Abstract: We review the transmission properties of carriers interacting with potential barriers in graphene. The tunneling of electrons and holes in quantum structures in graphene is found to display features that are in marked contrast with those of other systems. In particular, the interaction between the carriers with electrostatic potential barriers can be related to the propagation of electromagnetic waves in media with negative refraction indices, also known as metamaterials. This behavior becomes evident as one calculates the time evolution of wavepackets propagating across the barrier interface. In addition, we discuss the effect of trigonal warping on the tunneling through potential barriers.

Abstract: We performed ab initio density-functional theory calculations to investigate the process of hydrogenation of a bilayer of graphene. 50% hydrogen coverage is possible in case that the hydrogen atoms are allowed to adsorb on both sides of the bilayer. In this case interlayer chemical bonding occurs which stabilizes the structure. At maximum coverage, a bilayer of graphane is formed which has properties that are similar to those of a single layer of graphane.

Abstract: Intermediate-state flux structures in mesoscopic type-I superconductors are studied within the Ginzburg-Landau theory. In addition to well-established tubular and laminar structures, the strong confinement leads to the formation of (i) a phase of singly quantized vortices, which is typical for type-II superconductors and (ii) a ring of a normal domain at equilibrium. The stability region and the formation process of these intermediate-state structures are strongly influenced by the geometry of the sample.

Abstract: The use of inks containing organic solvents by the offset printing process implies in the release of volatile organic compounds to the work environment. Many of these compounds such as benzene, toluene, ethylbenzene, and the xylene isomers (well known by the acronym BTEX) are extremely toxic. In this study, the BTEX concentrations were determined in two different printing plants that use distinct types of inks: the conventional and the so-called ecological, which is manufactured based on vegetal oil. Concentration ranges were 4384, 153,480, 2133, 5459, and 2236 μg m−3 for benzene, toluene, ethylbenzene, m + p-xylene, and o-xylene, respectively, for the conventional printing plant. At the ecological printing plant, concentration ranges were below limit of detection (<LD)-31, <LD-618, <LD-1,690, <LD-10,500, <LD-3,360 μg m−3 for benzene, toluene, ethylbenzene, m + p-xylene, and o-xylene, respectively. BTEX concentrations are lower at the ecological printing environment than in the conventional, where mineral oil-based inks are used. However, the worker who cleans the printing matrices is exposed to high concentrations of ethylbenzene and xylenes, due probably to the cleaning products composition (containing high amounts of BTEX). Although the BTEX concentrations found in both printing work environments were below the limits considered by the Brazilian Law for Activities and Unhealthy Operations (NR-15), the exposure to such vapors characterizes risk to the workers health for some of the evaluated samples, mainly the personal ones.

Abstract: The influence of the composition on the crystallographic properties of deposited Mg(M)O (with M=Al, Cr, Ti, Y, and Zr) films is studied. For a flexible control of the composition, dual reactive magnetron sputtering was used as deposition technique. Two different approaches to predict the composition are discussed. The first is an experimental way based on the simple relationship between the deposition rate and the target-substrate distance. The second is a route using a Monte Carlo based particle trajectory code. Both methods require a minimal experimental input and enable the user to quickly predict the composition of complex thin films. Good control and flexibility allow us to study the compositional effects on the growth of Mg(M)O films. Pure MgO thin films were grown with a (111) preferential out-of-plane orientation. When adding M to MgO, two trends were noticed. The first trend is a change in the MgO lattice parameters compared to pure MgO. The second tendency is a decrease in the crystallinity of the MgO phase. The experimentally determined crystallographic properties are shown to be in correspondence with the predicted properties from molecular dynamics simulations.

Abstract: The transmission T and conductance G through one or multiple one-dimensional, ä-function barriers of two-dimensional fermions with a linear energy spectrum are studied. T and G are periodic functions of the strength P of the ä-function barrier V(x,y)/ℏvF=Pä(x). The dispersion relation of a Kronig-Penney (KP) model of a superlattice is also a periodic function of P and causes collimation of an incident electron beam for P=2ðn and n integer. For a KP superlattice with alternating sign of the height of the barriers the Dirac point becomes a Dirac line for P=(n+1/2)ð.

Abstract: Using scanning tunneling microscopy and Ginzburg-Landau simulations, we explore vortex configurations in magnetically coupled NbSe2/permalloy superconductor/ferromagnet bilayer. The permalloy film with stripe domain structure induces periodic local magnetic induction in the superconductor, creating a series of pinning-antipinning channels for externally added magnetic flux quanta. Such laterally confined Abrikosov vortices form quasi-one-dimensional arrays (chains). The transitions between multichain states occur through propagation of kinks at the intermediate fields. At high fields we show that the system becomes nonlinear due to a change in both the number of vortices and the confining potential. The longitudinal instabilities of the resulting vortex structures lead to vortices levitating in the antipinning channels.

Abstract: We present comprehensive calculations of the low-field hole mobility in Ge p-channel inversion layers with SiO2 insulator using a six-band k·p band-structure model. The cases of relaxed, biaxially, and uniaxially (both tensily and compressively) strained Ge are studied employing an efficient self-consistent methodmaking use of a nonuniform spatial mesh and of the Broyden second methodto solve the coupled envelope-wave function k·p and Poisson equations. The hole mobility is computed using the KuboGreenwood formalism accounting for nonpolar hole-phonon scattering and scattering with interfacial roughness. Different approximations to handle dielectric screening are also investigated. As our main result, we find a large enhancement (up to a factor of 10 with respect to Si) of the mobility in the case of uniaxial compressive stress similarly to the well-known case of Si. Comparison with experimental data shows overall qualitative agreement but with significant deviations due mainly to the unknown morphology of the rough Ge-insulator interface, to additional scattering with surface optical phonon from the high- insulator, to Coulomb scattering interface traps or oxide chargesignored in our calculationsand to different channel structures employed.

Abstract: We investigate theoretically the Dyakonov-Perel spin relaxation time by solving the eight-band Kane model and Poisson equation self-consistently. Our results show distinct behavior with the single-band model due to the anomalous spin-orbit interactions in narrow band-gap semiconductors, and agree well with the experiment values reported in recent experiment [K. L. Litvinenko et al., New J. Phys. 8, 49 (2006)]. We find a strong resonant enhancement of the spin relaxation time appears for spin align along [11̅ 0] at a certain electron density at 4 K. This resonant peak is smeared out with increasing the temperature.

Abstract: Analysis of the structural parameters of phases that adopt brownmillerite-type structures suggests the distribution of the different complex ordering schemes adopted within this structure type can be rationalized by considering both the size of the separation between the tetrahedral layers and the tetrahedral chain distortion angle. A systematic study using structural data obtained from La1−xAxMnO2,5 (A = Ba, Sr, Ca,) phases, prepared by the topotactic reduction of the analogous La1−xAxMnO3 perovskite phases, was performed to investigate this relationship. By manipulating the A-cation composition, both the tetrahedral layer separation and tetrahedral chain distortion angle in the La1−xAxMnO2,5 phases were controlled and from the data obtained a ¡°structure map¡± of the different brownmillerite variants was plotted as a function of these structural parameters. This map has been extended to include a wide range of reported brownmillerite phases showing the structural ideas presented are widely applicable. The complete structural characterization of La1−xAxMnO2,5 0.1 ¡Ü x ¡Ü 0.33, A = Ba; 0.15 ¡Ü x ¡Ü 0.5 A = Sr, and 0.22 ¡Ü x ¡Ü 0.5 A = Ca is described and includes compositions which exhibit complex intralayer ordered structures and Mn2+/Mn3+ charge ordering.

Abstract: In this study we investigate the effect of device topology on the ballistic current in n-channel metal-oxide-semiconductor field-effect transistors. Comparison of the nanoscale planar and double-gate devices reveals that, down to a certain thickness of the double gate film, the ballistic current flowing in the double gate device is twice as large compared to its planar counterpart. On the other hand, further thinning of the film beyond this threshold is found to change noticeably the confinement and transport characteristics, which are strongly depending on the film material and the surface orientation. For double gate Ge and Si devices there exists a critical film thickness below which the transverse gate field is no longer effectively screened by the inversion layer electron gas and mutual inversion of the two gates is turned on. In the case of GaAs and other similar IIIV compounds, a decrease in the film thickness may drastically change the occupation of the L-valleys and therefore amend the transport properties. The simulation results show that, in both cases, the ballistic current and the transconductance are considerably enhanced.

Abstract: We investigate a scattering of electron which is injected individually into an empty ballistic channel containing a cavity that is Coulomb coupled to a quantum ring charged with a single electron. We solve the time-dependent Schrödinger equation for the electron pair with an exact account for the electron-electron correlation. Absorption of energy and angular momentum by the quantum ring is not an even function of the external magnetic field. As a consequence we find that the electron backscattering probability is asymmetric in the magnetic field and thus violates Onsager symmetry.

Abstract: Stained glass windows incorporating dark blue and purple enamel paint layers are in some cases subject to severe degradation while others from the same period survived the ravages of time. A series of dark blue, greenblue and purple enamel glass paints from the same region (Northwestern Europe) and from the same period (16early 20th centuries) has been studied by means of a combination of microscopic X-ray fluorescence analysis, electron probe micro analysis and transmission electron microscopy with the aim of better understanding the causes of the degradation. The chemical composition of the enamels diverges from the average chemical composition of window glass. Some of the compositions appear to be unstable, for example those with a high concentration of K2O and a low content of CaO and PbO. In other cases, the deterioration of the paint layers was caused by the less than optimal vitrification of the enamel during the firing process. Recipes and chemical compositions indicate that glassmakers of the 1617th century had full control over the color of the enamel glass paints they made. They mainly used three types of coloring agents, based on Co (dark blue), Mn (purple) and Cu (light-blue or greenblue) as coloring elements. Bluepurple enamel paints were obtained by mixing two different coloring agents. The coloring agent for redpurple enamel, introduced during the 19th century, was colloidal gold embedded in grains of lead glass.

Abstract: The energy-loss near-edge structure (ELNES) spectra of several carbon allotropes (non-hydrogenated and hydrogenated face-centered cubic (FCC) carbon, rhombohedral carbon, glitter, hexagonite and lonsdaleite) are calculated within the supercell-core-excited density functional theory approach. In particular an experimental ELNES spectrum of new diamond (n-diamond) [Konyashin et al., Diamond Relat. Mater. 10, (2001) 99102] is compared with the ELNES spectra of FCC carbon, rhombohedral carbon and the so-called glitter structure. Our calculations show that the ELNES spectrum considered in that publication cannot be that of FCC carbon.