Electron transport in carbon nanotube quantum dots in an axial magnetic field

The quantum conductance of the quantum dots (QDs) made of two kinds of primary carbon nanotubes (CNTs), i.e., armchair and zigzag CNTs, threaded by an axial magnetic field, has been studied by using the tight binding approximation and constant interaction model. It is found that under increasing axial magnetic field, each conductance shell of the zigzag CNT-QDs could split into two groups with each group of two peaks moving up or down, respectively. And the up- and down-moving two peaks would re-group with other two peaks, down- and up-moving, in the neighboring shell, forming a new four-peak shell, and then re-splitting, re-grouping again due to the Aharonov-Bohm effect, which is in agreement with those of experiments. But, in contrast, the conductance shells of the armchair CNT-QDs do not split by the magnetic field. Our subsequent theoretical studies show further that the above phenomena, i.e., the conductance shell-splitting, re-grouping, and re-splitting again with increasing the magnetic field exist in all the CNT-QDs except for the armchair one.     

Effects of Contact Atomic Structure on Electronic Transport in Molecular Junction

Based on nonequilibrium Green＇s function and first-principles calculations, we investigate the change in molecular conductance caused by different adsorption sites with the presence of additional Au atom around the metal- molecule contact in the system that benzene sandwiched between two Au（111） leads. The motivation is the variable situations that may arise in break junction experiments. Numerical results show that the enhancement of conductance induced by the presence of additional Au is dependent on the adsorption sites of anchoring atom. When molecule is located on top site with the presence of additional Au atoms, it can increase molecular conductance remarkably and present negative differential resistance under applied bias which cannot be found in bridge and hollow sites. Furthermore, the effects of different distance between additional Au and sulfur atoms in these three adsorption sites are also discussed.     

Characterization of conductance under finite bias for a self-assembled monolayer coated Au quantized point contact

We have demonstrated that an experimental cross-wire junction set-up can be used to measure the I-V characteristics of a self-assembled monolayer (SAM) stabilized metal quantized point contact. The increased stability due to the presence of the SAM allows the measurement of the I-V characteristics. However, the SAM also provides additional conductance paths in addition to the pure metal point contact. The presence of the SAM may contribute to the non-integral quantum conductance transition and the non-linear I-V characteristics of the quantum contact. Nonetheless, a straight I-V curve is obtained for the Au quantized point contact from 0 to 1 V with a conductance of approximately 1G₀, in contrast to previous work reported in the literature.     

Quantum transport in honeycomb lattice ribbons with armchair and zigzag edges coupled to semi-infinite linear chain leads

We study quantum transport in honeycomb lattice ribbons with either armchair or zigzag edges. The ribbons are coupled to semi-infinite linear chains serving as the input and output leads and we use a tight-binding Hamiltonian with nearest-neighbor hops. The input and output leads are coupled to the ribbons through bar contacts. In narrow ribbons we find transmission gaps for both types of edges. The appearance of this gap is due to the enhanced quantum interference coming from the multiple channels in bar contacts. The center of the gap is at the middle of the band in ribbons with armchair edges. This particle-hole symmetry is because bar contacts do not mix the two sublattices of the underlying bipartite honeycomb lattice when the ribbon has armchair edges. In ribbons with zigzag edges the gap center is displaced to the right of the band center. This breakdown of particle-hole symmetry is the result of bar contacts now mixing the two sublattices. We also find transmission oscillations and resonances within the transmitting region of the band for both types of edges. Extending the length of a ribbon does not affect the width of the transmission gap, as long as the ribbon’s length is longer than a critical value when the gap can form. Increasing the width of the ribbon, however, changes the width of the gap. In ribbons with zigzag edges the gap width systematically shrinks as the width of the ribbon is increased. In ribbons with armchair edges the gap is not well-defined because of the appearance of transmission resonances. We also find only evanescent waves within the gap and both evanescent and propagating waves in the transmitting regions.     

Stabilization of Au quantum point contacts by self-assembled monolayers

We examine conductance phenomenon for Au quantum point contacts (QPC) formed using a crossed-wire geometry experimental set-up. When one of the wires is coated with a self-assembled monolayer of an alkanethiol, we find that a conductance plateau indicative of a QPC can be stable for tens of seconds, exceeding typical periods of stability by several orders of magnitude. This extended stability is attributed to the inhibition of the diffusion of Au atoms away from the contact area by the presence of the self-assembled monolayer.     

Phonon thermal conductance of disordered graphene strips with armchair edges

Based on the model of lattice dynamics together with the transfer matrix technique, we investigate the thermal conductances of phonons in quasi-one-dimensional disordered graphene strips with armchair edges using Landauer formalism for thermal transport. It is found that the contributions to thermal conductance from the phonon transport near von Hove singularities is significantly suppressed by the presence of disorder, on the contrary to the effect of disorder on phonon modes in other frequency regions. Besides the magnitude, for different widths of the strips, the thermal conductance also shows different temperature dependence. At low temperatures, the thermal conductance displays quantized features of both pure and disordered graphene strips implying that the transmission of phonon modes at low frequencies are almost unaffected by the disorder.     

Structural properties of defected ZnO nanoribbons under uniaxial strain: Molecular dynamics simulations

Structural properties of various type and position defected zinc oxide nanoribbons with armchair and zigzag edges have been investigated via classical molecular dynamics simulations. An atomistic potential energy function has been used to represent the interactions among the atoms. A uniaxial strain has been applied to the generated ZnO nanostructures at two different temperatures of 1 K and 300 K. It has been found that ZnO nanoribbons under strain application exhibit a structural change depending on the temperature; the position and type of the defect; and the edge geometries of the nanoribbons.     

Spin Polarization and Andreev Conductance through a Diluted Magnetic Semiconductor Quantum Wire with Spin--Orbit Interaction

Spin-dependent Andreev reflection and spin polarization through a diluted magnetic semiconductor quantum wire coupled to normal metallic and superconductor electrodes are investigated using scattering theory. When the spin-orbit coupling is considered, more Andreev conductance steps appear at the same Fermi energy. Magnetic semiconductor quantum wire separates the spin-up and spin-down electrons. The Fermi energy, at which different- spin-state electrons begin to separate, becomes lower due to the effect of the spin-orbit interaction. The spin filter effect can be measured more easily by investigating the Andreev conductance than by investigating the normal conductance.     

Spin currents and magnetoresistance of graphene-based magnetic junctions

Using the tight-binding approximation and the nonequilibrium Green’s function approach, we investigate the coherent spin-dependent transport in planar magnetic junctions consisting of two ferromagnetic (FM) electrodes separated by a graphene flake (GF) with zigzag or armchair interfaces. It is found that the electron conduction strongly depends on the geometry of contact between the GF and the FM electrodes. In the case of zigzag interfaces, the junction demonstrates a spin-valve effect with high magnetoresistance (MR) ratios and shows negative differential resistance features for a single spin channel at positive gate voltage. In the case of armchair interfaces, the current-voltage characteristics behave linearly at low bias voltages and hence, both spin channels are in on state with low MR ratios.     

H2 dissociative adsorption at the armchair edges of graphite

We investigate and discuss how hydrogen behaves at the edges of a graphite sheet, in particular the armchair edge. Our density functional theory-based calculations results show that, in contrast to the zigzag edge [W.A. Diño, H. Nakanishi, H. Kasai, T. Sugimoto, T. Kondoe, e-J. Surf. Sci. Nanotech. 2 (2004) 77. [25]], regardless of orientation, there is an activation barrier hindering H₂ dissociation at the armchair edges. And once they do get dissociatively adsorbed at the armchair edges, we find that it would be extremely hard to desorb the H from their adsorption sites at the armchair edges. Furthermore, we also found that, consistent with our earlier conclusions [W.A. Diño, Y. Miura, H. Nakanishi, H. Kasai, T. Sugimoto, J. Phys. Soc. Jpn. 72 (2003) 1867. [24]], it is unlikely that we would find a whole H₂ in between plain graphite sheets.     

Fano effect in a T-shaped double quantum dot structure in the presence of Rashba spin-orbit coupling

The influence of Rashba spin-orbit coupling on the Fano lineshape of the conductance spectrum in a T-shaped double quantum dot structure is theoretically studied. By second-quantizing the electron Hamiltonian in this structure, it is found that the Rashba interaction brings about a spin-flip interdot hopping term. With the enhancement of the Rashba interaction, this term separates the two resonant peaks in the conductance spectrum from each other. More importantly, it causes the broadening of the narrow Fano peak, and the narrowing of the broader peak. Finally, the asymmetric Fano lineshape changes into a symmetric profile in the global conductance spectrum.     

Single electron tunneling at large conductance: The semiclassical approach

We study the linear conductance of single electron devices showing Coulomb blockade phenomena. Our approach is based on a formally exact path integral representation describing electron tunneling nonperturbatively. The electromagnetic environment of the device is treated in terms of the Caldeira-Leggett model. We obtain the linear conductance from the Kubo formula leading to a formally exact expression which is evaluated in the semiclassical limit. Specifically we consider three models. First, the influence of an electromagnetic environment of arbitrary impedance on a single tunnel junction is studied focusing on the limits of large tunneling conductance and high to moderately low temperatures. The predictions are compared with recent experimental data. Second, the conductance of an array of N tunnel junctions is determined in dependence on the length N of the array and the environmental impedance. Finally, we consider a single electron transistor and compare our results for large tunneling conductance with experimental findings. Received 2 February 2000     

Electronic Transport in Molecular Junction Based on C20 Cages

Choosing closed-ended armchair （5, 5） single-wall carbon nanotubes （CCNTs） as electrodes, we investigate the electron transport properties across an all-carbon molecular junction consisting of C20 molecules suspended between two semi-infinite carbon nanotubes. It is shown that the conductances are quite sensitive to the number of C20 molecules between electrodes for both configuration CF1 and double-bonded models： the conductances of C20 dimers are markedly smaller than those of monomers. The physics is that incident electrons easily pass the C20 molecules and are predominantly scattered at the C20-C20 junctions. Moreover, we study the doping effect of such molecular junction by doping nitrogen atoms substitutionally. The bonding property of the molecular junction with configuration CF1 has been analysed by calculating the Mulliken atomic charges. Our results have revealed that the C atoms in N-doped junctions are more ionic than those in pure-carbon ones, leading to the fact that N-doped junctions have relatively large conductance.     

Contact conductance between graphene and quantum wires

The contact conductance between graphene and two quantum wires which serve as the leads to connect graphene and electron reservoirs is theoretically studied. Our investigation indicates that the contact conductance depends sensitively on the graphene-lead coupling configuration. When each quantum wire couples solely to one carbon atom, the contact conductance vanishes at the Dirac point if the two carbon atoms coupling to the two leads belong to the same sublattice of graphene. We find that such a feature arises from the chirality of the Dirac electron in graphene. Such a chirality associated with conductance zero disappears when a quantum wire couples to multiple carbon atoms. The general result irrelevant to the coupling configuration is that the contact conductance decays rapidly with the increase of the distance between the two leads. In addition, in the weak graphene-lead coupling limit, when the distance between the two leads is much larger than the size of the graphene-lead contact areas and the incident electron energy is close to the Dirac point, the contact conductance is proportional to the square of the product of the two graphene-lead contact areas, and inversely proportional to the square of the distance between the two leads.     

Transport properties of a double quantum dot molecule in the presence of impurity effects

In this Letter, we studied the electronic transport through a parallel-coupled double quantum dot (DQD) molecule including impurity effects at zero temperature. The linear conductance can be calculated by using the Green's function method. An obvious Fano resonance arising from the impurity state in the quantum dot is observed for the symmetric dot-lead coupling structure in the absence of the magnetic flux through the quantum device. When the magnetic flux is presented, two groups of conductance peaks appear in the linear conductance spectra. Each group is decomposed into one Breit-Wigner and one Fano resonances. Tuning the system parameters, we can control effectively the shapes of these conductance peaks. The Aharonov-Bohm (AB) oscillation for the magnetic flux is also studied. The oscillation period of the linear conductance with π, 2π or 4π may be observed by tuning the interdot tunneling coupling or the dot-impurity coupling strengths.     

Precise implementation of cluster transfer matrix method in the single electron box

The partition function of the single electron box (SEB), a small metallic island connected by a tunnel junction to the source lead and by a gate capacitor to the gate, can be expressed in path-integral form, which contains the effective action of the collective variable, phase, after integrating out the background electron degrees of freedom. The cluster transfer matrix method (CTM) is applied to the SEB. By using an improved numerical algorithm and more intensive calculations with larger cluster size, we obtained a highly accurate result for the effective charging energy of SEB up to a large barrier conductance. With a clear converging tendency and the fact that we do not use any approximation in calculation of the partition function, our CTM calculation is systematic and exact. The result is in excellent agreement with the real time renormalization group method of König and Schoeller.     

Valley polarization enhancement and oscillation in asymmetric T junctions

We studied the valley dependent transport in a T junction consisting of an armchair lead and two zigzag leads. Electrons transmitted from the armchair lead to the two outgoing zigzag leads can be valley polarized. When the two outgoing leads have different widths, electrons are pushed into the wider lead and as a result, the valley polarization of the current in the narrow lead is enhanced with an oscillatory dependence on energy. The oscillation pattern is determined by the widths of the two zigzag leads. We analyzed the total local density of states of the device region of the junction and cannot find features that attribute this enhancement to quasi-bound state formation.     

Effects of Contact Geometry on Electron Transport of 1,4-Diaminobenzene

The equilibrium electron transport of 1,4-diaminobenzene sandwiched between two Au electrodes is simulated by using a first principles analysis. The results show that equilibrium conductance increases with the molecule- electrode distance decreasing, and a platform occurs at the distance varying from 1.4 A to 1.9 A, implying the insensitiveness of 1,4-diaminobenzene equilibrium conductance to molecule-electrode distance. This is helpful to understand the improved reliability and reproducibility of conductance measurements using amines.     

Flux-dependent anti-crossing of resonances in parallel non-coupled double quantum dots

We present novel resonant phenomena through parallel non-coupled double quantum dots (QDs) embedded in each arm of an Aharonov-Bohm (AB) ring with magnetic flux passing through its center. The electron transmission through this AB ring with each QD formed by two short-range potential barriers is calculated using a scattering matrix at each junction and a transfer matrix in each arm. We show that as the magnetic flux modulates, a distortion of the grid-like square transmission occurs and an anti-crossing of the resonances appears. Hence, the modulation of magnetic flux in this system can have an equivalent effect to the control of inter-dot coupling between the two QDs.