9,624 research outputs found
Glimmers of a Quantum KAM Theorem: Insights from Quantum Quenches in One Dimensional Bose Gases
Real-time dynamics in a quantum many-body system are inherently complicated
and hence difficult to predict. There are, however, a special set of systems
where these dynamics are theoretically tractable: integrable models. Such
models possess non-trivial conserved quantities beyond energy and momentum.
These quantities are believed to control dynamics and thermalization in low
dimensional atomic gases as well as in quantum spin chains. But what happens
when the special symmetries leading to the existence of the extra conserved
quantities are broken? Is there any memory of the quantities if the breaking is
weak? Here, in the presence of weak integrability breaking, we show that it is
possible to construct residual quasi-conserved quantities, so providing a
quantum analog to the KAM theorem and its attendant Nekhoreshev estimates. We
demonstrate this construction explicitly in the context of quantum quenches in
one-dimensional Bose gases and argue that these quasi-conserved quantities can
be probed experimentally.Comment: 21 pages with appendices; 13 figures; version accepted by PR
Modal analysis of gravitational instabilities in nearly Keplerian, counter-rotating collisionless discs
We present a modal analysis of instabilities of counter-rotating,
self-gravitating collisionless stellar discs, using the recently introduced
modified WKB formulation of spiral density waves for collisionless systems
(Gulati \& Saini). The discs are assumed to be axisymmetric and in coplanar
orbits around a massive object at the common center of the discs. The mass in
both discs is assumed to be much smaller than the mass of the central object.
For each disc, the disc particles are assumed to be in near circular orbits.
The two discs are coupled to each other gravitationally. The perturbed dynamics
of the discs evolves on the order of the precession time scale of the discs,
which is much longer than the Keplerian time scale. We present results for the
azimuthal wave number and , for the full range of disc mass ratio
between the prograde and retrograde discs. The eigenspectra are in general
complex, therefore all eigenmodes are unstable. Eigenfunctions are radially
more compact for as compared to . Pattern speed of eigenmodes is
always prograde with respect to the more massive disc. The growth rate of
unstable modes increases with increasing mass fraction in the retrograde disc,
and decreases with ; therefore instability is likely to play the
dominant role in the dynamics of such systems.Comment: 24 pages, 8 figures, 1 tabl
Phase Diagram for Magnon Condensate in Yttrium Iron Garnet Film
Recently, magnons, which are quasiparticles describing the collective motion
of spins, were found to undergo Bose-Einstein condensation (BEC) at room
temperature in films of Yttrium Iron Garnet (YIG). Unlike other quasiparticle
BEC systems, this system has a spectrum with two degenerate minima, which makes
it possible for the system to have two condensates in momentum space. Recent
Brillouin Light scattering studies for a microwave-pumped YIG film of thickness
d=5 m and field H=1 kOe find a low-contrast interference pattern at the
characteristic wavevector of the magnon energy minimum. In this report, we
show that this modulation pattern can be quantitatively explained as due to
non-symmetric but coherent Bose-Einstein condensation of magnons into the two
energy minima. Our theory predicts a transition from a high-contrast symmetric
phase to a low-contrast non-symmetric phase on varying the and , and a
new type of collective oscillations.Comment: 6 figures. Accepted by Nature Scientific Report
- …