Superconducting Energy Gap from Infrared Response Measurements in Granular Tl2Ba2CaCu2O8 Films

The resistance response of polycrystalline Tl2Ba2CaCu2O8 films to short laser pulses has been investigated for a wide range of far-infrared frequencies. For temperatures T ll Tc a drastic change in response time is observed at a critical frequency νc simeq 4 THz. The time constant increases from about 1 ns at ν νc. The critical frequency is attributed to a superconducting energy gap with 2Δ(0)/kB Tc simeq 2