OPTICAL CONDUCTIVITY OF SINGLE-CRYSTALS OF BA(1-X)M(X)BIO(3) (M=K, RB, X=0.04, 0.37)

Reflectance data (0.001-4.0 eV) from several large (a typical surface area 3 x 3 mm(2)) single crystals of Ba1-xKxBiO3 (x = 0.04, 0.37) (BKB O) and Ba1-xRbxBiO3 (x = 0.37) (BRBO) were obtained by Fourier transfo rm infrared (FTIR) and ellipsometric methods. Normal-state optical con ductivities (sigma(1)) of these samples were obtained from infrared an d ellipsometric measurements using a Kramers-Kronig transform. A broad mid-IR band was observed that peaked at 0.3 eV for BKBO and at 0.16 e V for BRBO at room temperature. Each band was fitted with two Lorentz oscillators. The optical mass of the charge carriers was obtained from a Drude fit, and was found to be large (m = 28-33m(e)). These overda mped charge carriers can be viewed as polarons with a large effective mass. An optic phonon mode at 325 cm(-1) was also observed in the meta llic phase. This mode was identified as a disorder-induced lattice mod e, and was strongly enhanced at 8 K, favoring a strong coupling betwee n this phonon and itinerant electronic states. Low-frequency spectra b etween 10 and 400 cm(-1) observed below the superconducting temperatur e indicated an energy gap that agreed with the BCS-type mechanism. Int erpretations of low-temperature measurements on BKBO and BRBO were com plicated due to the change of color of the sample from bluish-green to bronze-red. Upon warming, samples revert to their original bluish-gre en color.