Tuning of the superconducting properties of FeSe_0.5Te_0.5thin films through the substrate effect

Abstract

Using a field-effect transistor (FET) configuration with solid Li-ion conductor (SIC) as gate di-electric, we have successfully tuned carrier density in FeSe0.5Te0.5 thin flakes, and the electronic phase diagram has been mapped out. It is found that electron doping controlled by SIC-FET leads to a suppression of the superconducting phase, and eventually gives rise to an insulating state in FeSe0.5Te0.5. During the gating process, the (001) peak in XRD patterns stays at the same position and no new diffraction peak emerges, indicating no evident Li + ions intercalation into the FeSe0.5Te0.5. It indicates that a systematic change of electronic properties in FeSe0.5Te0.5 arises from the electrostatic doping induced by the accumulation of Li + ions at the interface between FeSe0.5Te0.5 and solid ion conductor in the devices. It is striking that these findings are drastically different from the observation in FeSe thin flakes using the same SIC-FET, in which Tc is enhanced from 8 K to larger than 40 K, then the system goes into an insulating phase accompanied by structural transitions. PACS numbers: 74.25.F-, 74.70.Xa, 74.78.-w Tuning carrier concentration is one of the most powerful approaches in the condensed matter physics for the explorations of novel quantum phases and exotic electronic properties as well as their underlying physical mechanics [1-8]. To overcome the inherent doping limit in the material synthetic methods, field effect transistor (FET) configurations have been applied to tune material properties using gating by electric field [9]. Two types of FET, metal-insulator-semiconductor (MIS) FET and electric double layer (EDL) FET, are widely used to control the charge carrier density on the surface of materials [10, 11]. In order to change the carrier density in the bulk, the so-called ionic field-effect transistor (iFET) with gel-like electrolyte as the gate medium has been used to drive Li + ions into layered materials. This type of FET configuration can effectively modulate 1T-TaS 2 electronic properties by the tunable Li + ion intercalation [12]. However, the heavily-doped electronic states in all these FET configurations are confined at the interfaces or overlaid with electrolyte, which prevents them from being characterized by many physical measurements. On the other hand, conventional MIS-FET devices cannot provide sufficient carriers to induce novel phases, such as superconductivity, by electrostatic doping, and the liquid or gel-like electrolyte is not compatible with modern solid electronics and may react with samples when gating voltage is applied [11, 13, 14]. Recently, we have fabri-[ †] These authors contributed equally to this work.

Strain effects on the electronic structure of the FeSe0.5Te0.5 superconductor

“…2. At the same time, the relation between the T c and Z Se/T e (and also Fe-Se and Fe-Te bond lengths) is clearly linear, as was reported for FeSe 0.5 Te 0.5 thin films deposited on e.g., LaAlO 3 or SrTiO 3 [18,19]. Hydrostatic pressure of 2 GPa, investigated here, is related just to the maximum of bulk T c [14], whereas other values of stress were chosen arbitrarily.…”

“…In turn, non-hydrostatic strains also modify T c of the 11-type superconductors as it was revealed in lattice mismatched epitaxial films where tensile strain suppresses superconductivity (SC) [16,17]. It was an opposite effect to that of the compressive biaxial (ab-plane) [17,18,19] or uniaxial (c-axis) [20] strain, causing the increase of the T c 's.…”

“…The optimal conditions for SC are closely related to the imperfect tetrahedral coordination of an Fe atom, tuned by a chalcogenide anion height (free Z Se/T e position) [9,17,19].…”

Strain effects on electronic structure and superconductivity in the iron telluride

“…In FeSe tensile strain strongly decreases T c [4] but all kinds of compressive strain lead to an increase of its T c [4,5,6,7,8,9,10,11,12,13]. The influence of strain on SC of iron chalcogenides is somewhat related to the changes of tetrahedral coordination of Fe atoms [10,5,12]. Furthermore, a magnetic order of particular compounds is also connected with chalcogen atom position in the unit cell (u.c.)…”

Comparative Review on Thin Film Growth of Iron-Based Superconductors

“…Indeed, the films with best superconducting properties, are on LAO and MgO, whose the lattice misfit parameter M ≡ (a sub − 3.798)/a sub = -0.21% and 9.82%, respectively. It should be noted that Bellingeri et al reported positive correlation between the in-plane lattice parameters of the films and the substrates [99], while their films were not coherently strained to the underlying substrates. The discrepancy between the observed results may result from the difference of the growth temperatures; Bellingeri et al grew the films at much higher temperature of 550 • C than Imai et al (300 • C) [52].…”