The Dirac-Mott insulator 5d1 Ba2NaOsO6 (BNOO) offers an optimal playground to study the effect of the intricate interplay between electron correlations and spin-orbit (SO) coupling when their energy scales are comparable in size. As a result, this material shows local distortions from the cubic symmetry revealing the presence of a broken local point symmetry (BLPS) phase as a precursor to a canted-antiferromagnetic state driven by a multipolar order, most likely of the antiferro-quadrupolar type [1].
Here we investigate the evolution of the magnetic and BLPS phases as a function the electron charge doping induced by the Ca/Na heterovalent substitution of Ba2Na1−xCaxOsO6 in the whole 0<x<1 doping range by combining local experimental probes, namely nuclear magnetic resonance and muon spin spectroscopy. We show the persistence of the BLPS phase indicating that these materials preserve the multipolar character, most likely of the antiferro-quadrupolar type for x < 1 [3]. In addition, we reveal anomalous peaks in the spin-lattice and spin-spin relaxation rates, attributed to fluctuation of the electric field gradients with a doping dependent amplitude driven by a charge-related time dependent perturbation. With a very successful combination of ab initio calculations and measurements we give evidence for the first time [4] that an essential ingredient which characterizes the physics of Dirac-Mott insulators is the formation of spin-orbital bipolarons, responsible for blocking the Metal-Insulator Transition.
References
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[2] J. K. Kesavan, D. Fiore Mosca, S. Sanna, F. Borgatti, G. Schuck, P. M. Tran, P. M. Woodward, V. F. Mitrović, C. Franchini, and F. Boscherini, J. Phys. Chem. C 124, 16577 (2020).
[3] R.Cong, E.Garcia, P.C.Forino, A. Tassetti, G.Allodi, A.P.Reyes, P.M.Tran, P.M.Woodward, C.Franchini, S.Sanna, V.F.Mitrovic, Phys. Rev. Materials 7, 084409 (2023).
[4] L. Celiberti, D. Fiore Mosca, G. Allodi, L. V. Pourovskii, A. Tassetti, P. C. Forino, R. Cong, E. Garcia, P. M. Tran, R. De Renzi, P. M. Woodward, V. F. Mitrović, S. Sanna, C. Franchini, Nature Communications 15, 2429 (2024)