Theoretical study of the charge transport through C60-based single-molecule junctions

We present a theoretical study of the conductance and thermopower of single-molecule junctions based on C60 and C60-terminated molecules. We first analyze the transport properties of gold-C60-gold junctions and show that these junctions can be highly conductive (with conductances above 0.1G0, where G0 is the quantum of conductance). Moreover, we find that the thermopower in these junctions is negative due to the fact that the LUMO dominates the charge transport, and its magnitude can reach several tens of micro-V/K, depending on the contact geometry. On the other hand, we study the suitability of C60 as an anchoring group in single-molecule junctions. For this purpose, we analyze the transport through several dumbbell derivatives using C60 as anchors, and we compare the results with those obtained with thiol and amine groups. Our results show that the conductance of C60-terminated molecules is rather sensitive to the binding geometry. Moreover, the conductance of the molecules is typically reduced by the presence of the C60 anchors, which in turn makes the junctions more sensitive to the functionalization of the molecular core with appropriate side groups.Comment: 9 pages, 7 figure

Interaction of moving breathers with an impurity

We analyze the influence of an impurity in the evolution of moving discrete breathers in a Klein--Gordon chain with non-weak nonlinearity. Three different behaviours can be observed when moving breathers interact with the impurity: they pass through the impurity continuing their direction of movement; they are reflected by the impurity; they are trapped by the impurity, giving rise to chaotic breathers. Resonance with a breather centred at the impurity site is conjectured to be a necessary condition for the appearance of the trapping phenomenon.Comment: 4 pages, 2 figures, Proceedings of the Third Conference, San Lorenzo De El Escorial, Spain 17-21 June 200

Tuning the thermal conductance of molecular junctions with interference effects

We present an \emph{ab initio} study of the role of interference effects in the thermal conductance of single-molecule junctions. To be precise, using a first-principles transport method based on density functional theory, we analyze the coherent phonon transport in single-molecule junctions based on several benzene and oligo-phenylene-ethynylene derivatives. We show that the thermal conductance of these junctions can be tuned via the inclusion of substituents, which induces destructive interference effects and results in a decrease of the thermal conductance with respect to the unmodified molecules. In particular, we demonstrate that these interference effects manifest as antiresonances in the phonon transmission, whose energy positions can be controlled by varying the mass of the substituents. Our work provides clear strategies for the heat management in molecular junctions and more generally in nanostructured metal-organic hybrid systems, which are important to determine, how these systems can function as efficient energy-conversion devices such as thermoelectric generators and refrigerators

Critical generalized inverse participation ratio distributions

The system size dependence of the fluctuations in generalized inverse participation ratios (IPR's) $I_{\alpha}(q)$ at criticality is investigated numerically. The variances of the IPR logarithms are found to be scale-invariant at the macroscopic limit. The finite size corrections to the variances decay algebraically with nontrivial exponents, which depend on the Hamiltonian symmetry and the dimensionality. The large-$q$ dependence of the asymptotic values of the variances behaves as $q^2$ according to theoretical estimates. These results ensure the self-averaging of the corresponding generalized dimensions.Comment: RevTex4, 5 pages, 4 .eps figures, to be published in Phys. Rev.

Breathers and kinks in a simulated crystal experiment

We develop a simple 1D model for the scattering of an incoming particle hitting the surface of mica crystal, the transmission of energy through the crystal by a localized mode, and the ejection of atom(s) at the incident or distant face. This is the first attempt to model the experiment described in Russell and Eilbeck in 2007 (EPL, v. 78, 10004). Although very basic, the model shows many interesting features, for example a complicated energy dependent transition between breather modes and a kink mode, and multiple ejections at both incoming and distant surfaces. In addition, the effect of a heavier surface layer is modelled, which can lead to internal reflections of breathers or kinks at the crystal surface.Comment: 15 pages, 12 figures, based on a talk given at the conference "Localized Excitations in Nonlinear Complex Systems (LENCOS)", Sevilla (Spain) July 14-17, 200

Impulse-induced localized nonlinear modes in an electrical lattice

Intrinsic localized modes, also called discrete breathers, can exist under certain conditions in one-dimensional nonlinear electrical lattices driven by external harmonic excitations. In this work, we have studied experimentally the efectiveness of generic periodic excitations of variable waveform at generating discrete breathers in such lattices. We have found that this generation phenomenon is optimally controlled by the impulse transmitted by the external excitation (time integral over two consecutive zerosComment: 5 pages, 8 figure

Nonequilibrium Green's function theory for nonadiabatic effects in quantum electron transport

We develop nonequilibribrium Green's function based transport theory, which includes effects of nonadiabatic nuclear motion in the calculation of the electric current in molecular junctions. Our approach is based on the separation of slow and fast timescales in the equations of motion for the Green's functions by means of the Wigner representation. Time derivatives with respect to central time serves as a small parameter in the perturbative expansion enabling the computation of nonadiabatic corrections to molecular Green's functions. Consequently, we produce series of analytic expressions for non-adiabatic electronic Green's functions (up to the second order in the central time derivatives); which depend not solely on instantaneous molecular geometry but likewise on nuclear velocities and accelerations. Extended formula for electric current is derived which accounts for the non-adiabatic corrections. This theory is concisely illustrated by the calculations on a model molecular junction

Influence of moving breathers on vacancies migration

A vacancy defect is described by a Frenkel--Kontorova model with a discommensuration. This vacancy can migrate when interacts with a moving breather. We establish that the width of the interaction potential must be larger than a threshold value in order that the vacancy can move forward. This value is related to the existence of a breather centred at the particles adjacent to the vacancy.Comment: 11 pages, 10 figure

Bright and dark breathers in Fermi-Pasta-Ulam lattices

In this paper we study the existence and linear stability of bright and dark breathers in one-dimensional FPU lattices. On the one hand, we test the range of validity of a recent breathers existence proof [G. James, {\em C. R. Acad. Sci. Paris}, 332, Ser. 1, pp. 581 (2001)] using numerical computations. Approximate analytical expressions for small amplitude bright and dark breathers are found to fit very well exact numerical solutions even far from the top of the phonon band. On the other hand, we study numerically large amplitude breathers non predicted in the above cited reference. In particular, for a class of asymmetric FPU potentials we find an energy threshold for the existence of exact discrete breathers, which is a relatively unexplored phenomenon in one-dimensional lattices. Bright and dark breathers superposed on a uniformly stressed static configuration are also investigated.Comment: 11 pages, 16 figure