3,261 research outputs found
Properties of the energy landscape of network models for covalent glasses
We investigate the energy landscape of two dimensional network models for
covalent glasses by means of the lid algorithm. For three different particle
densities and for a range of network sizes, we exhaustively analyse many
configuration space regions enclosing deep-lying energy minima. We extract the
local densities of states and of minima, and the number of states and minima
accessible below a certain energy barrier, the 'lid'. These quantities show on
average a close to exponential growth as a function of their respective
arguments. We calculate the configurational entropy for these pockets of states
and find that the excess specific heat exhibits a peak at a critical
temperature associated with the exponential growth in the local density of
states, a feature of the specific heat also observed in real glasses at the
glass transition.Comment: RevTeX, 19 pages, 7 figure
Strongly enhanced shot noise in chains of quantum dots
We study charge transport through a chain of quantum dots. The dots are fully
coherent among each other and weakly coupled to metallic electrodes via the
dots at the interface, thus modelling a molecular wire. If the non-local
Coulomb interactions dominate over the inter-dot hopping we find strongly
enhanced shot noise above the sequential tunneling threshold. The current is
not enhanced in the region of enhanced noise, thus rendering the noise
super-Poissonian. In contrast to earlier work this is achieved even in a fully
symmetric system. The origin of this novel behavior lies in a competition of
"slow" and "fast" transport channels that are formed due to the differing
non-local wave functions and total spin of the states participating in
transport. This strong enhancement may allow direct experimental detection of
shot noise in a chain of lateral quantum dots.Comment: 4 pages, 2 figures, submitted to PR
The possible explanation of electric-field-doped C60 phenomenology in the framework of Eliashberg theory
In a recent paper (J.H. Schon, Ch. Kloc, R.C. Haddon and B. Batlogg, Nature
408 (2000) 549) a large increase in the superconducting critical temperature
was observed in C60 doped with holes by application of a high electric field.
We demonstrate that the measured Tc versus doping curves can be explained by
solving the (four) s-wave Eliashberg equations in the case of a finite,
non-half-filled energy band. In order to reproduce the experimental data, we
assume a Coulomb pseudopotential depending on the filling in a very simple and
plausible way. Reasonable values of the physical parameters involved are
obtained. The application of the same approach to new experimental data (J.H.
Schon, Ch. Kloc and B. Batlogg, Science 293 (2001) 2432) on electric
field-doped, lattice-expanded C60 single crystals (Tc=117 K in the hole-doped
case) gives equally good results and sets a theoretical limit to the linear
increase of Tc at the increase of the lattice spacing.Comment: latex2e, 6 pages, 7 figures, 1 table, revised versio
Kondo effect in quantum dots coupled to ferromagnetic leads
We study the Kondo effect in a quantum dot which is coupled to ferromagnetic
leads and analyse its properties as a function of the spin polarization of the
leads. Based on a scaling approach we predict that for parallel alignment of
the magnetizations in the leads the strong-coupling limit of the Kondo effect
is reached at a finite value of the magnetic field. Using an equation-of-motion
technique we study nonlinear transport through the dot. For parallel alignment
the zero-bias anomaly may be split even in the absence of an external magnetic
field. For antiparallel spin alignment and symmetric coupling, the peak is
split only in the presence of a magnetic field, but shows a characteristic
asymmetry in amplitude and position.Comment: 5 pages, 2 figure
Identifying Agile Requirements Engineering Patterns in Industry
Agile Software Development (ASD) is gaining in popularity in today´s business world. Industry is adopting agile methodologies both to accelerate value delivery and to enhance the ability to deal with changing requirements. However, ASD has a great impact on how Requirements Engineering (RE) is carried out in agile environments. The integration of Human-Centered Design (HCD) plays an important role due to the focus on user and stakeholder involvement. To this end, we aim to introduce agile RE patterns as main objective of this paper. On the one hand, we will describe our pattern mining process based on empirical research in literature and industry. On the other hand, we will discuss our results and provide two examples of agile RE patterns. In sum, the pattern mining process identifies 41 agile RE patterns. The accumulated knowledge will be shared by means of a web application.Ministerio de EconomÃa y Competitividad TIN2013-46928-C3-3-RMinisterio de EconomÃa y Competitividad TIN2016-76956-C3-2-RMinisterio de EconomÃa y Competitividad TIN2015-71938-RED
Can grey ravens fly? Beyond Frayling's categories
This paper analyses the effect of Christopher Frayling's (1993) categorisation of artistic research ‘research into art and design, research through art and design and research for art and design’ on the debate surrounding the efficacy of studio-based artistic research as being valid within the university. James Elkins (2009:128) describes this as ‘the incommensurability of studio art production and university life’. Through an exploration of the positive and negative responses to Frayling this paper seeks to explore the influence that these initial definitions have come to have on framing the scope of the debate. The paper presents a range of responses and analyses them and focuses especially on the alternative frameworks that have been suggested and examines why they have so far not created a coherent and uncontested frame-work for practice-led research in the art and design field especially in relation to fine art
Super-poissonian noise, negative differential conductance, and relaxation effects in transport through molecules, quantum dots and nanotubes
We consider charge transport through a nanoscopic object, e.g. single
molecules, short nanotubes, or quantum dots, that is weakly coupled to metallic
electrodes. We account for several levels of the molecule/quantum dot with
level-dependent coupling strengths, and allow for relaxation of the excited
states. The current-voltage characteristics as well as the current noise are
calculated within first-order perturbation expansion in the coupling strengths.
For the case of asymmetric coupling to the leads we predict
negative-differential-conductance accompanied with super-poissonian noise. Both
effects are destroyed by fast relaxation processes. The non-monotonic behavior
of the shot noise as a function of bias and relaxation rate reflects the
details of the electronic structure and level-dependent coupling strengths.Comment: 8 pages, 7 figures, submitted to Phys. Rev. B, added reference
The significance of work allocation in the professional apprenticeship of solicitors
It is a peculiarity of the solicitors’ profession that it has historically relied on methods of pre-qualification ‘training’ by way of apprenticeship and that an entirely respectable non-graduate route into the profession remains. In a political context, however, where the profession is called upon positively to demonstrate its standards of performance, the professional regulator seeks to attach a competence framework to the existing model; shifting the focus from how the trainee learns to what the trainee learns. This paper will explore the period of traineeship from the perspective of the trainees themselves, drawing on two small qualitative studies, focussing on the fundamental context factor of the allocation and structuring of their work. In the first study the context for this evaluation is the set of outcomes being tested by the professional regulator and in the second, the perceptions of qualified individuals looking back at their apprenticeship, The paper concludes that there remains work for the profession to do not only in fostering supportive and expansive apprenticeships, but in attending, however, supportive the surrounding environment, to the work being carried out by trainees and its relationship with the work carried out by newly qualified solicitors
Evidence for Insulating Behavior in the Electric Conduction of (NH)KC Systems
Microwave study using cavity perturbation technique revealed that the
conductivity of antiferromagnet (NH)KRbC at 200K is
already 3-4 orders of magnitude smaller than those of superconductors,
KC and (NH)NaRbC, and that the antiferromagnetic
compounds are {\it insulators} below 250K without metal-insulator transitions.
The striking difference in the magnitude of the conductivity between these
materials strongly suggests that the Mott-Hubbard transition in the ammoniated
alkali fullerides is driven by a reduction of lattice symmetry from
face-centered-cubic to face-centered-orthorhombic, rather than by the magnetic
ordering.Comment: accepted for publication in PR
Strong Tunneling in Double-Island Structures
We study the electron transport through a system of two low-capacitance metal
islands connected in series between two electrodes. The work is motivated in
part by experiments on semiconducting double-dots, which show intriguing
effects arising from coherent tunneling of electrons and mixing of the
single-electron states across tunneling barriers. In this article, we show how
coherent tunneling affects metallic systems and leads to a mixing of the
macroscopic charge states across the barriers. We apply a recently formulated
RG approach to examine the linear response of the system with high tunnel
conductances (up to 8e^2/h). In addition we calculate the (second order)
cotunneling contributions to the non-linear conductance. Our main results are
that the peaks in the linear and nonlinear conductance as a function of the
gate voltage are reduced and broadened in an asymmetric way, as well as shifted
in their positions. In the limit where the two islands are coupled weakly to
the electrodes, we compare to theoretical results obtained by Golden and
Halperin and Matveev et al. In the opposite case when the two islands are
coupled more strongly to the leads than to each other, the peaks are found to
shift, in qualitative agreement with the recent prediction of Andrei et al. for
a similar double-dot system which exhibits a phase transition.Comment: 12 page
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