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We report investigations on the elasticity of superconducting FeSe using picosecond ultrasonic technique. The tetragonal (001) FeSe thin film, deposited on a processed SrTiO3 substrate by the pulsed laser deposition, exhibits distinct c-axis preferred orientation and single-crystalline features as a result of the x-ray diffraction. The high-quality crystallinity thus enables quantitative examinations of anisotropic stiffness coefficients (C33) of FeSe, correlating to the interatomic interaction in the simplest iron-based superconductor. Our experiment indicates a room-temperature C33 of 40.9 ± 0.4 GPa and material stiffening of 4.3% at low temperatures, which can be explained by the weakening of anharmonic phonon-phonon interactions. [ABSTRACT FROM AUTHOR]

Ultrafast carrier thermalization in n-type indium nitride (InN) with an electron concentration of 3.8×1018 cm-3 was investigated by femtosecond transient transmission measurements at room temperature with different wavelengths. An extremely fast carrier external thermalization time on the order of 400 fs was observed, which is much faster than all previous reports. This observed femtosecond thermalization time is consistent with a prediction based on a Coulomb screening effect. Through wavelength dependent and power dependent studies, even with a 400 fs thermalization time, we did not observe any evidence of the existence of the hot phonon effect, which agrees with a recent report that a longitudinal optical phonon lifetime could be shorter than 300 fs in specific InN samples. [ABSTRACT FROM AUTHOR]

The authors demonstrated that the piezoelectric superlattice, can serve as narrow-band detectors for propagating coherent longitudinal acoustic phonons at multiple frequencies corresponding to the spatial frequency of the superlattice and its higher harmonics, and its detection bandwidth is determined by the total structure width. By optically exciting a broadband propagating longitudinal acoustic pulse from a thin Ni film, the authors studied the acoustic spectral sensitivity function of a ten-period In0.12Ga0.88N/GaN multiple quantum well. Because the barriers (19 nm) and wells (3.6 nm) are of different widths, the second detection band, corresponding to the second harmonic of the fundamental frequency, can be resolved. [ABSTRACT FROM AUTHOR]

The authors demonstrate the generation of coherent acoustic phonons in (111)-strained InGaAs/GaAs multiple quantum wells through piezoelectric coupling by an optical pump-probe technique. Dependence of the induced strain amplitude on the photocarrier density reveals that coherent acoustic phonons can be efficiently excited by the piezoelectric effect with an unscreened built-in electric field. Under high fluence excitation, the generation of coherent acoustic phonons saturates. The experimental results are supported by a macroscopic theoretical model. No backward Raman signal and no zone-folded phonon modes were observed in this low impedance-mismatch and finite-period heterostructure. [ABSTRACT FROM AUTHOR]

Carrier thermalization dynamics in heavily doped n-type InN was investigated at room temperature with a femtosecond transient transmission measurement. The dependence of hot carrier cooling time on the total electron density indicates that the plasma screening of electron-LO phonon interactions is responsible for the reduction in energy-loss rate. Under low excitation, the carriers with different excess energies can be found to cool down with a fixed thermalization time of 1.4 ps. Intensity dependent study suggests that this relatively slow carrier cooling time could be attributed to the screening effect by high-background-doping plasma rather than the hot phonon effect. [ABSTRACT FROM AUTHOR]