Quantum ferroelectricity in charge-transfer complex crystals

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 (QBr(4–n)I(n), n=0–4) to search for ferroelectric charge-transfer complexes with tetrathiafulvalene (TTF). Among them, TTF–QBr(2)I(2) 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–QCl(4) and quantum antiferroelectric of dimethyl–TTF–QBr(4). By contrast, TTF–QBr(3)I complex, another member of this compound family, shows complete suppression of the ferroelectric spin-Peierls-type phase transition.