Persistent current in a ring coupled to an external fermionic reservoir

We study the energy spectrum and the persistent current in an ideal one-dimensional mesoscopic ring coupled to a fermionic reservoir. We find that the tunnel coupling in general leads to the suppression of the persistent current. However, with increasing coupling, the effective level structure of the ring coupled to the reservoir changes and quasistates with a sharp eigenenergy develop. Depending on the number of ring states coupled to the reservoir this results in a nonzero persistent current even at very large tunneling between the ring and the reservoir.Comment: this is a short version of cond-mat/030713

Symmetry-restoring quantum phase transition in a two-dimensional spinor condensate

Bose Einstein condensates of spin-1 atoms are known to exist in two different phases, both having spontaneously broken spin-rotation symmetry, a ferromagnetic and a polar condensate. Here we show that in two spatial dimensions it is possible to achieve a quantum phase transition from a polar condensate into a singlet phase symmetric under rotations in spin space. This can be done by using particle density as a tuning parameter. Starting from the polar phase at high density the system can be tuned into a strong-coupling intermediate-density point where the phase transition into a symmetric phase takes place. By further reducing the particle density the symmetric phase can be continuously deformed into a Bose-Einstein condensate of singlet atomic pairs. We calculate the region of the parameter space where such a molecular phase is stable against collapse.Comment: 5 pages, 1 Figure + Supplemen

Decoherence induced by magnetic impurities in quantum Hall system

Scattering by magnetic impurities is known to destroy coherence of electron motion in metals and semiconductors. We investigate the decoherence introduced in a single act of electron scattering by a magnetic impurity in a quantum Hall system. To this end we introduce a fictitious nonunitary scattering matrix $\mathcal{S}$ for electrons that reproduces the exactly calculated scattering probabilities. The strength of decoherence is identified by the deviation of eigenvalues of the product $\mathcal{S}\mathcal{S}^{\dagger}$ from unity. Using the fictitious scattering matrix, we estimate the width of the metallic region at the quantum Hall effect inter-plateau transition and its dependence on the exchange coupling strength and the degree of polarization of magnetic impurities.Comment: 13 pages, 4 figure