Abstract
It is now well established that the microstructure of Fe-based chalcogenide KxFe2−ySe2 consists of, at least, a minor (~15 percent), nano-sized, superconducting KsFe2Se2 phase and a major (~85 percent) insulating antiferromagnetic K2Fe4Se5 matrix. Other intercalated A1−xFe2−ySe2 (A = Li, Na, Ba, Sr, Ca, Yb, Eu, ammonia, amide, pyridine, ethylenediamine etc.) manifest a similar microstructure. On subjecting each of these systems to a varying control parameter (e.g. heat treatment, concentration x,y, or pressure p), one obtains an exotic normal-state and superconducting phase diagram. With the objective of rationalizing the properties of such a diagram, we envisage a system consisting of nanosized superconducting granules which are embedded within an insulating continuum. Then, based on the standard granular superconductor model, an induced variation in size, distribution, separation and Fe-content of the superconducting granules can be expressed in terms of model parameters (e.g. tunneling conductance, g, Coulomb charging energy, Ec, superconducting gap of single granule, Δ, and Josephson energy J = πΔg/2). We show, with illustration from experiments, that this granular scenario explains satisfactorily the evolution of normal-state and superconducting properties (best visualized on a {boldsymbol{g}}{boldsymbol{-}}frac{{{boldsymbol{E}}}_{{boldsymbol{c}}}}{{boldsymbol{Delta }}}{boldsymbol{-}}{boldsymbol{T}} phase diagram) of AxFe2−ySe2 when any of x, y, p, or heat treatment is varied.
Highlights
Tunneling conductance, g, Coulomb charging energy, Ec, superconducting gap of single granule, Δ, and Josephson energy J = πΔg/2)
All points are plotted against the conductance g which is obtained from a fit of Eq (3) to measured ρ(T, control) curves: each obtained g − T phase diagram is discussed as being a projection of the generalized g − Ec/Δ − T diagram[33,35], the latter is most appropriate for the description of the normal and superconducting properties
We did not extend the granular superconducting scenario to the p > 9 GPa regime because, for above 9 GPa, (i) the structural symmetry is transformed from I4/m into I4/mmm, (ii) the resistivity is strongly flattened at high-temperature, (iii) a Kondo-like behavior is manifested at lower temperature, and (iv) the low-pressure superconductivity is being monotonically suppressed, a re-entrant superconducting state emerges above 9 GPa
Summary
The pseudo-monocrystalline character of KxFe2−ySe2 (best visualized in the electron micrographs[21,29,30,31] of Fig. 1) can be envisaged as a granular array wherein nano-sized superconducting granules of KsFe2Se2 are randomly dispersed within the insulating K2Fe4Se5 matrix[17]. The granular superconductor model[33] so as to rationalize the evolution of the normal and superconducting properties of KxFe2−ySe2 Within this simplifying scenario, we consider that a variation in control parameters (such as heat treatment, pressure p, and concentration x, y)[21,30,31,34] modifies the size, distribution, separation, and concentration of metallic granules and that the latter modification can be expressed in terms of the model parameters. (ii) the quantum confinement within each granule (with mean energy-level spacing δ and an inverse escape rate ( ) okBfThssaitnwghleergerTasnaut l=e
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