2,107 research outputs found
Motion of a condensate in a shaken and vibrating harmonic trap
The dynamics of a Bose-Einstein condensate (BEC) in a time-dependent harmonic
trapping potential is determined for arbitrary variations of the position of
the center of the trap and its frequencies. The dynamics of the BEC wavepacket
is soliton-like. The motion of the center of the wavepacket, and the spatially
and temporally dependent phase (which affects the coherence properties of the
BEC) multiplying the soliton-like part of the wavepacket, are analytically
determined.Comment: Accepted for publication in J. Phys. B: At Mol Opt Phy
Synchronization Transition in the Kuramoto Model with Colored Noise
We present a linear stability analysis of the incoherent state in a system of
globally coupled, identical phase oscillators subject to colored noise. In that
we succeed to bridge the extreme time scales between the formerly studied and
analytically solvable cases of white noise and quenched random frequencies.Comment: 4 pages, 2 figure
Denaturation of Circular DNA: Supercoil Mechanism
The denaturation transition which takes place in circular DNA is analyzed by
extending the Poland-Scheraga model to include the winding degrees of freedom.
We consider the case of a homopolymer whereby the winding number of the double
stranded helix, released by a loop denaturation, is absorbed by
\emph{supercoils}. We find that as in the case of linear DNA, the order of the
transition is determined by the loop exponent . However the first order
transition displayed by the PS model for in linear DNA is replaced by a
continuous transition with arbitrarily high order as approaches 2, while
the second-order transition found in the linear case in the regime
disappears. In addition, our analysis reveals that melting under fixed linking
number is a \emph{condensation transition}, where the condensate is a
macroscopic loop which appears above the critical temperature.Comment: 9 pages, 4 figure
Emission of Massive Scalar Fields by a Higher-Dimensional Rotating Black-Hole
We perform a comprehensive study of the emission of massive scalar fields by
a higher-dimensional, simply rotating black hole both in the bulk and on the
brane. We derive approximate, analytic results as well as exact numerical ones
for the absorption probability, and demonstrate that the two sets agree very
well in the low and intermediate-energy regime for scalar fields with mass
m_\Phi < 1 TeV in the bulk and m_\Phi < 0.5 TeV on the brane. The numerical
values of the absorption probability are then used to derive the Hawking
radiation power emission spectra in terms of the number of extra dimensions,
angular-momentum of the black hole and mass of the emitted field. We compute
the total emissivities in the bulk and on the brane, and demonstrate that,
although the brane channel remains the dominant one, the bulk-over-brane energy
ratio is considerably increased (up to 33%) when the mass of the emitted field
is taken into account.Comment: 28 pages, 18 figure
Exciton states in cylindrical nanowires
The exciton ground state and excited state energies are calculated for a
model system of an infinitely long cylindrical wire. The effective Coulomb
potential between the electron and the hole is studied as function of the wire
radius. Within the adiabatic approximation, we obtain `exact' numerical results
for the effective exciton potential and the lowest exciton energy levels which
are fitted to simple analytical expressions. Furthermore, we investigated the
influence of a magnetic field parallel to the nanowire on the effective
potential and the exciton energy.Comment: 9 pages, 9 figures. Submitted for publication to PRB. Figures must be
downloaded seperatel
Quasi-Ferromagnet Spintronics in Graphene Nanodisk-Lead System
A zigzag graphene nanodisk can be interpreted as a quantum dot with an
internal degree of freedom. It is well described by the infinite-range
Heisenberg model. We have investigated its thermodynamical properties. There
exists a quasi-phase transition between the quasi-ferromagnet and
quasi-paramagnet states, as signaled by a sharp peak in the specific heat and
in the susceptability. We have also analyzed how thermodynamical properties are
affected when two leads are attached to the nanodisk. It is shown that lead
effects are described by the many-spin Kondo Hamiltonian. There appears a new
peak in the specific heat, and the multiplicity of the ground state becomes
just one half of the system without leads. Another lead effect is to enhance
the ferromagnetic order. Being a ferromagnet, a nanodisk can be used as a spin
filter. Furthermore, since the relaxation time is finite, it is possible to
control the spin of the nanodisk by an external spin current. We then propose a
rich variety of spintronic devices made of nanodisks and leads, such as spin
memory, spin amplifier, spin valve, spin-field-effect transistor, spin diode
and spin logic gates such as spin-XNOR gate and spin-XOR gate. Graphene
nanodisks could well be basic components of future nanoelectronic and
spintronic devices.Comment: 12 pages, 13 figures, invited paper to "focus on graphene
The few-body problem in terms of correlated gaussians
In their textbook, Suzuki and Varga [Y. Suzuki and K. Varga, {\em Stochastic
Variational Approach to Quantum-Mechanical Few-Body Problems} (Springer,
Berlin, 1998)] present the stochastic variational method in a very exhaustive
way. In this framework, the so-called correlated gaussian bases are often
employed. General formulae for the matrix elements of various operators can be
found in the textbook. However the Fourier transform of correlated gaussians
and their application to the management of a relativistic kinetic energy
operator are missing and cannot be found in the literature. In this paper we
present these interesting formulae. We give also a derivation for new
formulations concerning central potentials; the corresponding formulae are more
efficient numerically than those presented in the textbook.Comment: 10 page
Coupled Oscillators with Chemotaxis
A simple coupled oscillator system with chemotaxis is introduced to study
morphogenesis of cellular slime molds. The model successfuly explains the
migration of pseudoplasmodium which has been experimentally predicted to be
lead by cells with higher intrinsic frequencies. Results obtained predict that
its velocity attains its maximum value in the interface region between total
locking and partial locking and also suggest possible roles played by partial
synchrony during multicellular development.Comment: 4 pages, 5 figures, latex using jpsj.sty and epsf.sty, to appear in
J. Phys. Soc. Jpn. 67 (1998
Coulomb corrected eikonal description of the breakup of halo nuclei
The eikonal description of breakup reactions diverges because of the Coulomb
interaction between the projectile and the target. This divergence is due to
the adiabatic, or sudden, approximation usually made, which is incompatible
with the infinite range of the Coulomb interaction. A correction for this
divergence is analysed by comparison with the Dynamical Eikonal Approximation,
which is derived without the adiabatic approximation. The correction consists
in replacing the first-order term of the eikonal Coulomb phase by the
first-order of the perturbation theory. This allows taking into account both
nuclear and Coulomb interactions on the same footing within the computationally
efficient eikonal model. Excellent results are found for the dissociation of
11Be on lead at 69 MeV/nucleon. This Coulomb Corrected Eikonal approximation
provides a competitive alternative to more elaborate reaction models for
investigating breakup of three-body projectiles at intermediate and high
energies.Comment: 19 pages, 9 figures, accepted for publication in Phys. Rev.
Soliton states in mesoscopic two-band-superconducting cylinders
In the framework of the Ginzburg-Landau approach, we present a
self-consistent theory of specific soliton states in mesoscopic (thin-walled)
two-band-superconducting cylinders in external parallel magnetic fields. Such
states arise in the presence of "Josephson-type" interband coupling, when phase
winding numbers are different for each component of the superconducting order
parameter. We evaluate the Gibbs free energy of the sysyem up to second-order
terms in a certain dimensionless parameter
, where
and are the magnetic and kinetic
inductance, respectively. We derive the complete set of exact soliton
solutions. These solutions are thoroughly analyzed from the viewpoint of both
local and global (thermodynamic) stability. In particular, we show that
rotational-symmetry-breaking caused by the formation of solitons gives rise to
a zero-frequency rotational mode. Although soliton states prove to be
thermodynamically metastable, the minimal energy gap between the lowest-lying
single-soliton states and thermodynamically stable zero-soliton states can be
much smaller than the magnetic Gibbs free energy of the latter states, provided
that intraband "penetration depths" differ substantially and interband coupling
is weak. The results of our investigation may apply to a wide class of
mesoscopic doubly-connected structures exhibiting two-band superconductivity.Comment: 15 pages, 3 figure
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