Mott variable range hopping and bad-metal in lightly doped spin-orbit Mott insulator BaIrO3

Abstract

The Mott transition and the vicinity of a Mott insulating phase have constantly been a fertile ground for exploring exotic quantum states, most notably the high-${T}_{c}$ cuprates. The layered iridate represents another intriguing example. The nature of the Mott insulator phase and transition mechanism to a metallic state is still under debate. Much of the challenge originates from a series of energy scales involved in the electronics phases. Here, we report synthetization, characterization, and transport measurements on doped and undoped $\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x}\mathrm{Ir}{\mathrm{O}}_{3}$ films grown on $\mathrm{SrTi}{\mathrm{O}}_{3}$. The films are fully strained up to 70 nm thick and have a tetragonal lattice structure. For a doping level of $x=0.1$, a bad-metal state with linear temperature dependence of the resistivity beyond the Mott-Ioffe-Regel limit emerges in a wide temperature range down to a critical temperature ${T}_{c}\ensuremath{\sim}30\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, below which the system shows Mott variable range hopping (Mott VRH) conduction behavior. The ground state is confirmed to be insulating by the Mobius criterion. A strong correlation between the bad-metal state and Mott VRH localized state is found as the slope of the linear resistivity is inversely proportional to film thickness and the size of the Mott VRH activation energy $\mathrm{\ensuremath{\Delta}}$ is linearly proportional to ${T}_{c}$. We further show that upon doping the spin-orbit Mott insulator, itinerant metallic regions coexist with localized regions within a nanoscale phase-separated ground state with a small activation energy. Our results shed light on the nature of the metallic state and a crossover to a bad-metal phase for doping the spin-orbit Mott insulator.