First-principles design of the spinel iridate Ir$_2$O$_4$ for high temperature quantum spin ice

Insulating magnetic rare-earth pyrochlores related to spin ice host emergent bosonic monopolar quasiparticles. These quantum spin ice monopoles obey a magnetic analogue of quantum electrodynamics, opening a route to a magnetic analogue of electronics. However, the energy scales of the interactions among rare-earth moments are so low as 1~K that the possible quantum coherence can only be achieved at sub-Kelvins. This too low energy scale hinders advances in fundamental understandings and potential applications. Here, we desgin high-temperature quantum spin ice materials from first principles. It is shown that the $A$-site deintercalated spinel iridate Ir$_2$O$_4$, which has been experimentally grown as epitaxial thin films, is a promising candidate for quantum spin ice with a spin-ice-rule interaction of a few tens of meV. Controlling electronic structures of Ir$_2$O$_4$ through substrates, it is possible to tune magnetic interactions so that a magnetic Coulomb liquid persists at high temperatures.