Quantum electric dipole glass and frustrated magnetism near a critical point in Li₂ZrCuO₄

We report a new peculiar effect of the interaction between a sublattice of frustrated quantum spin-1/2 chains and a sublattice of pseudospin-1/2 centers (quantum electric dipoles) uniquely co-existing in the complex oxide γ-Li₂ZrCuO₄ (≡Li₂CuZrO₄). ⁷Li nuclear magnetic resonance, Cu²⁺ electron spin resonance and a complex dielectric constant data reveal that the sublattice of Li⁺-derived electric dipoles orders glass like at T_g≃70 K yielding a spin site nonequivalency in the CuO₂ chains. We suggest that such a remarkable interplay between electrical and spin degrees of freedom might strongly influence the properties of the spiral spin state in Li₂ZrCuO₄ that is close to a quantum ferromagnetic critical point. In particular that strong quantum fluctuations and/or the glassy behavior of electric dipoles might renormalize the exchange integrals affecting this way the pitch angle of the spiral as well as be responsible for the missing multiferroicity present in other helicoidal magnets.