• Takeya, H.
• Tanaka, M.
• Soares, C. C.
• Elmassalami, M.
• Yanagisawa, Y.
• Takano, Y.

Abstract

<jats:title>Abstract</jats:title><jats:p>It is now well established that the microstructure of Fe-based chalcogenide K<jats:sub><jats:italic>x</jats:italic></jats:sub>Fe<jats:sub>2−<jats:italic>y</jats:italic></jats:sub>Se<jats:sub>2</jats:sub> consists of, at least, a minor (~15 percent), nano-sized, superconducting K<jats:sub><jats:italic>s</jats:italic></jats:sub>Fe<jats:sub>2</jats:sub>Se<jats:sub>2</jats:sub> phase and a major (~85 percent) insulating antiferromagnetic K<jats:sub>2</jats:sub>Fe<jats:sub>4</jats:sub>Se<jats:sub>5</jats:sub> matrix. Other intercalated <jats:italic>A</jats:italic><jats:sub>1−<jats:italic>x</jats:italic></jats:sub>Fe<jats:sub>2−<jats:italic>y</jats:italic></jats:sub>Se<jats:sub>2</jats:sub> (<jats:italic>A</jats:italic> = 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 <jats:italic>x</jats:italic>,<jats:italic>y</jats:italic>, or pressure <jats:italic>p</jats:italic>), 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, <jats:italic>g</jats:italic>, Coulomb charging energy, <jats:italic>E</jats:italic><jats:sub><jats:italic>c</jats:italic></jats:sub>, superconducting gap of single granule, Δ, and Josephson energy <jats:italic>J</jats:italic> = <jats:italic>π</jats:italic>Δ<jats:italic>g</jats:italic>/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 <jats:inline-formula><jats:alternatives><jats:tex-math>{{g}}{{-}}{{{{E}}}_{{{c}}}}{{}}{{-}}{{T}}</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>g</mml:mi><mml:mo>−</mml:mo><mml:mfrac><mml:mrow><mml:msub><mml:mrow><mml:mi>E</mml:mi></mml:mrow><mml:mrow><mml:mi>c</mml:mi></mml:mrow></mml:msub></mml:mrow><mml:mi>Δ</mml:mi></mml:mfrac><mml:mo>−</mml:mo><mml:mi>T</mml:mi></mml:math></jats:alternatives></jats:inline-formula> phase diagram) of <jats:italic>A</jats:italic><jats:sub><jats:italic>x</jats:italic></jats:sub>Fe<jats:sub>2−<jats:italic>y</jats:italic></jats:sub>Se<jats:sub>2</jats:sub> when any of <jats:italic>x</jats:italic>, <jats:italic>y</jats:italic>, <jats:italic>p</jats:italic>, or heat treatment is varied.</jats:p>

Topics

• impedance spectroscopy
• microstructure
• phase
• experiment
• phase diagram