title: Quantum ferroelectricity in charge-transfer complex crystals.
abstract: Quantum phase transition achieved by fine tuning the continuous phase transition down to zero kelvin is a challenge for solid state science. Critical phenomena distinct from the effects of thermal fluctuations can materialize when the electronic, structural or magnetic long-range order is perturbed by quantum fluctuations between degenerate ground states. Here we have developed chemically pure tetrahalo-p-benzoquinones of n iodine and 4-n bromine substituents (QBr4-nIn, n=0-4) to search for ferroelectric charge-transfer complexes with tetrathiafulvalene (TTF). Among them, TTF-QBr2I2 exhibits a ferroelectric neutral-ionic phase transition, which is continuously controlled over a wide temperature range from near-zero kelvin to room temperature under hydrostatic pressure. Quantum critical behaviour is accompanied by a much larger permittivity than those of other neutral-ionic transition compounds, such as well-known ferroelectric complex of TTF-QCl4 and quantum antiferroelectric of dimethyl-TTF-QBr4. By contrast, TTF-QBr3I complex, another member of this compound family, shows complete suppression of the ferroelectric spin-Peierls-type phase transition.
authors.affiliation: 1] National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8562, Japan [2] CREST, Japan Science and Technology Agency (JST), Tokyo 102-0076, Japan.
authors.forename: Sachio
authors.initials: S
authors.lastname: Horiuchi
authors.affiliation: Condensed Matter Research Center (CMRC) and Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan.
authors.forename: Kensuke
authors.initials: K
authors.lastname: Kobayashi
authors.affiliation: 1] CREST, Japan Science and Technology Agency (JST), Tokyo 102-0076, Japan [2] Condensed Matter Research Center (CMRC) and Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan.
authors.forename: Reiji
authors.initials: R
authors.lastname: Kumai
authors.affiliation: Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan.
authors.forename: Nao
authors.initials: N
authors.lastname: Minami
authors.affiliation: 1] CREST, Japan Science and Technology Agency (JST), Tokyo 102-0076, Japan [2] RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan.
authors.forename: Fumitaka
authors.initials: F
authors.lastname: Kagawa
authors.affiliation: 1] Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan [2] RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan.
authors.forename: Yoshinori
authors.initials: Y
authors.lastname: Tokura