Crystal growth,electrical and photophysical properties of Tl<Subscript>2</Subscript>S layered single crystals

The Tl₂S compound was prepared in a single crystal form using a special local technique, and the obtained crystals were analysed by X-ray diffraction. For the resultant crystals, the electrical properties (electrical conductivity and Hall effect) and steady-state photoconductivity were elucidated in this work. The electrical measurements extend from 170 to 430 K, where it was found that σ _⊥ = 8.82 × 10⁻⁵ Sm⁻¹ when current flow direction makes right angle to the cleavage plane of the crystals. In the same range of temperatures, it was found that σ _‖ = 4.73 × 10⁻⁵ Sm⁻¹ when the current flow is parallel to the cleavage plane. In line with the investigated range of temperatures, the widths of the band gaps were calculated and discussed as also the results of the electrical conductivity and Hall effect measurements. In addition, the anisotropy of the electrical conductivity (σ _⊥/σ _‖) for the obtained crystals was also studied in this work. Finally the photosensitivity was calculated for different levels of illumination as a result of the photoconductivity measurements, which showed that the recombination process in Tl₂S single crystals is a monomolecular process.      

ESR of V4+ in α-RbTiOPO4 single crystals

We have recorded and investigated the ESR spectrum of vanadium-doped α-RbTiOPO₄ single crystals in the temperature interval 77–300 K. Two types of structurally distinct centers, V1 and V2, with a 4:1 ratio of the peak intensities were observed. The angular dependences of the resonance magnetic fields are described by a spin Hamiltonian corresponding to axial symmetry with the parameters g _∥1=1.9305, g _⊥1=1.9565, A _∥1=−168.2×10⁻⁴cm⁻¹, and A _⊥1=−54.3×10⁻⁴cm⁻¹ for V1 centers and g _∥2=1.9340, g _⊥2=1.9523, A _∥2=−169.0×10⁻⁴cm⁻¹, and A _⊥2=−55.2×10⁻⁴cm⁻¹ for V2 centers. A model of a paramagnetic center is proposed: The vanadium ions replace titanium ions in two structurally distinct positions Ti1 and Ti2 (V1 and V2 centers, respectively). The possibility that a VO²⁺ ion forms when α-RbTiOPO₄ crystals and crystals of the KTP group (KTiOPO₄, NaTiOPO₄, α-and β-LiTiOPO₄), studied earlier, are doped with vanadium is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 534–536 (March 1998)     

On the Energy Growth of Some Periodically Driven Quantum Systems with Shrinking Gaps in the Spectrum

We consider quantum Hamiltonians of the form H(t)=H+V(t) where the spectrum of H is semibounded and discrete, and the eigenvalues behave as E _n ∼n ^α , with 0<α<1. In particular, the gaps between successive eigenvalues decay as n ^α−1. V(t) is supposed to be periodic, bounded, continuously differentiable in the strong sense and such that the matrix entries with respect to the spectral decomposition of H obey the estimate ‖V(t) _m,n ‖≤ε|m−n|^−p max {m,n}^−2γ for m≠n, where ε>0, p≥1 and γ=(1−α)/2. We show that the energy diffusion exponent can be arbitrarily small provided p is sufficiently large and ε is small enough. More precisely, for any initial condition Ψ∈Dom(H ^1/2), the diffusion of energy is bounded from above as 〈H〉 _Ψ (t)=O(t ^σ ), where . As an application we consider the Hamiltonian H(t)=|p| ^α +ε v(θ,t) on L ²(S ¹,dθ) which was discussed earlier in the literature by Howland.     

Dielectric properties of 4′-n-alkyl-4-cyanobiphenyls in their nematic phases

The principal dielectric constants of the pentyl to octyl derivatives of 4′-n-alkyl-4-cyanobiphenyl have been measured as functions of temperature in their nematic and isotropic phases. All the compounds exhibit a strong positive dielectric anisotropy due to the presence of a large dipole moment along the major molecular axis. The principal dielectric constantsε _‖ andε _⊥ as well as the anisotropy Δε decrease with increasing alkyl chain length. The experimental value of (ε_‖+2ε_⊥) decreases with decreasing temperature, and is throughout less than the extrapolated isotropic value, in conformity with the model of antiferroelectric short range order proposed by Madhusudana and Chandrasekhar.     

Influence of magnetic field on the superconducting transition in a Nd1.82Ce0.18CuO4−δ film

The temperature and magnetic-field dependences of the resistivity ρ and Hall effect R(j‖ab, B‖c) in a Nd_1.82Ce_0.18CuO_4−δ single crystal film (T _c =6 K) is investigated at temperatures 1.4≤T≤20 K and magnetic fields 0≤B≤5.5 T. At the lowest temperature T=1.4 K the resistive state (exhibiting resistivity and the Hall effect) arises in a magnetic field B=0.5 T. A transition to the normal state is completed at B _c 2≃3 T, where the Hall coefficient becomes nearly constant. The negative magnetoresistance due to the weak-localization effect in the normal state is observed for B>3 T. The nonmonotonic behavior and the inversion of the sign of R(B) in the mixed state are explained in a reasonable way by the flux-flow model with the effect of pinning taken into account. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 6, 407–411 (25 September 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

One-dimensional ionic conduction in solid Ag2 Tl6I10

The ionic and electronic conductivities of Ag₂Tl₆I₁₀ single crystals have been studied as a function of crystallographic orientation and temperature from 20 to 135°C. EMF as well as AC and DC techniques have been employed. The highly anisotropic material is predominantly an Ag⁺-ion conductor parallel toc-direction, with the Ag⁺ ions moving through linear channels that are not interconnected. The conductivity σ_‖c =1.6×10⁻⁷Ω⁻¹cm⁻¹ at 25°C, with an activation enthalpy for σ_‖c of 0.38 eV. The conduction perpendicular toc-direction has been found to be predominantly electronic with a value of σ_⊥c =3×10⁻⁹Ω⁻¹cm⁻¹ at 25°C and an activation enthalpy for σ_⊥c of 0.64 eV. This is the first observation of one-dimensional Ag⁺ conduction and this type of orientation-dependent change from ionic to electronic conduction. On leave from Institute of Physics, Academia Sinica, Peking, China.     

Variations in the electrical resistivity of La0.67Ca0.33MnO3 films and induced interconversions of ferromagnetic and nonferromagnetic inclusions in their bulk

A significant (∼1.8%) positive unit between the parameters of the crystal lattice is the reason of tetragonal distortion (a _⊥/a _‖ ≈ 1.04) and reduction in the volume of the unit cell of La_0.67Ca_0.33MnO₃ films (15 nm) quasicoherently grown on the (001) surface of a LaAlO₃ substrate. The films consist of single-crystal blocks with the lateral size of 30–50 nm. The atomically smooth LaAlO₃-La_0.67Ca_0.33MnO₃ interphase boundary has no misfit dislocations. At T = 4.2 K, the transformation of nonferromagnetic phase inclusions into ferromagnetic ones in a constant magnetic field H is accompanied by a stable reduction in the electrical resistivity ρ of manganite films with time, so that the curve ρ(t) is well approximated by the relationship ρ(t) ∼ ρ₁(t − t ₀)^1/2, (where t ₀ is the time for establishment of the specified value (μ₀ H = 5 T) of the magnetic field and ρ₁ is a coefficient independent of H). The magnetocrystalline anisotropy due to the elastic deformation of films by the substrate and stratification of electronic phases are the reasons of the distinct hysteresis in the dependences ρ(μ₀ H, T < 100 K) obtained on μ₀ H scanning in the sequence 5 T → 0 → −5 T → 0 → 5 T. At T = 50 K and μ₀ H = 0.4 T, the magnetoresistance MR = 100% [ρ(μ₀ H) − ρ(μ₀ H = 0)]/ρ(μ₀ H = 0) of LCMO films attains 150%.     

Static charged spheres with anisotropic pressure in general relativity

We report a generalization of our earlier formalism [Pramana, 54, 663 (1998)] to obtain exact solutions of Einstein-Maxwell’s equations for static spheres filled with a charged fluid having anisotropic pressure and of null conductivity. Defining new variables: w=(4π/3)(ρ+ε)r ², u=4πξr ², v _r=4πp _r r ², v _⊥=4πp _⊥ r ²[ρ, ξ(=−(1/2)F ₁₄ F ¹⁴), p _r, p _⊥ being respectively the energy densities of matter and electrostatic fields, radial and transverse fluid pressures whereas ε denotes the eigenvalue of the conformal Weyl tensor and interpreted as the energy density of the free gravitational field], we have recast Einstein’s field equations into a form easy to integrate. Since the system is underdetermined we make the following assumptions to solve the field equations (i) u=v _r=(a ²/2κ)r ⁿ⁺², v _⊥=k ₁ v _r, w=k ₂ v _r; a ², n(>0), k ₁, k ₂ being constants with κ=((k ₁+2)/3+k ₂) and (ii) w+u=(b ²/2)r ⁿ⁺², u=v _r, v _⊥−v _r=k, with b and k as constants. In both cases the field equations are integrated completely. The first solution is regular in the metric as well as physical variables for all values of n>0. Even though the second solution contains terms like k/r ² since Q(0)=0 it is argued that the pressure anisotropy, caused by the electric flux near the centre, can be made to vanish reducing it to the generalized Cooperstock-de la Cruz solution given in [14]. The interior solutions are shown to match with the exterior Reissner-Nordstrom solution over a fixed boundary. Dedicated to Prof. F A E Pirani.     

Magnetic breakdown and quantum magnetotransport with a constant pseudospin under tilted magnetic fields in an n -In x Ga1? x As/GaAs double quantum well

A procedure is proposed for precise scanning of the (B _⊥, B _‖) plane between the magnetic field projections that are perpendicular and parallel to (quasi-)two-dimensional layers when measuring their longitudinal and Hall magnetoresistances. Investigations of a n-In _x Ga_1−x As/GaAs double quantum well (x ≈ 0.2) performed using this procedure make it possible to reveal a number of the features of the magnetoresistance, which appear due to a complex energy spectrum of the double quantum well in a parallel field, and to separate them from the structures associated with the magnetic breakdown. The trajectories representing the features of the magnetoresistance in the (B _⊥, B _‖) plane are described by the semiclassical calculations of the quantization of the energy spectrum of the double quantum well under the action of the perpendicular field component. The structures appearing due to the magnetic breakdown are amplified with increasing the total magnetic field magnitude and, in the samples with low mobility, completely suppress the features caused by the motion of an electron with a constant pseudospin component. The peaks corresponding to the magnetic breakdown are split in a strong parallel field due to the spin splitting of the Landau levels. These splittings correspond to the effective Landé factor |g*| ≈ 3. Original Russian Text ? M.V. Yakunin, S.M. Podgornykh, V.N. Neverov, 2007, published in Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 132, No. 1, pp. 241–249.     

Electron beam irradiation induced changes in liquid-crystal compound 5CB

Electron beam irradiation studies on liquid crystal material 5CB have been carried out at a temperature where the compound exists in the isotropic liquid phase. In situ time-resolved spectroscopic characterization was carried out during the irradiation. Three different transients were observed during the 2-μs electron pulse. After about 50 μs, only one transient species was found to be present, which has an absorption peak at 360 nm. Radiolysed sample exhibits a broad absorption at ∼400 nm. The dielectric measurements show that even a low level of irradiation results in a dramatic increase in the component of dielectric permittivity normal to the long axes of the molecules ε_⊥^′, and a corresponding decrease in the dielectric anisotropy (Δε′=ε_∥^′−ε_⊥^′ ). These studies show that 5CB is prone to substantial radiation damage on exposure to the beam of high-energy electrons.     

Quasi-one dimensional electrical conductivity and thermoelectric power studies on a discotic liquid crystal

We have studied the electrical conductivity of well aligned samples of hexahexylthiotriphenylene (HHTT) in the pure as well as doped states. The dopant used was a small concentration (0.62 mole %) of the electron acceptor trinitrofluorenone (TNF). In the columnar phases, doping causes the AC(1 kHz) conductivity along the columnar axis (σ _‖) to increase by a factor of 10⁷ or more relative to that in undoped samples; σ _‖ attains a value of 10⁻²S/m, which was the maximum measurable limit of our experimental set up. On the other hand, in the isotropic phase doping makes hardly any difference to the conductivity. The frequency dependence of the conductivity has been investigated. The DC conductivity of doped samples exhibits an enormous anisotropy, σ _‖/σ _⊥ ≥ 10¹⁰, which is 7 orders higher than that reported for any liquid crystalline system, and, to our knowledge, the largest observed in an organic conductor. We also report the first thermoelectric power studies on these ‘molecular wires’. The sign of the thermoelectric power is in conformity with the expected nature of the charge carriers, namely, holes.     

Superhyperfine structure of the EPR spectra of Ce<Superscript>3+</Superscript> ions in Li<Emphasis Type="Italic">R</Emphasis>F<Subscript>4</Subscript> (<Emphasis Type="Italic">R</Emphasis> = Y,Lu, Tm) double fluorides

The EPR spectra of Ce³⁺ impurity ions in LiYF₄, LiLuF₄, and LiTmF₄ double-fluoride single crystals have been investigated at a frequency of ∼9.3 GHz in the temperature range 5–25 K. The effective g factors of the ground Kramers doublet of the cerium ions in three crystals are close to each other (g _‖ = 2.737, g _⊥ = 1.475 for LiYF₄:Ce³⁺). A superhyperfine structure of the EPR spectrum of Ce³⁺ ions in the LiTmF₄ Van Vleck paramagnet has been observed in the external magnetic field B oriented along the crystallographic axis c (B ‖ c). The superhyperfine structure of the EPR soectra of the Ce³⁺ ions in the LiYF₄ and LiLuF₄ diamagnetic matrices is resolved for B ⊥ c. Possible factors responsible for this pronounced difference in the properties of the systems studied have been discussed.     

The estimates of periodic potentials in terms of effective masses

LetG _n=(A _n ⁻ , A _n ⁺ ),n≧1, denote the gaps,M _n ^± be the effective masses and Σn=[A _n−1 ⁺ ,A _n ^- ],A ₀ ⁺ =0, be the spectral bands of the Hill operatorT=−d ²/dx ²+V(x) inL ² (R), whereV is a 1-periodic real potential fromL ²(0,1). Let the length gapL _n=|G_n|, h_n be the height of the corresponding slit on the quasimomentum domain and Δ_n=π²(2n−1)−∣Σ_n∣>0 be the band reduction. Let ,n≧1, denote the gap length for the operator . Introduce the sequencesL={L_n}, h={h_n}, l={l_n}, Δ={Δ_n},M ^±={M _n ^± } and the norms ,m≧0. The following results are obtained: i) The estimates of‖V‖, ‖L‖, ‖h‖ ₁, ‖l‖₁, ‖δ‖ in terms of ‖M^±‖₂, ii) identities for the Dirichlet integral of quasimomentum and integral of potentials and so on, iii) the generation of i), ii) for more general potentials. The research described in this publication was made possible in part by grant from the Russian Fund of Fundamental Research and INTAS.     

Bistability of quantum magnetotransport in a multilayer Ge/p-Ge1−x Six heterostructure with wide potential wells

Two metastable states of a multilayer Ge/p-Ge_1−x Si_x heterosystem with wide (∼ 35 nm) potential wells (Ge) are observed in strong magnetic fields B at low temperatures. In the first state, the Hall resistivity exhibits an inflection near the value ρ_xy=h/e ² scaled to one Ge layer. The longitudinal magnetoresistivity ρ_xx(B) possesses a minimum in the range of fields where this inflection occurs. The temperature evolution of the inflection in ρ_xy(B), the minimum of ρ _xx(B), and the value of ρ_xy at the inflection indicates a weakly expressed state of the quantum Hall effect with a uniform current distribution over the layers. In the second metastable state, an unusually wide plateau near h/2e ² with a very weak field dependence is observed in ρ_xy(B). Estimates show that in these samples the Fermi level lies below but close to the top of the inflection in the bottom of the well. For this reason, the second state can be explained by separation of a hole gas in the Ge layers into two sublayers, and the saturation of ρ_xy(B) near h/2e ² can be explained by the formation of a quantum Hall insulator state. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 4, 290–297 (25 August 1999)     

Time Quasi-Periodic Unbounded Perturbations¶of Schrödinger Operators and KAM Methods

We eliminate by KAM methods the time dependence in a class of linear differential equations in ℓ² subject to an unbounded, quasi-periodic forcing. This entails the pure-point nature of the Floquet spectrum of the operator H ₀+εP(ωt) for ε small. Here H ₀ is the one-dimensional Schr?dinger operator p ²+V, V(x)∼|x|^α, α <2 for |x|→∞, the time quasi-periodic perturbation P may grow as |x|^β, β <(α−2)/2, and the frequency vector ω is non resonant. The proof extends to infinite dimensional spaces the result valid for quasiperiodically forced linear differential equations and is based on Kuksin's estimate of solutions of homological equations with non-constant coefficients. Received: 3 October 2000 / Accepted: 20 December 2000     

Diffusion and transport phenomena in a collisional magnetoplasma having both streaming and temperature anisotropy: Collisional kinetics

Boltzmann-transport equation is analytically solved for two-component magnetoplasma using Chapman-Enskog analysis to include collisional diffusion transport having anisotropies in both streaming velocity and temperature components. The modified collisional integrals are analytically solved with flux integrals and perturbed kinetic equation to arrive at drift diffusion velocity and resulting transport coefficients which are markedly affected by both streaming and temperature anisotropy. The early isotropic results are recovered in the limit V ₀ = 0 and T _‖ = T _⊥ which reduce to eqs (11.30) and (11.31) of [1] and eqs (2.7) and (2.13) of [2]. The electrical resistivity (η_⊥) diminishes sharply in fusion temperature limit kT _⊥ = 1 keV. The shape of the curves for both electrical resistivity and thermal conductivity is rectangular hyperbolic. However, for low thermal ratio (T _‖/T _⊥ < 1), the curves are raised up and for high thermal ratio (T _‖/T _⊥ > 1), they are lowered down the isotropic case (T _‖/T _⊥ > 1), showing comparatively diminished magnitudes of the quantities.      

Magnetic circular dichroism in the absorption of theF A centre in KCl doped with Li and Na

Summary The spin-orbit structure ofF _A centres in KCl:Li and KCl:Na have been studied by means of the magnetic circular dichroism. Due to theirC _4V symmetry theF _A centres have two different spin-orbit parameters, Δ^‖ and Δ^⊥, which only in the KCl:Li case follow the relation: Δ^‖ < Δ_F < Δ^⊥ as expected from the theory. For a close comparison we have also reported our measurement of the spin-orbit coupling ofF centre in KCl. The spin-orbit parameters of theF andF _A centres have been determined using the method of moment.     

Influence of anisotropy on the dispersion of surface plasmon-phonon polaritons in silicon carbide

Surface plasmon-phonon polaritons (SPPP’s) of types 3 and 4 are investigated in doped anisotropic single crystals of hexagonal silicon carbide (6H-SiC) in the orientation corresponding to K⊥C and xy⊥C. It is shown that a dispersion dependence of the type-3 SPPP’s bounded by K appears in 6H-SiC when the plasmon frequency increases to ν _p⊥⩾350 cm⁻¹. At ν _p⊥⩾400 cm⁻¹, ν _s(K) exists for type-4 SPPP’s in the frequency range Ω _∥ ⁺ <ν< _⊥ ⁺ . When the concentration of free charge carriers is increased, the dispersion curves are displaced toward higher frequencies. The conditions for the existence of type-3 and type-4 SPPP’s in 6H-SiC are determined. Fiz. Tverd. Tela (St. Petersburg) 40, 636–639 (April 1998)     

Probing meson spectral functions with double differential dilepton spectra in heavy-ion collisions

The double differential dilepton spectrum d/(d ² d _⊥ ²) at fixed transverse mass M _⊥ allows a direct access to the vector meson spectral functions. Within a fireball model the sensitivity of d/(d ² d _⊥ ²) against variations of both the in-medium properties of mesons and the dynamics of the fireball is investigated. In contrast to the integrated invariant-mass spectrum d/d ², in the spectrum d/(d ² d _⊥ ²) with fixed M _⊥ the ω signal is clearly seen as bump riding on the ρ background even in case of strong in-medium modifications.[3mm] Received: 16 November 2000 / Accepted: 16 January 2001