Strain-induced energy band gap opening in two-dimensional bilayered silicon film

This work presents a theoretical study of the structural and electronic properties of bilayered silicon films under in-plane biaxial strain/stress using density functional theory. Atomic structures of the two-dimensional silicon films are optimized by using both the local-density approximation and generalized gradient approximation. In the absence of strain/stress, five buckled hexagonal honeycomb structures of the bilayered silicon film have been obtained as local energy minima and their structural stability has been verified. These structures present a Dirac-cone shaped energy band diagram with zero energy band gaps. Applying tensile biaxial strain leads to a reduction of the buckling height. Atomically flat structures with zero bucking height have been observed when the AA-stacking structures are under a critical biaxial strain. Increase of the strain between 10.7% ~ 15.4% results in a band-gap opening with a maximum energy band gap opening of ~168.0 meV obtained when 14.3% strain is applied. Energy band diagram, electron transmission efficiency, and the charge transport property are calculated.Comment: 18 pages, 5 figures, 1 tabl

Microstructural strain energy of α-uranium determined by calorimetry and neutron diffractometry

The microstructural contribution to the heat capacity of α-uranium was determined by measuring the heat-capacity difference between polycrystalline and single-crystal samples from 77 to 320 K. When cooled to 77 K and then heated to about 280 K, the uranium microstructure released (3±1) J/mol of strain energy. On further heating to 300 K, the microstructure absorbed energy as it began to redevelop microstrains. Anisotropic strain-broadening parameters were extracted from neutron-diffraction measurements on polycrystals. Combining the strain-broadening parameters with anisotropic elastic constants from the literature, the microstructural strain energy is predicted in the two limiting cases of statistically isotropic stress and statistically isotropic strain. The result calculated in the limit of statistically isotropic stress was (3.7±0.5) J/mol K at 77 K and (1±0.5) J/mol at room temperature. In the limit of statistically isotropic strain, the values were (7.8±0.5) J/mol K at 77 K and (4.5±0.5) J/mol at room temperature. In both cases the changes in the microstructural strain energy showed good agreement with the calorimetry

Strain controlled oxygen vacancy formation and ordering in CaMnO3_3

We use first-principles calculations to investigate the stability of bi-axially strained \textit{Pnma} perovskite CaMnO$_3$ towards the formation of oxygen vacancies. Our motivation is provided by promising indications that novel material properties can be engineered by application of strain through coherent heteroepitaxy in thin films. While it is usually assumed that such epitaxial strain is accommodated primarily by changes in intrinsic lattice constants, point defect formation is also a likely strain relaxation mechanism. This is particularly true at the large strain magnitudes ($>$4%) which first-principles calculations often suggest are required to induce new functionalities. We find a strong dependence of oxygen vacancy defect formation energy on strain, with tensile strain lowering the formation energy consistent with the increasing molar volume with increasing oxygen deficiency. In addition, we find that strain differentiates the formation energy for different lattice sites, suggesting its use as a route to engineering vacancy ordering in epitaxial thin films.Comment: 7 pages, 7 figure

Landau levels in deformed bilayer graphene at low magnetic fields

We review the effect of uniaxial strain on the low-energy electronic dispersion and Landau level structure of bilayer graphene. Based on the tight-binding approach, we derive a strain-induced term in the low-energy Hamiltonian and show how strain affects the low-energy electronic band structure. Depending on the magnitude and direction of applied strain, we identify three regimes of qualitatively different electronic dispersions. We also show that in a weak magnetic field, sufficient strain results in the filling factor ff=+-4 being the most stable in the quantum Hall effect measurement, instead of ff=+-8 in unperturbed bilayer at a weak magnetic field. To mention, in one of the strain regimes, the activation gap at ff=+-4 is, down to very low fields, weakly dependent on the strength of the magnetic field.Comment: 14 single-column pages, 5 figures, more details on material presented in arXiv:1104.502

Non-linear optimization of the material constants in Ogden's strain-energy function for incompressible isotropic elastic materials

The Levenberg—Marquardt non—linear least squares optimization algorithm is adapted to compute the material constants in Ogden' s strain—energy function for incompressible isotropic elastic materials. In previous papers, three terms have been included in the strain-energy function. In the present paper, four terms are used and it is shown that the optimal values of the eight material constants, which are determined using the Levenberg—Marquardt algorithm, give a much closer fit to experimental data than the strain-energy function with three terms

Strain energy calculations of hexagonal boron nanotubes: An ab-initio approach

An ab initio calculations have been carried out for examining the curvature effect of small diameter hexagonal boron nanotubes. The considered conformations of boron nanotubes are namely armchair (3,3), zigzag (5,0) and chiral (4,2), and consist of 12, 20, and 56 atoms, respectively. The strain energy is evaluated in order to examine the curvature effect. It is found that the strain energy of hexagonal BNT strongly depends upon the radius, whereas the strain energy of triangular BNTs depends on both radius and chirality.Comment: 7 pages, 4 figure

Elastic Properties of Carbon nanotubes : An atomistic approach

Energetically the single sheet of graphite (graphene) is more stable than the nanotube. The energy difference between the two systems can be directly related to the strain energy involved in rolling up the graphene sheet to form the nanotube. We have carried out first principle electronic structure calculations and evaluated the strain energy as a function of the nanotube radius. The dependence of the strain energy on the diameter of the nanotube has been found by several groups to be welldescribed by a continuum elasticity model. We attempt to examine why this is the case and show where atomistics enter the description.Comment: 10 pages, 4 figure