Origin of the emergence of higher T c than bulk in iron chalcogenide thin films

Abstract

Fabrication of epitaxial FeSexTe1−x thin films using pulsed laser deposition (PLD) enables improving their superconducting transition temperature (T c) by more than ~40% than their bulk T c. Intriguingly, T c enhancement in FeSexTe1−x thin films has been observed on various substrates and with different Se content, x. To date, various mechanisms for T c enhancement have been reported, but they remain controversial in universally explaining the T c improvement in the FeSexTe1−x films. In this report, we demonstrate that the controversies over the mechanism of T c enhancement are due to the abnormal changes in the chalcogen ratio (Se:Te) during the film growth and that the previously reported T c enhancement in FeSe0.5Te0.5 thin films is caused by a remarkable increase of Se content. Although our FeSexTe1−x thin films were fabricated via PLD using a Fe0.94Se0.45Te0.55 target, the precisely measured composition indicates a Se-rich FeSexTe1−x (0.6 < x < 0.8) as ascertained through accurate compositional analysis by both wavelength dispersive spectroscopy (WDS) and Rutherford backscattering spectrometry (RBS). We suggest that the origin of the abnormal composition change is the difference in the thermodynamic properties of ternary FeSexTe1−x, based on first principle calculations.

Universal scaling behavior of the upper critical field in strained FeSe0.7Te0.3 thin films

“…EDX line scans across the cross-section of the films confirmed the stoichiometry to be homogeneous over the film thickness, as shown in figure A1 in the appendix. It was found that the composition of the films is FeSe 0.7 Te 0.3 within the error-bars of the analysis (few percent) for all studied substrates due to the preference of Fe to bond with Se because of the low formation energy [23]. Electrical transport properties were measured in a Physical Property Measurement System [(PPMS) Quantum Design] by a standard four-probe method, for which 4 pins are collinearly aligned along the edge of the film.…”

Artificially engineered nanostrain in FeSexTe1-x superconductor thin films for supercurrent enhancement

“…First, the critical transition temperature (Tc) of FST abruptly increases due to an increasing Se ratio with the suppression of phase separation, which is generally observed in Se-rich FST bulk, when FST is fabricated as an epitaxial thin film. 11,12 In addition, the FeSe monolayer can achieve a Tc of 100 K, which is the maximum Tc for FeSCs. 13 Second, FST thin films exhibit a promising critical current density (Jc) greater than 1 MA/cm 2 , at the self-field regardless of the substrates including a coated conductor substrate.…”

Recent development of iron-based superconducting films

“…虽然铁基超导体的T c 低于铜氧化物超导体, 但是它 具有铜氧化物超导体所不具备的优点: 非磁性杂质的 掺杂和缺陷的引入对铁基超导体T c 的影响较小 [5] 、其 各向异性γ比铜氧化物超导体更小 [6] 、临界晶界角更 大 [7] 、制备简单、成本低廉等, 这些特性均使铁基超导 [8] 认为T c 会随着薄膜厚度 的减小而显著降低, 尤其是含Te的11薄膜, 原因可能是 在低温下四方基底会抑制较薄薄膜从四方相到正交相 的结构转变 [9] . Si等人 [10] Bellingeri等人 [11] 和Yuan等人 [12] 分别在不同的基底上 制备了名义成分为FeSe 0.5 Te 0.5 的薄膜, 并得出了相同的 [12,13] . 该现象基本符合Wu等 人 [8] 的观点, 即PLD制备的薄膜具有比靶材更高的Se/ Te比, 并且该成分被认为可以产生最高T c [13,14] .…”

“…Si等人 [10] Bellingeri等人 [11] 和Yuan等人 [12] 分别在不同的基底上 制备了名义成分为FeSe 0.5 Te 0.5 的薄膜, 并得出了相同的 [12,13] . 该现象基本符合Wu等 人 [8] 的观点, 即PLD制备的薄膜具有比靶材更高的Se/ Te比, 并且该成分被认为可以产生最高T c [13,14] . Seo等 人 [13] 通过计算FeSe、FeSe 0.5 Te 0.5 和FeTe的形成能来解 释这一现象: 三者中FeSe的形成能最低, 因此在薄膜生 长的过程中更易形成FeSe, 最终造成薄膜的Se/Te比高 于靶材.…”

“…该现象基本符合Wu等 人 [8] 的观点, 即PLD制备的薄膜具有比靶材更高的Se/ Te比, 并且该成分被认为可以产生最高T c [13,14] . Seo等 人 [13] 通过计算FeSe、FeSe 0.5 Te 0.5 和FeTe的形成能来解 释这一现象: 三者中FeSe的形成能最低, 因此在薄膜生 长的过程中更易形成FeSe, 最终造成薄膜的Se/Te比高 于靶材.…”