Synthesis and physical properties of the new potassium iron selenide superconductor K0.80Fe1.76Se2

TL;DR

This paper reviews the synthesis and physical properties of the new superconductor K0.80Fe1.76Se2, revealing its growth methods, anisotropic properties, high upper critical field, and coexistence of magnetism and superconductivity, suggesting a phase-separated structure.

Contribution

It provides a comprehensive analysis of the physical properties and phase behavior of K0.80Fe1.76Se2, highlighting its anisotropic superconductivity and magnetic order, and proposing a phase separation model.

Findings

Moderate anisotropy in magnetic susceptibility and resistivity.

High upper critical field (~55 T at 2 K).

Magnetic order coexists with superconductivity, indicating phase separation.

Abstract

In this article we review our studies of the K0.80Fe1.76Se2 superconductor, with an attempt to elucidate the crystal growth details and basic physical properties over a wide range of temperatures and applied magnetic field, including anisotropic magnetic and electrical transport properties, thermodynamic, London penetration depth, magneto-optical imaging and Mossbauer measurements. We find that: (i) Single crystals of similar stoichiometry can be grown both by furnace-cooled and decanted methods; (ii) Single crystalline K0.80Fe1.76Se2 shows moderate anisotropy in both magnetic susceptibility and electrical resistivity and a small modulation of stoichiometry of the crystal, which gives rise to broadened transitions; (iii) The upper critical field, Hc2(T) is ~ 55 T at 2 K for H||c, manifesting a temperature dependent anisotropy that peaks near 3.6 at 27 K and drops to 2.5 by 18 K; (iv) Mossbauer measurements reveal that the iron sublattice in K0.80Fe1.76Se2 clearly exhibits magnetic order, probably of the first order, from well below Tc to its Neel temperature of Tn = 532 +/- 2 K. It is very important to note that, although, at first glance there is an apparent dilemma posed by these data: high Tc superconductivity in a near insulating, large ordered moment material, analysis indicates that the sample may well consist of two phases with the minority superconducting phase (that does not exhibit magnetic order) being finely distributed, but connected with in an antiferromagnetic, poorly conducting, matrix, essentially making a superconducting aerogel.