Abstract
The persistent photoinduced effects in Y(SrxBa2-x)Cu3O7-□, (Y1-xCax)Ba2Cu3O7-□ and (Y1-xPrx)Ba2Cu3O7-□ films are studied. The optical source used for our experiment was provided by an Argon laser. Our experiments show the PPC effects are absent in fully oxygenated Y(SrxBa2-x)Cu3O7-□ films. However, the PPC effects do occur in both fully oxygenated (Y1-xCax)Ba2Cu3O7-□ and (Y1-xPrx)Ba2Cu3O7-□ films. We speculate that the PPC effects are negligibly small for any “123” system fully annealed in one atmosphere of oxygen even though the oxygen vacancies might be already present. This inference contradicts the photoinduced vacancy capture model which is the most commonly invoked model to explain the persistent photoinduced phenomena. We propose a modification of the phenomenological photoinduced vacancy capture model to explain why the PPC effects are negligibly small for fully oxygenated “123” systems. In the fully oxygenated “123” systems, the photoexcitation electron which is trapped at an oxygen vacancy in the Cu-O chain can easily hop to one of adjacent long perfect Cu-O chains. The electron can recombine with the related hole through the Cu-O chain. This is why the PPC is negligibly small in fully oxygenated “123” systems. The (Y1-xCax)Ba2Cu3O7-□ system can be changed from the overdoped regime to the underdoped regime by annealing in a controlled oxygen pressure. For the overdoped sample, both the Tc and resistance decrease after laser illumination. This behavior is consistent with the picture that the illumination introduces more holes in the CuO2 plane. We demonstrate that the laser illumination always introduces more holes in the CuO2 plane regardless of its overdoping or underdoping status. Therefore, the PPC effects can be used to optimize Tc for such a system. This also indicates that the PPC effects can be used as a sensitive indicator of whether any given “123” sample is overdoped, optimal doped, or underdoped. The PPC effects can demonstrate that the as-grown (Y1-xPrx)Ba2Cu3O7-□ films are overdoped.We used pulse terahertz (THz) radiation to study the complex conductivity of YBCO thin films with and without PPC effects. An ultrashort laser pulse (~ 100 fs duration) from a mode-locked Ti:Sapphire laser is used to generate a freely propagating THz pulse by photoexciting a semiconductor gap between two biased electrodes of a large aperture antenna fabricated on SI-GaAs. The transmitted THz radiation through a 2000-A-thick YBCO film on NdGaO3 substrate is measured by a similar antenna with a smaller photoconductive gap on low-temperature grown GaAs. The transmissivity yields the information about the complex conductivity of the thin film samples. The penetration depth of fully oxygenated YBCO film obtained from the imaginary part of the conductivity decreases linearly with temperature as expected from the d-wave theory in the clean limit. However, its zero-temperature value is larger than that of typical high quality single crystal YBCO. The temperature dependence of the real part of the conductivity below Tc exhibits a frequency-dependent broad peak. Using the two-fluid Drude model for the superconducting state, a temperature-dependent of the quasiparticle scattering rate is observed. In addition, THz time domain spectroscopy is also put to use to study the PPC effects in oxygen deficient YBCO thin film. Both real and imaginary parts of the conductivity of the oxygen deficient YBCO is enhanced after prolonged Argon laser illumination at low temperatures. These results are consistent with the standard interpretation of hole-concentration enhancement by photocarrier doping.