George Alexandru Nemnes

Nano-transistors in the Landauer–Büttiker formalism

We investigate transport in nanotransistors in the Landauer–Buttiker formalism. A systematic linearization of the general expression for the current response yields the quantum version of the small signal equivalent circuit. This equivalent circuit can be compared with classical schemes so that explicit quantum mechanical expressions for the circuit elements can be extracted. Reducing our analysis to an effective Y-parameter description of the equivalent circuit we find the multi-terminal Buttiker formula except for one extra term. We show that this extra term is essential for the operation of transistors. An application of our theory to a simple transistor model yields a description of mismatch oscillations in the source-drain current experimentally observed in nano-transistors.

Nonlinear I-V characteristics of nanotransistors in the Landauer-Büttiker formalism

We present the nonlinear I-V characteristics of a nanoscale metal-oxide-semiconductor field-effect transistor in the Landauer-Buttiker formalism. In our three-dimensional ballistic model the gate, source, and drain contacts are treated on an equal footing. As in the drift-diffusion regime for ballistic transport a saturation of the drain current results. We demonstrate the quantum mechanism for the ballistic drain current saturation. As a specific signature of ballistic transport we find a specific threshold characteristic with a close-to-linear dependence of the drain current on the drain voltage. This threshold characteristic separates the ON-state regime from a quasi-OFF-state regime in which the device works as a tunneling transistor. Long- and short-channel effects are analyzed in both regimes and compared qualitatively with existing experimental data by Intel [B. Doyle et al., Intel Technol. J. 6, 42 (2002)].

Dynamic electrical behavior of halide perovskite based solar cells

Abstract A dynamic electrical model is introduced to investigate the hysteretic effects in the J-V characteristics of perovskite based solar cells. By making a simple ansatz for the polarization relaxation, our model is able to reproduce qualitatively and quantitatively detailed features of measured J-V characteristics. Pre-poling effects are discussed, pointing out the differences between initially over- and under-polarized samples. In particular, the presence of the current overshoot observed in the reverse characteristics is correlated with the solar cell pre-conditioning. Furthermore, the dynamic hysteresis is analyzed with respect to changing the bias scan rate, the obtained results being consistent with experimentally reported data: the hysteresis amplitude is maximum at intermediate scan rates, while at very slow and very fast ones it becomes negligible. The effects induced by different relaxation time scales are assessed. The proposed dynamic electrical model offers a comprehensive view of the solar cell operation, being a practical tool for future calibration of tentative microscopic descriptions.

Spin current switching and spin-filtering effects in Mn-doped boron nitride nanoribbons

The spin transport properties are investigated by means of the first principle approach for boron nitride nanoribbons with one or two substitutional Mn impurities, connected to graphene electrodes. The spin current polarization is evaluated using the nonequilibrium Greens function formalism for each structure and bias. The structure with one Mn impurity reveals a transfer characteristics suitable for a spin current switch. In the case of two Mn impurities, the system behaves as an efficient spin-filter device, independent on the ferromagnetic or antiferromagnetic configurations of the magnetic impurities. The experimental availability of the building blocks as well as the magnitudes of the obtained spin current polarizations indicates a strong potential of the analyzed structures for future spintronic devices.

Adiabatic Edge Channel Transport in a Nanowire Quantum Point Contact Register

We report on a prototype device geometry where a number of quantum point contacts are connected in series in a single quasi-ballistic InAs nanowire. At finite magnetic field the backscattering length is increased up to the micron-scale and the quantum point contacts are connected adiabatically. Hence, several input gates can control the outcome of a ballistic logic operation. The absence of backscattering is explained in terms of selective population of spatially separated edge channels. Evidence is provided by regular Aharonov-Bohm-type conductance oscillations in transverse magnetic fields, in agreement with magnetoconductance calculations. The observation of the Shubnikov-de Haas effect at large magnetic fields corroborates the existence of spatially separated edge channels and provides a new means for nanowire characterization.

Self-consistent potentials and linear regime conductance of cylindrical nanowire transistors in the R-matrix formalism

One of the major difficulties in solving the coupled Schrodinger–Poisson equations for open quantum systems is providing the wave functions for a large energy set. In this context, the R-matrix formalism provides an alternative method to obtain efficiently the wave functions. In a first step, which is energy independent, the eigenvalue problem associated with the quantum system is solved only once using fixed boundary conditions. Then, in a second step, the wave functions and transmission coefficients are obtained with a much lower computational effort for each energy. As an application, self-consistent potential and charge distribution, as well as the ballistic source-drain conductance, are calculated for a cylindrical nanowire transistor. The numerical accuracy with respect to basis cardinality is also discussed.

Reversal of Thermoelectric Current in Tubular Nanowires

We calculate the charge current generated by a temperature bias between the two ends of a tubular nanowire. We show that in the presence of a transversal magnetic field the current can change sign; i.e., electrons can either flow from the hot to the cold reservoir, or in the opposite direction, when the temperature bias increases. This behavior occurs when the magnetic field is sufficiently strong, such that Landau and snaking states are created, and the energy dispersion is nonmonotonic with respect to the longitudinal wave vector. The sign reversal can survive in the presence of impurities. We predict this result for core-shell nanowires, for uniform nanowires with surface states due to the Fermi level pinning, and for topological insulator nanowires.

Electronic and thermal conduction properties of halogenated porous graphene nanoribbons

We investigate the electronic and thermal properties of porous graphene (PG) structures passivated with halogen atoms as possible candidates for efficient thermoelectric devices in the framework of density functional theory (DFT) calculations. Armchair and zigzag halogenated PG nanoribbons are analyzed comparatively. The electronic properties are consistent with the expected behavior for the two types of terminations, however with marked influences introduced by the different halogen atoms. Depending on the pore sizes and halogen type pseudo-gaps in the phononic band structure are visible in the low frequency range, which are particularly important for the thermal conduction at low temperatures. The gaps are systematically displaced towards lower energies as the atomic number of the halogen increases. At the same time, the electronic gap decreases, which is also essential for attaining a large figure of merit in a thermoelectric device. This opens the possibility of tuning both electronic and thermal properties of PG structures by halogen passivation.

Conductance oscillations of core-shell nanowires in transversal magnetic fields

This work was financially supported by the research funds of Reykjavik University and of the University of Iceland, and by the Icelandic Research Fund. T.O.R. acknowledges support from a European Research Council Starting Grant. We are thankful to Thomas Schapers and Sebastian Heedt for very interesting discussions [23].

Spin-filtering effects in würtzite and graphite-like AlN nanowires with Mn impurities

Spin transport properties of magnetic nanowire systems--atomic-sized AlN nanowires with additional Mn impurities--are investigated employing ab initio constrained spin density functional theory calculations and nonequilibrium Greens functions formalism. The analyzed nanowire structures exhibit a stress-induced phase transition, between wurtzite and graphite-like configurations. In these quasi-one dimensional systems, the surface states ensure the basic prerequisite in establishing spin and charge transfer, by reducing the relatively large bandgap of the group III nitride semiconductor. The results show in how far this phase transition affects the surface states, focusing on the consequences which appear in the spin-filtering processes.