Record High Critical Temperature Among Bismuthides of the 122 Family: BaAg1.8Bi2 with a Monoclinically Distorted CaBe2Ge2 Structure

Abstract

A new superconductor BaAg_1.8Bi₂ with a previously unknown variant of the monoclinically distorted CaBe₂Ge₂ structure (space group C2/m) has been crystallized in the form of single crystals from a bismuth flux. The temperature and magnetic field dependences of the magnetic susceptibility and resistance have shown that this compound transits to a superconducting state at the temperature T_c = 5.4 K. According to the found Ginzburg–Landau parameter κ = 27, this compound is a type-II superconductor with the first and second critical fields H_c1(0) = 53 Oe and H_c2(0) = 2.1 × 10⁴ Oe and the critical current density reaching 4.4 kA/cm² at 2.5 K. It can be assumed that, similar to some superconducting 112 type bismuthides, superconductivity in this compound is due to the planar square bismuth sublattice contained in the fluorite-like [BiAg_0.8] layer rather than to the [AgBi] layers where Ag atoms are locally disordered. This could explain the unusually high T_c value for bismuthides belonging to the CaBe₂Ge₂ structural type and its derivatives.