303 research outputs found
The scaling of the decoherence factor of a qubit coupled to a spin chain driven across quantum critical points
We study the scaling of the decoherence factor of a qubit (spin-1/2) using
the central spin model in which the central spin (qubit) is globally coupled to
a transverse XY spin chain. The aim here is to study the non-equilibrium
generation of decoherence when the spin chain is driven across (along) quantum
critical points (lines) and derive the scaling of the decoherence factor in
terms of the driving rate and some of the exponents associated with the quantum
critical points. Our studies show that the scaling of logarithm of decoherence
factor is identical to that of the defect density in the final state of the
spin chain following a quench across isolated quantum critical points for both
linear and non-linear variations of a parameter even if the defect density may
not satisfy the standard Kibble-Zurek scaling. However, one finds an
interesting deviation when the spin chain is driven along a critical line. Our
analytical predictions are in complete agreement with numerical results. Our
study, though limited to integrable two-level systems, points to the existence
of a universality in the scaling of the decoherence factor which is not
necessarily identical to the scaling of the defect density.Comment: 5 pages, 2 figures, Final and accepted versio
Quenching through Dirac and semi-Dirac points in optical Lattices: Kibble-Zurek scaling for anisotropic Quantum-Critical systems
We propose that Kibble-Zurek scaling can be studied in optical lattices by
creating geometries that support, Dirac, Semi-Dirac and Quadratic Band
Crossings. On a Honeycomb lattice with fermions, as a staggered on-site
potential is varied through zero, the system crosses the gapless Dirac points,
and we show that the density of defects created scales as , where
is the inverse rate of change of the potential, in agreement with the
Kibble-Zurek relation. We generalize the result for a passage through a
semi-Dirac point in dimensions, in which spectrum is linear in parallel
directions and quadratic in rest of the perpendicular directions. We
find that the defect density is given by where
and are the dynamical exponents and the correlation
length exponents along the parallel and perpendicular directions, respectively.
The scaling relations are also generalized to the case of non-linear quenching
Biology of Blepharida-group flea beetles with first notes on natural history of Podontia congregata Baly, 1865 an endemic flea beetle from southern India (Coleoptera, Chrysomelidae, Galerucinae, Alticini)
This is a publisher's version, also available electronically at http://www.pensoft.net/index.php.The biology, host plants, and pest status of Podontia Dalman, 1824 species are reviewed. Natural history of Podontia congregata Baly, 1865 a flea beetle endemic to southern India, is reported for the first time. It is distributed from the Western Ghats Mountains westward to the plains. Clusiaceae is reported as a new host plant family for Blepharida-group species, with Garcinia gummi-gutta (L.) N. Robson (Clusiaceae) as the host plant for P. congregata. Pentatomid bugs attack the larvae but not eggs, pupae, or adults. A new egg parasitoid species, Ooencyrtus keralensis Hayat and Prathapan, 2010 (Hymenoptera: Encyrtidae), was discovered. Aspects of P. congregata host selection, life cycle, and larval fecal defenses are consistent with its inclusion in the Blepharida-genus group
A NEW SPECIES-GROUP IN APHTHONA CHEVROLAT (COLEOPTERA: CHRYSOMELIDAE) WITH A DESCRIPTION OF A NEW SPECIES FROM SOUTHERN INDIA
A new species-group in the flea beetle genus Aphthona Chevrolat is defined and Aphthona yercaudensis Prathapan and Konstantinov, new species, from southern India is described. A key to Aphthona species-groups in the Oriental Region is provided. Herbaceous species of Phyllanthus L. (Euphorbiaceae) are recorded as the host plants of Aphthona bombayensis Scherer
Defect production due to quenching through a multicritical point
We study the generation of defects when a quantum spin system is quenched
through a multicritical point by changing a parameter of the Hamiltonian as
, where is the characteristic time scale of quenching. We argue
that when a quantum system is quenched across a multicritical point, the
density of defects () in the final state is not necessarily given by the
Kibble-Zurek scaling form , where is the
spatial dimension, and and are respectively the correlation length
and dynamical exponent associated with the quantum critical point. We propose a
generalized scaling form of the defect density given by , where the exponent determines the behavior of the
off-diagonal term of the Landau-Zener matrix at the multicritical
point. This scaling is valid not only at a multicritical point but also at an
ordinary critical point.Comment: 4 pages, 2 figures, updated references and added one figur
Quenching Dynamics of a quantum XY spin-1/2 chain in presence of a transverse field
We study the quantum dynamics of a one-dimensional spin-1/2 anisotropic XY
model in a transverse field when the transverse field or the anisotropic
interaction is quenched at a slow but uniform rate. The two quenching schemes
are called transverse and anisotropic quenching respectively. Our emphasis in
this paper is on the anisotropic quenching scheme and we compare the results
with those of the other scheme. In the process of anisotropic quenching, the
system crosses all the quantum critical lines of the phase diagram where the
relaxation time diverges. The evolution is non-adiabatic in the time interval
when the parameters are close to their critical values, and is adiabatic
otherwise. The density of defects produced due to non-adiabatic transitions is
calculated by mapping the many-particle system to an equivalent Landau-Zener
problem and is generally found to vary as , where is the
characteristic time scale of quenching, a scenario that supports the
Kibble-Zurek mechanism. Interestingly, in the case of anisotropic quenching,
there exists an additional non-adiabatic transition, in comparison to the
transverse quenching case, with the corresponding probability peaking at an
incommensurate value of the wave vector. In the special case in which the
system passes through a multi-critical point, the defect density is found to
vary as . The von Neumann entropy of the final state is shown to
maximize at a quenching rate around which the ordering of the final state
changes from antiferromagnetic to ferromagnetic.Comment: 8 pages, 6 figure
Descriptions of eight new species of Phaelota (Coleoptera: Chrysomelidae) with a new generic synonymy and a key to species of Indian subcontinent
Six new species of Phaelota Jacoby from India viz. P. assamensis, P. kottigehara, P. maculipennis, P. mauliki, P. saluki, and P. viridipennis and two new species from Sri Lanka viz. P. ogloblini and P. schereri are described and illustrated. Thrylaea Jacoby is treated as a new junior synonym of Phaelota. A lectotype for Phaelota variabilis (Jacoby) is desig- nated and the species is removed from the synonyms of P. flavipennis (Motschulsky). The consequences of loss of flight on host plant selection in Phaelota are discussed and a key to the species of the Indian subcontinent is provided
Orisaltata, a new genus of flea beetles from the Oriental Region (Coleoptera: Chrysomelidae)
A new genus, Orisaltata, with the type species Aphthona azurea Jacoby (the only known species) from the Oriental Region is described and illustrated. Comparative notes and host plant information are provided. Aphthona babai Kimoto is synonymized with Orisaltata azurea
Improving Performance of Quantum Heat Engines by Free Evolution
The efficiency of a quantum heat engine is maximum when the unitary strokes
are adiabatic. On the other hand, this may not be always possible due to small
energy gaps in the system, especially at the critical point where the gap
vanishes. With the aim to achieve this adiabaticity, we modify one of the
unitary strokes of the cycle by allowing the system to evolve freely with a
particular Hamiltonian till a time so that the system reaches a less excited
state. This will help in increasing the magnitude of the heat absorbed from the
hot bath so that the work output and efficiency of the engine can be increased.
We demonstrate this method using an integrable model and a non- integrable
model as the working medium. In the case of a two spin system, the optimal
value for the time till which the system needs to be freely evolved is
calculated analytically in the adiabatic limit. The results show that
implementing this modified stroke significantly improves the work output and
efficiency of the engine, especially when it crosses the critical point.Comment: 8 pages, 8 figure
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