The intermolecular lattice vibrations in small-molecule organic semiconductors have a strong impact on their functional properties. Existing models treat the lattice vibrations within the harmonic approximation. In this work, we use polarization-orientation (PO) Raman measurements to monitor the temperature-evolution of the symmetry of lattice vibrations in anthracene and pentacene single crystals. Combined with first-principles calculations, we show that at 10 K the lattice dynamics of the crystals are indeed harmonic. However, as the temperature is increased specific lattice modes gradually lose their PO dependence and become more liquid-like. This finding is indicative of a dynamic symmetry breaking of the crystal structure and shows clear evidence of the strongly anharmonic nature of these vibrations. Pentacene also shows a subtle phase transition between 80-150 K, indicated by a change in the vibrational symmetry of one of the lattice modes. Our findings lay the groundwork for accurate predictions and new design rules for high-mobility organic semiconductors at room temperature.
Ultrafast vibrational dynamics of inorganic-organic hybrid lead halide perovskite CH3NH3PbI$ in the room-temperature tetragonal phase is investigated under moderate photoexcitation slightly below the band gap. Time-resolved optical Kerr effect signal exhibits periodic modulations due to the libration and twisting of the methylammonium molecule at 4 and 8 THz, in addition to the inorganic sublattice deformation at 1.2 THz. These oscillations are heavily damped, and their frequencies exhibit blueshifts with increasing pump density, contrary to the expectation from conventional laser heating. Our…
The effect of electronic entropy on temperature peculiarities of the frequency characteristics of two interacting anharmonic vibrational modes in $β-$Zr
We present a vibrational dynamical mean-field theory (VDMFT) of the dynamics of atoms in solids with anharmonic interactions. Like other flavors of DMFT, VDMFT maps the dynamics of a periodic anharmonic lattice of atoms onto those of a self-consistently defined impurity problem with local anharmonicity and coupling to a bath of harmonic oscillators. VDMFT is exact in the harmonic and molecular limits, nonperturbative, systematically improvable through its cluster extensions, usable with classical or quantum impurity solvers (depending on the importance of nuclear quantum effects), and can be c…
We present the investigations on atomic dynamics and Li+ diffusion in crystalline and amorphous Li2Si2O5 using quasielastic (QENS) and inelastic neutron scattering (INS) studies supplemented by ab-initio molecular dynamics simulations (AIMD). The QENS measurements in the amorphous phase of Li2Si2O5 show a narrow temperature window (700 < T < 775 K), exhibiting significant quasielastic broadening corresponding to the fast Li+ diffusion and relaxation of SiO4 units to the crystalline phase. Our INS measurements clearly show the presence of large phonon density of states (PDOS) at low energy (low…
The efficient and accurate calculation of how ionic quantum and thermal fluctuations impact the free energy of a crystal, its atomic structure, and phonon spectrum is one of the main challenges of solid state physics, especially when strong anharmonicy invalidates any perturbative approach. To tackle this problem, we present the implementation on a modular Python code of the stochastic self-consistent harmonic approximation method. This technique rigorously describes the full thermodyamics of crystals accounting for nuclear quantum and thermal anharmonic fluctuations. The approach requires the…
Anharmonic Effects in the Low-Frequency Vibrational Modes of Aspirin and Paracetamol Crystals
振动光谱的低频范围对晶格的集体振动敏感。在分子晶体中，识别不同多晶型物的结构具有决定性作用，此外，它对这些晶体之间振动自由能差的温度依赖性分量的大小也起着重要作用。在这项工作中，我们研究了不同晶型的柔性阿司匹林和扑热息痛晶体的振动拉曼光谱和振动态密度基于色散校正的密度泛函理论，密度泛函微扰理论，和$ab\,initio$分子动力学。我们研究了准谐晶格展开的影响，并比较了振动谱的谐理论和时间相关形式主义的结果。晶格膨胀强烈影响300 cm $^{-1}$以下的集体振动，但在较高的频率下，它的重要性明显降低，而热核运动在整个振动范围内可能是重要的。我们还观察到，包含或忽略多体范德华色散相互作用不会导致拉曼光谱或振动态密度的低频范围内的大的差异，提供的晶格常数是固定的。我们获得定量协议与实验室温度低于300厘米$^{-1}$拉曼光谱的所有多晶型研究。检查不同的振动之间的二维相关性，我们发现哪些模式显示出更大程度的非谐耦合到别人，提供了一个可能的路线，以评估在不同情况下的谐波自由能评估的准确性。
Water ice is a unique material presenting intriguing physical properties, like negative thermal expansion and anomalous volume isotope effect (VIE). They arise from the interplay between weak hydrogen bonds and nuclear quantum fluctuations, making theoretical calculations challenging. Here, we employ the stochastic self-consistent harmonic approximation (SSCHA) to investigate how thermal and quantum fluctuations affect the physical properties of ice XI ab initio. Regarding the anomalous VIE, our work reveals that quantum effects on hydrogen are so strong to be in a nonlinear regime: when progr…
Proper consideration of anharmonicity is important for the calculation of the thermal conductivity. However, how the anharmonicity influences the thermal conduction in amorphous materials is still an open question. In this work, we uncover the role of anharmonicity on the thermal conductivity of amorphous silica (a-SiO2) by comparing the thermal conductivity predicted from the harmonic theory and the anharmonic theory. Moreover, we explore the effect of anharmonicity-induced frequency shift on the prediction of the thermal conductivity. It is found that the thermal conductivity calculated by t…
Organic semiconductors exhibit properties of individual molecules and extended crystals simultaneously. The strongly bound excitons they host are typically described in the molecular limit, but excitons can delocalize over many molecules, raising the question of how important the extended crystalline nature is. Using accurate Green's function based methods for the electronic structure and non-perturbative finite difference methods for exciton-vibration coupling, we describe exciton interactions with molecular and crystal degrees of freedom concurrently. We find that the degree of exciton deloc…
Mode-selective vibrational control of charge transport in $π$-conjugated molecular materials
有机材料的柔软特性导致了分子核动力学和电子动力学之间的强耦合。这种耦合开辟了通过诱导分子振动运动来控制有机电子器件中的电荷传输的途径。然而，尽管令人鼓舞的理论预测，这种控制的实验实现仍然难以捉摸。在这里，我们实验证明，在一个模型的有机光电子器件的光电导可以控制的分子振动的选择性激发。利用超快红外激光源在并五苯/C60光敏电阻中产生振动运动的相干叠加，我们观察到在1500-1700 cm $^{-1}$区域中的某些模式的激发导致光电流增强。受激振动主要影响被捕获的载流子。这种效应取决于振动的性质，其模式特异性可以很好地描述为分子间电子耦合的振动调制。因此，振动控制为研究（生物）分子材料中的电子-声子耦合和电荷动力学提供了一种新的工具。
The vibrational modes in organic/inorganic layered perovskites are of fundamental importance for their optoelectronic properties. The hierarchical architecture of the Ruddlesden-Popper phase of these materials allows for distinct directionality of the vibrational modes withrespect to the main axes of the pseudocubic lattice in the octahedral plane. Here, we study the directionality of the fundamental phonon modes in single exfoliated Ruddlesden-Popper perovskite flakes with polarized Raman spectroscopy at ultralow-frequencies. A wealth of Raman bands is distinguished in the range from 15-150 c…
We consider the problem of heat transport by vibrational modes (conduction) between Langevin thermostats connected by a central device. The latter is anharmonic and can be subject to large temperature differences and thus be out of equilibrium. We develop a self-consistent Green's function formalism to describe high-temperature and non-equilibrium transport, and derive a formula for the heat current for up to quartic anharmonicity (4th-order in the potential energy). We show the importance of including quartic terms in the anharmonic potential in order to properly describe thermal expansion an…
It is textbookly regarded that phonons, i.e., an energy quantum of propagating lattice waves, are the main heat carriers in perfect crystals. As a result, in many crystals, e.g., bulk silicon, the temperature-dependent thermal conductivity shows the classical 1/T relationship because of the dominant Umklapp phonon-phonon scattering in the systems. However, the thermal conductivity of many crystalline metal-organic frameworks is very low and shows no, a weakly negative and even a weakly positive temperature dependence (glass-like thermal conductivity). It has been in debate whether the thermal …
It is a persistent problem in condensed matter physics that glasses exhibit vibrational and thermal properties that are markedly different from those of crystals. While recent works have advanced our understanding of vibrational excitations in glasses at the harmonic approximation limit, efforts in understanding finite-temperature anharmonic processes have been limited. It is well known that phonons in crystals couple through phonon-phonon interactions, an extremely efficient mechanism for anharmonic decay that is also important in glasses. Here, however, we show that an additional anharmonic …
