Condensed Matter Seminar Series

Title: Shining the Light on Superconductors and Nano-systems

Jiufeng J. Tu

City College of New York

Monday March 6,  4:00pm,  Room 090,  Physics Building

Abstract: Electron-phonon (el-ph) 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. In this talk, three representative superconductors will be discussed: Nb (T_c = 9.2 K), MgB₂ (T_c = 39.6 K) and OP Bi2212 (T_c = 91.5 K). The robustness of a second derivative technique will be established first in Nb by determining a_tr²F(w) optically in this classic BCS superconductor. The electron-phonon interaction is more intriguing in MgB₂ and l_tr = 0.13 is determined from our optical data. This surprising result can be understood since the el-ph interaction is highly anisotropic in MgB₂. This anisotropic picture also has important implications for high-T_c cuprates. An electron-boson spectral function peaked at 43 meV is found in OP Bi2212 using the same second derivative technique and is shown to be involved in the pair formation. Our data are consistent with the spin-fluctuation models. The issue of inhomogeneity, particularly in the underdoped region, is investigated using the ultra-thin metal films (made of nanometer-sized clusters) as model systems. In summary, as far as pairing mechanisms are concerned, all these superconductors can be understood within an extended BCS framework.