Theoretical study of modified electron band dispersion and density of states due to high frequency phonons in graphene-on-substrates

We propose here a theoretical model for the study of band gap opening in graphene-on- polarizable substrate taking the effect of electron–electron and electron–phonon (EP) interactions at high frequency phonon vibrations. The Hamiltonian consists of hopping of electrons upto third nearest- neighbors and the effect substrate, where A sublattice site is raised by energy [Formula: see text] and B sublattice site is suppressed by energy [Formula: see text], hence producing a band gap energy of [Formula: see text]. Further, we have considered Hubbard type electron–electron repulsive interactions at A and B sublattices, which are considered within Hartree–Fock meanfield approximation. The electrons in the graphene plane interact with the phonon’s present in the polarized substrate in the presence of phonon vibrational energy within harmonic approximation. The temperature-dependent electron occupancies are computed numerically and self-consistently for both spins at both the sublattice sites. By using these electron occupancies, we have calculated the electron band dispersion and density of states (DOS), which are studied for the effects of EP interaction, high phonon frequency, Coulomb energy and substrate induced gap.

The Effect of the Size and Electronic Factor on Occupation and Magnetic Property of Co Addition in L10 FePt Alloy

The size and the electronic factor are used to analyze the occupation of doped Co atom in L10 FePt alloy as well as its influence on crystal structure and magnetic property. The Co atom located in Fe sublattice site is determined by the variance of the calculated structure and the doping energy results investigated by the first-principles calculations. The charge-density difference contour plot and Bader charge analysis can quantitatively distinguish the effects of the size and the electronic factor on physical properties of the doping alloy. The effect of the electronic factor on the doping energy is more dominant than that of the size factor. The size factor almost has no effect on saturation magnetization.

Reduction in Majority-Carrier Concentration in Lightly-Doped 4H-SiC Epilayers by Electron Irradiation

The mechanisms for the reduction in the hole concentration in lightly Al-doped p-type 4H-SiC epilayers by electron irradiation as well as in the electron concentration in lightly N-doped n-type 4H-SiC epilayers by electron irradiation are investigated. In the p-type 4H-SiC epilayers, the temperature dependence of the hole concentration, , is not changed by 100 keV electron irradiation, while the is decreased by 150 keV electron irradiation. The density of Al acceptors with energy level eV decreases with increasing fluence of 150 keV electrons, whereas the density of deep acceptors with energy level eV increases. In the n-type 4H-SiC epilayers, the temperature dependence of the electron concentration, , is decreased by 200 keV electron irradiation. The density of N donors located at hexagonal C-sublattice sites decreases significantly with increasing fluence of 200 keV electrons, whereas the density of N donors located at cubic C-sublattice site decreases slightly.

Site Occupation and Defect Structure of Fe2Nb Laves Phase in Fe-Nb-M Ternary Systems at Elevated Temperatures

For the Fe2Nb Laves phase with C14 structure in the Fe-Nb-M (M : Cr, Mn, Co, Ni) systems, the site occupation of M in Fe2Nb has been examined in terms of XRD Rietveld analysis, particularly paying attention to the two Fe sublattice sites of Fe1 (36-net in the triple layer : t) and Fe2 (kagome-net of the single layer : s) with the fraction of 0.25 and 0.75, respectively. In any these four ternary systems, the Fe2Nb Laves phase region largely extends along the equi-Nb concentration direction; for Mn complete solid solubility exists, and the solubility of Cr and Co in Fe2Nb is more than 50 at.% and that of Ni is 44 at.%. Thus, at least two thirds of all Fe sublattices in Fe2Nb are occupied by M in all cases. Rietveld analysis revealed that Cr and Mn with which have a larger atomic size than Fe prefer to occupy the Fe1 sublattice site when the amount in solution is less than 0.25 fraction of Fe in Fe2Nb and the preferred occupation site changes to the Fe2 sublattice site when the amount in solution increases beyond 0.25. In contrast, Co and Ni whose atomic size is smaller than Fe preferentially occupy the Fe2 sublattice site, regardless of the amount. The c/a ratio of stoichiometoric Fe2Nb increases and becomes closer to the ideal value (1.633) of the cubic C15 structure when the Fe1 sublattice site is occupied by Cr and Mn. However, the degree of symmetries of both tetrahedron and kagome-net formed by Fe atoms become better when Fe2 sublattice site is occupied by a certain amount of Ni.

The Role of Simulation in ALCHEMI

Contrast in zone axis channelling patterns, formed by variations in characteristic X-ray emission rates as an electron beam is scanned in angle, is related to projected sublattice site symmetries of ionized atoms within the unit cell. Whereas overall Brillouin zone geometry is identical to that observed in large angle convergent beam patterns, this incoherent X-ray channelling contrast is related to ADF or BSE contrast by integration over thickness of the signal generated within the specimen. Contrast is thus relatively stable and easily interpretable.1 This is useful in separating the response of dilute atoms from abundant atomic species, since the specific channelling pattern of each atom has its own overall fingerprint. Correlation of patterns formed from Ta-doped TiAl with bright field LACBED is shown in Fig. 1 (clearly Ta occupies Ti sublattice sites). No simulation of channelling contrast is necessary if minority atomic species are distributed over sublattice sites in a way that can be reconstructed from a linear superposition of majority (or host) atom sites that form part of the regular lattice structure. The distribution over sublattice sites may be determined by any one of a variety of analysis methods.2,3

Reliability of sublattice occupancies determined by ALCHEMI

The atom location by channeling enhanced microanalysis (ALCHEMI) technique is an attractive method for sublattice (site) occupancy determination, especially in multiphase materials. It has been applied to a wide range of ceramics, minerals, semiconductors and metallic alloys with varying degrees of success. The simplicity of the method originally proposed has not always been borne out in practice; applications of the technique have revealed limitations due to crystallography, microstructure, specimen preparation, ionization delocalization, and weak channeling discrimination. Some of these limitations have been encountered in work at ORNL on ALCHEMI of intermetallic alloys such as Ll2-ordered Ni3Al-based alloys (with additions of Hf, Co, and Fe) and Ll0-ordered Cu50Au50-y(X)y alloys with X = Pd or Ni. These applications have been motivated by the technological importance of ordered intermetallic alloys. Commercial materials often have complex compositions with ternary (or higher) additions present at substantial levels. The distribution of the various components on the different sublattices is an important facet of characterization for structure property correlations to help guide alloy development.