Superconducting gap excitations, acoustic and optical phonons probed with femtosecond time-resolved and Raman spectroscopy.

摘要

Spontaneous Raman scattering and femtosecond time-resolved pump-probe spectroscopy are used to study and characterize material properties in superconductors, bulk semiconductors and semiconducting quantum dots.; Superconducting gap oscillations are observed for the first time in MgB 2 (Tc = 39 K) and 2H-NbSe2 ( Tc = 7 K). In MgB2, multiple superconducting gap oscillations are observed at 24 cm-1, 103 cm -1 and ∼44 cm-1. These frequencies are in reasonable agreement with other reported experimental results and published theoretical models. Spontaneous Raman measurements on MgB2 show a single strong resonance at 107 cm-1 near the superconducting gap. Consistent with spontaneous Raman measurement, coherent superconducting gap oscillations in 2H-NbSe2 are observed at 18 cm-1 (E symmetry) and 14 cm-1 ( A symmetry). The analysis suggests that a non-Raman mechanism or impurities are responsible for the observed oscillations in MgB2.; Spontaneous Raman spectra of MgB2 show a large frequency shift in the optical E2g phonon from 580 cm-1 at 300 K to 630 cm-1 at 3 K due to anharmonicity. The resonance at the superconducting gap frequency shows a strong asymmetric lineshape near Tc, becoming Lorentzian-like near 3 K due to spectral weight gain at low frequencies.; In ZnO, a study of the anharmonic properties of the low-frequency E2 optical phonon, using pump-probe spectroscopy, show an unusually long lifetime ∼2 orders of magnitude larger for most optical modes in semiconductors. At 5 K, the frequency and lifetime are (2.9787 +/- 0.0002) THz and (211 +/- 7) ps. The temperature dependence of the lifetime is determined by up-conversion decay contributions, which vanish at zero temperature. The results show that the lifetime is limited by isotopic disorder and that nanosecond lifetimes may be achievable in isotopically-pure samples.; In CdTe1-xSex quantum dots, two longitudinal optical phonons and a mixed mode are identified. The amplitude of the optical modes as a function of the laser wavelength agrees with the impulsive stimulated Raman scattering mechanism. At low power densities, the frequency of the optical phonons deviates from spontaneous Raman scattering behavior, which is attributed to impurity state trapping. Size-selective excitation effects for the acoustic phonons are more noticeable in time-domain studies. The difference between the time-domain and spontaneous Raman studies is not understood. Acoustic phonons are observed in CdSe quantum dots at 37 cm-1, 22 cm-1 and 5 cm-1 at 560 nm. The linewidth of the mode at 37 cm-1 shows a strong dependence on dot size and a linear dependence with temperature.