95 research outputs found
Second-order susceptibility from a tight-binding Hamiltonian
Journals published by the American Physical Society can be found at http://journals.aps.org/Using a new formalism that modifies a tight-binding Hamiltonian to include interaction with a time-dependent electromagnetic field, we have obtained an analytical expression for the second-order susceptibility. This expression has been used to calculate the energy dependence of X-(2)(omega) for GaAs. The results are in agreement with previous calculations and with available experimental data. [S0163-1829(98)01848-7]
Enhancement of Friction between Carbon Nanotubes: An Efficient Strategy to Strengthen Fibers
Interfacial friction plays a crucial role in the mechanical properties of
carbon nanotube based fibers, composites, and devices. Here we use molecular
dynamics simulation to investigate the pressure effect on the friction within
carbon nanotube bundles. It reveals that the intertube frictional force can be
increased by a factor of 1.5 ~ 4, depending on tube chirality and radius, when
all tubes collapse above a critical pressure and when the bundle remains
collapsed with unloading down to atmospheric pressure. Furthermore, the overall
cross-sectional area also decreases significantly for the collapsed structure,
making the bundle stronger. Our study suggests a new and efficient way to
reinforce nanotube fibers, possibly stronger than carbon fibers, for usage at
ambient conditions.Comment: revtex, 5 pages, accepted by ACS Nano 10 Dec 200
Virtual money, practices and moral orders in Second Life
Virtual monies present a limit case in debates about money's moral and political entanglements between sociologists, anthropologists, and economists. Digitized virtual monies seem ephemeral, almost ideal typical examples of money as a pure medium of exchange. This paper begins with the premise that virtual monies are as value-laden and morally entangled as any other form of money. This assertion is demonstrated by exploring how one type of virtual money, the Linden dollar (L$), and some of its associated practices are bound up with research participants' moral categories and judgments in the virtual world of Second Life (SL). Participants' accounts of virtual money practices are linked to moral attributes, sometimes in stark âgoodâ or âbadâ dichotomies, but also in more nuanced terms. These framings reproduce classifications of people and practices along a continuum with virtuousness at one end and maliciousness or harm at the other, passing through various states of possible moral dubiousness. For respondents, these two judgments go together; people are what they do with money. As a result, respondents decide what âpeople like thatâ deserve. Evaluating someone's money practices means assessing the person. Participants' accounts of Linden dollar practices overlap with explanations of what SL is and how residents should live there. In SL, money is a form of material culture through which appropriate ways of being in the world are debated and reproduced
Radial Corrugations of Multi-Walled Carbon Nanotubes Driven by Inter-Wall Nonbonding Interactions
We perform large-scale quasi-continuum simulations to determine the stable cross-sectional configurations of free-standing multi-walled carbon nanotubes (MWCNTs). We show that at an inter-wall spacing larger than the equilibrium distance set by the inter-wall van der Waals (vdW) interactions, the initial circular cross-sections of the MWCNTs are transformed into symmetric polygonal shapes or asymmetric water-drop-like shapes. Our simulations also show that removing several innermost walls causes even more drastic cross-sectional polygonization of the MWCNTs. The predicted cross-sectional configurations agree with prior experimental observations. We attribute the radial corrugations to the compressive stresses induced by the excessive inter-wall vdW energy release of the MWCNTs. The stable cross-sectional configurations provide fundamental guidance to the design of single MWCNT-based devices and shed lights on the mechanical control of electrical properties
Dislocations and Grain Boundaries in Two-Dimensional Boron Nitride
A new dislocation structure-square-octagon pair (4|8) is discovered in
two-dimensional boron nitride (h-BN), via first-principles calculations. It has
lower energy than corresponding pentagon-heptagon pairs (5|7), which contain
unfavorable homo-elemental bonds. Based on the structures of dislocations,
grain boundaries (GB) in BN are investigated. Depending on the tilt angle of
grains, GB can be either polar (B-rich or N-rich), constituted by 5|7s, or
un-polar, composed of 4|8s. The polar GBs carry net charges, positive at B-rich
and negative at N-rich ones. In contrast to GBs in graphene which generally
impede the electronic transport, polar GBs have smaller bandgap compared to
perfect BN, which may suggest interesting electronic and optic applications
The influence of size effect on the electronic and elastic properties of diamond films with nanometer thickness
The atomic structure and physical properties of few-layered oriented
diamond nanocrystals (diamanes), covered by hydrogen atoms from both sides are
studied using electronic band structure calculations. It was shown that energy
stability linear increases upon increasing of the thickness of proposed
structures. All 2D carbon films display direct dielectric band gaps with
nonlinear quantum confinement response upon the thickness. Elastic properties
of diamanes reveal complex dependence upon increasing of the number of
layers. All theoretical results were compared with available experimental data.Comment: 16 pages, 5 figures, 3 table
Femtosecond-scale response of GaAs to ultrafast laser pulses RID A-7793-2008
Journals published by the American Physical Society can be found at http://journals.aps.org/We present nonadiabatic simulations of the coherent response of crystalline GaAs irradiated by intense femtosecond-scale laser pulses. Above a threshold fluence, which corresponds to promotion of about 12% of the valence electrons into the conduction band, the lattice is destabilized and the electronic band gap collapses to zero. This confirms the nonthermal nature of recently observed structural changes driven by electronic excitations and occurring during the first few hundred femtoseconds
Thermally-Active Screw Dislocations in Si Nanowires and Nanotubes
International audienceDislocations significantly influence the physical properties of nanomaterials. Nonequilibrium molecular dynamics simulations uncover significant reductions in thermal conductivity when Si nanowires contain axial screw dislocations. The effect can act in combination with other known thermal conductivity limiting mechanisms, and thus can enable the further optimization of the figure of merit for a new family of complex thermoelectric nanomaterials
Thermal transport along the dislocation line in silicon carbidev
International audienceWe elucidate thermal conductivity along the screw dislocation line, which represents a transport direction inaccessible to classical theories. By using equilibrium molecular dynamics simulations, we uncover a Burgers vector dependent thermal conductivity reduction in silicon carbide. The effect is uncorrelated with the classical modeling and originates in the highly deformed core region, which represents a significant source of anharmonic phonon-phonon scattering. High strain reduces the phonon relaxation time, especially in the longitudinal acoustic branches, and creates an effective internal thermal resistance around the dislocation axis. Our results have implications for designing materials useful for high-temperature electronics and thermoelectric applications
- âŠ