Lifetime and Coherence of Two-Level Defects in a Josephson Junction

We measure the lifetime ($T_{1}$) and coherence ($T_{2}$) of two-level defect states (TLSs) in the insulating barrier of a Josephson phase qubit and compare to the interaction strength between the two systems. We find for the average decay times a power law dependence on the corresponding interaction strengths, whereas for the average coherence times we find an optimum at intermediate coupling strengths. We explain both the lifetime and the coherence results using the standard TLS model, including dipole radiation by phonons and anti-correlated dependence of the energy parameters on environmental fluctuations.Comment: 4 pages, 4 figures and supplementary material (3 pages, 2 figures, 1 table

Increasing the critical temperature of Nb films by chemically linking magnetic nanoparticles using organic molecules

In type-II superconductors vortex pinning enhances the critical current density. One known method to induce pinning sites is the use of magnetic nanostructures that locally degrade the superconductivity via stray fields. In recent studies, we showed that both the critical temperature and critical current of Nb thin films can be enhanced by coupling Au nanoparticles via organic molecules and, concomitantly, a zero-bias peak appeared in the density of states. One suggested mechanism to explain these effects was the interaction of the induced pinning potential landscape with the Cooper pairs and vortices. To further examine this mechanism we study in the present work the effects of chemically linking magnetic nanoparticles to Nb films. Two types of magnetic nanoparticles are investigated, half-metal (Fe3O4) and metallic (Co). For high nanoparticle density, resulting in an effective continuous magnetic film, the critical temperature is reduced, as expected. However, for intermediate density, where the magnetic nanoparticles are well separated and a distinct pinning landscape is formed above the Nb film, critical temperature and current density enhancements are observed for both types of particles. Moreover, the tunnelling spectra acquired on the (metallic) Co nanoparticles exhibit a zero-bias conducting peak. The magnetic nanoparticles proximity through organic molecules presents similar behaviour to the non-magnetic Au nanoparticles inverse proximity results. This may suggest that pinning mechanisms play a role in the critical temperature enhancement

Dynamic Control of the Vortex Pinning Potential in a Superconductor Using Current Injection through Nanoscale Patterns

Control over the vortex potential at the nanoscale in a superconductor is a subject of great interest for both fundamental and technological reasons. Many methods for achieving artificial pinning centers have been demonstrated, for example, with magnetic nanostructures or engineered imperfections, yielding many intriguing effects. However, these pinning mechanisms do not offer dynamic control over the strength of the patterned vortex potential because they involve static nanostructures created in or near the superconductor. Dynamic control has been achieved with scanning probe methods on the single vortex level but these are difficult so scale up. Here, we show that by applying controllable nanopatterned current injection, the superconductor can be locally driven out of equilibrium, creating an artificial vortex potential that can be tuned by the magnitude of the injected current, yielding a unique vortex channeling effect