Picosecond dynamics of the superconducting state in MgB 2

Abstract

We present time-resolved pump–probe studies of carrier dynamics in superconducting MgB 2 thin films and single crystals. The samples were excited by 800-nm-wavelength, 100-fs-wide optical pulses and their photoinduced, transient reflectivity change was measured by the second, time-delayed, 100-fs-wide optical probe. In the superconducting state, the photoresponse of MgB 2 was governed by three relaxation processes: the initial sub-picosecond electron–phonon relaxation, followed by few-picosecond-long electron–phonon component, and the nanosecond-long, phonon cooling/escape time limited, recombination of quasiparticles into Cooper pairs. In thin films, the temperature dependence of the amplitude of the superconducting component followed the two-gap anomalous temperature dependence with the gaps closing at two different T c's. The picosecond electron–phonon relaxation time constant experienced a double-divergence behavior at the same T c values.