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At least 145 records · Page 8Linked to original sources

arXiv2008

Electronic structures of defective BN nanotubes under transverse electric fields

We investigate the electronic structures of some defective boron nitride nanotubes (BNNTs) under transverse electric fields within density-functional theory. (16,0) BNNTs with antisite, carbon substitution, single vacancy, and Stone-Wales 5775 defects are studied. Under transverse electric fields, the band gaps of the defective BNNTs are reduced, similar to the pristine ones. The energy levels of the defect states vary with the transverse electric field directions, due to the different electrostatic potential shift at the defect sites induced by the electric fields. Therefore, besides electronic structure and optical property engineering, the transverse electric field can be used to identify the defect positions in BNNTs.

cond-mat.mtrl-sci↗

arXiv2009

Single-band tight-binding parameters for Fe-MgO-Fe magnetic heterostructures

We present a computationally efficient transferable single-band tight-binding model (SBTB) for spin polarized transport in heterostructures with an effort to capture the band structure effects. As an example, we apply it to study transport through Fe-MgO-Fe(100) magnetic tunnel junction devices. We propose a novel approach to extract suitable tight-binding parameters for a material by using the energy resolved transmission as the benchmark, which inherently has the bandstructure effects over the two dimensional transverse Brillouin zone. The SBTB parameters for each of the four symmetry bands for bcc Fe(100) are first proposed which are complemented with the transferable tight-binding parameters for the MgO tunnel barrier for the Delta_1 and Delta_5 bands. The non-equilibrium Green's function formalism is then used to calculate the transport. Features like I-V characteristics, voltage dependence and the barrier width dependence of the tunnel magnetoresistance ratio are captured quantitatively and the trends match well with the ones observed by ab initio methods.

cond-mat.mes-hall↗

arXiv2008

Non-equilibrium Green's function based single-band tight-binding model for Fe-MgO-Fe magnetic tunnel junction devices

Motivated by observation of very high tunnel magnetoresistance (TMR) in Fe-MgO-Fe magnetic tunnel junction devices, we propose a theoretical model for these devices based on a single-band tight-binding approximation. An effort is made to capture the band dispersions over the two dimensional transverse Brillouin zone. In the transport direction, spin dependent Hamiltonian is prescribed for Delta_1 and Delta_5 bands. Non-equilibrium Green's function formalism is then used to calculate transport. Features like voltage dependence of TMR are captured quantitatively within this simple model and the trends match well with the ones predicted by ab-initio methods and experiments.

cond-mat.mes-hall↗

arXiv2008

Approach of the constitutive material behaviour of textile composites through simulation

A complete approach for the determination of the complex constitutive behaviour of textile composites through finite element simulation is presented in this paper. In this work, simulations of different loading cases are carried out on small samples of textile composites, taking into account all individual fibers of the fabric and interactions taking place between them. The most delicate issue is related to the modelling of contact-friction interactions between fibers,and the solution of the nonlinear problem that follows up by the means of robust algorithms. The efficiency of these algorithms, and the increase of capacities of high performance computing, allow to simulate samples made of several hundreds of interlocked fibers.

cond-mat.soft↗

arXiv2008

A finite element approach of the behaviour of woven materials at microscopic scale

A finite element simulation of the mechanical behaviour of woven textile materials at the scale of individual fibers is proposed in this paper. The aim of the simulation is to understand and identify phenomena involved at different scales in such materials. The approach considers small patches of woven textile materials as collections of fibers. Fibers are modelled by 3D beam elements, and contact-friction interactions are considered between them. An original method for the detection of contacts, and the use of efficient algorithms to solve the nonlinearities of the problem, allow to handle patches made of few hundreds of fibers. The computation of the unknown initial configuration of the woven structure is carried out by simulating the weaving process. Various loading cases can then be applied to the studied patches to identify their mechanical characteristics. To approach the mesoscopic behaviour of yarns, 3D strains are calculated at the scale of yarns, as post-processing. These strains display strong inhomogeinities, which raises the question of using continuous models at the scale of yarns.

cond-mat.soft↗

arXiv2008

Antibonding ground states in semiconductor artificial molecules

The spin-orbit interaction is a crucial element of many semiconductor spintronic technologies. Here we report the first experimental observation, by magneto-optical spectroscopy, of a remarkable consequence of the spin-orbit interaction for holes confined in the molecular states of coupled quantum dots. As the thickness of the barrier separating two coupled quantum dots is increased, the molecular ground state changes character from a bonding orbital to an antibonding orbital. This result is counterintuitive, and antibonding molecular ground states are never observed in natural diatomic molecules. We explain the origin of the reversal using a four band k.p model that has been validated by numerical calculations that account for strain. The discovery of antibonding molecular ground states provides new opportunities for the design of artificially structured materials with complex molecular properties that cannot be achieved in natural systems.

cond-mat.mtrl-sci↗

arXiv2008

Mesoscopic study on historic masonry

This paper presents a comprehensive approach to the evaluation of macroscopic material parameters for natural stone and quarry masonry. To that end, a reliable non-linear material model on a meso-scale is developed to cover the random arrangement of stone blocks and quasi-brittle behaviour of both basic components, as well as the impaired cohesion and tensile strength on the interface between the blocks and mortar joints. The paper thus interrelates the following three problems: (i) definition of a suitable periodic unit cell (PUC) representing a particular masonry structure; (ii) derivation of material parameters of individual constituents either experimentally or running a mixed numerical-experimental problem; (iii) assessment of the macroscopic material parameters including the tensile and compressive strengths and fracture energy.

cond-mat.mtrl-sci↗

arXiv2008

Force focusing in confined fibers and sheets

A thin fiber or sheet curled into a circular container has a detached region whose shape and force ratios are independent of material properties and container radius. We compute this shape and compare it with experiments. The discrete forces acting at either end of the detached region have a ratio that depends only on the length of the fiber or sheet relative to the circle radius. We calculate this force ratio in three regimes of circle radius.

cond-mat.mtrl-sci↗

arXiv2008

Site-selective conductance of sidewall functionalized carbon nanotubes

We use DFT to study the effect of molecular adsorbates on the conductance of metallic carbon nanotubes. The five molecules considered (NO2, NH2, H, COOH, OH) lead to similar scattering of the electrons. The adsorption of a single molecule suppresses one of the two available channels of the CNT at low bias conductance. If more molecules are adsorbed on the same sublattice, the remaining open channel can be blocked or not, depending on the relative position of the adsorbates. If a simple geometric condition is fulfilled this channel is still open, even after adsorbing an arbitrary number of molecules.

cond-mat.mes-hall↗

arXiv2008

Effective electromagnetic theory for dielectric media

Light in a dielectric medium moves slower than in vacuum. The corresponding electromagnetic field equations are then no longer invariant under ordinary Lorentz transformations, but only under such transformations corresponding to this reduced velocity. Based on this physical symmetry, an effective theory for low-energy electromagnetic phenomena in dielectrics is constructed. It has none of the problems of the old formulations of Minkowski and Abraham. Dispersion in the optical regime and the Kerr effect arise in a natural way from higher order interaction terms. The effective field theory is quantized by standard methods and quantum corrections can be calculated in a systematic way. Thus it relates many different classical and quantum optical phenomena into a unified description.

quant-ph↗

arXiv2008

Long-range antiferromagnetic interactions in ZnFe2O4 and CdFe2O4

For the first time, the Fe-Fe interactions in the geometrically frustrated antiferromagnetic systems of zinc and cadmium ferrites are determined quantitatively by the first-principles methods of density functional theory. Both the generalized gradient approximation (GGA) as well as GGA plus the onesite Coulomb interaction (GGA+U) are considered for the exchange-correlation energy functional. The interactions up to third neighbours are found to be all antiferromagnetic for both materials, regardless of which approximation scheme (GGA or GGA+U) is used. Surprisingly, the thirdneighbour interactions are estimated to be much stronger than the second-neighbour interactions and on the same order in magnitude as the first-neighbour interactions.

cond-mat.mtrl-sci↗

arXiv2008

Chemical Gas Sensors Based On Nanowires

Chemical gas sensors based on nanowires can find a wide range of applications in clinical assaying, environmental emission control, explosive detection, agricultural storage and shipping, and workplace hazard monitoring. Sensors in the forms of nanowires are expected to have significantly enhanced performance due to high surface-volume ratio and quasi-one-dimensional confinement in nanowires. Indeed, chemical gas sensors based on nanowires with a ppb level sensitivity have been demonstrated. In this review, the fundamental aspects on (i) methods of nanowire synthesis (ii) performance of nanowire sensors, (iii) chemiresistors, transistor sensors, and their sensing mechanism, and (iv) assembly technologies will be summarized and discussed. The prospects of the future research on chemical gas sensors based on nanowires will be also addressed.

cond-mat.mtrl-sci↗

arXiv2008

A Mems Tribometer for on-Chip Measurements of Dynamic Friction Loops

A MEMS based tribometer has been developed that can be read out with nanometer and nano-Newton resolution, approaching the resolution and sensitivity of a friction force microscope (FFM). It can be used to study friction of MEMS device sidewall surfaces. We find repeatable but irregular stick-slip behavior related to the surface roughness or surface modification (wear), depending on the normal force between the sliding surfaces. In addition, we show that normal force modulation can decrease the apparent friction even though the average normal force remains the same. This may be a technologically viable way to reduce friction and wear in microscopic contacts.

cond-mat.mtrl-sci↗

arXiv2008

Resistance Quenching in Graphene Interconnects

We investigated experimentally the high-temperature electrical resistance of graphene interconnects. The test structures were fabricated using the focused ion beam from the single and bi-layer graphene produced by mechanical exfoliation. It was found that as temperature increases from 300 to 500K the resistance of the single- and bi-layer graphene interconnects drops down by 30% and 70%, respectively. The quenching and temperature dependence of the resistance were explained by the thermal generation of the electron-hole pairs and acoustic phonon scattering. The obtained results are important for the proposed applications of graphene as interconnects in integrated circuits.

cond-mat.mtrl-sci↗

arXiv2008

Microtribological Property of Vertically Aligned Carbon Nanotube Film

Microtribological properties of vertically-aligned carbon-nanotube (VACNT) films have been studied. Adhesion forces were obtained by measuring force-displacement curves. Friction experiments were conducted in reciprocating sliding configurations. Effects of tip radius, applied force, scan speed, and relative humidity were investigated. A model of the friction of VACNT film is discussed on the basis of in-situ tribological experiments inside a scanning electron microscope (SEM).

cond-mat.mtrl-sci↗

arXiv2008

A Liquid Film Motor

It is well known that electro-hydrodynamical effects in freely suspended liquid films can flow the liquid. Here we report a purely electrically driven rotation in water and some other liquid suspended films with full control on the velocity and the chirality of the rotating vortices. The device, which is called ``film motor'', consists of a quasi two-dimensional electrolysis cell in an external in-plane electric field, crossing the mean electrolysis current density. If either the external field or the electrolysis voltage exceeds some threshold (while the other one is not zero), the liquid film begins to rotate. The device works perfectly with both DC and AC fields.

cond-mat.soft↗

arXiv2008

Pressure Effects on the Atomic and Electronic Structure of Aligned Small Diameter Carbon Nanotubes

Density functional methods have been used to calculate the electronic properties of aligned smalldiameter single-walled carbon nanotubes under hydrostatic pressures. Abrupt pressure induced semiconductor-metal and metal-semiconductor transitions concomitant with changes in structure are observed. Novel and unexpected unit cell nanotube cross-sections are found. These tubes are observed to form interlinking structures at very high pressures. The large changes in electronic structure and the ability of different small diameter nanotubes to respond in different pressure regimes suggest their use in high pressure multiple switching devices.

cond-mat.mtrl-sci↗

arXiv2010

K-edge XANES of substitutional and interstitial Mn atoms in (Ga,Mn)As

This work reports theoretical and experimental study of the X-ray absorption near-edge structure (XANES) at the Mn K-edge in (Ga,Mn)As diluted magnetic semiconductors. The spectra have been calculated from the first-principles using FLAPW including the core-hole effect, a special attention has been paid to consequences of coexistence of Mn impurities in substitutional and tetrahedral interstitial positions. We have performed quantitative component analysis of experimental spectra collected on the (Ga,Mn)As samples before/after annealing and etching, with the aim to determine the proportion of Mn impurity configurations. Comparison of the experimental data with theoretical computations indicates that even after annealing and etching some Mn atoms still reside in interstitial sites, although the concentration of interstitial defects has been reduced by annealing.

cond-mat.mtrl-sci↗