Elastic-to-plastic crossover below the peak effect in the vortex solid of YBa2Cu3O7 single crystals

We report on transport and ac susceptibility studies below the peak effect in twinned YBa2Cu3O7 single crystals. We find that disorder generated at the peak effect can be partially inhibited by forcing vortices to move with an ac driving current. The vortex system can be additionally ordered below a well-defined temperature where elastic interactions between vortices overcome pinning-generated stress and a plastic to elastic crossover seems to occur. The combined effect of these two processes results in vortex structures with different mobilities that give place to history effects.Comment: 4 pages, 4 figures. Published in PRB Rapid Comm., February 1, 200

History effects and pinning regimes in solid vortex matter

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^) 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.Comment: 11 pages, 7 figures. To be published in PR

Slow relaxations and history dependence of the transport properties of layered superconductors

We study numerically the time evolution of the transport properties of layered superconductors after different preparations. We show that, in accordance with recent experiments in BSCCO performed in the second peak region of the phase diagram (Portier et al, 2001), the relaxation strongly depends on the initial conditions and is extremely slow. We investigate the dependence on the pinning center density and the perturbation applied. We compare the measurements to recent findings in tapped granular matter and we interpret our results with a rather simple picture.Comment: 4 pages, 4 fig

History effect in inhomogeneous superconductors

A model was proposed to account for a new kind of history effect in the transport measurement of a sample with inhomogeneous flux pinning coupled with flux creep. The inhomogeneity of flux pinning was described in terms of alternating weak pinning (lower jc) and strong pinning region (higher jc). The flux creep was characterized by logarithmic barrier. Based on this model, we numerically observed the same clockwise V-I loops as reported in references. Moreover, we predicted behaviors of the V-I loop at different sweeping rates of applied current dI/dt or magnetic fields Ba, etc. Electric transport measurement was performed in Ag-sheathed Bi2-xPbxSr2Ca2Cu3Oy tapes immersed in liquid nitrogen with and without magnetic fields. V-I loop at certain dI/dt and Ba was observed. It is found that the area of the loop is more sensitive to dI/dt than to Ba, which is in agreement well with our numerical results.Comment: To appear in Phys Rev B, October 1 Issu

Hysteretic behavior of the vortex lattice at the onset of the second peak for HgBa2_2CuO4+δ_{4+\delta} superconductor

By means of local Hall probe ac and dc permeability measurements we investigated the phase diagram of vortex matter for the HgBa$_2$CuO$_{4+\delta }$ superconductor in the regime near the critical temperature. The second peak line, $H_{\rm sp}$, in contrast to what is usually assumed, doesn't terminate at the critical temperature. Our local ac permeability measurements revealed pronounced hysteretic behavior and thermomagnetic history effects near the onset of the second peak, giving evidence for a phase transition of vortex matter from an ordered qausilattice state to a disordered glass

Large-amplitude driving of a superconducting artificial atom: Interferometry, cooling, and amplitude spectroscopy

Superconducting persistent-current qubits are quantum-coherent artificial atoms with multiple, tunable energy levels. In the presence of large-amplitude harmonic excitation, the qubit state can be driven through one or more of the constituent energy-level avoided crossings. The resulting Landau-Zener-Stueckelberg (LZS) transitions mediate a rich array of quantum-coherent phenomena. We review here three experimental works based on LZS transitions: Mach-Zehnder-type interferometry between repeated LZS transitions, microwave-induced cooling, and amplitude spectroscopy. These experiments exhibit a remarkable agreement with theory, and are extensible to other solid-state and atomic qubit modalities. We anticipate they will find application to qubit state-preparation and control methods for quantum information science and technology.Comment: 13 pages, 5 figure

Metastability and Transient Effects in Vortex Matter Near a Decoupling Transition

We examine metastable and transient effects both above and below the first-order decoupling line in a 3D simulation of magnetically interacting pancake vortices. We observe pronounced transient and history effects as well as supercooling and superheating between the 3D coupled, ordered and 2D decoupled, disordered phases. In the disordered supercooled state as a function of DC driving, reordering occurs through the formation of growing moving channels of the ordered phase. No channels form in the superheated region; instead the ordered state is homogeneously destroyed. When a sequence of current pulses is applied we observe memory effects. We find a ramp rate dependence of the V(I) curves on both sides of the decoupling transition. The critical current that we obtain depends on how the system is prepared.Comment: 10 pages, 15 postscript figures, version to appear in PR