89 research outputs found
Decay of Superflow Confined in Thin Torus: A Realization of Tunneling Quantum Fields
The quantum nucleation of phase slips in neutral superfluids confined in a
thin torus is investigated by means of the collective coordinate method. We
have devised, with numerical justification, a certain collective coordinate to
describe the quantum nucleation process of a phase slip. Considering the
quantum fluctuation around the local minimum of the action, we calculate the
effective mass of the phase slip. Due to the coherence of the condensate
throughout the torus, the effective mass is proportional to the circumference L
of the torus, and the decay rate has a strong exponential L-dependence.Comment: 4 pages, 2 figures, REVTe
Dynamical Vortices in Superfluid Films
The coupling of vortices to phonons in a superfluid is a gauge coupling
dictated by topology. The density and current response to a moving vortex are
computed and contrasted with the standard backflow picture. Exploiting the
analogy to (2+1)-dimensional electrodynamics, we compute the effective vortex
mass and find it to be logarithmically divergent in the low
frequency limit, leading to a super-Ohmic dissipation in response to an
oscillating superflow. Numerical integration of the nonlinear Schroedinger
equation supports these conclusions. Interaction of vortices and impurities is
also discussed.Comment: 13 pages, 6 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
Phase Structure and Compactness
In order to study the influence of compactness on low-energy properties, we
compare the phase structures of the compact and non-compact two-dimensional
multi-frequency sine-Gordon models. It is shown that the high-energy scaling of
the compact and non-compact models coincides, but their low-energy behaviors
differ. The critical frequency at which the sine-Gordon model
undergoes a topological phase transition is found to be unaffected by the
compactness of the field since it is determined by high-energy scaling laws.
However, the compact two-frequency sine-Gordon model has first and second order
phase transitions determined by the low-energy scaling: we show that these are
absent in the non-compact model.Comment: 21 pages, 5 figures, minor changes, final version, accepted for
publication in JHE
Genes optimized by evolution for accurate and fast translation encode in Archaea and Bacteria a broad and characteristic spectrum of protein functions
BACKGROUND: In many microbial genomes, a strong preference for a small number of codons can be observed in genes whose products are needed by the cell in large quantities. This codon usage bias (CUB) improves translational accuracy and speed and is one of several factors optimizing cell growth. Whereas CUB and the overrepresentation of individual proteins have been studied in detail, it is still unclear which high-level metabolic categories are subject to translational optimization in different habitats. RESULTS: In a systematic study of 388 microbial species, we have identified for each genome a specific subset of genes characterized by a marked CUB, which we named the effectome. As expected, gene products related to protein synthesis are abundant in both archaeal and bacterial effectomes. In addition, enzymes contributing to energy production and gene products involved in protein folding and stabilization are overrepresented. The comparison of genomes from eleven habitats shows that the environment has only a minor effect on the composition of the effectomes. As a paradigmatic example, we detailed the effectome content of 37 bacterial genomes that are most likely exposed to strongest selective pressure towards translational optimization. These effectomes accommodate a broad range of protein functions like enzymes related to glycolysis/gluconeogenesis and the TCA cycle, ATP synthases, aminoacyl-tRNA synthetases, chaperones, proteases that degrade misfolded proteins, protectants against oxidative damage, as well as cold shock and outer membrane proteins. CONCLUSIONS: We made clear that effectomes consist of specific subsets of the proteome being involved in several cellular functions. As expected, some functions are related to cell growth and affect speed and quality of protein synthesis. Additionally, the effectomes contain enzymes of central metabolic pathways and cellular functions sustaining microbial life under stress situations. These findings indicate that cell growth is an important but not the only factor modulating translational accuracy and speed by means of CUB
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