Multipolar Order and Spin-Orbital Bipolarons in 5d Correlated Oxides*

The investigation of intricate magnetic orders in transition metal oxides, characterized by strong spin-orbital entanglement and electronic correlations, has emerged as a captivating area of scientific research. This endeavor has led to the discovery of novel quantum states resulting from interactions between effective pseudospins bearing high-rank multipoles.

Due to the elusive nature of these multipolar orders, conventional probes fall short in their ability to uncover them. As a result, theoretical and numerical approaches have become indispensable tools for gaining insight into the underlying physical mechanisms. Initially, significant progress in this field was driven by a synergistic interplay between experimental and theoretical methods employing semi-empirical effective Hamiltonians. More recently, there has been a shift toward material-specific quantitative techniques that combine magnetically constrained density functional theory and dynamical mean-field theory, proving vital in resolving the competition between a multitude of competing phases [1,2,3,4].

In this presentation, following an introduction to the methodology, we will delve into the formation of multipolar phases in double perovskite compounds based on osmium, specifically those with 5d¹ (Ba₂NaOsO₆)[2] and 5d² (Ba₂CaOsO₆)[3] electron configurations. We will also explore the coexistence of various J-effective states in doped phases (Ba₂Na_1-xCa_xOsO₆, where 0 < x < 1), mediated by relativistic bipolarons. Notably, the gradual increase in bipolaron density with increasing doping gives rise to robust in-gap states, preventing the transition to a metallic phase even at ultrahigh doping levels. This phenomenon preserves the Dirac-Mott gap across the entire doping range, spanning from d¹ to d² [5].

To validate our theoretical and numerical predictions, we will present results from nuclear magnetic resonance and muon spin rotation measurements.