Ab-initio Unveiling of Relativistic Ordered Phases

In condensed matter physics, hidden-order phases are those in which a clear thermodynamic phase transition is observed, yet the associated order parameter remains difficult to detect with conventional experimental probes. Closely related, but more exotic, are quantum spin liquids, phases where spins evade conventional long-range order even at the lowest temperatures due to strong quantum entanglement. These states exhibit fractionalized excitations and, in some cases, emergent gauge fields. Both hidden order and quantum spin liquids behaviour are naturally promoted by the interplay of strong spin-orbit coupling and electron correlations. Two material families of particular interest are 5d double perovskites and dipole-octupole pyrochlores. The former are known to host high-rank multipolar order (e.g., quadrupoles and octupoles) beyond conventional magnetic dipoles. Despite their elusive experimental signatures, theory and experiment are increasingly converging to map out the broader landscape of such multipolar phases. The latter class, which includes certain Ce-based pyrochlores, features Kramers doublets whose pseudospin components transform as magnetic dipole and octupole moments, due to combined crystal-field and spinorbit effects. Theory predicts that these doublets can realize two distinct symmetry-enriched U(1) quantum spin liquids. In this project, we will employ state-of-the-art first-principles and model-Hamiltonian approaches to address two central questions: How does doping influence hidden-order phases in 5d/5d double perovskites, and can it serve as a probe for their detection? Do dipoleoctupole pyrochlores truly host U(1) quantum spin-liquid phases? How can they be clearly detected and what design strategies could enable their realization? The research will be conducted by the PI in collaboration with a PhD student