Abstract
The bulk ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Co}}_{2}{\mathrm{O}}_{8}$ crystal is an insulator at low temperature and experiences an insulator-metal transition at ${T}_{C}\ensuremath{\approx}60\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The new experiment showed that ${T}_{C}$ increases to 140 K for a mechanically exfoliated nanosheet of four blocks and is beyond room temperature for nanosheets thinner than four blocks. We show that the thickness-dependent insulator-metal transition observed in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Co}}_{2}{\mathrm{O}}_{8}$ nanosheets can naturally be explained by the strongly correlated low-spin-state insulator (LS state) and intermediate-spin-low-spin-state metal (IS-LS state). In particular, the energy difference between the LS state and IS-LS state qualitatively reproduces the trend of the transition temperature with the nanosheet thickness. The predicted transition temperature of a nanosheet with three blocks is only slightly above room temperature, a result that can be used to check our proposed mechanism. Further experiments on the distinct magnetotransport properties and spin-fluctuation behaviors of the LS state and IS-LS state are also very helpful to resolve the issue. The weak interblock binding is also consistent with the layer-resolved partial densities of states.
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