Inhomogeneities and superconductivity in poly-phase Fe-Se-Te systems

Abstract

To study the influence of the sample preparation procedure on the superconducting properties of FeSe x Te 1−x , we have grown two FeSe 0.4 Te 0.6 crystals and investigated their superconducting properties. One of the crystals possessed a secondary phase of Fe 3 Se 2.1 Te 1.8 , while the other was a high-quality FeSe 0.4 Te 0.6 single crystal. We have checked the sample compositions and phases via energy dispersive x-ray spectroscopy to obtain a more sophisticated picture of the inclusions. Our susceptibility measurements under hydrostatic pressure show that neither pressure up to 9 kbar nor stress is sufficient to obtain a superconducting state in the homogeneous crystal. The critical temperature of two-phase FeSe 0.4 Te 0.6 increases at 8.9 kbar from 12.3 K to T c =17.9 K. Therefore, we conclude that inhomogeneities are a necessary feature for providing superconductivity in this iron chalcogenide system. We have prepared Fe 3 Se 2.1 Te 1.8 polycrystals and studied their magnetic properties for comparison.

Temporal Spinodal Decomposition of the Fe1+yTe1−xSex Crystals and its Impact on Superconducting Properties

“…They produce a strain and thus improve superconducting properties in accordance with the conclusions of other studies. [ 4,7 ] This manifests itself in the temperature dependence of resistance and susceptibility (see Figure 6 ).…”

“…The tilted thick blue arrow marked as SB1 points to the side band of (004) reflection. Short vertical arrows show the reflection positions of the possible Fe 3 Te 2.1 Se 1.8 impurity [ 4 ] and long one is for possible Fe 3 O 4 admixture. [ 35 ] The interplanar distance, corresponding to angle of lower abscissa axis, is plotted on the top abscissa axis.…”

“…Note, that the powder XRD studies detect often the presence of extraneous phases, such as Fe 3 O 4 , [ 35 ] Fe 7 Se 8 , [ 34 ] and Fe 3 Se 2.1 Te 1.8 . [ 4 ] The Bragg reflection positions of these possible admixtures are marked by arrows on the S2 diffraction pattern (Figure 3b). It should be mentioned that there no reflections corresponding to the (00l) planes among them.…”

“…Iron chalcogenide crystals FeTe 1Àx Se x often show weak superconductivity immediately after preparation, namely, they have relatively a small part of superconducting volume and lower transition temperature (see, e.g., refs. [1][2][3][4]) Bulk superconductivity was detected in crystals with secondary or extraneous phases only. [1,4] Attempts to improve the superconducting properties of these materials by removing defects and inhomogeneities using annealing often give the opposite effect, that is, they turn out with lower transition temperature and volume fraction of the superconducting phase.…”

Short-Lived Electron Excitations in FeTe1 –xSex as Revealed by Microwave Absorption

“…The excess Fe acts in two ways: on the one hand, it leads to the additional current carrier scattering, increases the resistivity and lowers the critical temperature [16,17]; on the other hand, impurity phases may arise and induce internal stress, which can improve the superconducting properties of a material (as it was shown in Ref. [18]). Our Fe 1+y Te 1−x Se x samples have a broad superconducting transition (3 ÷ 5 K) with a rather high onset temperature (see Table 1).…”