Quantum ferroelectricity in charge-transfer complex crystals.

Sachio Horiuchi,Fumitaka Kagawa,Kensuke Kobayashi,Reiji Kumai,Yoshinori Tokura,Nao Minami

doi:10.1038/ncomms8469

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.

Highlights

Quantum phase transition achieved by fine tuning the continuous phase transition down to zero kelvin is a challenge for solid state science
A quantum critical behaviour has been found in the neutral–ionic phase transition (NIT) system by pressurizing DMTTF–QBr4 crystals, the inter-stack antiferroelectric coupling, which leads to the antiferroelectrically ordered state, is responsible for the much smaller permittivity (B180) at the quantum critical point (QCP)
The TTF–QCl4 complex was partially replaced with tetraselenafulvalene (TSF), which has a larger molecular volume, to reduce the ferroelectric NIT temperature[32]

Summary

Introduction

Quantum phase transition achieved by fine tuning the continuous phase transition down to zero kelvin is a challenge for solid state science. The antiferromagnetically coupled S 1⁄4 1/2 spins residing on D þ and A– radical ions turn to spin-singlet D þ A– pairs on cooling, changing the chains from paramagnetic to nonmangetic This ferroelectricity, has a magnetic origin corresponding to the spin-Peierls (SP)-type phase transition that is manifested by the magnetoelectric phenomena such as magnetic control of electric polarization[16]. For both NIT and SP systems, one challenging issue is the quantum phase transition[17] achieved by fine tuning the continuous phase transition down to zero kelvin. A quantum critical behaviour has been found in the NIT system by pressurizing DMTTF–QBr4 crystals, the inter-stack antiferroelectric coupling, which leads to the antiferroelectrically ordered state, is responsible for the much smaller permittivity (B180) at the QCP

Methods

Results

Conclusion