X-ray fluorescence spectra of metals excited below threshold

X-ray scattering spectra of Cu and Ni metals have been measured using monochromatic synchrotron radiation tuned from far above to more than 10 eV below threshold. Energy conservation in the scattering process is found to be sufficient to explain the modulation of the spectral shape, neglecting momentum conservation and channel interference. At excitation energies close to and above threshold, the emission spectra map the occupied local partial density of states. For the sub-threshold excitations, the high-energy flank of the inelastic scattering exhibits a Raman-type linear dispersion, and an asymmetric low energy tail develops. For excitation far below threshold the emission spectra are proportional to a convolution of the occupied and unoccuppied local partial densities of states.Comment: 10 pages, 3 figures, http://link.aps.org/doi/10.1103/PhysRevB.68.04511

The role of singletons in S7S^7 compactifications

We derive the isometry irrep content of squashed seven-sphere compactifications of eleven-dimensional supergravity, i.e., the left-squashed ($LS^7$) with ${\mathcal N}=1$ and right-squashed ($RS^7$) with ${\mathcal N}=0$ supersymmetry, in a manner completely independent of the round sphere. Then we compare this result with the spectrum obtained by Higgsing the round sphere spectrum. This way we discover features of the spectra which makes it possible to argue that the only way the round spectrum can be related by a Higgs mechanism to the one of $LS^7$ is if the singletons are included in the round sphere spectrum. For this to work also in the $RS^7$ case it seems that the gravitino of the $LS^7$ spectrum must be replaced by a fermionic singleton present in the $RS^7$ spectrum.Comment: 24 pages including appendix with 12 figure, v2 minor typos correcte

Quantum-dot thermometry

We present a method for the measurement of a temperature differential across a single quantum dot that has transmission resonances that are separated in energy by much more than the thermal energy. We determine numerically that the method is accurate to within a few percent across a wide range of parameters. The proposed method measures the temperature of the electrons that enter the quantum dot and will be useful in experiments that aim to test theory which predicts quantum dots are highly-efficient thermoelectrics.Comment: 3 pages, 4 Figure

Electronic structure of GaAs1-xNx alloy by soft-X-ray absorption and emission: Origin of the reduced optical efficiency

The local electronic structure of N atoms in a diluted GaAs1-xNx (x=3%) alloy, in view of applications in optoelectronics, is determined for the first time using soft-X-ray absorption (SXA) and emission (SXE). Deviations from crystalline GaN, in particular in the conduction band, are dramatic. Employing the orbital character and elemental specificity of the SXE/SXA spectroscopies, we identify a charge transfer from the N atoms at the valence band maximum, reducing the overlap with the wavefunction in conduction band minimum, as the main factor limiting the optical efficiency of GaAs1-xNx alloys. Moreover, a k-conserving process of resonant inelastic x-ray scattering involving the L1 derived valence and conduction states is discovered.Comment: 3 pages, physica status solidi (Rapid Research Notes), in pres

On Making Good Games - Using Player Virtue Ethics and Gameplay Design Patterns to Identify Generally Desirable Gameplay Features

This paper uses a framework of player virtues to perform a theoretical exploration of what is required to make a game good. The choice of player virtues is based upon the view that games can be seen as implements, and that these are good if they support an intended use, and the intended use of games is to support people to be good players. A collection of gameplay design patterns, identified through their relation to the virtues, is presented to provide specific starting points for considering design options for this type of good games. 24 patterns are identified supporting the virtues, including RISK/REWARD, DYNAMIC ALLIANCES, GAME MASTERS, and PLAYER DECIDED RESULTS, as are 7 countering three or more virtues, including ANALYSIS PARALYSIS, EARLY ELIMINATION, and GRINDING. The paper concludes by identifying limitations of the approach as well as by showing how it can be applied using other views of what are preferable features in games

Levitated droplet dye laser

We present the first observation, to our knowledge, of lasing from a levitated, dye droplet. The levitated droplets are created by computer controlled pico-liter dispensing into one of the nodes of a standing ultrasonic wave (100 kHz), where the droplet is trapped. The free hanging droplet forms a high quality optical resonator. Our 750 nL lasing droplets consist of Rhodamine 6G dissolved in ethylene glycol, at a concentration of 0.02 M. The droplets are optically pumped at 532 nm light from a pulsed, frequency doubled Nd:YAG laser, and the dye laser emission is analyzed by a fixed grating spectrometer. With this setup we have achieved reproducible lasing spectra in the visible wavelength range from 610 nm to 650 nm. The levitated droplet technique has previously successfully been applied for a variety of bio-analytical applications at single cell level. In combination with the lasing droplets, the capability of this high precision setup has potential applications within highly sensitive intra-cavity absorbance detection.Comment: 6 pages including 3 figure

Electronic properties of graphene multilayers

We study the effects of disorder in the electronic properties of graphene multilayers, with special focus on the bilayer and the infinite stack. At low energies and long wavelengths, the electronic self-energies and density of states exhibit behavior with divergences near half-filling. As a consequence, the spectral functions and conductivities do not follow Landau's Fermi liquid theory. In particular, we show that the quasiparticle decay rate has a minimum as a function of energy, there is a universal minimum value for the in-plane conductivity of order e^2/h per plane and, unexpectedly, the c-axis conductivity is enhanced by disorder at low doping, leading to an enormous conductivity anisotropy at low temperatures.Comment: 4 pages, 4 figure. Reference to exciting new ARPES results on graphite added (we thank A. Lanzara for sharing the paper prior to its publication

Comment on ``Two Time Scales and Violation of the Fluctuation-Dissipation Theorem in a Finite Dimensional Model for Structural Glasses''

In cond-mat/0002074 Ricci-Tersenghi et al. find two linear regimes in the fluctuation-dissipation relation between density-density correlations and associated responses of the Frustrated Ising Lattice Gas. Here we show that this result does not seem to correspond to the equilibrium quantities of the model, by measuring the overlap distribution P(q) of the density and comparing the FDR expected on the ground of the P(q) with the one measured in the off-equilibrium experiments.Comment: RevTeX, 1 page, 2 eps figures, Comment on F. Ricci-Tersenghi et al., Phys. Rev. Lett. 84, 4473 (2000

High fidelity readout scheme for rare-earth solid state quantum computing

We propose and analyze a high fidelity readout scheme for a single instance approach to quantum computing in rare-earth-ion-doped crystals. The scheme is based on using different species of qubit and readout ions, and it is shown that by allowing the closest qubit ion to act as a readout buffer, the readout error can be reduced by more than an order of magnitude. The scheme is shown to be robust against certain experimental variations, such as varying detection efficiencies, and we use the scheme to predict the expected quantum fidelity of a CNOT gate in these solid state systems. In addition, we discuss the potential scalability of the protocol to larger qubit systems. The results are based on parameters which we believed are experimentally feasible with current technology, and which can be simultaneously realized.Comment: 7 pages, 5 figure