Inelastic collisions of relativistic electrons with atomic targets assisted by a laser field

We consider inelastic collisions between relativistic electrons and atomic targets assisted by a low-frequency laser field in the case when this field is still much weaker than the typical internal fields in the target. Concentrating on target transitions we show that they can be substantially affected by the presence of the laser field. This may occur either via strong modifications in the motion of the relativistic electrons caused by the electron-laser interaction or via the Compton effect when the incident electrons convert laser photon(s) into photons with frequencies equal to target transition frequencies.Comment: 4 pages, 2 figure

Irreducible Representations of Diperiodic Groups

The irreducible representations of all of the 80 diperiodic groups, being the symmetries of the systems translationally periodical in two directions, are calculated. To this end, each of these groups is factorized as the product of a generalized translational group and an axial point group. The results are presented in the form of the tables, containing the matrices of the irreducible representations of the generators of the groups. General properties and some physical applications (degeneracy and topology of the energy bands, selection rules, etc.) are discussed.Comment: 30 pages, 5 figures, 28 tables, 18 refs, LaTex2.0

Harmonic generation by atoms in circularly polarized two-color laser fields with coplanar polarizations and commensurate frequencies

The generation of harmonics by atoms or ions in a two-color, coplanar field configuration with commensurate frequencies is investigated through both, an analytical calculation based on the Lewenstein model and the numerical ab initio solution of the time-dependent Schroedinger equation of a two-dimensional model ion. Through the analytical model, selection rules for the harmonic orders in this field configuration, a generalized cut-off for the harmonic spectra, and an integral expression for the harmonic dipole strength is provided. The numerical results are employed to test the predictions of the analytical model. The scaling of the cut-off as a function of both, one of the laser intensities and frequency ratio $\eta$, as well as entire spectra for different $\eta$ and laser intensities are presented and analyzed. The theoretical cut-off is found to be an upper limit for the numerical results. Other discrepancies between analytical model and numerical results are clarified by taking into account the probabilities of the absorption processes involved.Comment: 8 figure

A Formal Analysis of a Business Contract Language

This paper presents a formal system for reasoning about violations of obligations in contracts. The system is based on the formalism for the representation of contrary-to-duty obligations. These are the obligations that take place when other obligations are violated as typically applied to penalties in contracts. The paper shows how this formalism can be mapped onto the key policy concepts of a contract specification language, called Business Contract Language (BCL), previously developed to express contract conditions for run time contract monitoring. The aim of this mapping is to establish a formal underpinning for this key subset of BCL

Relativistic ionization-rescattering with tailored laser pulses

The interaction of relativistically strong tailored laser pulses with an atomic system is considered. Due to a special tailoring of the laser pulse, the suppression of the relativistic drift of the ionized electron and a dramatic enhancement of the rescattering probability is shown to be achievable. The high harmonic generation rate in the relativistic regime is calculated and shown to be increased by several orders of magnitude compared to the case of conventional laser pulses. The energies of the revisiting electron at the atomic core can approach the MeV domain, thus rendering hard x-ray harmonics and nuclear reactions with single atoms feasible

Topological energy barrier for skyrmion lattice formation in MnSi

We report the direct measurement of the topological skyrmion energy barrier through a hysteresis of the skyrmion lattice in the chiral magnet MnSi. Measurements were made using small-angle neutron scattering with a custom-built resistive coil to allow for high-precision minor hysteresis loops. The experimental data was analyzed using an adapted Preisach model to quantify the energy barrier for skyrmion formation and corroborated by the minimum-energy path analysis based on atomistic spin simulations. We reveal that the skyrmion lattice in MnSi forms from the conical phase progressively in small domains, each of which consisting of hundreds of skyrmions, and with an activation barrier of several eV.Comment: Final accepted versio

Effects of Imperfect Reference Signal Recovery on Performance of SC and SSC Receivers over Generalized Fading Channels

This paper presents the study of the effects of imperfect reference signal recovery on the bit error rate (BER) performance of dual-branch switch-and-stay combining (SSC) and multibranch selection combining (SC) receivers in a generalized - fading channel. The average BER of binary and quaternary phase shift keying (BPSK and QPSK) is derived under the assumption that the reference carrier signal is extracted from the received modulated signal. For SSC receiver the optimal switching threshold (in a minimum BER sense) is numerically evaluated. Hereby we determine and discuss the simultaneous influence of the average signal-to-noise ratio (SNR) per bit, fading severity, product phase-locked loop (PLL) bandwidthbit duration (B_LT_b), switching threshold of SSC and diversity order of SC on BER performance. The influence of B_LT_b in different channel conditions and modulation formats is pointed out. The numerical results are confirmed by computer simulations