Griessen, R., Wen Hai-Hu, A. J. J. Van Dalen, B. Dam, J. Rector, H. G. Schnack, S. Libbrecht, E. Osquiguil, and Y. Bruynseraede. "Evidence for mean free path fluctuation induced pinning in YBa2Cu3O7 and YBa2Cu4O8 films." Physical Review Letters 72, no. 12 (1994): 1910–1913.
Abstract: The critical current jc and the pinning energy Uc have been determined for three types of yttrium-based superconducting films from current js and dynamic relaxation Q=d lnjs/d ln(dB/dt) data by means of the generalized inversion scheme. For B<2 T and T<80 K the temperature dependence of jc and Uc for all films is found to be in excellent agreement with a model of single vortices pinned by randomly distributed weak pinning centers via spatial fluctuations of the charge carrier mean free path. Pinning due to spatial fluctuations of Tc is not observed.
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Neerinck, D., K. Temst, M. Baert, E. Osquiguil, C. Van Haesendonck, Y. Bruynseraede, A. Gilabert, and I. K. Schuller. "Thermally induced transition in the vortex lattice of Ge/Pb multilayers." Physica C: Superconductivity and its applications 185-189, no. PART 3 (1991): 2061–2062.
Abstract: We report on a novel magnetic field dependence of the critical current density of M.B.E. grown Ge/Pb insulator/superconductor multilayers. When the magnetic field is applied perpendicular to the layers, the critical current density Jc drops to a minimum, followed by a broad maximum before finally decaying monotonously to zezero with increasing magnetic field. The dependence of this effect on pinning strength, temperature, and layer thickness suggests either a thermally driven coupling/decoupling transition, or melting of the vortex lattice.
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Nieva, G., E. Osquiguil, J. Guimpel, M. Maenhoudt, B. Wuyts, Y. Bruynseraede, M. B. Maple, and I. K. Schuller. "Photoinduced enhancement of superconductivity." Applied Physics Letters 60, no. 17 (1992): 2159–2161.
Abstract: We show clear and conclusive experimental evidence for the enhancement of superconductivity in GdBa2Cu3Ox and YBa 2Cu3Ox thin films by photoexcitation. Upon laser illumination the critical temperature increases while the resistivity of the material decreases. The relaxation back to the original state is very slow, of the order of days at room temperature. The existence of this effect opens the possibility of fabricating weak-link devices with in situ tunable superconducting characteristics.
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Dhalle, M., W. Boon, E. Osquiguil, C. Van Haesendonck, Y. Bruynseraede, J. Kwarciak, and O. Van Der Biest. "Correlation between critical current and resistivity in YBa2Cu3O7." Physica C: Superconductivity and its applications 185-189, no. PART 4 (1991): 2447–2448.
Abstract: Critical current versus temperature measurements were carried out in ceramic YBa2Cu3O7 samples. The samples were all prepared using the same sintering procedure, but starting from powders obtained by different wet and dry methods. All samples showed a linear Jc vs. T behaviour, typical of Josephson weak-link coupling, with a slope that depends on the preparation method. Using the Ambegaokar-Baratoff relation for Jc, the difference in slopes can be attributed to a change in the characteristic value of the junction resistance. This value turns out to be clearly correlated with the resistivity in the normal state.
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Neerinck, D., K. Temst, M. Baert, E. Osquiguil, C. Van Haesendonck, Y. Bruynseraede, A. Gilabert, and I. K. Schuller. "Transition in the flux lattice of artificially layered superconductors." Physical Review Letters 67, no. 18 (1991): 2577–2580.
Abstract: We report a novel magnetic-field dependence of the critical-current density Jc in artificially grown Ge/Pb superlattices. In these layered structures Jc exhibits an unexpected minimum as a function of perpendicular applied field, unlike any ever observed in other superconductor systems. The systematic evolution of this minimum as a function of layer thicknesses, temperature and pinning strength is related to either flux-lattice decoupling or melting.
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Libbrecht, S., A. J. J. van Dalen, B. Wuyts, M. Maenhoudt, Z. X. Gao, E. Osquiguil, R. Griessen, and Y. Bruynseraede. "Magnetic properties of oxygen deficient Yba2Cu3Ox thin films." Physica C: Superconductivity and its applications 235-240, no. PART 5 (1994): 2889–2890.
Abstract: We have invesitgated several superconducting properties of oxygen defiecient YBa2Cu3Ox thin films, with oxygen contents ranging from x = 6.55 to x = 7. By means of torque magnetometry, the temperature dependence of the superconducting current dendity js and relaxation rate of the magnetization Q is measured in an (almost perpendicular) applied magnetic induction of 0.6 T. As the oxygen content is reduced, js decreases, whereas Q increases. Both properties are influenced by the decreased dpoing level as well as the enhanced anisotropy of oxygen depleted YBa2Cu3Ox. Using the Generalixed Inversion Scheme derived by Schnak et al. we obtain the current dependence of the activation energy U(f). We show that U(j) is lowered when the oxygen content is reduced.
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van Dalen, A. J. J., R. Griessen, S. Libbrecht, E. Osquiguil, and Y. Bruynseraede. "Observation of quantum creep in oxygen deficient YBa2Cu3O7-? at fields up to 7 T." Physica C: Superconductivity and its applications 235-240, no. PART 5 (1994): 2959–2960.
Abstract: Superconducting current densities js and dynamic relaxation rates Q ? d ln js/d ln (dBe/dt) in an oxygen deficient YBa2Cu3O7-? epitaxial film were measured as a function of temperature (1.5K < T < Tirr) in magnetic fields up to 7 T by means of high sensitivity capacitance torquemeters. Below 5 K, the relaxation rate remains high and does not extrapolate to Q = 0 at T = 0K. This is associated to the quantum tunneling of vortices through the energy barrier separating two adjacent vortex configurations. In the case of ohmic dissipation the quantum creep rate is proportional to the normal state resistivity ?{variant}n(0) at T = 0 K. In order to check this theoretical prediction we measured Q as a function of the oxygen content of the sample. The quantum creep rate increases systematically with increasing ?{variant}n(0). However, Q remains finite even when ?{variant}n = 0.
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Gao, Z. X., E. Osquiguil, M. Maenhoudt, B. Wuyts, S. Libbrecht, and Y. Bruynseraede. "3D-2D dimensional crossover in YBa2Cu3O7 films." Physical Review Letters 71, no. 19 (1993): 3210–3213.
Abstract: We show that, in fully oxidized YBa2Cu3O7 c-axis oriented films, the changes induced in the angular and field dependence of the magnetoresistivity ?ab by increasing the temperature towards Tc are similar to those caused by a lowering in oxygen content. This remarkable behavior can be interpreted as due to a crossover from a three-dimensional to a two-dimensional system as T approaches Tc. Moreover, our critical current data mimic the behavior observed by Qi Li et al. in YBa2Cu3O7/(PrxY1-x)Ba2Cu3O7 multilayers, suggesting the possibility that chain planes are superconducting.
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Osquiguil, E., M. Maenhoudt, B. Wuyts, Y. Bruynseraede, D. Lederman, and I. K. Schuller. "Photoexcitation and oxygen ordering in YBa2Cu3Ox films." Physical Review B 49, no. 5 (1994): 3675–3678.
Abstract: We report on the photoinduced enhancement of the critical temperature ?Tc and the in-plane conductivity ??ab measured in a c-axis-oriented YBa2Cu3Ox film as a function of oxygen concentration (6.4?x?7). Both ?Tc and ??ab decrease with the oxygen content, display a plateaulike behavior for 6.6?x?6.75, and become zero at x=7. This dependence of ?Tc on x is similar to that observed for the Tc enhancement due to ordering effects in oxygen-deficient YBa2Cu3Ox single crystals. We propose that the persistent photoinduced changes in YBa2Cu3Ox films are due to photoassisted oxygen ordering.
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Wuyts, B., E. Osquiguil, M. Maenhoudt, S. Libbrecht, Z. X. Gao, and Y. Bruynseraede. "Influence of the oxygen content on the normal-state Hall angle in YBa2Cu3Oxn films." Physical Review B 47, no. 9 (1993): 5512–5515.
Abstract: We report on measurements of in-plane resistivity ?ab(T) and Hall coefficient RH(T) in c-axis-oriented oxygen-deficient YBa2Cu3Oxn thin films. Independent of the changes induced in RH(T) and ?ab(T) by the oxygen depletion, a quadratic temperature dependence is observed for the Hall angle ?H. The slope of cot?H versus T2 decreases as the oxygen content xn is reduced and saturates in the Tc(xn)=60 K plateau region. The results are discussed in the framework of the two-dimensional Luttinger liquid theory as proposed by Anderson.
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