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LiV$_2$O$_4$ is a mixed-valent spinel oxide and one of a few transition-metal compounds to host a heavy fermion phase at low temperatures. While numerous experimental studies have attempted to elucidate how its 3$d$ electrons undergo giant mass renormalization, spectroscopic probes that may provide crucial hints, such as scanning tunneling microscopy (STM), remain to be applied. A prerequisite is atomically flat and pristine surfaces, which, in the case of LiV$_2$O$_4$, are difficult to obtain by cleavage of small, three-dimensional crystals. We report the epitaxial growth of LiV$_2$O$_4$ thin films with bulklike properties on SrTiO$_3$(111) via pulsed laser deposition and stable STM imaging of the LiV$_2$O$_4$(111) surface. The as-grown films were transferred $ex$ $situ$ to a room-temperature STM, where subsequent annealing with optional sputtering in ultrahigh vacuum enabled compact islands with smooth surfaces and a hexagonal 1$\times$1 atomic lattice to be resolved. Our STM measurements provide insights into growth mechanisms of LiV$_2$O$_4$ on SrTiO$_3$(111), as well as demonstrate the feasibility of performing surface-sensitive measurements of this heavy fermion compound., Comment: 6 pages, 4 figures + SM

The mixed-valent spinel LiV$_{2}$O$_{4}$ is known as the first oxide heavy-fermion system. There is a general consensus that a subtle interplay of charge, spin, and orbital degrees of freedom of correlated electrons plays a crucial role in the enhancement of quasi-particle mass, but the specific mechanism has remained yet elusive. A charge-ordering (CO) instability of V$^{3+}$ and V$^{4+}$ ions that is geometrically frustrated by the V pyrochlore sublattice from forming a long-range CO down to $T$ = 0 K has been proposed as a prime candidate for the mechanism. To uncover the hidden CO instability, we applied epitaxial strain from a substrate on single-crystalline thin films of LiV$_{2}$O$_{4}$. Here we show a strain-induced crystallization of heavy fermions in a LiV$_{2}$O$_{4}$ film on MgO, where a charge-ordered insulator comprising of a stack of V$^{3+}$ and V$^{4+}$ layers along [001], the historical Verwey-type ordering, is stabilized by the in-plane tensile and out-of-plane compressive strains from the substrate. Our discovery of the [001] Verwey-type CO, together with previous realizations of a distinct [111] CO, evidence the close proximity of the heavy-fermion state to degenerate CO states mirroring the geometrical frustration of the pyrochlore lattice, which supports the CO instability scenario for the mechanism behind the heavy-fermion formation.