Quantum entanglment and symmetry breaking in a pyrochlore iridates

Quantum entanglement is a fundamental feature of quantum information, driving unconventional phenomena in microscopic systems. In condensed matter, the entanglement of many-body electrons can give rise to complex multi-electron ground states, leading to unexpected symmetry-breaking orders [1]. We investigate a pyrochlore system exhibiting a quantum entangled ground state of multi-electronic states, where magnetic order can transform easily under perturbations through collapse into an entangled state. As a building block, our resonant inelastic X-ray scattering (RIXS) reveals interference modulation driven by quasi-molecular electronic clustering [2,3]. Our cluster calculations and symmetry analysis suggest that the true ground state of this quasi-molecular system is not a pure all-in-all-out magnetic state, typical in low-temperature pyrochlores, but rather an entangled state with lower symmetry configurations, including 2-in-2-out, 3-in-1-out, and so on. These hidden states can emerge under external perturbations, such as magnetic fields. Our Raman spectroscopy, showing a broad split of Eg phonon at high temperatures, supports the presence of hidden states, while observations of an anomalous Hall effect under magnetic fields suggest the collapse of these states.