Universal critical exponents in percolation systems with tunneling

Some specific dependence of micro-crystalline solar cells efficiency on the crystallite size was reported recently [R. Beserman, A. Chak, R. Weil, T. Roschek, in: Proceedings of the 19th European Photovoltaic Solar Energy Conference, Paris, 2004, p. 1520]. The simple explanation of this dependence is suggested in the framework of percolation phenomena for non-equilibrium carriers. Percolation problem for non-equilibrium carriers was solved in continuous space with tunnel coupling between conducting grains. It was shown that local micro-geometry is not essential if the inter-grains transition time is less than the lifetime of the non-equilibrium carriers inside the conducting grains. In this case the main parameter of the problem is the lifetime of the non-equilibrium carriers in the conducting grains. A new bonding criterion was formulated. An amorphous hydrogenated silicon (a-Si:H) matrix with a big number of crystalline silicon (c-Si) inclusions was investigated as an example of the system. It was shown that universal dependence exists for non-equilibrium conductivity on the mean crystalline size. This prediction was verified by experiments with real solar cells produced from a-Si:H with c-Si inclusions.     

Microstructure and percolation threshold characteristics of reactive sputtered Au-TiO2 granular composite films

The Au/TiO₂ composite films were prepared by using reactive co-sputtering technique. The size and shape of the embedded Au particles and the absorption spectra of the composite films were investigated by using SEM, XRD, and UV-VIS-NIR spectrophotometer, respectively. The average size of Au particles and the electrical conductivity decrease as the sputtering pressure increases. The normalized conductivity of the films deposited at five different pressures with the Au concentration in the range of 0.15-0.91 were measured. The percolation threshold increases from 0.21 to 0.90 as the sputtering pressure increases from 2 × 10⁻² Torr to 9.5 × 10⁻² Torr.     

Investigation of near constant loss contribution to conductivity in lithium bismo-silicate glasses

Glasses having compositions 20Li₂O · (80 − x)Bi₂O₃ · xSiO₂ (x = 55, 60, 65, 70 mol%) were investigated using impedance spectroscopy in the frequency range from 20 Hz to 1 MHz and in the temperature range from 543 to 663 K. The ac and dc conductivities, activation energy of the dc conductivity and relaxation frequency are extracted from the impedance spectra. The increase in conductivity with increase in SiO₂ content is attributed to the change in the structural units of bismuth. Both electric modulus and the conductivity formalism have been employed to study the relaxation dynamics of charge carriers in these glasses. A single ‘master curve’ for normalized plots of all the modulus isotherms observed for a given composition indicates the temperature independence of the dynamic processes for ions in these glasses. Similar values of activation energy for dc conduction and for conductivity relaxation time indicates that the ions overcome same energy barrier while conducting and relaxing. The observed conductivity spectra follows power law with exponent ‘s’ which increases regularly with frequency and approaches unity at higher frequencies. Near constant losses (NCL) characterize this linearly dependent region of conductivity spectra. A deviation from ‘super curve’ for various isotherms of conductivity spectra was also observed in high frequency region and at low temperatures, which supports the existence of different dynamic processes like NCL in addition to the ion hopping processes in the investigated glass system.     

Rigidity percolation in glassy structures

We review the new concept of rigidity percolation and show that if local flexible units are joined together to form a network, the composite consists of floppy and rigid regions. When the rigid regions percolate, the whole network becomes rigid and resists attempts at elastic deformation. These ideas are applied to network glasses. It is shown that in the floppy region there exist low-frequency modes that should show up in inelastic neutron scattering.     

Diffusion kinetics of some glass systems used for the production of optical fibres

The rate at which the mobile ions in the core and cladding glasses interdiffuse during the drawing of an optical fibre by the double-crucible method, considerably affects the refractive index profile in the final product. In four glass systems suitable for the production of a low-loss graded-index compound glass fibre this quantity has been measured using interference microscopy.     

Specific heat of vitreous systems: Temperature dependence near absolute zero

Unlike the low temperature heat capacities (L.T. C_V) of crystals, those of vitreous substances near T = 0 K require the formula C_V/3R = C₁T+C₂T²+C₃T³+C₅T⁵+…, where the constants C₁ and C₂ represent the anomalous excess characteristic of vitreous systems. The C₃ and C₅ represent, respectively, Debye and dispersion terms as for crystals. Recent literature confirms C₁ ≠ 0 for various glasses and offers diverse theoretical justifications for this. The C₂T² term has been proposed but not evaluated by Leadbetter. As shown here, it is required to represent the experimental C_V of vitreous silica at T < 2 K, is inferred from its Raman spectrum and is supported by theoretical calculations.     

Water-network percolation transition in hydrated blue-green algae in vivo

We found that dc conductivity percolation process typical for low hydrated porous materials shows up in bulk viable blue-green algae, Arthrospira (Spirulina) platensis (strain Laporte 1963/M-132/2b) at unusually low hydrations, more than an order of magnitude lower than in, e.g., hydrated yeast [D. Sokolowska, A. Krol-Otwinowska, J.K. Moscicki, Phys. Rev. E 70 (2004) 052901]. The critical exponent is characteristic for two-dimensional network. Comparison with results for yeast and other similar materials shows that the hydration percolation threshold is a sensitive indicator of wettability of water accessible surface in porous bio- and abiotic materials.     

Analysis of the threshold current in nitride-based lasers

Using optical gain measurements and calculated optical gain spectra we analyse the various contributions to the threshold current observed for laser diodes. Our model is based on band-to-band transitions and includes internal polarization fields as well as multiple quantum wells. Besides good agreement between experiment and theory, our model explains the characteristic dependence of the threshold current on emission wavelength and well number.     

Electrical conductivity and crystallization in amorphous germanium

A simple model is presented which interprets conductivity variations due to crystallization in amorphous layers and allows the determination of the growth rate of crystallization. The validity of this model is demonstrated in the case of germanium.     

AC conductivity in amorphous germanium

The ac conductivity in evaporated amorphous germanium films has been measured as a function of annealing and has been found to obey the ω^0.8 law, in accordance with the hopping model. The dc conductivity measurements on the same samples show a $\text{T}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ law behaviour. The densities of localized states near the Fermi level g(E_F), obtained from both experiments are in reasonable agreement with each other. Both the measurements show a reduction by about a factor of 2 in g(E_F) when a freshly prepared film is fully annealed. High-temperature substrate films also show the ω^0.8 behaviour. This suggests that the frequency dependence of the ac conductivity is not caused by voids alone. Other possible explanations of our results are also discussed.     

Search for instantaneous radiation near the instant of break momentum of various fissioning nuclear systems at low excitation energies

The main results of studying the properties of “instantaneous” neutrons and γ photons during the fission of ^{233, 235}U(n _th, f) and ²³⁹Pu(n _th, f) nuclei and spontaneous fission of ²⁵²Cf, which were performed on the WWR-M reactor at the St. Petersburg Nuclear Physics Institute, Russian Academy of Sciences, are presented. Along with obtaining the main characteristics of the instantaneous radiation from fission fragments, these studies were also aimed at gaining deeper insight into such exotic processes as the emission of break neutrons and γ photons from a fissioning nucleus near the break point. These investigations were performed on different experimental setups using different analytical methods. This approach allowed us not only to find but also to reduce to minimum possible systematic effects. The yields of break neutrons were found to be about (5–7) × 10⁻² of the total number of neutrons per ^{233, 235}U(n, f) fission event and approximately twice as much for ²³⁹Pu(n, f) and ²⁵²Cf. The coefficient of T-odd asymmetry for γ photons is in agreement with the estimate obtained on the assumption that the observed effect is mainly related to the γ photons emitted by excited fragments with highly oriented angular momenta. This fact gave grounds to conclude that the desired break γ photons cannot be reliably selected (within the obtained experimental accuracy) against the much larger background of γ photons from fission fragments.     

Signatures of an extended rigidity percolation in the photo-degradation behavior and the composition dependence of photo-response of Ge-Te-In glasses

Time dependent photocurrent measurements have been undertaken on bulk Ge₁₅Te_85−xIn_x (1 ? x ? 11) series of glasses. It is found that samples with x < 3 do not exhibit any photo-degradation whereas a decrease in photo-conductivity under illumination is observed in samples with x ? 3. Further, the photosensitivity of Ge₁₅Te_85−xIn_x glasses is found to reveal specific signatures at compositions x = 3 and 7. The observed composition dependent photo-degradation behavior and photo-response of these glasses have been understood on the basis of an extended rigidity percolation and its influence on network related properties.     

Diffusion of chromium in chrysoberyl

Cr³⁺ diffusion in chrysoberyl (BeAl₂O₄) irradiated by H⁺ ions and electrons has been studied and compared with diffusion in non-irradiated samples. Chrysoberyl crystals were irradiated with 6 MeV H⁺ ions to fluencies of 1×10¹⁶ cm^–2 for 25 min and with 10 MeV electrons to fluencies of 2×10¹⁷ cm^–2 for 1 h. Three different types of samples, which were doped with Cr³⁺, were annealed in horizontal alumina tube furnaces by 50 K intervals in the temperature range from 1773 to 1923 K for 200 h. Scanning electron microscope–energy dispersive X-ray spectrometer (SEM–EDX) was used to measure the diffusion. Arrhenius equations for the diffusion coefficient for Cr³⁺ in the temperature range 1773–1923 K were developed:     

Diffusion of vanadium in GaAs

The diffusion of Vanadium has been studied in V-doped GaAs layers (GaAs:V) grown by Metal-Organic Chemical Vapour Deposition (MOCVD) using secondary ion mass spectroscopy (SIMS). The vanadium (V) concentration profiles of sandwiched structures made of alternatively undoped and V doped GaAs layers have shown a concentration independent diffusion coefficient (D_V) for varying V doping levels from 10¹⁸ to 10¹⁹ cm⁻³. Measurements of D_V at 550, 615 and 680 °C indicate that the temperature dependence of D_V can be represented by the Arrhenius equation:  cm² s⁻¹. It is suggested that V diffuses via interstitial sites.     

Diffusion in amorphous NiZrAl alloys

Tracer diffusion coefficients for large and small sized impurities were measured in the bulk metallic glass Ni_xZr_yAl_z for several compositions. Molecular beam deposition was employed to grow thin films and secondary ion mass spectrometry was used to determine the concentration-depth profiles of several tracers. A dependence of the atomic mobility of the tracer on its size was found, as observed in the binary Ni_xZr_y amorphous alloy system. The presence of the Al reduces the diffusion coefficients in general, with a stronger decrease for the small sized tracer. The results support the existence of two different diffusion mechanisms proposed for amorphous metallic alloys.     

On the Diffusion Anisotropy of Polycrystals

A model of diffusion anisotropy appearance in a textured polycrystalline aggregate under the grain boundary diffusion preponderance (the C type of kinetic) has been suggested. Although in the textured polycrystal there exists a distribution over 5 degrees of freedom describing the grain boundaries macroscopically, it is the misorientation distribution that has been shown to may give rise to the diffusion anisotropy. Presence of small-angle boundaries and a certain, sufficiently sharp texture is the key moment at this.