We report on far-infared (FIR) reflectivity measurements on sintered pressed poweder 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 Tc with, depending on the measurement criterion used, values of 2delta/kbTc 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.
Citation: Physical Review B
Pub Type: Journals