Search

We investigate carrier and collective mode dynamics in 2H-NbSe$_2$ using time-resolved optical pump-probe spectroscopy and compare the results with first-principle calculations. Broadband ultrafast reflectivity studies of 2H-NbSe$_2$ in a wide temperature interval covering the normal, charge density wave (CDW) and superconducting phase were performed. Spectral features observed in the transient reflectivity experiment were associated with specific optical transitions obtained from band structure calculations. Displacive excitation of coherent phonons showed CDW-associated coherent oscillations of the soft phonon mode across the whole spectral range. Temperature evolution of this coherent phonon mode in the low-excitation linear regime shows softening of the mode down to the CDW transition temperature T$_{CDW}$ with subsequent hardening below T$_{CDW}$. The global fit of the broadband probe data reveals four different relaxation times associated with characteristic electron-electron, electron-phonon and phonon-phonon relaxation processes. From first principle calculations of electron-phonon coupling we associate the few picosecond electron-phonon relaxation time $\tau_2$ with a specific group of phonons with frequencies around 20 meV. On the other hand, the anomalously long relaxation time of $\tau_3$~100 ps is associated with anharmonicity-driven phonon-phonon scattering. All relaxation processes result from anomalies near the second order CDW phase transition that are reflected in the temperature dependencies of the characteristic relaxation times and amplitudes of optical densities. At highest fluences we observe electronic melting of the CDW and disappearance of the mode hardening below T$_{CDW}$., Comment: 10 pages, 7 figures

We report on a transient optical reflectivity study of the charge density wave (CDW) in CuxTiSe2 single crystals. Our measurements reveal that the system undergoes a quantum phase transition at x=0.04 from a strong commensurate CDW (x0.04). We find that the cooperative driving mechanisms for the commensurate CDW, the excitonic insulator mechanism and the soft L$_1$$^-$ phonon mode, decouple at x=0.04 with the observed fluctuations in the folded Se-4p band dominating the transition. We also demonstrate a loss of coherence in the A$_{1g}$ phonon with increased copper intercalation of the parent lattice, indicating a loss of long-range lattice order. These findings provide compelling evidence that TiSe2 undergoes a quantum phase transition upon copper intercalation from a state of commensurate charge order to a state with a different symmetry in which the new charge order coexists with the superconducting phase.

We demonstrate for the first time that impact ionization (II) [the inverse of Auger recombination (AR)] occurs with very high efficiency in semiconductor nanocrystals (NCs). Interband optical excitation of PbSe NCs at low pump intensities, for which less than one exciton is initially generated per NC on average, results in the formation of two or more excitons (carrier multiplication) when pump photon energies are more than three times the NC band gap energy. Generation of multiexcitons from a single photon absorption event is observed to take place on an ultrafast (picosecond) timescale and occurs with up to 100% efficiency depending upon the excess energy of the absorbed photon. Efficient II in NCs can be used to considerably increase the power conversion efficiency of NC-based solar cells., Comment: 16 pages total, 4 figures. This manuscipt has been accepted for publication in Physical Review Letters