Thursday, August 2, 2007

Laura Greene: Andreev reflection in Ce 115's





Laura presented point-contact spectroscopy results on CeCoIn5/Au contact,
namely the data on normalized tunneling conductance for the wide range of
temperatures. Data shows an asymmetry which appears below certain temperature,T*, most probably associated with a formation of a heavy Fermi liquid. As CeCoIn5 becomes superconducting, the small (~13%) Andreev signal is
observed. Laura presented fits to the data based on the the Blonder-Tinkham-Klapwijk (BTK) model. To account for an asymmetry, the "Kondo-like" resonance peak is included in the density of states. The Andreev reflection data for various tunneling directions convincingly shows the d-wave symmetry of the order parameter. To account for an amplitude of Andreev signal it is assumed that there are two contributions coming from the light gapless Fermi surface and heavy Fermi surface which is gapped. Questions from the audience (C. Capan, A. Balatsky, A. Millis, Q. Si) were mostly concerned with the origin of an asymmetry in tunneling conductance and whether one might think of this asymmetry as a result of the formation of a heavy quasiparticles. Laura's BIG QUESTIONS are:

1. Why does BTK model works so well?

2. What would be theoretical justification for "two-fluid" picture of gapless
and gapped Fermi surfaces?

3. Does the peak in the density of states exist?

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