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The peak effect (PE) in the critical current density of type II superconductors has been related to an order-disorder transition in the vortex lattice (VL), but its underlying physics remains a controversial issue. Intrinsic to the PE are strong metastabilities that frequently mask the stationary VL configurations. We follow shaking and thermal protocols in NbSe2 single crystals to access these configurations and examine them by linear ac susceptibility measurements that avoid VL reorganization. We identify three different regions. For TT2(H), configurations are fully disordered and no metastability is observed. In the T1

We study the different dynamical regimes of a vortex lattice driven by ac forces in the presence of random pinning via numerical simulations. The behavior of the different observables is characterized as a function of the applied force amplitude for different frequencies. We discuss the inconveniences of using the mean velocity to identify the depinning transition and we show that instead, the mean quadratic displacement of the lattice is the relevant magnitude to characterize different ac regimes. We discuss how the results depend on the initial configuration and we identify hysteretic effects which are absent in the dc driven systems.

We present measurements of the angular dependence of the irreversible magnetization of YBa 2 Cu 3 O 7 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 angleindependent 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. ͓S0163-1829͑99͒09417-5͔A difficult aspect of the study of vortex dynamics in hightemperature superconductors in the presence of correlated disorder is the determination of flux structures for applied fields tilted with respect to the pinning potential. As threedimensional vortex configurations cannot be directly observed, our knowledge is mostly based on the analysis of the angular dependence of magnetization, susceptibility, or transport data. [1][2][3][4][5][6][7][8][9][10][11] According to theoretical models, 12,13 when the angle between the applied field H and the defects is smaller than the lock-in angle L vortices remain locked into the defects thus producing a transverse Meissner effect. For tilt angles larger than L and smaller than a trapping angle T , vortices form staircases with segments pinned into different defects and connected by unpinned or weakly pinned kinks. Beyond T , vortices will be straight and take the direction of the applied field, thus being unaffected by the correlated nature of the pinning. In principle, this picture should apply with minor differences to twins, columnar defects, and intrinsic pinning. 13 Many experiments have confirmed the directional pinning due to columnar defects, twins, and Cu-O planes. 1-11 Evidence for a locked-in phase arises from the observation of the transverse Meissner effect, 10 but a quantitative determination of L (H,T) for columnar defects had not been done until now. The introduction of columnar defects inclined with respect to the crystallographic axis has been used 1,6-8,10,11 to discriminate their pinning effects from those due to twin boundaries, and from anisotropy effects. However, the vortex staircase configurations resulting from the combined effect of the various correlated structures had not yet been explored in detail.In this work we report studies of the vortex pinning in YBa 2 Cu 3 O 7 crystals with inclined columnar defects, for the whole range of field orientations. This allows us to determine the misalignment between the applied and internal fields due to anisotropy, as well as to identify the angular range of influence of each correlated pinning structure. We present the first determination of the lock-in angle of tracks using irreversible magnetization.The crystal used in this study was grown by the self-flux method, 14 and ...