Narrow Results

We have recently reported on the synthesis and characterization of a new form of nanostructured graphene that we call "nanoperforated graphene". Nanoperforated graphene is fabricated by etching a periodic array of nanoscale holes into atomic membranes of graphene to create an ultrathin superlattice-like structure. Nanoperforated graphene demonstrates semiconductor-like behavior and we have realized room-temperature field-induced conductance modulation as high as 450 (compared with < 10 for unpatterned graphene) with field-effect mobilities of ~ 1 cm²V^-1s^-1. Here, we discuss the conduction mechanisms in nanoperforated graphene and the relevance of this new material for field-effect transistor devices. In nanoperforated graphene with 15 nm nanoconstrictions, we observe that the low-bias mobility is independent of temperature, consistent with elastic scattering-limited conduction. At low temperatures, a transport gap limits conduction in the sub-threshold regime and affects the threshold voltage for band conduction. We show that the high-bias electrical characteristics of nanoperforated graphene are similar to "artificial solids," a class of materials made of 2D arrays of Coulomb islands, consistent with observed Coulomb Blockade features in the sub-threshold regime. Currently, the device characteristics of the nanopatterned graphene material are found to be suitable for large-area, thin-film transistor applications. Future higher-performance applications are expected.

We find an analytical expression for the conductance of a single electron transistor in the regime when temperature, level spacing and charging energy of a grain are all of the same order. We consider the model of equidistant energy levels in a grain in the sequential tunneling approximation. In the case of spinless electrons, our theory describes transport through a dot in the quantum Hall regime. In the case of spin-½ electrons, we analyze the line shape of a peak, shift in the position of the peak's maximum as a function of temperature and the values of the conductance in the odd and even valleys.

The electrical transport and magnetoresistance properties of the polycrystalline La_0.67Ca_0.33MnO₃ film produced on a Pyrex substrate were investigated for the first time. X-ray powder diffraction indicated that the film sample has a perovskite structure. Scanning electron microscope indicated that La_0.67Ca_0.33MnO₃ film thickness is approximately 500 nm, and the average grain size of this sample varies between 40 and 50 nm. La_0.67Ca_0.33MnO₃ film showed a phase transition from paramagnetic to ferromagnetic at (T_C) 80 K and a metal–insulator transition at (T_MI) 77.5 K and at 2 mT magnetic field. The upturn of the resistance observed at low temperatures (<36 K) was attributed to the Coulomb blockade, and the strong structural disorder is due to the large lattice mismatch and strain relaxation. A large magnetoresistance ratio [MR (%)] of 780% was observed at 100 K and 6 T magnetic field.

An accurate theoretical treatment of electron-electron interactions in mesoscopic systems is available in very few cases and approximation schemes are developed in most of the applications, especially for many-level quantum dots. Here we present transport calculations within the random-phase approximation for the Coulomb interaction using the Keldysh Green's functions formalism. We describe the quantum dot systems by a tight-binding Hamiltonian. Our method is similar to the one used by Faleev and Stockman [Phys. Rev. B 66 085318 (2002)] in their study of the equilibrium properties of a homogeneous 2D electron gas. The important extension at the formal level is that we combine the RPA and the Keldysh formalism for studying non-linear transport properties of open quantum dots. Within the Keldysh formalism the polarization operator becomes a contour-ordered quantity that should be computed either from the non-interacting Green functions of the coupled quantum dot (the so-called G₀W approximation) either self-consistently (GW approximation). We performed both non-selfconsistent and self-consistent calculations and compare the results. In particular we recover the Coulomb diamonds for interacting quantum dots and we discuss the charge sensing effects in parallel quantum dots.

Graphene single electron transistor (SET) as a coulomb blockade (CB) device operates based on the quantum mechanical effect. Its desired current is achieved by overcoming the CB energy that depends on the total capacitance of SET. Therefore, small size of graphene quantum capacitance is suitable for SET structure because it plays a dominant role in the total capacitance. In this paper, the density of state (DOS) model of graphene SET is suggested because of its important effect on many physical properties. Furthermore, carrier concentration as a key factor in quantum capacitance is modeled. Finally, the quantum capacitance of graphene SET based on the fundamental parameters is modeled and compared to the experimental data, so an acceptable agreement between them is reported. As a result, silicon SET can be replaced with graphene SET because of its lower quantum capacitance and also higher operation speed than the silicon one.

We study the conductance spectrum of graphene quantum dots, both single- and multiple-dot cases. The single electron tunneling through a graphene dot is investigated and the periodicity, amplitude and line shape of the Coulomb blockade oscillations at low temperatures are obtained, which are consistent with the recent experimental observations. Further, we discuss the transport behavior when multiple dots are assembled in array and find a phase transition of conductance spectra from individual Coulomb blockade to collective Coulomb blockade.

In this study, we carried out the analysis of the thermal fluctuations on the dynamics of small Josephson junction (JJ). An expression for the fluctuation of Coulomb blockade edge in the case of low and high growing rate of voltage was obtained. It was shown that dynamics of small-sized JJ under thermal fluctuations is determined by the energy ratio-parameter, temperature and growing rate of voltage.

We report the results of electron tunneling and Coulomb blockade in nanocrystalline silicon (nc-Si) double-barrier floating-gate structure (SiO₂/nc-Si/SiO₂) fabricated in situ in a plasma-enhanced chemical-vapor-deposition (PECVD) system for the nanoelectronic devices application. The quantum confinement and Coulomb blockade effect have been demonstrated in the capacitance–voltage (C–V) characteristics, in which unique peak structures differ remarkably from the normal smooth C–V curves. The experimental results have been explained by band diagram and equivalent circuits. By contrasted with silicon single electron transistor memory made by using ultra fancy nanotechnology, nc-Si-based memory can be fabricated with a minimum perturbation of conventional silicon technology and may be closest to industrial application.

The present paper discusses the investigation of electronic properties of anthracene-based single electron transistor (SET) operating in coulomb blockade region using Density Functional Theory (DFT) based Atomistix toolkit-Virtual nanolab. The charging energies of anthracene molecule in isolated as well as electrostatic SET environments have been calculated for analyzing the stability of the molecule for different charge states. Study also includes the analysis of SET conductance dependence on source/drain and gate potentials in reference to the charge stability diagram. Our computed charging energies for anthracene in isolated environment are in good agreement with the experimental values and the proposed anthracene SET shows good switching properties in comparison to other acene series SETs.

In the framework of the density functional theory and method of nonequilibrium Green functions (DFT $+$ NEGF), the electric transport properties of the model nanojunction “Graphene–Fullerene C${}_{60}$–Graphene” were studied. The transmission spectra, the density of states, the current–voltage characteristic (CVC) and the differential conductivity of the nanojunction are determined. The appearance of a feature of the DOS nanotransition is revealed. This is due to the fact that the Lowest Unoccupied Molecular Orbital (LUMO) of C${}_{60}$ becomes closer to the Fermi level of metal substrates than its Highest Occupied Molecular Orbital (HOMO). It is shown that Coulomb stairs associated with the Coulomb blockade effect appear on the CVC of the nanotransition. The same changes are observed on the differential conductivity spectrum in the form of eight distinct peak structures arising with period $\mathrm{\Delta }V\approx 0.683$V. The comparison of the electric transport characteristics of single-fullerene nanodevices with various electrode materials (graphene, gold, platinum) are presented. It was found that the voltage period of Coulomb features $\mathrm{\Delta }V$ in a nanodevice with graphene electrodes is less than in nanodevices with platinum and gold electrodes. It was revealed that the considered nanotransition has negative differential conductivity. The results obtained can be useful in calculating promising elements of single-electronics.

The application of graphyne for a single-electron transistor (SET) that is operating in the Coulomb blockade regime is investigated in the first principles framework. Density functional theory modeling for graphyne has been used and the device environment has been described by a continuum model. The interaction between graphyne and the SET environment is treated with self-consistent Poisson equations. The charging energy as a function of gate voltage thus calculated has been used to obtain the charge stability diagram for the present system. The effect of electrode separation and the position of the molecule with respect to the dielectric on the gate coupling have been studied further. As compared with the previously studied systems on this line, graphyne has been observed to provide the gate coupling that is nearly close to that of benzene and graphene, but significantly greater than fullerene-based systems.

Using the Majorana representation and the Schwinger-Keldysh approach, we evaluate the distribution of current through a single-electron transistor for the large conductance. The strong quantum fluctuations induce the lifetime broadening of charge-state levels out of equilibrium, as well as the renormalization effect. We find that the lifetime broadening suppresses higher cumulants of current fluctuations.

We study the transport properties of a single InAs self-assembled quantum dot contacted with superconducting leads. The charging energy E_c of the quantum dot is much larger than the superconducting gap energy Δ. For the dot whose tunnel coupling Γ to the lead is much larger than Δ but smaller than E_c, we observe enhancement of first-order Andreev reflections by the Kondo effect. We find that the zero-bias conductance measured for various Δ's and Kondo temperature T_K's collapses onto a single curve with Δ/T_K as the only relevant energy scale, providing experimental evidence for universal scaling in this system. On the other hand, for the dot with Γ comparable to E_c we observe a supercurrent flowing through the dot, reflecting the charge fluctuation sufficiently greater that one.

Resonant tunneling through an open quantum dot in the Coulomb blockade regime T ≪ e²/C₀ is studied numerically using the path-integral Monte Carlo method. The quantum dot is connected to two bulk leads by single-mode point contacts. We consider on-site repulsion between electrons and analyze its influence on the resonant conductance. Our numerical results exhibit power-law dependence of the peak width and the phase transition between perfect conductor and insulator, which is consistent with the analytical results.

An accurate theoretical treatment of electron-electron interactions in mesoscopic systems is available in very few cases and approximation schemes are developed in most of the applications, especially for many-level quantum dots. Here we present transport calculations within the random-phase approximation for the Coulomb interaction using the Keldysh Green's functions formalism. We describe the quantum dot systems by a tight-binding Hamiltonian. Our method is similar to the one used by Faleev and Stockman [Phys. Rev. B 66 085318 (2002)] in their study of the equilibrium properties of a homogeneous 2D electron gas. The important extension at the formal level is that we combine the RPA and the Keldysh formalism for studying non-linear transport properties of open quantum dots. Within the Keldysh formalism the polarization operator becomes a contour-ordered quantity that should be computed either from the non-interacting Green functions of the coupled quantum dot (the so-called G₀W approximation) either self-consistently (GW approximation). We performed both non-selfconsistent and self-consistent calculations and compare the results. In particular we recover the Coulomb diamonds for interacting quantum dots and we discuss the charge sensing effects in parallel quantum dots.