25,086 research outputs found
Quantum density anomaly in optically trapped ultracold gases
We show that the Bose-Hubbard Model exhibits an increase in density with
temperature at fixed pressure in the regular fluid regime and in the superfluid
phase. The anomaly at the Bose-Einstein condensate is the first density anomaly
observed in a quantum state. We propose that the mechanism underlying both the
normal phase and the superfluid phase anomalies is related to zero point
entropies and ground state phase transitions. A connection with the typical
experimental scales and setups is also addressed. This key finding opens a new
pathway for theoretical and experimental studies of water-like anomalies in the
area of ultracold quantum gases
Molecular correlations and solvation in simple fluids
We study the molecular correlations in a lattice model of a solution of a
low-solubility solute, with emphasis on how the thermodynamics is reflected in
the correlation functions. The model is treated in Bethe-Guggenheim
approximation, which is exact on a Bethe lattice (Cayley tree). The solution
properties are obtained in the limit of infinite dilution of the solute. With
, , and the three pair correlation functions
as functions of the separation (subscripts 1 and 2 referring to solvent and
solute, respectively), we find for lattice steps that
. This illustrates a general
theorem that holds in the asymptotic limit of infinite . The three
correlation functions share a common exponential decay length (correlation
length), but when the solubility of the solute is low the amplitude of the
decay of is much greater than that of , which in turn is
much greater than that of . As a consequence the amplitude of the
decay of is enormously greater than that of . The
effective solute-solute attraction then remains discernible at distances at
which the solvent molecules are essentially no longer correlated, as found in
similar circumstances in an earlier model. The second osmotic virial
coefficient is large and negative, as expected. We find that the
solvent-mediated part of the potential of mean force between solutes,
evaluated at contact, , is related in this model to the Gibbs free energy
of solvation at fixed pressure, , by , where is the coordination number of the lattice, the
pressure, and the volume of the cell associated with each lattice site. A
large, positive associated with the low solubility is thus
reflected in a strong attraction (large negative at contact), which is the
major contributor to the second osmotic virial coefficient. In this model, the
low solubility (large positive ) is due partly to an unfavorable
enthalpy of solvation and partly to an unfavorable solvation entropy, unlike in
the hydrophobic effect, where the enthalpy of solvation itself favors high
solubility, but is overweighed by the unfavorable solvation entropy.Comment: 9 pages, 2 figure
Spacetime algebraic skeleton
The cosmological constant Lambda, which has seemingly dominated the primaeval
Universe evolution and to which recent data attribute a significant
present-time value, is shown to have an algebraic content: it is essentially an
eigenvalue of a Casimir invariant of the Lorentz group which acts on every
tangent space. This is found in the context of de Sitter spacetimes but, as
every spacetime is a 4-manifold with Minkowski tangent spaces, the result
suggests the existence of a "skeleton" algebraic structure underlying the
geometry of general physical spacetimes. Different spacetimes come from the
"fleshening" of that structure by different tetrad fields. Tetrad fields, which
provide the interface between spacetime proper and its tangent spaces, exhibit
to the most the fundamental role of the Lorentz group in Riemannian spacetimes,
a role which is obscured in the more usual metric formalism.Comment: 13 page
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