Optical study of superconducting Ga-rich layers in silicon
  We performed phase-sensitive terahertz (0.12 - 1.2 THz) transmission measurements of Ga-enriched layers in silicon. Below the superconducting transition, T_{c} = 6.7 K, we find clear signatures of the formation of a superconducting condensate and of the opening of an energy gap in the optical spectra. The London penetration depth, \lambda(T), and the condensate density, n_{s} = \lambda^{2} 0)/\lambda^{2}(T), as functions of temperature demonstrate behavior, typical for conventional superconductors with \lambda(0) = 1.8 \mu m. The terahertz spectra can be well described within the framework of Eliashberg theory with strong electron-phonon coupling: the zero-temperature energy gap is 2\Delta(0) = 2.64 meV and 2\Delta(0)/k_{B}T_{c} = 4.6 \pm 0.1, consistent with the amorphous state of Ga. At temperatures just above T_{c}, the optical spectra demonstrate Drude behavior. 
T. Fischer, A. V. Pronin, R. Skrotzki, T. Herrmannsd\"orfer, J.   Wosnitza, J. Fiedler, V. Heera, M. Helm, E. Schachinger
cond-mat.supr-con cond-mat.mtrl-sci