Ultrafast, Time-Resolved Quasiparticle Dynamics in Hg-Based High Temperature Superconductors

We present our all optical and optoelectronic time-domain studies of the quasiparticle and Cooper pair dynamics in Hg-based superconductors. The samples were mixed phase (Hg-1212 and Hg-1223), $c$-axis-oriented thin films, fabricated by magnetron sputtering, followed by \textit{ex-situ} mercuration. The films exhibited the onset of the superconducting transition at T$_{c}$ = 123 K and the zero-resistance at 110 K. Far below T$_{c}$, our femtosecond pump-probe spectroscopy studies clearly demonstrated the bi-molecular-type relaxation of photoexcited quasiparticles, governed by the direct Cooper pair formation process. At temperatures close to T$_{c}$, we observed a severe phonon bottleneck, and the quasiparticle relaxation time was limited by the phonon anharmonic decay/escape process. The photoresponse studies performed on current-biased microbridges, illuminated by femtosecond optical pulses demonstrated the picosecond nonequilibrium response on top of the much slower bolometric signal.