Anisotropy effects in a mixed quantum-classical Heisenberg model in two dimensions

We analyse a specific two dimensional mixed spin Heisenberg model with exchange anisotropy, by means of high temperature expansions and Monte Carlo simulations. The goal is to describe the magnetic properties of the compound (NBu_{4})_{2}Mn_{2}[Cu(opba)]_{3}\cdot 6DMSO\cdot H_{2}O which exhibits a ferromagnetic transition at $T_{c}=15K$. Extrapolating our analysis on the basis of renormalisation group arguments, we find that this transition may result from a very weak anisotropy effect.Comment: 8 pages, 10 Postscript figure

Ribosomal protein synthesis is not regulated at the translational level in Saccharomyces cerevisiae: balanced accumulation of ribosomal proteins L16 and rp59 is mediated by turnover of excess protein.

We have investigated the mechanisms whereby equimolar quantities of ribosomal proteins accumulate and assemble into ribosomes of the yeast Saccharomyces cerevisiae. Extra copies of the cry1 or RPL16 genes encoding ribosomal proteins rp59 or L16 were introduced into yeast by transformation. Excess cry1 or RPL16 mRNA accumulated in polyribosomes in these cells and was translated at wild-type rates into rp59 or L16 proteins. These excess proteins were degraded until their levels reached those of other ribosomal proteins. Identical results were obtained when the transcription of RPL16A was rapidly induced using GAL1-RPL16A promoter fusions, including a construct in which the entire RPL16A 5\u27-noncoding region was replaced with the GAL1 leader sequence. Our results indicate that posttranscriptional expression of the cry1 and RPL16 genes is regulated by turnover of excess proteins rather than autogenous regulation of mRNA splicing or translation. The turnover of excess rp59 or L16 is not affected directly by mutations that inactivate vacuolar hydrolases

Giant ambipolar Rashba effect in a semiconductor: BiTeI

We observe a giant spin-orbit splitting in bulk and surface states of the non-centrosymmetric semiconductor BiTeI. We show that the Fermi level can be placed in the valence or in the conduction band by controlling the surface termination. In both cases it intersects spin-polarized bands, in the corresponding surface depletion and accumulation layers. The momentum splitting of these bands is not affected by adsorbate-induced changes in the surface potential. These findings demonstrate that two properties crucial for enabling semiconductor-based spin electronics -- a large, robust spin splitting and ambipolar conduction -- are present in this material.Comment: 4 pages, 3 figure

Pnictogens Allotropy and Phase Transformation during van der Waals Growth

Pnictogens have multiple allotropic forms resulting from their ns2 np3 valence electronic configuration, making them the only elemental materials to crystallize in layered van der Waals (vdW) and quasi-vdW structures throughout the group. Light group VA elements are found in the layered orthorhombic A17 phase such as black phosphorus, and can transition to the layered rhombohedral A7 phase at high pressure. On the other hand, bulk heavier elements are only stable in the A7 phase. Herein, we demonstrate that these two phases not only co-exist during the vdW growth of antimony on weakly interacting surfaces, but also undertake a spontaneous transformation from the A17 phase to the thermodynamically stable A7 phase. This metastability of the A17 phase is revealed by real-time studies unraveling its thickness-driven transition to the A7 phase and the concomitant evolution of its electronic properties. At a critical thickness of ~4 nm, A17 antimony undergoes a diffusionless shuffle transition from AB to AA stacked alpha-antimonene followed by a gradual relaxation to the A7 bulk-like phase. Furthermore, the electronic structure of this intermediate phase is found to be determined by surface self-passivation and the associated competition between A7- and A17-like bonding in the bulk. These results highlight the critical role of the atomic structure and interfacial interactions in shaping the stability and electronic characteristics of vdW layered materials, thus enabling a new degree of freedom to engineer their properties using scalable processes

Three-potential formalism for the three-body scattering problem with attractive Coulomb interactions

A three-body scattering process in the presence of Coulomb interaction can be decomposed formally into a two-body single channel, a two-body multichannel and a genuine three-body scattering. The corresponding integral equations are coupled Lippmann-Schwinger and Faddeev-Merkuriev integral equations. We solve them by applying the Coulomb-Sturmian separable expansion method. We present elastic scattering and reaction cross sections of the $e^++H$ system both below and above the $H(n=2)$ threshold. We found excellent agreements with previous calculations in most cases.Comment: 12 pages, 3 figure

Structure determination of the (√3×√3)R30° boron phase on the Si(111) surface using photoelectron diffraction

A quantitative structural analysis of the system Si(111)(√3×√3)R30°−B has been performed using photoelectron diffraction in the scanned energy mode. We confirm that the substitutional S5 adsorption site is occupied and show that the interatomic separations to the three nearest-neighbor Si atoms are 1.98(±0.04)Å, 2.14(±0.13)Å, and 2.21(±0.12)Å. These correspond to the silicon atom immediately below the boron atom, the adatom immediately above, and the three atoms to which it is coordinated symmetrically in the first layer

Structural precursor to adsorbate-induced reconstruction: C on Ni(100)

The local structure around adsorbed carbon atoms on Ni(100) has been determined at low coverage as well as in the 0.5 monolayer (2×2)p4g “clock” reconstruction by scanned energy mode photoelectron diffraction. At low coverage, there is no radial strain of the Ni atoms surrounding the adsorbed carbon, contrary to previous suggestions. None of the C-Ni near-neighbor distances are changed by reconstruction, but the Ni-Ni nearest-neighbor distance in the top layer increases significantly, showing that the adsorbate-induced compressive stress is associated with Ni-Ni, rather than Ni-C, repulsion

Fully differential cross sections for photo-double-ionization of D2

We report the first kinematically complete study of the four-body fragmentation of the D2 molecule following absorption of a single photon. For equal energy sharing of the two electrons and a photon energy of 75.5 eV, we observed the relaxation of one of the selection rules valid for He photo-double-ionization and a strong dependence of the electron angular distribution on the orientation of the molecular axis. This effect is reproduced by a model in which a pair of photoionization amplitudes is introduced for the light polarization parallel and perpendicular to the molecular axis

How metal films de-wet substrates - identifying the kinetic pathways and energetic driving forces

We study how single-crystal chromium films of uniform thickness on W(110) substrates are converted to arrays of three-dimensional (3D) Cr islands during annealing. We use low-energy electron microscopy (LEEM) to directly observe a kinetic pathway that produces trenches that expose the wetting layer. Adjacent film steps move simultaneously uphill and downhill relative to the staircase of atomic steps on the substrate. This step motion thickens the film regions where steps advance. Where film steps retract, the film thins, eventually exposing the stable wetting layer. Since our analysis shows that thick Cr films have a lattice constant close to bulk Cr, we propose that surface and interface stress provide a possible driving force for the observed morphological instability. Atomistic simulations and analytic elastic models show that surface and interface stress can cause a dependence of film energy on thickness that leads to an instability to simultaneous thinning and thickening. We observe that de-wetting is also initiated at bunches of substrate steps in two other systems, Ag/W(110) and Ag/Ru(0001). We additionally describe how Cr films are converted into patterns of unidirectional stripes as the trenches that expose the wetting layer lengthen along the W[001] direction. Finally, we observe how 3D Cr islands form directly during film growth at elevated temperature. The Cr mesas (wedges) form as Cr film steps advance down the staircase of substrate steps, another example of the critical role that substrate steps play in 3D island formation

Symmetry Breaking in Few Layer Graphene Films

Recently, it was demonstrated that the quasiparticle dynamics, the layer-dependent charge and potential, and the c-axis screening coefficient could be extracted from measurements of the spectral function of few layer graphene films grown epitaxially on SiC using angle-resolved photoemission spectroscopy (ARPES). In this article we review these findings, and present detailed methodology for extracting such parameters from ARPES. We also present detailed arguments against the possibility of an energy gap at the Dirac crossing ED.Comment: 23 pages, 13 figures, Conference Proceedings of DPG Meeting Mar 2007 Regensburg Submitted to New Journal of Physic