Disorder-driven electronic localization and phase separation in superconductingFe1+yTe0.5Se0.5<

Abstract

We have investigated the influence of Fe excess on the electrical transport and magnetism of Fe 1+y Te 0.5 Se 0.5 ͑y = 0.04 and 0.09͒ single crystals. Both compositions exhibit resistively determined superconducting transitions ͑T c ͒ with an onset temperature of about 15 K. From the width of the superconducting transition and the magnitude of the lower critical field H c1 , it is inferred that excess of Fe suppresses superconductivity. The linear and nonlinear responses of the ac susceptibility show that the superconducting state for these compositions is inhomogeneous. A possible origin of this phase separation is a magnetic coupling between Fe excess occupying interstitial sites in the chalcogen planes and those in the Fe-square lattice. The temperature derivative of the resistivity d / dT in the temperature range T c Ͻ T Ͻ T a with T a being the temperature of a magnetic anomaly, changes from positive to negative with increasing Fe. A log 1 / T divergence of the resistivity above T c in the sample with higher amount of Fe suggests a disorder-driven electronic localization.

Doping Effects of Co, Ni, and Cu in FeTe0.65Se0.35Single Crystals

“…The low-T upturn of resistivity is usually caused by localization of carriers. For example, similar behavior has been observed in crystals Fe 1+δ Te 1−x Se x with x = 0.4 [10] or x = 0.5 [11], and it has been attributed to disorderdriven localization, presumably caused by the excess of Fe. However, the disorder-driven weak localization is an orbital eect which should be suppressed by the perpendicular magnetic eld, causing very characteristic negative magnetoresistance eect.…”

Structural inhomogeneities in FeTe0.6Se0.4: Relation to superconductivity

“…It was also shown that incommensurate magnetic order and superconductivity coexist for certain Se concentrations [15,16]. Both, superconducting and magnetic properties are known to critically depend on the iron excess [17,18,19,20,16]. The element Fe occupies two different crystallographic positions: position 2a at (3/4 1/4 0) denoted as a Fe1 position and position 2c at (1/4 1/4 z) with z ≈ 0.70, denoted as Fe2.…”

“…Many controversial reports concerning the mutual coexistence/interplay of AFM and SC exist in literature [9] which may be, at least in part, related to different preparation methods. Influence of annealing [23,24,25] and different contents of iron [17,18,19,20,16] have been studied intensively, however, very little attention has been given to the influence of cooling rates [26]. Although the majority of the studies [27,23] report, that the prepared polyor single-crystalline Fe 1+y Te 1−x Se x samples contain, depending on preparation conditions, secondary phases (mostly Fe 7 Se 8 , Fe 3 Se 4 or Fe 3 O 4 ), a study dedicated to clarify whether these phases are just unwanted complication or whether they may play a key role in superconductivity of this system is, so far, missing.…”

Interplay of structure, magnetism, and superconductivity in Se substituted iron telluride with excess Fe

“…Above x = 0.10, a weak magnetic transition is still present but shifts to lower temperature with increase in Se content and almost vanishes for x = 0.25. At still higher Se compositions, x = 0.4 and 0.5, superconductivity dominates [30]. The onset of superconductivity and the weakening of magnetic transition in compositions from x = 0.15 to 0.50 are illustrated in Figure 2 The dc magnetization of crystals with different composition were studied with magnetic field applied parallel to the crystallographic ab plane, see Figure 3.…”

“…The effect of substitution on the lattice is studied by XRD. From these data, the lattice parameters are extracted using a Rietveld analysis within the FullProf code and assuming the structure model of an earlier work [30]. Figure 1 (a) illustrates the refined powder XRD pattern for the various compositions.…”