Abstract:
Josephson junctions (JJs) with nontrivial current phase relation (CPR) have attracted a large interest over the last 20 years, due to the new fundamental physics and their potential for many applications in both classical and quantum circuits, e.g., as phase batteries or memory elements.
In this thesis, the properties of varphi Josephson junctions in the classical and in the quantum domain are studied. A varphi JJ is a junction with a doubly degenerate ground state phase phi=varphi, where 0<varphi<pi. Such a system can be obtained combining a 0 and a pi JJ, with phases phi=0 and phi=pi in the ground state, respectively. The two segments should not be very different, with a small asymmetry either in the geometrical lengths or in the different critical currents densities.
The experiments presented here have been performed on varphi junctions fabricated with two different technologies. The first one is based on superconductor-insulator-ferromagnet-superconductor (SIFS) JJs with a tailored ferromagnetic barrier. Realization of the varphi state with such a technology was already proven in the past. In the thesis, SIFS varphi junctions were used for two main experiments in the classical limit. First, we studied the retrapping dynamics of the Josephson phase upon returning from the resistive to the zero-voltage state. Since a varphi JJ has two possible ground state phases, it is not obvious where the phase ends when the junction jumps back to the zero-voltage state. Second, we demonstrated the operation of the varphi JJ as a deterministic ratchet. The energy U(phi) of a varphi JJ is tunable by an external magnetic field, and a ratchet potential with no reflection symmetry can be easily obtained.
In the quantum regime a varphi JJ can be regarded as macroscopic two-level system. Hence, it would also be interesting to investigate its quantum properties. However, several technological drawbacks affecting the SIFS varphi JJs (e.g., low j_c and high damping) prevented experiments in this domain. As a first attempt to improve our technology, we fabricated SIFS JJs with an additional thin superconducting interlayer s, obtaining SIsFS structures. It was proposed that such junctions can have parameters (e.g., j_c, characteristic voltage V_c) comparable to conventional superconductor-insulator-superconductor (SIS) junctions. Although we actually detected an improvement with respect to the SIFS JJs, the typical parameters obtained did not fulfill our purposes.
A more successful technology was attained with SIS JJs, where the phase discontinuity is artificially created by means of the current I_inj circulating through two microinjectors attached to the junction. In this thesis, I present the experiments carried out on such JJs in the quantum regime, where we investigated the escape mechanism of the Josephson phase from both ground states.
Finally, I give an outlook on measurements to be performed in the near future, where we want to create a varphi JJ with an energy profile which is fully tunable electronically. Simulations predict that such a junction can be realized out of a SIS JJ with three pairs of injectors. The Josephson potential can be then controlled by adjusting the current through the two additional injector pairs, I_inj2 and I_inj3, and the external magnetic flux. Preliminary experimental characterization of such multiple injector junctions in the classical limit, together with numerical fits, are shown.