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.

Figure 1. Chemical and crystal structures of tetrathiafulvalene (TTF) complexes of tetrahalo-p-benzoquinones (QBr_4–nI_n).

(a) Chemical form of QBr_4–nI_n. (b-e) Molecular packings of the TTF–QBr_4–nI_n complexes. (b) Neutral (TTF)₂(QBr₂I₂) projected along the c direction. (c) Ionic 1:1 TTF–QBr₃I projected along the a direction. (d,e) Neutral 1:1 TTF–QBr₂I₂ projected along the crystallographic a and b directions. For clarity, the halogen positions of the QBr₂I₂ molecules are shown only for preferable occupations.

Figure 2. Temperature dependence of dielectric and magnetic properties of 1:1 TTF–QBr_4–nI_n complexes.

(a) The spin susceptibility χ_s for ionic TTF–QBr₄ and TTF–QBr₃I crystals. The inset shows the corresponding χ_sT–T plot. (b) The relative permittivity measured with an a.c. electric field applied along the crystallographic b axis parallel to the DA stack of ionic TTF–QBr₄ and TTF–QBr₃I crystals. (c) The relative permittivity measured with an a.c. electric field applied along the DA stack (parallel to the crystallographic b axis) for neutral 1:1 TTF–QBr₂I₂, TTF–QBrI₃ and TTF–QI₄ crystals at ambient pressure.

Figure 3. Temperature- and pressure-dependent properties of TTF–QBr₂I₂ crystal.

(a) Temperature dependence of the relative permittivity under various hydrostatic pressures. The applied pressure value, corrected considering its thermal change in the medium for each measurement, is represented by the value at the transition point or lowest temperature when the phase transition is absent. The inset depicts the data in the low-pressure range. (b) Temperature–pressure phase diagram; linear extrapolation of the phase boundary at high-temperature region points to 1.64 GPa at room temperature. The inset represents the quantum critical behaviour of ferroelectrics obeying the relation T_c∝(p–p_c)^1/2 in the low-critical temperature region. The open square represents the pressure at which the room temperature conductivity exhibited a sharp peak caused by the pressure-induced NIT. (c) Inverse relative permittivity as the function of the square of the temperature. The solid line represents the quantum critical behaviour ɛ_r^–1∝T².

Figure 4. High-pressure properties of TTF–QBr₂I₂ crystal at low temperatures.

(a) Hydrostatic pressure dependence of the relative permittivity at 5 K, ɛ_r (T=5 K) on a TTF–QBr₂I₂ crystal (filled squares) in comparison with a quantum (anti)ferroelectric DMTTF–QBr₄ crystal (filled circles, redrawn based on data in ref. 29). The inset shows the inverse permittivity obeying a simple power law, ɛ_r (T=5 K)∝|p–p_c|^–1, as expected for quantum ferroelectricity. (b) Electric polarization (P) versus electric field (E) hysteresis loops with a triangular a.c. electric field at T=4 K and frequency f=1 kHz.