Itinerant type many-body theories for photo-induced structural phase transitions
Itinerant type many-body theories for photo-induced structural phase transitions
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DOI：
10.1088/0034-4885/67/9/r02
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发表时间：
2004-07
影响因子：
18.1
通讯作者：
K. Nasu
中科院分区：
文献类型：
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作者：
K. Nasu
结合与这种新光学现象相关的各种最新实验结果，对光致结构相变（PSPT）的巡回型量子多体理论进行了回顾。有两个关键概念：基态的隐藏多稳定性和光激发态的扩散。以有机电荷转移（CT）晶体TTF-CA中的离子（I）→中性（N）相变作为此类转变的典型例子，我们首先从理论上展示了一条从I相中的CT激子开始，最终到达宏观尺寸的N域的绝热路径。与这种I-N跃迁相关，还提出了初始条件敏感性的概念，以阐明实验观察到的该材料的非线性特性。接下来，我们使用更简化的多激子系统模型，从理论上研究了激子增殖的早期量子动力学，最终导致形成具有大量激子的域。为此，我们推导了一个逐步迭代方程来描述激子增殖，并阐明了初始条件敏感性的起源。结合最近关于有机金属复合晶体中光致相变的实验，还从一般和概念的角度阐明了高温下光致非平衡相和平衡相之间可能存在的差异。结果表明，光致相可以使新的相互作用仅在该相中表现为对称性破缺，即使这种相互作用几乎完全隐藏在所有平衡相中，例如基态和其他高温相。光致非平衡相和磁滞诱导非平衡相之间的关系也得到了定性讨论。我们将关注钙钛矿型量子电介质 SrTiO3 中光生电子引起的宏观宇称不守恒和铁电（或超对位电）。假设 Ti 3d 能带中的光生电子与软非简声 T1u 声子呈弱二次耦合，与呼吸 (A1g) 型高能声子呈强线性耦合。这两种类型的电子-声子耦合产生两种类型的极化子，一种是具有准全局宇称违例的超顺电（SPE）大极化子，另一种是仅具有局部宇称违例的偏心型自陷极化子。这种SPE大极化子相当于带电且导电的铁电域，极大地增强了准静态电敏感性和电子电导率。我们还简要回顾了最近通过使用X射线测量更直接地观察PSPT的成功。
Itinerant type quantum many-body theories for photo-induced structural phase transitions (PSPTs) are reviewed in close connection with various recent experimental results related to this new optical phenomenon. There are two key concepts: the hidden multi-stability of the ground state, and the proliferations of optically excited states. Taking the ionic (I) → neutral (N) phase transition in the organic charge transfer (CT) crystal, TTF–CA, as a typical example for this type of transition, we, at first, theoretically show an adiabatic path which starts from CT excitons in the I-phase, but finally reaches an N-domain with a macroscopic size. In connection with this I–N transition, the concept of the initial condition sensitivity is also developed so as to clarify experimentally observed nonlinear characteristics of this material.In the next, using a more simplified model for the many-exciton system, we theoretically study the early time quantum dynamics of the exciton proliferation, which finally results in the formation of a domain with a large number of excitons. For this purpose, we derive a stepwise iterative equation to describe the exciton proliferation, and clarify the origin of the initial condition sensitivity.Possible differences between a photo-induced nonequilibrium phase and an equilibrium phase at high temperatures are also clarified from general and conceptional points of view, in connection with recent experiments on the photo-induced phase transition in an organo-metallic complex crystal. It will be shown that the photo-induced phase can make a new interaction appear as a broken symmetry only in this phase, even when this interaction is almost completely hidden in all the equilibrium phases, such as the ground state and other high-temperature phases. The relation between the photo-induced nonequilibrium phase and the hysteresis induced nonequilibrium one is also qualitatively discussed.We will be concerned with a macroscopic parity violation and a ferro- (or super-para-) electricity, induced by a photogenerated electron in the perovskite type quantum dielectric SrTiO3. The photogenerated electron in the 3d band of Ti is assumed to couple weakly, but quadratically, with soft-anharmonic T1u phonons, and strongly but linearly to the breathing (A1g) type high energy phonons. These two types of electron–phonon coupling result in two types of polarons, a super-para-electric (SPE) large polaron with a quasi-global parity violation, and an off-centre type self-trapped polaron with only a local parity violation. This SPE large polaron, being equal to a charged and conductive ferroelectric domain, greatly enhances both the quasi-static electric susceptibility and the electronic conductivity.We also briefly review recent successes to observe the PSPTs more directly by using x-ray measurements.