9,996 research outputs found
Evolution of the Fermi surface in phase fluctuating d-wave superconductors
One of the most puzzling aspects of the high superconductors is the
appearance of Fermi arcs in the normal state of the underdoped cuprate
materials. These are loci of low energy excitations covering part of the fermi
surface, that suddenly appear above instead of the nodal quasiparticles.
Based on a semiclassical theory, we argue that partial Fermi surfaces arise
naturally in a d-wave superconductor that is destroyed by thermal phase
fluctuations. Specifically, we show that the electron spectral function
develops a square root singularity at low frequencies for wave-vectors
positioned on the bare Fermi surface. We predict a temperature dependence of
the arc length that can partially account for results of recent angle resolved
photo emission (ARPES) experiments.Comment: Journal ref. adde
Application of contractor directions for solving systems of nonlinear equations
AbstractOur approach combines a method of an inexact steepest descent with the method of contractor directions to obtain a backtracking linear-time convergence algorithm for solving systems of nonlinear equations. Using the fundamental inequality of contractor directions, we prove the convergence and give an error estimate for our method.The algorithm is well-suited for parallel computation. In fact, for systems with m equations and n unknowns, each iteration may be computed in parallel time O(log m + log n) with O(mn) processors
Household food security status in South Africa
The Human Sciences Research Council has established a policy research initiative to monitor household food security and to identify and evaluate policy options. In this special edition, a selection of articles from this project is assembled. While deep chronic hunger has fallen with the expansion of the social grants, under-nutrition is a very serious and widespread challenge. This special edition draws together the best available evidence on household food security with the aim of stimulating wider debate.food security, social grants, smallholder and subsistence production, poverty, Consumer/Household Economics,
Phase transition of one dimensional bosons with strong disorder
We study one dimensional disordered bosons at large commensurate filling.
Using a real space renormalization group approach we find a new random fixed
point which controls a phase transition from a superfluid to an incompressible
Mott-glass. The transition can be tuned by changing the disorder distribution
even with vanishing interactions. We derive the properties of the transition,
which suggest that it is in the Kosterlitz-Thouless universality class.Comment: 4 pages 3 embedded eps figure
Agreement between methods of measurement with multiple observations per individual
Limits of agreement provide a straightforward and intuitive approach to agreement between different methods for measuring the same quantity. When pairs of observations using the two methods are independent, i.e., on different subjects, the calculations are very simple and straightforward. Some authors collect repeated data, either as repeated pairs of measurements on the same subject, whose true value of the measured quantity may be changing, or more than one measurement by one or both methods of an unchanging underlying quantity. In this paper we describe methods for analysing such clustered observations, both when the underlying quantity is assumed to be changing and when it is not
Tableaux for Policy Synthesis for MDPs with PCTL* Constraints
Markov decision processes (MDPs) are the standard formalism for modelling
sequential decision making in stochastic environments. Policy synthesis
addresses the problem of how to control or limit the decisions an agent makes
so that a given specification is met. In this paper we consider PCTL*, the
probabilistic counterpart of CTL*, as the specification language. Because in
general the policy synthesis problem for PCTL* is undecidable, we restrict to
policies whose execution history memory is finitely bounded a priori.
Surprisingly, no algorithm for policy synthesis for this natural and
expressive framework has been developed so far. We close this gap and describe
a tableau-based algorithm that, given an MDP and a PCTL* specification, derives
in a non-deterministic way a system of (possibly nonlinear) equalities and
inequalities. The solutions of this system, if any, describe the desired
(stochastic) policies.
Our main result in this paper is the correctness of our method, i.e.,
soundness, completeness and termination.Comment: This is a long version of a conference paper published at TABLEAUX
2017. It contains proofs of the main results and fixes a bug. See the
footnote on page 1 for detail
Trusty URIs: Verifiable, Immutable, and Permanent Digital Artifacts for Linked Data
To make digital resources on the web verifiable, immutable, and permanent, we
propose a technique to include cryptographic hash values in URIs. We call them
trusty URIs and we show how they can be used for approaches like
nanopublications to make not only specific resources but their entire reference
trees verifiable. Digital artifacts can be identified not only on the byte
level but on more abstract levels such as RDF graphs, which means that
resources keep their hash values even when presented in a different format. Our
approach sticks to the core principles of the web, namely openness and
decentralized architecture, is fully compatible with existing standards and
protocols, and can therefore be used right away. Evaluation of our reference
implementations shows that these desired properties are indeed accomplished by
our approach, and that it remains practical even for very large files.Comment: Small error corrected in the text (table data was correct) on page
13: "All average values are below 0.8s (0.03s for batch mode). Using Java in
batch mode even requires only 1ms per file.
Analysis of two competing TCP/IP connections
Many mathematical models exist for describing the behavior of TCP/IP under an exogenous loss process that does not depend on the window size. The goal of this paper is to present a mathematical analysis of two asymmetric competing TCP connections where loss probabilities are directly related to their instantaneous window size, and occur when the sum of throughputs attains a given level. We obtain bounds for the stationary throughput of each connection, as well as an exact expression for symmetric connections. This allows us to further study the fairness as a function of the different round trip times. We avoid the simplifying artificial synchronization assumption that has frequently been used in the past to study similar problems, according to which whenever one connection looses a packet, the other one looses a packet as well
- …