Electronic structure and magnetic properties of the strong-rung spin-1 ladder compound Rb$_3$Ni$_2$(NO$_3$)$_7$

Small single crystals of Rb$_3$Ni$_2$(NO$_3$)$_7$ were obtained by crystallization from anhydrous nitric acid solution of rubidium nitrate and nickel nitrate hexahydrate. The basic elements of the crystal structure of this new compound are isolated spin-1 two-leg ladders of Ni$^{2+}$-ions connected by (NO$_3$)$^-$ groups. The experimental data show the absence of long range magnetic order at T $geq 2$~K. LDA+U calculations and the detailed analysis of the experimental data, i.e. of the magnetic susceptibility, the specific heat in magnetic fields up to 9~T, the magnetization, and of the high-frequency electron spin resonance data, enable quantitative estimates of the relevant parameters of the $S=1$ ladders in Rb$_3$Ni$_2$(NO$_3$)$_7$ . The rung-coupling $J_1 = 10.5$~K, the leg-coupling $J_2=1.6$~K, and the uniaxial anisotropy $|A| = 179$~GHz are obtained. The scenario of spin liquid quantum ground state is further corroborated by quantum Monte Carlo simulations of the magnetic susceptibility.