Abstract: We study the energy and temperature relaxation of electrons in graphene on a piezoelectric substrate. Scattering from the combined potential of extrinsic piezoelectric surface acoustical (PA) phonons of the substrate and intrinsic deformation acoustical phonons of graphene is considered for a (non) degenerate gas of Dirac fermions. It is shown that in the regime of low energies or temperatures the PA phonons dominate the relaxation and change qualitatively its character. This prediction is relevant for quantum metrology and electronic applications using graphene devices and suggests an experimental setup for probing electron-phonon coupling in graphene.

Abstract: The electronic properties of graphene nano-flakes (GNFs) with different edge passivation are investigated by using density functional theory. Passivation with F and H atoms is considered: C-Nc X-Nx (X = F or H). We studied GNFs with 10 < N-c < 56 and limit ourselves to the lowest energy configurations. We found that: (i) the energy difference Delta between the highest occupied molecular orbital and the lowest unoccupied molecular orbital decreases with N-c, (ii) topological defects (pentagon and heptagon) break the symmetry of the GNFs and enhance the electric polarization, (iii) the mutual interaction of bilayer GNFs can be understood by dipole-dipole interaction which were found sensitive to the relative orientation of the GNFs, (iv) the permanent dipoles depend on the edge terminated atom, while the energy gap is independent of it, and (v) the presence of heptagon and pentagon defects in the GNFs results in the largest difference between the energy of the spin-up and spin-down electrons which is larger for the H-passivated GNFs as compared to F-passivated GNFs. Our study shows clearly the effect of geometry, size, termination, and bilayer on the electronic properties of small GNFs. This study reveals important features of graphene nano-flakes which can be detected using Raman spectroscopy. (C) 2014 AIP Publishing LLC.

Abstract: We propose two coupled electron-hole sheets of few-layer graphene as a new nanostructure to observe superfluidity at enhanced densities and enhanced transition temperatures. For ABC stacked few-layer graphene we show that the strongly correlated electron-hole pairing regime is readily accessible experimentally using current technologies. We find for double trilayer and quadlayer graphene sheets spatially separated by a nano-thick hexagonal boron-nitride insulating barrier, that the transition temperature for electron-hole superfluidity can approach temperatures of 40 K.

Abstract: Recently, rhenium disulfide (ReS2) monolayers were experimentally extracted by conventional mechanical exfoliation technique from as-grown ReS2 crystals. Unlike the well-known members of transition metal dichalcogenides (TMDs), ReS2 crystallizes in a stable distorted-1T structure and lacks an indirect to direct gap crossover. Here we present an experimental and theoretical study of the formation, energetics, and stability of the most prominent lattice defects in monolayer ReS2. Experimentally, irradiation with 3-MeV He+2 ions was used to break the strong covalent bonds in ReS2 flakes. Photoluminescence measurements showed that the luminescence from monolayers is mostly unchanged after highly energetic a particle irradiation. In order to understand the energetics of possible vacancies in ReS2 we performed systematic first-principles calculations. Our calculations revealed that the formation of a single sulfur vacancy has the lowest formation energy in both Re and S rich conditions and a random distribution of such defects are energetically more preferable. Sulfur point defects do not result in any spin polarization whereas the creation of Re-containing point defects induce magnetization with a net magnetic moment of 1-3 mu B. Experimentally observed easy formation of sulfur vacancies is in good agreement with first-principles calculations.

Abstract: We present a systematic study of the energy spectra of graphene quantum rings having different geometries and edge types in the presence of a perpendicular magnetic field. Results are obtained within the tight-binding (TB) and Dirac models and we discuss which features of the former can be recovered by using the approximations imposed by the latter. Energy levels of graphene quantum rings obtained by diagonalizing the TB Hamiltonian are demonstrated to be strongly dependent on the rings geometry and the microscopical structure of the edges. This makes it difficult to recover those spectra by the existing theories that are based on the continuum (Dirac) model. Nevertheless, our results show that both approaches (i.e., TB and Dirac model) may provide similar results, but only for very specific combinations of ring geometry and edge types. The results obtained by a simplified model describing an infinitely thin circular Dirac ring show good agreement with those obtained for hexagonal and rhombus armchair graphene rings within the TB model. Moreover, we show that the energy levels of a circular quantum ring with an infinite mass boundary condition obtained within the Dirac model agree with those for a ring defined by a ring-shaped staggered potential obtained within the TB model.

Abstract: Moire patterns in the pseudo-magnetic field and in the strain profile of graphene (GE) when put on top of a hexagonal lattice substrate are predicted from elasticity theory. The van der Waals interaction between GE and the substrate induces out-of-plane deformations in graphene which results in a strain field, and consequently in a pseudo-magnetic field. When the misorientation angle is about 0.5 degrees, a three-fold symmetric strain field is realized that results in a pseudo-magnetic field very similar to the one proposed by F. Guinea, M. I. Katsnelson, and A. K. Geim [Nature Phys. 6, 30 (2010)]. Our results show that the periodicity and length of the pseudo-magnetic field can be tuned in GE by changing the misorientation angle and substrate adhesion parameters and a considerable energy gap (23 meV) can be obtained due to out-of-plane deformation of graphene which is in the range of recent experimental measurements (20-30 meV). (C) 2014 AIP Publishing LLC.

Abstract: In order to account explicitly for the existence of long-periodic layered structures and the strong structural relaxations in the most common binary and ternary alloys of the Bi-Sb-Te-Se system, we have developed a one-dimensional cluster expansion (CE) based on first-principles electronic structure calculations, which accounts for the Bi and Sb bilayer formation. Excellent interlayer distances are obtained with a van der Waals density functional. It is shown that a CE solely based on pair interactions is sufficient to provide an accurate description of the ground-state energies of Bi-Sb-Te-Se binary and ternary systems without making the data set of ab initio calculated structures unreasonably large. For the binary alloys A1−xQx (A=Sb, Bi; Q=Te, Se), a ternary CE yields an almost continuous series of (meta)stable structures consisting of consecutive A bilayers next to consecutive A2Q3 for 0<x<0.6. For x>0.6, the binary alloy segregates into pure Q and A2Q3. The Bi-Sb system is described by a quaternary CE and is found to be an ideal solid solution stabilized by entropic effects at T≠0 K but with an ordered structure of alternating Bi and Sb layers for x=0.5 at T=0 K. A quintuple CE is used for the ternary Bi-Sb-Te system, where stable ternary layered compounds with an arbitrary stacking of Sb2Te3, Bi2Te3, and Te-Bi-Te-Sb-Te quintuple units are found, optionally separated by mixed Bi/Sb bilayers. Electronic properties of the stable compounds were studied taking spin-orbit coupling into account.

Abstract: The effect of vacancy defects on the structural properties and the thermal stability of free standing silicene – a buckled structure of hexagonally arranged silicon atoms – is studied using reactive molecular dynamics simulations. Pristine silicene is found to be stable up to 1500 K, above which the system transits to a three-dimensional amorphous configuration. Vacancy defects result in local structural changes in the system and considerably reduce the thermal stability of silicene: depending on the size of the vacancy defect, the critical temperature decreases by more than 30%. However, the system is still found to be stable well above room temperature within our simulation time of 500 ps. We found that the, stability of silicene can be increased by saturating the dangling bonds at the defect edges by foreign atoms (e.g., hydrogen).

Abstract: Promising predictions are made for III-V tunnel-field-effect transistor (FET), but there is still uncertainty on the parameters used in the band-to-band tunneling models. Therefore, two simulators are calibrated in this paper; the first one uses a semi-classical tunneling model based on Kane's formalism, and the second one is a quantum mechanical simulator implemented with an envelope function formalism. The calibration is done for In0.53Ga0.47As using several p+/intrinsic/n+ diodes with different intrinsic region thicknesses. The dopant profile is determined by SIMS and capacitance-voltage measurements. Error bars are used based on statistical and systematic uncertainties in the measurement techniques. The obtained parameters are in close agreement with theoretically predicted values and validate the semi-classical and quantum mechanical models. Finally, the models are applied to predict the input characteristics of In0.53Ga0.47As n- and p-lineTFET, with the n-lineTFET showing competitive performance compared to MOSFET.

Abstract: The interband optical absorption in Si/SiO2 quantum wells is calculated as function of the well width (W) and the evolution from an indirect to a direct gap material as function of the well width is investigated. In order to compute the electron states in the conduction band, the 30-band k . p model is employed, whereas the 6-band Luttinger-Kohn model is used for the hole states. We found that the effective direct band gap in the quantum well agrees very well with the W-2 scaling result of the single-band model. The interband matrix elements for linear polarized light oscillate with the quantum well width, which agrees qualitatively with a single band calculation. Our theoretical results indicate that the absorption can be maximized by a proper choice of the well width. However, the obtained absorption coefficients are at least an order of magnitude smaller than for a typical direct semiconductor even for a well width of 2 nm. (C) 2014 AIP Publishing LLC.

Abstract: We analyze the combined effect of three ingredients of an aggregation model-surface tension, particle flow and particle source-representing typical characteristics of many aggregation growth processes in nature. Through extensive numerical experiments and for different underlying lattice structures we demonstrate that the location of incoming particles and their preferential direction of flow can significantly affect the resulting general shape of the aggregate, while the surface tension controls the surface roughness. Combining all three ingredients increases the aggregate shape plasticity, yielding a wider spectrum of shapes as compared to earlier works that analyzed these ingredients separately. Our results indicate that the considered combination of effects is fundamental for modeling the polymorphic growth of a wide variety of structures in confined geometries and/or in the presence of external fields, such as rocks, crystals, corals, and biominerals.

Abstract: Transverse magnetic focusing properties of graphene using a ballistic four terminal structure are investigated. The electric response is obtained using the semiclassical billiard model. The transmission exhibits pronounced peaks as a consequence of skipping orbits at the edge of the structure. When we add a pn-junction between the two probes, snake states along the pn-interface appear. Injected electrons are guided by the pn-interface to one of the leads depending on the value of the applied magnetic field. Oscillations in the resistance are found depending on the amount of particles that end up in each lead.

Abstract: Carbon-based molecular semiconductors are explored for application in spintronics because their small spinorbit coupling promises long spin lifetimes. We calculate the electronic transport from first principles through spin valves comprising bi-and tri-layers of the fullerene molecules C-60 and C-70, sandwiched between two Fe electrodes. The spin polarization of the current, and the magnetoresistance depend sensitively on the interactions at the interfaces between the molecules and the metal surfaces. They are much less affected by the thickness of the molecular layers. A high current polarization (CP > 90%) and magnetoresistance (MR > 100%) at small bias can be attained using C-70 layers. In contrast, the current polarization and the magnetoresistance at small bias are vanishingly small for C-60 layers. Exploiting a generalized Julliere model we can trace the differences in spin-dependent transport between C-60 and C-70 layers to differences between the molecule-metal interface states. These states also allow one to interpret the current polarization and the magnetoresistance as a function of the applied bias voltage.

Abstract: Using density functional theory, we obtain the mechanical and thermal properties of MX2 monolayers (where M = Cr, Mo, W and X = O, S, Se, Te). The C-centered phonon frequencies (i.e., A(1), A(2)'', E ', and E ''), relative frequency values of A(1), and E ' modes, and mechanical properties (i.e., elastic constants, Young modulus, and Poisson's ratio) display a strong dependence on the type of metal and chalcogenide atoms. In each chalcogenide (metal) group, transition-metal dichalcogenides (TMDCs) with W (O) atom are found to be much stiffer. Consistent with their stability, the thermal expansion of lattice constants for TMDCs with O (Te) is much slower (faster). Furthermore, in a heterostructure of these materials, the difference of the thermal expansion of lattice constants between the individual components becomes quite tiny over the whole temperature range. The calculated mechanical and thermal properties show that TMDCs are promising materials for heterostructures. (C) 2014 AIP Publishing LLC.