Civale, L., G. Pasquini, P. Levy, G. Nieva, D. Casa, and H. Lanza. "Time relaxation of persistent currents in YBa2Cu3O7−x crystals with columnar defects." Physica C: Superconductivity 263, no. 1-4 (1996): 389–395.
Abstract: We have measured the persistent currents and their time relaxation in YBa2Cu3O7−x single crystals with correlated disorder introduced by irradiation with high-energy heavy-ions. For vortex density lower than the defect density, and for applied fields either parallel or nonparallel to the columnar defects, we find clear evidence of a pinning-crossover between a regime of strong individual pinning at low temperatures and a regime of weaker collective pinning at high temperatures. We explore the vortex dynamics above and below this crossover.
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Civale, L., G. Pasquini, P. Levy, G. Nieva, and H. Lanza. "AC susceptibility of pinned vortices in YBa2Cu3O7 crystals with correlated disorder." Advances in Superconductivity: VIII: Proceedings of the 8th International Symposium on Superconductivity (1996): 553.
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Herbsommer, J. A., J. Luzuriaga, L. Civale, G. Nieva, G. Pasquini, H. Lanza, and P. Levy. "Angular variation of pinning near the irreversibility temperature in single crystal Y Ba2Cu3O7-? with splayed columnar defects." Physica C 304, no. 1-2 (1998): 112–118.
Abstract: We have studied vortex pinning through ac susceptibility measurements in single crystals of YBCO with columnar defects (CDs). The CDs have 0, 10 and 20 splay angle and average direction 108 off the c-axis. By studying the angular variation we can compensate for the anisotropy and effects of twins, etc. Using a simple expression we can obtain the angular spread when the field direction is outside the splay angle. An increase of pinning when the field direction is inside
the angle defined by the CDs can be attributed to vortex entanglement due to the splay and the suppression of the sliding of double kinks.
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Pasquini, G., L. Civale, H. Lanza, and G. Nieva. "Angular regimes of ac dynamics and pinning in YBa2Cu3O7 crystals with and without columnar defects." PHYSICAL REVIEW B 65, no. 21 (2002): 214517.
Abstract: The main source of vortex pinning near the solid-liquid transition at different orientations of a low dc field in YBa2Cu3O7 crystals with columnar defects is investigated by means of ac susceptibility measurements. The behavior is compared with that observed in nonirradiated samples. It is found that in a very wide angular region the tracks act as correlated defects and are the prevailing source of pinning. The results rule out the existence of an accommodation angle, determined by the competition between the pinning and elastic energies, in twined crystals: the interplay with natural correlated pinning centers has to be taken into account. The linear and nonlinear dynamic regimes when the field is tilted relative to the defects are analyzed. It is clearly shown that characterization of the dynamic regimes is crucial for the correct interpretation of the angular dependence in the ac response.
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Pasquini, G., L. Civale, H. Lanza, and G. Nieva. "Dynamic regimes in the ac response of YBa2Cu3O7 with columnar defects: Intra- and inter-valley vortex motion." PHYSICAL REVIEW B 59, no. 14 (1999): 9627–9636.
Abstract: We use ac susceptibility measurements to explore the dynamic response of vortices in YBCO single crystals pinned by aligned columnar defects at low de fields. Using a combination of techniques for the analysis of the data, we investigate the vortex motion as a function of the amplitude and frequency of the applied ac field h(ac), identifying the influence of both intra- and inter-valley motion. We build up a dynamic diagram in the h(ac)-T plane indicating the crossover lines among several regimes in the solid phase. At low h(ac), a linear response with very low dissipation arises from the oscillation of pinned vortices inside the tracks (Campbell regime). At high h(ac) a critical state develops. The large influence of thermal fluctuations in this limit is indicated by the frequency dependence, which allows us to obtain the current density dependence of the activation energy and the glassy exponent. We also discuss the origin of the large nonlinear transition regime observed at intermediate amplitudes. [S0163-1829(99)02613-2].
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Pasquini, G., P. Levy, L. Civale, G. Nieva, and H. Lanza. "Linear response in the AC susceptibility of vortices in YBa2Cu3O7 crystals with columnar defects." Physica C: Superconductivity 274, no. 1-2 (1997): 165–172.
Abstract: We explore the AC magnetic susceptibility of YBa2Cu3O7 single crystals with columnar defects. For vortices parallel to the defects the linear response is mainly determined by the Campbell penetration depth with a field independent Labusch parameter indicating that vortices are individually pinned. The small dissipation in this limit is likely to arise from thermal fluctuations of vortex segments between pairs of metastable states (two-level systems) in the Bose glass phase. Tilting vortices away from the tracks results in a reduction of the Bose glass temperature and an increase of the dissipation due to the reduction of the pinning forces. We discuss the onset of nonlinear response as the AC field increases.
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Silhanek, A., L. Civale, S. Candia, G. Nieva, G. Pasquini, and H. Lanza. "Evidence for vortex staircases in the whole angular range due to competing correlated pinning mechanisms." PHYSICAL REVIEW B 59, no. 21 (1999): 13620–13623.
Abstract: We present measurements of the angular dependence of the irreversible magnetization of YBa2Cu3O7 single crystals with columnar defects inclined with respect to the c axis. At high fields a sharp maximum centered at the tracks direction is observed. At low fields we identify a lock-in phase characterized by an angle-independent pinning strength and observe an angular shift of the peak towards the c axis that originates in the misalignment between vortices and applied field in anisotropic materials. The interplay among columnar defects, twins, and intrinsic pinning by the ab planes generates a variety of staircase structures. We show that correlated pinning dominates for all orientations of the applied field. [S0163-1829(99)09417-5].
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Silhanek, K., J. Guimpel, L. Civale, H. Lanza, and P. Levy. "Thickness effects on the pinning by columnar defects." PHYSICA C 341 (2000): 1217–1218.
Abstract: We compare the angular dependence of the pinning of equivalent columnar defects in GdBa2Cu2O7 thin films and a thicker YBa2Cu3O7 single crystal. The tracks' related enhancement of the critical currents is much smaller for the case of the thin films. We interpret this thickness dependence as resulting from a competition between the pinning of the correlated structures, related to total sample thickness, and surface boundary conditions, which force the vortices to depin from the columnar defects near the sample surface.
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