Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userHistory effects and pinning regimes in solid vortex matter
83
0
0.0
(
0
)
Added by
Sergio O. Valenzuela
Publication date
2002
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
We propose a phenomenological model that accounts for the history effects observed in ac susceptibility measurements in YBa2Cu3O7 single crystals [Phys. Rev. Lett. 84, 4200 (2000) and Phys. Rev. Lett. 86, 504 (2001)]. Central to the model is the assumption that the penetrating ac magnetic field modifies the vortex lattice mobility, trapping different robust dynamical states in different regions of the sample. We discuss in detail on the response of the superconductor to an ac magnetic field when the vortex lattice mobility is not uniform inside the sample. We begin with an analytical description for a simple geometry (slab) and then we perform numerical calculations for a strip in a transverse magnetic field which include relaxation effects. In calculations, the vortex system is assumed to coexist in different pinning regimes. The vortex behavior in the regions where the induced current density j has been always below a given threshold (j_c^>) is described by an elastic Campbell-like regime (or a critical state regime with local high critical current density, j_c^>). When the VS is shaken by symmetrical (e.g. sinusoidal) ac fields, the critical current density is modified to j_c^< (which is smaller than j_c^>) at regions where vortices have been forced to oscillate by a current density larger than j_c^>. Experimentally, an initial state with high critical current density (j_c^>) can be obtained by zero field cooling, field cooling (with no applied ac field) or by shaking the vortex lattice with an asymmetrical (e.g. sawtooth) field. We compare our calculations with experimental ac susceptibility results in YBa2Cu3O7 single crystals.
rate research
Read More
We examine the current driven dynamics for vortices interacting with conformal crystal pinning arrays and compare to the dynamics of vortices driven over random pinning arrays. We find that the pinning is enhanced in the conformal arrays over a wide range of fields, consistent with previous results from flux gradient-driven simulations. At fields above this range, the effectiveness of the pinning in the moving vortex state can be enhanced in the random arrays compared to the conformal arrays, leading to crossing of the velocity-force curves.
Recent studies have shown a number of surprising vortex dynamics phenomena both in low and high temperature superconductors, which include: low frequency noise, slow voltage oscillations, history dependent dynamic response, memory of the direction, amplitude, duration, and frequency of the previously applied current, suppression of a large ac vortex response by a very small dc bias, and a strong frequency dependence. Taken together, these phenomena are incompatible with the current understanding of bulk vortex dynamics. We propose a generic mechanism to account for these observations in terms of the competition between the injection of a disordered vortex phase through the surface barriers at the sample edges, and the annealing of this metastable disorder by the transport current. The model is confirmed by investigating the current distribution across NbSe2 single crystals using arrays of Hall sensors. For an ac current only narrow regions near the edges are in the pinned disordered phase resulting in a large response. In the presence of a dc bias a wide region of the sample is filled by the disordered phase preventing vortex motion. The resulting spatial variation of the disorder across the sample acts as an active memory of the previously applied current sequence.
We study numerically the evolution of the degree order and mobility of the vortex lattice under steady and oscillating applied forces. We show that the oscillatory motion of vortices can favor an ordered structure, even when the motion of the vortices is plastic when the same force is applied in a constant way. Our results relate the spatial order of the vortex lattice with its mobility and they are in agreement with recent experiments. We predict that, in oscillating applied forces, the lattice orients with a principal axis perpendicular to the direction of motion.
We study geometrical confinement effects in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8 +delta}$ mesoscopic vortex-matter with edge-to-surface ratio of $7-12$%. Samples have in-plane square and circular edges, 30,$mu$m widths, and $sim 2,mu$m thickness. Direct vortex imaging reveals the compact planes of the structure align with the sample edge by introducing topological defects. The defects density is larger for circular than for square edges. Molecular dynamics simulations suggest this density is not an out-of-equilibrium property but rather determined by the geometrical confinement.
Manipulating vortices in non-conventional superconductors is nowadays a challenging path toward controlling functionalities for superconducting nanodevices. Here, we directly observe and control single vortex core trajectories with unmatched resolution using a new scanning tunneling spectroscopy at very low temperature. Our data show the depinning threshold of a Bragg-glass in a weakly disordered superconductor, a clean 2H-NbSe2 crystal. We first experimentally capture the linear and collective response, the Campbell regime. Upon strong drives, the oscillating trajectories perform a series of stick-slip motions that mimics the lattice periodicity. We then theoretically elucidate this peculiar non-linear regime by solving the Langevin dynamics equations. We additionally explore the impact of initial conditions and reveal an enhancement of the long-range correlations with the cooling procedure. Finally, our work establishes a connection between theory of vortex pinning, memory effects and vortex lineshapes, thus offering a new platform to investigate the relationship between viscous media and individual controllable objects in any many-body systems.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
