Measurement of the superconducting energy gap in La-Ba-Cu oxide and La-Sr-Cu oxide

Abstract

We report on far-infrared (FIR) reflectivity measurements on sintered pressed powder samples of La-Ba-Cu oxide and La-Sr-Cu oxide. The best samples show fairly sharp transitions to zero measurable resistance with a transition midpoint of 28 K for the Ba alloy and 36 K for the Sr alloy. The FIR measurements reveal an energy gap below $T_c$ with, depending on the measurement criterion used, values of 2Δ/$k_B$ $T_c$ of 1.3–1.95 for La-Ba-Cu oxide and 1.6–2.7 for La-Sr-Cu oxide, which are lower than expected for a BCS superconductor.