Magnetoconductance in quantum waveguides with inhomogeneous magnetic fields

We discuss the properties of magnetotransport of electrons in quantum waveguides (QWs) in the presence of laterally inhomogeneous magnetic fields perpendicular to the QW plane. The inhomogeneous magnetic fields can be produced by the deposition, on top of a heterostructure, of ferromagnetic stripes with magnetization perpendicular and parallel to the two-dimensional electron gas. It is found that the magnetoconductance in such a device as a function of the Fermi energy of electrons exhibits square-wave-like oscillations that are strongly dependent on the geometrical arrangement and magnetic configurations in the QWs. Different dispersions can be observed. From analyses of the magnetic effective potential, the dispersion relations can be understood well. In some magnetic modulations, the energy spectrum exhibits a bump superimposed upon every bulk Landau level. It is these oscillatory structures in dispersions that lead to square-wave-like shaped modulation in magnetoconductance, which may serve as an energy filter of electrons. ©1999 American Institute of Physics.published_or_final_versio

Creation of partial band gaps in anisotropic photonic-band-gap structures

The photonic-band-gap (PBG) structure composed of an anisotropic-dielectric sphere in uniform dielectric medium is studied by solving Maxwell’s equations using the plane-wave expansion method. In particular, for a uniaxial material with large principal refractive indices and sufficient anisotropy between them, the photonic band structures possess a full band gap in the whole Brillouin zone for a diamond lattice. Furthermore, in the 1/3 partial Brillouin zone where the Bloch wave vector has a dominant component along the extraordinary axis of uniaxial sphere, the photonic band structures are found to exhibit full band gaps for all the other lattices such as face-centered-cubic, body-centered-cubic, and simple-cubic lattices, although a complete band gap does not open in the whole Brillouin zone. The partial band gaps persist at a very low filling fraction of uniaxial sphere. This phenomenon is attributed to the breakdown of the photonic band degeneracy at high-symmetry points of the Brillouin zone by the anisotropy of material dielectricity. The combination of such an anisotropic PBG structure with the self-arrangement technique of colloidal crystal may provide a possible way to fabricate the three-dimensional photonic crystal in visible and infrared regimes. The application of a strong electric field may bring into alignment the extraordinary axis of uniaxial sphere as this configuration of spheres is most favorable thermodynamically.published_or_final_versio

Parity anomaly of bound states and optical properties in semiconductor superlattices with structural defects

We investigate the property of the bound states with an infinite number of classical turning points in the semiconductor superlattices (SL’s) with the double-barrier and double-well structural defects. The anomalous parity sequence of these bound states violating the conventional parity rule is predicted. A physical mechanism on the origin of the bound states in the structural defect SL’s, different from the mechanism addressed in the previous literature, is proposed. Based upon it, the parity anomaly can be interpreted very well. In addition, we also study the property of the optical transition in this system, for instance, the evaluation of the optical transition probability from the bound states to the bound states, from the bound states to the delocalized scattering states, and from the scattering states to other scattering states, in detail.published_or_final_versio

Electronic states and magnetotransport in quantum waveguides with nonuniform magnetic fields

The electronic states and magnetotransport properties of quantum waveguides (QW’s) in the presence of nonuniform magnetic fields perpendicular to the QW plane are investigated theoretically. It is found that the magnetoconductance of those structures as a function of Fermi energy exhibits stepwise variation or square-wave-like oscillations, depending on the specific distributions (both in magnitude and direction) of nonuniform magnetic fields in QW’s. We have investigated the dual magnetic strip structures and three magnetic strip structures. The character of the magnetotransport is closely related to the effective magnetic potential and the energy-dispersion spectrum of electron in the structures. It is found that dispersion relations seem to be combined by different sets of dispersion curves that belong to different individual magnetic subwaveguides. The magnetic effective potential leads to the coupling of states and the substantial distortion of the original dispersion curves at the interfaces in which the abrupt change of magnetic fields appears. Magnetic scattering states are created. Only in some three magnetic strip structures, these scattering states produce the dispersion relations with oscillation structures superimposed on the bulk Landau levels. It is the oscillatory behavior in dispersions that leads to the occurrence of square-wave-like modulations in conductance.published_or_final_versio

Tunneling transmission in two quantum wires coupled by a magnetically defined barrier

A numerical analysis of an electron waveguide coupler based on two quantum wires coupled by a magnetically defined barrier is presented with the use of the scattering-matrix method. For different geometry parameters and magnetic fields, tunneling transmission spectrum is obtained as a function of the electron energy. Different from that of conventional electron waveguide couplers, the transmission spectrum of the magnetically coupled quantum wires does not have the symmetry with regard to those geometrically symmetrical ports. It was found that the magnetic field in the coupling region drastically enhances the coupling between the two quantum wires for one specific input port while it weakens the coupling for the other input port. The results can be well understood by the formation of the edge states in the magnetically defined barrier region. Thus, whether these edge states couple or decouple to the electronic propagation modes in the two quantum wires, strongly depend on the relative moving directions of electrons in the propagating mode in the input port and the edge states in the magnetic region. This leads to a big difference in transmission coefficients between two quantum wires when injecting electrons via different input ports. Two important coupler specifications, the directivity and uniformity, are calculated which show that the system we considered behaves as a good quantum directional coupler. ©1997 American Institute of Physics.published_or_final_versio

Rigorous electromagnetic analysis of a microcylindrical axilens with long focal depth and high transverse resolution

We first present nonparaxial designs for a microcylindrical axilens with different long focal depths and rigorously analyze electromagnetic field distributions of the axilens using integral equations and the boundary-element method. Numerical results show that the designed axilenses indeed have the special feature of attaining a long focal depth while keeping high transverse resolution for numerical apertures of 2.4, 2.0, and 1.0. The ratio between the extended focal depth of the designed axilens and the focal depth of the conventional focal lens is 1.41, the corresponding maximal extended focal depth of the axilens can reach 28 μm, and the spot size of the focal beam is ∼10 μm over the focal range. © 2001 Optical Society of America.published_or_final_versio

Electronic structures of free-standing nanowires made from indirect bandgap semiconductor gallium phosphide

We present a theoretical study of the electronic structures of freestanding nanowires made from gallium phosphide (GaP)--a III-V semiconductor with an indirect bulk bandgap. We consider [001]-oriented GaP nanowires with square and rectangular cross sections, and [111]-oriented GaP nanowires with hexagonal cross sections. Based on tight binding models, both the band structures and wave functions of the nanowires are calculated. For the [001]-oriented GaP nanowires, the bands show anti-crossing structures, while the bands of the [111]-oriented nanowires display crossing structures. Two minima are observed in the conduction bands, while the maximum of the valence bands is always at the $\Gamma$-point. Using double group theory, we analyze the symmetry properties of the lowest conduction band states and highest valence band states of GaP nanowires with different sizes and directions. The band state wave functions of the lowest conduction bands and the highest valence bands of the nanowires are evaluated by spatial probability distributions. For practical use, we fit the confinement energies of the electrons and holes in the nanowires to obtain an empirical formula.Comment: 19 pages, 10 figure

Bacteria-Killing Type IV Secretion Systems

Bacteria have been constantly competing for nutrients and space for billions of years. During this time, they have evolved many different molecular mechanisms by which to secrete proteinaceous effectors in order to manipulate and often kill rival bacterial and eukaryotic cells. These processes often employ large multimeric transmembrane nanomachines that have been classified as types I–IX secretion systems. One of the most evolutionarily versatile are the Type IV secretion systems (T4SSs), which have been shown to be able to secrete macromolecules directly into both eukaryotic and prokaryotic cells. Until recently, examples of T4SS-mediated macromolecule transfer from one bacterium to another was restricted to protein-DNA complexes during bacterial conjugation. This view changed when it was shown by our group that many Xanthomonas species carry a T4SS that is specialized to transfer toxic bacterial effectors into rival bacterial cells, resulting in cell death. This review will focus on this special subtype of T4SS by describing its distinguishing features, similar systems in other proteobacterial genomes, and the nature of the effectors secreted by these systems and their cognate inhibitor

Functional Diversification of Paralogous Transcription Factors via Divergence in DNA Binding Site Motif and in Expression

BACKGROUND: Gene duplication is a major driver of evolutionary innovation as it allows for an organism to elaborate its existing biological functions via specialization or diversification of initially redundant gene paralogs. Gene function can diversify in several ways. Transcription factor gene paralogs in particular, can diversify either by changes in their tissue-specific expression pattern or by changes in the DNA binding site motif recognized by their protein product, which in turn alters their gene targets. The relationship between these two modes of functional diversification of transcription factor paralogs has not been previously investigated, and is essential for understanding adaptive evolution of transcription factor gene families. FINDINGS: Based on a large set of human paralogous transcription factor pairs, we show that when the DNA binding site motifs of transcription factor paralogs are similar, the expressions of the genes that encode the paralogs have diverged, so in general, at most one of the paralogs is highly expressed in a tissue. Moreover, paralogs with diverged DNA binding site motifs tend to be diverged in their function. Conversely, two paralogs that are highly expressed in a tissue tend to have dissimilar DNA binding site motifs. We have also found that in general, within a paralogous family, tissue-specific decrease in gene expression is more frequent than what is expected by chance. CONCLUSIONS: While previous investigations of paralogous gene diversification have only considered coding sequence divergence, by explicitly quantifying divergence in DNA binding site motif, our work presents a new paradigm for investigating functional diversification. Consistent with evolutionary expectation, our quantitative analysis suggests that paralogous transcription factors have survived extinction in part, either through diversification of their DNA binding site motifs or through alterations in their tissue-specific expression levels

Modes of Gene Duplication Contribute Differently to Genetic Novelty and Redundancy, but Show Parallels across Divergent Angiosperms

BACKGROUND: Both single gene and whole genome duplications (WGD) have recurred in angiosperm evolution. However, the evolutionary effects of different modes of gene duplication, especially regarding their contributions to genetic novelty or redundancy, have been inadequately explored. RESULTS: In Arabidopsis thaliana and Oryza sativa (rice), species that deeply sample botanical diversity and for which expression data are available from a wide range of tissues and physiological conditions, we have compared expression divergence between genes duplicated by six different mechanisms (WGD, tandem, proximal, DNA based transposed, retrotransposed and dispersed), and between positional orthologs. Both neo-functionalization and genetic redundancy appear to contribute to retention of duplicate genes. Genes resulting from WGD and tandem duplications diverge slowest in both coding sequences and gene expression, and contribute most to genetic redundancy, while other duplication modes contribute more to evolutionary novelty. WGD duplicates may more frequently be retained due to dosage amplification, while inferred transposon mediated gene duplications tend to reduce gene expression levels. The extent of expression divergence between duplicates is discernibly related to duplication modes, different WGD events, amino acid divergence, and putatively neutral divergence (time), but the contribution of each factor is heterogeneous among duplication modes. Gene loss may retard inter-species expression divergence. Members of different gene families may have non-random patterns of origin that are similar in Arabidopsis and rice, suggesting the action of pan-taxon principles of molecular evolution. CONCLUSION: Gene duplication modes differ in contribution to genetic novelty and redundancy, but show some parallels in taxa separated by hundreds of millions of years of evolution