Isothermal decay of colour centres in microcrystalline powders of alkali bromides

The bleaching of F-centres on storing in darkness of electrolytically coloured single crystals and microcrystalline powders of solid solutions of KBr with other alkali bromides is reported. The colouration of the microcrystalline powders decays isothermally while that of single crystals is comparatively stable. The decay proceeds slower in the powders of mixed crystals than in the pure materials. A tentative explanation is suggested.     

Deformation-induced excitation of the luminescence centres in coloured alkali halide crystals

The present paper reports the deformation-induced excitation of the luminescence centres in coloured alkali halide crystals. The peaks of the mechanoluminescence (ML) in γ-irradiated KCl, KBr, KI, NaCl and LiF crystals lie at 455, 463, 472, 450 and 485 nm, i.e. at 2.71, 2.67, 2.62, 2.75 and 2.56 eV, respectively. From the similarity between the ML spectra and the thermoluminescence (TL) and afterglow spectra, the ML of KCl, KBr, KI, NaCl and LiF crystals can be assigned to the deformation-induced excitation of the halide ions in V₂-centres or any other hole centres. For the deformation-induced excitation of the halide ions in V₂-centres, or in other centres, the following four models may be considered: (i) free electron generation model, (ii) electron–hole recombination model, (iii) dislocation exciton radiative decay model and (iv) dislocation exciton energy transfer model. The dislocation exciton energy transfer model is found to be suitable for the coloured alkali halide crystals. According to the dislocation exciton energy transfer model, during the deformation of solids the moving dislocations capture electrons from the F-centres and then they capture holes from the hole centres and consequently the formation of dislocation excitons takes place. Subsequently, the energy released during the decay of dislocation excitons excites the halide ions of the V₂-centres or any other hole centres and the light emission occurs during the de-excitation of the excited halide ions, which is the characteristic of halide ions. The mechanism of ML in irradiated alkali halide crystals is different from that of the TL in which the electrons released form F-centres due to the thermal vibrations of lattices reach the conduction band and the energy released during the electron–hole recombination excites the halide ions in V₂-centres or in any other hole centres. It is shown that the phenomenon of ML may give important information about the dislocation bands in coloured alkali halide crystals.     

The thermoelastic behaviour of the orientationally disordered ionic crystals in two dimensions

The anomalous temperature dependence of the elastic constantsc ₁₁ andc ₆₆ and the elastic instability temperatureT _c are obtained in the two-dimensional microscopic model of the alkali cyanides crystals. It has been found, by means of the Michel and Naudts' theory of the translation rotation coupling, that the orientational disorder in the high-temperature phase leads to the critical softening of the shear elastic constantc ₆₆.     

Possibility of elastico-mechanoluminescence dosimetry using alkali halides and other crystals

The elastico-mechanoluminescence (EML) intensity of X or γ-irradiated alkali halide crystals can be used in radiation dosimetry. The EML intensity of X or γ-irradiated alkali halide crystals increases linearly with the strain of the crystals, and when the crosshead of the testing machine deforming an X or γ-irradiated crystal is stopped, then the EML intensity decreases with time. The semilog plot of the EML intensity versus (t − t_c) (where t_c is the time where the crosshead of the testing machine is stopped) indicates that, in the post-deformation region, the EML intensity initially decreases exponentially at a fast rate and later on it decreases exponentially at a slow rate. The EML intensity increases linearly with the density of the F-centres in the crystals. This fact indicates that elastico-ML can suitably be used for the radiation dosimetry. The EML spectra of X or γ-irradiated alkali halide crystals are similar to their thermoluminescence spectra. Based on the detrapping of electrons during the mechanical interaction between the dislocation segments and F-centres, an expression is derived, which indicates that the EML intensity should increase linearly with the density of F-centres in the crystals. The expression derived for the decay of EML indicates that the decay time for the fast decrease of EML should gives the pinning time of dislocation segments (lifetime of interacting F-centres), and the decay time for the slow decrease of EML intensity should gives the lifetime of electrons in the shallow traps. As the elastic deformation is non-destructive phenomenon and the EML intensity depends on the radiation dosage given to the alkali halide crystals, similar to the thermoluminescence and photo-stimulated luminescence, the EML of alkali halide crystals and other crystals may be used for the radiation dosimetry. In EML dosimetry, the same crystal can be used number of times because the elastic deformation does not cause permanent deformation in the crystals, and moreover, comparatively the devices needed for the EML measurements are of low cost and very simple. In recent years, a large number of elastico mechanoluminescent materials have been investigated, and the study of their suitability for the radiation dosimetry may be interesting.     

Mechanoluminescence response to the plastic flow of coloured alkali halide crystals

The present paper reports the luminescence induced by plastic deformation of coloured alkali halide crystals using pressure steps. When pressure is applied onto a γ-irradiated alkali halide crystal, then initially the mechanoluminescence (ML) intensity increases with time, attains a peak value and later on it decreases with time. The ML of diminished intensity also appears during the release of applied pressure. The intensity I_m corresponding to the peak of ML intensity versus time curve and the total ML intensity I_T increase with increase in value of the applied pressure. The time t_m corresponding to the ML peak slightly decreases with the applied pressure. After t_m, initially the ML intensity decreases at a fast rate and later on it decreases at a slow rate. The decay time of the fast decrease in the ML intensity is equal to the pinning time of dislocations and the decay time for the slow decrease of ML intensity is equal to the diffusion time of holes towards the F-centres. The ML intensity increases with the density of F-centres and it is optimum for a particular temperature of the crystals. The ML spectra of coloured alkali halide crystals are similar to the thermoluminescence and afterglow spectra. The peak ML intensity and the total ML intensity increase drastically with the applied pressure following power law, whereby the pressure dependence of the ML intensity is related to the work-hardening exponent of the crystals. The ML also appears during the release of the applied pressure because of the movement of dislocation segments and movements of dislocation lines blocked under pressed condition. On the basis of the model based on the mechanical interaction between dislocation and F-centres, expressions are derived for the ML intensity, which are able to explain different characteristics of the ML. From the measurements of the plastico ML induced by the application of loads on γ-irradiated alkali halide crystals, the pinning time of dislocations, diffusion time of holes towards F-centres, the energy gap E_a between the bottom of acceptor dislocation band and the energy level of interacting F-centres, and work-hardening exponent of the crystals can be determined. As in the elastic region the strain increases linearly with stress, the ML intensity also increases linearly with stress, however, as in the plastic region, the strain increases drastically with stress and follows power law, the ML intensity also increases drastically with stress and follows power law. Thus, the ML is intimately related to the plastic flow of alkali halide crystals.     

Isothermal stability of colour centers in microcrystalline powder of NaCl:KCl

A correlation between thermoluminescence and isothermal decay of colouration in NaCl:KCl microcrystalline powder is reported. No isothermal bleaching of colouration is observed for all the compositions. However, a rapid isothermal decay is seen in K₉₅N₅Cl and K₉₀Na₁₀Cl, when the first of the two glow peaks is thermally bleached. It is suggested that the interaction between colour centers and dislocations is inhibited upon mixing.     

Pecularities in Optical Properties of ns2 Ions

The behavior of an octahedral center created in alkali chloride and bromide single crystals by doping with divalent lead is analyzed using the experimental results from absorption and luminescence measurements. We focus on the questions of local distortion both in ground and excited states, the regularities in A_T and A_X emissions and differences in the decay shape of high energy emission.     

Nuclear spin-electron coupling of copper at microkelvin temperatures

The nuclear spin-electron coupling of copper has been determined by establishing stationary temperature differences between nuclei and electrons at electronic temperatures of 15 K to 5 mK in magnetic fields of 2 mT to 440 mT using a well-defined heat flow from the electrons to the nuclear spin system. The measured Korringa constantk increases proportional toB/T _e ² with decreasing electronic temperatureT _e and with increasing magnetic fieldB. At the lowest temperaturesk is more than an order of magnitude larger than its high temperature value.     

Effect of dislocation velocity on the nuclear spin relaxation

Using the Jeener pulse sequence, dislocation motion at various velocities and temperatures in alkali halide single crystals was studied by measuring the resultant variation in the zero-field relaxation time T₁₀, which is known to be influenced by slow atomic movements.It was found that T₁₀ is strongly dependent on the plastic deformation rate g?3 — although not on the plastic strain ε. The theory of Rowland and Fradin for atomic diffusion is used as a basis for an explanation of the results of this new type of experiment.     

On the relaxation behaviour of a cluster glass aboveT f

The cluster glass Y(Fe _x Al_1−x )₂,x=0.75, 0.65 and 0.40 was investigated in external fields between 4.5 and 13.5 T at temperatures above the freezing temperatureT _f. To explain the relaxation effects, which dominate the magnetic behaviour up toT∼7·T _f, a model is proposed, where magnetic clusters are formed stochastically and decay after a mean life time.     

The superconducting transition temperature of Pb1?xMnx films

Results of the superconducting transition temperatureT _c of amorphous and microcrystalline films of lead doped with manganese as magnetic impurity are reported in this work. The amorphous films show an Abrikosov-Gor'kov behaviour, whereas for the crystalline films there is a much smaller depression and a peak for higher Mn concentrations, which indicates a region of coexistence of superconductivity and magnetic ordering as a spinglass.     

Dislocation unpinning model of acoustic emission from alkali halide crystals

The present paper reports the dislocation unpinning model of acoustic emission (AE) from alkali halide crystals. Equations are derived for the strain dependence of the transient AE pulse rate, peak value of the AE pulse rate and the total number of AE pulse emitted. It is found that the AE pulse rate should be maximum for a particular strain of the crystals. The peak value of the AE pulse rate should depend on the volume and strain rate of the crystals, and also on the pinning time of dislocations. Since the pinning time of dislocations decreases with increasing strain rate, the AE pulse rate should be weakly dependent on the strain rate of the crystals. The total number of AE should increase linearly with deformation and then it should attain a saturation value for the large deformation. By measuring the strain dependence of the AE pulse rate at a fixed strain rate, the time constantτ _s for surface annihilation of dislocations and the pinning timeτ _p of the dislocations can be determined. A good agreement is found between the theoretical and experimental results related to the AE from alkali halide crystals.     

Some properties of a-C:N films prepared by KrF laser ablation

Thin a-C:N films were prepared by pulsed laser ablation using a KrF excimer laser and a graphite target. The influence of the substrate temperatureT _s (27°C and 330°C), and the nitrogen gas ambient pressurep n (1.5 Pa, 13.3 Pa and 40 Pa) on the film properties was studied. The films were characterized by scanning electron microscopy, Rutherford backscattering, spectroscopic ellipsometry and by Raman scattering. The nitrogen content in the films was in the range from 20 to 31 at.%, and those of oxygen from 5.6 to 10.7 at.%. For lowerp n andT _s the film delamination was observed.The authors wish to thank J. Sobota from the Institute of Scientific Instruments of ASCR, Brno, for valuable discussions.This work was supported by Grant Agency of the Academy of Sciences of the Czech Republic under Grant No. 202/93/0464.     

On the pressure dependence of the effective ionic charge of cesium halides

The importance of temperature effects in the use of the second Szigeti relation to calculate the volume dependence of the effective ionic charge of cesium halide crystals is studied. The effect obtained is larger than the one for alkali halide crystals of the NaCl structure. Positive values of (? ln s/? In v) are obtained using low temperature experimental data with the Grüineisen parameter γ_t(T → 0K) calculated using the generalized first Szigeti relation. Values of γ_t are also obtained with a rigid ion model and they differ little from the previous ones.     

Analyticity of correlation functions for the two-dimensional Ising model

Analyticity of correlation functions for the two-dimensional Ising model as a function of the inverse temperature except for the singularity at the critical temperature is proved. A crucial step is the establishment of the correspondence between extremal equilibrium states of the model and pure ground states of a one-dimensional spin system below the critical temperatureT _c . An exact decay rate of the clustering property along axes is also determined for allTT _c .     

Theory of melting,vacancy model

A theory of melting based on vacancy model is formulated. The polymer solution theory is used for derivation of the melting equation for a two-species model of melting solid. Under simplifying assumptions the analysis leads to a simple correlation betweenT _m and 〈v〉, the average energy of interaction between the vibrating atoms. Pseudopotential method is used for calculating 〈v〉 for the alkali metals lithium, sodium, potassium and rubidium at temperatureT _m. The calculated values ofT _m〈v〉 are in accord with those expected from our model. Application to the high pressure melting curves of solids is also discussed.     

Exact quantum partition function of the BCS model

The exact calculation of the reduced BCS model quantum partition function (QPF) in the thermodynamic limit is carried out by the path integration method. The expression for the QPF and the phase transition temperatureT _c in the regular phase coincide with the results of Bogolyubov. In the nonregular phase a temperature singularity appears in the expression for the QPF: the QPF diverges in the region of temperaturesT _c which are smaller than some critical temperatureT _c ^* , and it turns out that in all casesT _c ^* > T _c and the differenceT _c ^* –T _c is not small. The interpretation of the temperatureT _c ^* is given.     

Correlation between deformation bleaching and mechanoluminescence in coloured alkali halide crystals

The present paper reports the correlation between deformation bleaching of coloration and mechanoluminescence (ML) in coloured alkali halide crystals. When the F-centre electrons captured by moving dislocations are picked up by holes, deep traps and other compatible traps, then deformation bleaching occurs. At the same time, radiative recombination of dislocation captured electrons with the holes gives rise to the mechanoluminescence. Expressions are derived for the strain dependence of the density of colour centres in deformed crystals and also for the number of colour centres bleached. So far as strain, temperature, density of colour centres, E _a and volume dependence are concerned, there exists a correlation between the deformation bleaching and ML in coloured alkali halide crystals. From the strain dependence of the density of colour centres in deformed crystals, the value of coefficient of deformation bleaching D is determined and it is found to be 1.93 and 2.00 for KCl and KBr crystals, respectively. The value of (D+χ) is determined from the strain dependence of the ML intensity and it is found to be 2.6 and 3.7 for KCl and KBr crystals, respectively. This gives the value of coefficient of deformation generated compatible traps χ to be 0.67 and 1.7 for KCl and KBr crystals, respectively.     

Optical absorption spectrum of Co2+ in ammonium perchlorate single crystal

Single crystals of cobalt — doped ammonium perchlorate were grown at room temperature. The electronic absorption bands observed at room and liquid air temperatures have been assigned transitions from the ground⁴T₁(F) state to the excited⁴T₂(F),⁴A₂(F),²T₁(G) and⁴T₁(P) states. The crystal parameters derived areDq=880 cm^–1,B=865 cm^–1 andC= 4·63B.The authors wish to express their thanks to the authorities of the Indian Institute of Science, Bangalore for kind permission to use their spectrophotometer. Two of the authors (B. C. V.Reddy and J. L.Rao) express their greatful thanks to the council of scientific and Industrial Research (New Delhi) for financial assistance.     

Failure of reflection positivity in the quantum Heisenberg ferromagnet

We show that reflection positivity does not hold in the finite-volume spin 1/2 quantum Heisenberg ferromagnet in two cases: (i) for any dimension or degree ofS ¹-invariant anisotropy, above a volume independent temperatureT ₀, and (ii) for the isotropic model in one dimension, below a volume dependent temperatureT ₁.