Technological Education Institute of Ionian Islands

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Apostolopoulos P.S.,Technological Education Institute of Ionian Islands
General Relativity and Gravitation | Year: 2017

The implications from the existence of a proper Homothetic Vector Field on the dynamics of vacuum anisotropic models in F(R) gravitational theory are studied. The fact that every Spatially Homogeneous vacuum model is equivalent, formally, with a “flux”-free anisotropic fluid model in standard gravity and the induced power-law form of the functional F(R) due to self-similarity enable us to close the system of equations. We found some new exact anisotropic solutions that arise as fixed points in the associated dynamical system. The non-existence of Kasner-like (Bianchi type I) solutions in proper F(R)-gravity (i.e. R≠ 0) strengthens the belief that curvature corrections will prevent the shear influence into the past thus permitting an isotropic singularity. We also discuss certain issues regarding the lack of vacuum models of type III, IV, VIIh in comparison with the corresponding results in standard gravity. © 2017, Springer Science+Business Media New York.


Katerelos D.T.G.,Lulea University of Technology | Katerelos D.T.G.,Technological Education Institute of Ionian Islands | Varna J.,Technological Education Institute of Ionian Islands
International Journal of Damage Mechanics | Year: 2013

Intralaminar matrix cracking is recognized as the primary mode of damage appearing in laminates. Local Raman strain measurements in neighboring layers show that the distribution shape in the vicinity of the crack tip is smoothened with increasing magnitude of the applied strain. This behavior is attributed to secondary damage development in the neighboring supporting plies close to the transverse crack tip. In this article, an approximate analytical variational model, based on the principle of minimum complementary energy, is proposed for the investigation of the stress state in a glass/ epoxy cross-ply laminate with cracks in 90° layer and with homogenized damage (reduced elastic properties) in part of the 0° layer. The extent of damage (fiber breaks) is estimated combining modeling with measured local strain distributions. The model is applied in order to examine the effect of ply thickness on the magnitude of the secondary damage at the transverse crack tip. © 2012 The Author(s).


Baskoutas S.,University of Patras | Garoufalis C.,University of Patras | Garoufalis C.,Technological Education Institute of Ionian Islands | Terzis A.F.,University of Patras
European Physical Journal B | Year: 2011

In the present theoretical study, the linear and third-order nonlinear optical absorption coefficients have been calculated in GaAs/Ga 1-x Al x As inverse parabolic quantum wells (single and double) subjected to an external electric field. Our calculations are based on the potential morphing method in the effective mass approximation. The systematic theoretical investigation contains results with all possible combinations of the involved parameters, as quantum well width, quantum barrier width, Al concentration at each well center and magnitude of the external electric field. Our results indicate that in most cases investigated, the increase of the electric field blue-shifts the peak positions of the total absorption coefficient. In all cases studied it became apparent that the incident optical intensity considerably affects the total absorption coefficient. © 2011 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.


Baskoutas S.,University of Patras | Zeng Z.,University of Patras | Garoufalis C.S.,University of Patras | Garoufalis C.S.,Technological Education Institute of Ionian Islands | Bester G.,Max Planck Institute for Solid State Research
Journal of Physical Chemistry C | Year: 2012

In the present work, we have performed a systematic theoretical study of the electronic and optical properties of ZnO nanorods as a function of an externally applied hydrostatic pressure. Our calculations are based on the empirical pseudopotential method and configuration interaction to consider the excitonic effects. The optical emission polarization is found to exhibit a transition between in-plane and out-of-plane polarization with increasing pressure. The critical value of the hydrostatic pressure at which the crossover takes place appears to depend strongly on the nanorod's radius. In particular, when the nanorod's radius is smaller than the ZnO exciton Bohr radius, the necessary pressure to reach the crossover point is significantly smaller than the pressure required for a nanorod with radius larger than the exciton Bohr radius. For large pressures, the lowest exciton state is a nearly pure state (0,0), where electron and hole have dominant S-orbital character and the hole has a nearly pure parentage from the bulk Bloch C-band. © 2012 American Chemical Society.


Zeng Z.,University of Patras | Garoufalis C.S.,University of Patras | Garoufalis C.S.,Technological Education Institute of Ionian Islands | Baskoutas S.,University of Patras | Bester G.,Max Planck Institute for Solid State Research
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

In the present work, we studied the electronic and optical properties of ZnO quantum dots (QDs) subjected to externally applied hydrostatic pressure. Our single-particle calculations are based on the empirical pseudopotential method and the excitonic effects are considered by employing the configuration interaction approach. The optical band gap, Stokes shift, and optical emission polarization have been investigated as a function of the applied pressure. It is found that the applied pressure causes a linear increase in the optical band gap. The pressure coefficient appears to be highly size dependent, exhibiting a monotonic increase with increasing QD size. In contrast to this monotonic behavior, the applied pressure induces a nonmonotonic Stokes shift which presents a minimum value at a critical pressure. For pressures larger than this critical value, the optical emission polarization exhibits a sharp transition from in-plane to out-of-plane polarization. Finally, it is found that the critical pressure at which the crossing takes place strongly depends on the QD size, showing larger values for larger QD sizes. Beyond this crossing point, the lowest optically bright exciton state mainly originates from one Slater determinant, where both the single-particle electron and hole states have an S-type envelope function and the hole state originates mainly from the bulk Bloch C band. © 2013 American Physical Society.


Zeng Z.,University of Patras | Garoufalis C.S.,University of Patras | Garoufalis C.S.,Technological Education Institute of Ionian Islands | Terzis A.F.,University of Patras | Baskoutas S.,University of Patras
Journal of Applied Physics | Year: 2013

In the present work, we investigated theoretically the linear, nonlinear, and total absorption coefficients and refractive index changes associated with intersubband transitions in ZnO/ZnS core shell quantum dot (CSQD) and ZnS/ZnO inverted CSQD (ICSQD), emphasizing on the influence of the shell thickness, impurity, and dielectric environment. The effect of the polarization charges due to the possible existence of the dielectric mismatch between the system and its surrounding matrix is considered. The electronic structures are numerically calculated by employing the potential morphing method in the framework of effective mass approximation. We find that in both impurity-free CSQD and ICSQD, increasing the shell thickness red shifts significantly the threshold energy and enhances drastically the nonlinear absorption coefficients and all the refractive index changes, independently on the dielectric environments. Similar behaviour has also been observed in most of the cases studied when the impurity is displaced from the core center to the shell center. In contrast, comparing to a dielectrically homogeneous system, dispersing the systems into a matrix with a lower dielectric constant blue shifts all the peak positions of the absorption coefficients and refractive index changes. However, the corresponding magnitudes (in absolute value) are substantially reduced. Finally, we find that the nonlinear properties are more sensitive to the external perturbations, while at a weak radiation intensity, the variation of the total quantities is generally dominated by that of the corresponding linear terms. © 2013 AIP Publishing LLC.


Zeng Z.,University of Patras | Garoufalis C.S.,University of Patras | Garoufalis C.S.,Technological Education Institute of Ionian Islands | Baskoutas S.,University of Patras
Journal of Physics D: Applied Physics | Year: 2012

We have performed a systematic study on the ground-state binding energy of an on-center donor impurity confined in a GaAs/Al 0.3Ga 0.7As cylindrical quantum dot (QD), subjected to simultaneously applied electric and magnetic fields. The two fields are tilted with respect to the QD growth direction and they are either parallel or perpendicular to each other. All the calculations are based on the potential morphing method which is employed within the framework of the effective-mass approximation. Our results show that when the tilted electric and magnetic fields are parallel, the magnetic shift of the donor binding energy is a monotonic function of the magnetic field strength. On the other hand, when the two fields are perpendicular to each other, the magnetic shift of the donor binding energy varies nonmonotonically with respect to the magnetic field strength, exhibiting a minimum value at a critical magnetic field strength. The position of this minimum value and its dependence on the QD size, its aspect ratio and the orientation of the tilted magnetic field is systematically investigated. Moreover, we discuss in detail the competition effects which appear in the presence of the two fields, showing that the critical line which corresponds to zero shift of the donor binding energy can be manipulated by suitably adjusting the QD size, the aspect ratio and the relative orientation of the two fields. © 2012 IOP Publishing Ltd.


Schindler S.,University of Vienna | Schindler S.,University of Porto | Von Wehrden H.,Lüneburg University | Poirazidis K.,WWF Greece | And 3 more authors.
Ecological Indicators | Year: 2013

Landscape metrics are widely used to investigate the spatial structure of landscapes. Numerous metrics are currently available, yet only little empirical research has comparatively examined their indicator value for species richness for several taxa at several scales. Taking a Mediterranean forest landscape - Dadia National Park (Greece) - as a case study area, we explored the performance of 52 landscape level landscape metrics as indicators of species richness for six taxa (woody plants, orchids, orthopterans, amphibians, reptiles, and small terrestrial birds) and for overall species richness. We computed the landscape metrics for circular areas of five different extents around each of 30 sampling plots. We applied linear mixed models to evaluate significant relations between metrics and species richness and to assess the effects of the extent of the considered landscape on the performance of the metrics. Our results showed that landscape metrics were good indicators for overall species richness, woody plants, orthopterans and reptiles. Metrics quantifying patch shape, proximity, texture and landscape diversity resulted often in well-fitted models, while those describing patch area, similarity and edge contrast rarely contributed to significant models. Spatial scale affected the performance of the metrics, since woody plants, orthopterans and small terrestrial birds were usually better predicted at smaller extents of surrounding landscape, and reptiles frequently at larger ones. The revealed pattern of relations and performances will be useful to understand landscape structure as a driver and indicator of biodiversity, and to improve forest and landscape management decisions in Mediterranean and other forest mosaics. © 2012 Elsevier Ltd. All rights reserved.


Toki C.J.,Technological Education Institute of Ionian Islands
Journal of Fluids Engineering, Transactions of the ASME | Year: 2014

The problem of boundary layer flow of an incompressible fluid over a moving porous flat plate is investigated, by taking into account the heat due to viscous dissipation. The governing boundary layer equations of this flow field were solved analytically using the Laplace transform technique. These new exact analytical solutions for velocity and temperature were obtained with arbitrary Prandtl number and dissipation parameter (or Eckert number Ec). The corresponding solutions for nonporous plate are discussed. Applying numerical values into the analytical expressions of the temperature and heat transfer coefficient, we also discussed the effects of the dissipation parameter in the cases of water, gas, and ammonia flow. We can finally deduce that the fluid temperature of the present problem will increase in the case of viscous dissipation with positive Ec, but this temperature will decrease with negative Ec. Copyright © 2014 by ASME.


Vitoratos A.G.,Technological Education Institute of Ionian Islands
Journal of Phytopathology | Year: 2014

Ustilago maydis strains, with low to moderate resistance to fluazinam (Rf ranging from 11.8 to 80), were isolated in a mutation frequency of 0.75 × 10-7 after chemical mutagenesis with N-methyl-N-nitro-N-nitrosoguanidine (MNNG). Genetic analysis resulted in the identification of two chromosomal genes. A study of the effect of mutant genes in the phytopathogenic fitness of U. maydis revealed that the resistance mutations had no apparent effect on mycelia growth rate and pathogenicity on young corn plants. Cross-resistant studies showed that the mutations for resistance to fluazinam were also responsible for resistance to oligomycin, but not to dinitrophenol. A dose-dependent inhibition of glucose oxidation in whole cells was observed by both fluazinam and oligomycin, and a complete inhibition was found at 40 μg/ml. The results obtained provide strong evidence that the mode of action of fluazinam consists of the inhibition the fungal cell's energy production process through direct inhibition of the ATP synthetase. © 2014 Blackwell Verlag GmbH.

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