Dynamical Quantum Properties of Phonons in Crystals

本论文的目的是对晶体中声子的动力学量子特性进行实验和理论相结合的研究。柏林的联合实验活动（Michael Woerner）和卢森堡的理论计算（Alexandre Tkatchenko）旨在了解晶体物质中超快原子运动和电子电荷重新分配过程的基本微观机制，声子的量子特性在其中发挥着重要作用。实验和理论将集中在声子的相干时间依赖过程。计划活动的一部分侧重于利用相干n阶拉曼激发和相干太赫兹发射相结合，对晶体中多声子模式的密度矩阵进行相干操纵和完全相位分辨探测，从而可以激发和探测多声子模式密度矩阵中的“非经典”状态。这项计划的另一个科学目标是详细的时空表征（原子长度和时间分辨率）的铁电体和分子晶体中的软模声子。与晶体中的“常规”光学声子相比，软模具有核运动和电子电荷移位的明显混合特征。多维太赫兹光谱和飞秒x射线衍射是该项目的主要实验技术。由于电子对软模电流的贡献比原子核的贡献大几个数量级，在软模的理论描述中，玻恩-奥本海默近似不再是一个很好的近似，因为原子核动能的期望值包含了重要的电子间势能表面项。该方案的理论部分以货车德瓦耳斯包含密度泛函理论（DFT）为基础，从理论上研究声子晶体的“非玻恩-奥本海默”效应。

The present proposal aims at a combined experimental and theoretical study of the dynamical quantum properties of phonons in crystals. The joint experimental activities in Berlin (Michael Woerner) and theoretical calculations in Luxembourg (Alexandre Tkatchenko) aim at understanding fundamental microscopic mechanisms which underlie ultrafast atomic motions and electronic charge redistribution processes in crystalline matter for which quantum properties of phonons play a significant role. Experiments and theory will focus on coherent time-dependent processes of phonons. One part of the planned activities focuses on the coherent manipulation and fully phase-resolved detection of the density matrix of multi-phonon modes in crystals using a combination of coherent n-th-order Raman excitations and coherent THz emission which allows for exciting and probing "non-classical" states in the density matrix of multi-phonon modes. Another scientific goal of this proposal is the detailed spatio-temporal characterization (with atomic length and time resolution) of the soft mode phonons in ferroelectrics and molecular crystals. In contrast to "conventional" optical phonons in crystals soft modes have a pronounced hybrid character of nuclear motions and electronic charge relocations. Multi-dimensional terahertz spectroscopy and femtosecond x-ray diffraction are the main experimental techniques of the project. Since the electronic contribution to soft-mode currents is by orders of magnitudes larger than that of the nuclei, the Born Oppenheimer approximation in a theoretical description of soft modes is no longer a good approximation because the expectation value of the kinetic energy of the nuclei contains significant inter-electronic potential surface terms. The theoretical part of this proposal which starts with a well-established background based on van der Waals inclusive density functional theory (DFT) will be explored and extended into a direction to study theoretically such "non-Born-Oppenheimer" effects of phonons crystals.