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arXiv · 2103.06043

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

Keishi Sunami·Tomohiro Baba·Kazuya Miyagawa·Sachio Horiuchi·Kazushi Kanoda
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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 $β$ 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)].

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Keishi Sunami, Tomohiro Baba, Kazuya Miyagawa, Sachio Horiuchi, Kazushi Kanoda. 2021-03-10. Enhanced lattice fluctuations prior to a nonmagnetic ferroelectric order in an ionic spin-chain system. https://doi.org/10.1103/physrevb.104.l081102

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