ABSTRACT
In this chapter, we review our studies of the K0.80Fe1.76Se2 super-
conductor, with an attempt to elucidate the crystal growth details
and basic physical properties over a wide range of temperatures and
appliedmagnetic field, including anisotropic magnetic and electrical
transport properties, thermodynamic, London penetration depth,
magneto-optical imaging and Mo¨ssbauer 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 suscep-
tibility and electrical resistivity and a smallmodulation of stoichiom-
etry of the crystal, which gives rise to broadened transitions; (iii)
The upper critical field, Hc2(T) is∼55 T at 2 K forH ‖ c, manifesting a temperature dependent anisotropy that peaks near 3.6 at 27 K
and drops to 2.5 by 18 K; (iv) Mo¨ssbauer measurements reveal
that the iron sublattice in K0.80Fe1.76 Se2 clearly exhibits magnetic
order, probably of the first order, from well below Tc to its Ne´el 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.
