Static and resonant properties of decorated square kagome lattice compound KCu$_7$(TeO$_4$)(SO$_4$)$_5$Cl

Abstract: The magnetic subsystem of nabokoite, KCu$7$(TeO$_4$)(SO$_4$)$_5$Cl, is constituted by copper ions forming a buckled square kagom\'e lattice decorated by quasi-isolated ions. This combination determines peculiar physical properties of this compound evidenced in electron spin resonance (ESR) spectroscopy, dielectric permittivity $\varepsilon$, magnetization $M$ and specific heat $C_p$ measurements. At lowering temperature, the magnetic susceptibility $\chi = M/H$ passes through a broad hump inherent for low-dimensional magnetic systems at about 150 K and a sharp peak at antiferromagnetic phase transition at $T_N = 3.2 $K. The $C_p(T,H)$ curves demonstrate additional peak-like anomaly at $T{peak}= 5.7$K robust to magnetic field. The latter can be ascribed to low-lying singlet excitations filling the singlet-triplet gap in magnetic excitation spectrum of the square kagom\'e lattice [J.Richter, O.Derzhko and J.Schnack, Phys. Rev. B \textbf{105} (2022) 144427]. ESR spectroscopy provides indications that antiferromagnetic structure below $T_N$ is non-collinear. Separate issue is the observation of antiferroelectric-type behavior in $\varepsilon$ at low temperatures, which tentatively reduces the symmetry and partially lifts frustration of magnetic interactions of decorating copper ions with buckled square kagom\'e lattice. These complex thermodynamic and resonant properties signal the presence of two weakly coupled magnetic subsystems in nabokoite, namely a spin-liquid in square kagom\'e lattice layers and an antiferromagnet represented by decorating ions.