5,118 research outputs found
Competing Antiferromagnetic and Spin-Glass Phases in a Hollandite Structure
We introduce a simple lattice model with Ising spins to explain recent
experimental results on spin freezing in a hollandite-type structure. We argue
that geometrical frustration of the lattice in combination with
nearest-neighbour antiferromagnetic (AFM) interactions is responsible for the
appearance of a spin-glass phase in presence of disorder. We investigate this
system numerically using parallel tempering. The model reproduces the magnetic
behaviour of oxides with hollandite structure, such as
and presents a rich phenomenology: in absence of disorder three types of ground
states are possible, depending on the relative strength of the interactions,
namely AFM ordered and two different disordered, macroscopically degenerate
families of ground states. Remarkably, for sets of AFM couplings having an AFM
ground state in the clean system, there exists a critical value of the disorder
for which the ground state is replaced by a spin-glass phase while maintaining
all couplings AFM. To the best of our knowledge this is the only existing model
that presents this kind of transition with short-range AFM interactions. We
argue that this model could be useful to understand the relation between AFM
coupling, disorder and the appearance of a spin-glass phase.Comment: 8 pages, 7 figure
Incommensurate, helical spin ground states on the Hollandite lattice
We present a model of classical Heisenberg spins on a Hollandite lattice,
which has been developed to describe the magnetic properties of
-MnO and similar compounds. The model has nearest neighbor
interacting spins, however the strength and the sign of spin-spin interactions
is anisotropic and depends on the nature of the bonds. Our analysis shows that
the Hollandite lattice supports four different incommensurate and helical
magnetic ground states depending on the relative strengths and signs of
spin-spin interactions. We show that the incommensurate helical ground states
appear due to the geometrical frustration present in the model. We demonstrate
that each of the four helical incommensurate magnetic phases are continuously
connected to four different collinear antiferromagnetic ground states as the
strength of spin-spin interaction along some bonds is increased. The present
results give support to the presence of helical states that have been
previously suggested experimentally for Hollandite compounds. We provide an
in-depth analysis of the magnetic form factors for each helical phase and
describe how it could be used to identify each of these phases in neutron
diffraction experiments.Comment: 11 pages, 8 figure
Dispersion managed mode-locking dynamics in a Ti:Sapphire laser
We present what is to our knowledge the most complete 1-D numerical analysis
of the evolution and the propagation dynamics of an ultrashort laser pulse in a
Ti:Sapphire laser oscillator. This study confirms the dispersion managed model
of mode-locking, and emphasizes the role of the Kerr nonlinearity in generating
mode-locked spectra with a smooth and well-behaved spectral phase. A very good
agreement with preliminary experimental measurements is found.Comment: 11 pages, 4 figures, submitted to Optics Letter
A flexible approach for measuring author-level publishing performance
We propose a framework to evaluate, in relative terms, author-level publishing performance. To that end we introduce the publishing performance index (PPI) and the publishing performance box (PPB), and discuss the associated publishing profiles. We illustrate our approach conducting an extensive empirical application covering 472 top economists and developing several robustness tests. Instead of using a pre-designed measure without lexibility to adjust to the circumstances of each specific case, our approach accommodates alternative evaluation criteria, as defined by the evaluators. Beyond this key characteristic, our approach has some other important advantages: (1) it is easy to apply; (2) it is sensitive to the full list of publications and citations; (3) it is able to include additional dimensions of scientific performance beyond papers and citations; (4) it is a high granularity measure, providing a complete ranking of the authors under analysis.info:eu-repo/semantics/acceptedVersio
The problem of credit in research evaluation – the case of Economics
The measurement of scientific performance is usually done giving the full credit of each paper to all its authors. Aiming
to analyze the impact of the number of authors on the performance, we propose an adjustment to the h-index that is flexible enough to allow the consideration of distinct co-authorship weighting schemes. We then evaluate the publication performance of the members of the departments of economics of the top 10 world universities. Our results show that the number of authors per paper is rapidly increasing and that this dimension measurably affects the final ranking of authors even in a subject area where the average number of authors is lower than in physical and life sciences.info:eu-repo/semantics/publishedVersio
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