Nonequilibrium Green Function Method Research Articles

Size-dependent magnetic order and giant magnetoresistance in organic titanium–benzene multidecker cluster

Using density functional theory and non-equilibrium Green's function method, we investigated the magnetic and transport properties of small organic titanium-benzene sandwich clusters TinBzn+1 (n = 1-3). The results show that TiBz2 is nonmagnetic while Ti2Bz3 and Ti3Bz4 are ferromagnetic, and our prediction is in agreement with experimental observation. The double exchange mechanism plays a key role in the ferromagnetism of larger clusters. With Ni as the two electrodes, significant spin-filter efficiency (SFE) and giant magnetoresistance (GMR) were found in the TinBzn+1 molecular junction. These transport properties could be controlled by cluster size, bias voltage or gate voltage. Specially, a sign-reversible GMR effect was observed in the Ti2Bz3 molecular junction. Finally, the microscopic mechanisms of SFE and GMR were suggested.

Electron transport through T-shaped double quantum dot molecule Aharonov-Bohm interferometer

Using non-equilibrium Green's function method, the charge and spin transport properties through T-shape double quantum dot molecule Aharonov-Bohm (A-B) interference are theoretically investigated. Resonance or anti-resonance can occur at the same location in conductance spectrum by controlling coupling or uncoupling between two quantum dots in T-shape double quantum dot molecule, which is the basis for designing quantum switches. When two identical T-shaped double quantum dot molecules are embedded in two arms of A-B interferometer, respectively, totally destructive interference can appear by taking appropriate magnetic flux. Spin current through the system can be regulated by adjusting quantum dot level, bias between two electrodes and Rashba spin-orbit interaction.

Spin-filtering and charge- and spin-switching effects in a quantum wire with periodically attached stubs

Spin-dependent electron transport in a periodically stubbed quantum wire in the presence of Rashba spin-orbit interaction (SOI) is studied via the nonequilibrium Green's function method combined with the Landauer-Buttiker formalism. The coexistence of spin filtering, charge and spin switching are found in the considered system. The mechanism of these transport properties is revealed by analyzing the total charge density and spin-polarized density distributions in the stubbed quantum wire. Furthermore, periodic spin-density islands with high polarization are also found inside the stubs, owing to the interaction between the charge density islands and the Rashba SOI-induced effective magnetic field. The proposed nanostructure may be utilized to devise an all-electrical multifunctional spintronic device.

Spin-dependent ballistic transport properties and electronic structures of pristine and edge-doped zigzag silicene nanoribbons: large magnetoresistance

The electronic structure and conductance of substitutionally edge-doped zigzag silicene nanoribbons (ZSiNRs) are investigated using the nonequilibrium Green's function method combined with the density functional theory. Two-probe systems of ZSiNRs in both ferromagnetic and antiferromagnetic states are considered. Doping effects of elements from groups III and V, in a parallel or antiparallel magnetic configuration of the two electrodes, are discussed. By switching on and off the external magnetic field, we may convert the metallic ferromagnetic ZSiNRs into insulating antiferromagnetic ZSiNRs. In the ferromagnetic state, even- or odd-width ZSiNRs exhibit a drastically different magnetoresistance. In an odd-width edge-doped ZSiNR a large magnetoresistance occurs compared to that in a pristine ZSiNR. The situation is reversed in even-width ZSiNRs. These phenomena result from the drastic change in the conductance in the antiparallel configuration.

Theoretical calculation of electron transport properties of the Au-Si60-Au molecular junctions

The ground structure of Si60 clusters, which was obtained by optimization when using the density functional theory method, is a fullerene structure with C1 point group, a diameter 1.131 nm, the average bond length 0.239 nm, and the difference between the energies of the lowest unoccupied molecular orbital and the highest occupied molecular orbital is 0.72 eV. A Si60 cluster with optimized structure is sandwiched between two semi-infinite Au(100)-44 electrodes, and the Au-Si60-Au molecular junctions is constructed, whose electron transport properties is investigated with a combination of density functional theory and non-equilibrium Green's function method. When the distance between the two electrodes is 1.74 nm, the equilibrium conductance of the junctions is 1.93 G0 (G0=2e2/h). In the range of voltage from -2.02.0 V, we have calculated the current and conductance under different voltages, and find that the Ⅰ-Ⅴ curve of the junctions show linear characteristics. We also analyze the properties of transport from transmission and frontier molecular orbitals, and discuss the relationship of transfer charge with conductance.

Calculation of electron transport in GaAs nanoscale junctions using first-principles

Electron transport properties of GaAs cluster, which is sandwiched between two semi-infinite Au(100)-3×3 electrodes in four different anchoring configurations (top-top, top-hollow, hollow-top, hollow-hollow), is investigated using the combination of density functional theory and non-equilibrium Green's function method. We optimize the geometry of junctions at different distances, simulate the breaking process of Au-GaAs-Au junctions, calculate the cohesion energy and conductance of the junctions as functions of distance dz, and obtain the most stable structure when the distances are set at 1.389 nm, 1.145 nm, 1.145 nm, 0.861 nm, respectively. For stable structures, the Ga-As bond lengths of the junctions is 0.222 nm, 0.235 nm, 0.227 nm, 0.235 nm, respectively. The equilibrium conductances are 2.33 G0, 1.20 G0, 1.90 G0, 1.69 G0,respectively. All junctions have large conductance. In the range of voltage from -1.2–1.2 V, the I-V curve of the junctions shows linear characteristics.

Electronic and transport properties of carbon and boron-nitride ferrocene nanopeapods

Electronic and transport properties of novel ferrocene based carbon nanotube (CNT) and boron-nitride nanotube (BNNT) nanopeapods, including Fe(Cp)2@CNT, Fe2(Cp)3@CNT, Fe(Cp)2@BNNT, and Fe2(Cp)3@BNNT (where Cp refers as cyclopentadiene), are investigated using the density functional theory and non-equilibrium Green's function methods.

Atomistic Calculations of Heat Transport in a Silicon Crystal

We have performed atomistic calculations of phonon transport properties. The dynamical matrix is obtained from ab initio calculations, and phonon transport properties are analyzed by the nonequilibrium Green's function method. As an example of the applications of the method, we present the temperature dependence of heat transport properties and the phonon lifetime due to phonon-phonon scatterings by the anharmonic terms of interatomic forces for a silicon crystal. [DOI: 10.1380/ejssnt.2014.154]

Ab initio calculations of quantum transport of Au–GaN–Au nanoscale junctions

We investigate the contact geometry and electronic transport properties of a GaN pair sandwiched between Au electrodes by performing density functional theory plus the non-equilibrium Green's function method.

Effects of Interference between Energy Absorption Processes of Molecule and Surface Plasmons on Light Emission Induced by Scanning Tunneling Microscopy

A theoretical analysis of plasmon and molecular luminescence induced by scanning tunneling microscopy is carried out in the case of luminescence from clean metal surfaces using a molecule-covered tip and from molecular layers. The effects of coupling between molecular exciton and surface plasmon (exciton–plasmon coupling) on the luminescence properties of the molecule and surface plasmons are investigated using the nonequilibrium Green's function method. The enhancement by molecular electronic and vibrational modes (molecular modes) and the energy absorption by the creation of molecular excitons lead to peak and dip structures in the luminescence spectra of surface plasmons. The re-emission of energy in surface plasmons by exciton annihilation leads to a dent structure in their luminescence spectra. It is found that the processes of energy absorption by the creation of molecular excitons and the subsequent re-emission by exciton annihilation interfere with each other, resulting in the enhancement and suppression of these energy-transfer processes. Owing to exciton–plasmon coupling, excitation channels of the molecule appear in the energy range lower than the energy of molecular exciton. It is found that electronic transitions via these excitation channels give rise to molecular luminescence and vibrational excitations at a bias voltage lower than the ratio of the energy of molecular exciton to the elementary charge. The results also indicate that vibrational excitations assist in the emission of photons whose energy exceeds the product of the elementary charge and bias voltage (upconverted luminescence). The trends in the luminescence spectra predicted by the developed model are found to be in good agreement with the experimental data reported recently.

Structural and tunneling properties of Si nanowires

We investigate the electronic structure and electron transport properties of Si nanowires attached to Au electrodes from first principles using density functional theory and the nonequilibrium Green's function method. We systematically study the dependence of the transport properties on the diameter of the nanowires, on the growth direction, and on the length. At the equilibrium Au-nanowire distance we find strong electronic coupling between the electrodes and nanowires, which results in a low contact resistance. With increasing nanowire length we study the transition from metallic to tunneling conductance for small applied bias. For the tunneling regime we investigate the decay of the conductance with the nanowire length and rationalize the results using the complex band structure of the pristine nanowires. The conductance is found to depend strongly on the growth direction, with nanowires grown along the $\ensuremath{\langle}110\ensuremath{\rangle}$ direction showing the smallest decay with length and the largest conductance and current.

Structural, magnetic and transport properties of carbon chains sandwiched between zigzag graphene nanoribbons

We investigate the structural, magnetic properties and spin polarized transport of monatomic carbon chains covalently interconnected between zigzag graphene nanoribbon (ZGNR) leads with first-principles and non-equilibrium Green's function methods. It is revealed that an even numbered carbon chain is nonmagnetic when connected at the center of the armchair side of ZGNRs, but it becomes spin polarized and a net magnetic moment (about 0.1 μB) is developed when it moves to the edge. In contrast, an odd-numbered carbon chain is magnetic when it lies at the center of the armchair side and the magnetic moment decreases when the carbon chain moves to the edge. The total magnetic moment of the ZGNR/Cn/ZGNR junction depends on the magnetic alignment between two zigzag edges. Compared with that of ZGNR/C9/ZGNR with C9 chain anchoring at the center, the magnetic moment of the junction increases by 2.2 μB in the antiferromagnetic configuration and decreases by 1.5 μB in the ferromagnetic alignment when the carbon chains migrate from the center to one edge. Thus, the ZGNR/Cn/ZGNR junctions show rich magnetic varieties by modulating the carbon chain anchoring sites and the zigzag edge magnetic coupling. Further, spin-dependent transport calculations indicate that the ZGNR/C9/ZGNR junction behaves as a perfect spin-filter in spintronic circuits.

Spintronic transport of a non-magnetic molecule between magnetic electrodes

The spintronic transport properties of a junction system composed of a non-magnetic molecule sandwiched between ferromagnetic metal electrodes are investigated theoretically using a non-equilibrium Green's function method based on density functional theory. It is revealed that in such a system, the molecular magnetic properties induced by hybridization with the magnetic electrodes play a crucial role. Alignment of the induced molecular spin-split levels is strongly related to the spin injection and tunneling magneto-resistance effects. It is found that in the system with weaker molecule-electrode interaction, stronger spintronic effects of the spin injection and tunneling magneto-resistance are observed.

Spin-valve giant magnetoresistance in scandium-benzene sandwich cluster

Using density functional theory and non-equilibrium Green's function method, we investigate the magnetic and transport properties of organic Scn(C6H6)n+1 () sandwich clusters. The results show that the sandwiches possess high stabilities and large magnetic moments, and our prediction is in agreement with the experimental observation. With Ni as two electrodes, significant spin-valve giant magnetoresistance was found in Sc3(C6H6)4 molecular junction. Furthermore, all the sandwiches can be viewed as a new kind of spin filter. Specially, by changing the magnetization orientation of one electrode, Sc2(C6H6)3 molecular junction could effectively control the spin orientation of the electron in the system.

Spin transport and spin polarization properties in double-stranded DNA

We study the spin-dependent electron transport through a double-stranded DNA (dsDNA) using the Bogoliubov-de Gennes equations and non-equilibrium Green's function method. We calculate the spin-dependent electron conductance and spin-polarization for different lengths, helix angles, twist angles of dsDNA, the environment-induced dephasing factors, and hopping integral. It is shown that the conductance decreases by increasing the length and dephasing factor. Also, we show that the spin-polarization depends on the helical symmetry and the length of DNA. It is shown that the double-stranded DNA can act as a perfect spin filter. Finally, we show that the sign of spin polarization can be inverted from +1 (−1) to −1 (+1) for some values of hopping integral.

Energy level alignment and quantum conductance of functionalized metal-molecule junctions: Density functional theory versus GW calculations

We study the effect of functional groups (CH3*4, OCH3, CH3, Cl, CN, F*4) on the electronic transport properties of 1,4-benzenediamine molecular junctions using the non-equilibrium Green function method. Exchange and correlation effects are included at various levels of theory, namely density functional theory (DFT), energy level-corrected DFT (DFT+Σ), Hartree-Fock and the many-body GW approximation. All methods reproduce the expected trends for the energy of the frontier orbitals according to the electron donating or withdrawing character of the substituent group. However, only the GW method predicts the correct ordering of the conductance amongst the molecules. The absolute GW (DFT) conductance is within a factor of two (three) of the experimental values. Correcting the DFT orbital energies by a simple physically motivated scissors operator, Σ, can bring the DFT conductances close to experiments, but does not improve on the relative ordering. We ascribe this to a too strong pinning of the molecular energy levels to the metal Fermi level by DFT which suppresses the variation in orbital energy with functional group.

Phonon-assisted tunneling and two-channel Kondo effect in a vibrating molecular dot coupled to Luttinger liquid leads

We study the joint effects of the electron–phonon interaction (EPI) and intralead electron interaction (IEI) on the density of states (DOS) of a single-molecular quantum dot weakly coupled to Luttinger liquid leads in the Kondo regime by using the extended non-equilibrium Green's function method. The introduction of the EPI yields satellite peaks around the Kondo peak. With the increase of the IEI, all the peak heights reduce and then turn to dips. The full crossover in the DOS from the phonon-assisted one-channel physics to two-channel physics is exhibited. The inelastic tunneling will dominate electron transport for a certain region of interaction strength.

Thermal spin current through a double quantum dot molecular junction in the Coulomb blockade regime

Based on non-equilibrium Green's function methods, we investigate the thermal spin current through a double quantum dot (DQD) molecular junction in the Coulomb blockade regime. An external magnetic field and a temperature difference are utilized to manipulate the electron spin degree of freedom in the DQD device. When the chemical potentials are aligned with the electron-hole symmetry point, a very steady pure-spin-current thermal generator is achieved. This is because the transmission nodes of different spin channels relative to chemical potentials have a perfect mirror symmetry configuration. In addition, the pure spin current also appears near resonant regions induced by the molecular states. Particularly interesting is that the sign of the pure spin current in the electron-hole symmetry point is opposite to those appearing near resonant regions in the strong Coulomb interaction regime.

Effects of Contact Geometry on the Transport Properties of a Silicon Atom

Contact geometry and electronic transport properties of a silicon atom sandwiched between Au electrodes in three different anchoring configurations are investigated by using the density functional theory combined with the non-equilibrium Green function method. We simulate the nanoscale junction breaking process and calculate the corresponding cohesion energy, obtain the equilibrium conductance and the projected density of states of junctions in an optimal postion. We also calculate the conductance and the current of junctions at the equilibrium position under small bias voltage. It is found that all junctions have large conductance and show a linear I—V relationship, but the current and conductance of a hollow-hollow configuration is always the biggest under the voltage range of −1.2 V∼1.2V. The calculated results proved that the coupling morphology of a silicon atom connected with electrodes has an important effect on the electronic transport properties of the nanoscale junction.

Possible origin of thermoelectric response fluctuations in single-molecule junctions

The thermoelectric response of molecular junctions exhibits large fluctuations, as observed in recent experiments (e.g. Malen J A et al 2009 Nano Lett. 10 3406). These were attributed to fluctuations in the energy alignment between the highest occupied molecular orbital (HOMO) and the Fermi level at the electrodes. By analyzing these fluctuations assuming resonant transport through the HOMO level, we demonstrate that fluctuations in the HOMO level alone cannot account for the observed fluctuations in the thermopower, and that the thermo-voltage distributions obtained using the non-equilibrium Green's function method, arguably the most common method at practice, are qualitatively different than those observed experimentally. We suggest that this inconsistency between theory and experiment is due to the level broadening that is inherently built-in to the calculation method, and smears out any variations of the transmission on energy scales smaller than the level broadening. We show that although this smearing only weakly affects the transmission, it has a large effect on the calculated thermopower. To amend this discrepancy, we introduce the method of open quantum systems and use it to calculate the thermopower and its fluctuations. We account for both the magnitude of the variations in the thermopower and the qualitative form of the distributions, and show that they arise not only from variations in the HOMO-Fermi level offset, but also from variations of the local density of states at the contact point between the molecule and the electrode.