Hidenori Takagi,Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany

Thursday May 23 at 2:00 pm (Paris time), Room Boreau, at LPEM in hybrid format :

Crystallization of heavy fermions via epitaxial strain in spinel LiV2O4 thin film

The mixed-valent spinel LiV2O4 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 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 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. Our recent transport (Hall coefficient RH and thermopower S) studies on LiV2O4 single crystals at low temperatures below 2K points to a semimetallic ground state with almost equally heavy electrons and holes, which we argue gives us a hint for the origin of CO instability in k-space.

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).

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