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Events on Thursday, May 13th, 2010

R. G. Herb Condensed Matter Seminar
Spin-triplet superconductivity in Co-based Josephson junctions
Time: 10:00 am
Place: 5310 Chamberlin
Speaker: Norman Birge, Michigan State University

Superconducting/Ferromagnetic (S/F) hybrid systems exhibit a number of interesting properties due to the interplay between the competing symmetries of their order parameters. With conventional spin-singlet superconductors, the proximity effect in S/F systems decays over an extremely short length scale in the ferromagnet due to the large exchange splitting between the spin-up and spin-down electron bands. In S/F/S Josephson junctions, the critical current oscillates and decays rapidly as a function of the ferromagnetic layer thickness. If there were spin-triplet superconducting correlations present, however, then both the proximity and Josephson effects would persist over much longer distances. Such correlations have been predicted to occur in S/F systems with certain forms of magnetic inhomogeneity near the S/F interface. Moreover, these correlations exhibit a strange symmetry never before observed: they are odd in frequency or time. In this talk I will discuss our efforts to produce and measure these elusive spin-triplet correlations in S/F/S Josephson junctions, culminating in our very recent success.1

1 T.S. Khaire, M.A. Khasawneh, W.P. Pratt, Jr., and N.O. Birge, Phys. Rev. Lett. 104, 137002 (2010).

Host: Robert McDermott
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Atomic Seminar
Polarization and hyperfine transitions of metastable <sup>129</sup><i>Xe</i> in discharge cell, & Nonlinear Pressure Shifts of Cs Atoms in Neon
Time: 2:00 pm
Place: 5280 Chamberlin
Speaker: Tian Xia, Princeton University

Part I: Polarization and hyperfine transitions of metastable 129Xe in discharge cell

The polarization and relaxation rates of metastable 129Xe atoms are measured with magnetic resonance spectroscopy, at both microwave frequencies, where ΔF = 1 transitions are induced between the sublevels, and at radiofrequencies, corresponding to ΔF = 0 transitions. Unlike earlier work, which mostly used ΔF = 0 radiofrequency (rf) resonances for monitoring spin polarization of even isotopes of the metastable atoms, the work reported here was focused on the isotope 129Xe , where we used both ΔF = 0 rf resonances and Δ F = 1 microwave resonances. This permits us to more unambiguously determine the degree of nuclear spin polarization of the optically pumped metastable atoms. The nuclear spin polarization of the resonant velocity group is measured to be 22 +/- 2%. The relaxation of metastable xenon atoms is dominated by depolarizing collisions with ground state atoms, with lesser contributions from metastability exchange collisions.

Part II: Nonlinear Pressure Shifts of Cs Atoms in Neon

I demonstrate that the hyperfine resonance frequency of ground state Cs atoms have a nonlinear dependence on the pressure of the buffer gas Ne at a fixed temperature. The hyperfine resonance frequency of alkali-metal atoms is shifted by an amount, which had long been assumed to be linear with the buffer gas pressure until Fei Gong discovered that the shift of Rb and Cs hyperfine resonance frequency has a nonlinear dependence on the pressure of the buffer gas Ar and Kr. I found that the nonlinear pressure shift of Cs hyperfine frequency in Ne is positive. The reason of the nonlinear shift is the three body collision (eg: Cs-Ne-Ne) and the formation of Van der Waals molecules of a Cs atom and a buffer gas atom of Ar, Kr, or Ne. The hyperfine precession rate of a Cs atom bound in molecule has a shift respect to a free Cs atom.

Host: Mark Saffman & Thad Walker
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NPAC (Nuclear/Particle/Astro/Cosmo) Forum
Joint Seminar with String Theory
Inflationary Cosmology on Trial
Time: 4:00 pm
Place: 4274
Speaker: Paul Steinhardt, Princeton University
Host: Gary Shiu
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