H. Sevinçli

Superlattice structures of graphene-based armchair nanoribbons

Based on first-principles calculations we predict that periodically repeated junctions of armchair graphene nanoribbons of different widths form multiple quantum well structures. In these superlattice heterostructures the width as well as the energy-band gap is modulated in real space and specific states are confined in certain segments. Not only the size modulation, but also composition modulation, such as periodically repeated and commensurate heterojunctions of boron nitride and graphene honeycomb nanoribbons, results in a multiple quantum well structure. The geometrical features of the constituent nanoribbons, namely, their widths and lengths, the form of the junction, as well as the symmetry of the resulting superlattice, are the structural parameters available to engineer electronic properties of these quantum structures. We present our analysis regarding the variation of the band gaps and the confined states with these structural parameters. Calculation of transmission coefficient through a double barrier resonant tunneling device formed from a finite segment of such a multiple quantum well structure and placed between metallic electrodes yields resonant peaks which can be identified with electronic states confined in the well. We show that these graphene-based quantum structures can introduce interesting concepts to design nanodevices. Relevance of the quantum structures are discussed in view of the most recent experimental results.

Spin confinement in the superlattices of graphene ribbons

Based on first-principles calculations, we showed that repeated heterostructures of zigzag graphene nanoribbons of different widths form multiple quantum well structures. Edge states of specific spin directions can be confined in these wells. The electronic and magnetic state of the ribbon can be modulated in real space. In specific geometries, the absence of reflection symmetry causes the magnetic ground state of whole heterostructure to change from antiferromagnetic to ferrimagnetic. These quantum structures of different geometries provide unique features for spintronic applications.

Dynamics of phononic dissipation at the atomic scale: Dependence on internal degrees of freedom

Dynamics of dissipation local vibrations to the surrounding substrate is a key issue in friction between sliding surfaces as well as in boundary lubrication. We consider a model system consisting of an excited nano-particle which is weakly coupled with a substrate. Using three different methods, we solve the dynamics of energy dissipation for different types of coupling between the nanoparticle and the substrate, where different types of dimensionality and phonon densities of states were also considered for the substrate. In this paper, we present a microscopic analysis of transient properties of energy dissipation via phonon discharge toward the substrate. Finally, important conclusions of our theoretical analysis are verified by a realistic study, where the phonon modes and interaction parameters involved in the energy dissipation from an excited benzene molecule to the graphene are calculated by using first-principles methods. The methods used are applicable also to dissipative processes in the contexts of infrared Raman spectroscopy and atomic force microscopy of molecules on surfaces.

Functionalization of graphene nanoribbons

With the synthesis of a single atomic plane of graphite, namely, graphene honeycomb structure, a new perspective for carbon-based electronics is opened. The one-dimensional graphene nanoribbons (GNRs) have different band-gap values depending on their edge shape and width. In this contribution, we report our results showing that repeated heterostructures of GNRs of different widths form multiple quantum-well structures. The widths of the constituent parts as well as the bandgap, and also the magnetic ground state of the superlattices are modulated in direct space. We provide detailed analysis of these structures and show that superlattices with armchair edge shapes can be used as resonant tunneling devices and those with zigzag edge shape have unique features for spintronic applications. We also discuss another route of functionalizing 2D graphene, 1D GNR, and superlattices with 3d-transition metal (TM) atom adsorption.

Oscillatory exchange coupling in magnetic molecules

Recently, first-principles calculations based on the spin-dependent density functional theory (DFT) have revealed that the magnetic ground state of a finite linear carbon chain capped by two transition metal (TM) atoms alternates between ferromagnetic and antiferromagnetic configurations depending on the number of carbon atoms. The character of indirect exchange coupling in this nanoscale, quasi-zero-dimensional system is different from those analogous extended structures consisting of magnetic layers separated by a nonmagnetic spacer (or magnetic impurities in a non-magnetic host material) and a formulation based on an atomic picture is needed. We present a tight-binding model which provides a theoretical framework to the underlying mechanism of the exchange coupling in molecular structures. The model calculations are capable of reproducing the essential features of the DFT results for the indirect exchange coupling and the atomic magnetic moments in the TM‐Cn‐TM structures as functions of the number of carbon atoms. In nanostructures consisting of a few atoms the concepts of extended wavefunctions and the band theory lose their validity, and hence the oscillatory exchange coupling turns out to be a consequence of quantum interference effects due to the spin-dependent onsite and hopping energies. (Some figures in this article are in colour only in the electronic version)

The off-resonant aspects of decoherence and a critique of the two-level approximation

Conditions in favour of a realistic multilevelled description of a decohering quantum system are examined. In this regard the first crucial observation is that the thermal effects, contrary to the conventional belief, play a minor role at low temperatures in the decoherence properties. The system–environment coupling and the environmental energy spectrum dominantly affect the decoherence. In particular, zero temperature quantum fluctuations or non-equilibrium sources can be present and influential on the decoherence rates in a wide energy range allowed by the spectrum of the environment. A crucial observation against the validity of the two-level approximation is that the decoherence rates are found to be dominated not by the long time resonant but the short time off-resonant processes. This observation is demonstrated in two stages. Firstly, our zero temperature numerical results reveal that the calculated short time decoherence rates are Gaussian-like (the time dependence of the density matrix is led by the second time derivative at t = 0). Exact analytical results are also permitted in the short time limit, which, consistent with our numerical results, reveal that this specific Gaussian-like behaviour is a property of the non-Markovian correlations in the environment. These Gaussian-like rates have no dependence on any spectral parameter (position and the width of the spectrum) except, in totality, the spectral area itself. The dependence on the spectral area is a power law. Furthermore, the Gaussian-like character at short times is independent of the number of levels (N), but the numerical value of the decoherence rates is a monotonic function of N. In this context, we demonstrate that leakage, as a characteristic multilevel effect, is dominated by the non-resonant processes. The long time behaviour of decoherence is also examined. Since our spectral model allows Markovian environmental correlations at long times, the decoherence rates in this regime become exponential independently from the number of levels. The latter and the coupling strengths play the major role in the quantitative values of the rates and the rates are independent of the other spectral parameters. The validity of the presented results is restricted only by their reliance on the Born–Oppenheimer approximation. This approximation is strongly dependent on the external observational time and its reliability depends on an additional timescale. In the rest of the work, the crossover between the short and the long time behaviour of the density matrix of the multilevelled system is examined using an intuitive argument. It is shown that the Born approximation weakens as the resonant couplings become more effective at long times. This implies that, in calculations made with this approximation in the long time regime, a need for a justification arises for the reliability of the results. This justification is made for the present work.