Dynamics of Phononic Dissipation at the Atomic Scale: Dependence on Internal Degrees of Freedom

Dynamics of dissipation of a local phonon distribution to the 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 nano-particle and the substrate, where different types of dimensionality and phonon densities of states were also considered for the substrate. In this paper, we present our analysis of transient properties of energy dissipation via phonon discharge in the microscopic level towards the substrate. Our theoretical analysis can be extended to treat realistic lubricant molecules or asperities, and also substrates with more complex densities of states. We found that the decay rate of the nano-particle phonons increases as the square of the interaction constant in the harmonic approximation.Comment: 10 pages, 6 figures, submitted to Phys. Rev.

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. © Springer-Verlag Berlin Heidelberg 2013

Superlattice Structures of Graphene based Nanoribbons

Based on first-principles calculations we predict that periodically repeated junctions of armchair graphene nanoribbons of different widths form superlattice structures. In these superlattice heterostructures the width and the energy gap are modulated in real space and specific states are confined in certain segments. Orientation of constituent nanoribbons, their width and length, the symmetry of the junction are the structural parameters to engineer electronic properties of these quantum structures. Not only the size modulation, but also composition modulation, such as periodically repeated, commensurate heterojunctions of BN and graphene honeycomb nanoribbons result in a multiple quantum well structure. We showed that these graphene based quantum structures can introduce novel concepts to design nanodevices.Comment: amended versio

Coherent Population Trapping with Controlled Interparticle Interactions

We investigate Coherent Population Trapping in a strongly interacting ultracold Rydberg gas. Despite the strong van der Waals interactions and interparticle correlations, we observe the persistence of a resonance with subnatural linewidth at the single-particle resonance frequency as we tune the interaction strength. This narrow resonance cannot be understood within a meanfield description of the strong Rydberg--Rydberg interactions. Instead, a many-body density matrix approach, accounting for the dynamics of interparticle correlations, is shown to reproduce the observed spectral features

First-principles approach to monitoring the band gap and magnetic state of a graphene nanoribbon via its vacancies

Using first-principles plane-wave calculations we predict that electronic and magnetic properties of graphene nanoribbons can be modified by the defect-induced itinerant states. Structure optimization gives rise to significant reconstruction of atomic structure, which is in good agreement with transmission electron microscope images. The band gaps of armchair nanoribbons can be modified by hydrogen-saturated holes. The band-gap changes depend on the width of the ribbon as well as on the position of the hole relative to the edges of the ribbon. Defects due to periodically repeating vacancy or divacancies induce metallization as well as magnetization in nonmagnetic semiconducting nanoribbons due to the spin polarization of local defect states. Antiferromagnetic ground state of semiconducting zigzag ribbons can change to ferrimagnetic state upon creation of vacancy defects, which reconstruct and interact with edge states. Even more remarkable is that all these effects of vacancy defects are found to depend on their geometry and position relative to the edges. It is shown that these effects can, in fact, be realized without really creating defects. © 2008 The American Physical Society

Electronic and magnetic properties of 3d transition-metal atom adsorbed graphene and graphene nanoribbons

In this paper, we theoretically studied the electronic and magnetic properties of graphene and graphene nanoribbons functionalized by 3d transition-metal (TM) atoms. The binding energies and electronic and magnetic properties were investigated for the cases where TM atoms adsorbed to a single side and double sides of graphene. We found that 3d TM atoms can be adsorbed on graphene with binding energies ranging between 0.10 and 1.95 eV depending on their species and coverage density. Upon TM atom adsorption, graphene becomes a magnetic metal. TM atoms can also be adsorbed on graphene nanoribbons with armchair edge shapes (AGNR's). Binding of TM atoms to the edge hexagons of AGNR yields the minimum energy state for all TM atom species examined in this work and in all ribbon widths under consideration. Depending on the ribbon width and adsorbed TM atom species, AGNR, which is a nonmagnetic semiconductor, can either be a metal or a semiconductor with ferromagnetic or antiferromagnetic spin alignment. Interestingly, Fe or Ti adsorption makes certain AGNR's half-metallic with a 100% spin polarization at the Fermi level. Present results indicate that the properties of graphene and graphene nanoribbons can be strongly modified through the adsorption of 3d TM atoms. © 2008 The American Physical Society

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. © 2006 IOP Publishing Ltd

Non-Markovian decoherence: A critique of the two-level approximation

The environmental decoherence in multilevelled systems in the context of two-level approximation is examined. It is found that the environmental temperature plays a minor role in the magnitudes of the decoherence rates whereas, the system-environment coupling and the environmental energy spectrum are dominant. Particularly, the latter is important in zero temperature quantum fluctuations and/or the nonequilibrium noise sources due to the large range of energies present in the environmental modes. Decoherence is found to be dominated by the short time nonresonant processes and this observation severely questions the use of the two-levelled models on decoherence. © 2005 Elsevier B.V. All rights reserved