Non-Existence of Black Hole Solutions¶for a Spherically Symmetric, Static Einstein–Dirac–Maxwell System

We consider for j=?, … a spherically symmetric, static system of (2j+1) Dirac particles, each having total angular momentum j. The Dirac particles interact via a classical gravitational and electromagnetic field. The Einstein–Dirac–Maxwell equations for this system are derived. It is shown that, under weak regularity conditions on the form of the horizon, the only black hole solutions of the EDM equations are the Reissner–Nordstr?m solutions. In other words, the spinors must vanish identically. Applied to the gravitational collapse of a “cloud” of spin-?-particles to a black hole, our result indicates that the Dirac particles must eventually disappear inside the event horizon. Received: 2 November 1998 / Accepted: 23 February 1999     

Electrical detection of coherent nuclear spin oscillations in phosphorus-doped silicon using pulsed ENDOR

We demonstrate the electrical detection of pulsed X-band electron nuclear double resonance (ENDOR) in phosphorus-doped silicon at 5 K. A pulse sequence analogous to Davies ENDOR in conventional electron spin resonance is used to measure the nuclear spin transition frequencies of the (31)P nuclear spins, where the (31)P electron spins are detected electrically via spin-dependent transitions through Si/SiO(2) interface states, thus not relying on a polarization of the electron spin system. In addition, the electrical detection of coherent nuclear spin oscillations is shown, demonstrating the feasibility to electrically read out the spin states of possible nuclear spin qubits.     

Static observers in curved spaces and non-inertial frames in Minkowski spacetime

Static observers in curved spacetimes may interpret their proper acceleration as the opposite of a local gravitational field (in the Newtonian sense). Based on this interpretation and motivated by the equivalence principle, we are led to investigate congruences of timelike curves in Minkowski spacetime whose acceleration field coincides with the acceleration field of static observers of curved spaces. The congruences give rise to non-inertial frames that are examined. Specifically, we find, based on the locality principle, the embedding of simultaneity hypersurfaces adapted to the non-inertial frame in an explicit form for arbitrary acceleration fields. We also determine, from the Einstein equations, a covariant field equation that regulates the behavior of the proper acceleration of static observers in curved spacetimes. It corresponds to an exact relativistic version of the Newtonian gravitational field equation. In the specific case in which the level surfaces of the norm of the acceleration field of the static observers are maximally symmetric two-dimensional spaces, the energy?Cmomentum tensor of the source is analyzed.     

Crossover in the slow decay of dynamic correlations in the lorentz model

The long-time behavior of transport coefficients in a model for spatially heterogeneous media in two and three dimensions is investigated by molecular dynamics simulations. The behavior of the velocity autocorrelation function is rationalized in terms of a competition of the critical relaxation due to the underlying percolation transition and the hydrodynamic power-law anomalies. In two dimensions and in the absence of a diffusive mode, another power-law anomaly due to trapping is found with an exponent -3 instead of -2. Further, the logarithmic divergence of the Burnett coefficient is corroborated in the dilute limit; at finite density, however, it is dominated by stronger divergences.     

Identifying the underlying structure and dynamic interactions in a voting network

We analyse the structure and behaviour of a specific voting network using a dynamic structure-based methodology which draws on Q-Analysis and social network theory. Our empirical focus is on the Eurovision Song Contest over a period of 20 years. For a multicultural contest of this kind, one of the key questions is how the quality of a song is judged and how voting groups emerge. We investigate structures that may identify the winner based purely on the topology of the network. This provides a basic framework to identify what the characteristics associated with becoming a winner are, and may help to establish a homogenous criterion for subjective measures such as quality. Further, we measure the importance of voting cliques, and present a dynamic model based on a changing multidimensional measure of connectivity in order to reveal the formation of emerging community structure within the contest. Finally, we study the dynamic behaviour exhibited by the network in order to understand the clustering of voting preferences and the relationship between local and global properties.     

Electrical and microscopic characterization of ZnO films on p-SiC substrates

ZnO/p- SiC heterojunctions were fabricated by thermal evaporation from ZnO high quality powder (99.99%) onto 4H and 6H p-SiC polytypes. We find that, despite the low cost technique employed for the deposition of the ZnO film, the devices exhibited breakdown voltages in excess of 100 V, high rectification ratio (forward to reverse current ratio, I_F/I_R) and low leakage current, respectively, 2×10⁵ and 4.5×10⁻⁷ A/cm² (for the 4H p-SiC based device) and 5×10⁴ and 5×10⁻⁷ A/cm² (for the 6H p-SiC based device). The current-voltage (I×V) characteristics were also measured at the nanometer scale by means of conductive atomic force microscopy. A simple Schottky diode model and conductance divided by current versus conductance plots (G/I×G plots) was used to analyze device characteristics. This analysis shows that, when probing at the nanometric scale, fluctuations of the effective barrier height and/or surface states across individual grains or grain boundaries cause deviations from linear G/I×G plots. These fluctuations are smeared out when probing at the macroscale and thus it becomes possible to obtain linear plots and extract diode parameters.     

Spin echoes in the charge transport through phosphorus donors in silicon

The electrical detection of spin echoes via echo tomography is used to observe coherent processes associated with the electrical readout of the spin state of phosphorus donor electrons in silicon near a SiO2 interface. Using the Carr-Purcell pulse sequence, an echo decay with a time constant of 1.7+/-0.2 micros is observed and discussed in terms of decoherence and recombination times. Electrical spin echo tomography thus can be used to study the dynamics of the spin-dependent transport processes, e.g., in realistic spin qubit devices for quantum information processing.     

Feasibility of probing boundary morphology of structured materials by 2D NMR q-space imaging

It is well known that one-dimensional (1D) q-space imaging allows retrieval of structural information at cellular resolution. Here we demonstrate by simulation that boundary morphology of structured materials can be derived from 2D q-space mapping. Based on a finite-difference model for restricted diffusion, 2D q-space maps obtained from water diffusion inside apertures at various levels of asperity were simulated. The results indicate that the observed ring patterns (diffraction minima) reveal the boundary profiles of the apertures but become blurred in the case of significant variation in aperture size. For uniform size distribution of apertures, a quantitative measure of surface roughness can be established by means of spatial autocorrelation analysis. The results suggest that 2D q-space imaging may allow probing of the boundary morphology of structured materials and possibly biological cells.