Photoinduced phase transition in tetrathiafulvalene-p-chloranil observed in femtosecond reflection spectroscopy
Photoinduced phase transition in tetrathiafulvalene-p-chloranil observed in femtosecond reflection spectroscopy
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DOI：
10.1103/physrevb.70.165202
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发表时间：
2004-10-01
影响因子：
3.7
通讯作者：
Tokura, Y
中科院分区：
文献类型：
--
作者：
Okamoto, H;Ishige, Y;Tokura, Y
用飞秒泵浦-探测反射光谱研究了有机电荷转移(CT)络合物四硫富瓦烯对氯苯醚(TTF-CA)中离子相(I)到中性相(N)和中性相(N)到离子相(I)的光致跃迁。测量了TTF的供体分子和CA的受体分子之间的分子内跃迁带随激发能量、激发密度和温度的变化。采用多层模型对获得的瞬时反射率谱进行了分析，得到了I(N)相中光生N(I)态量的时间特性和空间分布。结果表明，CT能带在4K的共振激发引起的I-N(IN)跃迁由三个过程组成：(1)在光激发后形成一个受限的一维(1D)N区，即D(0)A(0)对序列；(2)在激发光的吸收深度内，将一维N区倍增到高达20ps的半宏观N态；(3)沿垂直于样品表面的方向进行IN跃迁。在NI转变温度附近的77K(T-c=81K)，初始产生的一维N磁畴的尺寸增大，其倍增过程强烈增强。随着激发能的增加，初始光产物从受限的一维N域转变为带正负电荷的N态。后者的空间尺寸比前者大得多，表明电荷载流子的引入使邻近的I态具有很强的不稳定性。还研究了光致N到I(NI)跃迁的动力学。1D I磁区最初是由光产生的，然而，即使I磁区的密度增加，它们也会在20ps内衰减。结果表明，光致In和NI跃迁的动力学特性有明显的区别。在这些光致跃迁中，在光致反射率变化上检测到了三种周期分别为0.6、50和85ps的相干振荡，它们合理地归结为与自旋-Peierls不稳定性有关的分子的动态二聚体位移、由光致跃迁引起的突然体积变化引起的冲击波以及Ni界的振荡。在此基础上，详细讨论了光致In和NI跃迁的动力学性质。
Photoinduced transitions from ionic (I) to neutral (N) and neutral (N) to ionic (I) phases in an organic charge transfer (CT) complex, tetrathiafulvalene-p-chloranil (TTF-CA), were investigated by femtosecond pump-probe reflection spectroscopy. Transient reflectivity changes of the intramolecular transition band of TTF sensitive to the degree of CT between a donor molecule of TTF and an acceptor molecule of CA are measured as a function of excitation energy, excitation density, and temperature. By adopting the multilayer model for the analysis of the obtained transient reflectivity spectra, we have derived the time characteristics of amounts and spatial distributions of photoinduced N (I) states in the I (N) phase. The results reveal that the I to N (IN) transition induced by the resonant excitation of the CT band at 4 K is composed of three processes; (1) formation of a confined one-dimensional (1D) N domain, that is, a sequence of D(0)A(0) pairs, just after the photoexcitation, (2) multiplication of the 1D N domains to the semimacroscopic N states up to 20 ps within the absorption depth of the excitation light, and (3) proceeding of the IN transition along the direction normal to the sample surface. At 77 K near the NI transition temperature (T-c=81 K), the size of the 1D N domain initially produced is enlarged and its multiplication process is strongly enhanced. When the excitation energy is increased, the initial photoproduct is changed from the confined 1D N domain to the positively and negatively charged N states. The spatial size of the latter is considerably larger than that of the former, indicating that the introduction of charge carriers makes the neighboring I state strongly unstable. The dynamics of the photoinduced N to I (NI) transition has also been investigated. The 1D I domains are initially produced by lights, however, they decay within 20 ps even if the density of the I domains is increased. The results demonstrate that there is a clear difference of the dynamics between the photoinduced IN and NI transitions. In these photoinduced transitions, three kinds of coherent oscillations with the period of similar to0.6, similar to50, and similar to85 ps have been detected on the photoinduced reflectivity changes, which are reasonably assigned to the dynamical dimeric displacements of molecules associated with the spin-Peierls instability, the shock wave driven by the sudden volume change due to the photoinduced transitions, and the oscillation of the NI domain boundary. On the basis of the results, dynamical aspects of the photoinduced IN and NI transitions have been discussed in detail.