The mixed-valent spinel LiV₂O₄ is known as the first oxide heavy-fermion system with an enhanced specific heat coefficient γ ~400 mJ/molK². 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 V3+ and V4+ 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. We uncover the hidden CO instability by applying epitaxial strain on single-crystalline LiV2O4 thin films [1]. We find a crystallization of heavy fermions in a LiV2O4 film on MgO, where a charge-ordered insulator comprising of a stack of V3+ and V4+ 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 in bulk single crystals under hydrostatic pressures, evidence the close proximity of the heavy-fermion state to degenerate CO states mirroring the geometrical frustration of the V pyrochlore lattice, which supports the CO instability scenario for the mechanism behind the heavy fermion formation.

[1] U.Nieman et al., PNAS 120, e2215722120 (2023).

+work done in collaboration with U.Nieman, Y.M.Wu, R.Oka, D.Hirai, Y.Wanga, Y.E.Suyolocua, M.Kim, P.v.Aken.