Electronic transport in Si nanowires: Role of bulk and surface disorder

We calculate the resistance and mean free path in long metallic and semiconducting silicon nanowires (SiNWs) using two different numerical approaches: A real space Kubo method and a recursive Green's function method. We compare the two approaches and find that they are complementary: depending on the situation a preferable method can be identified. Several numerical results are presented to illustrate the relative merits of the two methods. Our calculations of relaxed atomic structures and their conductance properties are based on density functional theory without introducing adjustable parameters. Two specific models of disorder are considered: Un-passivated, surface reconstructed SiNWs are perturbed by random on-site (Anderson) disorder whereas defects in hydrogen passivated wires are introduced by randomly removed H atoms. The un-passivated wires are very sensitive to disorder in the surface whereas bulk disorder has almost no influence. For the passivated wires, the scattering by the hydrogen vacancies is strongly energy dependent and for relatively long SiNWs (L>200 nm) the resistance changes from the Ohmic to the localization regime within a 0.1 eV shift of the Fermi energy. This high sensitivity might be used for sensor applications.Comment: 9 pages, 7 figures, submitted to Phys. Rev.

On the structure of covariant phase observables

We study the mathematical structure of covariant phase observables. Such an observable can alternatively be expressed as a phase matrix, as a sequence of unit vectors, as a sequence of phase states, or as an equivalent class of covariant trace-preserving operations. Covariant generalized operator measures are defined by structure matrices which form a W*-algebra with phase matrices as its subset. The properties of the Radon-Nikodym derivatives of phase probability measures are studied.Comment: 11 page

Competition for Popularity in Bipartite Networks

We present a dynamical model for rewiring and attachment in bipartite networks in which edges are added between nodes that belong to catalogs that can either be fixed in size or growing in size. The model is motivated by an empirical study of data from the video rental service Netflix, which invites its users to give ratings to the videos available in its catalog. We find that the distribution of the number of ratings given by users and that of the number of ratings received by videos both follow a power law with an exponential cutoff. We also examine the activity patterns of Netflix users and find bursts of intense video-rating activity followed by long periods of inactivity. We derive ordinary differential equations to model the acquisition of edges by the nodes over time and obtain the corresponding time-dependent degree distributions. We then compare our results with the Netflix data and find good agreement. We conclude with a discussion of how catalog models can be used to study systems in which agents are forced to choose, rate, or prioritize their interactions from a very large set of options.Comment: 13 Pages, 19 Figure

The norm-1-property of a quantum observable

A normalized positive operator measure $X\mapsto E(X)$ has the norm-1-property if \no{E(X)}=1 whenever $E(X)\ne O$. This property reflects the fact that the measurement outcome probabilities for the values of such observables can be made arbitrary close to one with suitable state preparations. Some general implications of the norm-1-property are investigated. As case studies, localization observables, phase observables, and phase space observables are considered.Comment: 14 page

New Environmental Demands and the Future of the Helsinki−Tallinn Freight Route

The environmental friendliness of short sea shipping has been justified in Europe by the ensuing lower congestion at hinterlands and unneeded large-scale infrastructure investments on roads and railways. However, the attractiveness of short sea shipping is about to change. This is because of increasing environmental regulations (International Maritime Organization (IMO) sulfur regulation in the Baltic Sea and planned CO2 emissions trading) and increased world market oil prices. In this research, we analyze this potential change using data envelopment analysis on the existing transportation chain alternatives in the Helsinki (Finland)−Tallinn (Estonia) short sea route (chains using either roro, ropax or container ships). The analysis also includes the planned railway tunnel between the two cities. On the basis of our findings, the current truck and semi-trailer-based transportation is challenged by containers, irrespective of how they are carried (ship type). In the long term, for reasons of emissions and oil independency, the possibility of tunnel construction would make it vital to have container ship operations available along this route. The forthcoming change is not radical, but rather evolutionary and long term oriented

On the Definition of Effective Permittivity and Permeability For Thin Composite Layers

The problem of definition of effective material parameters (permittivity and permeability) for composite layers containing only one-two parallel arrays of complex-shaped inclusions is discussed. Such structures are of high importance for the design of novel metamaterials, where the realizable layers quite often have only one or two layers of particles across the sample thickness. Effective parameters which describe the averaged induced polarizations are introduced. As an explicit example, we develop an analytical model suitable for calculation of the effective material parameters $\epsilon_{\rm{eff}}$ and $\mu_{\rm{eff}}$ for double arrays of electrically small electrically polarizable scatterers. Electric and magnetic dipole moments induced in the structure and the corresponding reflection and transmission coefficients are calculated using the local field approach for the normal plane-wave incidence, and effective parameters are introduced through the averaged fields and polarizations. In the absence of losses both material parameters are purely real and satisfy the Kramers-Kronig relations and the second law of thermodynamics. We compare the analytical results to the simulated and experimental results available in the literature. The physical meaning of the introduced parameters is discussed in detail.Comment: 6 pages, 5 figure

Intervalling-Effect Bias and Competition Policy

The purpose of this paper is twofold. First, it aims to investigate whether the security's systematic risk beta estimates change as the infrequent trading phenomenon appears. Second, it attempts to provide useful insight on the impact of mergers and acquisitions on competition policy. For this reason, we employ the models of Scholes and Williams (1977), Dimson (1979), Cohen et al. (1983a) and Maynes and Rumsey (1993) on a small stock exchange with thickly infrequent trading stocks. The empirical results reveal that for some securities the models employed by Scholes and Williams (1977) and Cohen et al. (1983a) improve the biasness of the Ordinary Least Squares Market Model (Maynes and Rumsey, 1993). We argue that competitors gain while merged entities loose or at least do not gain from the clearness of the investigated mergers

Accelerating networks

Evolving out-of-equilibrium networks have been under intense scrutiny recently. In many real-world settings the number of links added per new node is not constant but depends on the time at which the node is introduced in the system. This simple idea gives rise to the concept of accelerating networks, for which we review an existing definition and -- after finding it somewhat constrictive -- offer a new definition. The new definition provided here views network acceleration as a time dependent property of a given system, as opposed to being a property of the specific algorithm applied to grow the network. The defnition also covers both unweighted and weighted networks. As time-stamped network data becomes increasingly available, the proposed measures may be easily carried out on empirical datasets. As a simple case study we apply the concepts to study the evolution of three different instances of Wikipedia, namely, those in English, German, and Japanese, and find that the networks undergo different acceleration regimes in their evolution.Comment: 12 pages, 8 figure