FOCUS

The BCS theory of superconductivity is a mean field theory in which the physical properties of the superconductor arise from the influence of interactions between the charge carriers, which is approximated by an average over the charge carriers in the superconductor. This is similar to the Hartree approximation in metals. In this way, fluctuations about this average influence are ignored. For low-dimensional superconductors, such as the cuprate superconductors, that are quasi­two dimensional, and the organic superconductors, some of which are quasi­two dimensional and others of which are quasi­one dimensional, there are corrections to this mean field approximation that show up in experiments. These corrections can be identified in measurements of the current across superconductor-insulator-superconductor tunnel junctions made from hi-T_c superconductors. These differences give information on the nature of the superconducting state and is a new probe of superconductivity in the cuprate and organic superconductors.

A second project involves the normal state of conductors. Band structure effects in low-dimensional systems, such as the organic and cuprate materials, can lead to large deviations from the frequency and temperature dependencies determined for a Fermi liquid based on a free particle dispersion. In such circumstances, it is important to determine what Fermi liquid dependencies are for the interpretation of experiments.

SELECTED PROJECTS

• Role of collective excitations in enhancing quasiparticle interactions in low-dimensional superconductors
• Nature of corrections to the leading temperature, magnetic field, and
  frequency dependencies of properties of low-dimensional Fermi liquids
• Influence of band structures on many-body effects in normal and superconducting states
• Electronic and lattice degrees of freedom in semiconductor heterostructures
• Properties of mesoscopic magnetic systems and the influence of quantum fluctuations on classical results

PUBLICATIONS

• D. Coffey and N. Bock. Intrinsic electric fields and raman scattering in Wurtzite semiconductor heterostructures. Phys. Rev. B 59:5799 (1999).
• J. S. Kim and D. Coffey. Single-particle properties of a two-dimensional Fermi liquid at finite frequencies and temperature. Phys. Rev. B 57:542 (1998).
• D. Coffey. Strong coupling features due to quasiparticle interactions in two-dimensional superconductors. Physica C 305:139 (1998).
• D. Coffey. Quasiparticle interactions in two- and three-dimensional superconductors. Europhys. Lett. 40:563 (1997).
• J. S. Kim and D. Coffey. Two-dimensional Fermi liquids at low density: The single-particle self-energy. Phil. Mag. B 74:477 (1996).