9 research outputs found
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, Ď„E, 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 Ď„E 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