Nonequilibrium Superconductivity and Quasiparticle Dynamics Studied by Photoinduced Activation of mm-Wave Absorption

The occurrence of zero’s in the superconducting gap for certain values of the momentum ¯~k at the Fermi surface of high Tc superconductors has a number of intriguing consequences for the dynamical behavior and lifetime of the quasi-particles at low temperatures, which has only recently begun to attract the attention of researchers in the field. Due to the presence of these zero’s (or nodes) the reduction in the superfluid fraction (ρs) [1,2] and specific heat [3] is proportional to H 1/2 , where H is the magnetic field. Also, a strong reduction of the quasi-particle scattering rate (1/τ) below Tc [4–6] provides evidence that the dominant scattering mechanism has an electronic signature. In this Letter we present a study of the quasi-particle dynamics using Photo Induced Activation of Mm-wave Absorption (PIAMA). In this pump/probe experiment we use a free electron laser [7] (FELIX) which is continuously tunable from 100 to 2000 cm −1 as a pump to create a temporary excited state of a superconductor. FELIX produces macro-pulses with a stepwise off-on-off intensity profile (’on’ for 3step-response of the complex dielectric constant is monitored at 5 cm −1 using the combination of a Backward Wave Oscillator (BWO) and a fast waveguide diode detector as a probe to measure the transmission through a superconducting film as a function of time. The mm-wave detector-circuit was selected as a compromise between sensitivity and speed of detection, resulting in an overall time resolution of 1 � s. This choice of experimental parameters is optimal for the detection of small changes induced in the dielectric function by the infrared (IR) pulse at, as we will see, the scale of the lifetime of nonequilibrium superconductivity in high Tc’s. We used films of DyBa2Cu3O7−� which were prepared by RF sputtering on LaAlO3 substrates. The film thickness was 20 nm and Tc was 88 K. Optimal surface quality was obtained by using Dy instead of Y. This substitution does not affect the superconducting properties. A detailed description of the preparation, characterization and the mm-wave dielectric properties of these films has