Spin-orbital Jahn-Teller bipolarons

Abstract

Polarons and spin-orbit (SO) coupling are distinct quantum effects that play a critical role in charge transport and spin-orbitronics. Polarons originate from strong electron-phonon interaction and are ubiquitous in polarizable materials featuring electron localization, in particular $3d$ transition metal oxides (TMOs). On the other hand, the relativistic coupling between the spin and orbital angular momentum is notable in lattices with heavy atoms and develops in $5d$ TMOs, where electrons are spatially delocalized. Here we combine ab initio calculations and magnetic measurements to show that these two seemingly mutually exclusive interactions are entangled in the electron-doped SO-coupled Mott insulator $\mathrm{B}{\mathrm{a}}_2\mathrm{N}{\mathrm{a}}_{1-\mathrm{x}}\mathrm{C}{\mathrm{a}}_{\mathrm{x}}\mathrm{Os}{\mathrm{O}}_6$ ($0<x<1$), unveiling the formation of spin-orbital bipolarons. Polaron charge trapping, favoured by the Jahn-Teller lattice activity, converts the Os $5{\mathrm{d}}^1$ spin-orbital ${\mathrm{J}}_{\mathrm{eff}}=3/2$ levels, characteristic of the parent compound $\mathrm{B}{\mathrm{a}}_2\mathrm{NaOs}{\mathrm{O}}_6$ (BNOO), into a bipolaron $5{\mathrm{d}}^2$ ${\mathrm{J}}_{\mathrm{eff}}=2$ manifold, leading to the coexistence of different J-effective states in a single-phase material. The gradual increase of bipolarons with increasing doping creates robust in-gap states that prevents the transition to a metal phase even at ultrahigh doping, thus preserving the Mott gap across the entire doping range from ${\mathrm{d}}^1$ BNOO to ${\mathrm{d}}^2$ $\mathrm{B}{\mathrm{a}}_2\mathrm{CaOs}{\mathrm{O}}_6$ (BCOO).

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Cited by in corpus (6)

• Hidden orders in spin-orbit entangled correlated insulators
• Dynamic Jahn-Teller Phenomena in Heavy Transition Metal Compounds
• Comparison between first-principles supercell calculations of polarons and the ab initio polaron equations
• Sr4Os3O12- A Layered Osmate (V,VI) that is Magnetic close to Room Temperature
• Polaron-driven switching of octupolar order in doped 5d${}^2$ double perovskite
• Extending the Feynman variational principle and analytical methods to lattice polarons