2,416 research outputs found
Majorana spin-flip transitions in a magnetic trap
Atoms confined in a magnetic trap can escape by making spin-flip Majorana
transitions due to a breakdown of the adiabatic approximation. Several papers
have studied this process for atoms with spin or . The present
paper calculates the escape rate for atoms with spin . This problem has
new features because the perturbation which allows atoms to escape
satisfies a selection rule and multi-step
processes contribute in leading order. When the adiabatic approximation is
satisfied the leading order terms can be summed to yield a simple expression
for the escape rate.Comment: 16page
Mediated tunable coupling of flux qubits
It is sketched how a monostable rf- or dc-SQUID can mediate an inductive
coupling between two adjacent flux qubits. The nontrivial dependence of the
SQUID's susceptibility on external flux makes it possible to continuously tune
the induced coupling from antiferromagnetic (AF) to ferromagnetic (FM). In
particular, for suitable parameters, the induced FM coupling can be
sufficiently large to overcome any possible direct AF inductive coupling
between the qubits.
The main features follow from a classical analysis of the multi-qubit
potential. A fully quantum treatment yields similar results, but with a
modified expression for the SQUID susceptibility.
Since the latter is exact, it can also be used to evaluate the
susceptibility--or, equivalently, energy-level curvature--of an isolated
rf-SQUID for larger shielding and at degenerate flux bias, i.e., a (bistable)
qubit. The result is compared to the standard two-level (pseudospin) treatment
of the anticrossing, and the ensuing conclusions are verified numerically.Comment: REVTeX 4, 16 pp., 4 EPS figures. N.B.: "Alec" is my first, and
"Maassen van den Brink" my family name. v2: major expansion and rewriting,
new title and co-author; to appear in New Journal of Physics special issue
(R. Fazio, ed.
Collective Dipole Bremsstrahlung in Fusion Reactions
We estimate the dipole radiation emitted in fusion processes. We show that a
classical bremsstrahlung approach can account for both the preequilibrium and
the thermal photon emission. We give an absolute evaluation of the
pre-equilibrium component due to the charge asymmetry in the entrance channel
and we study the energy and mass dependence in order to optimize the
observation. This dynamical dipole radiation could be a relevant cooling
mechanism in the fusion path. We stress the interest in experiments with the
new available radioactive beams.Comment: 4 pages (LATEX), 4 Postscript figures, minor text modification
Odd-even mass differences from self-consistent mean-field theory
We survey odd-even nuclear binding energy staggering using density functional
theory with several treatments of the pairing interaction including the BCS,
Hartree-Fock-Bogoliubov, and the Hartree-Fock-Bogoliubov with the Lipkin-Nogami
approximation. We calculate the second difference of binding energies and
compare with 443 measured neutron energy differences in isotope chains and 418
measured proton energy differences in isotone chains. The particle-hole part of
the energy functional is taken as the SLy4 Skyrme parametrization and the
pairing part of the functional is based on a contact interaction with possible
density dependence. An important feature of the data, reproduced by the theory,
is the sharp gap quenching at magic numbers. With the strength of the
interaction as a free parameter, the theory can reproduce the data to an rms
accuracy of about 0.25 MeV. This is slightly better than a single-parameter
phenomenological description but slightly poorer than the usual two-parameter
phenomenological form C/A^alpha . The following conclusions can be made about
the performance of common parametrization of the pairing interaction: (i) there
is a weak preference for a surface-peaked neutron-neutron pairing, which might
be attributable to many-body effects; (ii) a larger strength is required in the
proton pairing channel than in the neutron pairing channel; (iii) pairing
strengths adjusted to the well-known spherical isotope chains are too weak to
give a good overall fit to the mass differences.Comment: 13 pages, 9 figure
BRST Algebra Quantum Double and Quantization of the Proper Time Cotangent Bundle
The quantum double for the quantized BRST superalgebra is studied. The
corresponding R-matrix is explicitly constucted. The Hopf algebras of the
double form an analytical variety with coordinates described by the canonical
deformation parameters. This provides the possibility to construct the
nontrivial quantization of the proper time supergroup cotangent bundle. The
group-like classical limit for this quantization corresponds to the generic
super Lie bialgebra of the double.Comment: 11 pages, LaTe
Generalization of a result of Matsuo and Cherednik to the Calogero-Sutherland- Moser integrable models with exchange terms
A few years ago, Matsuo and Cherednik proved that from some solutions of the
Knizhnik-Zamolodchikov (KZ) equations, which first appeared in conformal field
theory, one can obtain wave functions for the Calogero integrable system. In
the present communication, it is shown that from some solutions of generalized
KZ equations, one can construct wave functions, characterized by any given
permutational symmetry, for some Calogero-Sutherland-Moser integrable models
with exchange terms. Such models include the spin generalizations of the
original Calogero and Sutherland ones, as well as that with -function
interaction.Comment: Latex, 7 pages, Communication at the 4th Colloquium "Quantum Groups
and Integrable Systems", Prague (June 1995
Observation of macroscopic Landau-Zener transitions in a superconducting device
A two-level system traversing a level anticrossing has a small probability to
make a so-called Landau-Zener (LZ) transition between its energy bands, in
deviation from simple adiabatic evolution. This effect takes on renewed
relevance due to the observation of quantum coherence in superconducting qubits
(macroscopic "Schrodinger cat" devices). We report an observation of LZ
transitions in an Al three-junction qubit coupled to a Nb resonant tank
circuit.Comment: REVTeX4, 4pp., 4 EPS figures. v2: clarifications added; final, to
appear in EP
Crystal chemistry aspects of the magnetically induced ferroelectricity in TbMn2O5 and BiMn2O5
The origin of magnetic frustration was stated and the ions whose shift is
accompanied by emerging magnetic ordering and ferroelectricity in TbMn2O5 and
BiMn2O5 were determined on the basis of calculation of magnetic coupling
parameters by using the structural data. The displacements accompanying the
magnetic ordering are not polar, they just induce changes of bond valence
(charge disordering) of Mn1 and Mn2, thus creating instability of the crystal
structure. To approximate again the bond valence to the initial value (charge
ordering) under magnetic ordering conditions is possible only due to polar
displacement of Mn2 (or O1) and O4 ions along the b axis that is the cause of
ferroelectric transition.Comment: 17 pages, 3 figures, 5 table
Knizhnik-Zamolodchikov equations and the Calogero-Sutherland-Moser integrable models with exchange terms
It is shown that from some solutions of generalized Knizhnik-Zamolodchikov
equations one can construct eigenfunctions of the Calogero-Sutherland-Moser
Hamiltonians with exchange terms, which are characterized by any given
permutational symmetry under particle exchange. This generalizes some results
previously derived by Matsuo and Cherednik for the ordinary
Calogero-Sutherland-Moser Hamiltonians.Comment: 13 pages, LaTeX, no figures, to be published in J. Phys.
Resonant hyper-Raman scattering in spherical quantum dots
A theoretical model of resonant hyper-Raman scattering by an ensemble of
spherical semiconductor quantum dots has been developed. The electronic
intermediate states are described as Wannier-Mott excitons in the framework of
the envelope function approximation. The optical polar vibrational modes of the
nanocrystallites (vibrons) and their interaction with the electronic system are
analized with the help of a continuum model satisfying both the mechanical and
electrostatic matching conditions at the interface. An explicit expression for
the hyper-Raman scattering efficiency is derived, which is valid for incident
two-photon energy close to the exciton resonances. The dipole selection rules
for optical transitions and Fr\"ohlich-like exciton-lattice interaction are
derived: It is shown that only exciton states with total angular momentum
and vibrational modes with angular momentum contribute to the
hyper-Raman scattering process. The associated exciton energies, wavefunctions,
and vibron frequencies have been obtained for spherical CdSe zincblende-type
nanocrystals, and the corresponding hyper-Raman scattering spectrum and
resonance profile are calculated. Their dependence on the dot radius and the
influence of the size distribution on them are also discussed.Comment: 12 pages REVTeX (two columns), 2 tables, 8 figure
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