Distinguishing quantum and classical transport through nanostructures

We consider the question of how to distinguish quantum from classical transport through nanostructures. To address this issue we have derived two inequalities for temporal correlations in nonequilibrium transport in nanostructures weakly coupled to leads. The first inequality concerns local charge measurements and is of general validity; the second concerns the current flow through the device and is relevant for double quantum dots. Violation of either of these inequalities indicates that physics beyond that of a classical Markovian model is occurring in the nanostructure.     

Electronic transport properties of the Ising quantum Hall ferromagnet in a Si quantum well

Magnetotransport properties are investigated for a high mobility Si two-dimensional electron system in the vicinity of a Landau level crossing point. At low temperatures, the resistance peak having a strong anisotropy shows large hysteresis which is attributed to Ising quantum Hall ferromagnetism. The peak is split into two peaks in the paramagnetic regime. A mean field calculation for the peak positions indicates that electron scattering is strong when the pseudospin is partially polarized. We also study the current-voltage characteristics which exhibit a wide voltage plateau.     

Electronic transport in graphene nanostructures on SiO2

We report two experiments on graphene nanostructures. The first was performed on a graphene nanoribbon, where the nature of electronic transport was investigated in detail. Electrons or holes are found to localize in pockets of the potential along the ribbon. Transport is governed by the joint action of localization and Coulomb interaction. The temperature-dependence of the conductance shows activated behavior at temperatures above a few Kelvin. The activation energy retraces the edges of Coulomb blockade diamonds found in nonlinear transport. In the second experiment the metallic tip of a low-temperature scanning force microscope was scanned above a graphene quantum dot. In addition to the familiar Coulomb blockade fringes, localized states are detected forming in the constrictions connecting the dot to source and drain.     

Electronic and optical properties of fullerene nanostructures

Two new types of molecular/electronic fullerene nanostructures are considered: 1) highly stable hydrated clusters (I _h symmetry group) and microcrystals (T _h symmetry group) of fullerene C₆₀ in water solution and 2) the single-walled carbon nanotube from C₆₀ fullerenes. The vibrational spectra of these fullerene nanostructures are calculated using molecular dynamics. The electronic properties of a single-walled fullerene nanotube are investigated using the tight-binding method. The theoretical results obtained were compared with available experimental data. Fiz. Tverd. Tela (St. Petersburg) 41, 885–887 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Monte Carle simulation of quantum transport through nanostructures

We describe a numerical scheme of combining Monte Carlo procedure and quantum scattering theory to simulate electron transport processes through nanostructures. The transport of electrons through a nanostructure is a highly nontrivial nonequilibrium process in which we should consider the interplay of (i) complicated many-body quantum states in nanostructure, (ii) thermal relaxation processes keeping the leads (electron reservoirs) in local equilibrium, (iii) the coupling between the leads and the nanostructure, and (iv) the bias causing nonequilibrium, current, and evolution of quantum states in the nanostructure. Considering the quantum coherence within the nanostructure, we include the degrees of freedom of the nanostructure and a single tunneling electron and solve the Schrödinger equation for the many-body states to obtain the scattering matrix in the Fock space from which both the transmission of the electron and the variation of the states in nanostructure can be full quantum-mechanically calculated. The transport is investigated by the Monte Carlo simulation of successive scattering events of single electrons which are sampled with the Metropolis scheme governed by the scattering probabilities, the thermal statistics in the leads, and the applied bias. By this way from a given initial nanostructure state we can calculate the time evolutions of the current and the internal state. As examples we investigate the transmission of electrons through a two-level system. It is shown that the proposed method can properly deal with the inelastic effects in transport processes.     

Graphene-based superconducting quantum point contacts

We investigate the Josephson effect in the graphene nanoribbons of length L smaller than the superconducting coherence length and an arbitrary width W. We find that in contrast to an ordinary superconducting quantum point contact (SQPC), the critical supercurrent I_c is not quantized for the nanoribbons with smooth and armchair edges. For a low concentration of the carriers, I_c decreases monotonically with lowering W/L and tends to a constant minimum for a narrow nanoribbon with . The minimum I_c is zero for the smooth edges but for the armchair edges. At higher concentrations of the carriers this monotonic variation acquires a series of peaks. Further analysis of the current-phase relation and the Josephson coupling strength I_cR_N in terms of W/L and the concentration of carriers revels significant differences with those of an ordinary SQPC. On the other hand for a zigzag nanoribbon, we find that, similar to an ordinary SQPC, I_c is quantized but to the half-integer values . PACS 74.45.+c; 74.50.+r; 73.63.-b; 74.78.Na     

Electronic structure and quantum transport in carbon nanotubes

Received: 3 April 1998     

Real-space calculations for electron transport properties of nanostructures

Recent developments in the fabrication and investigation of conductors of atomic dimensions have stimulated a large number of experimental and theoretical studies on these nanoscale devices. In this paper, we introduce examples presenting the efficiencies and advantages of a first-principles transport calculation scheme based on the real-space finite-difference (RSFD) formalism and the overbridging boundary-matching (OBM) method. The RSFD method does not suffer from the artificial periodicity problems that arise in methods using plane-wave basis sets or the linear dependence problems that occur in methods using atomic basis sets. Moreover, the algorithm of the RSFD method is suitable for massively parallel computers and, thus, the combination of the RSFD and OBM methods enables us to execute first-principles transport calculations using large models. To demonstrate the advantages of this method, several applications of the transport calculations in various systems ranging from jellium nanowires to the tip and surface system of scanning tunneling microscopy are presented.     

Electronic transport in a side-coupled triple quantum dot array

The electronic transport through a side-coupled triple quantum dot array (QDA) is investigated by means of Green function technique within the tight-binding framework. We obtain the formula of the linear conductance of QDA. The linear conductance spectrum is numerically studied. We discuss the feasibility of applying our structure to the electron spin polarized device and calculate the ratio of the spin polarized current flows.     

Electronic transport and quantum hall effect in bipolar graphene p-n-p junctions

We have developed a device fabrication process to pattern graphene into nanostructures of arbitrary shape and control their electronic properties using local electrostatic gates. Electronic transport measurements have been used to characterize locally gated bipolar graphene p-n-p junctions. We observe a series of fractional quantum Hall conductance plateaus at high magnetic fields as the local charge density is varied in the p and n regions. These fractional plateaus, originating from chiral edge states equilibration at the p-n interfaces, exhibit sensitivity to interedge backscattering which is found to be strong for some of the plateaus and much weaker for other plateaus. We use this effect to explore the role of backscattering and estimate disorder strength in our graphene devices.     

Nonlinear transport properties of quantum dots

The influence of excited levels on nonlinear transport properties of a quantum dot weakly coupled to leads is studied using a master-equation approach. A charging model for the dot is compared with a quantum mechanical model for interacting electrons. The currentvoltage curve shows Coulomb lockade and additional finestructure that is related to the excited states of the correlated electrons. Unequal coupling to the leads causes asymmetric conductance peaks. Negative differential conductances are predicted due to the existence of excited states with different spins.     

Nonlocal optical properties in AlGaAs/GaAs coupled quantum well nanostructures

Nonlocal intersubband optical absorption properties in AlGaAs/GaAs coupled quantum well (CQW) nanostructures are investigated for a p-polarized light in the case of taking spatial nonlocality of optical response into account. The numerical results show that the spatial nonlocality of optical response can lead optical spectrum lines to have a radiation shift due to the nonlocal effects, and the spatial nonlocality is associated closely with the coupling effects between the potential wells of the CQW system. The dependence of the radiation shift on structure parameters of the CQW is clarified. It is also demonstrated that the maximal radiation shift and the least optical absorbance can be obtained by optimizing structure parameters of the CQW. These results may be constructive in designing nanomaterials with various nonlocality and observing the spatial nonlocal effects in experiment.     

Assembly of quantum dots on peptide nanostructures and their spectroscopic properties

We present a chemical process for the decoration of self-assembled two-dimensional peptide fibrils with two different sizes of CdSe@ZnS core–shell quantum dots (Qdots) capped with trioctylphosphine oxide molecules. The attachment of the semiconducting nanoparticles to the fibrils is directed via disulfide bond between the quantum dot and cysteine aminoacids that are deliberately impeded in the peptide structures. A significant red shift in the emission spectra of the quantum dots is observed and attributed to the synergistic interaction between adjacent nanoparticles arranged on peptide film templates extending over hundreds of nanometers.     

Electronic transport through a ring-shaped array of quantum dots

Making use of the equation of motion method and Keldysh Green function technique, we have developed a calculation method for the ring-shaped array of quantum dots with arbitrary dots. A general formula for the current under dc bias is obtained; the transmission probability and the differential conductance are numerically studied.     

Electronic states in magnetic nanostructures

The electronic states responsible for oscillatory magnetic coupling, giant magnetoresistance (GMR), and spin-polarized tunneling are explored. They occupy well-defined locations in (E, k) space. Their energy E has to be within a few kT of the Fermi level, a range that is now becoming accessible to high-resolution photoemission. Particular attention is paid to k-regions near the Fermi level crossings of the s, p-band, where a sizable group velocity is combined with a d-like magnetic splitting and spin-polarization. These electronic states can be tailored by quantization in structures with single-digit nanometer dimensions, such as two-dimensional quantum wells and one-dimensional arrays of stripes and dots. Such arrays can be produced by self-assembly on top of stepped silicon surfaces.     

Electronic and transport properties of boron-doped graphene nanoribbons

We report a spin polarized density functional theory study of the electronic and transport properties of graphene nanoribbons doped with boron atoms. We considered hydrogen terminated graphene (nano)ribbons with width up to 3.2 nm. The substitutional boron atoms at the nanoribbon edges (sites of lower energy) suppress the metallic bands near the Fermi level, giving rise to a semiconducting system. These substitutional boron atoms act as scattering centers for the electronic transport along the nanoribbons. We find that the electronic scattering process is spin-anisotropic; namely, the spin-down (up) transmittance channels are weakly (strongly) reduced by the presence of boron atoms. Such anisotropic character can be controlled by the width of the nanoribbon; thus, the spin-up and spin-down transmittance can be tuned along the boron-doped nanoribbons.