329 research outputs found
Normal-superfluid interaction dynamics in a spinor Bose gas
Coherent behavior of spinor Bose-Einstein condensates is studied in the
presence of a significant uncondensed (normal) component. Normal-superfluid
exchange scattering leads to a near-perfect local alignment between the spin
fields of the two components. Through this spin locking, spin-domain formation
in the condensate is vastly accelerated as the spin populations in the
condensate are entrained by large-amplitude spin waves in the normal component.
We present data evincing the normal-superfluid spin dynamics in this regime of
complicated interdependent behavior.Comment: 5 pages, 4 fig
Electric field induced strong localization of electrons on solid hydrogen surface: possible applications to quantum computing
Two-dimensional electron system on the liquid helium surface is one of the
leading candidates for constructing large analog quantum computers (P.M.
Platzman and M.I. Dykman, Science 284, 1967 (1999)). Similar electron systems
on the surfaces of solid hydrogen or solid neon may have some important
advantages with respect to electrons on liquid helium in quantum computing
applications, such as larger state separation , absence of
propagating capillary waves (or ripplons), smaller vapor pressure, etc. As a
result, it may operate at higher temperatures. Surface roughness is the main
hurdle to overcome in building a realistic quantum computer using these states.
Electric field induced strong localization of surface electrons is shown to be
a convenient tool to characterize surface roughness.Comment: 4 pages, 3 figure
Castaing Instability and Precessing Domains in Confined Alkali Gases
We explore analogy between two-component quantum alkali gases and
spin-polarized helium systems. Recent experiments in trapped gases are put into
the frame of the existing theory for Castaing instability in transverse channel
and formation of homogeneous precessing domains in spin-polarized systems.
Analogous effects have already been observed in spin-polarized and
mixtures systems. The threshold effect of the confining
potential on the instability is analyzed. New experimental possibilities for
observation of transverse instability in a trap are discussed.Comment: 6 RevTex pages, no figure
Optical excitations in a non-ideal Bose gas
Optical excitations in a Bose gas are demonstrated to be very sensitive to
many-body effects. At low temperature the momentum relaxation is provided by
momentum exchange collisions, rather than by elastic collisions. A collective
excitation mode forms, which in a Boltzmann gas is manifest in a collision
shift and dramatic narrowing of spectral lines.
In the BEC state, each spectral line splits into two components. The doubling
of the optical excitations results from the physics analogous to that of the
second sound. We present a theory of the line doubling, and calculate the
oscillator strengths and linewidth.Comment: 5 pages, 3 eps figure
Guiding and Trapping Electron Spin Waves in Atomic Hydrogen Gas
We present a high magnetic field study of electron spin waves in atomic
hydrogen gas compressed to high densities of 10^18 cm^-3 at temperatures
ranging from 0.26 to 0.6 K. We observed a variety of spin wave modes caused by
the identical spin rotation effect with strong dependence on the spatial
profile of the polarizing magnetic field. We demonstrate confinement of these
modes in regions of strong magnetic field and manipulate their spatial
distribution by changing the position of the field maximum.Comment: 5 pages, 4 figure
Collective dynamics of internal states in a Bose gas
Theory for the Rabi and internal Josephson effects in an interacting Bose gas
in the cold collision regime is presented. By using microscopic transport
equation for the density matrix the problem is mapped onto a problem of
precession of two coupled classical spins. In the absence of an external
excitation field our results agree with the theory for the density induced
frequency shifts in atomic clocks. In the presence of the external field, the
internal Josephson effect takes place in a condensed Bose gas as well as in a
non-condensed gas. The crossover from Rabi oscillations to the Josephson
oscillations as a function of interaction strength is studied in detail.Comment: 18 pages, 2 figure
Single Particle and Fermi Liquid Properties of He-3/--He-4 Mixtures: A Microscopic Analysis
We calculate microscopically the properties of the dilute He-3 component in a
He-3/--He-4 mixture. These depend on both, the dominant interaction between the
impurity atom and the background, and the Fermi liquid contribution due to the
interaction between the constituents of the He-3 component. We first calculate
the dynamic structure function of a He-3 impurity atom moving in He-3. From
that we obtain the excitation spectrum and the momentum dependent effective
mass. The pole strength of this excitation mode is strongly reduced from the
free particle value in agreement with experiments; part of the strength is
distributed over high frequency excitations. Above k > 1.7^{-1}$ the
motion of the impurity is damped due to the decay into a roton and a low energy
impurity mode. Next we determine the Fermi--Liquid interaction between He-4
atoms and calculate the pressure-- and concentration dependence of the
effective mass, magnetic susceptibility, and the He-3--He-3 scattering phase
shifts. The calculations are based on a dynamic theory that uses, as input,
effective interactions provided by the Fermi hypernetted--chain theory. The
relationship between both theories is discussed. Our theoretical effective
masses agree well with recent measurements by Yorozu et al. (Phys. Rev. B 48,
9660 (1993)) as well as those by R. Simons and R. M. Mueller (Czekoslowak
Journal of Physics Suppl. 46, 201 (1996)), but our analysis suggests a new
extrapolation to the zero-concentration limit. With that effective mass we also
find a good agreement with the measured Landau parameter F_0^a.Comment: 47 pages, 15 figure
Macroscopic quantum tunneling of two-component Bose-Einstein condensates
We show theoretically the existence of a metastable state and the possibility
of decay to the ground state through macroscopic quantum tunneling in
two-component Bose-Einstein condensates with repulsive interactions. Numerical
analysis of the coupled Gross-Pitaevskii equations clarifies the metastable
states whose configuration preserves or breaks the symmetry of the trapping
potential, depending on the interspecies interaction and the particle number.
We calculate the tunneling decay rate of the metastable state by using the
collective coordinate method under the WKB approximation. Then the height of
the energy barrier is estimated by the saddle point solution. It is found that
macroscopic quantum tunneling is observable in a wide range of particle
numbers. Macroscopic quantum coherence between two distinct states is
discussed; this might give an additional coherent property of two-component
Bose condensed systems. Thermal effects on the decay rate are estimated.Comment: 11 pages, 10 figures, revtex
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