Crystal growth, transport phenomena and two-gap superconductivity in the mixed alkali metal (K1−zNaz)xFe2−ySe2 iron selenide

Abstract

Using the self-flux technique we grew superconducting (K_1−zNa_z)_xFe_2−ySe₂ (z = 0.3) single crystals. EDX mapping revealed the uniform elements distribution on the crystal surface while XRD measurements indicate that the crystals are compositionally inhomogeneous on the nanoscale. The physical properties of the as-prepared sample are characterized by electrical resistivity, magnetization and specific heat measurements. Resistivity measurements show the onset of the superconducting transition at 33 K and zero resistivity at 31.7 K. The large upper critical field H_c2(0) was estimated as high as about 140 T for the in-plane field and 38 T for the out-of-plane field. The anisotropy of H^ab_c2(0)/H^c_c2(0) and coherence lengths ξ^ab(0)/ξ^c(0) was found to be around 3.7. The pioneering studies by multiple Andreev reflections effect spectroscopy (“break-junction” technique) revealed the presence of two anisotropic superconducting gaps Δ_L = (9.3 ± 1.5) meV, Δ_S = (1.9 ± 0.4) meV, and provided a measurement of the Δ_L(T) temperature dependence. The Bardeen–Cooper–Schrieffer (BCS) theory ratio ratio for the large gap 2Δ_L/k_BT^bulk_c ≈ 6.3 points to a strong electron–boson coupling in the “driving” condensate characterized by the Δ_L order parameter.