Enhanced lattice fluctuations prior to a nonmagnetic ferroelectric order in an ionic spin-chain system
Abstract
We investigated microscopic lattice states in the donor-acceptor ionic Mott insulator, TTF-BA, by 79Br-NQR spectroscopy to explore cross-correlated fluctuations between spin, charge and lattice. A ferroelectric transition with lattice dimerization is captured by a NQR line splitting with the critical exponent β of 0.40, as expected in the 3D Ising universality class, and a peak formation in the spin-lattice relaxation rate T1-1 at the transition temperature, Tc, of 53 K. Notably, T1-1 does not obey the conventional T2 law expected for the Raman process of phonons even far above Tc, indicating the emergence of extraordinary lattice fluctuations. They are very probably associated with polar fluctuations in the paraelectric and paramagnetic phase of TTF-BA and explain the previous observation of the anomalously suppressed paramagnetic spin susceptibility, which was conjectured to be due to the local spin-singlet pairing prior to the nonmagnetic ferroelectric order [K. Sunami et al., Phys. Rev. Res. 2, 043333 (2020)].