Small polaron formation in 5d spin-orbit coupled omsates*

Small polarons (SP) have been thoroughly investigated in 3d transition metal oxides [1]. However, very little is known about these quasi-particles in 5d systems, since the more delocalised nature of the 5d orbitals reduces the strength of the Electronic Correlation (EC), making SP formation in these compounds rather unexpected. The spin-orbit coupled Dirac-Mott insulator Ba₂NaOsO₆ (BNOO) represents a candidate material for enabling polaron formation in a relativistic background, due to the relatively large EC (U ~ 3 eV) and Jahn-Teller activity [2]. We verify this hypothesis by combining first principles calculations with nuclear magnetic resonance (NMR) and muons measurements. We find that excess electrons injected into BNOO through chemical doping are trapped in Os sites and distort the local phonon field, typical hallmark of SP formation. SP are subjected to thermally activated hopping, revealed by anomalous peaks in the spin-lattice and spin-spin relaxation rates, attributed to fluctuation of the electric field gradients driven by a charge-related time dependent perturbation.

References

[1] C. Franchini et al., Nat. Rev. Mater., 1-27 (2021) 10.1038/s41578-021-00289-w.

[2] D. Fiore Mosca et al., Phys. Rev. B 103, 104401 (2021) 10.1103/PhysRevB.103.104401.