Cosmic ray anisotropies at high energies

The directional anisotropies of the energetic cosmic ray gas due to the relative motion between the observers frame and the one where the relativistic gas can be assumed isotropic is analyzed. The radiation fluxes formula in the former frame must follow as the Lorentz invariance of dp/E, where p, E are the 4-vector momentum-energy components; dp is the 3-volume element in the momentum space. The anisotropic flux shows in such a case an amplitude, in a rotating earth, smaller than the experimental measurements from say, EAS-arrays for primary particle energies larger than 1.E(14) eV. Further, it is shown that two consecutive Lorentz transformations among three inertial frames exhibit the violation of dp/E invariance between the first and the third systems of reference, due to the Wigner rotation. A discussion of this result in the context of the experimental anisotropic fluxes and its current interpretation is given

Quantum Hall states under conditions of vanishing Zeeman energy

We report on magneto-transport measurements of a two-dimensional electron gas confined in a Cd$_{0.997}$Mn$_{0.003}$Te quantum well structure under conditions of vanishing Zeeman energy. The electron Zeeman energy has been tuned via the $s-d$ exchange interaction in order to probe different quantum Hall states associated with metallic and insulating phases. We have observed that reducing Zeeman energy to zero does not necessary imply the disappearing of quantum Hall states, i.e. a closing of the spin gap. The spin gap value under vanishing Zeeman energy conditions is shown to be dependent on the filling factor. Numerical simulations support a qualitative description of the experimental data presented in terms of a crossing or an avoided-crossing of spin split Landau levels with same orbital quantum number $N$

Nonsingular systems of generalized Sylvester equations: An algorithmic approach

We consider the uniqueness of solution (i.e., nonsingularity) of systems of r generalized Sylvester and ⋆-Sylvester equations with n×n coefficients. After several reductions, we show that it is sufficient to analyze periodic systems having, at most, one generalized ⋆-Sylvester equation. We provide characterizations for the nonsingularity in terms of spectral properties of either matrix pencils or formal matrix products, both constructed from the coefficients of the system. The proposed approach uses the periodic Schur decomposition and leads to a backward stable O(n3r) algorithm for computing the (unique) solution

EPR and ferromagnetism in diluted magnetic semiconductor quantum wells

Motivated by recent measurements of electron paramagnetic resonance (EPR) spectra in modulation-doped CdMnTe quantum wells, [F.J. Teran {\it et al.}, Phys. Rev. Lett. {\bf 91}, 077201 (2003)], we develop a theory of collective spin excitations in quasi-two-dimensional diluted magnetic semiconductors (DMSs). Our theory explains the anomalously large Knight shift found in these experiments as a consequence of collective coupling between Mn-ion local moments and itinerant-electron spins. We use this theory to discuss the physics of ferromagnetism in (II,Mn)VI quantum wells, and to speculate on the temperature at which it is likely to be observed in n-type modulation doped systems.Comment: 4 pages, 1 figur

Tailoring magnetic anisotropy in epitaxial half metallic La0.7Sr0.3MnO3 thin films

We present a detailed study on the magnetic properties, including anisotropy, reversal fields, and magnetization reversal processes, of well characterized half-metallic epitaxial La0.7Sr0.3MnO3 (LSMO) thin films grown onto SrTiO3 (STO) substrates with three different surface orientations, i.e. (001), (110) and (1-18). The latter shows step edges oriented parallel to the [110] (in-plane) crystallographic direction. Room temperature high resolution vectorial Kerr magnetometry measurements have been performed at different applied magnetic field directions in the whole angular range. In general, the magnetic properties of the LSMO films can be interpreted with just the uniaxial term with the anisotropy axis given by the film morphology, whereas the strength of this anisotropy depends on both structure and film thickness. In particular, LSMO films grown on nominally flat (110)-oriented STO substrates presents a well defined uniaxial anisotropy originated from the existence of elongated in-plane [001]-oriented structures, whereas LSMO films grown on nominally flat (001)-oriented STO substrates show a weak uniaxial magnetic anisotropy with the easy axis direction aligned parallel to residual substrate step edges. Elongated structures are also found for LSMO films grown on vicinal STO(001) substrates. These films present a well-defined uniaxial magnetic anisotropy with the easy axis lying along the step edges and its strength increases with the LSMO thickness. It is remarkable that this step-induced uniaxial anisotropy has been found for LSMO films up to 120 nm thickness. Our results are promising for engineering novel half-metallic magnetic devices that exploit tailored magnetic anisotropy.Comment: 10 pages, 10 figures, 1 tabl

Collective character of spin excitations in a system of Mn2+^{2+} spins coupled to a two-dimensional electron gas

We have studied the low energy spin excitations in n-type CdMnTe based dilute magnetic semiconductor quantum wells. For magnetic fields for which the energies for the excitation of free carriers and Mn spins are almost identical an anomalously large Knight shift is observed. Our findings suggests the existence of a magnetic field induced ferromagnetic order in these structures, which is in agreement with recent theoretical predictions [J. K{\"o}nig and A. H. MacDonald, submitted Phys. Rev. Lett. (2002)]Comment: 4 figure

Enhancement of the spin-gap in fully occupied two-dimensional Landau levels

Polarization-resolved magneto-luminescence, together with simultaneous magneto-transport measurements, have been performed on a two-dimensional electron gas (2DEG) confined in CdTe quantum well in order to determine the spin-splitting of fully occupied electronic Landau levels, as a function of the magnetic field (arbitrary Landau level filling factors) and temperature. The spin splitting, extracted from the energy separation of the \sigma+ and \sigma- transitions, is composed of the ordinary Zeeman term and a many-body contribution which is shown to be driven by the spin-polarization of the 2DEG. It is argued that both these contributions result in a simple, rigid shift of Landau level ladders with opposite spins.Comment: 4 pages, 3 figure

An unholy alliance: Christian identity extremists and ICTs

The rise of extremism has been a global concern, but white extremism, in particular, has been expanding rapidly within the United States (U.S.). Even more concerning is how white extremist groups have utilized Information and Communication Technologies (ICTs) to broaden their reach and spread their ideologies to larger audiences. This research examines the Christian Identity Movement (CIM) subset within the larger white extremist community. The CIM has utilized Christianity to justify extremist actions, a problem notably reflected in recent white extremist shooter manifestos. As extremist propaganda continues making its way through the digital landscape, this study aims to understand how the CIM has infiltrated the greater white extremist digital communities. This is particularly relevant as extremist groups are proliferating across numerous social media platforms. Due to the lack of scholarly literature currently discussing the nexus between white extremists, Christian Identity, technology, and social media, this research is necessary to understand the information flow between these groups online

Tire/road noise, texture, and vertical accelerations: Surface assessment of an urban road

Pavements are made up of several layers with different mechanical and functional characteristics. The correct design of the surface layerof a roadmay leadto pavements with bettercharacteristics regarding ride qualityand safety, but also pavements that may be used as a measure against noise.The use oflow-noise pavements maybe aneffective measure to reduce the generation of acoustic pollutionby roadtraffic.This work aims to assesssome functionalcharacteristicsof a rehabilitated urban street,after two months in service conditions.The pavement was fabricated with a gap-graded bituminous mixture type Stone Mastic Asphalt(SMA) with crumb rubber(CR)from end-of-life (EOL) tires.This work studies the acoustic performance of the pavement, as well as other surface characteristics such as the macrotexturedepth(MPD)and the unevenness(IRI), establishing the relationship between them and the tire/road noise at different frequencies.Finally, the mainvertical acceleration frequencies of the pavement/vehiclesystemat 50 km/h were also assessedand related to the pavement unevennessand conservation.According to the results, this mixture might be used as a noise mitigation measure within the Action Plans of some urban areas with problems related to noise. The macrotexturedepthofthe mixture contributes to its acoustic performance at low frequencies;however, its acoustic performance cannot be totally explained from a macrotexture point of vie