Transmission through a benzene ring, connected by sulfur contact atoms to gold leads, is calculated by a tight-binding model by means of the renormalization method. Attention is focused on the parameters associated with the contact atoms, namely their site energy and their bond energies with the ring and the leads. These parameters are found to have significant effects on the transmission probability function.
By expressing the discrete Schrodinger equation as a second-order finite-difference equation with constant coefficients, the renormalization equations for substituted benzene dimers are derived via the c_n-coefficient elimination procedure. On subjecting the benzene molecule to a linear applied field, the resulting field-modified site energies are obtained, by projecting the site-energy locations onto a corresponding benzene dimer axis. Incorporating these modified site energies into the Lippmann-Schwinger scattering theory, enables the field effect to manifest itself in the substituted benzen…
The Huckel molecular-orbital method (with overlap S) is used to derive the S-modified version of the renormalization equations, which are then employed to introduce overlap into the para-, meta- and ortho-benzene dimers' parameters. Invoking the Lippmann-Schwinger scattering theory enables the spectral energy transmission function T(E) to be found for each of the benzene types. The effect of overlap on the behaviour of the various T(E) curves is, indeed, marked, even for low values of S, where all the curves' symmetries become permanently broken. As S increases, the graphs become more distorte…
The renormalization method is applied to investigate the electron transmission properties of a circuit containing a benzene molecule, in which one of the carbon atoms has been modified so as to simulate displacement in position or replacement by another atom. Consideration of the different possible attachments of the leads, and the relative location of the modified atom, results in 9 distinct configurations to examine. For each configuration, the number and locations of anti-resonances, and whether they shift upon variation of the parameters, is seen to be the key to determining the shape of t…
The renormalization method has been previously used in conjunction with the Lippman-Schwinger equation to calculate transmission probabilities for benzene molecules, within the tight-binding approximation. Those results are extended here by presenting explicit formulas for the transmission probability functions, which are subsequently analysed for their anti-resonances, resonances and other maxima and minima.
Using the tight-binding model and the generalized Green's function formalism, the effect of quantum interference on the electron transport through the benzene molecule in a semiconductor/benzene/semiconductor junction is numerically investigated. We show how the quantum interference sources, different contact positions and local gate, can control the transmission characteristics of the electrode/molecule/electrode junction. We also study the occurrence of anti-resonant states in the transmission probability function using a simple graphical scheme (introduced in Ref.[Phys. Chem. Chem. Phys, 20…
The renormalization equations emerge from a Greenian-matrix solution of the discretized Schrodinger equation. A by-product of these equations is the decimation process, which enables substituted-benzenes to be mapped onto corresponding dimers, that are used to construct the series and parallel circuits of single-, double- and triple-dimers. The transmittivities of these circuits are calculated by the Lippmann-Schwinger theory, which yields the transmission-energy function T(E). The average value of T(E) provides a measure of the electron transport in the circuit in question. The undulating nat…
The transport through a quantum-scale device may be characterized by the transmission eigenvalues. These values constitute a junction PIN code where, for example, in single-atom metallic contacts the number of transmission channels is also the chemical valence of the atom. Recently, highly conductive single-molecule junctions (SMJ) with multiple transport channels have been formed from benzene molecules between Pt electrodes. Transport through these multi-channel SMJs is a probe of both the bonding properties at the lead-molecule interface and of the molecular symmetry. Here we utilize a many-…
For the first time we calculate the electron transmission phase through a quantum point contact (QPC). The QPC is considered in the saddle point approximation in the single-electron picture. We show that when the electron energy is close to the height of the potential barrier the transmission-phase depends linearly on the energy. The coefficient in the linear dependence is logarithmically enhanced by the ratio of the height over the curvature of the barrier potential. We compare the calculated transmission phase with the first experimental measurements of the phase.
We measure the transmission phase of a quantum point contact (QPC) at a low carrier density in which electron interaction is expected to play an important role and anomalous behaviors are observed. In the first conductance plateau, the transmission phase shifts monotonically as the carrier density is decreased by the gate voltage. When the conductance starts to decrease, in what is often called the 0.7 regime, the phase exhibits an anomalous increase compared with the noninteracting model. The observation implies an increase in the wave vector as the carrier density is decreased, suggesting a …
While mesoscopic conducting loops are sensitive to external magnetic fields, as seen by observations of the Aharonov-Bohm (AB) effect in such structures, the field needed to observe the AB periodicity in small molecular rings is unrealistically large. The present study aims to identify conditions under which magnetic field dependence can be observed in electronic conduction through such molecules. We consider molecular ring structures modeled both within the tight-binding (Hückel) model and as continuous rings. In fact, much of the observed qualitative behavior can be rationalized in terms of …
We calculate transmission eigenvalue distributions for Pt-benzene-Pt and Pt-butadiene-Pt junctions using realistic state-of-the-art many-body techniques. An effective field theory of interacting $π$-electrons is used to include screening and van der Waals interactions with the metal electrodes. We find that the number of dominant transmission channels in a molecular junction is equal to the degeneracy of the molecular orbital closest to the metal Fermi level.
Evidence is presented for the finite wave vector crossing of the two lowest one-dimensional spin-split subbands in quantum point contacts fabricated from two-dimensional hole gases with strong spin-orbit interaction. This phenomenon offers an elegant explanation for the anomalous sign of the spin polarization filtered by a point contact, as observed in magnetic focusing experiments. Anticrossing is introduced by a magnetic field parallel to the channel or an asymmetric potential transverse to it. Controlling the magnitude of the spin-splitting affords a novel mechanism for inverting the sign o…
We predict an enormous order-dependent quantum enhancement of thermoelectric effects in the vicinity of a higher-order `supernode' in the transmission spectrum of a nanoscale junction. Single-molecule junctions based on 3,3'-biphenyl and polyphenyl ether (PPE) are investigated in detail. The nonequilibrium thermodynamic efficiency and power output of a thermoelectric heat engine based on a 1,3-benzene junction are calculated using many-body theory, and compared to the predictions of the figure-of-merit ZT.
We study the conductance $g$ of an electron interferometer created in a two dimensional electron gas between a nanostructured contact and the depletion region induced by the charged tip of a scanning gate microscope. Using non-interacting models, we study the beating pattern of interference fringes exhibited by the images giving $g$ as a function of the tip position when a parallel magnetic field is applied. The analytical solution of a simplified model allows us to distinguish between two cases: (i) If the field is applied everywhere, the beating of Fabry-Pérot oscillations of opposite spins …
Using density functional theory calculations, we demonstrate that the effect of anisotropic magnetoresistance (AMR) can be enhanced by orders of magnitude with respect to conventional bulk ferromagnets in junctions containing molecules sandwiched between ferromagnetic leads. We study ballistic transport in metal-benzene complexes contacted by $3d$ transition-metal wires. We show that the gigantic AMR can arise from spin-orbit coupling effects in the leads, drastically enhanced by orbital-symmetry filtering properties of the molecules. We further discuss how this molecular anisotropic magnetore…
We investigate near-equilibrium ballistic transport through a quantum point contact (QPC) along a GaAs/AlGaAs heterojunction with a transfer matrix technique, as a function of temperature and the shape of the potential barrier in the QPC. Our analysis is based on a three-dimensional (3D) quantum-mechanical variational model within the Hartree-Fock approximation that takes into account the vertical depletion potential from ionized acceptors in GaAs and the gate-induced transverse confinement potential that reduce to an effective slowly-varying one-dimensional (1D) potential along the narrow con…
Using scanning tunneling microscopy, we reproducibly obtained electrical conductance spectra of superconductor- superconductor atomic point contacts formed on various atomic sites of the substrate. From the analysis of the subharmonic features due to the multiple Andreev reflection, spatial distributions of the number of the conduction channels and their transmission probabilities were obtained. It is found that the number of channels and the transmission probabilities strongly depend on an atomic site where the contact is formed. We also revealed how the conduction channels and their transmis…
Transmission through a quantum point contact (QPC) in the quantum Hall regime usually exhibits multiple resonances as a function of gate voltage and high nonlinearity in bias. Such behavior is unpredictable and changes sample by sample. Here, we report the observation of a sharp transition of the transmission through an open QPC at finite bias, which was observed consistently for all the tested QPCs. It is found that the bias dependence of the transition can be fitted to the Fermi-Dirac distribution function through universal scaling. The fitted temperature matches quite nicely to the electron…
Motivated by experiments with current biased superconducting atomic point contacts the general problem of nonadiabatic transitions between adiabatic surfaces in presence of strong dissipation is studied. For a single channel device the supercurrent is determined by the diffusive motion of the superconducting phase difference on two Andreev levels. These surfaces are uncoupled only in the adiabatic limit of low to moderate transmissions, while for high transmissions curve crossings are important. Starting from a general master equation of the full density matrix an approximate time evolution eq…
The effect of dephasing on electron transport through a benzene molecule is carefully examined using a phenomenological model introduced by Büttiker. Within a tight-binding framework all the calculations are performed based on the Green's function formalism. We investigate the influence of dephasing on transmission probability and current-voltage characteristics for three different configurations ({\em ortho}, {\em meta} and {\em para}) of the molecular system depending on the locations of two contacting leads. The presence of dephasing provides a significant change in the spectral properties …
Contact resistance and current crowding are important to nanoscale electrical contacts. In this paper, we present a self-consistent model to characterize partially overlapped parallel contacts with varying specific contact resistivity along the contact length. For parallel tunneling contacts formed between contacting members separated by a thin insulating gap, we examine the local voltage-dependent variation of potential barrier height and tunneling current along the contact length, by solving the lumped circuit transmission line model (TLM) equations coupled with the tunneling current self co…
We study the ballistic transmission and the contact resistance ($R_c$) of a graphite-graphene contact in a top contact geometry from first principles. We find that the calculated $R_c$'s depend on the amount of graphene-graphite overlap, but quickly saturate for transfer lengths of the order of 20 Å. For contacts overlapping more than this transfer length, the $R_c$ can be lower than the 100 Ω$\cdot$μm mark. On the other hand, edge graphite-graphene contacts are expected to have very low contact resistance.
The current $I$ through nickelocene molecules and its noise are measured with a low temperature scanning tunneling microscope on a Cu(100) substrate. Density functional theory calculations and many-body modeling are used to analyze the data. During contact formation, two types of current evolution are observed, an abrupt jump to contact and a smooth transition. These data along with conductance spectra ($dI/dV$) recorded deep in the contact range are interpreted in terms of a transition from a spin-1 to a spin-1/2 state that is Kondo screened. Many-body calculations show that the smooth transi…
We investigate energy and momentum non-contact exchanges between two arbitrary flat media separated by a gap. This problem is revisited as a transmission problem of individual system eigenmodes weighted by a transmission probability obtained either from fluctuational electrodynamics or quantum field theory. An upper limit for energy and momentum flux is derived using a general variational approach. The corresponding optimal reflectivity coefficients are given both for identical and different media in interaction.
We find the charge and heat currents caused by a temperature difference applied to a superconducting point contact or to a quantum point contact between a superconducting and normal conductors. The results are formulated in terms of the properties of the electron scattering matrix of the quantum point contact in its normal state, and are valid at any transmission coefficient. In the low-transmission limit, the new theory provides reliable results, setting the limits for the use of the popular method of tunnel Hamiltonian.
We have investigated the transmission of electrons and holes through interfaces between superconducting aluminum (Tc = 1.2 K) and various normal non-magnetic metals (copper, gold, palladium, platinum, and silver) using Andreev-reflection spectroscopy at T = 0.1 K. We analyzed the point contacts with the modified BTK theory that includes Dynes' lifetime as a fitting parameter G in addition to superconducting energy gap 2D and normal reflection described by Z. For contact areas from 1 nm^2 to 10000 nm^2 the BTK Z parameter was 0.5, corresponding to transmission coefficients of about 80 %, indepe…
Conserving GW scheme for nonequilibrium quantum transport in molecular contacts
我们给出了一个详细的介绍，我们最近的计划，包括在分子输运计算中使用的GW近似的非平衡Keldysh形式主义的相关效应。我们将GW的自能限制在中心区域，并采用密度泛函理论（DFT）对其进行了描述。一个最小的最大本地化的Wannier函数的基础上应用在中央GW区域和铅。使用一个保守的，即完全自洽的，GW自能的重要性，证明了分析和数值例子。我们引入了一个有效的自旋相关的相互作用，自动减少自相互作用的错误的所有订单的相互作用。该计划适用于安德森模型在平衡和出。在零温度下的平衡，我们发现GW描述的近藤共振相当好的中间相互作用强度。平衡，我们证明了一杆G 0 W 0近似可以产生严重的错误，特别是在高偏置。最后，我们考虑一个苯分子之间的无特征的线索。据发现，分子的HOMO-LUMO间隙计算GW显着减少耦合到引线的增加，反映了更有效的屏蔽在强耦合结。对于结的IV特性，我们发现HF和G 0 W 0 [G_HF]产生的结果比DFT和G 0 W 0 [G_DFT]更接近GW。这是解释在自相互作用的影响和寿命减少，由于电子-电子相互作用。
The contact resistance between two dissimilar semiconductors is determined by the carrier transmission through their interface. Despite the ubiquitous presence of interfaces, quantitative simulation of charge transport across such interfaces is difficult, limiting the understanding of interfacial charge transport. This work employs Green's functions to study the charge transport across representative Si/Ge interfaces. For perfect interfaces, it is found that the transmittance is small and the contact resistance is high, not only because the mismatch of carrier pockets makes it hard to meet the…
