Reassembled phase diagram for Fe _y Se _x Te_1−x superconducting films grown by pulsed laser deposition

Abstract

FeySexTe1−x targets with different Fe contents have been prepared by the solid state reaction method followed by the hot-pressed sintering, and corresponding single-phase FeySexTe1−x films with high quality have been deposited at different substrate temperatures by the pulsed laser deposition. The evolution of the structure, superconductivity and pinning mechanism of the FeySexTe1−x superconductors with the Fe content has been investigated in detail. It can be found that the excess Fe can form the residual Fe impurities scattered in the polycrystalline bulks when the Fe content in the nominal composition exceeds 1.36. More importantly, a reassembled phase diagram for FeySexTe1−x superconducting films based on the change of both Fe and Se contents is constructed for the first time. It can be clearly seen that both the Fe and Se contents can greatly affect the superconductivity of FeySexTe1−x films within a wide range of Fe content (0.67 ≤ y ≤ 1.48). The Fe vacancies caused by the Fe deficiency is beneficial to the improvement of both critical temperature and activation energy, while with the increase of the Fe content, the superconductivity degrades because of the introduction of the interstitial Fe and the planar defects. The optimal Fe content at constant Se content shifts to the higher value with the increase of Se content. Our results facilitate the rational regulation of the superconducting properties, as well as the study of the underlying superconducting mechanism of FeSeTe superconductors.

Unveiling the growth mechanism of FeSeTe films by pulsed laser deposition technique

“…Therefore, we infer that the crystallinity has little effect on the superconductivity of the FeSeTe films deposited in the T s range of 475 of (Se+Te):Fe ratio. The highest T c,onset = 15.8 K is obtained at the (Se+Te):Fe ratio of 1.4 (T s = 550 • C) due to the introduction of appropriate Fe vacancies, which is consistent with the result in the previous study [11]. In addition, the good inplane texture and the well aligned grains (figure 3(b)) may also contribute to its high T c .…”

“…When the T s is above 625 • C, the (Se+Te):Fe ratio increases again, which is related to the active desorption of Fe atoms at high T s . In general, T s can serve as an effective means to regulate the (Se+Te):Fe ratio and therefore the superconductivity of the FeSeTe film [11,21,[39][40][41][42][43].…”

“…To further explore the T s effect on flux pinning of the FeSeTe films, the R-T curves for all the superconducting samples under various magnetic fields were measured. Figure 5(a) shows an example of the FeSeTe film deposited at 550 • C. It can be found that the T c decreases and the transition width broadens with increasing magnetic field, which can be explained by the thermally activated flux flow (TAFF) mechanism [11,49,50]. The ∆T c broadens from 1.1 K at 0 T to 1.6 K at 5 T. In addition, the temperature and magnetic field dependence of the resistance in the TAFF region follows the Arrhenius relation:…”

“…Most importantly, the FeSeTe films deposited on CaF 2 substrates exhibit higher T c values compared to those deposited on other substrates [18,27,30]. The target is a high-quality Fe 1.14 Se 0.45 Te 0.55 polycrystalline bulk, which was prepared by the solid-state reaction method followed by the hot-pressed sintering, as described in detail elsewhere [11,31]. During the film deposition process, the vacuum environment was maintained at 10 −7 Torr, and the target-substrate distance was set at 50 mm.…”

“…research and practical applications [2,8]. Pulsed laser deposition (PLD) is one of the most common methods to prepare 11-type Fe-based superconducting films [9][10][11][12][13], and involves many preparation parameters, including substrate temperature (T s ), target to substrate distance, laser fluence and so on. These preparation parameters jointly determine the properties of the films [14][15][16][17][18][19][20].…”