Group theory for structural analysis and lattice vibrations in phosphorene systems

Group theory analysis for two-dimensional elemental systems related to phosphorene is presented, including (i) graphene, silicene, germanene and stanene, (ii) dependence on the number of layers and (iii) two stacking arrangements. Departing from the most symmetric $D_{6h}^{1}$ graphene space group, the structures are found to have a group-subgroup relation, and analysis of the irreducible representations of their lattice vibrations makes it possible to distinguish between the different allotropes. The analysis can be used to study the effect of strain, to understand structural phase transitions, to characterize the number of layers, crystallographic orientation and nonlinear phenomena.Comment: 24 pages, 3 figure

Group Theory analysis of phonons in two-dimensional Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDCs) have emerged as a new two dimensional materials field since the monolayer and few-layer limits show different properties when compared to each other and to their respective bulk materials. For example, in some cases when the bulk material is exfoliated down to a monolayer, an indirect-to-direct band gap in the visible range is observed. The number of layers $N$ ($N$ even or odd) drives changes in space group symmetry that are reflected in the optical properties. The understanding of the space group symmetry as a function of the number of layers is therefore important for the correct interpretation of the experimental data. Here we present a thorough group theory study of the symmetry aspects relevant to optical and spectroscopic analysis, for the most common polytypes of TMDCs, i.e. $2Ha$, $2Hc$ and $1T$, as a function of the number of layers. Real space symmetries, the group of the wave vectors, the relevance of inversion symmetry, irreducible representations of the vibrational modes, optical selection rules and Raman tensors are discussed.Comment: 32 pages, 4 figure

Optical-phonon resonances with saddle-point excitons in twisted-bilayer graphene

Twisted-bilayer graphene (tBLG) exhibits van Hove singularities in the density of states that can be tuned by changing the twisting angle $\theta$. A $\theta$-defined tBLG has been produced and characterized with optical reflectivity and resonance Raman scattering. The $\theta$-engineered optical response is shown to be consistent with persistent saddle-point excitons. Separate resonances with Stokes and anti-Stokes Raman scattering components can be achieved due to the sharpness of the two-dimensional saddle-point excitons, similar to what has been previously observed for one-dimensional carbon nanotubes. The excitation power dependence for the Stokes and anti-Stokes emissions indicate that the two processes are correlated and that they share the same phonon.Comment: 5 pages, 6 figure

Group theory analysis of electrons and phonons in N-layer graphene systems

In this work we study the symmetry properties of electrons and phonons in graphene systems as function of the number of layers. We derive the selection rules for the electron-radiation and for the electron-phonon interactions at all points in the Brillouin zone. By considering these selection rules, we address the double resonance Raman scattering process. The monolayer and bilayer graphene in the presence of an applied electric field are also discussed.Comment: 8 pages, 6 figure

Design of compact microstrip bandpass filter using square DMS slots for Wi-Fi and bluetooth applications

This paper presents the design of a compact bandpass filter based on two identical rectangular resonators and is implemented on microstrip technology for Wi-Fi and bluetoothapplications. To reduce the size of the filter, the defected microstrip structure (DMS) technique is proposed. This technique consists of etching slots in the rectangular resonator, which results in a change in the line properties and increase of the effective inductance and capacitance. This feature is used for miniaturization. The designed filter has a compact size (6.82x8.3) mm² with a low insertion loss of -0.1 dB and a good return loss of -36 dB. The simulation results are realized using the (computer simulation technology) CST Microwave software

Designing rigid carbon foams

We use ab initio density functional calculations to study the stability, elastic properties and electronic structure of sp2 carbon minimal surfaces with negative Gaussian curvature, called schwarzites. We focus on two systems with cubic unit cells containing 152 and 200 carbon atoms, which are metallic and very rigid. The porous schwarzite structure allows for efficient and reversible doping by electron donors and acceptors, making it a promising candidate for the next generation of alkali ion batteries. We identify schwarzite structures that act as arrays of interconnected quantum spin dots or become magnetic when doped. We introduce two interpenetrating schwarzite structures that may find their use as the ultimate super-capacitor.Comment: 6 pages, 5 figure

Displacement damages created by γγγγ particles radiation in n type GaAs

In this work, we present a study of the effect of γ particles radiation in n type gallium arsenide (GaAs) doped with silicon (SiGa ). For this, we have irradiated samples of GaAs doped with 1015cm-3 and 1016cm-3 of SiGa at different fluencies of γ radiation. We have used photoluminescence (PL) measurement at 8.8K to identify defects induced by γ radiation in these samples. We found that this type of radiation induces the gallium vacancy VGa in GaAs and causes the transfer of the silicone impurity from Ga site to As site. These two defects are displacement damages created by γ radiation and are the same of displacement damages created by the other type of radiation (charged particles and neutral particles). The difference between the effect of particles is the introduction rate b of the defect. Then, we found that b or γ particles is ten times weaker than 7MeV electron particles. γ ray are photons, so they can’t interact with GaAs atoms to product displacement damages by Rutherford diffusion (charged particles) or diffusion from hard spheres model (neutral particles). We suggest that recoil electrons produced in GaAs by photoelectric effect and Compton effect are responsible to the creation of these displacement damages. Indeed, these electrons have enough energy (~ 1 MeV) to product the same damages.In this work, we present a study of the effect of γ particles radiation in n type gallium arsenide (GaAs) doped with silicon (SiGa ). For this, we have irradiated samples of GaAs doped with 1015cm-3 and 1016cm-3 of SiGa at different fluencies of γ radiation. We have used photoluminescence (PL) measurement at 8.8K to identify defects induced by γ radiation in these samples. We found that this type of radiation induces the gallium vacancy VGa in GaAs and causes the transfer of the silicone impurity from Ga site to As site. These two defects are displacement damages created by γ radiation and are the same of displacement damages created by the other type of radiation (charged particles and neutral particles). The difference between the effect of particles is the introduction rate b of the defect. Then, we found that b or γ particles is ten times weaker than 7MeV electron particles. γ ray are photons, so they can’t interact with GaAs atoms to product displacement damages by Rutherford diffusion (charged particles) or diffusion from hard spheres model (neutral particles). We suggest that recoil electrons produced in GaAs by photoelectric effect and Compton effect are responsible to the creation of these displacement damages. Indeed, these electrons have enough energy (~ 1 MeV) to product the same damages

THEORETICAL INVESTIGATION OF THE LEVEL ENERGIES FOR IDEAL Ga AND As VACANCIES IN GaAs

The bound state energies of ideal Ga and As vacancies in GaAs have been studied in the Green’s-function framework in conjunction the tight-binding method. Using existing data for band structures, the Koster-Slater parameters have been estimated, the energy levels at critical points and the density of states in perturbed and unperturbed crystal have been calculated. The unrelaxed vacancy of Ga and As introduces bound state at 0.05 eV and 1.46 eV respectively above the valence band edge. These results provide justification to experimental data based on irradiation of GaAs by energetic neutrons.The bound state energies of ideal Ga and As vacancies in GaAs have been studied in the Green’s-function framework in conjunction the tight-binding method. Using existing data for band structures, the Koster-Slater parameters have been estimated, the energy levels at critical points and the density of states in perturbed and unperturbed crystal have been calculated. The unrelaxed vacancy of Ga and As introduces bound state at 0.05 eV and 1.46 eV respectively above the valence band edge. These results provide justification to experimental data based on irradiation of GaAs by energetic neutrons