215 research outputs found
Superfluidity and excitations at unitarity
We present lattice results for spin-1/2 fermions at unitarity, where the
effective range of the interaction is zero and the scattering length is
infinite. We measure the spatial coherence of difermion pairs for a system of
6, 10, 14, 18, 22, 26 particles with equal numbers of up and down spins in a
periodic cube. Using Euclidean time projection, we analyze ground state
properties and transient behavior due to low-energy excitations. At
asymptotically large values of t we see long-range order consistent with
spontaneously broken U(1) fermion-number symmetry and a superfluid ground
state. At intermediate times we see exponential decay in the t-dependent signal
due to an unknown low-energy excitation. We probe this low-energy excitation
further by calculating two-particle correlation functions. We find that the
excitation has the properties of a chain of particles extending across the
periodic lattice.Comment: 40 pages, 19 figures, revised version includes new data on
two-particle density correlation
Looking back at superfluid helium
A few years after the discovery of Bose Einstein condensation in several
gases, it is interesting to look back at some properties of superfluid helium.
After a short historical review, I comment shortly on boiling and evaporation,
then on the role of rotons and vortices in the existence of a critical velocity
in superfluid helium. I finally discuss the existence of a condensate in a
liquid with strong interactions, and the pressure variation of its superfluid
transition temperature.Comment: Conference "Bose Einstein Condensation", Institut henri Poincare,
Paris, 29 march 200
Image method in the calculation of the van der Waals force between an atom and a conducting surface
Initially, we make a detailed historical survey of van der Waals forces,
collecting the main references on the subject. Then, we review a method
recently proposed by Eberlein and Zietal to compute the dispersion van der
Waals interaction between a neutral but polarizable atom and a perfectly
conducting surface of arbitrary shape. This method has the advantage of
relating the quantum problem to a corresponding classical one in electrostatics
so that all one needs is to compute an appropriate Green function. We show how
the image method of electrostatics can be conveniently used together with the
Eberlein and Zietal mehtod (when the problem admits an image solution). We then
illustrate this method in a couple of simple but important cases, including the
atom-sphere system. Particularly, in our last example, we present an original
result, namely, the van der Waals force between an atom and a boss hat made of
a grounded conducting material.Comment: This is a pedagogical and introductory paper on van der Waals forces
between an atom and a conducting surfac
A search for stable strange quark matter nuggets in helium
A search for stable strange quark nuggets has been conducted in helium and
argon using a high sensitivity mass spectrometer. The search was guided by a
mass formula for strange quark nuggets which suggested that stable strange
helium might exist at a mass around 65 u. The chemical similarity of such
``strangelets'' to noble gas atoms and the gravitational unboundedness of
normal helium result in a large enhancement in the sensitivity of such a
search. An abundance limit of no more than strangelets per
normal nucleus is imposed by our search over a mass region from 42 to 82 u,
with much more stringent limits at most (non-integer) masses.Comment: 11 pages RevTeX, Accepted for publication in Physics Letters B. 2
updated references added. Air abundance to cosmic abundance ratios now
reflect updated references. No change in results or figures. Also see
ftp://www-physics.mps.ohio-state.edu/pub/nucex/sq
Condensate and superfluid fractions for varying interactions and temperature
A system with Bose-Einstein condensate is considered in the frame of the
self-consistent mean-field approximation, which is conserving, gapless, and
applicable for arbitrary interaction strengths and temperatures. The main
attention is paid to the thorough analysis of the condensate and superfluid
fractions in a wide region of interaction strengths and for all temperatures
between zero and the critical point T_c. The normal and anomalous averages are
shown to be of the same order for almost all interactions and temperatures,
except the close vicinity of T_c. But even in the vicinity of the critical
temperature, the anomalous average cannot be neglected, since only in the
presence of the latter the phase transition at T_c becomes of second order, as
it should be. Increasing temperature influences the condensate and superfluid
fractions in a similar way, by diminishing them. But their behavior with
respect to the interaction strength is very different. For all temperatures,
the superfluid fraction is larger than the condensate fraction. These coincide
only at T_c or under zero interactions. For asymptotically strong interactions,
the condensate is almost completely depleted, even at low temperatures, while
the superfluid fraction can be close to one.Comment: Latex file, 22 pages, 5 figure
Magnetization of ferrofluids with dipolar interactions - a Born--Mayer expansion
For ferrofluids that are described by a system of hard spheres interacting
via dipolar forces we evaluate the magnetization as a function of the internal
magnetic field with a Born--Mayer technique and an expansion in the dipolar
coupling strength. Two different approximations are presented for the
magnetization considering different contributions to a series expansion in
terms of the volume fraction of the particles and the dipolar coupling
strength.Comment: 19 pages, 11 figures submitted to PR
Cosmic-ray strangelets in the Earth's atmosphere
If strange quark matter is stable in small lumps, we expect to find such
lumps, called ``strangelets'', on Earth due to a steady flux in cosmic rays.
Following recent astrophysical models, we predict the strangelet flux at the
top of the atmosphere, and trace the strangelets' behavior in atmospheric
chemistry and circulation. We show that several strangelet species may have
large abundances in the atmosphere; that they should respond favorably to
laboratory-scale preconcentration techniques; and that they present promising
targets for mass spectroscopy experiments.Comment: 28 pages, 4 figures, revtex
Extended Ensemble Theory, Spontaneous Symmetry Breaking, and Phase Transitions
In this paper, we suppose a possible extension of Gibbs ensemble theory so
that it can provide a reasonable description to phase transitions and
spontaneous symmetry breaking. The extension is founded on three hypotheses,
and can be regarded as a microscopic edition of the Landau phenomenological
theory of phase transitions. Within its framework, the state of a system is
determined by the evolution of order parameter with temperature according to
such a principle that the entropy of the system will reach its minimum in this
state. The evolution of order parameter can cause change in representation of
the system Hamiltonian, different phases will realize different
representations. Physically, it turns out that phase transition originates from
the automatic interference among matter waves as temperature is cooled down.
Typical quantum many-body systems are studied with this extended ensemble
theory. In particular, the theory predicts that the specific heat Cp of liquid
He II will vanish linearly as T tends to zero, which is anticipating
experimental verifications.Comment: 41 pages, 8 figures, content change
Drastic enhancement of magnon thermal conductivity in the Bose-Einstein condensed state of TlCuCl
We have measured the thermal conductivity of a TlCuCl single crystal in
magnetic fields up to 14 T. It has been found that the temperature dependence
of the thermal conductivity exhibits a sharp peak at 4 K in zero field, which
is suppressed by the application of magnetic fields up to 7 T. The peak is
concluded to be attributable to the enhancement of the thermal conductivity due
to phonons because of the formation of a spin-gap state. In high magnetic
fields above 7 T, on the other hand, another sharp peak appears around 4 K and
this is enhanced with increasing magnetic field. This peak is regarded as being
attributable to the enhancement of the thermal conductivity due to magnons
and/or phonons because of the drastic extension of the mean free path of
magnons and/or phonons in the Bose-Einstein condensed state.Comment: 4 pages, 4 figure
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