Breakdown of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>J</mml:mi><mml:mo>=</mml:mo><mml:mn>0</mml:mn></mml:mrow></mml:math>nonmagnetic state in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi>d</mml:mi><mml:mn>4</mml:mn></mml:msup></mml:math>iridate double perovskites: A first-principles study

Through first-principles calculations, we study the electronic structure of double-perovskite iridates with Ir in the ${d}^{4}$ valence state. Contrary to the expected strong spin-orbit driven $J=0$ nonmagnetic state, we find finite moment at the Ir site, exhibiting breakdown of the $J=0$ state. We further find the band structure effect rather than the crystal field effect to be responsible for this breakdown. The antiferromagnetic superexchange interaction between Ir moments, in general, makes these compounds insulating.