Electrical conduction along dislocations in semiconductors has attracted much attention both from fundamental and practical viewpoints. Fundamentally, such dislocations can be one-dimensional electronic systems and their conduction mechanism has already been investigated. On the other hand, the issue of the dislocation conduction should be practically important because it might degrade the performance of semiconductor devices. Here, local current conduction due to dislocations in GaN has been studied by scanning spreading resistance microscopy (SSRM). Fresh dislocations, induced by plastic deformation, were used to see their intrinsic properties.

The SSRM images showed many bright spots with high conductivity. The spots form chains along the slip direction and appear to be due to the edge dislocations introduced by deformation, which are terminated at the observed surface. Here, the line direction and Burgers vector of the dislocations are l = [0001] and b = (a/3) [1 10], resp. Previous studies have shown that grown-in screw dislocations with l = b = c[0001] are conductive but that grown-in edge dislocations with l = [0001] and b = (a/3) [1 10] are not, in contrast to our results. This apparent discrepancy should arise from the difference in the core structure between deformation-induced fresh dislocations and grown-in dislocations possibly decorated with native point defect or impurities. Theoretically, the electronic structures of the dislocations in GaN have been shown to change sensitively with the core structure, which should bring large conductivity differences. Local current-voltage spectra, measured at conductive spot positions, indicated a Frenkel-Poole mechanism for the conduction. These findings provide new insights into the issue of electrical conduction along dislocations in semiconductors.

For more than a century, physicists have searched for a unique and general form for the force density that an electromagnetic field imposes on a medium. The existing expressions for this quantity, obtained, e.g., by Minkowski, Einstein and Laub, Abraham, and Helmholtz, are different, and, as such, give different predictions in particular physical situations. The theories of Abraham and Minkowski, for example, ignore the existence of electro- and magnetostriction. Moreover, real media with dispersion, dissipation, and nonlinearities have not been addressed much.

We present an unambiguous general equation for the electromagnetic force density f = -∇T- (dG/dt) expressed in terms of a new three-dimensional energy-momentum tensor T and momentum density G of the field. The tensor T can be written as T = T_M + I_V, where T_M is the Minkowski tensor, I the unit tensor, and V the density of the field-matter interaction potential that is responsible for electro- and magnetostriction. Remarkably, if the medium is not magnetic, the momentum density G is given by Abraham’s expression G = ExH/c². If the material obeys the Clausius-Mossotti law, the tensor T becomes the Helmholtz tensor that to our knowledge has not been contradicted in any experiment so far.

The general equation obtained for the force density can be applied to essentially any natural or designed material whether inhomogeneous, anisotropic, nonlinear, dispersive, or dissipative, and even to materials providing optical gain. We also calculate the rate of work done on a medium by an electromagnetic field, and using the result, obtain the four-dimensional energy-momentum tensor T₄ in spacetime. Interestingly, this tensor is physically very close to the almost forgotten tensor of Einstein and Laub.

Nitrogen is a common contaminant species in fusion reactors such as the ITER (International Thermonuclear Experimental Reactor). Thus, the collisional properties of nitrogen-containing plasma compounds are widely studied experimentally and theoretically. Here we show the results of absolute cross section measurements for the electron impact dissociative excitation of ND⁺ yielding D⁺, especially at low electron energies near the onset of the dissociation. The identification of indirect and resonant processes is a particular challenge in that energy regime. Excitation is likely to be influenced by vibrationally excited levels populated within the X²Π ground state of ND⁺(v). Two mechanisms can produce these levels: (i) the endothermic reaction D₂⁺(v') + N and (ii) the ion-molecule reaction D₂(v") + N⁺. The latter is the first step in a sequence of molecular activated processes, which confirms that such a reaction chain is important for an understanding of the overall plasma chemistry. The low experimental energy threshold observed in the present studies indicates that the importance of contribution of Rydberg states via the capture of the incoming electron into doubly excited electronic states of (ND)**.

The dynamics of electron ionization are an important topic in attosecond science. Applying a strong laser pulse, electrons can quantum mechanically tunnel through the potential barrier created by the combined Coulomb field of the atom and the laser field. At the tunnel exit, it is commonly assumed that the electron velocity parallel to the electric field is zero, contrary to the well-described distribution of transverse momenta.

After ionization, electrons propagate in the remainder of the laser pulse, where they acquire a momentum spread due to the different phases of the field at their individual exit time. However, the longitudinal momentum spread measured in experiments on helium is considerably larger than that.

Monte Carlo simulations with zero initial longitudinal momentum agree with the theoretical predictions of acquired spread, while simulations that include a longitudinal momentum spread at the tunnel exit are compatible with experimental data. The authors introduced a new method to investigate electron velocity spreads after ionization. Applying this method to experimental data lead to a more accurate reconstruction of the electron wave packet immediately after tunnelling.

The interaction of intense laser fields with atoms gives rise to characteristic strong-field effects, most notably above-threshold ionization and high-harmonic generation. After the electron tunnels out of the atomic potential, it is accelerated by the laser field before being driven back to the parent ion by the same field. The returning electron energy ranges from a few to a few hundred eV depending on the field parameters. Upon rescattering, parts of the electron wave-packet may recombine with the atom under emission of energetic radiation of harmonics of the laser frequency. The harmonic radiation carries characteristics of the atomic potential, as well as of the rescattering electron. This sequence of processes can be directly visualized by a quantum phase space analysis. Using a Wigner phase-space distribution, one can relate the coordinate-momentum space distribution of the returning electron to the temporal emission pattern of the harmonic spectrum (see Figure).

Such relation between the time-frequency vs. the coordinate-momentum space distribution has been calculated for different atomic model potentials, the Coulomb potential supporting a Rydberg series and short-range potentials, with a finite number of bound states. The important role of dynamic bound-state polarization effects in the harmonic emission can be identified. The long-range part of the Coulomb spectrum modifies the local momentum distribution at the moment of rescattering. This might become important for an improved description of experiments aiming towards a tomographic reconstruction of the electronic structure of a system using the harmonic spectrum.

Thanks to the great accuracy in predicting experimental data, the standard model of particle physics is widely considered to be a building block of our current knowledge of the structure of matter. In spite of this success, we are still lacking an essential piece of evidence, namely the detection of the Higgs boson, a hypothetical massive elementary particle whose existence makes it possible to explain how most of the known elementary particles become massive. In this paper, an alternative electroweak model is presented that assumes running coupling constants described by energy-dependent entire functions. Contrary to the conventional formulation the action contains no physical scalar fields and no Higgs particle, even if the foreseen masses for particles are compatible with known experimental values. In addition the vertex couplings possess an energy scale for predicting scattering amplitudes that can be tested in current particle accelerators. As a result the paper provides an essential alternative to the current established knowledge in the field and addresses an issue that might soon be resolved, as the Large Hadron Collider could provide the experimental evidence of the existence or non-existence of the Higgs boson.