Quantum Phenomena Emerging Near a Ferroelectric Critical Point in a Donor-Acceptor Organic Charge-Transfer Complex

When a second-order transition point is decreased to zero temperature, a continuous quantum phase transition between different ground states is realized at a quantum critical point (QCP). A recently synthesized organic charge-transfer complex, TTF-2,5-QBr(2)I(2), provides a platform for the exploration of the quantum phenomena that accompany a ferroelectric QCP. Here, we summarize the recent results showing the quantum phenomena associated with the ferroelectric QCP in TTF-2,5-QBr(2)I(2). Whereas the enhanced quantum fluctuations lead to quantitative changes in the critical exponents of the critical phenomena, they qualitatively change the nature of the domain-wall kinetics from thermally activated motion to temperature-independent tunneling motion. The present findings highlight the great influence of quantum fluctuations on the low-temperature physical properties and suggest that TTF-2,5-QBr(2)I(2) is a model system for the uniaxial ferroelectric QCP.