Person:

Demler, Eugene

Condensates of atoms with spins can have vortices of several types; these are related to the symmetry group of the atoms’ ground state. We discuss how, when a condensate is placed in a small magnetic field that breaks the spin symmetry, these vortices may form bound states. Using symmetry classification of vortex charge and rough estimates for vortex interactions, one can show that some configurations that are stable at zero temperature can decay at finite temperatures by crossing over energy barriers. Our focus is cyclic spin-2 condensates which have tetrahedral symmetry.

The superfluid-insulator transition of bosons is strongly modified by the presence of Fermions. Through an imaginary-time path integral approach, we derive the self-consistent mean-field transition line, and account for both the static and the dynamic screening effects of the fermions. We find that an effect akin to the fermionic orthogonality catastrophe, arising from the fermionic screening fluctuations, suppresses superfluidity. We analyze this effect for various mixture parameters and temperatures, and consider possible signatures of the orthogonality catastrophe effect in other measurables of the mixture.

In this paper we discuss the interplay of quantum fluctuations and dissipation in uniform superconducting nanowires. We consider a phenomenological model with superconducting and normal components, and a finite equilibration rate between these two-fluids. We find that phase-slip dipoles proliferate in the wire, and decouple the two-fluids within its bulk. This implies that the the normal fluid only couples to the superconductor fluid through the leads at the edges of the wire, and the {\it local} dissipation is unimportant. Therefore, while long wires have a superconductor-metal transition tuned by local properties of the superconducting fluid, short wires have a transition when the total resistance is (R_{total}) = (R_{Q}) = (h/4e^{2}).

(d)-wave Cooper pairs are believed to be the key for understanding the phenomenon of high-temperature superconductivity in cuprates. These superconductors are an example of the emergence of strong pairing in systems with purely repulsive interactions, similar to superfluid helium 3 and the newly discovered iron oxypnictides. Despite intense studies, there is currently no consensus as to what causes the formation of (d)-wave Cooper pairs in these materials. Here we propose a novel experimental scheme in which recently demonstrated methods for realizing optical lattices and superlattices are combined to create and to detect, in a controlled way, ultracold-atom (d)-wave Cooper pairs. Our scheme starts from arrays of isolated plaquettes which incorporate the required (d)-wave correlations on a short length scale. By tuning the parameters of the potentials, these plaquettes can be coupled to achieve long-range (d)-wave superfluid correlations, finally arriving at the generic Hubbard model.