172 research outputs found
Superlattice switching from parametric instabilities in a driven-dissipative BEC in a cavity
We numerically obtain the full time-evolution of a parametrically-driven
dissipative Bose-Einstein condensate in an optical cavity and investigate the
implications of driving for the phase diagram. Beyond the normal and
superradiant phases, a third nonequilibrium phase emerges as a manybody
parametric resonance. This dynamical normal phase switches between two
symmetry-broken superradiant configurations. The switching implies a breakdown
of the system's mapping to the Dicke model. Unlike the other phases, the
dynamical normal phase shows features of nonintegrability and thermalization.Comment: 5 pages, 3 figure
Effect of random interactions in spin baths on decoherence
We study the decoherence of a central spin 1/2 induced by a spin bath with
intrabath interactions. Since we are interested in the cumulative effect of
interaction and disorder, we study baths comprising Ising spins with random
ferro- and antiferromagnetic interactions between the spins. Using the
resolvent operator method which goes beyond the standard Born-Markov master
equation approach, we show that, in the weak coupling regime, the decoherence
of the central spin at all times is entirely determined by the local-field
distribution or equivalently, the dynamical structure factor of the Ising bath.
We present analytic results for the Ising spin chain bath at arbitrary
temperature for different distributions of the intrabath interaction strengths.
We find clear evidence of non-Markovian behavior in the low temperature regime.
We also consider baths described by Ising models on higher-dimensional
lattices. We find that interactions lead to a significant reduction of the
decoherence. An important feature of interacting spinbaths is the saturation of
the asymptotic Markovian decay rate at high temperatures, as opposed to the
conventional Ohmic boson bath.Comment: 13 page
Effect of storage and processing on phytic acid levels in legumes and its interference with the utilisation of protein and iron
xv
During prolonged storage of the legumes studied, the most notable change
observed was the loss of phytic acid. Chickpea, green gram, and soybean stored
at 25 and 37°C showed marked decreases in protein digestibility (in vitro). The
legumes stored at 25 and 37°C required prolonged cooking times; however,
legumes stored at 5'C showed only a slight increase in cooking time. PCMP
number relating the contents of pectin, calcium, magnesium and phytin increased
during storage. When phytic acid disappears during prolonged storage, chelation
diminishes and Ca and Mg are freed as cations. Probably free Ca and Mg
associated with pectic substances causing the hard-to-cook phenomena. However,
under storage conditions of low temperature (5"C), these changes were
rninimised. The phytic acid level can thus be indicative of the cookability of
legumes.
Results indicate that the genotypes of pulses with low phytic acid content
could be identified and used in breeding programs to improve their nutritive
value and utilisation
Decoherence induced by an ordered environment
This Letter deals with the time evolution of a qubit weakly coupled to a
reservoir which has a symmetry broken state with long range order at finite
temperatures. In particular, we model the ordered reservoir by a standard BCS
superconductor with s-wave pairing. We study the reduced density matrix of a
qubit using both the time-convolutionless and Nakajima-Zwanzig approximations.
We study different kinds of couplings between the qubit and the superconducting
bath. We find that ordering in the superconducting bath generically leads to an
unfavorable non- Markovian faster-than-exponential decay of the qubit
coherence. On the other hand, a coupling of the qubit to the non-ordered sector
of the bath can result in a Markovian decoherence of the qubit with a drastic
reduction of the decoherence rate. Since these behaviors are endemic to the
ordered phase, qubits can serve as useful probes of continuous phase
transitions in their environment. We also briefly discuss the validity of our
main result, faster than exponential decay of the qubit coherences, for a qubit
coupled to a generic ordered bath with a spontaneously broken continuous
symmetry at finite temperatures.Comment: 6 pages, 3 figure
Effect of a gap on the decoherence of a qubit
We revisit the problem of the decoherence and relaxation of a central spin
coupled to a bath of conduction electrons. We consider both metallic and
semiconducting baths to study the effect of a gap in the bath density of states
(DOS) on the time evolution of the density matrix of the central spin. We use
two weak coupling approximation schemes to study the decoherence. At low
temperatures, though the temperature dependence of the decoherence rate in the
case of a metallic bath is the same irrespective of the details of the bath,
the same is not true for the semiconducting bath. We also calculate the
relaxation and decoherence rates as a function of external magnetic fields
applied both on the central spin and the bath. We find that in the presence of
the gap, there exists a certain regime of fields, for which surprisingly, the
metallic bath has lower rates of relaxation and decoherence than the
semiconducting bath.Comment: 9 pages, 9 figure
Frustrated antiferromagnetic quantum spin chains for spin length S > 1
We investigate frustrated antiferromagnetic Heisenberg quantum spin chains at
T=0 for S=3/2 and S=2 using the DMRG method. We localize disorder and Lifshitz
points, confirming that quantum disorder points can be seen as quantum remnants
of classical phase transitions. Both in the S=3/2 and the S=2 chain, we observe
the disappearance of effectively free S=1/2 and S=1 end spins respectively. The
frustrated spin chain is therefore a suitable system for clearly showing the
existence of free end spins S'=[S/2] also in half-integer antiferromagnetic
spin chains with S>1/2. We suggest that the first order transition found for
S=1 in our previous work is present in all frustrated spin chains with S>1/2,
characterized by the disappearance of effectively free end spins with S'=[S/2].Comment: 6 pages, 8 ps figures, uses RevTeX, submitted to PR
Numerical and approximate analytical results for the frustrated spin-1/2 quantum spin chain
We study the frustrated phase of the quantum spin-
system with nearest-neighbour and next-nearest-neighbour isotropic exchange
known as the Majumdar-Ghosh Hamiltonian. We first apply the coupled-cluster
method of quantum many-body theory based on a spiral model state to obtain the
ground state energy and the pitch angle. These results are compared with
accurate numerical results using the density matrix renormalisation group
method, which also gives the correlation functions. We also investigate the
periodicity of the phase using the Marshall sign criterion. We discuss
particularly the behaviour close to the phase transitions at each end of the
frustrated phase.Comment: 17 pages, Standard Latex File + 7 PostScript figures in separate
file. Figures also can also be requested from [email protected]
Fictive Impurity Models: an Alternative Formulation of the Cluster Dynamical Mean Field Method
"Cluster" extensions of the dynamical mean field method to include longer
range correlations are discussed. It is argued that the clusters arising in
these methods are naturally interpreted not as actual subunits of a physical
lattice but as algorithms for computing coefficients in an orthogonal function
expansion of the momentum dependence of the electronic self-energy. The
difficulties with causality which have been found to plague cluster dynamical
mean field methods are shown to be related to the "ringing" phenomenon familiar
from Fourier analysis. The analogy is used to motivate proposals for simple
filtering methods to circumvent them. The formalism is tested by comparison to
low order perturbative calculations and self consistent solutions
Dynamical Structure Factors of the S=1/2 Bond-Alternating Spin Chain with a Next-Nearest-Neighbor Interaction in Magnetic Fields
The dynamical structure factor of the S=1/2 bond-alternating spin chain with
a next-nearest-neighbor interaction in magnetic field is investigated using the
continued fraction method based on the Lanczos algorithm. When the plateau
exists on the magnetization curve, the longitudinal dynamical structure factor
shows a large intensity with a periodic dispersion relation, while the
transverse one shows a large intensity with an almost dispersionless mode. The
periodicity and the amplitude of the dispersion relation in the longitudinal
dynamical structure factor are sensitive to the coupling constants. The
dynamical structure factor of the S=1/2 two-leg ladder in magnetic field is
also calculated in the strong interchain-coupling regime.
The dynamical structure factor shows gapless or gapful behavior depending on
the wave vector along the rung.Comment: 8 pages, 4 figures, to appear in Journal of the Physical Society of
Japan, vol. 69, no. 10, (2000
- …