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Field-Tuned Superconductor-Insulator Transition in BaPb1-xBixO3

BaPb$_{1-x}$Bi$_x$O$_3$ is found to exhibit a field-tuned superconductor to insulator transition for Bi compositions 0.24 $\leq x \leq$ 0.29. The magnetoresistance of optimally doped samples manifests a temperature-independent crossing point and scaling of the form $ρ(T,H)=ρ_c F(|H-H_{c}|T^{-1/zν})$, where $H_c$ is the field determined by the temperature-independent crossing point, and $zν$ = 0.69 $\pm$ 0.03. High resolution transmission electron microscopy measurements reveal a complex intergrown nanostructure comprising tetragonal and orthorhombic polymorphs. Data are analyzed in terms of both a classical effective medium theory and a field-tuned quantum phase transition, neither of which provides a completely satisfactory explanation for this remarkable phenomenology.