Superconducting energy gap and a normal-state excitation in Ba0.6K0.4BiO3.

Abstract

We report the first measurement of the energy gap of the high-temperature bismuth oxide superconductor ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_{3}$ (${T}_{c}$\ensuremath{\simeq}30 K). With temperature- and magnetic-field-dependent infrared reflectivity measurements, we obtain a reduced energy gap of 2\ensuremath{\Delta}/${\mathrm{kT}}_{c}$\ensuremath{\simeq}3.5\ifmmode\pm\else\textpm\fi{}0.5, consistent with a BCS-type mechanism with moderate or weak coupling. In the normal state a broad peak in the infrared conductivity is observed near 5000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. This unusual infrared behavior is common to both the ${\mathrm{BaPb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Bi}}_{\mathrm{x}}$${\mathrm{O}}_{3}$ and the ${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{K}}_{\mathrm{x}}$${\mathrm{BiO}}_{3}$ systems and may provide a fundamental clue to the mechanism of superconductivity in the cubic bismuth oxides.