Electron-hole pairs during the adsorption dynamics of O2 on Pd(100) - Exciting or not?

During the exothermic adsorption of molecules at solid surfaces dissipation of the released energy occurs via the excitation of electronic and phononic degrees of freedom. For metallic substrates the role of the nonadiabatic electronic excitation channel has been controversially discussed, as the absence of a band gap could favour an easy coupling to a manifold of electronhole pairs of arbitrarily low energies. We analyse this situation for the highly exothermic showcase system of molecular oxygen dissociating at Pd(100), using time-dependent perturbation theory applied to first-principles electronic-structure calculations. For a range of different trajectories of impinging O2 molecules we compute largely varying electron-hole pair spectra, which underlines the necessity to consider the high-dimensionality of the surface dynamical process when assessing the total energy loss into this dissipation channel. Despite the high Pd density of states at the Fermi level, the concomitant non-adiabatic energy losses nevertheless never exceed about 5% of the available chemisorption energy. While this supports an electronically adiabatic description of the predominant heat dissipation into the phononic system, we critically discuss the non-adiabatic excitations in the context of the O2 spin transition during the dissociation process.Comment: 20 pages including 7 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html [added two references, changed V_{fsa} to V_{6D}, modified a few formulations in interpretation of spin asymmetry of eh-spectra, added missing equals sign in Eg.(2.10)

Non-perturbative QEG Corrections to the Yang-Mills Beta Function

We discuss the non-perturbative renormalization group evolution of the gauge coupling constant by using a truncated form of the functional flow equation for the effective average action of the Yang-Mills-gravity system. Our result is consistent with the conjecture that Quantum Einstein Gravity (QEG) is asymptotically safe and has a vanishing gauge coupling constant at the non-trivial fixed point.Comment: To appear in the proceedings of CORFU 200

Gluon Condensation in Nonperturbative Flow Equations

We employ nonperturbative flow equations for an investigation of the effective action in Yang-Mills theories. We compute the effective action $\Gamma[B]$ for constant color magnetic fields $B$ and examine Savvidy's conjecture of an unstable perturbative vacuum. Our results indicate that the absolute minimum of $\Gamma[B]$ occurs for B=0. Gluon condensation is described by a nonvanishing expectation value of the regularized composite operator $F_{\mu\nu}F^{\mu\nu}$ which agrees with phenomenological estimates.Comment: 64 pages, late

Running Gauge Coupling in Asymptotically Safe Quantum Gravity

We investigate the non-perturbative renormalization group behavior of the gauge coupling constant using a truncated form of the functional flow equation for the effective average action of the Yang-Mills-gravity system. We find a non-zero quantum gravity correction to the standard Yang-Mills beta function which has the same sign as the gauge boson contribution. Our results fit into the picture according to which Quantum Einstein Gravity (QEG) is asymptotically safe, with a vanishing gauge coupling constant at the non-trivial fixed point.Comment: 27 page

Fabrication of quantum point contacts by engraving GaAs/AlGaAs-heterostructures with a diamond tip

We use the all-diamond tip of an atomic force microscope for the direct engraving of high quality quantum point contacts in GaAs/AlGaAs-heterostructures. The processing time is shortened by two orders of magnitude compared to standard silicon tips. Together with a reduction of the line width to below 90 nm the depletion length of insulating lines is reduced by a factor of two with the diamond probes. The such fabricated defect free ballistic constrictions show well resolved conductance plateaus and the 0.7 anomaly in electronic transport measurements.Comment: 3 pages, 3 figure

Specific Heat of a Fractional Quantum Hall System

Using a time-resolved phonon absorption technique, we have measured the specific heat of a two-dimensional electron system in the fractional quantum Hall effect regime. For filling factors $\nu = 5/3, 4/3, 2/3, 3/5, 4/7, 2/5$ and 1/3 the specific heat displays a strong exponential temperature dependence in agreement with excitations across a quasi-particle gap. At filling factor $\nu = 1/2$ we were able to measure the specific heat of a composite fermion system for the first time. The observed linear temperature dependence on temperature down to $T = 0.14$ K agrees well with early predictions for a Fermi liquid of composite fermions.Comment: 4 pages, 4 figures (version is 1. resubmission: Added a paragraph to include the problems which arise by the weak temperature dependence at \nu = 1/2, updated affiliation

Effective Average Action in N=1 Super-Yang-Mills Theory

For N=1 Super-Yang-Mills theory we generalize the effective average action Gamma_k in a manifest supersymmetric way using the superspace formalism. The exact evolution equation for Gamma_k is derived and, introducing as an application a simple truncation, the standard one-loop beta-function of N=1 SYM theory is obtained.Comment: 17 pages, LaTeX, some remarks added, misprints corrected, to appear in Phys. Rev.

Polarization-preserving confocal microscope for optical experiments in a dilution refrigerator with high magnetic field

We present the design and operation of a fiber-based cryogenic confocal microscope. It is designed as a compact cold-finger that fits inside the bore of a superconducting magnet, and which is a modular unit that can be easily swapped between use in a dilution refrigerator and other cryostats. We aimed at application in quantum optical experiments with electron spins in semiconductors and the design has been optimized for driving with, and detection of optical fields with well-defined polarizations. This was implemented with optical access via a polarization maintaining fiber together with Voigt geometry at the cold finger, which circumvents Faraday rotations in the optical components in high magnetic fields. Our unit is versatile for use in experiments that measure photoluminescence, reflection, or transmission, as we demonstrate with a quantum optical experiment with an ensemble of donor-bound electrons in a thin GaAs film.Comment: 9 pages, 7 figure