Domain decomposition methods in image denoising using Gaussian curvature

AbstractA domain decomposition method is used to solve a Gaussian curvature driven flow to denoise digital images. The algorithm is embarrassingly parallel and is ideal for parallel computers. We use the scheme of Lee–Keun [Noise removal based on nonlinear diffusion with Gauss curvature conductance, 2003, Preprint] in which the diffusion coefficient contains a term involving the Gaussian curvature. This minimizes the occurrence of patches and tips of cones are not flattened

Alternative approach to computing transport coefficients: application to conductivity and Hall coefficient of hydrogenated amorphous silicon

We introduce a theoretical framework for computing transport coefficients for complex materials. As a first example, we resolve long-standing inconsistencies between experiment and theory pertaining to the conductivity and Hall mobility for amorphous silicon and show that the Hall sign anomaly is a consequence of localized states. Next, we compute the AC conductivity of amorphous polyanaline. The formalism is applicable to complex materials involving defects and band-tail states originating from static topological disorder and extended states. The method may be readily integrated with current \textit{ab initio} methods.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let

Optical properties of small polarons from dynamical mean-field theory

The optical properties of polarons are studied in the framework of the Holstein model by applying the dynamical mean-field theory. This approach allows to enlighten important quantitative and qualitative deviations from the limiting treatments of small polaron theory, that should be considered when interpreting experimental data. In the antiadiabatic regime, accounting on the same footing for a finite phonon frequency and a finite electron bandwidth allows to address the evolution of the optical absorption away from the well-understood molecular limit. It is shown that the width of the multiphonon peaks in the optical spectra depends on the temperature and on the frequency in a way that contradicts the commonly accepted results, most notably in the strong coupling case. In the adiabatic regime, on the other hand, the present method allows to identify a wide range of parameters of experimental interest, where the electron bandwidth is comparable or larger than the broadening of the Franck-Condon line, leading to a strong modification of both the position and the shape of the polaronic absorption. An analytical expression is derived in the limit of vanishing broadening, which improves over the existing formulas and whose validity extends to any finite-dimensional lattice. In the same adiabatic regime, at intermediate values of the interaction strength, the optical absorption exhibits a characteristic reentrant behavior, with the emergence of sharp features upon increasing the temperature -- polaron interband transitions -- which are peculiar of the polaron crossover, and for which analytical expressions are provided.Comment: 16 pages, 6 figure

A Variational Approach to Nonlocal Exciton-Phonon Coupling

In this paper we apply variational energy band theory to a form of the Holstein Hamiltonian in which the influence of lattice vibrations (optical phonons) on both local site energies (local coupling) and transfers of electronic excitations between neighboring sites (nonlocal coupling) is taken into account. A flexible spanning set of orthonormal eigenfunctions of the joint exciton-phonon crystal momentum is used to arrive at a variational estimate (bound) of the ground state energy for every value of the joint crystal momentum, yielding a variational estimate of the lowest polaron energy band across the entire Brillouin zone, as well as the complete set of polaron Bloch functions associated with this band. The variation is implemented numerically, avoiding restrictive assumptions that have limited the scope of previous assaults on the same and similar problems. Polaron energy bands and the structure of the associated Bloch states are studied at general points in the three-dimensional parameter space of the model Hamiltonian (electronic tunneling, local coupling, nonlocal coupling), though our principal emphasis lay in under-studied area of nonlocal coupling and its interplay with electronic tunneling; a phase diagram summarizing the latter is presented. The common notion of a "self-trapping transition" is addressed and generalized.Comment: 33 pages, 11 figure

Optical absorption and activated transport in polaronic systems

We present exact results for the optical response in the one-dimensional Holstein model. In particular, by means of a refined kernel polynomial method, we calculate the ac and dc electrical conductivities at finite temperatures for a wide parameter range of electron phonon interaction. We analyze the deviations from the results of standard small polaron theory in the intermediate coupling regime and discuss non-adiabaticity effects in detail.Comment: 7 pages, 8 figure

Comment on `Dynamical properties of small polarons'

We show that the conclusion on the breakdown of the standard small polaron theory made recently by E.V. deMello and J. Ranninger (Phys. Rev. B 55, 14872 (1997)) is a result of an incorrect interpretation of the electronic and vibronic energy levels of the two-site Holstein model. The small polaron theory, when properly applied, agrees well with the numerical results of these authors. Also we show that their attempt to connect the properties of the calculated correlation functions with the features of the intersite electron hopping is unsuccessful.Comment: To appear in Phys. Rev.

Electric Field Effect in Atomically Thin Carbon Films

We report a naturally-occurring two-dimensional material (graphene that can be viewed as a gigantic flat fullerene molecule, describe its electronic properties and demonstrate all-metallic field-effect transistor, which uniquely exhibits ballistic transport at submicron distances even at room temperature

Momentum average approximation for models with boson-modulated hopping: Role of closed loops in the dynamical generation of a finite quasiparticle mass

We generalize the momentum average approximation to study the properties of single polarons in models with boson affected hopping, where the fermion-boson scattering depends explicitly on both the fermion's and the boson's momentum. As a specific example, we investigate the Edwards fermion-boson model in both one and two dimensions. In one dimension, this allows us to compare our results with exact diagonalization results, to validate the accuracy of our approximation. The generalization to two-dimensional lattices allows us to calculate the polaron's quasiparticle weight and dispersion throughout the Brillouin zone and to demonstrate the importance of Trugman loops in generating a finite effective mass even when the free fermion has an infinite mass.Comment: 15 pages, 14 figure

Disorder-driven superconductor-normal metal phase transition in quasi-one-dimensional organic conductors

Effects of non-magnetic disorder on the critical temperature T_c and on diamagnetism of quasi-one-dimensional superconductors are reported. The energy of Josephson-coupling between wires is considered to be random, which is typical for dirty organic superconductors. We show that this randomness destroys phase coherence between wires and that T_c vanishes discontinuously at a critical disorder-strength. The parallel and transverse components of the penetration-depth are evaluated. They diverge at different critical temperatures T_c^{(1)} and T_c, which correspond to pair-breaking and phase-coherence breaking respectively. The interplay between disorder and quantum phase fluctuations is shown to result in quantum critical behavior at T=0, which manifests itself as a superconducting-normal metal phase transition of first-order at a critical disorder strength.Comment: 12 pages, 3 figure

Two Dimensional Electron and Hole Gases at the Surface of Graphite

We report high-quality two-dimensional (2D) electron and hole gases induced at the surface of graphite by the electric field effect. The 2D carriers reside within a few near-surface atomic layers and exhibit mobilities up to 15,000 and 60,000 cm2/Vs at room and liquid-helium temperatures, respectively. The mobilities imply ballistic transport at micron scale. Pronounced Shubnikov-de Haas oscillations reveal the existence of two types of carries in both 2D electron and hole gases.Comment: related to cond-mat/0410631 where preliminary data for this experimental system were reporte