A series of experiments have been conducted in order to evaluate the relative importance of several recent theories of vibrational dephasing in solids. The theories are discussed briefly, and are used to interpret the temperature dependence of the C–H and C–D stretch bands in the spontaneous Raman spectra of h14- and d14-1,2,4,5-tetramethyl benzene (durene). The infrared spectra of these same molecules are also reported in the region of the combination bands involving C–H (or C–D) stretches and low-frequency modes. The results support the applicability of the model of Harris et al., [C. B. Harris, R. M. Shelby and P. A. Cornelius, Phys. Rev. Lett. 38, 1415 (1977); Chem Phys. Lett. 57, 8 (1978); R. M. Shelby, C. B. Harris, and P. A. Cornelius, J. Chem. Phys. 70, 34 (1979)], based on energy exchange in anharmonically coupled low-frequency modes. This theory is then used, in connection with Raman spectra obtained in isotopically mixed samples of durene, to elucidate the vibrational dynamics underlying the dephasing. It is found that the results are consistent with the hypothesis that some low-frequency modes in this molecule are significantly delocalized or ’’excitonic’’ in character, and that this delocalization may be studied by means of Raman spectroscopy on the low-frequency modes themselves, as well as by exchange analysis of the coupled high-frequency modes. These conclusions represent a generalization and extension of the previously published exchange model [R. M. Shelby, C. B. Harris, and P. A. Cornelius, J. Chem Phys. 70, 34 (1979)].

The collective dynamics of topological structures [1][2][3][4][5][6] have been of great interest from both fundamental and applied perspectives. For example, the studies of dynamical properties of magnetic vortices and skyrmions 3,4 not only deepened the understanding of many-body physics but also led to potential applications in data processing and storage 7 . Topological structures constructed from electrical polarization rather than spin have recently been realized in ferroelectric superlattices 5,6 , promising for ultrafast electric-field control of topological orders. However, little is known about the dynamics of such complex extended nanostructures which in turn underlies their functionalities. Using terahertz-field excitation and femtosecond x-ray diffraction measurements, we observe ultrafast collective polarization dynamics that are unique to polar vortices, with orders of magnitude higher frequencies and smaller lateral size than those of experimentally realized magnetic vortices 3 . A previously unseen soft mode, hereafter referred to as a vortexon, emerges as transient arrays of nanoscale circular patterns of atomic displacements, which reverse their vorticity on picosecond time scales. Its frequency is significantly reduced at a critical strain, indicating a condensation of structural dynamics. First-principles-based atomistic calculations and phase-field modeling reveal the microscopic atomic arrangements and frequencies of the vortex modes. The discovery of subterahertz collective dynamics in polar vortices opens up opportunities for applications of electric-field driven data processing in topological structures with ultrahigh speed and density.Precisely engineered (PbTiO3)n/(SrTiO3)n oxide superlattices provide a controllable platform to host novel phenomena such as negative capacitance 8,9 and light-induced supercrystals 10 as well as unique polarization topologies including vortex 5 and skyrmion 6 structures. In comparison with their counterparts in magnetic systems 3,4,11 , the building elements of these

Optical excitation leads to ultrafast stress generation in the prototypical multiferroic BiFeO 3 . The time scales of stress generation are set by the dynamics of the population of excited electronic states and the coupling of the electronic configuration to the structure. X-ray free-electron laser diffraction reveals high-wavevector subpicosecond-time scale stress generation following ultraviolet excitation of a BiFeO 3 thin film. Stress generation includes a fast component with a 1/e rise time with an upper limit of 300 fs and longer-rise time components extending to 1.5 ps. The contributions of the fast and delayed components vary as a function of optical fluence, with a reduced a fast-component contribution at high fluence. The results provide insight into stress-generation mechanisms linked to the population of excited electrons and point to new directions in the application of nanoscale multiferroics and related ferroic complex oxides. The fast component of the stress indicates that structural parameters and properties of ferroelectric thin film materials can be optically modulated with 3 dB bandwidths of at least 0.5 THz.

A pillared layer network containing amide functional groups (Cu­(pzdc)­(pia); pzdc = pyrazine-2,3-dicarboxylate; pia = N-(4-pyridyl)­isonicotinamide) was used to test a postsynthesis metalation rationale to insert lithium and create a porous surface with enhanced CO2 adsorption capacity. Synchrotron powder X-ray diffraction (XRD) was used to determine variations after lithiation in long-range and textural properties. CO2 adsorption measurements at room temperature showed a concave up isotherm shape with an increasing adsorption at high pressures, surpassing by 1 order of magnitude the values previously reported for the unmodified material. There was significant hysteresis upon desorption, which suggests structural variations consequent to different or stronger adsorption sites. Results from elemental, thermal gravimetric, and crystal refinement analyses indicate that the lithium content is ca. 3 Li atoms per asymmetric unit. Raman scattering showed N–Li and Li–O stretching bands, a shift of pia amide- and pyridyl-related bands, and other significant skeletal vibrations associated with nitrogen and oxygen lone pair variations. In situ XRD and CO2 adsorption observations at up to 50 bar at ambient temperature were consistent with the anticipated structural dynamic variation. The lattice changes observed at pressures below 10 bar following lithiation may be directly related to an enhancement in the CO2 adsorption amount.