Magnetic relaxation and collective vortex creep in FeTe _0.6 Se _0.4 single crystal

Abstract

-We study the vortex dynamics in high-quality FeTe0.6Se0.4 single crystal by performing magnetization measurements of the screening current density J s and flux creep rate S. Temperature dependence of S shows a plateau in the intermediate temperature region with a high creep rate ∼ 0.03, which is interpreted in the framework of the collective creep theory. A crossover from elastic to plastic creep is observed. The glassy exponent and barrier height for flux creep are directly determined by extended Maley's method. J s with flux creep, obtained from magnetic hysteresis loops, is successfully reproduced based on the collective creep analysis. We also approach critical current density without flux creep by means of the generalized inversion scheme, which proves that the δl and δT c pinning coexist in FeTe0.6Se0.4 single crystal.Recently discovered iron-based superconductors (IBSs) with superconducting transition temperature T c above 55 K is another member of the high temperature superconductors (HTS) after cuprate superconductors [1,2]. IBSs share some similarities with cuprate superconductors like layered structure, very high upper critical fields and unconventional paring mechanism [2,3]. The study of IBSs is helpful to solve some puzzles remained in HTS and testify some notions and theories originated from the cuprate superconductors. Among these, vortex dynamics is one of the central issues related to both basic science and technological applications. For cuprate superconductors, due to large anisotropy, short coherence lengths and high operation temperature, the vortex motion and fluctuations are quite strong [4]. This leads to a collective pinning with small characteristic pinning energy, which give rise to intriguing experimental results, such as plateau observed in the temperature dependence of normalized relaxation rate (S ≡|dlnM /dlnt |) [5] in contrast to the linear increase with temperature predicted by the Anderson-Kim model in low temperature superconductors (LTS). To understand these behaviors, some theories have been proposed in the past decades [4]. Among these, the collective creep theory successfully interpreted the plateau region, and the large creep rate [5,6]. The discovery of IBSs provides another opportunity to study the vortex dynamics as well as collective creep theory in HTS, and its intermediate T c is also meaningful to understand the crossover between LTS and HTS.

Direct evidence of two superconducting gaps in FeSe0.5Te0.5: SnS-Andreev spectroscopy and the lower critical field

“…However, as it was shown for several types of 11 family single crystal materials (FeSe - [30], FeS, FeSeS - [32], FeSeTe - [33,34] magnetic hysteresis loops are symmetric close to the critical temperatures of these materials, which means the absence of surface barriers and thus validating the determination of H c1 (T ) by means of deviation from linear Meissner behaviour criterium.…”

Magneto-optical characterizations of FeTe0.5Se0.5thin films with critical current density over 1 MA cm−2

“…In Figure 5, we compare the temperature dependence of J c 's for FeTe 0.5 Se 0.5 thin films grown on LAO and YSZ substrates together with that from high-quality single crystal, which also manifests a roof-top pattern in MO images [15,23]. The values of J c 's for thin films are over 10 6 A/cm 2 , one order of magnitude higher than that of the single crystal.…”

Critical current density, vortex dynamics, and phase diagram of single-crystal FeSe

“…Here, we should point out that weak fish-tail effect was observed in a related material FeTe 1−x Se x , 29,30 which is also an isovalently doped system. This exception may come from the sample quality, in which the interstitial Fe (excess Fe) in FeTe 1−x Se x may work as the charged quasiparticle-scattering centers.…”

“…However, the crossover to fast creep occurs at progressively higher fields with lowering temperature. Here, we should point out that the value of S shows monotonic behavior with field, different from YBa 2 Cu 3 O 7−δ , 39 iron-based "122", 8,40 and FeTe 1−x Se x , 30,41 in which S at low temperatures shows an upturn with decreasing field to zero. Such an increase in S usually corresponds to the smaller value of µ.…”

“…30 It is obvious that all the curves can be well scaled together without introducing any more factors as shown in Fig. 4(b).…”