Josephson Coupling and Fiske Dynamics in Ferromagnetic Tunnel Junctions

We report on the fabrication of Nb/AlO_x/Pd_{0.82}Ni_{0.18}/Nb superconductor/insulator/ferromagnetic metal/superconductor (SIFS) Josephson junctions with high critical current densities, large normal resistance times area products, high quality factors, and very good spatial uniformity. For these junctions a transition from 0- to \pi-coupling is observed for a thickness d_F ~ 6 nm of the ferromagnetic Pd_{0.82}Ni_{0.18} interlayer. The magnetic field dependence of the \pi-coupled junctions demonstrates good spatial homogeneity of the tunneling barrier and ferromagnetic interlayer. Magnetic characterization shows that the Pd_{0.82}Ni_{0.18} has an out-of-plane anisotropy and large saturation magnetization, indicating negligible dead layers at the interfaces. A careful analysis of Fiske modes provides information on the junction quality factor and the relevant damping mechanisms up to about 400 GHz. Whereas losses due to quasiparticle tunneling dominate at low frequencies, the damping is dominated by the finite surface resistance of the junction electrodes at high frequencies. High quality factors of up to 30 around 200 GHz have been achieved. Our analysis shows that the fabricated junctions are promising for applications in superconducting quantum circuits or quantum tunneling experiments.Comment: 15 pages, 9 figure

Tuned Transition from Quantum to Classical for Macroscopic Quantum States

The boundary between the classical and quantum worlds has been intensely studied. It remains fascinating to explore how far the quantum concept can reach with use of specially fabricated elements. Here we employ a tunable flux qubit with basis states having persistent currents of 1???A carried by a million pairs of electrons. By tuning the tunnel barrier between these states we see a crossover from quantum to classical. Released from nonequilibrium, the system exhibits spontaneous coherent oscillations. For high barriers the lifetime of the states increases dramatically while the tunneling period approaches the phase coherence time and the oscillations fade away.QN/Quantum NanoscienceApplied Science

Strong Coupling of a Quantum Oscillator to a Flux Qubit at Its Symmetry Point

A flux qubit biased at its symmetry point shows a minimum in the energy splitting (the gap), providing protection against flux noise. We have fabricated a qubit of which the gap can be tuned fast and have coupled this qubit strongly to an LC oscillator. We show full spectroscopy of the qubit-oscillator system and generate vacuum Rabi oscillations. When the gap is made equal to the oscillator frequency ?osc we find the largest vacuum Rabi splitting of ?0.1?osc. Here being at resonance coincides with the optimal coherence of the symmetry point.Kavli Institute of NanoscienceApplied Science

Quasiparticles relaxation processes in Nb/CuNi bilayers

The dynamic instability of the moving vortex lattice at high driving currents has been studied in superconductor (S)/weak ferromagnet (F) bilayer, Nb/Cu0.38Ni0.62. Voltage-current, V(I), characteristics have been acquired as a function of both the temperature, T, and the magnetic field, H, and interpreted in the framework of the model proposed by Larkin and Ovchinnikov. From these analysis the values of the quasiparticle relaxation time, τE, have been estimated. The results confirm the high performance of S/F hybrids in terms of velocity in the energy relaxation process, compared to corresponding single superconducting thin films. Moreover the temperature dependence of τE is extremely smooth, also if compared with the data reported in literature for other weak ferromagnet S/F based systems. This last result has been tentatively ascribed to the disorder present in the CuNi alloy