Mohamed Ismail Mohamed-Ibrahim

Muon sites in Ce(Ru,Rh)2Al10 investigated by using Density Functional Theory from the view point of electronic potential

Numerical investigations on muon sites in Ce-based Kondo semiconductors, Ce(Ru,Rh)2Al10 were carried out by using the Density Functional Theory. From the view point of simple electrostatic potential calculations, we found all the previously reported muon sites, suggested by different groups (Kambe S et al. 2010 J. Phys. Soc. Jpn. 79 053708 and Khalyavin D D et al., 2010 Phys. Rev. B 82 100405(R)), can be possibly chosen as muon stopping sites. We also investigated the changes in the potential of the Rh-doped case. We discovered that the electronic potential around the nearest Ru atom to the substituted Rh atom is affected and the potential becomes asymmetric around the nearest Ru ion. Although big changes in hyperfine fields at muon sites have been reported (Guo H et al. 2013 Phys. Rev. B 88 115206), the muon positions estimated from the potential calculations do not change much.

Effects of size on the structure and the electronic properties of graphene nanoribbons

In view of the current interest in graphene, especially graphene nanoribbons (GNRs), we carried out investigations using the molecular orbital cluster approach to find suitable models for use in simulations involving GNRs. Two types of nanoribbons were modeled, namely armchair- and zigzag-edged GNRs. We used four properties in determining the suitable models: molecular orbitals, spin densities, charges, and bond lengths. Among these criteria, it was found that size has a highly prominent effect on the molecular orbitals and bond lengths. The results show that models of sizes C126H32 and C110H30 are needed to represent the electronic and structural properties of infinite zigzag-edged and armchair-edged GNRs.Graphical abstract

Muon Site Estimations by DFT Calculations in Metal and Insulating Systems

A way to estimate muon sites in materials is reported. Since the muon has a positive charge, one easiest and conventional way to estimate muon positions is to calculate minimum potential positions. We applied our developed method to estimate those potential minimum positions to some systems which have been studied by mons at the RIKEN-RAL Muon Facility and showed well defined muon-spin precession behavior in magnetically ordered states. Tentative calculation results by using the density functional theory are reported.

An investigation of muon sites in YBa2Cu3O6 by using Density Functional Theory

The electrostatic potential has been investigated in YBa2Cu3O6 by applying the density functional theory in order to estimate possible muon sites. We found five minimum potential positions around the apical oxygen of the CuO5 polyhedral. In addition to those, we also found another minimum potential position near the yttrium atom which is in between the CuO2 layers. Those estimated positions are different from those suggested from previous μSR studies on the basis of the dipole-field estimation.

An Effect of the Supercell Calculation on Muon Positions and Local Deformations of Crystal Structure in La2CuO4

Muon positions in La2CuO4 were examined by using the density functional theory. Potential minimum positions near apical and plane oxygen have been determined as possible initial muon stopping positions. We found that final muon stopping positions were different from those initial positions due to effects of the local deformation of crystal structure which was induced by injected muons. This means that injected muons relax their positions deforming local crystal structures and minimizing the total energy of the system. We also found that the estimation of those final muon positions had to be done in the large scale area as a supercell which contained 27 unit cells in order to achieve realistic situations of the system with the muon as a dilute impurity.

Tris[3,5-bis­(trifluoro­meth­yl)phen­yl]phosphine oxide

In the title compound, C24H9F18OP, an intramolecular C—H⋯O short contact generates a five-membered ring, producing an S(5) ring motif. The dihedral angles between the benzene rings are 57.68 (10), 77.80 (11) and 79.48 (10)°. Each of the six trifluoromethyl substituents shows rotational disorder over two positions with refined site-occupany ratios of 0.64 (3)/0.36 (3), 0.649 (14)/0.351 (14), 0.52 (2)/0.48 (2), 0.545 (16)/0.455 (16), 0.774 (9)/0.226 (9) and 0.63 (5)/0.37 (5). The crystal structure is stabilized by intermolecular C—H⋯O and C—H⋯F interactions.

First Principles Density Functional Theory Investigation on the Structural, Energetic and Electronic Properties of 6–Bromo–4–Oxo–4H–Chromene–3–Carbaldehyde

In this paper, we report the first principles Density Functional Theory (DFT) calculation to study the structural, energetic, and electronics properties of the 6–Bromo–4–Oxo–4H–Chromene–3–Carbaldehyde, C10H5BrO3 molecular framework. Geometry optimization technique was carried out to find the local energy minimum of the title compound using four hybrid DFT functionals with the basis set of 6–311++G**. The optimized molecular structure of C10H5BrO3 cluster was then used to determine the HOMO–LUMO gaps, Molecular Electrostatic Potential (MEP), Mulliken atomic charges, and others. Using the four hybrid DFT techniques, the optimized geometries of C10H5BrO3 molecular cluster is close to that of measurement data. Our calculation results also show that the total energies obtained are close to each other with the four hybrid DFT procedures. The diagram of electrostatic potential surface show that the regions of negative electrostatic potential around the oxygen atoms, O1 and O2. Using the scheme of Mulliken Population Analysis (MPA), the distributions of atomic charges follow the same arguments for the B3LYP/6–311++G**, B3PW91/6–311++G**, M06/6–311++G**, and PBE1PBE/6–311++G** simulation approaches. For example, the atom of C5 has the highest positively charge, whereas the highest negatively charge was found in the C4 atom. For Br atom, the atomic charge values obtained are –0.158, –0.222, –0.277, and –0.224, respectively for the B3LYP/6–311++G**, B3PW91/6–311++G**, M06/6–311++G**, and PBE1PBE/6–311++G** computational methods.

Computational Studies of Electronic Structures and Hyperfine Interactions of Muonium in Tetraphenylgermane

The equilibrium structure of muoniatedtetraphenylgermane (GePh4Mu) was studied using the first principle Density Functional Theory (DFT) method. Three muonium (Mu) trapping sites were considered, namely ortho, meta, and para positions on one of the phenyl rings. Geometry optimization procedure was utilized to determine the local energy minimum for all the systems. The total energies corresponding to Mu at the three positions are very similar to each other. For the meta case, the corresponding energy is higher than the other two sites by only about 0.03 eV. The hyperfine parameters of Mu were also calculated. The Mu isotropic hyperfine coupling constants were found to be 441.85 MHz, 449.80 MHz, and 439.01 MHz for the ortho, meta, and para cases, respectively. The anisotropic value was calculated to be very small.

The RIKEN-RAL Muon Facility and the Application of Muons for Studies of Magnetic Properties of Nano-Materials

An overview of the RIKEN-RAL Muon Facility is reported. The RIKEN-RAL Muon Facility has been established at the Rutherford-Appleton Laboratory in the UK in 1992 and started to be in used for material science studies from 1994 by applying intense pulsed muon beams. More than 380 publications have been published in international journals and more than 100 domestic and international collaborations have been organized. As an example of those collaborative studies, a preliminary result of one recent experimental study on the magnetism in gold nano-cluster is reported. Additionally, a recent computational result on a local effect induced by an injected muon is reported in order to show how the presence of the muon leads to a local deformation of the crystal structure, thus changing the muon’s own position in finding the final stopping site self consistently.

Polymorphs of Tolfenamic Acids: Stability Analysis Using Cluster Method

We report results of the relative stability between form I and form II of tolfenamic acid. By performing systematic cluster calculations at the B3LYP/6-31 level of theory and including the corrections to the dispersion and basis set superposition error, we found that form II is energetically more stable than form I. Furthermore, we found that the formation of dimers has a stabilizing effect compared to individual monomers in the clusters that we have considered.