METHODS OF PHYSICAL INVESTIGATION

Record high critical temperature among bismuthides of the 122 family: BaAg$_{1.8}$Bi$_2$ with a monoclinically distorted CaBe$_2$Ge$_2$ structure

A. V. Sadakov^a, A. S. Usoltsev^a, V. A. Vlasenko^a, S. Yu. Gavrilkin^a, A. I. Shilov^a, K. S. Pervakov^a, E. O. Rakhmanov^b, I. V. Morozov<sup>b

a</sup> Ginzburg Center for High-Temperature Superconductivity and Quantum Materials, Lebedev Physical Institute, Russian Academy of Sciences, Moscow, 119991 Russia
^b Faculty of Chemistry, Moscow State University, Moscow, 119991 Russia

Abstract: A new superconductor BaAg$_{1.8}$Bi$_2$ with a previously unknown variant of the monoclinically distorted CaBe$_2$Ge$_2$ 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 $\kappa= 27$, this compound is a type-II superconductor with the first and second critical fields $H_{c1}(0)= 53$ Oe and $H_{c2}(0) = 2.1\times10^4$ Oe and the critical current density reaching $4.4$ kA/cm$^2$ 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 CaBe$_2$Ge$_2$ structural type and its derivatives.

Received: 12.11.2024
Revised: 18.11.2024
Accepted: 20.11.2024

DOI: 10.31857/S0370274X25010128

 English version: Journal of Experimental and Theoretical Physics Letters, 2025, 121:1, 72–77