Abstract: We theoretically demonstrate that the transport inefficiency recently found experimentally for branched-out mesoscopic networks can also be observed in a quantum ring of finite width with an attached central horizontal branch. This is done by investigating the time evolution of an electron wave packet in such a system. Our numerical results show that the conductivity of the ring does not necessary improve if one adds an extra channel. This ensures that there exists a quantum analog of the Braess paradox, originating from quantum scattering and interference.

Abstract: Transition metal dichalcogenides (TMDs) are considered as promising candidates for next generation of electronic and optoelectronic devices. To make use of these materials, for instance in field effect transistor applications, it is mandatory to know the detailed properties of contacts of such TMDs with metal electrodes. Here, we investigate the role of the contact structure on the electronic and transport properties of metal-MoSe2 interfaces. Two different contact types, namely face and edge contacts, are studied. We consider both low (Sc) and high (Au) work function metals in order to thoroughly elucidate the role of the metal work function and the type of metal. First principles plane wave calculations and transport calculations based on nonequilibrium Green's function formalism reveal that the contact type has a large impact on the electronic and transport properties of metal-MoSe2 interfaces. For the Sc electrode, the Schottky barrier heights are around 0.25 eV for face contact and bigger than 0.6 eV for edge contact. For the Au case, we calculate very similar barrier heights for both contact types with an average value of 0.5 eV. Furthermore, while the face contact is found to be highly advantageous as compared to the edge contact for the Sc electrode, the latter contact becomes a better choice for the Au electrode. Our findings provide guidelines for the fabrication of TMD-based devices.

Abstract: The Dirac equation is solved for triangular and hexagonal graphene quantum dots for different boundary conditions in the presence of a perpendicular magnetic field. We analyze the influence of the dot size and its geometry on their energy spectrum. A comparison between the results obtained for graphene dots with zigzag and armchair edges, as well as for infinite-mass boundary condition, is presented and our results show that the type of graphene dot edge and the choice of the appropriate boundary conditions have a very important influence on the energy spectrum. The single-particle energy levels are calculated as a function of an external perpendicular magnetic field that lifts degeneracies. Comparing the energy spectra obtained from the tight-binding approximation to those obtained from the continuum Dirac equation approach, we verify that the behavior of the energies as a function of the dot size or the applied magnetic field are qualitatively similar, but in some cases quantitative differences can exist.

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: The interplay between quantum Hall states and Cooper pairs is usually hindered by the suppression of the superconducting state due to the strong magnetic fields needed to observe the quantum Hall effect. From this point of view, graphene is special since it allows the creation of strong pseudomagnetic fields due to strain. We show that in a Josephson junction made of strained graphene, Cooper pairs will diffuse into the strained region. The pair correlation function will be sublattice polarized due to the polarization of the local density of states in the zero pseudo-Landau level. We uncover two regimes: (1) one in which the cyclotron radius is larger than the junction length, in which case the supercurrent will be enhanced, and (2) the long junction regime where the supercurrent is strongly suppressed because the junction becomes an insulator. In the latter case quantized Hall states form and Andreev scattering at the normal/superconducting interface will induce edge states. Our numerical calculation has become possible due to an extension of the Chebyshev-Bogoliubovde Gennes method to computations on video cards (GPUs).

Abstract: The thermodynamic properties of the electron gas in multilayer graphene depend strongly on the number of layers and the type of stacking. Here we analyze how those properties change when we vary the number of layers for rhombohedral stacked multilayer graphene and compare our results with those from a conventional two-dimensional electron gas. We show that the highly degenerate zero-energy Landau level which is partly filled with electrons and partly with holes has a strong influence on the values of the different thermodynamic quantities.

Abstract: When the energy difference between two electric subbands in a quasi-two-dimensional electron system equals a LO-phonon energy, resonant scattering will occur. This leads to an enhancement of the scattering rate and, consequently, to a suppression of the conductivity. Changing the energy difference between the electric subbands (e.g., through a gate) leads to a series of electrophonon resonances in the conductivity. A detailed study is made of this effect for different confinement potentials. We found that the scattering processes where the emission of a phonon is involved are very important for the electrophonon resonance and that the size of the effect decreases with increasing temperature.

Abstract: Surface effects of a doped thin film made of a strongly correlated material are investigated both in the absence and presence of a perpendicular electric field. We use an inhomogeneous Gutzwiller approximation for a single-band Hubbard model in order to describe correlation effects. For low doping, the bulk value of the quasiparticle weight is recovered exponentially deep into the slab, but with increasing doping, additional Friedel oscillations appear near the surface. We show that the inverse correlation length has a power-law dependence on the doping level. In the presence of an electrical field, considerable changes in the quasiparticle weight can be realized throughout the system. We observe a large difference (as large as five orders of magnitude) in the quasiparticle weight near the opposite sides of the slab. This effect can be significant in switching devices that use the surface states for transport. DOI: 10.1103/PhysRevB.87.035131

Abstract: Multifrequency electron paramagnetic resonance (EPR) and high resolution transmission electron microscopy (HRTEM) investigations were performed on small (2 nm) cubic ZnS nanocrystals (quantum dotsQDs) doped with 0.2% mol Mn2+, self-assembled into a mesoporous structure. The EPR data analysis shows that the substitutional Mn2+ ions are localized at Zn2+ sites subjected to a local axial lattice distortion, resulting in the observed zero-field-splitting parameter |D|=41×10−4 cm−1. The local distortion is attributed to the presence in the second shell of ligands of a stacking fault or twin, which alters the normal stacking sequence of the cubic structure. The HRTEM results confirm the presence of such extended planar defects in a large percentage of the investigated QDs, which makes possible the proposed substitutional Mn2+ impurity ions localization model. Based on these results it is suggested that the high doping levels of Mn2+ ions observed in cubic ZnS and possible in other II-VI semiconductor QDs prepared at low temperatures can be explained by the assistance of the extended lattice defects in the impurities incorporation.

Abstract: Dielectric properties of materials are crucial in describing the electromagnetic response of materials. As devices are becoming considerably smaller than the optical wavelength, the conventional measuring methods based on optical response are limited by their spatial resolution. Electron energy loss spectroscopy performed in a scanning transmission electron microscope is a good alternative to obtain the dielectric properties with excellent spatial resolution. Due to the overlap of diffraction discs in scanning transmission electron microscopy, it is difficult to apply conventional experimental settings to suppress retardation losses. In this contribution, a relativistic dielectric model for the loss function is presented which is used in a model based optimization scheme to estimate the complex dielectric function of a material. The method is applied to experiments on bulk diamond and SrTiO3 and shows a good agreement with optical reference data when retardation effects are included. Application of this technique to nanoparticles is possible but several theoretical assumptions made in the model of the loss function are violated and interpretation becomes problematic.

Abstract: Electrical transport properties of a two-dimensional electron gas axe studied in the presence of a perpendicular magnetic field B modulated weakly and periodically along one direction, B = (B + B0 cos Kx)z, with B0 much less than B, K = 2pi/a, and a being the period of the modulation. B0 is taken constant or proportional to B. The Landau levels broaden into bands and their width, proportional to the modulation strength B0, oscillates with B and gives rise to oscillations in the magnetoresistance at low B. These oscillations reflect the commensurability between the cyclotron diameter at the Fermi level and the period a and consequently hey are distinctly different from the Shubnikov-de Ha.as ones, at higher B, in period and temperature dependence. The bandwidth at the Fermi energy can be one order of magnitude larger, at low B, than that of the electric case for equal modulation strengths. The resulting magnetoresistance oscillations have a much higher amplitude than those of the electric case with which they are out of phase. Explicit asymptotic expressions are derived for the temperature dependence of the transport coefficients. The case when both electric and magnetic modulations are present is also considered. The position of the resulting oscillations depends on the ratio delta between the two modulation strengths. When the modulations are out of phase there is no shift in the position of the oscillations when delta varies and for a particular value of delta the oscillations are suppressed.