Superconducting gap excitations in MgB/sub 2/ revealed via time-domain THz spectroscopy

Summary form only given. We present the first measurements of the far-infrared conductivity of MgB/sub 2/ that encompass a frequency range spanning excitations across the fundamental superconducting energy gap. MgB/sub 2/ films of different thicknesses are grown on Al/sub 2/O/sub 3/ substrates using e-beam evaporation and ex-situ annealing, with T/sub c/ of 30-34 K. We utilize terahertz time-domain spectroscopy to directly obtain the complex conductivity /spl sigma/(/spl omega/) in the spectral range of interest, without the need for Kramers-Kronig transformations. Picosecond far-IR pulses are generated and detected via optical rectification and electro-optic sampling in ZnTe using near-IR pulses from a 250 kHz Ti:sapphire amplifier. The THz conductivity of MgB/sub 2/ in the vicinity of its superconducting energy gap 2/spl utri//sub 0/ exhibits a BCS-like shape but an unusually small gap ratio 2/spl utri//sub 0//k/sub B/T/sub c/ /spl ap/ 1.9 pointing to complex behavior of this novel superconductor.