Anisotropic magnetism, resistivity, London penetration depth and magneto-optical imaging of superconducting K_0.80Fe_1.76Se₂ single crystals

Single crystals of K_0.80Fe_1.76Se₂ were successfully grown from a ternary solution. We show that, although crystals form when cooling a near-stoichiometric melt, crystals are actually growing out of a ternary solution that remains liquid to at least 850 °C. We investigated their chemical composition, anisotropic magnetic susceptibility and resistivity, specific heat, thermoelectric power, London penetration depth and flux penetration via magneto-optical imaging. Whereas the samples appear to be homogeneously superconducting at low temperatures, there appears to be a broadened transition range close to T_c ∼ 30 K that may be associated with small variations in stoichiometry.