Josephson Junctions by Electrostatic tuning of Dichalcogenides – JJEDi

Fabrication of gate tunable JJ : We will develop the device presented in figure 1. On the one hand, a 2D superconducting phase will be modulated by ionic gating through a topgate, VTG. On the other hand, the junction properties will be controlled by tuning the weak link to an insulating or a metallic state with a local backgate, VCG. The ultimate device will exhibit few nanometer-wide weak links (only limited by the spatial drop of the electrostatic potential) obtained by using nanotubes as local gates. The standard backgate, VBG, will be useful to tune finely the superconducting properties at low temperature (when the ions are frozen in the electrolyte).
An incremental approach is used: transistors with metal-insulator transition, then metal-superconductor transition, development of local gating, combination of all the gates.
Setup design: from DC Josephson effect to coherent radiofrequency device response; In a similar fashion, the project will progressively aim at electronic transport measurements with temperature, field orientation and high frequency resolution toward quantum control. Starting from the existing low noise magnetoresistance (DC and low frequency) capabilities available in the team (up to 13 T magnetic field), we will first develop a sample rotation control system together with RF excitation in a variable transfer cryostat (VTI) with temperatures from 300 K to 1.7 K, then at 300mK in an Helium3 system. Ultimately, the device will be connected directly to the RF excitation to probe the coherence of the quantum states.

We managed to evidence and study carefully the contact resistance and metal-insulator transitions in monolayer TMDs grown by CVD.

We started the development of local gates and ionic gating and plan to study the metal-superconductor transition and local gating.
New promising TMDs have been identified and will be tested through the development of a transfer setup for high quality TMDs.
It is reasonnable to expect the demonstration of tunable JJ before the end of the project and a definite assesment of which phases are topological.

The first two articles are under progress.

Josephson Junctions (JJs) are studied in a huge variety of systems for their applications in magnetometry and voltage-frequency conversion. Such junctions are obtained by combining two superconductors separated by another material or a “weak link”. The coupling of JJs with radiofrequency photons, the capability to design topological junctions, have strengthened the interest in these devices for quantum computation.
Besides, there is a rich underlying physics in transition metal dichalcogenides (TMDs) multilayers. The 2D superconductivity in these materials coexists with a strong spin-orbit coupling, orienting the spins out-of-plane, whereas the electrostatic field orients the spins in plane. This combination is predicted to lead to various unconventional phases, and in particular topological superconductivity. Experimentally, superconductivity in these materials still needs to be investigated in details.
Tuning electrostatically the density of charges in semiconducting TMDs by designing Electrical double layer transistors (EDLT), exotic superconductivity was observed below a few Kelvin. Local gating using nanotubes also demonstrated tunable p-n-p junctions in TMDs. Therefore, TMDs offer a new promising platform to design in-plane topological JJs.

This project tackles two main challenges: combining various advanced nanofabrication techniques and demonstrating quantum manipulation in a new type of JJs. These two challenges can be realistically lifted by combining a systematic and incremental approach, which will maintain alternative routes open toward these objectives. The main objective of this project is to develop gate tunable JJs (from S-I-S to S-N-S and even S-S’-S) based on TMDs: MoS2 and WS2. The focus will be put on sulfide based TMDs because they exhibit the highest TC and the lowest doping needed to achieve superconductivity. Single crystals will be used as the unique material forming the junction (including weak link) and the superconductivity will be tuned by local gates. Hence, these JJs will be controlled without interfaces imperfection. This approach will be pushed down to the limit of a few nanometers wide JJs by local gating with nanotubes. Thanks to the advantage of TMDs to be easily gateable, these JJs can be envisioned as building blocks for spintronics and quantum information.
We will study our devices with low-noise transport experiments combining low temperature, magnetic field orientation, and radiofrequency excitation and probing. A deep understanding of the phase and spin properties of our JJs will be obtained by measuring current-voltage characteristics, under oriented field and radiofrequency excitation. The demonstration of anomalies in these dependencies will point toward topological states or other bound states. Finally, the demonstration of coherent coupling with long-lived quantum states will be done through the observation of anti-crossing in the RF response coupled to a waveguide.

Chances to achieve the best conditions for the observation and control of topological bound states will be maximized by this approach. Indeed, this approach allows controlling the superconducting and weak link regions by varying the in-plane spin orbit coupling, the intrinsic out-of-plane spin-orbit coupling and the type of doping (electron or holes). Moreover, the material thickness will allow choosing between different pairing mechanisms (phonon mediated or Ising). Temperature and magnetic field will allow controlling the tunneling supercurrent (and its phase) in the JJs. Finally, the radiofrequency measurements will allow us to confirm the onset of topological superconductivity and the associated extremely long lifetimes needed for their manipulation in applications.