Search

Kondo resonances in heterostructures formed by magnetic molecules on a metal require free host electrons to interact with the molecular spin and create delicate many-body states. Unlike graphene, semiconducting graphene nanoribbons do not have free electrons due to their large bandgaps, and thus they should electronically decouple molecules from the metal substrate. Here, we observe unusually well-defined Kondo resonances in magnetic molecules separated from a gold surface by graphene nanoribbons in vertically stacked heterostructures. Surprisingly, the strengths of Kondo resonances for the molecules on graphene nanoribbons appear nearly identical to those directly adsorbed on the top, bridge and threefold hollow sites of Au(111). This unexpectedly strong spin-coupling effect is further confirmed by density functional calculations that reveal no spin-electron interactions at this molecule-gold substrate separation if the graphene nanoribbons are absent. Our findings suggest graphene nanoribbons mediate effective spin coupling, opening a way for potential applications in spintronics.Semiconducting graphene nanoribbon provides a platform for band-gap engineering desired for electronic and optoelectronic applications. Here, Li et al. show that graphene nanoribbon can effectively mediate the interaction of molecular magnetic moment and electronic spin in underlying metallic substrates.

Rare‐earth complexes are vital for separation chemistry and useful in many advanced applications including emission and energy upconversion. Here, 2D rare‐earth clusters having net charges are formed on a metal surface, enabling investigations of their structural and electronic properties on a one‐cluster‐at‐a‐time basis using scanning tunneling microscopy. While these ionic complexes are highly mobile on the surface at ≈100 K, their mobility is greatly reduced at 5 K and reveals stable and self‐limiting clusters. In each cluster, a pair of charged rare‐earth complexes formed by electrostatic and dispersive interactions act as a basic unit, and the clusters are chiral. Unlike other non‐ionic molecular clusters formed on the surfaces, these rare‐earth clusters show mechanical stability. Moreover, their high mobility on the surface suggests that they are in a 2D liquid‐like state. [ABSTRACT FROM AUTHOR]

Evaluation of the conductance oscillations in a quantum box indicates that controllable impurity in quantum box induces a quantum interference effect that activates well defined conductance oscillations. Modifications in the impurity potential give rise to electronic oscillations from destructive and constructive interference processes. The oscillations depend on impurity size while strong oscillations also arise in the presence of realistic structural parameters.

We calculate the static longitudinal polarizability of single-wall carbon tubes in the long wavelength limit taking into account spin-orbit effects. We use a four-orbital orthogonal tight-binding formalism to describe the electronic states and the random phase approximation to calculate the dielectric function. We study the role of both the Rashba as well as the intrinsic spin-orbit interactions on the longitudinal dielectric response, i.e., when the probing electric field is parallel to the nanotube axis. The spin-orbit interaction modifies the nanotube electronic band dispersions, which may especially result in a small gap opening in otherwise metallic tubes. The bandgap size and state features, the result of competition between Rashba and intrinsic spin-orbit interactions, result in drastic changes in the longitudinal static polarizability of the system. We discuss results for different nanotube types and the dependence on nanotube radius and spin-orbit couplings. [ABSTRACT FROM AUTHOR]

This study presents a quantum-mechanical free electron model for analyzing a spin polarized transport and current-perpendicular-to-the-plane giant magnetoresistance (CPP-GMR) in a more realistic way. The CPP-GMR is evaluated by using three spin resolved conductance parameters based on the Landauer conductance formula. In a ballistic regime, a transfer-matrix method is used to calculate the spin dependent transmission probability as a function of the transverse mode. A spin dependent conduction band structure is constructed by extracting parameters of the free electron model, such as the atomic magnetic moments and the conduction electron densities, from the spin dependent layer-decomposed density of states of the Cu and Co interfacial layers in a Cu5/Co11 slab; these calculations are derived from the density functional theory. As a result, this study shows that the CPP-GMR in a [Cu(5 ML)/Co(11 ML)]n magnetic multilayer (n=2–5) with a 35 ML×35 ML cross section is in the range of 60%–111%. It is qualitatively comparable to the calculation results of first principles. This study also uses transmission probability to explain the increase of spin dependent scattering and CPP-GMR as a function of the number of layers in the [Cu/Co]n magnetic multilayer. Moreover, the study confirms that modification of the free electron model by quantum-mechanical methods can be applied to calculations of a spin polarized transport and CPP-GMR in a specific material system. [ABSTRACT FROM AUTHOR]

We present a detailed study of quasi-ballistic transport in submicron semiconductor channels. The electron distribution in such channels differs significantly from a near-equilibrium, shifted Maxwell-Boltzmann distribution function, and displays a large broadening, as well as pronounced features, peaks and shoulders, in the high-velocity tail of the distribution associated with the nonequilibrium and quasi-ballistic nature of the electron transport. For submicron channel lengths, analyses of the spatial dependence of the velocity distribution function show that scattering in the entire channel is important and that the scattering efficiency has a strong spatial dependence. In this article, we particularly study the crossover between the diffusive and quasi-ballistic regimes of transport and find characteristic signatures in (i) the electrostatics, which manifests as a redistribution of the voltage drop from the channel to the contact regions; (ii) the electron density, where exponential and linear spatial dependences of the source-injected and channel- and drain-backscattered electron densities are signatures of diffusive and quasi-ballistic transport, respectively, and (iii) the electron distribution function, where the source-injected ballistic peak observed in the quasi-ballistic regime disappears at the onset of diffusive transport. [ABSTRACT FROM AUTHOR]

A new no-barrier quantum interferometer design reveals high quantum interference in ballistic areas where the structure examined is smaller than the elastic mean free path and the phase coherence length. Recursive Green's function method calculations validate the production of strong quantum interference conductance oscillation by varying the width of one branch. These conductance oscillations are useful for device length and liquid-helium temperatures.

Kondo resonances in heterostructures formed by magnetic molecules on a metal require free host electrons to interact with the molecular spin and create delicate many-body states. Unlike graphene, semiconducting graphene nanoribbons do not have free electrons due to their large bandgaps, and thus they should electronically decouple molecules from the metal substrate. Here, we observe unusually well-defined Kondo resonances in magnetic molecules separated from a gold surface by graphene nanoribbons in vertically stacked heterostructures. Surprisingly, the strengths of Kondo resonances for the molecules on graphene nanoribbons appear nearly identical to those directly adsorbed on the top, bridge and threefold hollow sites of Au(111). This unexpectedly strong spin-coupling effect is further confirmed by density functional calculations that reveal no spin–electron interactions at this molecule-gold substrate separation if the graphene nanoribbons are absent. Our findings suggest graphene nanoribbons mediate effective spin coupling, opening a way for potential applications in spintronics. [ABSTRACT FROM AUTHOR]

The decoration of graphene samples with adatoms or nanoparticles leads to the enhancement of spin-orbit interactions as well as to the introduction of symmetry-breaking effects that could have drastic effects on spin and electronic transport phenomena. We present an analysis based on symmetry considerations and examine the impact on the scattering matrix for graphene systems containing defects that enhance spin-orbit interactions, while conserving the electronic total angular momentum. We show that the appearance and dominance of skew scattering, and the related observation of valley and/or spin Hall effect in decorated graphene samples, suggests the set of symmetries that adatom perturbations should satisfy. We further show that detailed measurements of the transport and elastic times as a function of carrier concentration make it possible to not only extract the strength of the spin-orbit interaction, as suggested before, but also obtain the amplitude of the symmetry-breaking terms introduced. To examine how different terms would affect measurements, we also present calculations for typical random distributions of impurities with different perturbations, illustrating the detailed energy dependence of different observables. [ABSTRACT FROM AUTHOR]

Broken symmetries in graphene affect the massless nature of its charge carriers. We present an analysis of scattering by defects in graphene in the presence of spin-orbit interactions (SOIs). A characteristic constant ratio (≃2) of the transport to elastic times for massless electrons signals the anisotropy of the scattering. We show that SOIs lead to a drastic decrease of this ratio, especially at low carrier concentrations, while the scattering becomes increasingly isotropic. As the strength of the SOI determines the energy (carrier concentration) where this drop is more evident, this effect could help evaluate these interactions through transport measurements in graphene systems with enhanced spin-orbit coupling. [ABSTRACT FROM AUTHOR]

Electron optics exploits the analogies between rays in geometrical optics and electron trajectories, leading to interesting insights and potential applications. Graphene, with its two dimensionality and photon-like behavior of its charge carriers, is the perfect candidate for the exploitation of electron optics. We show that a circular gate-or doping-controlled region in the presence of Rashba spin-orbit interaction in graphene may indeed behave as a Veselago electronic lens but with two different indices of refraction. We demonstrate that this birefringence results in complex caustics patterns for a circular gate, selective focusing of different spins, and the possible direct measurement of the Rashba coupling strength in scanning-probe experiments. [ABSTRACT FROM AUTHOR]

The article discusses a study on the one-orbital Anderson magnetic impurity embedded in a two-dimensional electron bath with Rashba spin-orbit (RSO) interactions in the Kondo regime. It aims to demonstrate that the presence of these interactions reduces the Anderson Hamiltonian to an effective two-band Anderson model coupled to the impurity. Results show that the spin SU(2) symmetry-breaking term couples the impurity to a two-band electron gas.

Abstract: In this letter, we develop a model to describe the Rabi oscillations observed in a quantum-dot photodiode. Using a multi-level density matrix formulation, which includes multi-exciton and single particle states, we show that the damping observed in recent experiments is the result of a non-resonant excitation from or to the continuum of the wetting layer states. [Copyright &y& Elsevier]

Resonant energy transfer is a mechanism for excitation exchange in quantum dot arrays. We explore this process by calculating the coupling strength, and by studying the detailed dynamics of extended coherent exciton states created in an array of quantum dots. Estimates of the Förster coupling VF are obtained from a microscopic description of the exciton levels in the quantum dot. We present results for different materials (CdS, CdSe, GaAs, and InP) as function of dot features. VF shows a nonmonotonic variation with a maximum value that depends on the specific dot sizes, material parameters, and dot separation. Analysis of coupled quantum dots using the time evolution of the density matrix when the Förster channel is open shows that a coherent coupling appears in different regimes. Realistic parameters, used throughout the study of the dynamics, suggest the practical realization of these phenomena in actual systems. [ABSTRACT FROM AUTHOR]

We report on a study of quasiballistic transport in deep submicron, inhomogeneous semiconductor structures, focusing on the analysis of signatures found in the full nonequilibrium electron distribution. We perform self-consistent numerical calculations of the Poisson-Boltzmann equations for a model n+-n--n+ GaAs structure and realistic, energy-dependent scattering. We show that, in general, the electron distribution displays significant, temperature dependent broadening and pronounced structure in the high-velocity tail of the distribution. The observed characteristics have a strong spatial dependence, related to the energy dependence of the scattering, and the large inhomogeneous electric field variations in these systems. We show that in this quasiballistic regime, the high-velocity tail structure is due to pure ballistic transport, whereas the strong broadening is due to electron scattering within the channel, and at the source(drain) interfaces. [ABSTRACT FROM AUTHOR]

Studies the Zeeman splitting of the donor spectra in cubic and hexagonal GaN using an effective mass theory approach. Description of the chemical shift of the different substitutional dopants using soft-core pseudopotentials; Donor ground states calculated.

Studies the simulation of hyperthermal deposition of Si and C on SiC surfaces. Sticking probabilities of adatoms; Efficient transfer of adatom energy to the substrate; Attractive forces experienced by the impinging adatom over the entire surface.

The possibility to effect valley splitting of an electronic current in graphene represents the essential component in the new field of valleytronics in such two-dimensional materials. Based on a symmetry analysis of the scattering matrix, we show that if the spatial distribution of multiple potential scatterers breaks mirror symmetry about the axis of incoming electrons, then a splitting of the current between two valleys is observed. This leads to the appearance of the valley Hall effect. We illustrate the effect of mirror-symmetry breaking in a minimal system of two symmetric impurities, demonstrating the splitting between valleys via the differential cross sections and nonvanishing skew parameter. We further discuss the role that these effects may play in transport experiments. [ABSTRACT FROM AUTHOR]

We present a theoretical analysis of the role that strain plays on the electronic structure of chromium nitride (CrN) crystals. We use local spin-density approximation + U calculations to study the elastic constants, deformation potentials, and strain dependence of electron and hole masses near the fundamental gap. We consider the lowest energy antiferromagnetic models believed to describe CrN at low temperatures, and apply strain along different directions. We find relatively large deformation potentials for all models, and find increasing gaps for tensile strain along most directions. Most interestingly, we find that compressive strains should be able to close the relatively small indirect gap (≃100 meV) at moderate amplitudes ≃1.3%. We also find large and anisotropic changes in the effective masses with strain, with principal axes closely related to the magnetic ordering of neighboring layers in the antiferromagnet. It would be interesting to consider the role that these effects may have on typical film growth on different substrates, and the possibility of monitoring optical and transport properties of thin films as strain is applied. [ABSTRACT FROM AUTHOR]

An Anderson model for a magnetic impurity in a two-dimensional electron gas with bulk Rashba spin-orbit interaction is solved using the numerical renormalization group under two different experimental scenarios. For a fixed Fermi energy, the Kondo temperature TK varies weakly with Rashba coupling λR, as reported previously. If instead the band filling is low and held constant, increasing λR can drive the system into a helical regime with exponential enhancement of TK. Under either scenario, thermodynamic properties at low temperatures T exhibit the same dependencies on T/TK as are found for λR=0. Unlike the conventional Kondo effect, however, the impurity exhibits static spin correlations with conduction electrons of nonzero orbital angular momentum about the impurity site. We also consider a magnetic field that Zeeman splits the conduction band but not the impurity level, an effective picture that arises under a proposed route to access the helical regime in a driven system. The impurity contribution to the system's ground-state angular momentum is found to be a universal function of the ratio of the Zeeman energy to a temperature scale that is not TK (as would be the case in a magnetic field that couples directly to the impurity spin), but rather is proportional to TK divided by the impurity hybridization width. This universal scaling is explained via a perturbative treatment of field-induced changes in the electronic density of states. [ABSTRACT FROM AUTHOR]