3,258 research outputs found

    Measurements of single top-quark production in pp collisions by the CMS experiment

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    A summary of studies of electroweak top-quark production performed by the CMS collaboration is presented. The results include measurements of production cross sections, extraction of the value of ∣Vtb∣|V_{tb}|, determination of W-boson helicity in top-quark decays and top-quark spin asymmetry, and a search for anomalous couplings in the Wtb vertex. No deviations from predictions of the standard model are found.Comment: Proceedings of the 8th International Workshop on the CKM Unitarity Triangle (CKM 2014), Vienna, Austria, September 8-12, 201

    Identification of signal events in a search for H→bbˉH\to b\bar b produced in association with single top quarks

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    The CMS collaboration has performed a search for an associated production of Higgs bosons and single top quarks in the H→bbˉH\to b\bar b decay channel. The measurement is challenged by a complex multijet final state and an overwhelming background from top-quark pair production. These proceedings address a particular aspect of the search and describe the procedure deployed to discriminate the signal process from backgrounds.Comment: Presented at 7th international workshop on top quark physics (TOP2014); 4 pages, 3 figure

    Nanotube-based scanning rotational microscope

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    A scheme of the scanning rotational microscope is designed. This scheme is based on using carbon nanotubes simultaneously as a probe tip and as a bolt/nut pair which converts translational displacements of two piezo actuators into pure rotation of the probe tip. First-principles calculations of the interaction energy between movable and rotational parts of the microscope confirms the capability for its operation. The scanning rotational microscope with a chemically functionalized nanotube-based tip can be used to study how the interaction between individual molecules or a molecule and a surface depends on their relative orientation.Comment: 4 pages, 3 figure

    Dynamics of a coefficient of friction during non-stationary sliding of a parabolic indenter on visco-elastic foundation

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    This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in AIP Conference Proceedings 1783, 020041 (2016) and may be found at https://doi.org/10.1063/1.4966334.We have studied the coefficient of friction between a rigid parabolic indenter and a visco-elastic Kelvin foundation under step-wise change of sliding velocity. We have obtained analytical estimations for normal and tangential forces in a contact and their limiting values during transition process that occurs after a jump of sliding velocity. The results of numerical simulation are in good agreement with analytical estimates

    A model of fretting wear in the contact of an axisymmetric indenter and a visco-elastic half-space

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    This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in AIP Conference Proceedings 1683, 020040 (2015) and may be found at https://doi.org/10.1063/1.4932730.We propose a simple and efficient model of wear of axially symmetric bodies in contact with a visco-elastic foundation based on the method of dimensionality reduction. The results of simulation of wear of a parabolic indenter have been demonstrated. It has been shown that dissipation due to viscosity of a material leads to increase the size of the worn region of an indenter. The noted effect is conditioned with an increase of effective shear modulus of visco-elastic material under sufficiently high velocities of tangential loading. The model can be generalized to a wide range of materials with complex visco-elastic properties

    Comparison of performance of van der Waals-corrected exchange-correlation functionals for interlayer interaction in graphene and hexagonal boron nitride

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    Exchange-correlation functionals with corrections for van der Waals interactions (PBE-D2, PBE-D3, PBE-D3(BJ), PBE-TS, optPBE-vdW and vdW-DF2) are tested for graphene and hexagonal boron nitride, both in the form of bulk and bilayer. The characteristics of the potential energy surface, such as the barrier to relative sliding of the layers and magnitude of corrugation, and physically measurable properties associated with relative in-plane and out-of-plane motion of the layers including the shear modulus and modulus for axial compression, shear mode frequency and frequency of out-of-plane vibrations are considered. The PBE-D3(BJ) functional gives the best results for the stackings of hexagonal boron nitride and graphite that are known to be ground-state from the experimental studies. However, it fails to describe the order of metastable states of boron nitride in energy. The PBE-D3 and vdW-DF2 functionals, which reproduce this order correctly, are identified as the optimal choice for general studies. The vdW-DF2 functional is preferred for evaluation of the modulus for axial compression and frequency of out-of-plane vibrations, while the PBE-D3 functional is somewhat more accurate in calculations of the shear modulus and shear mode frequency. The best description of the latter properties, however, is achieved also using the vdW-DF2 functional combined with consideration of the experimental interlayer distance. In the specific case of graphene, the PBE-D2 functional works very well and can be further improved by adjustment of the parameters.Comment: 22 pages, 4 figue

    Dislocations in stacking and commensurate-incommensurate phase transition in bilayer graphene and hexagonal boron nitride

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    Dislocations corresponding to a change of stacking in two-dimensional hexagonal bilayers, graphene and boron nitride, and associated with boundaries between commensurate domains are investigated using the two-chain Frenkel-Kontorova model on top of ab initio calculations. Structural transformations of bilayers in which the bottom layer is stretched and the upper one is left to relax freely are considered for gradually increased elongation of the bottom layer. Formation energies of dislocations, dislocation width and orientation of the boundary between commensurate domains are analyzed depending on the magnitude and direction of elongation. The second-order phase transition from the commensurate phase to the incommensurate one with multiple dislocations is predicted to take place at some critical elongation. The order parameter for this transition corresponds to the density of dislocations, which grows continuously upon increasing the elongation of the bottom layer above the critical value. In graphene and metastable boron nitride with the layers aligned in the same direction, where elementary dislocations are partial, this transition, however, is preceded by formation of the first dislocation at the elongation smaller than the critical one. The phase diagrams including this intermediate state are plotted in coordinates of the magnitude and direction of elongation of the bottom layer.Comment: 15 pages, 9 figure
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