Condensed Matter Physics Seminar
Infrared Exploration of Electrons in Superconductors
The City College of New York
Electron-phonon coupling and in general electron-boson coupling, are essential for the formation of Cooper-pairs in superconductors. Infrared spectroscopy is a direct probe to study such interactions and their influences on superconductivity. Recently with much improved signal- to-noise, it has become possible to derive the electron- boson spectral function from infrared reflectivity measurements using a second derivative technique. In this talk, three representative superconductors will be discussed: Nb (Tc = 9.2 K), MgB2 (Tc = 39.6 K) and OP Bi2212 (Tc = 91.5 K). The robustness of the second derivative technique will be established first in Nb by determining a2F(w) in this classic BCS superconductor. The electron-phonon interaction is more intriguing in MgB2 and ltr = 0.13 is determined from our optical data. This surprising result can be understood since the electron-phonon interaction is highly anisotropic in MgB2. The anisotropic picture could also have important implications in high-Tc cuprates. An electron-boson spectral function peaked at 43 meV is found in OP Bi2212 using the second derivative technique and this boson is shown to be involved in the pair formation. In contrary to the recent suggestion, our optical data on OP Bi2212 indicate that the zone-boundary phonons (~ 80 meV) alone are not sufficient to explain the superconducting properties in high-Tc cuprates and our data are consistent with the spin-fluctuation models.
Thursday, November 13, 2003
Gant Science Complex
(The seminar is followed by coffee/tea, cookies, and an informal discussions.)