Photodoping of YBa₂Cu₃O_x: dependence on temperature and photon energy

Abstract

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 × 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.

Enhancement of the critical current of intrinsic Josephson junctions by carrier injection

“…This explains why the doping process starts only beyond a certain threshold value, which was indeed observed [6]. If our β value would be closer to the larger value, the probability of electron trapping would be too small, since in the short-range case, the effective dimensionality of the configuration space, in which the relaxation or trapping takes place, is limited to a single unit cell [4]. According to this picture, our present results are in good agreement with our earlier interpretation, which is associated with the trapping of electrons [6].…”

“…Usually, doping in high-T c cuprates is achieved by chemical substitution or a variation of the oxygen excess δ. However, the carrier concentration can be also changed by physical processes such as photodoping [4], electric-field effect [5], or the recently discovered carrier injection along the c-axis [6]. For cuprate superconductors, physical doping can be persistent at low temperatures.…”

Persistent photoconductivity in oxygen-deficient and ion-irradiated YBa2Cu3Ox

“…In several of our previous studies [23,25,29,34] we observed that the resistance decrease (or conductivity increase) resulting from photodoping strongly depended on the temperature T dop at which the photodoping was carried out. The resistance decrease was large at low temperatures, relatively small at intermediate temperatures, and large again at high temperatures.…”

“…The data were normalized to the value at 70 K. R max /R 0 at 70 K was 9.5% (green laser), 8.5% (He-Ne laser), and 11% (white light). Data were taken from [25,34]. The line is a guide to the eye.…”

“…The resistance decrease was large at low temperatures, relatively small at intermediate temperatures, and large again at high temperatures. Figure 1, which was created from the raw data used in [25,34], shows the measured changes of the resistance of 60 K YBCO, R max , achieved by illumination with three different light sources, plotted as a function of T dop . Within each of the three measurement series, the illumination time 1.…”