Temperature dependence of the a.c. conductivity of Naβ-alumina

Low temperature a.c. conductivity of β-alumina is of the form σ(ω) = σ(ω) + Aωⁿ. Both σ(0) and A are found to be thermally activated. Simple relationships, dependent on the value of n, are shown to exist between (i) the relative magnitudes of σ(0) and A and (ii) their respective activation energies.     

Frequency Dependence of the Dielectric Loss Angle in Disordered Semiconductors in the Terahertz Frequency Range

Frequency dependence of the real part of the conductivity σ₁(ω) in the region of the transition from almost linear (s < 1) to quadratic (s ≈ 2) can indicate a change in the conduction mechanism (the transition from the variable-range to the fixed-range hopping with increasing frequency); in this case, the sharpness of the change in the slope of the frequency characteristic is related to the dependence of the preexponential factor of the resonance integral on the intercenter distance in the pair. The frequency dependence of the imaginary part of the conductivity σ₂(ω) has no kink in the vicinity of the transition frequency ωcr, remaining almost linear. A large dielectric loss angle |cotγ| = |σ₂|/σ₁ can indicate that the imaginary part of the conductivity at ω < ω_cr is defined by the larger zero-phonon contribution in σ₂^res the region of weak variation in the loss angle γ(ω), which significantly exceeds the relaxation contribution σ₂^res.     

Kotani theory for one dimensional stochastic Jacobi matrices

We consider families of operators,H _ω, on ?₂ given by (H _ω u)(n)=u(n+1)+u(n?1)+V _ω(n)u(n), whereV _ω is a stationary bounded ergodic sequence. We prove analogs of Kotani's results, including that for a.e. ω,σ_ac(H _ω) is the essential closure of the set ofE where γ(E) the Lyaponov index, vanishes and the result that ifV _ω is non-deterministic, then σ_ac is empty.     

Dielectric and AC ionic conductivity investigations in K3H(SeO4)2 single crystal

Optical observation under the polarizing microscope and DSC measurements on K₃H(SeO₄)₂ single crystal have been carried out in the temperature range 25-200 °C. It reveals a high-temperature structural phase transition at around 110 °C. The crystal system transformed from monoclinic to trigonal. Electrical impedance measurements of K₃H(SeO₄)₂ were performed as a function of both temperature and frequency. The electrical conduction and dielectric relaxation have been studied. The temperature dependence of electrical conductivity indicates that the sample crystal became a fast ionic conductor in the high-temperature phase. The frequency dependence of conductivity follows the Jonscher's universal dynamic law with the relation σ(ω)=σ(0)+Aωⁿ, where ω is the frequency of the AC field, and n is the exponent. The obtained n values decrease from 1.2 to 0.1 from the room temperature phase to fast ionic phase. The high ionic conductivity in the high-temperature phase is explained by the dynamical disordering of protons between the neighboring SeO₄ groups, which provide more vacant sites in the crystal.     

On the dynamic conductivity for an electron-impurity system

The correlation function formula for the dynamic conductivity of a system of non-interacting electrons in the field of impurities is analyzed in terms of proper connected diagrams. By selecting those diagrams appropriate in the region of weak coupling and low impurity concentration, a set of coupled equations for the energy broadening γ (ω, ε, n_s) and the energy shift Δ(ω, ε, n_s) is derived, where both γ and Δ depend on the frequency ω of a probing field, the energy ε of the electron, and the concentration, n_g, of impurities. With the assumption of a finite range potential, these equations are solved. It is found that γ (ω, n_s) is smaller than that extrapolated value which the conventional expression γ₀ for the low-concentration collision frequency would predict, in the entire region studied, that the difference γ₀-γ becomes appreciable when the ratio of the average time between scatterings, τ_c, to the average duration of a scattering, τ_d, is 100 or less, that γ (ω, n_s) decreases monotonically from its static value γ (0, n_s), and becomes vanishingly small in the region ω≈1/τ_d, and that in the static limit (ω=0), γ=γ₀[1?(2/π) (γ₀τ_d)+…], that the energy shift Δ is positive, and increases from 0 and reach a peak of magnitude γ₀ as ω is raised from 0. By using the γ and Δ obtained, the dynamic conductivity σ(ω, n_s) for degenerated electrons is calculated. The deviation, σ-σ₀, from the conventional expression σ₀=(?i) (n_ee²/M) [ω-iΓ₀]^?1, (n_e]=number density of electrons), for 0°K, is appreciable when the ratio τ_c/τ_d is 100 or less. The field-term correction, which arises from the modification of the scattering due to the probing field, is found to be negligible in the entire region studied.     

Thermally assisted hopping transport in disordered systems

The response to an external field of localized electrons coupled to phonons is investigated. The low frequency (ω<T) linear response function is shown to obey a kinetic equation. The transition probabilities (including multiphonon contributions) can be expressed in terms of the dynamical correlation functions(k, ?) of the phonons. The low temperature d.c. conductivity in three dimensions obeys a law σ(0)=σ₀ · exp(? (T ₀/T)^1/4). By a combined variational and “nearest neighbor” approximation upper limits for the exponential as well as the pre-exponential factor are obtained. In two dimensions the 1/4 in the exponent has to be replaced by 1/3. The one-dimensional case requires separate considerations which do not simply lead to an exponent 1/2. An expression for the thermopower in the hopping regime is derived and evaluated.     

Momentum conservation,time dependent Ginzburg Landau equation and paraconductivity

Superconductors exhibit increasing electrical conductivity as the temperature approachesT _c from above, due to superconducting fluctuations. The functions σ_f1=σ(ω, ?)-σ _n (ω), ?=(T-T _c )/T _c , have been derived by Schmidt phenomenologically using the time dependent Ginzburg-Landau equation (TDGL). These functions fail to vanish in the absolute clean limit τ → ∞ as they must. We have therefore reinvestigated the derivation of the linearized TDGL-equation and the corresponding current expression in the presence of a time dependent vector potential. We find several new terms, which are important for the rather clean superconductor only and are easily interpreted physically in terms of momentum conservation. Applying these corrected equations to the paraconductivity problem, we derive σ_fl(ω, ?) which has an extra factor (1 —iωτ)^?2 compared to Schmidt's result. There is also an additional term, which is connected to the problem of the contribution calculated by Maki. By comparison with the linear response function belowT _c , we show that this term is valid in the limit ¦ω¦?¦Δ¦ only and may not be continued to ω=0. There remains, however, a problem connected with this term, which cannot be solved within the present phenomenological framework.     

Optical characterization of the deep Cr level in GaAs

Comparison between the excitation spectra of luminescence (ESL) of the 0.57 eV band (typical of p or SI materials) and of the 0.62 eV band (typical of n samples) with the photoionization cross-sections σ⁰_n(hv) and σ⁰_p(hv) of the chronium level leads us to attribute unambiguously these luminescence transitions to the (Cr³⁺?Cr²⁺) center in chromium doped GaAs.     

Four-wave mixing and hyper-Raman scattering in CuCl

Non-degenerate four-wave mixing using two non-collinear laser beams with frequencies (wavevectors) ω_p, ω_t (k_p, k_t) respectively is studied in CuCl. Two emission lines at frequencies ω⁽¹⁾=2ω_t-ω_p, and ω⁽²⁾=2ω_p-ω_t are observed. Their excitation spectrum is sharply peaked if the phase-match condition k⁽¹⁾=2k_t-k_p is fulfilled. This is the case, if ω_p coincides with the hyper-Raman lines (R⁺_T, R^-_T) of the laser labelled (t) in a well-defined geometrical configuration.     

AC conductivity and dielectric behavior of bulk Furfurylidenemalononitrile

AC conductivity and dielectric behavior for bulk Furfurylidenemalononitrile have been studied over a temperature range (293–333 K) and frequency range (50–5×10⁶ Hz). The frequency dependence of ac conductivity, σ_ac, has been investigated by the universal power law, σ_ac(ω)=Aω^s. The variation of the frequency exponent (s) with temperature was analyzed in terms of different conduction mechanisms, and it was found that the correlated barrier hopping (CBH) model is the predominant conduction mechanism. The temperature dependence of σ_ac(ω) showed a linear increase with the increase in temperature at different frequencies. The ac activation energy was determined at different frequencies. Dielectric data were analyzed using complex permittivity and complex electric modulus for bulk Furfurylidenemalononitrile at various temperatures.     

Fe deep level optical spectroscopy in InP

The optical cross-section σ_n⁰(hv) and σ_p⁰(hv) associated with the (Fe³⁺ ? Fe²⁺) deep level have been measured by Deep Level Optical Spectroscopy in n-type Fe doped samples of InP. Optical transitions are interpreted as transitions from the Fe²⁺ ground state to the Γ and L point minima of the conduction band for σ_n⁰(hv) and from the valence band to the ground and excited state for Fe²⁺ for σ_p(hv). A theoretical model which accounts for the main features of the experimental data is proposed.     

Nonlinear susceptibilities of vapors and solutions of organic dyes

Results of theoretical studies of laser and Kerr nonlinear susceptibilities of vapors and solutions of organic dyes using a series of polycyclic arenes as an example are presented. Nonlinear susceptibilities of the third χ⁽³⁾ (?3ω; ω, ω, ω) and the fifth χ⁽⁵⁾ (?3ω; ω, ω, ω, ω, ?ω) orders of a series of organic dyes responsible for third harmonic generation of Nd:YAG laser radiation are calculated within the context of the free electron model. Results of calculations of their Kerr third-order nonlinear susceptibilities χ⁽³⁾ (?ω; ω, ?ω, ω) and non-linear refractive indices n ₂ are presented. The calculation results are compared with experimental data on third harmonic generation in naphthalene vapors and with χ⁽³⁾ (?ω; ω, ?ω, ω) as well as n ₂ of paraterphenyl and naphthalene solutions.     

Electrical and electromechanical properties of lead-potassium-yttrium-niobate ceramics—piezoelectric applications

Tungsten bronze (TB)-type oxide ceramic Pb_0.74K_0.13Y_0.13Nb₂O₆ (PKYN) has been synthesized by the standard solid state reaction method. Single phase formation, orthorhombic crystal structure was confirmed by X-ray diffraction (XRD). The substitution of Y³⁺ in Pb_0.74K_0.52Nb₂O₆ (PKN) decreased the unit cell volume and T_C=260 °C. PKYN exhibited the remnant polarization, P_r=8.5 μC/cm², and coercive field, E_c=28.71 kV/cm. Electrical spectroscopy studies were carried out over the temperature (35-595 °C) and frequency (45 Hz-5 MHz) ranges, and the charge carrier phenomenon, grain-grain boundary contribution and non-Debye-type relaxation were analyzed. The relaxation species are immobile charges in low temperature and oxygen vacancies at higher temperature. The theoretical values computed using the relations, ε′=ε_∞+sin(n(T)π/2)(a(T)/ε₀)(ω^n(T)−1); σ(ω)=σ_dc+Aωⁿ are fitted with the experimental one. The n and A parameters suggested that the charge carrier's couple with the soft mode and become mobile at T_C. The activation enthalpy, H_m=0.38 eV, has been estimated from the hopping frequency relation ω_p=ω_e exp(−H_m/k_BT). The piezoelectric constants K_t=35.4%, d₃₃=69×10⁻¹² C/N, d₃₁=−32×10⁻³ mV/N, S₁₁^E=17.8 pm²/N, etc., achieved in PKYN indicate the material is interesting for transducer applications. The activation energies from different formalisms confirmed the ionic-type conduction.     

Transport and magnetic properties of multiwall carbon nanotubes before and after bromination

Bulk samples of oriented carbon nanotubes were prepared by electric arc evaporation of graphite in a helium environment. The temperature dependence of the conductivity σ(T), as well as the temperature and field dependences of the magnetic susceptibility χ(T, B) and magnetoresistance ρ(B, T), was measured for both the pristine and brominated samples. The pristine samples exhibit an anisotropy in the conductivity σ_∥(T)/σ_⊥>50, which disappears in the brominated samples. The χ(T, B) data were used to estimate the carrier concentration n ₀ in the samples: n _0ini ～3×10¹⁰ cm^?2 for the pristine sample, and n _0Br～10¹¹ cm^\t—2 for the brominated sample. Estimation of the total carrier concentration n=n _e+n _p from the data on ρ(B, T) yields n _ini=4×10¹⁷ cm^?3 (or 1.3×10¹⁰ cm^?2) and n _Br=2×10¹⁸ cm^?3 (or 6.7×10¹⁰ cm^?2). These estimates are in good agreement with one another and indicate an approximately fourfold increase in carrier concentration in samples after bromination.     

Quasi-Debye response of ion-hopping conduction at lowest frequencies

On the basis of the relaxation mode theory, quasi-Debye response in the dynamic conductivity by hopping ions is theoretically investigated in three-dimensional random lattices with a uniform distribution of activation energies. It is shown at lowest frequencies that the dynamic conductivity is approximately given by σ(ω)∼σ(0)+A″ω^s″, s″=2−n, where n is a non-integer exponent originated from the mode diffusion length and density of states. The numerical analyses give rise to some values of s″<2, for example s″∼1.5, which behaves as s″→2 with increasing temperature.     

Zum Kernphotoeffekt am Ar40 und Ar38

Liquid Argon was irradiated with bremsstrahlung from 18 to 31 MeV endpoint energy in steps of 2 MeV. The yields of the reactions Ar⁴⁰(γ, n)+(γ, p) and Ar³⁸(γ, n) were measured by detecting the 269a and the 35d rest activity with a low-level-counter. Cross section curves for the (γ, n)-processes could be found from the yield values, and they allowed together withσ _N, σ(γ, p) andσ(γ, np) a determination ofσ(γ, 2n) and σ_γabs for Ar⁴⁰. The integrated cross section forσ(γ, n) from threshold to 33 MeV yields (200±40) MeVmb for Ar⁴⁰ and (210±40) MeVmb for Ar³⁸, the corresponding value for σ_γabs being (450±60) MeVmb for Ar⁴⁰.     

Charge carrier mobility in crystals of the Pb<Subscript>0.67</Subscript>Cd<Subscript>0.33</Subscript>F<Subscript>2</Subscript> superionic conductor

The frequency (ν = 10^?1–10⁷ Hz) dependences σ(ν) of the conductivity of single crystals of the Pb_0.67Cd_0.33F₂ superionic conductor with the fluorite-type structure (CaF₂) in the temperature range of 132–395 K have been studied. The dependences σ(ν) have been discussed in the framework of the hopping relaxation of ionic carriers, which are mobile anions F^?. From experimental curves σ(ν), the direct-current (dc) conductivity σ_dc and the average charge carrier hopping frequency ν_h have been determined. This has made it possible to calculate the charge carrier mobility μ_mob and charge carrier concentration n _mob in these crystals. At room temperature (293 K), the electrical parameters are σ_dc = 1.6 × 10^?4 S/cm, ν_h = 2.7 × 10⁷ Hz, μ_mob = 2.0 × 10^?7 cm²/(s V), and n _mob = 5.1 × 10²¹ cm^?3.     

χ polarization induced in molecular glass of conjugated compound by all-optical poling

The paper presents the first report on χ⁽²⁾ polarization induced in molecular glass of conjugated compound by all-optical poling. Transparent thin film of molecular glass of 1,4-bis[2-[4-[N,N-di(p-tolyl)amino]phenyl]vinyl]benzene (BTAPVB) was prepared using a spin-cast technique. Dipolar as well as octupolar components in BTAPVB contributed to the formation of photoinduced χ⁽²⁾ polarization. Growth rate of χ⁽²⁾ polarization has good linear relation with E_ω⁴E_2ω, which suggested that the simultaneous processes of two-photon (ω + 2ω) and three-photon (ω + ω + ω) excitation on the same electronic level contributed to the formation of photoinduced χ⁽²⁾ polarization.     

量子化极限情况下MOS反型层二维电子气定域态电子电导率σxx的低频效应

本文分析了量子化极限情况下MOS反型层二维电子气(2DEG)定域态电子电导率的频率特性。主要内容:(1)用费密分布函数随频率变化导出了在2DEG两种主要导电过程的频率特性。即向迁移率边激发导电过程电导率σ_ME(ω)和可变程跳跃导电过程电导率σ_VRH(ω)。发现:σ_ME(ω)和σ_VRH(ω)是一个复数。(2)说明了σ_ME(ω)有较长时间常数τ_n,对应一个低频过程。σ_VRH(ω)有较短时间常数,对应一个高频过程。(3)理论与实验作了比较,拟合结果表明,在实验条件下,向迁移率边E_c激发时间常数τ_n 3.6×10^-6。 关键词：     

The energy dependence of single particle spectra in e+e− annihilation

We present explicit predictions of the statistical bootstrap model for inclusive single particle spectra in e⁺e^? annihilation. The distribution (ω/σ_tot·E) (dσ/p² dp) is found to become, for E ? 3 GeV, a function only of the secondary energy ω, independent of the incident e⁺d^? energy E.