5,349 research outputs found
Low-lying excitations in superconducting bilayer systems
The ground and first excited state of two superconducting layers in
interaction are studied considering two different coupling terms, one
represented by the standard Josephson interaction, and one new, which is a
superexchange pairing force between bilayer pairs. It is shown that a
moderate-to strong Josephson interaction produces a low-lying collective state,
pictured as an out-of-phase oscillation of the BCS gauge angles of the two
layers. This antisymmetric angular oscillation might explain the 41 meV
resonance observed in the neutron scattering experiments. The bilayer pairs are
formed by electrons from different layers with an antiparallel orientation of
the spins, being related to the antiferromagnetic arrangement. The pair
operators within the layers together with the bilayer pairs generate by
commutation an so(5) algebra. It is shown that the transition between the
superconducting and antiferromagnetic phases can be explained assuming the
dependence on concentration of the bilayer pairing strength, with maximum at
half-filling.Comment: Latex, three postscript figures; replaced to correct page format
error
Variational Principle for Mixed Classical-Quantum Systems
An extended variational principle providing the equations of motion for a
system consisting of interacting classical, quasiclassical and quantum
components is presented, and applied to the model of bilinear coupling. The
relevant dynamical variables are expressed in the form of a quantum state
vector which includes the action of the classical subsystem in its phase
factor. It is shown that the statistical ensemble of Brownian state vectors for
a quantum particle in a classical thermal environment can be described by a
density matrix evolving according to a nonlinear quantum Fokker-Planck
equation. Exact solutions of this equation are obtained for a two-level system
in the limit of high temperatures, considering both stationary and
nonstationary initial states. A treatment of the common time shared by the
quantum system and its classical environment, as a collective variable rather
than as a parameter, is presented in the Appendix.Comment: 16 pages, LaTex; added Figure 2 and Figure
Quantum Coherence Oscillations in Antiferromagnetic Chains
Macroscopic quantum coherence oscillations in mesoscopic antiferromagnets may
appear when the anisotropy potential creates a barrier between the
antiferromagnetic states with opposite orientations of the Neel vector. This
phenomenon is studied for the physical situation of the nuclear spin system of
eight Xe atoms arranged on a magnetic surface along a chain. The oscillation
period is calculated as a function of the chain constant. The environmental
decoherence effects at finite temperature are accounted assuming a dipole
coupling between the spin chain and the fluctuating magnetic field of the
surface. The numerical calculations indicate that the oscillations are damped
by a rate , where is the number of spins and is
the relaxation time of a single spin.Comment: 10 pages, Latex, two postscript figures; submitted to Phys. Rev.
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