Felix Zeides

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

We measure the lifetime (T₁) and coherence (T₂) 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 anticorrelated dependence of the energy parameters on environmental fluctuations.

Increased superconducting transition temperature of a niobium thin film proximity coupled to gold nanoparticles using linking organic molecules.

The superconducting critical temperature, T(C), of thin Nb films is significantly modified when gold nanoparticles (NPs) are chemically linked to the Nb film, with a consistent enhancement when using 3 nm long disilane linker molecules. The T(C) increases by up to 10% for certain linker length and NP size. No change is observed when the nanoparticles are physisorbed with nonlinking molecules. Electron tunneling spectra acquired on the linked NPs below T(C) typically exhibit zero-bias peaks. We attribute these results to a pairing mechanism coupling electrons in the Nb and the NPs, mediated by the organic linkers.

dc and ac magnetic properties of thin-walled Nb cylinders with and without a row of antidots

dc and ac magnetic properties of two thin-walled superconducting Nb cylinders with a rectangular cross-section are reported. Magnetization curves and the ac response were studied on as-prepared and patterned samples in magnetic fields parallel to the cylinder axis. A row of micron-sized antidots (holes) was made in the film along the cylinder axis. Avalanche-like jumps of the magnetization are observed for both samples at low temperatures for magnetic fields not only above H c1, but in fields lower than H c1 in the vortex-free region. The positions of the jumps are not reproducible and they change from one experiment to another, resembling vortex lattice instabilities usually observed for magnetic fields larger than H c1. At temperatures above [Formula: see text] and [Formula: see text] the magnetization curves become smooth for the patterned and the as-prepared samples, respectively. The magnetization curve of a reference planar Nb film in the parallel field geometry does not exhibit jumps in the entire range of accessible temperatures. The ac response was measured in constant and swept dc magnetic field modes. Experiment shows that ac losses at low magnetic fields in a swept field mode are smaller for the patterned sample. For both samples the shapes of the field dependences of losses and the amplitude of the third harmonic are the same in constant and swept field near H c3. This similarity does not exist at low fields in a swept mode.

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.