Photodoping of YBa2Cu3Ox: dependence on temperature and photon energy

We present studies of the photodoping process in underdoped metallic YBa2Cu3Ox, where x∼6.6. Measurements of the time-dependent electrical resistivity during photoexcitation were performed at several temperatures and with two different lasers as the light source. The effect of photodoping on the samples was characterized by the resistivity reduction ΔR(N)/R(0) and by the enhancement ΔTc of the superconducting transition temperature for a fixed photon dose of N = 1.2 x 1023 cm-2. At low temperatures, we observed a large ΔR(N)/R(0) but a small ΔTc. In the intermediate temperature range, both quantities were relatively small. At the highest studied temperature (270 K), ΔTc reached the largest values in these experiments, and ΔR(N)/R(0) was almost as large as at low temperatures. Interestingly, only at low temperatures did we observe a significant dependence of ΔR(N)/R(0) on the laser wavelength λ, whereas ΔTc is essentially independent of λ. The time dependence of the resistivity during the photoexcitation showed a stretched-exponential decrease at all temperatures, with values of the dispersion parameter β between 0.3 and 0.55, which are distinctly smaller than observed in persistent photoconductivity relaxation in YBa2Cu3Ox. β was essentially the same for the two laser wavelengths and showed only a weak temperature dependence. We observed a tendency of β towards smaller values (∼0.4) at low temperatures and to slightly higher values (∼0.5) above approximately 200 K. We interpret this finding within a microscopic theory of the stretched-exponential parameters, and conclude that there is a long-range interaction between the photoexcited electrons and the chain defects, which act as trapping centres and that the traps become mobile at high temperatures