Ferromagnetic insulator-based superconducting junctions as sensitive electron thermometers

We present an exhaustive theoretical analysis of charge and thermoelectric transport in a normal metal-ferromagnetic insulator-superconductor (NFIS) junction, and explore the possibility of its use as a sensitive thermometer. We investigated the transfer functions and the intrinsic noise performance for different measurement configurations. A common feature of all configurations is that the best temperature noise performance is obtained in the non-linear temperature regime for a structure based on an europium chalcogenide ferromagnetic insulator in contact with a superconducting Al film structure. For an open-circuit configuration, although the maximal intrinsic temperature sensitivity can achieve $10$nKHz$^{-1/2}$, a realistic amplifying chain will reduce the sensitivity up to $10$$\mu$KHz$^{-1/2}$. To overcome this limitation we propose a measurement scheme in a closed-circuit configuration based on state-of-art SQUID detection technology in an inductive setup. In such a case we show that temperature noise can be as low as $35$nKHz$^{-1/2}$. We also discuss a temperature-to-frequency converter where the obtained thermo-voltage developed over a Josephson junction operated in the dissipative regime is converted into a high-frequency signal. We predict that the structure can generate frequencies up to $\sim 120$GHz, and transfer functions up to $200$GHz/K at around $\sim 1$K. If operated as electron thermometer, the device may provide temperature noise lower than $35$nKHz$^{-1/2}$ thereby being potentially attractive for radiation sensing applications.Comment: 11 pages, 10 color figure

Physics of the Josephson effect in junctions with ferromagnetic barriers towards quantum circuits and RF applications

Since its first discovering, several key superconducting applications directly use the Josephson effect. The improvement in material science and nanotechnologies allowed to build novel types of hybrid Josephson junctions. A traditional research path first aims at a complete understanding of the processes occurring in hybrid and unconventional Josephson devices, to be integrated in a second stage into real applications, and hopefully in frontier quantum circuits. In my work, I have addressed some key aspects of the physics in Josephson junctions with ferromagnetic barriers (SFS JJs), which fully falls in this category of unconventional junctions. In particular, I discuss the possibility to identify novel self-consistent and complementary protocols for the study of the fundamental physics in a special class of SFS JJs: the tunnel-SFS JJs, which use insulating ferromagnetic or multi-layered insulator-ferromagnet barriers. A special focus is given on the study of the dissipation mechanisms and the unconventional spin-triplet pairing that arises in these novel devices. I here show that the coexistence between tunnel conduction mechanisms and the ferromagnetic ordering in the barrier can be also exploited in quantum coherent devices, such as qubits

Tuning of Magnetic Activity in Spin-Filter Josephson Junctions Towards Spin-Triplet Transport.

The study of superconductor-ferromagnet interfaces has generated great interest in the last decades, leading to the observation of spin-aligned triplet supercurrents and 0-π transitions in Josephson junctions where two superconductors are separated by an itinerant ferromagnet. Recently, spin-filter Josephson junctions with ferromagnetic barriers have shown unique transport properties, when compared to standard metallic ferromagnetic junctions, due to the intrinsically nondissipative nature of the tunneling process. Here we present the first extensive characterization of spin polarized Josephson junctions down to 0.3 K, and the first evidence of an incomplete 0-π transition in highly spin polarized tunnel ferromagnetic junctions. Experimental data are consistent with a progressive enhancement of the magnetic activity with the increase of the barrier thickness, as neatly captured by the simplest theoretical approach including a nonuniform exchange field. For very long junctions, unconventional magnetic activity of the barrier points to the presence of spin-triplet correlations

Study of 0-π\pi phase transition in hybrid superconductor-InSb nanowire quantum dot devices

Hybrid superconductor-semiconducting nanowire devices provide an ideal platform to investigating novel intragap bound states, such as the Andreev bound states (ABSs), Yu-Shiba-Rusinov (YSR) states, and the Majorana bound states. The competition between Kondo correlations and superconductivity in Josephson quantum dot (QD) devices results in two different ground states and the occurrence of a 0-$\pi$ quantum phase transition. Here we report on transport measurements on hybrid superconductor-InSb nanowire QD devices with different device geometries. We demonstrate a realization of continuous gate-tunable ABSs with both 0-type levels and $\pi$-type levels. This allow us to manipulate the transition between 0 and $\pi$ junction and explore charge transport and spectrum in the vicinity of the quantum phase transition regime. Furthermore, we find a coexistence of 0-type ABS and $\pi$-type ABS in the same charge state. By measuring temperature and magnetic field evolution of the ABSs, the different natures of the two sets of ABSs are verified, being consistent with the scenario of phase transition between the singlet and doublet ground state. Our study provides insights into Andreev transport properties of hybrid superconductor-QD devices and sheds light on the crossover behavior of the subgap spectrum in the vicinity of 0-$\pi$ transition