Quasi-equilibrium optical nonlinearities in spin-polarized GaAs

Semiconductor Bloch equations, which microscopically describe the dynamics of a Coulomb interacting, spin-unpolarized electron-hole plasma, can be solved in two limits: the coherent and the quasi-equilibrium regime. These equations have been recently extended to include the spin degree of freedom, and used to explain spin dynamics in the coherent regime. In the quasi-equilibrium limit, one solves the Bethe-Salpeter equation in a two-band model to describe how optical absorption is affected by Coulomb interactions within a spin-unpolarized plasma of arbitrary density. In this work, we modified the solution of the Bethe-Salpeter equation to include spin-polarization and light holes in a three-band model, which allowed us to account for spin-polarized versions of many-body effects in absorption. The calculated absorption reproduced the spin-dependent, density-dependent and spectral trends observed in bulk GaAs at room temperature, in a recent pump-probe experiment with circularly polarized light. Hence our results may be useful in the microscopic modelling of density-dependent optical nonlinearities in spin-polarized semiconductors.Comment: 7 pages, 6 figure

Understanding highly excited states via parametric variations

Highly excited vibrational states of an isolated molecule encode the vibrational energy flow pathways in the molecule. Recent studies have had spectacular success in understanding the nature of the excited states mainly due to the extensive studies of the classical phase space structures and their bifurcations. Such detailed classical-quantum correspondence studies are presently limited to two or quasi two dimensional systems. One of the main reasons for such a constraint has to do with the problem of visualization of relevant objects like surface of sections and Wigner or Husimi distributions associated with an eigenstate. This neccesiates various alternative techniques which are more algebraic than geometric in nature. In this work we introduce one such method based on parametric variation of the eigenvalues of a Hamiltonian. It is shown that the level velocities are correlated with the phase space nature of the corresponding eigenstates. A semiclassical expression for the level velocities of a single resonance Hamiltonian is derived which provides theoretical support for the correlation. We use the level velocities to dynamically assign the highly excited states of a model spectroscopic Hamiltonian in the mixed phase space regime. The effect of bifurcations on the level velocities is briefly discussed using a recently proposed spectroscopic Hamiltonian for the HCP molecule.Comment: 12 pages, 9 figures, submitted to J. Chem. Phy

COBE ground segment gyro calibration

Discussed here is the calibration of the scale factors and rate biases for the Cosmic Background Explorer (COBE) spacecraft gyroscopes, with the emphasis on the adaptation for COBE of an algorithm previously developed for the Solar Maximum Mission. Detailed choice of parameters, convergence, verification, and use of the algorithm in an environment where the reference attitudes are determined form the Sun, Earth, and star observations (via the Diffuse Infrared Background Experiment (DIRBE) are considered. Results of some recent experiments are given. These include tests where the gyro rate data are corrected for the effect of the gyro baseplate temperature on the spacecraft electronics

Shot noise suppression at room temperature in atomic-scale Au junctions

Shot noise encodes additional information not directly inferable from simple electronic transport measurements. Previous measurements in atomic-scale metal junctions at cryogenic temperatures have shown suppression of the shot noise at particular conductance values. This suppression demonstrates that transport in these structures proceeds via discrete quantum channels. Using a high frequency technique, we simultaneously acquire noise data and conductance histograms in Au junctions at room temperature and ambient conditions. We observe noise suppression at up to three conductance quanta, with possible indications of current-induced local heating and $1/f$ noise in the contact region at high biases. These measurements demonstrate the quantum character of transport at room temperature at the atomic scale. This technique provides an additional tool for studying dissipation and correlations in nanodevices.Comment: 15 pages, 4 figures + supporting information (6 pages, 6 figures

Stabilizing single atom contacts by molecular bridge formation

Gold-molecule-gold junctions can be formed by carefully breaking a gold wire in a solution containing dithiolated molecules. Surprisingly, there is little understanding on the mechanical details of the bridge formation process and specifically on the role that the dithiol molecules play themselves. We propose that alkanedithiol molecules have already formed bridges between the gold electrodes before the atomic gold-gold junction is broken. This leads to stabilization of the single atomic gold junction, as observed experimentally. Our data can be understood within a simple spring model.Comment: 14 pages, 3 figures, 1 tabl