Study of dielectric and photoelastic properties of fluorite crystals

Summary The photoelastic constants and strain derivatives of the static and high-frequency dielectric constants of fluorite structure solids have been computed by means of a three-body potential which consists of the long-range Coulomb, three-body interactions, the short-range van der Waals attraction and overlap repulsion operative up to the second-neighbour ions and represented by the Hafemeister and Flygare potential. The agreement between available experimental and our calculated values of photoelastic constants and strain and pressure derivatives of the static dielectric constants is reasonably good.

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Riassunto Le costanti fotoelastiche e le derivate di deformazione delle costanti dielettriche statiche ed ad alta frequenza dei solidi con struttura dei cristalli di fluoruro sono state calcolate per mezzo di un potenziale a tre corpi che è formato dall'interazione di Coulomb e a tre corpi a lungo raggio e dall'attrazione di van der Waals a breve raggio e la repulsione di sovrapposizione operativa fino al secondo ione adiacente e rappresentata dal potenziale di Hafemeister e Flygare. L'accordo tra i valori sperimentali disponibili e i nostri valori calcolati delle costanti fotoelastiche e delle derivate di pressione e deformazione è ragionevolmente buono.

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Резюме Используь трех-частицный потенциал, который содержит длиннодействующее кулоновское и трех-частичное взаимодействие и короткодействующее притяжение ван дер Ваальса и отталкивание вплоть до второго ?соседа?, описываемое потенциалом Хафемайстера и Флигера, вычисляются фотоупрутие постоянные и производные по деформации статических и высокочастотных диэлектрических постоянных для твердых тел со структурой флюорита. Получается удовлетворительное согласие между имеющимися экспериментальными данными и вычисленными нами фотоупругими постоянными и производными статических диэлектрических постоянных по деформации и давлению.

     

Multiplicity of dielectric relaxation times of dispersed ice microcrystals

Summary The multiplicity of relaxation times of the dielectric relaxation of water molecules in dispersed ice microcrystals has been studied by means of the thermally stimulated depolarization (TSD) current method. Using several experimental techniques offered by the TSD method we have shown that the relaxation mechanism is characterized by a continuous distribution of relaxation times with both the activation energy.W and the pre-exponential factorτ ₀ in the Arrhenius equation being distributed parameters. A linear relationship has been found to exist betweenW and lnτ ₀. The dielectric behaviour of ice emulsions has been found to resemble strongly in some aspects that of HF-doped ice and ice samples with high concentrations of crystal imperfections. The multiplicity of relaxation times has been explained by the interaction of intrinsic ionic defects with water molecules. Work supported in part by the Greek Ministry of Research and Technology.     

Evolution with time of the dielectric properties of dispersed ice microcrystals

Summary The evolution with time of the characteristics of the dielectric relaxation of water molecules in dispersions of ice microcrystals has been studied by means of the thermally stimulated depolarization current (TSDC) method. With advancing stage of evolution,i.e. with increasing preservation time at constant preservation temperature and with increasing preservation temperature at constant preservation time, the mean values of relaxation time and activation energy have been found to decrease. The extent and the rate of the evolution have been compared to those in other forms of ice: ice samples with high content of disturbance, polycrystalline ice samples and single ice crystals. The observed similarities and differences in the behaviour of the different ice forms have been discussed in terms of the influence of extrinsic physical and chemical defects on the generation and motion of orientational Bjerrum defects.     

Ionic,electronic and ion-diffusion controlled relaxation processes in CaF2, BaF2 and LiBaF3 crystals

Abstract

The ionic, electronic and anion-diffusion controlled thermally stimulated relaxation (TSR) processes at 80—700 K in CaF₂ BaF₂ and LiBaF₃ crystals (X-ray irradiated or non-irradiated) have been investigated by means of ionic conductivity, ionic thermally stimulated (TS) depolarization current (TSDC); as well as current (TSC), luminescence (TSL) and bleaching (TSB) techniques. Above 250—290 K broad and overlapping anion TSDC peaks and correlated TSB stages are detected. The TSB kinetics is initiated and controlled by anion detrapping and interaction with the localized charges, i.e., the anion-diffusion controlled TSR processes take place in fluorides. The TSL and TSC data for LiBaF₃ indicate that the lifetime and drift of electrons at 80—250 K is very small because of deep retrapping. The main TSL peaks at 132K, 170K and 220 K are caused by V_k center detrapping and hole-diffusion controlled tunnel recombination within pairs like {D_n e^?…V_k }.     

Sizes and electronic polarizabilities of divalent ions in crystals

The Pauling’s form for the overlap repulsive energy is used to calculate the sizes of isoelectronic divalent ions in MgO, CaS, SrSe and BaTe crystals by minimising the next nearest neighbour repulsive interaction. The radii calculated by this method differ significantly from the conventional sets of ionic radii in a consistent way, being larger for cations and smaller for anions. The polarizability-radius cube relation is also utilized to calculate the electronic polarizabilities of ions. The electronic polarizabilities, thus obtained agree with the values determined from the refractivity measurements.     

Analysis of ionicity parameters and photoelastic behaviour of AB type crystals

Values of ionicity parameters have been obtained using the revised energy gap model due to Grimes and Cowley. We have used a power law as well as an exponential law for representing the dependences of homopolar energies on interatomic distances. Values of fractional ionic character of the chemical bond are calculated in 68 A^NB^8–N type diatomic solids. The heteropolar energy parameters have been found to exhibit systematic relationships with the interatomic separations for diatomic solids with the common cations in I–VII and II–VI crystals. The strain derivatives of electronic dielectric constant related to the photoelastic behaviour of solids have also been studied with the help of the ionicity parameters within the framework of Phillips–van Vechten dielectric theory. The results obtained from the power law and the exponential law are in close agreement with each other.     

Effect of the crystal potential on electronic polarizabilities of ions in alkaline earth chalcogenides

The electronic polarizabilities of ions in alkaline earth chalcogenides are estimated by taking account of the effect of the crystalline potential. The polarizabilities thus obtained are found to present a good agreement with experimental data. It has been shown that the polarizabilities and radii of alkaline earth and chalcogenide ions follow the polarizability-radius cube relation approximately well.     

两体和三体相互作用下的非线性Landau-Zener隧穿的圈结构研究

在加速光晶格中玻色-爱因斯坦凝聚体（BEC）,当同时考虑两体和三体相互作用时,其能级结构、隧穿率出现了独特的特性.在一维加速光晶格中的BEC,当两体和三体相互作用参数满足一定条件时,非线性两能级体系的能级结构中出现了圈结构,研究得出了圈的宽度随两体和三体相互作用参数变化的关系,并由此分析圈结构的出现及其大小对BEC隧穿率的影响.     

两体和三体相互作用下的非线性Landau-Zener隧穿的圈结构研究

在加速光晶格中玻色-爱因斯坦凝聚体（BEC）,当同时考虑两体和三体相互作用时,其能级结构、隧穿率出现了独特的特性.在一维加速光晶格中的BEC,当两体和三体相互作用参数满足一定条件时,非线性两能级体系的能级结构中出现了圈结构,研究得出了圈的宽度随两体和三体相互作用参数变化的关系,并由此分析圈结构的出现及其大小对BEC隧穿率的影响.     

Luminescence quantum yield and multiplication of electronic excitations in the corundum crystals

The excitation spectra of luminescence in irradiated and nonirradiated corundum crystals are investigated by means of highly polarised synchrotron radiation in 5 to 30 eV region. In the fundamental absorption region the double-exciton peaks are observed in the region 8.5-9.2 eV for irradiated (especially by neutrons) crystals at temperature 90 K. At 9.5 eV sharp drops appeared, in the luminescence spectrum space which were interpreted as nonirradiative near-surface recombination, the probability of which sharply increased at high absorption coefficients. The enhancement of the luminescence efficiency in the high-energy region was connected with the decay effect of electron excitations as well as with the influence of volumetric excitations leading to the enhancement of recombination glow. Received 16 October 1998 and Received in final form 20 January 1999     

Effect of deformation on the electronic energy spectrum in crystals

Electronic energy spectra in deformed crystals are studied by a perturbation method according to the scheme of Pikus and Bir. Terms of order k²e_ij are included in addition. An analytical expression is obtained for the deformation dependence of a carrier effective mass in a case of no degeneracy.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 61–64, December, 1972.     

Dynamics of entanglement in one-dimensional Ising chains with two- and three-body interactions

The evolution of entanglement in a one-dimensional Ising chain with both two-body and three-body interactions, under two types of initial states, is numerically simulated. We analyse three problems concerning the dynamics of pairwise entanglement: (i) the possibility of generating large entanglement from an initial separable state by the use of a selective irradiation scheme; (ii) the effect of three-body interaction on the generation of entanglement from an initial separable state; (iii) the effect of three-body interaction on the decay of the entanglement from a state with only (m,n)-pair maximal entangled, and the rest in product form. It is shown that a large pairwise concurrence C_mn can be obtained when the resonant, transverse radio-frequency fields are selectively switched on from the mth to nth spins. Three-body interaction will decrease the oscillation amplitude of the nearest neighbour concurrence, while the oscillation amplitude of remote pairwise concurrence will be greatly increased with the consideration of three-body interactions. For an initial (m,n)-pair maximal entangled state, a slow decay of the pairwise concurrence C_mn is found with the introduction of three-body interactions.     

Effect of three-body interactions on the formation entropy of monovacancies in copper,silver and gold

The vacancy formation entropy in Cu, Ag and Au is estimated from model calculations with explicit treatment of three-body interactions. The three-body interactions cause a rather strong relaxation around the vacancies and therefore lead to lower values for the formation entropy than usual pair-potential calculations.     

Effect of pressure on electronic and thermoelectric properties of magnesium silicide: A density functional theory study

We study the effect of pressure on electronic and thermoelectric properties of Mg_2Si using the density functional theory and Boltzmann transport equations. The variation of lattice constant, band gap, bulk modulus with pressure is also analyzed. Further, the thermoelectric properties(Seebeck coefficient, electrical conductivity, electronic thermal conductivity) have been studied as a function of temperature and pressure up to 1200 K. The results show that Mg_2Si is an n-type semiconductor with a band gap of 0.21 eV. The negative value of the Seebeck coefficient at all pressures indicates that the conduction is due to electrons. With the increase in pressure, the Seebeck coefficient decreases and electrical conductivity increases. It is also seen that, there is practically no effect of pressure on the electronic contribution of thermal conductivity.The paper describes the calculation of the lattice thermal conductivity and figure of merit of Mg_2Si at zero pressure. The maximum value of figure of merit is attained 1.83 × 10~(-3) at 1000 K. The obtained results are in good agreement with the available experimental and theoretical results.     

Effect of surface electric field on the anchoring of nematic liquid crystals

We analyse the influence of adsorbed ions and the resulting surface electric field and its gradient on the anchoring properties of nematics with ionic conductivity. We take into account two physical mechanisms for the coupling of the nematic director with the surface electric field: (i) the dielectric anisotropy and (ii) the coupling of the quadrupolar component of the flexoelectric coefficient with the field gradient. It is shown that for sufficiently large fields near saturated coverage of the adsorbed ions, there can be a spontaneous curvature distortion in the cell even when the anchoring energy is infinitely strong. We also discuss the director distortion when the anchoring energy of the surface is finite. Received: 29 September 1997 / Received in final from: 10 November 1997 / Accepted: 18 November 1997     

Effect of mean-square amplitude of atomic vibrations on the creep behaviour of cubic crystals

Correlation between the activation energyQ of high-temperature creep and the Debye-Waller thermal parameterB, which is proportional to the mean-square amplitude of atomic vibrations, has been examined in the case of 17 faced-centred cubic, body-centred cubic and diamond-structure cubic elements. It is observed thatQ is a function ofB, irrespective of the crystal structure;Q decreases asB increases. The correlation is governed by the power-lawQ=Q ₀(B/B ₀) ^M0 , whereQ ₀=0.095eV andM ₀=−0.62 are numerical constants determined by least-squares fit method, and the constantB ₀=1 nm².