68,516 research outputs found
Low-energy modes of spin-imbalanced Fermi gases in BCS phase
The low-energy modes of a spin-imbalanced superfluid Fermi gas in the
Bardeen-Cooper-Schrieffer (BCS) side are studied. The gas is assumed to be
sufficiently dilute so that the pairing of atoms can be considered effective
only in s-wave between fermions of different internal state. The order
parameter at equilibrium is determined by the mean-field approximation, while
the properties of the collective modes are calculated within a Gaussian
approximation for the fluctuations of the order parameter. In particular we
investigate the effects of asymmetry between the populations of the two
different components and of temperature on the frequency and damping of
collective modes. It is found that the temperature does not much affect the
frequency and the damping of the modes, whereas an increase of the imbalance
shifts the frequency toward lower values and enhances the damping sensitively.
Besides the Bogoliubov-Anderson phonons, we observe modes at zero frequency for
finite values of the wave-number. These modes indicate that an instability
develops driving the system toward two separate phases, normal and superfluid.Comment: 7 pages, 4 figures, submitted to European Physical Journal D for
publicatio
The effect of internal and global modes on the radial distribution function of confined semiflexible polymers
The constraints imposed by nano- and microscale confinement on the
conformational degrees of freedom of thermally fluctuating biopolymers are
utilized in contemporary nano-devices to specifically elongate and manipulate
single chains. A thorough theoretical understanding and quantification of the
statistical conformations of confined polymer chains is thus a central concern
in polymer physics. We present an analytical calculation of the radial
distribution function of harmonically confined semiflexible polymers in the
weakly bending limit. Special emphasis has been put on a proper treatment of
global modes, i.e. the possibility of the chain to perform global movements
within the channel. We show that the effect of these global modes significantly
impacts the chain statistics in cases of weak and intermediate confinement.
Comparing our analytical model to numerical data from Monte Carlo simulations
we find excellent agreement over a broad range of parameters.Comment: 6 pages, 3 figures typo corrected, slightly revised line of
reasoning, results unchange
Quantum mechanical calculations of rotational-vibrational scattering in homonuclear diatom-atom collisions
Most calculations of the vibrational scattering of diatom-atom collisions use the breathing sphere approximation (BSA) of orientation averaging the intermolecular potential. The resulting angularly symmetric
potential can not cause rotational scattering. We determine the error introduced by the BSA into observables of the vibrational scattering of low-energy homonuclear diatom-atom collisions by comparing two quantum mechanical calculations, one with the BSA and the other with the full angularly asymmetric intermolecular potential. For ·reasons of economy the rotational scattering of the second calculation is restricted by the use of special incomplete channel sets in the expansion of the scattering wavefunction.
Three representative collision systems are studied: H_2-Ar, O_2-He, and I_2-He. From our calculations, we
reach two conclusions. First, the BSA can be used to analyze accurately experimental measurements of
vibrational scattering. Second, measurements most sensitive to the symmetric part of the intermolecular
potential are, in order, elastic cross sections, inelastic cross sections, and inelastic differential cross sections.
Elastic differential cross sections are sensitive to the potential only if the collision is "sticky," with scattering
over a wide range of angles; I_2-He is such a collision. Otherwise the potential sensitivity of elastic differential
cross sections is concentrated in the experimentally difficult region of very small angle scattering
The Theory of Scanning Quantum Dot Microscopy
Electrostatic forces are among the most common interactions in nature and
omnipresent at the nanoscale. Scanning probe methods represent a formidable
approach to study these interactions locally. The lateral resolution of such
images is, however, often limited as they are based on measuring the force
(gradient) due to the entire tip interacting with the entire surface. Recently,
we developed scanning quantum dot microscopy (SQDM), a new technique for the
imaging and quantification of surface potentials which is based on the gating
of a nanometer-size tip-attached quantum dot by the local surface potential and
the detection of charge state changes via non-contact atomic force microscopy.
Here, we present a rigorous formalism in the framework of which SQDM can be
understood and interpreted quantitatively. In particular, we present a general
theory of SQDM based on the classical boundary value problem of electrostatics,
which is applicable to the full range of sample properties (conductive vs
insulating, nanostructured vs homogeneously covered). We elaborate the general
theory into a formalism suited for the quantitative analysis of images of
nanostructured but predominantly flat and conductive samples
Chemistry of the Spring Waters of the Ouachita Mountains Excluding Hot Springs, Arkansas
This report is based on the chemical analysis of the waters from 93 springs and 9 wells. Springs, when free from metal plumbing, provide an uncontaminated source of the ground water and it was desired to obtain water uncontaminated with metals. A few wells were added to the list, usually because of their unique location in the sampling grid
Errors in Hellmann-Feynman Forces due to occupation number broadening, and how they can be corrected
In ab initio calculations of electronic structures, total energies, and
forces, it is convenient and often even necessary to employ a broadening of the
occupation numbers. If done carefully, this improves the accuracy of the
calculated electron densities and total energies and stabilizes the convergence
of the iterative approach towards self-consistency. However, such a boardening
may lead to an error in the calculation of the forces. Accurate forces are
needed for an efficient geometry optimization of polyatomic systems and for ab
initio molecular dynamics (MD) calculations. The relevance of this error and
possible ways to correct it will be discussed in this paper. The first approach
is computationally very simple and in fact exact for small MD time steps. This
is demonstrated for the example of the vibration of a carbon dimer and for the
relaxation of the top layer of the (111)-surfaces of aluminium and platinum.
The second, more general, scheme employs linear-response theory and is applied
to the calculation of the surface relaxation of Al(111). We will show that the
quadratic dependence of the forces on the broadening width enables an efficient
extrapolation to the correct result. Finally the results of these correction
methods will be compared to the forces obtained by using the smearing scheme,
which has been proposed by Methfessel and Paxton.Comment: 6 pages, 5 figures, Scheduled tentatively for the issue of Phys. Rev.
B 15 15 Dec 97 Other related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
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