Electrochemical gating for superconductivity engineering in materials towards the 2D limit

In this thesis work we explored the capability of electrochemical gating to reliably control the ground state of several chosen materials, with a specific focus on the engineering of the superconducting state. We also experimented with different electrolyte compositions in order to best match the electrochemical requirements of the various materials under study (e.g., chemical stability). In the presentation of the results, we will move from the thicker, bulk-like materials down to the truly two-dimensional properties of thin exfoliated single crystals. Chapter 1 presents a general analysis of the field-effect technique based on an electrolytic gate. We discuss the basic principle that allows for the existence of ultrahigh electric fields at the device surface, together with the several pratical limitations and criticalities the technique entails. In particular, we consider the critical distinction between purely electrostatic gating and the regimes where various types of electrochemical interactions are activated between the sample and the electrolyte. We also discuss in detail a purely electrochemical measurement that can be performed on the complete devices in order to determine the amount of charge accumulated in the electric double layer. Chapter 2 shows a selection of our results on superconducting thin films. We analyze extensively the response of conventional BCS superconductor niobium nitride to EDL gating as a function of film thickness (∼ 40−10 nm), and we interpret our data in the framework of a bulk control of the superconducting transition mediated by proximity effect. We then extend our analysis to more complex materials. We show preliminary results on state-of-the-art thin films (∼ 20 nm) of two-gap superconductor magnesium diboride. Finally, we consider thin films of iron-based superconductor barium iron arsenide and show how its Tc can be modulated by the electric field only in the smallest thicknesses available by state-of-the-art growth techniques (∼ 10 nm). Chapter 3 presents our results on thin flakes (∼ 5−10 nm) of transition metal dichalcogenides. We explore via EDL gating the valley occupation in the conduction and of semiconducting molybdenum and tungsten disulphides at high carrier densities. We show preliminary evidence linking the emergence of EDL-induced superconductivity with the population of secondary minima in the bandstructure for molybdenum disulphide. We also exploit electrochemical gating beyond the electrostatic regime to perform field-assisted intercalation of molybdenum disulphide with alkali ions, in an effort to demonstrate both surface and bulk gate-controlled superconductivity in the same device architecture. We find preliminary evidence for the onset of a possible Charge-Density-Wave phase at very high ion doping. Chapter 4 is entirely devoted to our results on few-layer graphene. While we did not observe any gate-induced superconductivity (down to T= 3.5 K) even at the highest induced carrier densities ∼ 6 · 1014 cm-2, we were able to extensively study the dominant scattering mechanisms both in the high and low temperature regimes; in particular, we showed that inelastic scattering for T . 90 K is dominated by electron-electron collisions, in contrast with what was found in the literature for single-layer graphene. Moreover, we observed the emergence of quantum coherence phenomena (weak localization) for T . 20 K in these previously unreached conditions of ultrahigh carrier doping. Finally, in the Conclusions we summarize the most significant results obtained during this thesis work together with the questions that are still left open. Furthermore, we consider some perspectives and future lines of research that could be pursued in the framework of electrolyte gating.