The light-cone expansion and the parton model in polarized muon-production

The relation between the light-cone expansion and the QCD improved parton model formalism for polarized deep inelastic muon-production is discussed in detail. The regulator dependence of the relevant quantities is explicity studied at the two-loop level for the first moment ofg ₁ ^P . It is shown that the anomalous gluon component is well defined if the quark term is specified as a conserved quantity that does not evolve inQ ². The behaviour of the first moment ofg ₁ ^P at the threshold for heavy quark production is also discussed.     

Opportunities for resonant elastic X-ray scattering at X-ray free-electron lasers

X-ray Free-Electron Lasers (FELs) are beginning to deliver a revolution in X-ray experiments, thanks to their ultra-bright (peak brightness exceeding 10³³ photons/s/mm²/mrad²/0.1%BW), ultrashort (down to a few fs), spatially coherent X-ray pulses. Presently operational facilities cover wide spectral ranges, from the VUV and soft X-ray wavelengths of FLASH in Hamburg (down to 4.2 nm), to the hard X-rays delivered by the LCLS in Stanford (wavelengths of 0.15 nm or shorter). The basic properties of the new sources are briefly reviewed, and the impact on resonant scattering experiments is discussed. The perspective of investigating ultrafast magnetism, and, more generally, the time-dependent response of strongly correlated electron systems, in a pump-and-probe mode at the L edges of 3d transition metals, would be very attractive. In the hard X-ray range, the very recent proposal of self-seeded X-ray FELs, with 10⁻⁵ intrinsic bandwidth, tunable wavelength, 100 fs pulses and number of photons per pulse of order 10¹² also opens exciting possibilities for resonant scattering.     

Precision tests of the standard model,the Higgs,and new physics

We present a concise review of the status of the standard model and of the search for new physics.     

On the splitting of the exciton ground state in silicon

The exciton ground state in silicon is calculated taking into accoun the effect of the split-off valence band. We show that this effect is very important. The anisotropy splitting of the ground state is found to be 0.32 meV, while a previous analysis, which neglected the split-off band, gave 0.46 meV. The new result is in good agreement with recent experimental data.     

Local field corrections to binding energies of substitutional and interstitial donors in Si and Ge

Intervalley Umklapp matrix elements, with inclusion of local field screening effects, are computed for substitutional and interstitial point-charge impurity potentials in Si and Ge. The shallow character of substitutional donors is not affected in Ge and is even reinforced in Si, where a 20% reduction of the binding energy is obtained as a consequence of the local field effect. In both semiconductors the local field corrections strongly reinforce the non-effective-mass, deep-level character of interstitial point-charge donors.     

Effect of disorder on direct and indirect band gaps of semiconductor alloys

A simple analysis of the effect of disorder on different band edges of ternary III–V alloys is performed in terms of the symmetry properties of Bloch functions. The effect of disorder on the X₁ conduction band minimum is shown to be strongly dependent on the valence of the alloyed element, in agreement with available experimental data. The implications for indirect gaps and for the calculations of crossover compositions are briefly discussed.     

Camel's back excitons in GaP

The influence of the recently proposed camel's back structure of the GaP conduction band edge on the exciton spectrum is investigated theoretically. The results are in good agreement with differential absorption data and strongly support a camel's back structure, with a 4 meV central hump. The computed exciton binding energy is 18.5 meV, and when combined with recent experimental data, indicates a binding of about 32 meV for the electron-hole liquid.     

Interpretation of far-infrared absorption spectra in germanium

The high resolution far-infrared absorption spectra of free excitons in germanium are analyzed for the first time using a model which accurately includes both the anisotropy of the valence and condition bands and the degeneracy of the valence band. The observed structure is in good agreement with the calculated transitions between the anisotropy split ground state and the various p-like excited states.