Spectral Representation for the Effective Macroscopic Response of a Polycrystal: Application to Third-Order Nonlinear Susceptibility

Erratum: In our paper, we show that the spectral representation for isotropic two-component composites also applies to uniaxial polycrystals. We have learned that this result was, in fact, first conjectured by G.W. Milton. While our derivation is more detailed, our result for the spectral function is the same as Milton's. We very much regret not having been aware of this work at the time of writing our paper. Original abstract: We extend the spectral theory used for the calculation of the effective linear response functions of composites to the case of a polycrystalline material with uniaxially anisotropic microscopic symmetry. As an application, we combine these results with a nonlinear decoupling approximation as modified by Ma et al., to calculate the third-order nonlinear optical susceptibility of a uniaxial polycrystal, assuming that the effective dielectric function of the polycrystal can be calculated within the effective-medium approximation.Comment: v2 includes erratum and the original preprin

Influence of spin polarization on resistivity of a two-dimensional electron gas in Si MOSFET at metallic densities

Positive magnetoresistance (PMR) of a silicon MOSFET in parallel magnetic fields B has been measured at high electron densities n >> n_c where n_c is the critical density of the metal-insulator transition (MIT). It turns out that the normalized PMR curves, R(B)/R(0), merge together when the field is scaled according to B/B_c(n) where B_c is the field in which electrons become fully spin polarized. The values of B_c have been calculated from the simple equality between the Zeeman splitting energy and the Fermi energy taking into account the experimentally measured dependence of the spin susceptibility on the electron density. This extends the range of validity of the scaling all the way to a deeply metallic regime far away from MIT. The subsequent analysis of PMR for low n >~ n_c demonstrated that the merging of the initial parts of curves can bee achieved only with taking into account the temperature dependence of B_c. It is also shown that the shape of the PMR curves at strong magnetic fields is affected by a crossover from a purely two-dimensional (2D) electron transport to a regime where out-of-plane carrier motion becomes important (quasi-three-dimensional regime).Comment: 5 pages, including 6 figures; misprints corrected; Europhys. Lett. (in press

Mechanistic basis for the activation of plant membrane receptor kinases by SERK-family coreceptors.

Plant-unique membrane receptor kinases with leucine-rich repeat ectodomains (LRR-RKs) can sense small molecule, peptide, and protein ligands. Many LRR-RKs require SERK-family coreceptor kinases for high-affinity ligand binding and receptor activation. How one coreceptor can contribute to the specific binding of distinct ligands and activation of different LRR-RKs is poorly understood. Here we quantitatively analyze the contribution of SERK3 to ligand binding and activation of the brassinosteroid receptor BRI1 and the peptide hormone receptor HAESA. We show that while the isolated receptors sense their respective ligands with drastically different binding affinities, the SERK3 ectodomain binds the ligand-associated receptors with very similar binding kinetics. We identify residues in the SERK3 N-terminal capping domain, which allow for selective steroid and peptide hormone recognition. In contrast, residues in the SERK3 LRR core form a second, constitutive receptor-coreceptor interface. Genetic analyses of protein chimera between BRI1 and SERK3 define that signaling-competent complexes are formed by receptor-coreceptor heteromerization in planta. A functional BRI1-HAESA chimera suggests that the receptor activation mechanism is conserved among different LRR-RKs, and that their signaling specificity is encoded in the kinase domain of the receptor. Our work pinpoints the relative contributions of receptor, ligand, and coreceptor to the formation and activation of SERK-dependent LRR-RK signaling complexes regulating plant growth and development

Comparative investigation of NbN and Nb–Si–N films: experiment and theory

NbN and Nb–Si–N films have been deposited by magnetron sputtering of the Nb and Si targets on silicon wafers at various powers supplied to the Nb target. The films have been investigated by an atomic force microscope, X-ray diffraction, X-ray photoelectron spectroscopy, nanoindentaion and microindentation. The NbN films were nanostructured, and the Nb–Si–N films represented an aggregation of δ-NbNx nanocrystallites embedded into the amorphous CSi₃N₄ matrix (nc-δ-NbNx/a-CSi₃N₄). The annealing of the films in vacuum showed that their intensive oxidation occurred at annealing temperature higher than 600 °C. To explain the experimental results on the Nb–Si–N films, first-principles molecular dynamics simulations of the NbN(001)/CSi₃N₄ heterostructures have been carried out.NbN і Nb–Si–N плівки осаджували на кремнієві пластини методом магнетронного розпилення мішеней Nb і Si при різних потужностях розряду на мішені із Nb. Плівки були досліджені за допомогою атомно-силового мікроскопа, дифракції рентгенівських променів, рентгенівської фотоелектронної спектроскопії, нано- і мікроіндентування. NbN плівки були наноструктуровані, тоді як Nb–Si–N плівки являли агрегацію δ-NbNx нанокристалітів, вкраплених в аморфну CSi₃N₄ матрицю (nc-δ-NbNx/ a-CSi₃N₄). Відпал плівок у вакуумі показав, що їх інтенсивне окислення відбувається при температурі вищій, ніж 600 °C. Для пояснення експериментальних результатів по Nb–Si–N плівках проведено моделювання NbN (001)/CSi₃N₄ гетероструктури із перших принципів в рамках молекулярної динаміки.NbN и Nb–Si–N пленки осаждали на кремниевые пластины методом магнетронного распыления мишеней Nb и Si при различных мощностях разряда на мишени с Nb. Пленки были исследованы с помощью атомно-силового микроскопа, дифракции рентгеновских лучей, рентгеновской фотоэлектронной спектроскопии, нано- и микроиндентирования. NbN пленки были наноструктурированные, тогда как Nb–Si–N пленки представляли агрегацию δ-NbNx нанокристаллитов, вкрапленных в аморфную CSi₃N₄ матрицу (nc-δ-NbNx/a-CSi₃N₄). Отжиг пленок в вакууме показал, что их интенсивное окисление происходит при температуре выше, чем 600 °C. Для объяснения экспериментальных результатов по Nb–Si–N пленках проведено моделирование NbN (001)/CSi₃N₄ гетероструктуры из первых принципов в рамках молекулярной динамики.This work was partially supported by STCU Contract No. 5539. The authors are grateful to Dr. Timofejeva, I. I. and Dr. Dub, S. N. for XRD investigations and nanoindentation of the films. The authors are grateful to the directorate of the Summery Institute at Jackson State University for financial support and the possibility to perform large-scale calculations

Electron transport in a slot-gate Si MOSFET

The transversal and longitudinal resistance in the quantum Hall effect regime was measured in a Si MOSFET sample in which a slot-gate allows one to vary the electron density and filling factor in different parts of the sample. In case of unequal gate voltages, the longitudinal resistances on the opposite sides of the sample differ from each other because the originated Hall voltage difference is added to the longitudinal voltage only on one side depending on the gradient of the gate voltages and the direction of the external magnetic field. After subtracting the Hall voltage difference, the increase in longitudinal resistance is observed when electrons on the opposite sides of the slot occupy Landau levels with different spin orientations.Comment: To appear in Europhys. Let

Theory of current-driven motion of Skyrmions and spirals in helical magnets

We study theoretically the dynamics of the spin textures, i.e., Skyrmion crystal (SkX) and spiral structure (SS), in two-dimensional helical magnets under external current. By numerically solving the Landau-Lifshitz-Gilbert equation, it is found that (i) the critical current density of the motion is much lower for SkX compared with SS in agreement with the recent experiment, (ii) there is no intrinsic pinning effect for SkX and the deformation of the internal structure of Skyrmion reduces the pinning effect dramatically, (iii) the Bragg intensity of SkX shows strong time-dependence as can be observed by neutron scattering experiment.Comment: 4 pages, 3 figure

Viral discovery and diversity in trypanosomatid protozoa with a focus on relatives of the human parasite <i>Leishmania</i>.

Knowledge of viral diversity is expanding greatly, but many lineages remain underexplored. We surveyed RNA viruses in 52 cultured monoxenous relatives of the human parasite &lt;i&gt;Leishmania&lt;/i&gt; ( &lt;i&gt;Crithidia&lt;/i&gt; and &lt;i&gt;Leptomonas&lt;/i&gt; ), as well as plant-infecting &lt;i&gt;Phytomonas&lt;/i&gt; &lt;i&gt;Leptomonas pyrrhocoris&lt;/i&gt; was a hotbed for viral discovery, carrying a virus (Leptomonas pyrrhocoris ostravirus 1) with a highly divergent RNA-dependent RNA polymerase missed by conventional BLAST searches, an emergent clade of tombus-like viruses, and an example of viral endogenization. A deep-branching clade of trypanosomatid narnaviruses was found, notable as &lt;i&gt;Leptomonas seymouri&lt;/i&gt; bearing Narna-like virus 1 (LepseyNLV1) have been reported in cultures recovered from patients with visceral leishmaniasis. A deep-branching trypanosomatid viral lineage showing strong affinities to bunyaviruses was termed " &lt;i&gt;Leishbunyavirus&lt;/i&gt; " (LBV) and judged sufficiently distinct to warrant assignment within a proposed family termed " &lt;i&gt;Leishbunyaviridae&lt;/i&gt; " Numerous relatives of trypanosomatid viruses were found in insect metatranscriptomic surveys, which likely arise from trypanosomatid microbiota. Despite extensive sampling we found no relatives of the totivirus &lt;i&gt;Leishmaniavirus&lt;/i&gt; (LRV1/2), implying that it was acquired at about the same time the &lt;i&gt;Leishmania&lt;/i&gt; became able to parasitize vertebrates. As viruses were found in over a quarter of isolates tested, many more are likely to be found in the &gt;600 unsurveyed trypanosomatid species. Viral loss was occasionally observed in culture, providing potentially isogenic virus-free lines enabling studies probing the biological role of trypanosomatid viruses. These data shed important insights on the emergence of viruses within an important trypanosomatid clade relevant to human disease

Dynamics of Gaseous Disks in a Non-axisymmetric Dark Halo

The dynamics of a galactic disk in a non-axisymmetric (triaxial) dark halo is studied in detail using high-resolution, numerical, hydrodynamical models. A long-lived, two-armed spiral pattern is generated for a wide range of parameters. The spiral structure is global, and the number of turns can be two or three, depending on the model parameters. The morphology and kinematics of the spiral pattern are studied as functions of the halo and disk parameters. The spiral structure rotates slowly, and its angular velocity varies quasi-periodically. Models with differing relative halo masses, halo semi-axis ratios, distributions of matter in the disk, Mach numbers in the gaseous component, and angular rotational velocities of their halos are considered.Comment: 22 pages, 11 figure

Extensive molecular tinkering in the evolution of the membrane attachment mode of the Rheb GTPase

Rheb is a conserved and widespread Ras-like GTPase involved in cell growth regulation mediated by the (m)TORC1 kinase complex and implicated in tumourigenesis in humans. Rheb function depends on its association with membranes via prenylated C-terminus, a mechanism shared with many other eukaryotic GTPases. Strikingly, our analysis of a phylogenetically rich sample of Rheb sequences revealed that in multiple lineages this canonical and ancestral membrane attachment mode has been variously altered. The modifications include: (1) accretion to the N-terminus of two different phosphatidylinositol 3-phosphate-binding domains, PX in Cryptista (the fusion being the first proposed synapomorphy of this clade), and FYVE in Euglenozoa and the related undescribed flagellate SRT308; (2) acquisition of lipidic modifications of the N-terminal region, namely myristoylation and/or S-palmitoylation in seven different protist lineages; (3) acquisition of S-palmitoylation in the hypervariable C-terminal region of Rheb in apusomonads, convergently to some other Ras family proteins; (4) replacement of the C-terminal prenylation motif with four transmembrane segments in a novel Rheb paralog in the SAR clade; (5) loss of an evident C-terminal membrane attachment mechanism in Tremellomycetes and some Rheb paralogs of Euglenozoa. Rheb evolution is thus surprisingly dynamic and presents a spectacular example of molecular tinkering