Small Moments Without Long-Range Magnetic Ordering in the Zero-Temperature Ground State of the Double Perovskite Iridate Ba2YIrO6

The spin-orbit coupled double perovskite iridate Ba_2YIrO_6 with d^4 occupancy of Ir is considered as a candidate material for a non-magnetic J=0 ground state. The issue of existence of such a state in Ba_2YIrO_6 however has opened up intense debates both in experimental and theoretical studies. In this study, we revisit the issue using ab-initio density functional combined with dynamical mean-field theory to investigate the magnetic properties of Ba_2YIrO_6 down to zero temperature. To reach the ground state, a recently developed impurity solver based on tensor-product states working directly at zero temperature is employed. We find that Ba_2YIrO_6 has a small instantaneous non-zero magnetic moment, both at T=0 K as well as at room temperature. We did not observe any evidence of magnetic ordering, not even at T=0 K. From the calculated local magnetic susceptibility we see that the quantum fluctuations are very strong and effective in screening the instantaneous moments. This dynamical screening, together with frustration effects in the fcc lattice that can lead to almost degenerate magnetic ground states, prevents any long-range ordering.