Conductance and Kondo Interference beyond Proportional Coupling.

The transport properties of nanostructured systems are deeply affected by the geometry of the effective connections to metallic leads. In this work we derive a conductance expression for a class of interacting systems whose connectivity geometries do not meet the Meir-Wingreen proportional coupling condition. As an interesting application, we consider a quantum dot connected coherently to tunable electronic cavity modes. The structure is shown to exhibit a well-defined Kondo effect over a wide range of coupling strengths between the two subsystems. In agreement with recent experimental results, the calculated conductance curves exhibit strong modulations and asymmetric behavior as different cavity modes are swept through the Fermi level. These conductance modulations occur, however, while maintaining robust Kondo singlet correlations of the dot with the electronic reservoir, a direct consequence of the lopsided nature of the device.

Emergence of photoswitchable states in a graphene-azobenzene-Au platform.

The perfect transmission of charge carriers through potential barriers in graphene (Klein tunneling) is a direct consequence of the Dirac equation that governs the low-energy carrier dynamics. As a result, localized states do not exist in unpatterned graphene, but quasibound states can occur for potentials with closed integrable dynamics. Here, we report the observation of resonance states in photoswitchable self-assembled molecular(SAM)-graphene hybrid. Conductive AFM measurements performed at room temperature reveal strong current resonances, the strength of which can be reversibly gated on- and off- by optically switching the molecular conformation of the mSAM. Comparisons of the voltage separation between current resonances (∼ 70-120 mV) with solutions of the Dirac equation indicate that the radius of the gating potential is ∼ 7 ± 2 nm with a strength ≥ 0.5 eV. Our results and methods might provide a route toward optically programmable carrier dynamics and transport in graphene nanomaterials.

Spin-orbit interaction and isotropic electronic transport in graphene.

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.

2D Ionic Liquid-Like State of Charged Rare-Earth Clusters on a Metal Surface.

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.

Quantum manipulation via atomic-scale magnetoelectric effects.

Magnetoelectric effects at the atomic scale are demonstrated to afford unique functionality. This is shown explicitly for a quantum corral defined by a wall of magnetic atoms on a metal surface where spin-orbit coupling is observable. We show these magnetoelectric effects allow one to control the properties of systems placed inside the corral as well as their electronic signatures; they provide powerful alternative tools for probing electronic properties at the atomic scale.

Enhancement of the Kondo effect through Rashba spin-orbit interactions.

We study a one-orbital Anderson impurity in a two-dimensional electron bath with Rashba spin-orbit interactions in the Kondo regime. The spin SU(2) symmetry-breaking term couples the impurity to a two-band electron gas. A Schrieffer-Wolff transformation shows the existence of the Dzyaloshinsky-Moriya interaction away from the particle-hole symmetric impurity state. A renormalization group analysis reveals a two-channel Kondo model with ferro- and antiferromagnetic couplings. The parity-breaking Dzyaloshinsky-Moriya term renormalizes the antiferromagnetic Kondo coupling with an exponential enhancement of the Kondo temperature.

Inducing chiral superconductivity on honeycomb lattice systems.

Superconductivity in graphene-based systems has recently attracted much attention, as either intrinsic behavior or induced by proximity to a superconductor may lead to interesting topological phases and symmetries of the pairing function. A prominent system considers the pairing to have chiral symmetry. The question arises as to the effect of possible spin-orbit coupling on the resulting superconducting quasiparticle (QP) spectrum. Utilizing a Bogolyubov-de Gennes (BdG) Hamiltonian, we explore the interplay of different interaction terms in the system, and their role in generating complex Berry curvatures in the QP spectrum, as well as non-trivial topological behavior. We demonstrate that the topology of the BdG Hamiltonian in these systems may result in the appearance of edge states along the zigzag edges of nanoribbons in the appropriate regime. For suitable chemical potential and superconducting pairing strength, we find the appearance of robust midgap states at zigzag edges, well protected by large excitation gaps and momentum transfer.

Negative differential resistance observed on the charge density wave of a transition metal dichalcogenide.

Charge density waves and negative differential resistance are seemingly unconnected physical phenomena. The former is an ordered quantum fluid of electrons, intensely investigated for its relation with superconductivity, while the latter receives much attention for its potential applications in electronics. Here we show that these two phenomena can not only coexist but also that the localized electronic states of the charge density wave are essential to induce negative differential resistance in a transition metal dichalcogenide, 1T-TaS2. Using scanning tunneling microscopy and spectroscopy, we report the observation of negative differential resistance in the commensurate charge density wave state of 1T-TaS2. The observed phenomenon is explained by the interplay of interlayer and intra-layer tunneling with the participation of the atomically localized states of the charge density wave maxima and minima. We demonstrate that lattice defects can locally affect the coupling between the layers and are therefore a mechanism to realize NDR in these materials.

A chiral molecular propeller designed for unidirectional rotations on a surface.

Synthetic molecular machines designed to operate on materials surfaces can convert energy into motion and they may be useful to incorporate into solid state devices. Here, we develop and characterize a multi-component molecular propeller that enables unidirectional rotations on a material surface when energized. Our propeller is composed of a rotator with three molecular blades linked via a ruthenium atom to a ratchet-shaped molecular gear. Upon adsorption on a gold crystal surface, the two dimensional nature of the surface breaks the symmetry and left or right tilting of the molecular gear-teeth induces chirality. The molecular gear dictates the rotational direction of the propellers and step-wise rotations can be induced by applying an electric field or using inelastic tunneling electrons from a scanning tunneling microscope tip. By means of scanning tunneling microscope manipulation and imaging, the rotation steps of individual molecular propellers are directly visualized, which confirms the unidirectional rotations of both left and right handed molecular propellers into clockwise and anticlockwise directions respectively.

Spin-orbit interaction and controlled singlet-triplet dynamics in silicon double quantum dots.

We undertake a theoretical study of the role of spin orbit interactions in a silicon double quantum dot. We propose that an accurate estimate of the strength of this interaction can be obtained through the study of the return probability of the double occupation singlet state in a magnetic field, as the system is gated dynamically across the relevant states in the low energy two-electron manifold. Landau-Zener type of processes involving appropriate control of voltage pulses across neighboring avoided crossings in the energy spectrum of the system are utilized to explore the system dynamics. Our description takes into account Zeeman splitting, intervalley mixing and spin-orbit interaction present in the structure. Using a density matrix equation of motion approach, we carry out numerical calculations for the return probability of the double occupation singlet state. The analysis in terms of Landau-Zener theory allows the determination of the spin-orbit coupling strength for different Zeeman splitting regimes.

Anomalous Kondo resonance mediated by semiconducting graphene nanoribbons in a molecular heterostructure.

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.

Charge transport in DNA molecules: Cooperative interplay between the disordered base-pair channel and the ordered backbone.

Charge transport in DNA molecules has raised considerable interest because of its importance in biological processes and potential applications in nanoscale devices. A DNA molecule can be viewed as a quasi-one-dimensional system composed of stacked base pairs (AT , CG) together with backbones of sugar phosphates. Motivated by recent experimental observations on the importance of the backbone integrity, we investigate the interplay between charge transport through the ordered backbone and disordered base stacks with random sequences. By analytical and numerical calculations, we find that the coupling between the backbone and base-pair channels plays an important role in charge transport. The backbone can generate effective hopping constants well beyond the adjacent base pairs, enhancing charge transport through the base-pair channel. The corresponding enhancement of the localization length is nearly independent of the length of the DNA and increases with increasing coupling between backbone and base pair. Our model can explain qualitatively several experimental observations.

Tunable pseudogap Kondo effect and quantum phase transitions in Aharonov-Bohm interferometers.

We study two quantum dots embedded in the arms of an Aharonov-Bohm ring threaded by a magnetic flux. This system can be described by an effective one-impurity Anderson model with an energy- and flux-dependent density of states. For specific values of the flux, this density of states vanishes at the Fermi energy, yielding a controlled realization of the pseudogap Kondo effect. The conductance and transmission phase shifts reflect a nontrivial interplay between wave interference and interactions, providing clear signatures of quantum phase transitions between Kondo and non-Kondo ground states.

Zero-field kondo splitting and quantum-critical transition in double quantum dots.

Double quantum dots offer unique possibilities for the study of many-body correlations. A system containing one Kondo dot and one effectively noninteracting dot maps onto a single-impurity Anderson model with a structured (nonconstant) density of states. Numerical renormalization-group calculations show that, while band filtering through the resonant dot splits the Kondo resonance, the singlet ground state is robust. The system can also be continuously tuned to create a pseudogapped density of states and access a quantum-critical point separating Kondo and non-Kondo phases.

Decoherence of rabi oscillations in a single quantum dot.

We develop a realistic model of Rabi oscillations in a quantum-dot photodiode. Based in a multiexciton density matrix formulation we show that for short pulses the two-level model fails and higher levels should be taken into account. This affects some of the experimental conclusions, such as the inferred efficiency of the state rotation (population inversion) and the deduced value of the dipole interaction. We also show that the damping observed cannot be explained using constant rates with fixed pulse duration. We demonstrate that the damping observed is in fact induced by an off-resonant excitation to or from the continuum of wetting layer states. Our model describes the nonlinear decoherence behavior observed in recent experiments.