Fabrication and femtosecond photoresponse studies of MgB 2 superconducting thin films

Abstract

We present fabrication and optical time-resolved photoresponse characterization of MgB 2 superconducting thin films. The films were prepared on crystalline and flexible plastic substrates by vacuum co-deposition of B and Mg precursors and high-temperature annealing in an Ar or vacuum atmosphere. The post-annealed films exhibited very smooth surfaces and amorphous structures with nanocrystal inclusions. The best films exhibited the critical temperature Tc of up to 38 K, the transition width of 1 K, and the current density j c at 4.2 K of about 10 6 A/cm 2 . In our pump-probe photoresponse experiments, we used 100-fs-wide optical pulses generated by a Ti:Sapphire laser. The pump and the probe beams had 800-nm wavelength and the measurements were performed in the temperature range from 3.5 K to room temperature. The transient reflectivity change (ΔR/R) signals exhibited around 300-fs (10%-90%) risetime. At room temperature and far above Tc, (ΔR/R) the transient reflectivity change was characterized by a ~160-fs, single-exponential decay, interpreted as the electron-Debye-phonon interaction time. Below 60 K and in the superconducting state, the ΔR/R photoresponse was biexponential, with the initial femtosecond decay followed by a much slower, several-ps-long relaxation. We associate the latter slow relaxation with the electron-phonon interaction related to the Cooper pair recombination dynamics. The existence of this signal above the nominal T c of our films, we tentatively interpret as the presence of superconducting fluctuations in our MgB 2 films. Our work gives the first insight into the carrier dynamics in MgB 2 by time-resolved experimental studies of the Cooper pair breaking and thermalization mechanisms for the films perturbed by femtosecond optical excitations.