2,471 research outputs found

    First-principles structure determination of interface materials: The NixInAs nickelides

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    This is the final version of the article. Available from American Physical Society via the DOI in this record.We present here a first-principles study of the ternary compounds formed by Ni, In, and As, a material of great importance for self-aligned metallic contacts in next-generation InAs-based MOS transistors. The approach we outline is general and can be applied to study the crystal structure and properties of a host of other new interface compounds. Using the ab initio random structure searching approach we find the previously unknown low-energy structures of NixInAs and assess their stability with respect to the known binary compounds of Ni, In, and As. Guided by experiments, we focus on Ni3InAs and find a rich energy landscape for this stoichiometry. We consider the five lowest-energy structures, with space groups Pmmn, Pbcm, P21/m, Cmcm, and R3¯. The five low-energy structures for Ni3InAs are all found to be metallic and nonmagnetic. By comparison to previously published TEM results we identify the crystal structure observed in experiments to be Cmcm Ni3InAs. We calculate the work function for Cmcm Ni3InAs and, according to the Schottky-Mott model, expect the material to form an Ohmic contact with InAs. We further explicitly consider the interface between Cmcm Ni3InAs and InAs and find it to be Ohmic with an n-type Schottky barrier height of -0.55eV.This work was supported in part by the EPSRC Grants No. EP/G007489/2, No. EP/J010863/1, and No. EP/I009973/1. All data supporting this study are provided as Supplemental Material accompanying this paper [25]. Computational resources from the University College London and London Centre for Nanotechnology Computing Services as well as HECToR and Archer as part of the UKCP consortium are gratefully acknowledged

    Structure-specific mode-resolved phonon coherence and specularity at graphene grain boundaries

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    In spite of their importance for understanding phonon transport phenomena in thin films and polycrystalline solids, the effects of boundary roughness scattering on phonon specularity and co- herence are poorly understood because there is no general method for predicting their dependence on phonon momentum, frequency, branch and boundary morphology. Using the recently formulated atomistic S-matrix method, we develop a theory of boundary roughness scattering to determine the mode-resolved phonon coherence and specularity parameters from the scattering amplitudes. To illustrate the theory, we apply it to phonon scattering in realistic nonsymmetric graphene grain boundary (GB) models derived from atomic structure predictions. The method is validated by comparing its predictions with frequency-resolved results from lattice dynamics-based calculations. We prove that incoherent scattering is almost perfectly diffusive. We show that phonon scattering at the graphene GB is not diffuse although coherence and specularity are significantly reduced for long-wavelength flexural acoustic phonons. Our approach can be generalized to other atomistic boundary models

    Government ideology and international migration

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    We provide the first empirical evidence that differences in government ideology play an important role in the choice of cross-border migration destinations. In absence of first-hand experience, immigrants rely on information about the political landscape of the origin and host countries to form expectations about the context of reception in the host society. We use data on bilateral migration and government ideology for 36 OECD countries between 1990 and 2016. Our analysis shows that bilateral migration flows are higher when the government at the destination is more left-wing than the government at the origin, especially when we consider proximate countries

    Follicular lymphoma genomics

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    The growth kinetics of xenografts of human colorectal tumours in immune deprived mice.

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    The technique of labelled mitoses was used to examine cell proliferation within grafts of human colonic and rectal tumours in immune deprived mice. Most of the data were obtained on the first passage but in some cases up to the third passage was used. It was found to be difficult to obtain precise kinetic data on this type of tumour material, but the results did allow some estimates to be made, particularly of the duration of the G2 and S phases of the mitotic cycle. The average G2 duration was 6 h and the average S phase was 14 h. It is concluded that whilst xenografts may differ in a number of respects from the tumour in the patient, they nevertheless constitute a type of experimental tumour that is worthy of further study

    Experimental evidence of new tetragonal polymorphs of silicon formed through ultrafast laser-induced confined microexplosion

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    Ordinary materials can transform into novel phases at extraordinary high pressure and temperature. The recently developed method of ultrashort laser-induced confined microexplosions initiates a non-equilibrium disordered plasma state. Ultra-high quenching rates overcome kinetic barriers to the formation of new metastable phases, which are preserved in the surrounding pristine crystal for subsequent exploitation. Here we demonstrate that confined microexplosions in silicon produce several metastable end phases. Comparison with an ab initio random structure search reveals six energetically competitive potential phases, four tetragonal and two monoclinic structures. We show the presence of bt8 and st12, which have been predicted theoretically previously, but have not been observed in nature or in laboratory experiments. In addition, the presence of the as yet unidentified silicon phase, Si-VIII and two of our other predicted tetragonal phases are highly likely within laser-affected zones. These findings may pave the way for new materials with novel and exotic properties
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