45 research outputs found
Disordered Correlated Kondo-lattice model
We propose a self-consistent approximate solution of the disordered
Kondo-lattice model (KLM) to get the interconnected electronic and magnetic
properties of 'local-moment' systems like diluted ferromagnetic semiconductors.
Aiming at compounds, where magnetic (M) and non-magnetic (A)
atoms distributed randomly over a crystal lattice, we present a theory which
treats the subsystems of itinerant charge carriers and localized magnetic
moments in a homologous manner. The coupling between the localized moments due
to the itinerant electrons (holes) is treated by a modified RKKY-theory which
maps the KLM onto an effective Heisenberg model. The exchange integrals turn
out to be functionals of the electronic selfenergy guaranteeing selfconsistency
of our theory. The disordered electronic and magnetic moment systems are both
treated by CPA-type methods.
We discuss in detail the dependencies of the key-terms such as the long range
and oscillating effectice exchange integrals, 'the local-moment' magnetization,
the electron spin polarization, the Curie temperature as well as the electronic
and magnonic quasiparticle densities of states on the concentration of
magnetic ions, the carrier concentration , the exchange coupling , and
the temperature. The shape and the effective range of the exchange integrals
turn out to be strongly -dependent. The disorder causes anomalies in the
spin spectrum especially in the low-dilution regime, which are not observed in
the mean field approximation.Comment: Accepted by JMM
Characterization of poled and non-poled β-PVDF films using thermal analysis techniques
ß-poly(vinylidene fluoride)—ß-PVDF—exhibits ferroelectric properties due to the special arrangement of the chain units in the crystalline phase. The ferroelectric phase can be optimised by poling the original stretched film, that tends to align the randomly organised crystallites
against the applied field. In this work, polarised and non-polarised ß-PVDF from the same batch are characterised by mechanical tests and a series of thermal analysis techniques, including DMA, TMA and DSC. The films exhibit mechanical anisotropy, and in the longitudinal direction the poled film presents larger mechanical properties, due to the higher structural organisation. Poled and non-poled show similar crystallinity levels but the melting temperature probed by DSC is higher for the non-poled film; for both films the melting peak exhibits a complex shape, indicating a heterogeneous crystalline organization. Two relaxation processes were found (ß and alpha_c) by dynamic mechanical analysis (DMA). The ß process, attributed to segmental motions in the amorphous phase, appears at the same temperature in both directions,
but its intensity was found to be higher for the films tested in the longitudinal direction. For a given direction, the poled films exhibit lower peaks, due to the more organised amorphous structure. At higher temperature, the alpha_c-relaxationwas related to the contraction of the films in the longitudinal direction during heating, recorded by thermal mechanical analysis (TMA). The onset of molecular mobility within the crystalline
phase allows for cooperative diffusion processes in the amorphous phase, generating the randomisation of the initially oriented structure.Fundação para a Ciência e Tecnologia (FCT) - Programa Operacional "Ciência, Tecnologia, Inovação" (POCTI) - POCTI/CTM/33501/99
Effects of dilution and disorder on magnetism in diluted spin systems
The influence of configurational disorder on the magnetic properties of
diluted Heisenberg spin systems is studied with regard to the ferromagnetic
stability of diluted magnetic semiconductors. The equation of motion of the
magnon Green's function is decoupled by Tyablikov approximation. With supercell
approach, the concentrations of magnetic ions are determined by the size of the
supercell in which there is only one magnetic ion per supercell in our method.
In order to distinguish the influence of dilution and disorder, there are two
kinds of supercells being used: the \textit{diluted and ordered} case and the
\textit{diluted and disordered} case. The configurational averaging of magnon
Green function due to disorder is treated in the augmented space formalism. The
random exchange integrals between two supercells are treated as a matrix. The
obtained magnon spectral densities are used to calculate the temperature
dependence of magnetization and Curie temperature. The results are shown as
following: (i) dilution leads to increasing the averaged distance of two
magnetic ions, further decreases the effective exchange integrals and is main
reason to reduce Curie temperature; (ii) spatial position disorder of magnetic
ions results in the dispersions of the exchange integrals between two
supercells and slightly changes ferromagnetic transition temperature; (iii) the
exponential damping of distance dependence obviously reduces Curie temperature
and should be set carefully in any phenomenological model.Comment: 8 pages, 3 figures, accepted by physica status solidi (b); added
references and the corresponding remark
Defect structures in sine-Gordon-like models
We investigate several models described by real scalar fields, searching for
topological defects. Some models are described by a single field, and support
one or two topological sectors, and others are two-field models, which support
several topological sectors. Almost all the defect structures that we find are
stable and finite energy solutions of first-order differential equations that
solve the corresponding equations of motion. In particular, for the double
sine-Gordon model we show how to find small and large BPS solutions as
deformations of the BPS solution of the model. And also, for most of
the two field models we find the corresponding integrating factors, which lead
to the complete set of BPS solutions, nicely unveiling how they bifurcate among
the several topological sectors.Comment: RevTex, 18 pages, 17 figures; Version to appear in Physica
Schizophrenia detection and classification by advanced analysis of EEG recordings using a single electrode approach.
Electroencephalographic (EEG) analysis has emerged as a powerful tool for brain state interpretation and diagnosis, but not for the diagnosis of mental disorders; this may be explained by its low spatial resolution or depth sensitivity. This paper concerns the diagnosis of schizophrenia using EEG, which currently suffers from several cardinal problems: it heavily depends on assumptions, conditions and prior knowledge regarding the patient. Additionally, the diagnostic experiments take hours, and the accuracy of the analysis is low or unreliable. This article presents the "TFFO" (Time-Frequency transformation followed by Feature-Optimization), a novel approach for schizophrenia detection showing great success in classification accuracy with no false positives. The methodology is designed for single electrode recording, and it attempts to make the data acquisition process feasible and quick for most patients
Discrimination accuracy for each tested methodology.
<p>Discrimination accuracy for each tested methodology.</p
Discrimination accuracy per tested methodology.
<p>The accuracy achieved by the TFFO methodology is significantly better than other reaction latency related implementations, proving the added value of the analysis beyond latency differences between healthy subjects and schizophrenia patients (P-Value<1%). Moreover, the results obtained did not include any false positives, i.e., no healthy subject was classified as a schizophrenia patient. Therefore, it is desirable in such problems to have high specificity rather than high sensitivity. Generally, frontal area EEG input correlation to cognitive-related features and mental disorders is evident. Recent work in patients with schizophrenia [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123033#pone.0123033.ref013" target="_blank">13</a>;<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123033#pone.0123033.ref022" target="_blank">22</a>;<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123033#pone.0123033.ref024" target="_blank">24</a>] and Parkinson’s disease [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123033#pone.0123033.ref025" target="_blank">25</a>] has shown high input variance in that segment. Such evident correlations can explain the obtained results.</p><p>Discrimination accuracy per tested methodology.</p
Accuracy of methodology prediction as a function of time window within each stimulus recording.
<p>The most variance (or local peaks of methodology accuracy) is best achieved with a ~300 ms window placed in the recording window post-stimulus.</p
In addition to the direct connectivity approach (left panel), the connectivity maps approach brings features involving a reference electrode when addressing bi-electrode connectivity.
<p>Adding such an electrode expands the connectivity approach, as the relative changes in connectivity are uncovered.</p
Phases of the connectivity maps methodology.
<p>Phases of the connectivity maps methodology.</p