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An excitonic insulator (EI) is an intriguing correlated electronic phase of condensed excitons. Ta2NiSe5 is a model material for investigating condensed excitonic states. Herein, femtosecond pump–probe spectroscopy is used to study the coherent phonon dynamics and associated exciton–phonon coupling in single-crystal Ta2NiSe5. The reflectivity time series consists of exponential decay due to hot carriers and damped oscillations due to the Ag phonon vibration. Given the in-plane anisotropic thermal conductivity of Ta2NiSe5, coherent phonon oscillations are stronger with perpendicular polarization to its quasi-one-dimensional chains. The 1-, 2-, and 4-THz vibration modes show coherent amplitude responses in the EI phase of Ta2NiSe5 with increasing temperature, totally different from those of normal coherent phonons (the 3- and 3.7-THz modes). The amplitude modes at higher frequencies decouple with the EI order parameter at lower temperatures, as supported by theoretical analysis with a model Hamiltonian of the exciton–phonon coupling system. Our work provides valuable insights into the character of the EI order parameter and its coupling to multiple coherent amplitude modes. [ABSTRACT FROM AUTHOR]

Lattice dynamics plays a significant role in manipulating the unique physical properties of materials. In this work, femtosecond transient optical spectroscopy is used to investigate the generation mechanism and relaxation dynamics of coherent phonons in Fe1.14Te—a parent compound of chalcogenide superconductors. The reflectivity time series consist of the exponential decay component due to hot carriers and damped oscillations caused by the A1g phonon vibration. The vibrational frequency and dephasing time of the A1g phonons are obtained as a function of temperature. With increasing temperature, the phonon frequency decreases and can be well described with the anharmonicity model. Dephasing time is independent of temperature, indicating that the phonon dephasing is dominated by phonon-defect scattering. The impulsive stimulated Raman scattering mechanism is responsible for the coherent phonon generation. Owing to the resonance Raman effect, the maximum photosusceptibility of the A1g phonons occurs at 1.590 eV, corresponding to an electronic transition in Fe1.14Te. [ABSTRACT FROM AUTHOR]

Metal oxide vertically aligned nanocomposite thin films have been of considerable interest for hot-carrier-based light harvesting and active plasmonic applications. Herein, we present femtosecond pump–probe spectroscopy studies on a nanocomposite thin film with plasmonic Au nanopillars vertically aligned in a ferromagnetic La0.67Sr0.33MnO3 (LSMO) matrix. Our measurements reveal that the carrier decay of Au/LSMO films qualitatively follows that of pure LSMO with a two-component relaxation. In the presence of Au nanopillars, the charge generation within the LSMO film is enhanced by a factor of ∼40, and the spin–lattice relaxation time associated with demagnetization is substantially decreased from 259.6 to 7.7 ps. The enhanced nonequilibrium dynamics and energy transfer between the lattice and spin system in the Au/LSMO films can be well modeled using an extended three-temperature model. These Au/LSMO thin films can be potentially useful in ultrafast photoelectric and magnetooptical applications. [ABSTRACT FROM AUTHOR]

In the present study, we summarize indications for the existence of kinked post-perovskite structures in the MAS system. X-ray diffraction data and Raman spectra of aluminous magnesium metasilicate post-perovskite are inconsistent with the Cair[O.sub.3] structure. Instead the observations are consistent with structures intermediate between the perovskite and the Cair[O.sub.3] structure. Ab initio calculations show that the enthalpies of the kinked structures are slightly higher than the Calr[O.sub.3] structure at 0 K. Finite temperature, minor element chemistry, kinetics of phase transformation, and actual stress regime are plausible reasons for the observed differences between the present and the previously reported post-perovskite phases. Keywords: Crystal structure, post-perovskite, XRD data, quantum mechanical calculation, high-pressure studies, Raman spectroscopy