Enhanced lattice fluctuations prior to a nonmagnetic ferroelectric order in an ionic spin-chain system

TL;DR

This study reveals enhanced lattice fluctuations in TTF-BA, an ionic spin-chain system, prior to its nonmagnetic ferroelectric transition, highlighting the role of polar fluctuations and unconventional lattice dynamics.

Contribution

It provides microscopic evidence of lattice fluctuations preceding ferroelectric order in an ionic Mott insulator, linking spin, charge, and lattice dynamics with critical behavior.

Findings

Lattice dimerization captured by NQR line splitting with 3D Ising critical exponent

Peak in spin-lattice relaxation rate at transition temperature

Unconventional lattice fluctuations observed above transition temperature

Abstract

We investigated microscopic lattice states in the donor-acceptor ionic Mott insulator, TTF-BA, by $^{79}$Br-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 $\beta$ of 0.40, as expected in the 3D Ising universality class, and a peak formation in the spin-lattice relaxation rate $T_1^{-1}$ at the transition temperature, $T_\mathrm{c}$, of 53 K. Notably, $T_1^{-1}$ does not obey the conventional $T^2$ law expected for the Raman process of phonons even far above $T_\mathrm{c}$, 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)].