15,587 research outputs found
On the Spectrum of the XXZ-chain at roots of unity
In a recent paper (cond-mat/0009279), Fabricius and McCoy studied the
spectrum of the spin 1/2 XXZ-model at Delta = (q+q^{-1})/2 and q^{2N}=1 for
integer N >1. They found a certain pattern of degeneracies and linked it to the
sl(2)-loop symmetry present in the commensurable spin sector (N divides S^z).
We show that the degeneracies are due to zero-energy, transparent excitations,
the cyclic bound states. These exist both in commensurable and incommensurable
sectors, indicating a symmetry, of which sl(2)-loop is a partial manifestation.
Our approach treats both sectors on even footing and yields an analytical
expression for the degeneracies in the case N = 3.Comment: 27 page
Parity-Affected Persistent Currents in Superconducting Nanorings
We argue that at sufficiently low temperatures T superconducting parity
effect may strongly influence equilibrium persistent currents (PC) in isolated
superconducting nanorings containing a weak link with few conducting modes. For
a single channel quantum point contact at T=0 we predict a novel effect of
parity-induced blocking of PC. In nanorings with SNS junctions a -state
can occur for the odd number of electrons. Changing this number from even to
odd yields spontaneous supercurrent in the ground state of such rings without
any externally applied magnetic flux.Comment: Published versio
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