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Special Seminar

Exploring Exotic Superfluidity of Polarized Ultracold Fermions in Optical Lattices

by: Prof. Yan Chen

Date: Friday January 04, 2008

Time: 12:00 pm – 1:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

Enormous interest has been paid to ultracold Fermi gases due to the interplay between Cooper pairing and strong correlations. Beautiful experiments on the superfluidity have been performed in these systems with unequal spin populations. Arrestingly, it was found that the superfluid paired core is surrounded by a shell of normal unpaired fermions while the density distribution of the difference of the two components becomes bimodal. Here we explore theoretically the novel superfluidity of harmonically- trapped polarized ultracold fermionic atoms in a two-dimensional optical lattice by solving the Bogoliubov-de Gennes equations. The pairing amplitude is found to oscillate along the radial direction at low particle density and along the angular direction at high density. The former is able to account for the existing experimental observations, while the latter predicts a new kind of Fulde-Ferrell-Larkin- Ovchinnikov states, which can be tested in experiments.

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Special Seminar

Enhanced ferroelectricity in doped niobium clusters

by: Prof. Ramiro Moro

Date: Friday January 04, 2008

Time: 3:00 pm – 4:00 pm

Location: Houston Science Center – Building 593 — Room 102

Overview

In September of 2002 we observed a surprising effect in free niobium clusters: some of them acquired an electric dipole moment at low temperature. It is a surprising phenomenon because metallic particles are supposed to screen electric fields and because electronic transitions in small particles usually have very high transition temperatures, but the most surprising characteristic of the effect is that it seems to be related with superconductivity: It happens in the same metals, with similar transition temperatures, it is affected by magnetic dopants and it is enhanced for particles with even number of electrons (suggesting pairing).

It was already suggested in 1992 [Friedel, 1992] that a weakly interacting lattice of superconducting nanoparticles or clusters could yield a very high temperature superconductor. That work was prompted by the superconductivity seen in doped fullerenes, but the idea can be extended further. Recently there has been a discovery of superconductivity in a gallium cluster compound [Bakharev, 2006] which could be the first realization of Friedel?s proposal. Moreover there is recent theoretical work done by V.Z. Kresin and Ovchinnikov [Ovchinnikov, 2005] that suggest that small particles of certain metals could have an enhanced Tc due to shell effects.

Barring an incredible coincidence, Nb clusters are indeed superconducting, but its superconductivity is manifested as ferroelectricity. And some alloys, like gold-doped niobium have transition temperatures at more than 300K!

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