Fabrication and femtosecond photoresponse studies of MgB2 superconducting thin films

We present fabrication and optical time-resolved photoresponse characterization of MgB2 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 jc at 4.2 K of about 10 A/cm. 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 ( R/R) signals exhibited around 300-fs (10%–90%) risetime. At room temperature and far above Tc, R/R 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 R/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 Tc of our films, we tentatively interpret as the presence of superconducting fluctuations in our MgB2 films. Our work gives the first insight into the carrier dynamics in MgB2 by time-resolved experimental studies of the Cooper pair breaking and thermalization mechanisms for the films perturbed by femtosecond optical excitations.