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Origin of a Superlattice Observed in Li$_{0.9}$Mo$_{6}$O$_{17}$ by Scanning Tunneling Microscopy

We use scanning tunneling microscopy to study the lithium purple bronze (Li$_{0.9}$Mo$_{6}$O$_{17}$) at room temperature. Our measurements allow us to identify the single-crystal cleave plane and show that it is possible to obtain clean cleaved surfaces reflecting the crystal structure without the complications of nanoscale surface disorder. In addition to the crystal lattice, we observe a coexisting discommensurate superlattice with wavevectors q = 0.5a* $\pm$ 0.25b*. We propose that the origin of the superstructure is a surface reconstruction which is driven by cleaving along a crystal plane which contains in-plane MoO$_{4}$ tetrahedra connected to out-of-plane MoO$_{6}$ octahedra through corner-sharing oxygens. When combined with spectroscopic measurements, our studies show a promising avenue through which to study the complex physics within Li$_{0.9}$Mo$_{6}$O$_{17}$.