The mixed-valent spinel LiV2O4 is known as the first oxide heavy-fermion system. There is a 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. In this talk, we uncover the hidden CO instability by applying epitaxial strain on single-crystalline LiV2O4 thin films. 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, evidence the proximity of the heavy-fermion state to degenerate CO states mirroring the geometrical frustration of the V pyrochlore lattice. Our recent transport (Hall coefficient RH and thermopower S) studies on LiV2O4 single crystals at low temperatures below 2 K points to a semimetallic ground state with almost equally heavy electrons and holes, which gives us a hint for the origin of CO instability in k-space. We note that the semimetallic ground state is the natural consequence of the even number of electrons in the primitive cell containing 4 V ions (1.5 x 4 =6). The volume of semimetallic Fermi surfaces inferred from transport appears to respect roughly the LDA semimetallic Fermi surfaces, though the mass is two orders of magnitude enhanced. We argue that the coupling of itinerant electrons in the almost half-filled two eg bands and almost Mott-localized electrons in the narrow a1g bands below the Fermi level gives rise to the formation of extremely narrow semimetallic quasi-particle bands at the Fermi level. LiV2O4 may bridge the 4f Kondo heavy fermion physics with the correlated d-electron physics.

U. Niemann, Y.-M. Wu, R. Oka, D. Hirai, Y. Wanga, Y. E. Suyolcua, M. Kim, P. A. van Aken, and H. Takagi, Proceedings of the National Academy of Sciences 120, e2215722120 (2023).