Magnetoelastic couplings in the deformed kagome quantum spin lattice of volborthite

Microscopic spin interactions on a deformed kagome lattice of volborthite are investigated through magnetoelastic couplings. A negative longitudinal magnetostriction ΔL/L<0 in the b axis is observed, which depends on the magnetization M with a peculiar relation of ΔL/L�M1.3. Based on the exchange striction model, it is argued that the negative magnetostriction originates from a pantographlike lattice change of the Cu-O-Cu chain in the b axis, and that the peculiar dependence arises from the local spin correlation. This idea is supported by DFT+U calculations simulating the lattice change and a finite-size calculation of the spin correlation, indicating that the recently proposed coupled-trimer model is a plausible one.