Initial Conditions of Planet Formation: Lifetimes of Primordial Disks

The statistical properties of circumstellar disks around young stars are important for constraining theoretical models for the formation and early evolution of planetary systems. In this brief review, I survey the literature related to ground-based and Spitzer-based infrared (IR) studies of young stellar clusters, with particular emphasis on tracing the evolution of primordial (``protoplanetary'') disks through spectroscopic and photometric diagnostics. The available data demonstrate that the fraction of young stars with optically thick primordial disks and/or those which show spectroscopic evidence for accretion appears to approximately follow an exponential decay with characteristic time ~2.5 Myr (half-life = 1.7 Myr). Large IR surveys of ~2-5 Myr-old stellar samples show that there is real cluster-by-cluster scatter in the observed disk fractions as a function of age. Recent Spitzer surveys have found convincing evidence that disk evolution varies by stellar mass and environment (binarity, proximity to massive stars, and cluster density). Perhaps most significantly for understanding the planeticity of stars, the disk fraction decay timescale appears to vary by stellar mass, ranging from ~1 Myr for >1.3 Msun stars to ~3 Myr for <0.08 Msun brown dwarfs. The exponential decay function may provide a useful empirical formalism for estimating very rough ages for YSO populations and for modeling the effects of disk-locking on the angular momentum of young stars.Comment: 8 pages, 1 figure, invited review, Proceedings of the 2nd Subaru International Conference "Exoplanets and Disks: Their Formation and Diversity", Keauhou - Hawaii - USA, 9-12 March 200

Exciton dissociation at donor-acceptor polymer heterojunctions: quantum nonadiabatic dynamics and effective-mode analysis

The quantum-dynamical mechanism of photoinduced subpicosecond exciton dissociation and the concomitant formation of a charge-separated state at a TFB:F8BT polymer heterojunction is elucidated. The analysis is based upon a two-state vibronic coupling Hamiltonian including an explicit 24-mode representation of a phonon bath comprising high-frequency (C$=$C stretch) and low-frequency (torsional) modes. The initial relaxation behavior is characterized by coherent oscillations, along with the decay through an extended nonadiabatic coupling region. This region is located in the vicinity of a conical intersection hypersurface. A central ingredient of the analysis is a novel effective mode representation, which highlights the role of the low-frequency modes in the nonadiabatic dynamics. Quantum dynamical simulations were carried out using the multiconfiguration time-dependent Hartree (MCTDH) method

Phonon-driven ultrafast exciton dissociation at donor-acceptor polymer heterojunctions

A quantum-dynamical analysis of phonon-driven exciton dissociation at polymer heterojunctions is presented, using a hierarchical electron-phonon model parameterized for three electronic states and 24 vibrational modes. Two interfering decay pathways are identified: a direct charge separation, and an indirect pathway via an intermediate bridge state. Both pathways depend critically on the dynamical interplay of high-frequency C=C stretch modes and low-frequency ring-torsional modes. The ultrafast, highly non-equilibrium dynamics is consistent with time-resolved spectroscopic observations

Magnetic field effects on two-dimensional Kagome lattices

Magnetic field effects on single-particle energy bands (Hofstadter butterfly), Hall conductance, flat-band ferromagnetism, and magnetoresistance of two-dimensional Kagome lattices are studied. The flat-band ferromagnetism is shown to be broken as the flat-band has finite dispersion in the magnetic field. A metal-insulator transition induced by the magnetic field (giant negative magnetoresistance) is predicted. In the half-filled flat band, the ferromagnetic-paramagnetic transition and the metal-insulator one occur simultaneously at a magnetic field for strongly interacting electrons. All of the important magnetic fields effects should be observable in mesoscopic systems such as quantum dot superlattices.Comment: 10 pages, 4 figures, and 1 tabl

Topological Phases in Graphitic Cones

The electronic structure of graphitic cones exhibits distinctive topological features associated with the apical disclinations. Aharonov-Bohm magnetoconductance oscillations (period Phi_0) are completely absent in rings fabricated from cones with a single pentagonal disclination. Close to the apex, the local density of states changes qualitatively, either developing a cusp which drops to zero at the Fermi energy, or forming a region of nonzero density across the Fermi energy, a local metalization of graphene.Comment: 4 pages, RevTeX 4, 3 PostScript figure

Quantum Phonon Optics: Coherent and Squeezed Atomic Displacements

In this paper we investigate coherent and squeezed quantum states of phonons. The latter allow the possibility of modulating the quantum fluctuations of atomic displacements below the zero-point quantum noise level of coherent states. The expectation values and quantum fluctuations of both the atomic displacement and the lattice amplitude operators are calculated in these states---in some cases analytically. We also study the possibility of squeezing quantum noise in the atomic displacement using a polariton-based approach.Comment: 6 pages, RevTe

Influence of the scattering potential model on low energy electron diffraction from Cu(001)−c(2 × 2)-Pb

A dynamical LEED intensity analysis is reported for Cu(001)−c(2 × 2)-Pb. The adsorbate layer distance from the substrate is determined as 2.29 Å, and the topmost interlayer spacing for the substrate is the same as in bulk Cu, in contrast to a contraction for clean Cu(001). This structural result is, within the accuracy reached, insensitive to changes in the assumed scattering potential models. The r-factors suggest a weak preference for an energy-dependent exchange correlation and a moderate one for adding a localized adsorption part inside the muffin-tin spheres. The sensitivity of spectra and r-factors to changes in the assumed isotropic Debye temperature for Pb suggests that vibrational anisotropy should be taken into account in order to improve the accuracy of the analysis. Calculated spin polarization spectra are very sensitive to the exchange approximation, the localized absorption and the Debye temperature. Together with experimental data, they should be useful in particular for determining the vibrational anisotropy