Tuning electron correlation strength by charge doping of 5d¹ double perovskites

Competing interactions between spin-orbit coupling (SOC) and strong electron correlations have recently led to widespread studies of various emergent novel quantum phases. Double perovskites, particularly those containing 5d transition metal ions, present a useful opportunity to study such interactions since they display comparable magnitudes of electron correlations, crystal field, and SOC. Here, we present a systematic study of the electron doped powder Mott insulator Ba₂Na_1-xCa_xOsO₆ through the Na/Ca partial substitution using ²³Na nuclear magnetic resonance at various doping concentrations. These results complement our previous muon spin relaxation (μSR) and superconducting quantum interference device (SQUID) measurements. Ba₂Na_1-xCa_xOsO₆ displays a crossover from an exotic canted ferromagnetic state driven by multipolar ordering at x = 0 to an antiferromagnetic state at x = 1. Moreover, the magnetic transition increases monotonically from 5 K to 40 K with increasing Ca concentration. The experimental results will be compared to theoretical work and their implications will be discussed.