Tuning and characterizing the superconducting phases of iron selenide telluride thin films induced via gate-driven hydrogen intercalation

The discovery of iron-based superconductors marks an important turning point in solid-state physics. The superconducting transition temperature (Tc) of these compounds is among the highest known. FeSe is one of the simplest ones but nevertheless presents a rich phase diagram. Although stoichiometric FeSe is not superconducting, it can display superconductivity under pressure, strain, partial substitution of Se with isovalent elements like Te or aliovalent elements like S, and even under electron doping. In the latter case, it develops high-Tc phases whose nature remains a subject of intense debate. My thesis work is positioned within this debate. It focuses on the study of thin films of FeSe0.45Te0.55, with a Tc= 14.5 K, before and after electron doping (obtained through hydrogen intercalation). Indeed, FeSe and Fe(Se,Te) are layered compounds that can be intercalated rather easily. Here, I used the so-called ionic liquid-gating technique to insert H+ ions in the lattice. The idea is to immerse the FeSeTe film and a metallic gate, in a suitable ionic liquid that acts as an electrolyte. When a bias Vg is applied, the ionic liquid releases H+ ions pushing them into the film by the strong electric field at the surface. This insertion allows changing the electronic properties of the material and tuning the superconducting phase, i.e. increasing Tc. The work starts by a characterization of the pristine FeSe0.55Te0.45 thin films by means of electric transport measurements as a function of temperature, to study the normal-state resistivity and determine their Tc. The superconducting phase is further studied by point-contact Andreev-reflection spectroscopy (PCARS), which allows gathering direct information on the number, amplitude and symmetry of the superconducting energy gap(s) by analyzing the differential conductance dI/dV of a small contact between a normal metal (N) and the superconductor (S) as a function of the bias voltage V. The dI/dV vs V spectra contain information of the superconducting gap Δ thanks to a quantum phenomenon called Andreev reflection: an electron injected from N into S with an energy smaller than Δ is reflected as a hole, while a Cooper pair is created in S. By analyzing the PCARS spectra obtained in FeSeTe pristine films, I identify structures that can be associated with the superconducting gap. A fitting of the spectra with a suitable model in literature gives Δ≃2.4k_B T_c for contacts along the c axis and Δ≃2.9 k_B T_c for contacts along the ab plane. The spectra additionally show structures that allow extracting the characteristic energy of the boson that mediates the superconducting coupling (E_p≃11 meV). A closer inspection of the low-energy conductance also allows identifying a smaller gap Δ=1.4 k_B T_c. These two gaps, as well as the position of the electron-boson structure, agree very well with results reported in literature. Subsequently, the FeSeTe films are intercalated with H+ by using the ionic liquid gating technique, to inducing a higher-Tc phase. Despite the volatility of the H+ insertion, I demonstrate that applying a gate voltage 2.4 ≤Vg≤2.7V leads to an increase in Tc from 14.5 K to 20.7 K. Finally, I attempt to perform PCARS measurements in situ ( i.e. in the cell used for ionic liquid gating) on the intercalated sample, so as to overcome the volatility of H+ insertion. This approach could open new avenues for investigating the spectroscopic properties of protonated materials, which are expected to exhibit entirely new characteristics.