title: Far-infrared optical conductivity gap in superconducting MgB2 films.
abstract: We report the first study of the optical conductivity of MgB2 covering the range of its lowest-energy superconducting gap. Terahertz time-domain spectroscopy is utilized to determine the complex, frequency-dependent conductivity sigma(omega) of thin films. The imaginary part reveals an inductive response due to the emergence of the superconducting condensate. The real part exhibits a strong depletion of oscillator strength near 5 meV resulting from the opening of a superconducting energy gap. The gap ratio of 2Delta0/k(B)TC approximately 1.9 is well below the weak-coupling value, pointing to complex behavior in this novel superconductor.
authors.affiliation: Department of Physics, University of California at Berkeley, and Materials Sciences Division, E. O. Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors.forename: Robert A
authors.initials: RA
authors.lastname: Kaindl
authors.forename: Marc A
authors.initials: MA
authors.lastname: Carnahan
authors.forename: Joseph
authors.initials: J
authors.lastname: Orenstein
authors.forename: Daniel S
authors.initials: DS
authors.lastname: Chemla
authors.forename: Hans M
authors.initials: HM
authors.lastname: Christen
authors.forename: Hong-Ying
authors.initials: HY
authors.lastname: Zhai
authors.forename: Mariappan
authors.initials: M
authors.lastname: Paranthaman
authors.forename: Doug H
authors.initials: DH
authors.lastname: Lowndes