Multipolar Ordering in CaIrO₃ Thin Films

5d-transition metal oxides, where correlations and spin-orbit coupling play at the same energy scale, create a great playground in search for high-T_C and unconventional superconductivity, spin liquids and novel magnetic ordering phenomena.
The metastable perovskite phase of CaIrO₃ was previously grown in thin-film form and characterized as semimetallic by ab-initio calculations (Yang et al., J. Phys. Condens. Matter (2010) ), although the optical conductivity spectra were inconclusive in this respect, leaving open the possibility of a small-gap insulating state. We performed first-principles calculations which yield a small insulating gap, despite a large spatial extent of Ir 5d wavefunctions. Ordering of higher-order magnetic multipoles is shown to go hand-in-hand with the gap formation.
In order to verify our calculations we have grown CaIrO₃ thin films on SrTiO₃ by pulsed laser deposition. Due to the high volatility of IrO₃ significant off-stoichiometry is observed despite a high crystalline quality and the formation of a stable perovskite structure. In order to obtain stoichiometric CaIrO₃ thin films, we had to adjust the growth kinetics while reducing the oxygen partial pressure to prevent the formation of volatile IrO₃. We will discuss the electrical and magnetic properties of our films in the framework of the presence of multipolar ordering, gap formation and possible structural defects.