Characterization and Detection of Novel Phases in Fermionic Superfluids with Density Imbalance between Pairing Species

Kun Yang, Florida State University, DMR-0225698

Superfluidity in nature often originates from pairing between fermions of different species; for example, electrons with opposite spins pair up in superconductors. Thus the most favorable situation for pairing is when the two species have equal density. We have studied possible pairing states when the two species of fermions have different densities, so that some of them are necessarily unpaired. In particular, we have studied how to characterize these new phases, and detect them experimentally in superconductors and trapped cold atom systems.

References: K. Yang, Phys. Rev. Lett. 95, 218903 (2005); K. Yang and S. Sachdev, Phys. Rev. Lett. 96, 187001 (2006); S. Sachdev and K. Yang, Phys.Rev. B 73, 174504 (2006); Q. Cui, C.-R. Hu, J.Y.T. Wei, and K. Yang, Phys. Rev. B 73, 214514 (2006).

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One possible new phase is the Fulde-Ferrell-Larkin-Ovchinnikov phase, in which the pairing order parameter carries a momentum q. We showed that for d-wave superconductors, q can be measured in electron tunneling experiment, because q shifts the energies of the surface bound states formed due to the sign change of order parameter (- to +) (Q. Cui et al., 06).

Personnel Development: The PI’s graduate student, Qinghong Cui, was actively involved in this work. He is expected to graduate in fall of 2006.

Broad Scientific Impact: The issue studied here originally arose in superconductors subject to a strong magnetic field, which favors electrons with spins parallel to the field. More recently the same issue is being studied in the context of superfluidity in nuclear matter, neutron stars, quark matter, and cold atom systems. Thus this is a fundamental issue of importance to all branches of physics. Due to its broad significance, there was a workshop at the Inst. for Nuclear Theory of Univ. of Washington (Sept. 05) devoted exclusively to this issue, attended by physicists from all fields, and the PI was an invited speaker representing the condensed matter/materials physics community.

Outreach Activities: In June of 2006 the PI delivered this year’s first T. D. Lee Lecture at the Chinese Academy of Science in Beijing. The T. D. Lee Lectures are attended by all first-year and many more senior graduate students from all disciplines at the academy; the PI’s lecture was attended by over 300 students. The content and level of the lecture are designed to be accessible to people with basic physics knowledge at the undergraduate level, so that the students can understand and appreciate the exciting advancements of our field.

Characterization and Detection of Novel Phases in Fermionic Superfluids with Density Imbalance between Pairing Species

Kun Yang, Florida State University, DMR-0225698