C. Stockinger

PHOTOINDUCED ENHANCEMENT OF THE C-AXIS CONDUCTIVITY IN OXYGEN-DEFICIENT YBA2CU3OX THIN FILMS

High quality thin films of oxygen-depleted YBa2Cu3Ox (x≈6.6) were prepared by pulsed-laser deposition on SrTiO3, substrates that were cut with tilt angles of 10° and 20° with respect to the [001] direction, resulting in a steplike growth of the layers. The resistance showed a semiconducting behavior along the projection of the c axis to the film surface, but a metallic behavior in the perpendicular direction, indicating that the former is dominated by the c-axis resistivity and the latter by the ab-plane resistivity of YBa2Cu3Ox. Long-term illumination of the samples with a 100 W halogen lamp resulted in a significant conductivity enhancement in both directions. The photoinduced change of the out-of-plane resistance vs temperature characteristics is comparable to the effect of large hydrostatic pressure, introducing structural changes similar to that of photoexcitation.

Dynamics of photodoping and photoconductivity relaxation in oxygen-deficient YBa2Cu3Ox

Abstract Studies of the time evolution of the photodoping process and of the relaxation of persistent photoconductivity (PPC) in oxygen-deficient YBa 2 Cu 3 O x are presented. We show that the resistance decrease during white light illumination of the samples does not saturate up to photon doses of 10 23 –10 24 cm −2 . The efficiency of the photodoping process is weakly temperature dependent, decreasing at low temperatures for extended illumination. The relaxation of PPC follows a Kohlrausch law with the time constant of several hours near room temperature and more than 1200 h at 254 K. Similarly to semiconducting samples, the temperature dependence of the relaxation rate follows the thermal activation process across an energy barrier of ≈0.9 eV. However, the dispersion parameter exhibits in our case the opposite temperature dependence — it increases with the temperature decrease. There is no indication of a threshold temperature around 270 K. The observed behavior suggests that two different mechanisms contribute to the photodoping process.

In-situ resistivity and Hall effect studies of persistent photoconductivity in oxygen-depleted YBa2Cu3Ox

We report on in-situ studies of the resistivity and the Hall effect in partially oxygen-depleted, metallic YBa2Cu3Ox (YBCO) thin films during illumination with white light. The measurements were performed at temperatures of 100 K, 200 K, and 290 K and showed that the resistivity as well as the Hall coefficient decreased as a function of the illumination time. The photo-induced reduction of both quantities was largest at 290 K. Evaluation of the results of the Hall effect measurements within a simple one-band model shows that both the carrier mobility and the carrier concentration are enhanced by photodoping at 100 K and 200 K, with the enhancement of the carrier concentration dominating. At 290 K, however, the mobility decreases at large illumination times whereas the carrier concentration increases substantially, over-compensating for the loss of mobility. From the qualitatively different time dependencies of the carrier mobility and the carrier concentration, we conclude that two co-existing mechanisms contribute to photodoping: The first mechanism is related to a change of the electronic structure and is tentatively attributed to photo-assisted oxygen ordering. The second mechanism resembles the photogeneration of carriers in semiconductors and is ascribed to a photo-induced charge transfer. At 290 K, only the charge transfer process drives photodoping, whereas oxygen ordering seems to be hampered by thermal disordering. Additional evidence for the co-existence of the two persistent photoconductivity mechanisms is derived from measurements of the spectral efficiency of photodoping at 253 K. We observe a finite photodoping effect at photon energies above and below the charge transfer gap of YBCO. Above the gap energy (approximately equals 1.6 eV), however, the efficiency of photodoping increases remarkably. Our conclusion is that two mechanisms contribute to photodoping in metallic YBCO. One is related to oxygen ordering and does not involve interband transitions. Additionally, photo-induced charge transfer is responsible for the enhanced spectral efficiency of photodoping when the photon energy of the exciting light exceeds the charge transfer gap.

Large photoinduced enhancement of the electrical anisotropy in semiconducting YBa2Cu3Ox

Abstract The impact of long-term light excitation on YBa 2 Cu 3 O x (YBCO) ( x ≈6.4) thin films grown on “wedged” SrTiO 3 substrates was studied at several temperatures between 50 and 275 K. From the anisotropic resistance of the films, the in-plane ( ρ ab ) and the out-of-plane ( ρ c ) resistivities were calculated. The electrical anisotropy ρ c / ρ ab increased substantially during the light excitation at all studied temperatures. At 50 K, an increase of more than a factor of two was observed, whereas at higher temperatures the increases were smaller. At 275 K, ρ c / ρ ab showed a slightly falling tendency after about 1 h of illumination indicating oxygen ordering. The results strongly support our previously proposed two-mechanism picture of persistent photoconductivity in YBCO.

Spectroscopic Studies of the Persistent In-Plane and Out-of-Plane Photoconductivity in 60 K YBa2Cu3Ox

Investigations of the wavelength dependence of the so-called photodoping effect in 60 K YBa2Cu3Ox were performed using samples grown on “tilted” substrates, allowing to study the photo-induced changes of the in-plane and out-of-plane resistivities, ρab and ρc, respectively. The photo-excitation was carried out at photon energies from 1.16 to 2.05 eV and at several temperatures between 70 and 305 K. At 100 up to 305 K, the spectral efficiencies of the in-plane and out-of-plane photodoping showed a single maximum at about 1.8 eV. The anisotropy of the spectral efficiency at this maximum was significant at low temperatures, but decreased rapidly when the temperature approached room temperature, indicating an additional contribution to the out-of-plane spectral efficiency at high temperatures. At 70 K, we observed an additional maximum at 1.4 to 1.5 eV that is not predicted by the proposed models for the persistent photoconductivity in YBa2Cu3Ox, which assume electronic excitations across the charge-transfer gap of 1.7 eV.

Photodoping effect in Y-Ba-Cu-O Josephson junctions and thin films

We report our studies on the photo doping effect in YBa/sub 2/Cu/sub 3/O/sub x/ (YBCO) grain-boundary weak links and partially oxygen-depleted (T/sub c,mid/ /spl ap/52 K) thin films, We focus our attention on the erasable process, in which persistent photoconductivity (PPC) introduced by light at low temperatures relaxes back above 260 K. In the tested 2-/spl mu/m-wide step-edge grain-boundary junctions, PPC manifested itself as above 30% increase of the critical current, while the illuminated films exhibited up to 2.5 K enhancement of T/sub c/s. In the case of films, we also observed upon illumination a continuous decrease (no saturation effect) of both the longitudinal and transversal (Hall) resistivities. In-situ Hall effect band model, PPC at low temperatures results in the increase of both the hole mobility and concentration, while at temperatures above 250 K, the mobility is reduced after prolonged illumination, whereas the carrier concentration is enhanced even more than allow temperatures. We have also demonstrated that the T/sub c/ enhancement due to photodoping is a function of the carrier concentration rather than the mobility, Our experimental results indicate that the physical origin of PPC is same in both YBCO grain-boundary junctions and partially oxygen-depleted films, and can be understood as the coexistence of photoassisted oxygen ordering and photoinduced charge transfer.

Pulsed-laser deposition and characterization of high-temperature superconducting films

Pulsed-laser deposition (PLD) is a unique technique that has been employed for thin film growth of a broad variety of materials. In this contribution, PLD of high-temperature superconducting films of YBA2Cu3O7-(delta ), Bi2Sr2CaCu2O8+(delta ) and (Hg,Re)Ba2CaCu2O6+(delta ) is reported. Emphasis is put on the optimization of deposition parameters and the growth of so-called tilted films on vicinal cut substrates. Such films offer the unique possibility to measure in-plane and out-of- plane transport properties which is especially important for materials not available as single crystals. Experiments on photodoping and on vortex string channeling are presented. The electrical properties and the microstructure of the vicinal films are investigated with respect to the tilt angle and the film thickness.

Photoinduced changes of the in-plane and out-of-plane electronic properties in YBa2Cu3Ox

We report on the photoinduced changes of the electronic properties of YBa2Cu3Ox (x ≈ 6.6) thin films, focussing on the electrical anisotropy in the normal and the superconducting state. The latter was determined from magnetoconductivity measurements, performed before and after light excitation, and fitting the experimental results to the Aslamazov–Larkin theory of superconducting order-parameter fluctuations. The normal-state anisotropy σab/σc was calculated from resistance measurements carried out on samples grown on vicinal substrates. We observed that the photodoping process enhanced superconducting coherence lengths ξc(0) and ξab(0), but reduced the superconducting anisotropy ξab(0)/ξc(0) at all studied temperatures. In contrast, σab/σc was enhanced by photodoping at low temperatures, but reduced at high temperature. Our results strongly suggest that two different mechanisms contribute to the photodoping process, with one affecting only the normal-state properties.

Photodoping and relaxation of persistent photoconductivity in YBa2Cu3Ox studied byin-situ transport measurements

We report on in-situ resistivity and Hall-effect studies of partially oxygen-depleted YBa2Cu3Ox thin films during white light illumination. The measurements were performed at various temperatures between the superconducting transition and room temperature. At all temperatures the resistivity and the Hall coefficient decreased as a function of the illumination time. The Hall number pH and the Hall mobility μH, calculated within a simple one-band model, showed an increase of pH during the illumination without a saturation. At low temperatures, μH behaved similarly to pH. At 200 K, however, μH became constant after several hours of illumination, while at 260 K and 290 K, we observed a short increase of μH followed by a long-term decrease. From the different time dependences of μH and pH, we conclude that two mechanisms contribute to the photodoping effect. Additionally, we observed that after termination of the illumination at 290 K the photodoped values of pH and μH relaxed with substantially different time constants.