Photoinduced changes in the transport properties of oxygen-deficientYBa2Cu3

Abstract

We show that illumination of oxygen-deficient metallic YBa2Cu30" films produces a change in the Hall coefficient, an increase in the critical temperature, and a decrease in the electrical resistivity. These changes relax to equilibrium with characteristic times of the order of days, and are due to variations in both the carrier density and mobility. The relaxation times are of the same order of magnitude as the ones measured in nonilluminated oxygen-deficient films immediately after quenching.Photoconductivity has a long history and occurs in a variety of insulating and semiconducting materials. The existence of a metal-insulator (M-I) transition in high-T, superconductors make these materials attractive systems for photoexcitation experiments. Raman scattering measurements indicate the appearance of normally forbidden modes in fully oxygenated photoexcited samples. Transient photoinduced changes of more than ten orders of magnitude in the surface resistivity of YBa2Cu30 (YBCO) single crystals have been reported earlier. Later on, persistent photoconductivity in insulating YBCO films was observed. ' It was shown that laser illumination induces a systematic decrease with long relaxation times in the electrical resistivity p""( T) of oxygendeficient YBCO films. These experimental findings opened up the possibility for the existence of photoinduced superconductivity.Recently we have unambiguously shown that the decrease in resistivity in superconducting YBCO films is accompanied by a simultaneous increase in T, . This confirmed the expectations of a photoinduced transition to the metallic state and of photoinduced superconductivity in high-T, oxides.In this letter, we explicitly show that under laser or halogen lamp illumination the nonequilibrium carrier density is indeed increased and that changes in the carrier mobility are also induced. These conclusions are reached through photoinduced Hall coefficient RH and electrical resistivity p measurements in oxygen-deficient YBCO films. A comparison of these results with those obtained as a function of time in nonilluminated quenched YBCO films indicates that the relaxation towards equilibrium occurs with similar relaxation times. These measure-ments allow precise comparisons between changes in resistivity and the Hall coefBcient which may give a clue to the origin of the puzzling normal transport properties in high-T, superconductors.Photoconductivity and photoinduced Hall effect measurements were performed as a function of time on oxygen-deficient YBCO films on both sides of the M-I transition. Superconducting YBCO films were prepared as described earlier by off-axis sputtering on MgO substrates. ' The oxygen content of the films was adjusted to the desired value by controlled temperature T and oxygen partial pressure Po annealing, following a constant 2 oxygen content line in the Po -T phase diagram. The 2 nominally 1100-A-thick films were patterned (see schematic drawing in Fig. 1) using standard photolithographic techniques into lines 0.5 mm wide and 5 mm long allo...

Persistent photoconductivity in high-temperature superconductors with nano-scale fragmented copper-oxide chains

“…It was observed that long-term illumination, with illumination times of hours and even days, changes the electronic properties of all samples with 6 6 x 6 7 (see [1][2][3][4] and the references therein). One of the most interesting features of this effect is that it is persistent up to surprisingly high temperatures.…”

Photoconductivity of Bi12Ti1-xPbxO20Single Crystal

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Photon-induced switching and tunneling phenomena in a YBCO thin film junction

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