NMR investigation of spin and charge dynamics in the electron doped Dirac-Mott insulator Ba2Na1-xCaxOsO6 double Perovskite

Materials which possess both strong spin orbit coupling and strong electron correlation reveal exceptional magnetic and conductive properties, such as the case of the transition metal oxide object of this study. In the present dissertation we use Nuclear Magnetic Resonance technique in order to study the effect of charge doping on the Dirac Mott insulator double perovskites Ba2Na1-xCaxOsO6. We analyse in depth the material characteristic relaxation time T1 and T2 as a function of doping and temperature to investigate the spin and charge dynamics of the system. The study has revealed an unexpected and anomalous dependence at high temperature, well above the magnetic transitions, fingerprint of a thermally activated processes. The comparison with previous muSR data and a detailed analysis through Bloembergen-Purcell-Pound theory indicate that they are due to thermally activated fluctuation of the crystalline electric field gradient. The temperature and doping dependence of the observed dynamics, the material properties, along with the comparison with earlier manganites studies, suggests that the phenomenon can be identified with the presence of small polarons, which has never been observed before for double perovskites.