Quasiparticle mirages in the tunneling spectra of d-wave superconductors

We illustrate the importance of many-body effects in the Fourier transformed local density of states (FT-LDOS) of d-wave superconductors from a model of electrons coupled to an Einstein mode with energy Omega_0. For bias energies significantly larger than Omega_0 the quasiparticles have short lifetimes due to this coupling, and the FT-LDOS is featureless if the electron-impurity scattering is treated within the Born approximation. In this regime it is important to include boson exchange for the electron-impurity scattering which provides a `step down' in energy for the electrons and allows for long lifetimes. This many-body effect produces qualitatively different results, namely the presence of peaks in the FT-LDOS which are mirrors of the quasiparticle interference peaks which occur at bias energies smaller than ~ Omega_0. The experimental observation of these quasiparticle mirages would be an important step forward in elucidating the role of many-body effects in FT-LDOS measurements.Comment: revised text with new figures, to be published, Phys Rev

An overview of web accessibility in Greece: a comparative study 2004-2008

This communication reports on the results of a web accessibility audit of public Web sites in Greece. The audit was conducted in 2008 by the Human Computer Interaction Laboratory of the Institute of Computer Science of FORTH, in its capacity as the Greek National Contact Centre of the European Design for All e-Accessibility Network. In an earlier e-Accessibility study in 2004, that evaluated approximately 250 public and commercial Web sites in Greece, 73% of the sample failed to meet the most basic requirements for web accessibility (http://www.infosoc.gr/NR/rdonlyres/0B306F9C-A819-4F96-ABB1-A21945D1D2B3/1092/final_report.pdf). Four years later, in the context of a re-audit, a similar sample was examined for compliance with the same web accessibility standard (WCAG 1.0), as set by the Web Accessibility Initiative of the World Wide Web Consortium. The accessibility checks were conducted during the period from March to September 2008. A comparison of the results indicates that although new web development technologies may have been employed, instead of improving the overall status of e-accessibility, this has resulted in a serious deterioration in overall accessibility levels. Such findings—85% of sites failed to comply with Level A—suggest that Greek Web sites are likely to present even more significant access barriers to people with disability than in the past. As Web Accessibility is poorly understood and, until today, new platforms have failed to deliver products that conform to WCAG, the need emerges for a concrete legislative framework to set accessibility specifications for all public Web sites in Greece

White Paper: promoting Design for All and e-Accessibility in Europe.

This White Paper results from the collaboration of Design for All (DfA) and e-Accessibility expert bodies representing the European Union member states [ref: the coordination centres of the national European Design for All e-Accessibility network (EDeAN) in 23 European Union Member States, in this document referred to as NCCs]. The objectives of this White Paper are threefold: (1) to present the current state of affairs of EDeAN, active since 2002 as part of the European Commission’s action line supporting the development of the European Information Society; (2) to outline a roadmap for future initiatives, actively reflecting views of the EDeAN network in specific domains related to DfA, e-Accessibility and e-Inclusion; and (3) to bring forward a set of suggestions and recommendations regarding the activities of the network, aiming to support the creation of a fully inclusive European Information Society. The paper is organised as follows: Section 1 provides a short introduction to DfA and Accessibility in Information Society (IS) and Information Society Technology (IST) developments in Europe. Section 2 provides an overview of policy as well as R&D activities related to DfA and accessibility in IST in Europe. Section 3 briefly describes the role of EDeAN and the new challenges that emerge as a result of the changing European policy environment. Section 4 presents current challenges that emerge in five domains directly related to DfA and accessibility, such as policy and legislation, industry, monitoring and evaluation—benchmarking, standardisation, R&D, and education and training. Finally, Sect. 5 puts forward a number of recommendations with regard to future activities for EDeAN. The contents of this paper reflect results from the open discussion forums supported in the EDeAN website (ref: EDeAN Special Interest Groups, SIGs). The draft paper was written by the EDeAN Secretariat for 2005, FORTH-ICS (Greece), and completed with the support of all NCCs. Support was also provided by the European Commission DG Information Society and Media and by the D4ALLnet project (ref: an EC funded project with the code IST-2001-38833)

Online Communities for All: The Role of Design for All in the Formation and Support of Inclusive Online Communities

An online community is a virtual community composed of individuals who form and maintain online communication, share common goals, interests and needs that provide the reason of community existence, and use common interactive tools that support their networking and communication activities. The term “networking” is traditionally used to describe the formation and maintenance of connections and relationships between individuals, organizations or other interested parties. Virtual or online networking is a form of interpersonal communication that is characterized by the use of electronic means of communication between the different parties and the exchange of messages and information in electronic forms [11]. Hence, virtual or online communities perform virtual networking. This paper aims to examine how the principles of Design for All can support the formation and operation of online communities that are fully inclusive, multi-cultural and multiplatform, thus providing an infrastructure essentially accessible to all their members to carry out virtual networking activities. Following an introduction to online communities and community formation, the paper briefly builds the case for Design for All and the challenges for new information and communication technologies towards a fully inclusive Information Society. Then, based on practical experience and case studies, the paper examines the benefits and challenges of applying the principles of Design for All in the design, development and support of virtual interaction spaces, in order to cater for the diversity of users-community members, as well as of their environments of use. The paper concludes by proposing a set of design issues to be taken into account in the development of online communities for all

Facilitating virtual networking for Design for All in Europe: The HERMES platform

This paper reports on HERMES, the virtual networking platform of the European Design for All eAccessibility Network (EDeAN) that was established in 2002 by the European Commission in order to stimulate European network activities within the area of Design for All, with particular focus on ICT and e-Accessibility. The paper reports on the development and utilisation of the networking platform developed to enable the cooperation of EDeAN members through an accessible interface over the World Wide Web. HERMES is available at http://www.edean.or

Spin exchange in quantum rings and wires in the Wigner-crystal limit

We present a controlled method for computing the exchange coupling in strongly correlated one-dimensional electron systems. It is based on the asymptotically exact relation between the exchange constant and the pair-correlation function of spinless electrons. Explicit results are obtained for thin quantum rings with realistic Coulomb interactions, by calculating this function via a many-body instanton approach.Comment: 7 pages, 2 figures. Changes in the text and figures to improve readability; added reference

Nuclear magnetic resonance probes for the Kondo scenario for the 0.7 feature in semiconductor quantum point contact devices

We propose a probe based on nuclear relaxation and Knight shift measurements for the Kondo scenario for the "0.7 feature" in semiconductor quantum point contact (QPC) devices. We show that the presence of a bound electron in the QPC would lead to a much higher rate of nuclear relaxation compared to nuclear relaxation through exchange of spin with conduction electrons. Furthermore, we show that the temperature dependence of this nuclear relaxation is very non-monotonic as opposed to the linear-T relaxation from coupling with conduction electrons. We present a qualitative analysis for the additional relaxation due to nuclear spin diffusion (NSD) and study the extent to which NSD affects the range of validity of our method. The conclusion is that nuclear relaxation, in combination with Knight shift measurements, can be used to verify whether the 0.7 feature is indeed due to the presence of a bound electron in the QPC.Comment: Published version. Appears in a Special Section on the 0.7 Feature and Interactions in One-Dimensional Systems. 16 page

Electron waves in chemically substituted graphene

We present exact analytical and numerical results for the electronic spectra and the Friedel oscillations around a substitutional impurity atom in a graphene lattice. A chemical dopant in graphene introduces changes in the on-site potential as well as in the hopping amplitude. We employ a T-matrix formalism and find that disorder in the hopping introduces additional interference terms around the impurity that can be understood in terms of bound, semi-bound, and unbound processes for the Dirac electrons. These interference effects can be detected by scanning tunneling microscopy.Comment: 4 pages, 7 figure

Wigner crystal physics in quantum wires

The physics of interacting quantum wires has attracted a lot of attention recently. When the density of electrons in the wire is very low, the strong repulsion between electrons leads to the formation of a Wigner crystal. We review the rich spin and orbital properties of the Wigner crystal, both in the one-dimensional and quasi-one-dimensional regime. In the one-dimensional Wigner crystal the electron spins form an antiferromagnetic Heisenberg chain with exponentially small exchange coupling. In the presence of leads the resulting inhomogeneity of the electron density causes a violation of spin-charge separation. As a consequence the spin degrees of freedom affect the conductance of the wire. Upon increasing the electron density, the Wigner crystal starts deviating from the strictly one-dimensional geometry, forming a zigzag structure instead. Spin interactions in this regime are dominated by ring exchanges, and the phase diagram of the resulting zigzag spin chain has a number of unpolarized phases as well as regions of complete and partial spin polarization. Finally we address the orbital properties in the vicinity of the transition from a one-dimensional to a quasi-one-dimensional state. Due to the locking between chains in the zigzag Wigner crystal, only one gapless mode exists. Manifestations of Wigner crystal physics at weak interactions are explored by studying the fate of the additional gapped low-energy mode as a function of interaction strength.Comment: 37 pages, 15 figures; v2: references adde

Techniques for Arbuscular Mycorrhiza Inoculum Reduction

It is well established that arbuscular mycorrhizal (AM) fungi can play a significant role in sustainable crop production and environmental conservation. With the increasing awareness of the ecological significance of mycorrhizas and their diversity, research needs to be directed away from simple records of their occurrence or casual speculation of their function (Smith and Read 1997). Rather, the need is for empirical studies and investigations of the quantitative aspects of the distribution of different types and their contribution to the function of ecosystems. There is no such thing as a fungal effect or a plant effect, but there is an interaction between both symbionts. This results from the AM fungi and plant community size and structure, soil and climatic conditions, and the interplay between all these factors (Kahiluoto et al. 2000). Consequently, it is readily understood that it is the problems associated with methodology that limit our understanding of the functioning and effects of AM fungi within field communities. Given the ubiquous presence of AM fungi, a major constraint to the evaluation of the activity of AM colonisation has been the need to account for the indigenous soil native inoculum. This has to be controlled (i.e. reduced or eliminated) if we are to obtain a true control treatment for analysis of arbuscular mycorrhizas in natural substrates. There are various procedures possible for achieving such an objective, and the purpose of this chapter is to provide details of a number of techniques and present some evaluation of their advantages and disadvantages. Although there have been a large number of experiments to investigated the effectiveness of different sterilization procedures for reducing pathogenic soil fungi, little information is available on their impact on beneficial organisms such as AM fungi. Furthermore, some of the techniques have been shown to affect physical and chemical soil characteristics as well as eliminate soil microorganisms that can interfere with the development of mycorrhizas, and this creates difficulties in the interpretation of results simply in terms of possible mycorrhizal activity. An important subject is the differentiation of methods that involve sterilization from those focussed on indigenous inoculum reduction. Soil sterilization aims to destroy or eliminate microbial cells while maintaining the existing chemical and physical characteristics of the soil (Wolf and Skipper 1994). Consequently, it is often used for experiments focussed on specific AM fungi, or to establish a negative control in some other types of study. In contrast, the purpose of inoculum reduction techniques is to create a perturbation that will interfere with mycorrhizal formation, although not necessarily eliminating any component group within the inoculum. Such an approach allows the establishment of different degrees of mycorrhizal formation between treatments and the study of relative effects. Frequently the basic techniques used to achieve complete sterilization or just an inoculum reduction may be similar but the desired outcome is accomplished by adjustments of the dosage or intensity of the treatment. The ultimate choice of methodology for establishing an adequate non-mycorrhizal control depends on the design of the particular experiments, the facilities available and the amount of soil requiring treatment