Spin-singlet ground state of the coupled J_eff = 1/2 alternating chain system Sr₂ Co(SeO₃)₃

We report a detailed study of the structural, magnetic, thermodynamic, and electronic properties of a coupled J_eff = 1/2 alternating chain Sr2Co(SeO₃)₃ compound using magnetic susceptibility χ(T), magnetic specific heat C_m(T), magnetization, and neutron diffraction measurements along with first-principles calculations. The first-principles calculations based on the density functional theory suggest that Sr₂Co(SeO₃)₃ forms a quasi-one-dimensional chain with bond alternation and interchain interactions. χ(T), C_m(T), and neutron powder diffraction measurements confirm that no long-range magnetic ordering occurs down to 100 mK. Instead, a maximum in χ(T) and C_m(T) and an exponential drop of χ(T) and C_p(T) as T → 0 K point to a spin-singlet ground state. The analysis of χ(T) and C_m(T) based on a J₁-J₂ alternating Heisenberg model shows the bond alternation α = J₂/J₁ ≈ 0.7 and a spin gap of Δ ≈ 3 K. Our work demonstrates that Sr₂Co(SeO₃)₃ is a coupled alternating chain system based on spin-orbit entangled J_eff = 1/2.