Investigation of dc hopping conduction in TlGaS2 and TlInS2 single crystals

It is established that variable-range hopping conduction takes place between states localized near the Fermi level in layered TlGaS₂ and TlInS₂ single crystals both along and across their natural layers in a constant electric field at T⩽200 K. The densities of states near the Fermi level and the hopping distances at different temperature are estimated. The occurrence of activationless hopping conduction is established in TlGaS₂ and TlInS₂ single crystals in the temperature range 110–150 K. Fiz. Tverd. Tela (St. Petersburg) 40, 612–615 (April 1998)     

One-dimensional localization in porous a-Si1−c Mnc

The temperature dependence of the conductance of porous silicon doped with manganese up to densities corresponding to the metallic side of the Anderson transition is investigated. It is found that in the temperature range below T=40–60 K the conductance decreases with T as G(T)∝T ^−1/3. This behavior corresponds to one-dimensional electron localization in silicon wires under conditions of inelastic electron-electron collisions with a small energy transfer. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 4, 265–269 (25 February 1998)     

EPR investigation of a-Si:H aerosol particles formed under silane thermal decomposition

The dangling bond defects were investigated in a-Si:H particles formed under silane thermal decomposition in flow reactor. EPR together with hydrogen evolution method were used. The experimental results allowed us to conclude that there are two kinds of dangling bond defects in a-Si:H aerosol particles. The defects of the first kind are localized on the surface of interconnected microvoids and microchannels (surface dangling bonds) and those of the second kind are embedded in amorphous silicon network (volume dangling bonds). The thermal equilibration of dangling bonds and temperature dependence of equilibrium dangling bond concentration were investigated. It was found that at temperatures > 400 K the dangling bond concentrationNApplied Magnetic Resonance _s reversibly depends on sample temperature. The volume dangling bond concentration increases with temperature increasing (the effective activation energy of dangling bond formationU > 0), and the surface dangling bond concentration decreases with temperature increasing (U < 0). It has been found that EPR line is considerably asymmetric for samples with high hydrogen content and for low hydrogen content the EPR line is weakly asymmetric. A conclusion was drawn that the asymmetry degree depends on amorphous silicon lattice distortions. This conclusion has been confirmed by EPR spectra simulations.     

Anisotropic negative magnetoresistance in one-dimensional channels of porous silicon

It is found that the magnetoresistance of manganese-doped porous amorphous silicon in fields 0–5 T is negative and depends on the orientation of the magnetic field. The experimental curves of the magnetic-field dependence are described well by the theory of quantum corrections to the conductivity in the one-dimensional case. The phase coherence length in the material is ≈25 nm at T=4.2 K. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 3, 189–193 (10 February 1999)     

Enhancement of fluorescence of porous silicon upon saturation by liquid crystal

The fluorescence of samples of porous silicon of various morphologies that are filled with a liquid crystal (LC), n-pentyl-n′-cyanobiphenyl (5CB), is studied. The fluorescence spectra of the sample, along with the long-wavelength band of porous silicon with a maximum in the range 627–667 nm, exhibit a short-wavelength band of 5CB with a maximum in the range 385–410 nm. The radiative relaxation times of porous silicon and 5CB lie in the micro- and nanosecond ranges, respectively. It is found that the filling of pores with 5CB enhances the fluorescence of porous silicon by two to three times. This enhancement is caused by non-radiative energy transfer from 5CB to the porous matrix as a result of efficient interactions between LC molecules and pore walls. Using IR spectroscopy, it is shown that the formation of hydrogen bonds between cyano groups of 5CB molecules and silanol groups of pore surface is the predominant type of these interactions. A transfer mechanism is suggested according to which excited associates of 5CB molecules transfer their energy via surface channels to excitons of porous silicon, enhancing its fluorescence.     

Electrical properties of amorphous (Co45Fe45Zr10)x(Al2O3)1?x nanocomposites

The electrical properties of (Co₄₅Fe₄₅Zr₁₀)_x(Al₂O₃)_1−x granular nanocomposites have been studied. The concentration dependences of electrical resistivity are S-shaped (in accordance with the percolation theory of conduction) with a threshold at a metallic component concentration of ∼41 at. %. An analysis of the temperature behavior carried out in the range 300–973 K revealed that structural relaxation and crystallization of the amorphous phase are accompanied by a decrease in the electrical resistivity of the composites above the percolation threshold and by its increase below the percolation threshold. For metallic phase concentrations x<41 at. %, variable range hopping conduction over localized states near the Fermi level was found to be dominant at low temperatures (77–180 K). A further increase in temperature brings about a crossover of the conduction mechanism from Mott’s law ln(σ) ∝ (1/T)^1/4 to ln(σ) ∝ (1/T)^1/2. A model of inelastic resonance tunneling over a chain of localized states of the dielectric matrix was used to find the average number of localized states involved in the charge transport between metallic grains. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 11, 2004, pp. 2076–2082. Original Russian Text Copyright ? 2004 by Kalinin, Remizov, Sitnikov.     

dielectric relaxation in VDF<Subscript>60</Subscript>/Tr<Subscript>40</Subscript> and VDF<Subscript>88</Subscript>/Te<Subscript>12</Subscript> polar copolymers embedded in porous glass matrices

The dielectric properties of composite materials prepared by embedding P(VDF₆₀/Tr₄₀) and P(VDF₈₈/Te₁₂) polar copolymers in porous glass matrices with a mean flow-through pore diameter of around 320 nm were investigated in the temperature range 200–450 K. Strong dielectric relaxation, the characteristic time of which conformed to the Williams-Landel-Ferry law, was observed in the vicinity of glass transition point T _g of an amorphous fraction of polymer inclusions. An increase (≈10 K) in the T _g temperature of the amorphous fraction of incorporated polymeric materials was detected.     

Theory of the oscillatory dependence of the conductivity of two-component dielectric composites at room temperatures

It is predicted that at room temperatures a hopping mechanism of charge transfer plays a very important role and leads to temperature oscillations of the conductivity σ(T) of a dielectric composite. The dependence of the conductivity σ(ω) on the frequency of an alternating electric field is calculated. The relation obtained can be used to determine, first, the electron relaxation times and, second, and more importantly, the frequency of electron tunneling through the dielectric matrix from measurements of the conductivity in various frequency ranges. Zh. Tekh. Fiz. 69, 31–34 (March 1999)     

Onset of a nonoptimal hopping regime for ac conductivity in amorphous gallium antimonide

A possible explanation has been found for the typical discrepancy between the parameters of localized states as determined from the static and dynamic hopping conductivities in tetrahedral amorphous semiconductors. It is shown for the example of a-GaSb that the Mott hopping length R _opt, the correlation length for nonoptimal hops L _T , and the ac hopping length R _ω are related as R _opt <L _T <R _ω , as a result of which the Mott law holds for the dc conductivity and the Zvyagin regime of nonoptimal hops holds for the ac conductivity σ(ω). The observed value of σ(ω) is two orders of magnitude lower than the conductivity calculated by the Austin-Mott formula for the parameters of localized states found from dc measurements. A model that quantitatively describes the static and dynamic conductivity of a-GaSb with the aid of a single set of parameters characterizing the Miller-Abrahams resistor network is proposed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 4, 322–327 (25 February 1997)     

Paramagnetic properties of single-crystal silicon implanted with iron-group transition metals

Samples of single-crystal silicon implanted with iron-group ions are investigated by electron paramagnetic resonance (EPR). The change in relative permittivity μ_r of the silicon layer modified by implanted nickel ions is found. The accumulation kinetics of paramagnetic centers of amorphous regions of silicon implanted with Co, Ni, and Fe are shown to be different. Magnetic hysteresis for both the wide EPR line due to implanted impurities and the EPR line due to dangling chemical silicon bonds is found. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 2, pp. 197–201, March–April, 2008.     

Magnetotransport properties of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> with different grain sizes

The magnetotransport and magnetoresistive (MR) properties of manganese-based La_0.67Ca_0.33MnO₃ perovskite with different grain sizes are reported. The electrical resistivity was measured as a function of temperature in magnetic fields of 0.5 and 1 T. The insulator–metal transition temperature, T _IM, shifted to a higher temperature with the application of the magnetic field. In zero field, T _IM is almost constant (∼271 K) for all samples except for the sample with the largest grain size, where T _IM=265 K. The temperature dependence of resistivity was fitted with several equations in the metallic (ferromagnetic) region and the insulating (paramagnetic) region. The density of states at the Fermi level, N(E _F), and the activation energy of electron hopping were estimated by fitting the resistivity versus temperature curves. The ρ–T ² curves are nearly linear in the metallic regime, but the ρ–T ^2.5 curves exhibit a deviation from linearity. The variable range hopping model and small polaron hopping model fit the data well in the high-temperature region, indicating the existence of the Jahn–Teller distortion that localizes the charge carriers. MR was found to increase with an increase in the magnetic field, an effect which is attributed to the intergrain spin tunneling effect.     

Erbium electroluminescence excitation in amorphous hydrogenated silicon under thermally stimulated deep-center tunneling ionization

A study of the electroluminescence of erbium-doped, amorphous hydrogenated silicon, a-Si:H 〈Er〉, is reported. It has been found that the electroluminescence intensity at the wavelength λ=1.54 μm corresponding to the ⁴ I _13/2→⁴ I _15/2 intra-4f shell transition in Er passes through a maximum near room temperature. The unusual temperature and field dependences of the electroluminescence indicate electric-field induced multi-phonon tunneling emission of electrons from deep centers. The electroluminescence of Er³⁺ ions is due to their becoming excited as conduction-band electrons are captured by neutral dangling bonds (D ⁰ centers), which form when erbium is incorporated into the amorphous matrix. This Auger process transforms the center from its neutral state, D ⁰, to a negatively charged state, D ⁻, and the energy released in the capture is transferred by Coulomb interaction into the erbium-ion 4f shell. The steady-state current through the electroluminescent structure is supported by the reverse process of multi-phonon tunneling-electron emission from the D ⁻ center to the conduction band. The proposed theoretical model is in a good agreement with experimental data. Fiz. Tverd. Tela (St. Petersburg) 41, 210–217 (February 1999)     

Hydrogen in amorphous and crystalline ytterbium films

Ytterbium vapor condensation on a liquid-helium cooled substrate in a hydrogen atmosphere is used to obtain Yb-H films containing up to 55 at.% hydrogen. Various thermodynamic and kinetic parameters of the transition of these films from the amorphous to the crystalline state (a→c transition) are investigated along with the electrical conductivity of these states. It is shown that the investigated properties of Yb-H films containing up to 40 at.% hydrogen are essentially indistinguishable from those of pure Yb films in the temperature interval 4.2–293 K. Increasing the hydrogen concentration to 55 at.% leads to an insignificant increase in the electrical resistivity, the kinetic temperature, and the activation energy of the a→c transition, and also to a decrease of the propagation speed of self-maintaining avalanche (explosive) crystallization. Reasons for the observed influence of hydrogen on the properties of Yb-H films are analyzed. The examined low-temperature Yb-H condensates can be characterized as a “frozen” solid solution of hydrogen in ytterbium in the temperature interval 4.2–293 K. Storing such films at room temperature leads to the formation of ionic ytterbium dihydride YbH₂. Fiz. Tverd. Tela (St. Petersburg) 41, 177–182 (February 1999)     

Corresponding states theory for the prediction of surface tension of ionic liquids

The surface tension of ionic liquids is studied according to phenomenological scaling and the law of corresponding states. The reduced coordinates γ*–T*, where γ* represents the reduced surface tension and T* is the reduced temperature, are introduced for the prediction of the surface tension from the melting point up to boiling point. It has been shown that the correlation can be expressed as a unique straight-line plot with a linear correlation coefficient of 0.984 that requires only the melting and boiling point parameters and can predict the surface tension accurately.     

Near-ordering structural transitions in amorphous metal alloys

The temperature variations in the modulus of elasticity (Young’s modulus) E and internal friction Q ⁻¹ of the amorphous metal alloys Ti₅₀Cu_50−x Ni_x (5⩽x⩽20) are studied at temperatures of 300–800 K. There is an anomalous increase in E(T) at temperatures above T _x (which varies from 440 to 525 K, depending on the composition). When the amount of nickel in the alloy is high (x>12 at. %), a small peak shows up in Q ⁻¹(T). These effects are related to structural transitions in near-ordering regions (clusters). A model for structural relaxation of near ordering in amorphous alloys is proposed on the basis of these experiments. Fiz. Tverd. Tela (St. Petersburg) 40, 389–392 (March 1998)     

Metastability of amorphous silicon from silicon network rebonding

We propose a network rebonding model for light-induced metastability in amorphous silicon, involving bonding rearrangements of silicon and hydrogen atoms. Nonradiative recombination breaks weak silicon bonds and generates dangling bond-floating bond pairs, with very low activation energies. The transient floating bonds annihilate, generating local hydrogen motion. Charged defects are also found. Support for these processes is found with tight-binding molecular dynamics simulations. The model accounts for major experimental features of the Staebler-Wronski effect including electron-spin resonance data, the t(1/3) kinetics of defect formation, two types of metastable dangling bonds, and hysteretic annealing.     

Paramagnetic labeling as a method for the soft spectroscopy of electronic states

A self-consistent microscopic theory of the relaxation of the crystal-field levels of an impurity ion in a state with an integer valence implanted in a normal metal is devised. A microscopic approach based on the Coqblin-Schrieffer-Cooper approach, rather than the formal model of the sf exchange interaction, makes it possible to take into account the specific details of both the crystal-field states of the impurity ion and the electronic band spectrum of the metal. A new method for the soft spectroscopy of electronic states based on measurements of the temperature dependence of the width Γ_MM′(T) of transitions between the crystal-field states |M〉 of a paramagnetic ion implanted in the compound being studied is proposed. To make specific use of this method in neutron and optical spectroscopy, a classification of the types of temperature dependence of the natural relaxation width γ _M (T) of the levels is devised, and procedures for possible experimental methods are proposed. A nonzero value of the natural relaxation width γ _G (T) of the crystal-field ground state | G〉 of an impurity ion at zero temperature is obtained within the proposed self-consistent model, but is beyond the scope of perturbation theory. It is shown that the widely accepted estimate of the characteristic temperature of Kondo systems T*=Γ _G(T=0)/2 from the quasielastic scattering width at zero temperature Γ _G (T=0)/2 is incorrect in the case of strong relaxation in a system with soft crystal fields. The proposed model is applied to the quantitative analysis of the relaxation of the crystal-field levels of paramagnetic Pr³⁺ ions implanted in CeAl₃ and LaAl₃. The results of the calculations are in quantitative agreement with the experimental data. Zh. éksp. Teor. Fiz. 113, 1843–1865 (May 1998)     

Long-lived excited impurity states in diamond-like semiconductors

The lifetime of charge carriers in the lowest excited states of some impurities of groups III and V in diamond, silicon, and germanium can be several (four to six) orders of magnitude longer that the lifetime of free carriers. Accumulation of carriers in these long-lived states may give rise to several new effects, such as hopping photoconductivity via long-lived excited states of impurities in dc and microwave electric fields, slow relaxation of induced absorption, and infrared absorption at energies lower than the impurity ionization energy. Zh. éksp. Teor. Fiz. 112, 221–236 (July 1997)     

Temperature dependence of electric resistance and magnetoresistance of pressed nanocomposites of multilayer nanotubes with the structure of nested cones

Bulk samples of carbon multilayer nanotubes with the structure of nested cones (fishbone structure) suitable for transport measurements, were prepared by compressing under high pressure (∼25 kbar) a nanotube precursor synthesized through thermal decomposition of polyethylene catalyzed by nickel. The structure of the initial nanotube material was studied using high-resolution transmission electron microscopy. In the low-temperature range (4.2–100 K) the electric resistance of the samples changes according to the law ln R ∝ (T ₀/T)^1/3, where T ₀∼7 K. The measured magnetoresistance is quadratic in the magnetic field and linear in the reciprocal temperature. The measurements have been interpreted in terms of two-dimensional variable-range hopping conductivity. It is suggested that the space between the inside and outside walls of nanotubes acts as a two-dimensional conducting medium. Estimates suggest a high value of the density of electron states at the Fermi level of about 5×10²¹ eV⁻¹ cm⁻³. Zh. éksp. Teor. Fiz. 113, 2221–2228 (June 1998)