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The complex band structure (CBS), although not directly observable, determines many properties of a material where the periodicity is broken, such at surfaces, interfaces and defects. Furthermore, its knowledge helps in the interpretation of electronic transport calculations and in the study of topological materials. Here we extend the transfer matrix method, often used to compute the complex bands, to electronic structures constructed using an atomic non-orthogonal basis set. We demonstrate that when the overlap matrix is not the identity, the non-orthogonal case, spurious features appear in the analytic continuation of the band structure to the complex plane. The properties of these are studied both numerically and analytically and discussed in the context of existing literature. Finally, a numerical implementation to extract the CBS from periodic calculations carried out with the density functional theory codesiestais presented. This is constructed as a simple post-processing tool, and it is therefore amenable to high-throughput studies of insulators and semiconductors., (Creative Commons Attribution license.)

We perform a numerical study of interacting one-dimensional Hubbard rings with a single impurity potential and pierced by a magnetic flux. Our calculations are carried out at the level of current lattice density functional theory (CLDFT) for the Hubbard model and compared to known results obtained in the thermodynamical limit from the Bethe ansatz. In particular, we investigate the effects of disorder and Coulomb interaction on the persistent current (PC) and the Drude weight. It is found that CLDFT is able to accurately describe qualitative and quantitative features of these ground state properties in the presence of disorder and electronic interaction. When the impurity potential is switched off, the CLDFT approach describes well the velocity of the Luttinger liquid excitations as a function of both interaction strength and electron filling. Then, when the impurity scattering potential is finite, we find the PC to vanish as [Formula: see text] for large L and independent on the strength of the scattering potential, in good agreement with Luttinger liquid theory.

Pyrene, the smallest two-dimensional mesh of aromatic rings, with various terminal thiol substitutions, has been considered as a potential molecular interconnect. Charge transport through two terminal devices has been modeled using density functional theory (with and without self interaction correction) and the non-equilibrium Green's function method. A tetra-substituted pyrene, with dual thiol terminal groups at opposite ends, has been identified as an excellent candidate, owing to its high conductance, virtually independent of bias voltage. The two possible extensions of its motif generate two series of graphene nanoribbons, with zigzag and armchair edges and with semimetallic and semiconducting electron band structure, respectively. The effects related to the wire length and the bias voltage on the charge transport have been investigated for both sets. The conductance of the nanoribbons with a zigzag edge does not show either length or voltage dependence, owing to an almost perfect electron transmission with a continuum of conducting channels. In contrast, for the armchair nanoribbons a slow exponential attenuation of the conductance with the length has been found, due to their semiconducting nature.

We present a time-dependent study of electron transport through a strongly correlated quantum dot, which combines adiabatic lattice density functional theory in the Bethe ansatz local-density approximation (BALDA) to the Hubbard model, with the multiple-probe battery method for open-boundary simulations in the time domain. In agreement with the recently proposed dynamical picture of Coulomb blockade, a characteristic driven regime, defined by regular current oscillations, is demonstrated for a certain range of bias voltages. We further investigate the effects of systematically improving the approximation for the electron-electron interaction at the dot site (going from non-interacting, through Hartree-only to adiabatic BALDA) on the transmission spectrum and the I-V characteristics. In particular, a negative differential conductance is obtained at large bias voltages and large Coulomb interaction strengths. This is attributed to the combined effect of the electron-electron interaction at the dot and the finite bandwidth of the electrodes.

The origin and the distance dependence of the transition voltage of gold-vacuum-gold junctions are investigated by employing first-principles quantum transport simulations. Our calculations show that atomic protrusions always exist on the electrode surface of gold-vacuum-gold junctions fabricated using the mechanically controllable break junction (MCBJ) method. The transition voltage of these gold-vacuum-gold junctions with atomically sharp electrodes is determined by the local density of states (LDOS) of the apex gold atom on the electrode surface rather than by the vacuum barrier shape. More specifically, the absolute value of the transition voltage roughly equals the rising edge of the LDOS peak contributed by the 6p atomic orbitals of the gold atoms protruding from the electrode surface, whose local Fermi level is shifted downwards when a bias voltage is applied. Since the LDOS of the apex gold atom depends strongly on the exact shape of the electrode, the transition voltage is sensitive to the variation of the atomic configuration of the junction. For asymmetric junctions, the transition voltage may also change significantly depending on the bias polarity. Considering that the occurrence of the transition voltage requires the electrode distance to be larger than a critical value, the interaction between the two electrodes is actually rather weak. Consequently, the LDOS of the apex gold atom is mainly determined by its local atomic configuration and the transition voltage only depends weakly on the electrode distance as observed in the MCBJ experiments.

The Bethe ansatz local density approximation (LDA) to lattice density functional theory (LDFT) for the one-dimensional repulsive Hubbard model is extended to current-LDFT (CLDFT). The transport properties of mesoscopic Hubbard rings threaded by a magnetic flux are then systematically investigated by this scheme. In particular we present calculations of ground state energies, persistent currents and Drude weights for both a repulsive homogeneous and a single impurity Hubbard model. Our results for the ground state energies in the metallic phase compare favorably well with those obtained with numerically accurate many-body techniques. Also the dependence of the persistent currents on the Coulomb and the impurity interaction strength, and on the ring size are all well captured by LDA-CLDFT. Our study demonstrates the value of CLDFT in describing the transport properties of one-dimensional correlated electron systems. As its computational overheads are rather modest, we propose this method as a tool for studying problems where both disorder and interaction are present.

The energetic stability, electronic and magnetic properties of chiral graphene nanoribbons (GNRs) with hydrogen-terminated edges are investigated using density functional theory. Our calculations show that the percentage of carbon atoms at the zigzag sites (P(z)) is the key factor determining the electronic and magnetic properties of chiral GNRs. Within the local spin density approximation, chiral GNRs with P(z) ≥ 50% have a semiconducting antiferromagnetic ground state. Otherwise, chiral GNRs are spin degenerate semiconductors. Thus, the critical chiral angle for the occurrence of spin polarization is determined to be 13.9°. In contrast to the antiferromagnetic state that is independent of the width of GNRs investigated, size effects occur for the ferromagnetic metastable state. These findings are helpful for the design of GNR-based spintronic devices., (© 2011 IOP Publishing Ltd)

The spin crossover transition in π-conjugated polymers is a complex phenomenon involving a balance between Coulomb interaction and collective lattice distortions. We explore such a transition with a minimal electronic model comprising a Hubbard-U on-site repulsive potential and both electron-phonon and hyperfine interactions. The model is then solved numerically for small molecules at finite temperature by Monte Carlo methods in the search for the spin crossover. This is done at the mean field level in the Hubbard-U interaction at half filling. We demonstrate that for a certain region of the parameter space there is a spin crossover, where the system transits from a low-spin to a high-spin state as the temperature increases. In close analogy to standard spin crossover in divalent magnetic molecules such a transition is entropy driven, with both the spin and the vibrational contributions to the entropy being relevant. Such a transition is practically unaffected by the hyperfine interaction, which only plays a minor role in determining the electronic properties.

The electronic transport properties of a single benzene molecule connected to gold and platinum electrodes through the direct Au-C or Pt-C bond are investigated by using a self-consistent ab initio approach that combines the non-equilibrium Green's function (NEGF) formalism with density functional theory (DFT). Our calculations show that the benzene molecule can bind to the Au(111) surface via direct Au-C bond at the adatom, atop and bridge sites. The largest zero-bias conductance is calculated for the bridge site but it is only G = 0.37G(0) (G(0) = 2e(2)/h). In contrast benzene binds to the Pt(111) surface via direct Pt-C bond only at the adatom and atop sites. When the binding site is the adatom a stable molecular junction forms with a zero-bias conductance as large as 1.15G(0). This originates from the efficient coupling between the extended π-type highest occupied molecular orbital of benzene and the conducting states of the Pt electrodes via the 5d(xz) atomic orbital of the adatoms. The calculated transmission is robust to the choice of DFT functionals, illustrating the potential of the Pt-C bond for constructing future molecular electronic devices.

During the last decade the design of biosensors, based on quantum transport in one-dimensional nanostructures, has developed as an active area of research. Here we investigate the sensing capabilities of a DNA nanosensor, designed as a semiconductor single walled carbon nanotube (SWCNT) connected to two gold electrodes and functionalized with a DNA strand acting as a bio-receptor probe. In particular, we have considered both covalent and non-covalent bonding between the DNA probe and the SWCNT. The optimized atomic structure of the sensor is computed both before and after the receptor attaches itself to the target, which consists of another DNA strand. The sensor's electrical conductance and transmission coefficients are calculated at the equilibrium geometries via the non-equilibrium Green's function scheme combined with the density functional theory in the linear response limit. We demonstrate a sensing efficiency of 70% for the covalently bonded bio-receptor probe, which drops to about 19% for the non-covalently bonded one. These results suggest that a SWCNT may be a promising candidate for a bio-molecular FET sensor.

The low-bias transport properties of a single 1,4-phenylene diisocyanide (PDI) molecule connected to two platinum (Pt) electrodes are investigated using a self-consistent ab initio approach that combines the non-equilibrium Green's function formalism with density functional theory. Our calculations demonstrate that the zero-bias conductance of an asymmetric Pt-PDI-Pt junction, where the PDI molecule is attached to the atop site at one Pt(111) electrode and to a Pt adatom at the other, is 2.6 x 10( - 2)G(0), in good agreement with the experimental value (3 x 10( - 2)G(0)) measured with break junctions. Although the highest occupied and the lowest unoccupied molecule orbitals in PDI are both pi-type, delocalized along the entire molecule, their electronic coupling with the highly conducting states of the Pt electrode is blocked at the atop site, leading to the small transmission. This indicates that more efficient electronic contacts are needed to fabricate molecular devices with a high conductance using Pt electrodes and aromatic isocyanides such as PDI.

We present a theoretical study of spin transport in a series of organometallic iron-cyclopentadienyl, Fe(n)Cp(n+1), multidecker clusters sandwiched between either gold or platinum electrodes. Ab initio modeling is performed by combining the non-equilibrium Green's function formalism with spin density functional theory. Due to the intrinsic bonding nature, the low-bias conductance of the Fe(n)Cp(n+1) clusters contacted to gold electrodes is relatively small even for strong cluster-electrode coupling. However, a nearly 100% spin polarization of the transmitted electrons can be achieved for the Fe(n)Cp(n+1) (n>2) clusters. In contrast, the Fe(n)Cp(n+1) (n>2) clusters attached to platinum electrodes through Pt adatoms not only can act as nearly perfect spin filters but also show a much larger transmission around the Fermi level, demonstrating their promising applications in future molecular spintronics.

The effects of spin-orbit coupling on the conductance of molecular devices made with Au electrodes are investigated using a fully self-consistent ab initio approach, which combines the non-equilibrium Green's function formalism with density functional theory. In general, we find that the extent to which spin-orbit interaction affects the transport depends on the specific materials system investigated and on the dimensionality of the electrodes. For one-dimensional electrodes contacting benzene-dithiol molecules the spin-orbit coupling induces changes in the low-bias conductance up to about 20%. These originate mostly from changes in the electrode band structure. In contrast when three-dimensional electrodes are used, the bands near the Fermi level are only weakly modified by spin-orbit coupling and most of the variations are due to symmetry changes at the molecule-electrode interface. For this reason strongly coupled systems, such as Au atomic nanowires sandwiched between Au (100) surfaces and benzene-dithiol molecules bonded at the Au (111) hollow site, are rather insensitive to spin-orbit effects. In contrast, in junctions where the coupling between the molecule and the electrodes is weaker, as in the case of benzene-dithiol bonded to Au (111) at adatom positions, the transmission coefficient at the Fermi level can be modified by as much as 14%.

We present results of electronic structure and transport calculations for metallic interfaces, based on density functional theory and the non-equilibrium Green's function method. Starting from the electronic structure of smooth Al, Cu, Ag, and Au interfaces, we study the effects of different kinds of interface roughness on the transmission coefficient and the I-V characteristic. In particular, we compare prototypical interface distortions, including vacancies and metallic impurities.