Electronic and magnetic ground state of 4d^3 double perovskite ruthenates A_2LaRuO_6 (A = Ca, Sr, Ba)

4d transition metal oxide (TMO) offers an intriguing puzzle for their electronic and magnetic ground state. They are in the cross-over regime of strong spin orbit interaction (SOI) and electron-electron correlation (U) with quenched orbital angular momentum. Our work unravels the electronic and magnetic ground state of the less investigated 4d^3 double perovskite ruthenates A_2LaRuO_6 (A = Ca, Ba). The negligible effect of SOI is evident from the bulk magnetic, specific heat measurements and density functional theory (DFT) calculations, indicating a classical spin-only magnetic ground state (S = 3/2) for the materials. Magnetization measurements show that both materials have long range antiferromagnetic order with high degree of magnetic frustration (f ≈13 -15). Interestingly, a near T^2- behavior is observed in low-T magnetic heat capacity measurement, indicating the presence of low-dimensional spin-wave exciation and magnetic frustration in both materials. The temperature dependent resistivity measurements and electronic band structure calculations confirm a conventional Mott insulating ground state in these two systems. Moreover, our experimental investigation and DFT calculations highlight the reason for the nonexistence of Sr_2LaRuO_6.