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Statistical Physics, Neural Networks, Brain Studies, Gerard Toulouse
Statistical Physics, Neural Networks, Brain Studies, Gerard Toulouse
Turkish Journal of Physics
An overview of some aspects of a vast domain, located at the crossroads of physics, biology and computer science is presented: 1) During the last fifteen years, physicists advancing along various pathways have come into contact with biology (computational neurosciences) and engineering (formal neural nets). 2) This move may actually be viewed as one component in a larger picture. A prominent trend of recent years, observable over many countries, has been the establishment of interdisciplinary centers devoted to the study of: cognitive sciences; natural and artificial intelligence; brain, mind and behaviour; perception and action; learning and memory; robotics; man-machine communication, …
Neural Classifiers For Learning Higher-Order Correlations, Marifi Güler
Neural Classifiers For Learning Higher-Order Correlations, Marifi Güler
Turkish Journal of Physics
Studies by various authors suggest that higher-order networks can be more powerful and are biologically more plausible with respect to the more traditional multilayer networks. These architectures make explicit use of nonlinear interactions between input variables in the form of higher-order units or product units. If it is known a priori that the problem to be implemented possesses a given set of invariances like in the translation, rotation, and scale invariant pattern recognition problems, those invariances can be encoded, thus eliminating all higher-order terms which are incompatible with the invariances. In general, however, it is a serious set-back that the …
Ordering Due To Disorder In Frustrated Quantum Magnetic Systems, Taner Yildirim
Ordering Due To Disorder In Frustrated Quantum Magnetic Systems, Taner Yildirim
Turkish Journal of Physics
The phenomenon of order by disorder in frustrated magnetic systems is reviewed. Disorder (thermal or quantum fluctuations) may sometimes give rise to long range ordering in systems with frustration, where one must often consider the selection among classically degenerate ground states which are not equivalent by any symmetry. The lowest order effects of quantum fluctuations in such frustrated systems usually resolves the continues degeneracy of the ground state manifold into discrete Ising--type degeneracy. A unique ground state selection out of this Ising degenerate manifold then occurs due to higher order effects of quantum fluctuations. For systems such as face-centered cubic …
The Renormalization-Group Microscope: The Local Statistical Mechanics Of Heterogeneous Systems, Dicle Yeşi̇lleten, A. Nihat Berker
The Renormalization-Group Microscope: The Local Statistical Mechanics Of Heterogeneous Systems, Dicle Yeşi̇lleten, A. Nihat Berker
Turkish Journal of Physics
Renormalization-group theory is developed to yield all local microscopic thermodynamic densities in heterogeneous systems. Local energy densities and local magnetizations are thus obtained for random-bond systems, random-field systems, and spin-glasses, in two and three dimensions. Different order-disorder mechanisms in these diverse systems, such as chaotic ordering and domain-wall melting, become quantitatively evident.
Diverging Strains Near Threshold: Breakdown Of The Elastic Description Of A Charge Density Wave Model, Muhittin Mungan, Susan N. Coppersmith, Valeri M. Vinokur
Diverging Strains Near Threshold: Breakdown Of The Elastic Description Of A Charge Density Wave Model, Muhittin Mungan, Susan N. Coppersmith, Valeri M. Vinokur
Turkish Journal of Physics
We analyze the strains near threshold in 1-d charge density wave models at zero temperature and strong pinning. We show that in these models local strains diverge near the depinning threshold and characterize the scaling behavior of the phenomenon. This helps quantify when the underlying elastic description breaks down and plastic effects have to be included.
Angular Momentum Evolution In Close Late-Type Binaries, Carla Maceroni
Angular Momentum Evolution In Close Late-Type Binaries, Carla Maceroni
Turkish Journal of Physics
Accurate monitoring of the orbital period of late-type close binaries can provide important information on their secular evolution and on the structural changes of their components on very short timescales (i.e. decades). The orbital period turns out to be a powerful tool of investigation: it can be measured to great accuracy even with simple equipments and its change produces an easily detectable cumulative effect over time. This paper focuses on two physical mechanisms that cause orbital periods changes in late type binaries with negligible mass loss from the system: the dynamical evolution due to magnetic braking in tidally locked systems, …
Photon Counting In Astrophotometry. Fundamentals And Some Advices For Beginners, Romualdas Kalytis
Photon Counting In Astrophotometry. Fundamentals And Some Advices For Beginners, Romualdas Kalytis
Turkish Journal of Physics
The beginners in astrophotometry are reminded of fundamentals of photon counting. Several advices for new users of electrophotometric technics are given.
The Light And Colour Variations Of Mm Her, Günay Taş
The Light And Colour Variations Of Mm Her, Günay Taş
Turkish Journal of Physics
RS CVn type eclipsing binary MM Herculis have been observed using B, V, R filters in 1997 and obtained light and colour curves were examined together with the earlier 12 light and colour curves obtained between 1976 and 1997. It was seen that the system is bluer, while it is fainter. Such finding probably reveals the evolutionary effect of the photospheric bright facular structures surrounding starspots.
Dynamical Evolution Of The Rs Cvn-Type Binaries, Osman Demi̇rcan
Dynamical Evolution Of The Rs Cvn-Type Binaries, Osman Demi̇rcan
Turkish Journal of Physics
The orbital angular momentum (orbital AM) of a sample of forty RS CVn-type binaries with the orbital periods P \leq 10 days were estimated and the orbital AM distribution with respect to P was critically interpreted as the orbital AM evolution of these systems caused by the magnetic breaking process with the existence of spin-orbit coupling. The empirical relations (between the orbital AM loss, mass loss and period variation) derived from the diagram of the orbital AM distribution were used in deriving a semi-empirical formula for the dynamical evolution of the RS CVn-type binaries. The magnetic breaking induced dynamical evolution …
Kinematics Of Rs Cvn And W Uma Binaries In The Hipparcos Catalogue, Zeki̇ Aslan, Tuncay Özdemi̇r, Cahi̇t Yeşi̇lyaprak, Ahmet İskender
Kinematics Of Rs Cvn And W Uma Binaries In The Hipparcos Catalogue, Zeki̇ Aslan, Tuncay Özdemi̇r, Cahi̇t Yeşi̇lyaprak, Ahmet İskender
Turkish Journal of Physics
Space motions of RS CVn and W UMa binaries have been calculated using parallaxes and proper motions from Hipparcos catalogue and radial velocities from the literature. We have calculating the mean motions and velocity dispersions. We have excluded those binaries with relative parallax errors \geq 0.5. The comparison of the asymmetric drift, velocity ellipsoids and vertex deviation with those of main sequence stars given by Dehnen and Binney indicates that both RS CVn and W UMa binaries have kinematics similar to main sequence stars slightly bluer than the Parenago's discontinuity and B-V \simeq 0.62. Implication of this has been discussed.
Co-Ordinated Multi-Wavelength Observations Of The Rs Cvn System Cf Tuc, Edwin Budding, K. L. Jones, O. B. Slee, L. Watson
Co-Ordinated Multi-Wavelength Observations Of The Rs Cvn System Cf Tuc, Edwin Budding, K. L. Jones, O. B. Slee, L. Watson
Turkish Journal of Physics
A `multi-site, multi-wavelength' campaign on the eclipsing RS CVn binary CF Tuc was carried out in 1996 over both optical (photometry and spectroscopy) and radio (microwave) ranges. The microwave data was taken with the Australia Telescope Compact Array (ATCA). It covered slightly more than one complete orbital cycle at 4.8 and 8.64 GHz in one continuous run. There was also coverage of about 25% of the light curve at 1.38 and 2.38 GHz. High dispersion spectroscopy was obtained using the McLellan 1m telescope and échelle spectrograph at Mt John University Observatory (New Zealand). Supporting photometry came from various smaller-scale facilities …
Magnetodielectrical Polymer Compositional Materials, Sh. V. Mamedov, Y. Lenger, Sevi̇l Bolcal, Vilayet A. Alekperov, M. A. Ramazanov, R. Z. Sadikhov
Magnetodielectrical Polymer Compositional Materials, Sh. V. Mamedov, Y. Lenger, Sevi̇l Bolcal, Vilayet A. Alekperov, M. A. Ramazanov, R. Z. Sadikhov
Turkish Journal of Physics
Magnetodielectrical polymer compositional materials containing carbonyl iron (CI) were obtained. The changes of permittivity and permeability, loss angle tangent, resistivity, mechanical and electrical properties, depending on the content of carbonyl iron have been investigated. It is shown that the magnetodielectrical materials with 20-40% mass content of carbonyl iron have possible practical use.
Heat Pulse Studies Of The Emission And Absorption Of Acoustic Phonons In Gaas Quantum Wells And Wires, A. J. Kent
Heat Pulse Studies Of The Emission And Absorption Of Acoustic Phonons In Gaas Quantum Wells And Wires, A. J. Kent
Turkish Journal of Physics
This paper reviews some recent experiments in which heat pulse techniques have been used to study the interaction of acoustic phonons with electrons in semiconductor quantum wells and wires. Heat pulse experiments provide temporal and spatial resolution of the phonons that are emitted or absorbed by the electrons and so give more detailed information about the electron-phonon interaction than can be obtained by other methods (e.g. transport measurements). Phonon experiments demonstrate clearly the effect of the electron confinement on the carrier-phonon interaction and the overall energy loss rate due to acoustic phonon emission. A qualitative explanation of the results is …
Electron Transport In Gaas Quantum Wells: Effect Of Interface Roughness Scatterin, Rita Gupta
Electron Transport In Gaas Quantum Wells: Effect Of Interface Roughness Scatterin, Rita Gupta
Turkish Journal of Physics
The importance of interface roughness (IFR) scattering of electrons and LO-phonons for electron transport in semiconductor quantum wells is discussed. Modulation doping of quantum wells minimizes the effect of impurity scattering on the low-field electron mobility so that IFR scattering of electrons in the well becomes the major limiting factor. A model calculation of IFR scattering of electrons in quantum wells is presented and it is shown that (both)\Lambda and \Delta, the parameters defining IFR, can be estimated by comparing the theoretical and experimental electron mobilities. The application of high electric field leads to a distribution of hot electrons which …
Exchange-Correlation Effects In The Impurity-Limited Mobility Of Gaas Quantum Wires, B. Tanatar
Exchange-Correlation Effects In The Impurity-Limited Mobility Of Gaas Quantum Wires, B. Tanatar
Turkish Journal of Physics
We study the many-body effects described by the local-field corrections on the mobility of quasi-one dimensional electron systems. The low temperature mobility due to remote-impurity doping and interface-roughness scattering is calculated within the relaxation time approximation. We find that correlation effects significantly reduce the mobility at low density.
Well-Width Dependence Of Warm Electron Relaxation And Interface Roughness Scattering In Gaas/Ga_{1-X}Al_Xas Multiple Quantum Wells, M. Cankurtaran, H. Çeli̇k, E. Tiraş, A. Bayrakli, N. Balkan
Well-Width Dependence Of Warm Electron Relaxation And Interface Roughness Scattering In Gaas/Ga_{1-X}Al_Xas Multiple Quantum Wells, M. Cankurtaran, H. Çeli̇k, E. Tiraş, A. Bayrakli, N. Balkan
Turkish Journal of Physics
We review our recent results concerning the well-width dependence of the acoustic-phonon-assisted energy relaxation of two-dimensional (2D) warm electrons in modulation-doped GaAs/Ga_{1-x}Al_xAs multiple quantum wells. Electron energy-loss rates via the emission of acoustic phonons are determined from the amplitude of Shubnikov-de Haas (SdH) oscillations, measured as a function of lattice temperature and applied electric field. Experimental results are compared with the existing theoretical models that involve deformation-potential and screened and unscreened piezoelectric scattering. Well-width dependence of the quantum and transport mobilities of 2D electrons in the same samples have also been determined by measuring the quantum oscillations in both the …
Large-Bandgap Semiconductors, B. K. Ridley
Large-Bandgap Semiconductors, B. K. Ridley
Turkish Journal of Physics
The general features of large-bandgap semiconductors that make them interesting for electronic and optoelectronic applications are briefly reviewed. The nitrides AIN, GaN and InN are singled out for a more focused treatment.
Energy Relaxation Rate Of Hot Electrons In N-Type Gan Epilayers Using Heat Pulse Techniques, P. Hawker, A. J. Kent, T. S. Cheng, C. T. Foxon
Energy Relaxation Rate Of Hot Electrons In N-Type Gan Epilayers Using Heat Pulse Techniques, P. Hawker, A. J. Kent, T. S. Cheng, C. T. Foxon
Turkish Journal of Physics
We have measured the energy relaxation rate of hot electrons in MBE grown bulk GaN epilayers on GaAs and sapphire substrates over the electron temperature range 1 - 130 K. The measurements were made using heat pulse techniques. For layers grown on GaAs substrates the results show that the carriers reside in the substrate, probably as in a GaAs/AlGaAs heterojunction. For layers grown on sapphire substrates we obtain a P \propto T^4_e dependence for the relaxation rate in the low temperature limit, consistent with piezoelectric coupling in the so-called `dirty' regime, changing to a linear dependence in the high temperature, …
Feasibility Of The Gainp/Ingaas/Gaas System For Modulation Doped Field-Effect Transistors, C. Besi̇kci̇, Y. Ci̇van
Feasibility Of The Gainp/Ingaas/Gaas System For Modulation Doped Field-Effect Transistors, C. Besi̇kci̇, Y. Ci̇van
Turkish Journal of Physics
Si-doped single Ga_{0.51}In_{0.49}P layers and GaInP/InGaAs/GaAs modulation doped field-effect transistor structures grown by gas source molecular beam epitaxy were characterized in detail through Hall-Effect and Deep Level Transient Spectroscopy techniques. Electrical characterization of the undoped and moderately Si-doped (N_D = 3 x 10^{17} cm^{-3}) GaInP layers yielded an electron trap with an activation energy of 0.75 eV and a temperature dependent capture cross section with a capture barrier of 0.593 eV showing DX center properties. The trap was not detected in highly Si doped (N_D = 4 x 10^{18} cm^{-3}) as grown layers suggesting that it is a defect complex …
Giant Efficiency Of Fröhlich Interaction In Self-Assembled Quantum Dots, A. W. E. Minnaert, A. Yu. Silov, J. E. M. Haverkort, J. H. Wolter, A. Garcia, V. N. Gladilin, V. M. Fomn, J. T. Devreese
Giant Efficiency Of Fröhlich Interaction In Self-Assembled Quantum Dots, A. W. E. Minnaert, A. Yu. Silov, J. E. M. Haverkort, J. H. Wolter, A. Garcia, V. N. Gladilin, V. M. Fomn, J. T. Devreese
Turkish Journal of Physics
A transition is being observed from an inhomogeneously broadened photoluminescence band under non-resonant excitation of InAs/GaAs Self-Assembled quantum Dots (SAD's) into up to five phonon-assisted emission bands under selective excitation. A similar effect is obtained from photoluminescence excitation experiments (PLE). We interpret the phonon-assisted PL as being due to a giant efficiency of the Fröhlich interaction between an exciton polarized by strain in the SAD and LO-phonons. The model is consistent with the pronounced p-type polarization of the emission observed in our cleaved-side PL-measurements. Further support is obtained from our calculations in which a different localization of the electrons and …
Modelling Of Devices For Optoelectronic Applications: The Quantum Confined Stark Effect And Self-Electrooptic Effect Devices, Eckehard Scholl
Modelling Of Devices For Optoelectronic Applications: The Quantum Confined Stark Effect And Self-Electrooptic Effect Devices, Eckehard Scholl
Turkish Journal of Physics
Electro-optical effects, such as the Franz-Keldysh effect in bulk materials or the quantum confined Stark effect in quantum well structures, lead to strong optoelectronic nonlinearities which form the basis for optical modulators and optically bistable devices. They result from a modification of the optical absorption properties by an applied electric field and are particularly pronounced in the case of low dimensional semiconductors. We review theoretical modelling and computer simulations of such optoelectronic devices in particular for ZnSe based quantum well structures, where excitonic features dominate even at room temperature. The field dependent absorption spectra are calculated by a many-body theory …
Experimental Separation Of Phonon And Extrinsic Scattering In 2d Carrier Gases In Gaas, F. F. Ouali, H. R. Francis, H. C. Rhodes
Experimental Separation Of Phonon And Extrinsic Scattering In 2d Carrier Gases In Gaas, F. F. Ouali, H. R. Francis, H. C. Rhodes
Turkish Journal of Physics
A new method which separates phonon scattering from extrinsic scattering in 2D gases in GaAs is presented. In contrast to previous ones, the technique makes no assumptions about the temperature dependence of the extrinsic scattering. The preliminary measurements of the phonon limited mobility \mu_{ph} on two electron gas samples show a Bloch-Grüneisen regime in the temperature range 1.2-4.5K and the results agree reasonably well with other experimental and theoretical work.
Optimisation Of The Tunable Wavelength Hot Electron Light Emitter, Ali̇ Teke, Naci̇ Balkan
Optimisation Of The Tunable Wavelength Hot Electron Light Emitter, Ali̇ Teke, Naci̇ Balkan
Turkish Journal of Physics
We report on the optimization of the hot electron tunable wavelength surface light emitting device developed by us. The device consists of a p-GaAs, and n-Ga_{1-x}Al_xAs heterojunction containing an inversion layer on the p- side, and GaAs quantum wells on the n- side, and is referred to as HELLISH-2 (Hot Electron Light Emitting and Lasing in Semiconductor Heterojunction). The device utilises hot electron longitudinal transport and, therefore, light emission is independent of the polarity of the applied voltage. In order to optimise the operation of the device a theoretical model that calculates the tunnelling and the thermionic components of the …
Wigner Crystalization In Semiconductor Quantum Wires, I. Al-Hayek, B. Tanatar, M. Tomak
Wigner Crystalization In Semiconductor Quantum Wires, I. Al-Hayek, B. Tanatar, M. Tomak
Turkish Journal of Physics
We study the Wigner crystallization in semiconductor quantum wires within the density functional approach. As the density of electrons in quasi-one-dimensional structures is lowered, we find that the system favors the crystalline phase as envisioned by Wigner.
Binding Energies Of Excitons In Symmetric And Asymmetric Coupled Double-Quantum Well Structures In A Magnetic Field, Esi̇n Kasapoğlu, Hüseyi̇n Sari, Yüksel Ergün, Sezai̇ Elagöz, Naci̇ Balkan, İsmai̇l Sökmen
Binding Energies Of Excitons In Symmetric And Asymmetric Coupled Double-Quantum Well Structures In A Magnetic Field, Esi̇n Kasapoğlu, Hüseyi̇n Sari, Yüksel Ergün, Sezai̇ Elagöz, Naci̇ Balkan, İsmai̇l Sökmen
Turkish Journal of Physics
The binding energy of excitons in the symmetric and asymmetric coupled double GaAs/Ga_{1-x}Al_xAs quantum wells is calculated by using variational approach. Results have been obtained as a function of the potential symmetry, the size of the quantum well, and the coupling parameter of the wells in the presence of a magnetic field applied parallel to the growth direction. The role of the asymmetric barriers, magnetic field, barrier and well width in determining the tunability of the excitonic binding parameters of the GaAs/Ga_{1-x}Al_xAs system is discussed.
Stability Of Quasi-Two-Dimensional Bipolarons, R. Tuğrul Senger, Ati̇lla Erçelebi̇
Stability Of Quasi-Two-Dimensional Bipolarons, R. Tuğrul Senger, Ati̇lla Erçelebi̇
Turkish Journal of Physics
The stability criteria of quasi-two-dimensional dimensional bipolarons have been studied within the framework of strong coupling and path-integral theories. It is shown that the critical values of the electron-phonon coupling constant (\alpha), and the ratio of dielectric constants (\eta = \epsilon_{\infty}/\epsilon_{0}) exhibit some non-trivial features as the effective dimensionality is tuned from three to two.
Band-Gap Renormalization In Quantum Wires, K. Güven, C. R. Bennett, B. Tanatar
Band-Gap Renormalization In Quantum Wires, K. Güven, C. R. Bennett, B. Tanatar
Turkish Journal of Physics
Improved techniques in semiconductor fabrication increased the interest in quantum wire structures, because of their opto-electronic device application possibilities. Many-body interactions among the electrons and holes in the wire lead to the band-gap renormalization (BGR), which in turn affect the optical properties of the system. We study the BGR within the random-phase approximation incorporating the dynamical effects, and investigate the density dependence.
Light Emission From Travelling Space Charge Domains, M. Hostut, N. Balkan
Light Emission From Travelling Space Charge Domains, M. Hostut, N. Balkan
Turkish Journal of Physics
We report light emission from n-doped GaAs epilayers associated with impact ionization when the device is biased in the negative difference resistance (NDR) regime (F > 3.5 kV cm^{-1}). Spectral distribution of the emitted light (electroluminescence) has been measured to identify the energy distribution of the recombining electron-hole pairs. Electron temperatures, calculated from the high energy tail of the electroluminescence spectra, show that the light emission is due to the recombination of the impact ionized holes with the background, channel, and the travelling space charge electrons. The range of the electron temperatures obtained indicate that the contribution to the light emission …
The Orbit Center Dependence Of The Energy Levels In A Multiple Quantum Wells Under External Tilted Magnetic And Electric Fields, R. Amca, Y. Ergün, H. Sari, S. Elagöz, İ. Sökmen
The Orbit Center Dependence Of The Energy Levels In A Multiple Quantum Wells Under External Tilted Magnetic And Electric Fields, R. Amca, Y. Ergün, H. Sari, S. Elagöz, İ. Sökmen
Turkish Journal of Physics
An analytical solution to the Schrödinger equation for a multiple quantum well system subjected to an externally applied electric field the growth direction and an externally applied tilted magnetic field are obtained and the results are discussed. The energy dependence energy spectrum of the system as a function of the external electric field and the orbit center is also discussed.
Bands Of Localized Electromagnetic Waves In Random Collections Of Dielectric Particles, Marian Rusek, Arkadiusz Orlowski
Bands Of Localized Electromagnetic Waves In Random Collections Of Dielectric Particles, Marian Rusek, Arkadiusz Orlowski
Turkish Journal of Physics
Anderson localization of electromagnetic waves in random arrays of dielectric cylinders is studied. An effective theoretical approach based on the finite size scaling analysis of transmission is developed. The disordered dielectric medium is modeled by a system of randomly distributed 2D electric dipoles. The appearance of the band of localized waves emerging in the limit of an infinite medium is discovered. It suggests deeper insight into existing experimental and theoretical results.