant value. This
leads to an elegant and simple proof of the ratio limit theorem for these
random fields, which states that the ratio of the probabilities that the
cluster of the origin has sizes n+1 and n converges as n tends to infinity.
Implications for the maximal cluster in a finite box are discussed.Comment: 23 pages, 2 figure
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