Excitonic insulator: dependence of energy gap width on carrier density

On the basis of the Ashcroft empty core model potential, the equation for the constant of Coulomb interaction in the theory of excitonic insulator is modified. It is shown that in this case the dependence of the energy gap width on the charge carrier density obeys the Mott criterion in the limit of low densities. The conformity of the theory with some experimental data concerning metal–insulator phase transitions in doped semiconductors and transition metal compounds is discussed.     

Optical band gap width in GaAs in megagauss magnetic fields

The band gap width in GaAs in magnetic fields of up to 10 MG is calculated using a five-band kp model. The selection rules for interband electron transitions in strong magnetic fields are found, and the dependences of the interband transition probabilities on a magnetic field are calculated. The electronic spectra calculated in the five-band model are compared with those calculated in the Kane model and in the tight-coupling approximation. The calculations are shown to agree with experimental data if the contribution from the density-of-states tails and excitonic effects to light absorption is taken into account.     

Determination of temperature dependence of energy gap in HgTe by oscillatory magnetotransmission measurements

Interband Γ₆ → Γ₈ magnetotransmission measurements have been performed of HgTe for ten values of temperature, between 8 K and 92 K, in the magnetic field up to 60 kG. The results are interpreted using the many-level model of Pidgeon and Brown. The energy gap dependence versus temperature has been found. The others kp parameters show no systematic temperature variation within the experimental uncertainty.     

Study op electrical conductivity op some chalkogenide glasses under hydrostatic pressure up to 1000 MPa

The study of electrical properties of amorphous materials under high pressure can contribute to understanding of processes in these materials and helps to build the theory of amorphous materials. Properties of these materials under high pressure are studied for instance in papers /1–6/. The change of electrical conductivity is usually ascribed to the change of activation energy due to hydrostatic pressure.     

Near-thermal energy width of moderated high energy positrons

More than 10^?7 of the positrons emitted by ¹¹C (produced by proton bombardment of boron) emerge from the room-temperature boron target with a low energy (less than 1 eV) and a measured energy width of (0.100+0.010) eV.     

The thermal photo-electric phenomenon in semi-conductors

On illuminating homogeneous samples cut from single crystals of germanium an emf was observed when a thermal current flowed through them. This phenomenon was called a thermal photo-electric phenomenon and is caused by the fact that illumination changes the thermal emf of the semi-conductor. Using equations from the thermodynamics of irreversible processes a theory of this phenomenon was elaborated. Its conclusions are in agreement with preliminary measurements. It is shown that the measurement of a thermal photo emf renders possible the determination of Q ₁ ^* +Q ₂ ^* where Q ₁ ^* , Q ₂ ^* are the mean kinetic energies transferred by the electron and hole respectively. The type of scattering of the current carriers can be judged from the magnitude of Q ₁ ^* +Q ₂ ^* .     

Mass gap without vacuum energy

We consider soft nonlocal deformations of massless theories that introduce a mass gap. By use of a renormalization scheme that preserves the ultraviolet softness of the deformation, renormalized quantities of low mass dimension, such as normal mass terms, vanish via finite counterterms. The same applies to the renormalized cosmological constant. We connect this discussion to gauge theories, since they are also subject to a soft nonlocal deformation due to the effects of Gribov copies. These effects are softer than usually portrayed.     

Superconducting energy gap anomalies in NbN films

Nine NbN films ranging in thickness from 0.02 to 0.03 μm were studied by measuring the temperature dependence from 1.5 K to over 20 K of the ultrasonic attenuation of 700 MHz surface acoustic waves. The resulting curves did not fit the predictions of BCS. When the curves were numerically inverted to show the temperature dependence that the superconducting energy gap would need to have to produce the observed attenuation, they were found to be nearly constant at a value less than half of the BCS value. An analysis prompted by the Anderson theories of 2-D localizations shows the attenuation per film thickness to be proportional to the logarithm of the thickness.     

Electronic phenomena in semi-conductors with a temperature gradient

Phenomena connected with the transfer of current carrier concentrations as a result of a temperature gradient in semi-conductors with two kinds of current carriers are theoretically studied. The paper first deals with the general equations which are valid at a certain point of the semi-conductor with a temperature gradient for the concentrations of current carriers and density of electric and thermal current. From this the distribution of the concentrations of current carriers is found for a typical case, the phenomenon of thermal emission of minority current carriers is discussed, relations are derived for the dependence of the thermal emf on the magnitude of the temperature gradient, the possibility is discussed of thermal rectification and equations are given for the thermal conductivity of a semi-conductor with non-equilibrium concentrations of current carriers.     

Variable energy gap of SiCN nanopowders

SiCN and SiC nanopowders were prepared by infrared laser pyrolysis of gaseous precursors starting from a SiH₄C₂H₂NH₃ gas mixture. The SiCN powders were composed of an amorphous phase ascribed to the ternary compound with a β-SiC crystalline phase, while the SiC powders consisted of β phase only. The chemical bonding and the local atomic order in the SiCN powders are much more complicated than those of pure SiCSi₃N₄ mixtures and are strongly dependent on the variation in the initial gas composition. The average grain size was estimated from X-raydiffraction (XRD) patterns and atomic force microscopy (AFM). The UV-VIS transmittance data were used for the bandgap determination in these new materials. Absorption studies suggested that the direct energy gap is more favourable in SiCN and SiC nanoparticles and is blueshifted with regard to the crystalline bulk (SiC). The change of type of band-to-band transition in SiC nanopowders, due to the size effects, and increasing of the energy gap value in SiCN powders with different local atomic arrangement, makes them attractive for blue-UV optoelectronic applications. PACS 61.46.+w; 78.70.Ck; 68.3.Ps; 78.40.-q     

Magnon energy gap in a four-layer ferromagnetic superlattice

The magnon energy band in a four-layer ferromagnetic superlattice is studied by using the linear spin-wave approach and Green＇s function technique. It is found that three modulated energy gaps exist in the magnon energy band along Kx direction perpendicular to the superlattice plane. The spin quantum numbers and the interlayer exchange couplings all affect the three energy gaps. The magnon energy gaps of the four-layer ferromagnetic superlattice are different from those of the three-layer one. For the four-layer ferromagnetic superlattice, the disappearance of the magnon energy gaps △ω12, △ω23 and △ω34 all correlates with the symmetry of this system. The zero energy gap △ω23 correlates with the symmetry of interlayer exchange couplings, while the vanishing of the magnon energy gaps △ω12 and △ω34 corresponds to a translational symmetry of x-direction in the lattice. When the parameters of the system deviate from these symmetries, the three energy gaps will increase.     

Plasmon-assisted sound amplification in semi-conductors under strong magnetic fields

We derive an expression for the contribution from plasma effects to the sound amplification in piezo-electric semiconductors subjected to a strong d.c. magnetic field applied parallel to the drift field. It is shown that this contribution largely differs from that in the absence of a magnetic field both in the strength of the effect and in angular distribution of the emitted phonons.     

The theory of a bulk photo-voltaic phenomenon in semi-conductors

It is shown theoretically that for a photo-voltaic phenomenon in semi-conductors to exist neither the presence of localised potential barriers with rectifying properties at the contact of a semi-conductor with a metal (Mott's theory 1939) or at a p-n junction (Sosnowski, 1947) nor the presence of non-rectifying contacts (Dember's phenomenon, Lashkarev's theory 1948) is necessary. The case is considered that the illuminated part of the semi-conductor is sufficiently distant from both contacts so that these will not assert themselves; further, it is assumed that there are no localised potential barriers in the semi-conductor. Under these conditions a photo-voltage is produced in a semi-conductor if its conductivity varies throughout its length in such a way that in the dark it is different at the beginning and end of the illuminated part. For this type of non-barrier and non-contact photo-voltage we propose the name bulk photo-voltage. The underlying law of this phenomenon, the general procedure for calculating the photo-voltage, an approximation for weak illumination and the dependence on the intensity of illumination are derived and its physical aspects are discussed.     

Electronic structure and hyperfine interaction of muonium in semi-conductors

The Unrestricted Hartree-Fock self-consistent field cluster procedure is being utilized for first-principle investigations of the electronic structures and hyperfine interactions in normal and anomalous muonium states in semi-conductors. Our results for the total energy for the normal muonium state for a twenty-seven atom cluster in diamond, including the muonium and its neighboring atoms, show a minimum at the tetrahedral site and a maximum at the hexagonal site indicating that normal muonium is located in the tetrahedral region and avoids the hexagonal region. Using the calculated spin-density as a function of the position of muonium and carrying out averaging over the vibrational motion of the muon governed by the total energy curve obtained from our work, we have derived a muon hyperfine constant which is about 75% of that in free muonium, in good agreement with experiment. The natures of the total energy and spindensity curves permit us to draw conclusions regarding the origin of the observed trend in the hyperfine constants for normal muonium in diamond, silicon and germanium. The UHF cluster procedure is also applied to study a model of a muon in a positively charged environment for the anomalous muonium center in diamond. This model leads to a hyperfine interaction tensor with the observed feature of strong anisotropy but significantly weaker than experiment. The results obtained for this model indicate the importance for the anomalous muonium state with its relatively weak hyperfine interaction, of exchange polarization effects inherent in the UHF procedure.     

Slow light with symmetric gap plasmon guides with finite width metal claddings

We study the dispersion relation and the modes of a symmetric gap plasmon guide, where a dielectric planar slab is coated with finite metallic layers on both top and bottom. The finite conductivity of the metal is taken into account. The modes of the structure exhibit significant differences from those of dielectric waveguides with air or metal as the bounding media. Avoided level crossing phenomenon between the plasmon and the guided modes is shown to exist, leading to leaky modes. The structure sandwiched between two high index media is shown to lead to slow light in transmission. The group delay is shown to be larger for higher order modes.      

The control of gap width in the low-dimensional systems with charge density waves instability

The strong photoeffect was discovered experimentally in InSe crystals at temperature 4.2 K as a result of laser radiation action at fixed wavelengths of 337 and 195 micron. Effect was explained with appearance of the gap in the continuum of conduction band. The photoeffect value was found to depend strongly on electric current through the crystal.