Designer Dirac fermions and topological phases in molecular graphene

The observation of massless Dirac fermions in monolayer graphene has generated a new area of science and technology seeking to harness charge carriers that behave relativistically within solid-state materials. Both massless and massive Dirac fermions have been studied and proposed in a growing class of Dirac materials that includes bilayer graphene, surface states of topological insulators and iron-based high-temperature superconductors. Because the accessibility of this physics is predicated on the synthesis of new materials, the quest for Dirac quasi-particles has expanded to artificial systems such as lattices comprising ultracold atoms. Here we report the emergence of Dirac fermions in a fully tunable condensed-matter system-molecular graphene-assembled by atomic manipulation of carbon monoxide molecules over a conventional two-dimensional electron system at a copper surface. Using low-temperature scanning tunnelling microscopy and spectroscopy, we embed the symmetries underlying the two-dimensional Dirac equation into electron lattices, and then visualize and shape the resulting ground states. These experiments show the existence within the system of linearly dispersing, massless quasi-particles accompanied by a density of states characteristic of graphene. We then tune the quantum tunnelling between lattice sites locally to adjust the phase accrual of propagating electrons. Spatial texturing of lattice distortions produces atomically sharp p-n and p-n-p junction devices with two-dimensional control of Dirac fermion density and the power to endow Dirac particles with mass. Moreover, we apply scalar and vector potentials locally and globally to engender topologically distinct ground states and, ultimately, embedded gauge fields, wherein Dirac electrons react to 'pseudo' electric and magnetic fields present in their reference frame but absent from the laboratory frame. We demonstrate that Landau levels created by these gauge fields can be taken to the relativistic magnetic quantum limit, which has so far been inaccessible in natural graphene. Molecular graphene provides a versatile means of synthesizing exotic topological electronic phases in condensed matter using tailored nanostructures.     

论人的潜能与素质教育

我们每个人身上都存在着巨大的潜能,如何认识、发掘、更好地利用,素质教育就是发挥人的潜能的最佳途径.     

基于数学建模的高校专业教学改革

阐述数学建模及对高校专业教学改革的深刻背景和现实意义,探讨人才培养模式改革价值链模型,分析广西高校数学建模教学与竞赛活动,提出了基于数学建模的高校专业教学改革思路.     

几内亚共和国博凯地区红土型铝土矿地质特征和成矿机理初探

几内亚博凯地区红土型铝土矿为基性火山岩经风化淋滤形成的残积型铝土矿,其地质特征为地表形成铁铝质风化壳（铁帽）,下部为松散状铁红土,其形成机理为：在植被发育,微生物滋生的炎热潮湿环境,随着富铝质岩石的分解,“SiO2”在碱性溶液环境中形成硅酸胶体活化迁移,铁铝沉淀富集形成红土型铝土矿。     

Aminotransferase study ofNeurospora crassa ser-3 versus wild type ST4A

Summary TheNeurospora crassa serine auxotrophser-3 and the wild type ST4A were compared with respect to their phosphoserine and serine aminotransferase activities. The results obtained indicate no deficiency of either of these enzymes in the mutant strain.This work was supported in part by a grant to D.G. from the California Foundation for Biochemical Research. The authors also wish to thank Dr Robert G. Wolcott, formerly of the Department of Chemistry, California State University, Northridge, for his valuable advice and for the use of his department's liquid scintillation counting system. We acknowledge with appreciation the skilled technical assistance of Mr Tom Harper.     

Problem Structuring Methods: A Review of Advances Over the Last Decade

Systemic Practice and Action Research - The Problem Structuring Methods (PSMs) are a set of interactive and participatory modeling approaches for dealing with unstructured complex problems, which...