24,225 research outputs found
Quantum Phase Interference for Quantum Tunneling in Spin Systems
The point-particle-like Hamiltonian of a biaxial spin particle with external
magnetic field along the hard axis is obtained in terms of the potential field
description of spin systems with exact spin-coordinate correspondence. The
Zeeman energy term turns out to be an effective gauge potential which leads to
a nonintegrable pha se of the Euclidean Feynman propagator.
The phase interference between clockwise and anticlockwise under barrier
propagations is recognized explicitly as the Aharonov-Bohm effect. An
additional phase which is significant for quantum phase interference is
discovered with the quantum theory of spin systems besides the known phase
obtained with the semiclassical treatment of spin. We also show the energ y
dependence of the effect and obtain the tunneling splitting at excited states
with the help of periodic instantons.Comment: 19 pages, no figure, to appear in PR
Enhancement of Quantum Tunneling for Excited States in Ferromagnetic Particles
A formula suitable for a quantitative evaluation of the tunneling effect in a
ferromagnetic particle is derived with the help of the instanton method. The
tunneling between n-th degenerate states of neighboring wells is dominated by a
periodic pseudoparticle configuration. The low-lying level-splitting previously
obtained with the LSZ method in field theory in which the tunneling is viewed
as the transition of n bosons induced by the usual (vacuum) instanton is
recovered. The observation made with our new result is that the tunneling
effect increases at excited states. The results should be useful in analyzing
results of experimental tests of macroscopic quantum coherence in ferromagnetic
particles.Comment: 18 pages, LaTex, 1 figur
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Phenotypic and Genotypic Characteristics of Methicillin-Resistant Staphylococcus aureus (MRSA) Related to Persistent Endovascular Infection.
Persistent methicillin-resistant Staphylococcus aureus (MRSA) bacteremia (PB) represents an important subset of S. aureus infection and correlates with poor clinical outcomes. MRSA isolates from patients with PB differ significantly from those of resolving bacteremia (RB) with regard to several in vitro phenotypic and genotypic profiles. For instance, PB strains exhibit less susceptibility to cationic host defense peptides and vancomycin (VAN) killing under in vivo-like conditions, greater damage to endothelial cells, thicker biofilm formation, altered growth rates, early activation of many global virulence regulons (e.g., sigB, sarA, sae and agr) and higher expression of purine biosynthesis genes (e.g., purF) than RB strains. Importantly, PB strains are significantly more resistant to VAN treatment in experimental infective endocarditis as compared to RB strains, despite similar VAN minimum inhibitory concentrations (MICs) in vitro. Here, we review relevant phenotypic and genotypic characteristics related to the PB outcome. These and future insights may improve our understanding of the specific mechanism(s) contributing to the PB outcome, and aid in the development of novel therapeutic and preventative measures against this life-threatening infection
Periodic Bounce for Nucleation Rate at Finite Temperature in Minisuperspace Models
The periodic bounce configurations responsible for quantum tunneling are
obtained explicitly and are extended to the finite energy case for
minisuperspace models of the Universe. As a common feature of the tunneling
models at finite energy considered here we observe that the period of the
bounce increases with energy monotonically. The periodic bounces do not have
bifurcations and make no contribution to the nucleation rate except the one
with zero energy. The sharp first order phase transition from quantum tunneling
to thermal activation is verified with the general criterions.Comment: 17 pages, 5 postscript figures include
Quantum Phase Transition in Finite-Size Lipkin-Meshkov-Glick Model
Lipkin model of arbitrary particle-number N is studied in terms of exact
differential-operator representation of spin-operators from which we obtain the
low-lying energy spectrum with the instanton method of quantum tunneling. Our
new observation is that the well known quantum phase transition can also occur
in the finite-N model only if N is an odd-number. We furthermore demonstrate a
new type of quantum phase transition characterized by level-crossing which is
induced by the geometric phase interference and is marvelously periodic with
respect to the coupling parameter. Finally the conventional quantum phase
transition is understood intuitively from the tunneling formulation in the
thermodynamic limit.Comment: 4 figure
Dynamics and Berry phase of two-species Bose-Einstein condensates
In terms of exact solutions of the time-dependent Schrodinger equation for an
effective giant spin modeled from a coupled two-mode Bose-Einstein condensate
(BEC) with adiabatic and cyclic time-varying Raman coupling between two
hyperfine states of the BEC, we obtain analytic time-evolution formulas of the
population imbalance and relative phase between two components with various
initial states, especially the SU(2)coherent state. We find the Berry phase
depending on the number parity of atoms, and particle number dependence of the
collapse revival of population-imbalance oscillation. It is shown that
self-trapping and phase locking can be achieved from initial SU(2) coherent
states with proper parameters.Comment: 18 pages,5 figure
Periodic instanton method and macroscopic quantum tunneling between two weakly-linked Bose-Einstein condensates
A new method is used to investigate the tunneling between two weakly-linked
Bose-Einstein condensates confined in double-well potential traps. The
nonlinear interaction between the atoms in each well contributes to a finite
chemical potential, which, with consideration of periodic instantons, leads to
a remarkably high tunneling frequency. This result can be used to interpret the
newly found Macroscopic Quantum Self Trapping (MQST) effect. Also a new kind of
first-order crossover between different regions is predicted.Comment: 4 pages, 2 eps figures, final version to appear in Phys. Rev.
Preparation and properties of resistant starch from corn starch with enzymes
Corn starch was subjected to enzymatic biotechnology with thermostable á-amylase and pullulanase and followed by retrogradation, to prepare resistant starch. The properties of selected resistant starch (RS) samples were also investigated. The result showed that, appropriate amount (0.5 U/g) of thermostable á-amylase was good for resistant starch formation. The optimal condition for pullulanase hydrolysis was carried out with 0.8 PUN/g (dry starch) pullulanase in pH 5.5 starch gel at 60°C for 12 h. The highest yield of resistant starch could be obtained (19.02%) under optimal condition. The effect of pullulanase on resistant starch formation was the most significant. Compared with native starch, amylose content was increased by 17.0 to 28.1% and it did not increased with  enhancement of resistant starch content. The native starch showed A-type X-ray diffraction pattern, whereas RS products exhibited B-type pattern with strong intensity at the peak of 17.2°C 2è and two board peak at 20.1 and 23.9°C 2è. The transformation enthalpy of the second peak increased gradually with increasing RS content and the transformation peak temperature and Tc-T0 was also similar to this trend.Key words: Resistant starch, thermostable á-amylase, pullulanase, properties
Chemical composition and antioxidant properties of the essential oil and methanol extracts of rhizoma Alpinia officinarum from China in vitro
This study was designed to examine the in vitro antioxidant activities of the essential oil and methanol extracts of rhizoma Alpinia officinarum (small galanga) from China. The essential oil was analyzed by gas chromatography/ mass spectrometry (GC/MS) and 46 constituents were identified. Methanol extract from rhizoma A. officinarum was fractionated by chromatography of silica gel using ethyl acetate, acetone-methanol (8:1, V/V) and methanol, respectively. The antioxidant activities of essential oil and methanol extracts were evaluated with reducing power, diphenylpicrylhydrazyl (DPPH) assay and ferric thiocyanate test. In the above three assays, the essential oil and methanol extracts showed antioxidant potential to varying degrees; and acetone-methanol (8:1, V/V) subfraction exhibited better antioxidant potency than others. Owing to having antioxidative components, the essential oil exhibited satisfying antioxidant activities. A positive correlation was observed between the antioxidant activity potential and total phenolic contents of the extracts
Critical Property of Geometric Phase in the Dicke Model
We obtain the ground-state energy level and associated geometric phase in the
Dicke model analytically by means of the Holstein-Primakoff transformation and
the boson expansion approach in the thermodynamic limit. The non-adiabatic
geometric phase induced by the photon field is derived with the time-dependent
unitary transformation. It is shown that the quantum phase transition
characterized by the non-analyticity of the geometric phase is remarkably of
the first-order. We also investigate the scaling behavior of the geometric
phase at the critical point, which can be measured in a practical experiment to
detect the quantum phase transition.Comment: 1 figur
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