Self-field effects upon the critical current density of flat superconducting strips

We develop a general theory to account self-consistently for self-field effects upon the average transport critical current density Jc of a flat type-II superconducting strip in the mixed state when the bulk pinning is characterized by a field-dependent depinning critical current density Jp(B), where B is the local magnetic flux density. We first consider the possibility of both bulk and edge-pinning contributions but conclude that bulk pinning dominates over geometrical edge-barrier effects in state-of-the-art YBCO films and prototype second-generation coated conductors. We apply our theory using the Kim model, JpK(B) = JpK(0)/(1+|B|/B0), as an example. We calculate Jc(Ba) as a function of a perpendicular applied magnetic induction Ba and show how Jc(Ba) is related to JpK(B). We find that Jc(Ba) is very nearly equal to JpK(Ba) when Ba > Ba*, where Ba* is the value of Ba that makes the net flux density zero at the strip's edge. However, Jc(Ba) is suppressed relative to JpK(Ba) at low fields when Ba < Ba*, with the largest suppression occurring when Ba*/B0 is of order unity or larger.Comment: 9 pages, 4 figures, minor revisions to add four reference

Metastability and uniqueness of vortex states at depinning

We present results from numerical simulations of transport of vortices in the zero-field cooled (ZFC) and the field-cooled (FC) state of a type-II superconductor. In the absence of an applied current $I$, we find that the FC state has a lower defect density than the ZFC state, and is stable against thermal cycling. On the other hand, by cycling $I$, surprisingly we find that the ZFC state is the stable state. The FC state is metastable as manifested by increasing $I$ to the depinning current $I_{c}$, in which case the FC state evolves into the ZFC state. We also find that all configurations acquire a unique defect density at the depinning transition independent of the history of the initial states.Comment: 4 pages, 4 figures. Problem of page size correcte

Magnetic field of an in-plane vortex outside a layered superconductor

We present the solution to London's equations for the magnetic fields of a vortex oriented parallel to the plane, and normal to a crystal face, of a layered superconductor. These expressions account for flux spreading at the superconducting surface, which can change the apparent size of the vortex along the planes by as much as 30%. We compare these expressions with experimental results.Comment: 13 pages, 5 figure

Vortex trapping and expulsion in thin-film YBCO strips

A scanning SQUID microscope was used to image vortex trapping as a function of the magnetic induction during cooling in thin-film YBCO strips for strip widths W from 2 to 50 um. We found that vortices were excluded from the strips when the induction Ba was below a critical induction Bc. We present a simple model for the vortex exclusion process which takes into account the vortex - antivortex pair production energy as well as the vortex Meissner and self-energies. This model predicts that the real density n of trapped vortices is given by n=(Ba-BK)/Phi0 with BK = 1.65Phi0/W^2 and Phi0 = h/2e the superconducting flux quantum. This prediction is in good agreement with our experiments on YBCO, as well as with previous experiments on thin-film strips of niobium. We also report on the positions of the trapped vortices. We found that at low densities the vortices were trapped in a single row near the centers of the strips, with the relative intervortex spacing distribution width decreasing as the vortex density increased, a sign of longitudinal ordering. The critical induction for two rows forming in the 35 um wide strip was (2.89 + 1.91-0.93)Bc, consistent with a numerical prediction

Remarkable change of tunneling conductance in YBCO films in fields up to 32.4T

We studied the tunneling density of states in YBCO films under strong currents flowing along node directions. The currents were induced by fields of up to 32.4T parallel to the film surface and perpendicular to the $CuO_{2}$ planes. We observed a remarkable change in the tunneling conductance at high fields where the gap-like feature shifts discontinuously from 15meV to a lower bias of 11meV, becoming more pronounced as the field increases. The effect takes place in increasing fields around 9T and the transition back to the initial state occurs around 5T in decreasing fields. We argue that this transition is driven by surface currents induced by the applied magnetic field.Comment: 4 pages, 7 figure

Reducing microwave loss in superconducting resonators due to trapped vortices

Microwave resonators with high quality factors have enabled many recent breakthroughs with superconducting qubits and photon detectors, typically operated in shielded environments to reduce the ambient magnetic field. Insufficient shielding or pulsed control fields can introduce vortices, leading to reduced quality factors, although increased pinning can mitigate this effect. A narrow slot etched into the resonator surface provides a straightforward method for pinning enhancement without otherwise affecting the resonator. Resonators patterned with such a slot exhibited over an order of magnitude reduction in the excess loss due to vortices compared with identical resonators from the same film with no slot

Field Induced Nodal Order Parameter in the Tunneling Spectrum of YBa2_2Cu3_3O7−x_{7-x} Superconductor

We report planar tunneling measurements on thin films of YBa$_2$Cu$_3$O$_{7-x}$ at various doping levels under magnetic fields. By choosing a special setup configuration, we have probed a field induced energy scale that dominates in the vicinity of a node of the d-wave superconducting order parameter. We found a high doping sensitivity for this energy scale. At Optimum doping this energy scale is in agreement with an induced $id_{xy}$ order parameter. We found that it can be followed down to low fields at optimum doping, but not away from it.Comment: 9 pages, 8 figures, accepted for publication in Phys. Rev.

Vector magnetic hysteresis of hard superconductors

Critical state problems which incorporate more than one component for the magnetization vector of hard superconductors are investigated. The theory is based on the minimization of a cost functional ${\cal C}[\vec{H}(\vec{x})]$ which weighs the changes of the magnetic field vector within the sample. We show that Bean's simplest prescription of choosing the correct sign for the critical current density $J_c$ in one dimensional problems is just a particular case of finding the components of the vector $\vec{J}_c$. $\vec{J}_c$ is determined by minimizing ${\cal C}$ under the constraint $\vec{J}\in\Delta (\vec{H},\vec{x})$, with $\Delta$ a bounded set. Upon the selection of different sets $\Delta$ we discuss existing crossed field measurements and predict new observable features. It is shown that a complex behavior in the magnetization curves may be controlled by a single external parameter, i.e.: the maximum value of the applied magnetic field $H_m$.Comment: 10 pages, 9 figures, accepted in Phys. Rev.