Nonequilibrium carrier dynamics in single exfoliated graphene layers on muscovite substrates are studied by ultrafast optical pump-probe spectroscopy and compared with microscopic theory. The very high 10-fs-time resolution allows for mapping the ultrafast carrier equilibration into a quasi-Fermi distribution and the subsequent slower relaxation stages. Coulomb-mediated carrier-carrier and carrier-optical phonon scattering are essential for forming hot separate Fermi distributions of electrons and holes which cool by intraband optical phonon emission. Carrier cooling and recombination are influenced by hot phonon effects.

Using density-matrix theory, we microscopically calculate the relaxation dynamics of photoexcited electrons in graphene. Electron-phonon coupling leads to an initially ultrafast energy dissipation and to a nonthermal phonon occupation of the highest optical phonon modes. We also calculate the temporal evolution of the electronic temperature and find good agreement with recent experimental work.

We study the Anderson model of localization with anisotropic hopping in three dimensions for weakly coupled chains and weakly coupled planes. The eigenstates of the Hamiltonian, as computed by Lanczos diagonalization for systems of sizes up to 48 3 , show multifractal behavior at the metal-insulator transition even for strong anisotropy. The critical disorder strength W c determined from the system size dependence of the singularity spectra is in a reasonable agreement with a recent study using transfer matrix methods.But the respective spectrum at W c deviates from the "characteristic spectrum" determined for the isotropic system. This indicates a quantitative difference of the multifractal properties of states of the anisotropic as compared to the isotropic system. Further, we calculate the Kubo conductivity for given anisotropies by exact diagonalization. Already for small system sizes of only 12 3 sites we observe a rapidly decreasing conductivity in the directions with reduced hopping if the coupling becomes weaker. 71.30.+h, 72.15.Rn

We present a medium-dependent quantum optics approach to describe the influence of electronacoustic phonon coupling on the emission spectra of a strongly coupled quantum-dot cavity system. Using a canonical Hamiltonian for light quantization and a photon Green function formalism, phonons are included to all orders through the dot polarizability function obtained within the independent Boson model. We derive simple user-friendly analytical expressions for the linear quantum light spectrum, including the influence from both exciton and cavity-emission decay channels. In the regime of semiconductor cavity-QED, we study cavity emission for various exciton-cavity detunings and demonstrate rich spectral asymmetries as well as cavity-mode suppression and enhancement effects. Our technique is nonperturbative, and non-Markovian, and can be applied to study photon emission from a wide range of semiconductor quantum dot structures, including waveguides and coupled cavity arrays. We compare our theory directly to recent and apparently puzzling experimental data for a single site-controlled quantum dot in a photonic crystal cavity and show good agreement as a function of cavity-dot detuning and as a function of temperature.

We study the influence of non-Markovian electron-acoustic-phonon scattering on the vacuum Rabi splitting of a semiconductor single-quantum dot and a planar photonic crystal nanocavity. The regimes of strong coupling and side-coupled light transmission are explored as a function of temperature. At elevated temperatures, when the quantum dot has left the strong-coupling regime, a spectral splitting continues to be observed in transmission. The effects of non-Markovian scattering are shown to significantly vary the characteristic transmissivity compared to purely model Lorentzian line shapes for the electron-phonon interaction.