Yamakawa H, Miyamoto T, Morimoto T, Takamura N, Liang S, Yoshimochi H, Terashige T, Kida N, Suda M, Yamamoto H M, Mori H, Miyagawa K, Kanoda K, Okamoto H
Department of Advanced Materials Science, University of Tokyo, Chiba, 277-8561, Japan.
Department of Applied Physics, University of Tokyo, Bunkyo-Ku, 113-8656, Japan.
Nat Commun. 2021 Feb 11;12(1):953. doi: 10.1038/s41467-021-20925-x.
Ultrafast electronic-phase change in solids by light, called photoinduced phase transition, is a central issue in the field of non-equilibrium quantum physics, which has been developed very recently. In most of those phenomena, charge or spin orders in an original phase are melted by photocarrier generations, while an ordered state is usually difficult to be created from a non-ordered state by a photoexcitation. Here, we demonstrate that a strong terahertz electric-field pulse changes a Mott insulator of an organic molecular compound in κ-(ET)Cu[N(CN)]Cl (ET = bis(ethylenedithio)tetrathiafulvalene), to a macroscopically polarized charge-order state; herein, electronic ferroelectricity is induced by the collective intermolecular charge transfers in each dimer. In contrast, in an isostructural compound, κ-(ET)Cu(CN), which shows the spin-liquid state at low temperatures, a similar polar charge order is not stabilized by the same terahertz pulse. From the comparative studies of terahertz-field-induced second-harmonic-generation and reflectivity changes in the two compounds, we suggest the possibility that a coupling of charge and spin degrees of freedom would play important roles in the stabilization of polar charge order.
光在固体中引起的超快电子相变，即光致相变，是最近才发展起来的非平衡量子物理领域的核心问题。在大多数这类现象中，初始相中的电荷或自旋序会因光生载流子的产生而被破坏，而通过光激发通常很难从无序态产生有序态。在此，我们证明强太赫兹电场脉冲能将κ-(ET)Cu[N(CN)]Cl（ET = 双(乙撑二硫)四硫富瓦烯）有机分子化合物的莫特绝缘体转变为宏观极化的电荷序状态；在此，电子铁电性是由每个二聚体中分子间的集体电荷转移诱导产生的。相比之下，在一种低温下呈现自旋液体状态的同结构化合物κ-(ET)Cu(CN)中，相同的太赫兹脉冲并不能稳定类似的极性电荷序。通过对这两种化合物中太赫兹场诱导的二次谐波产生和反射率变化的对比研究，我们提出电荷和自旋自由度的耦合可能在极性电荷序的稳定中起重要作用。
