The calculation of fractional parentage coefficients for many-boson systems

A second quantised operator approach has been used to calculate formulae for one-body fractional parentage coefficients (fpc) for a system of many spinless bosons. A number of examples have been discussed and compared with some of those already calculated by a different technique when used to simplify Jahn-Teller problems in atomic physics. It has been shown explicitly that this procedure may be generalised to find two-boson fpc which are useful when calculating matrix elements of two-body boson operators.     

Jahn-Teller effect of K atoms isolated in Ar matrices

Optical absorption spectra have been observed for K in Ar matrices in the temperature range 9–30 K. Some absorption lines disappear irreversibly at annealing temperatures below 12 K. The major part of the remaining lines shows reversible changes with temperature. The lineshapes of the so-called blue and red triplets can be quantitatively described in terms of the dynamic Jahn-Teller effect.     

Dynamic Jahn-Teller effects in isolated C60- studied by near-infrared spectroscopy in a storage ring

We have measured the near-infrared absorption spectrum for isolated C60- ions at room temperature. Two bands, at 9145 cm(-1) and 10460 cm(-1), have been identified in addition to the main absorption band at 9382 cm(-1), seen also at low temperature in a matrix. An interpretation based on the theory of dynamic Jahn-Teller effects is proposed.     

Cooperative Jahn-Teller effect in the mixed crystal system (Tbx,Tm1−x)AsO4

Single crystals of the system (Tb_x,Tm_1?x)AsO₄ have been investigated by means of optical absorption spectroscopy, birefringence and specific heat. Over the entire concentration range we observe a cooperative Jahn-Teller distortion. The phase transition may be approximated in mean field theory, taking into account that due to local strains part of the Tb and Tm ions are rendered Jahn-Teller inactive, an effect which is much more pronounced on the Tm (x ? 0) than on the Tb (x ? 1) side.     

Phase transitions in crystals with competitive Jahn-Teller subsystems

A statistical thermodynamic model of phase transformations caused by the cooperative Jahn-Teller effect in spinels containing two types of Jahn-Teller cations has been proposed. It has been shown that the specific features of the phase diagrams of compounds similar to Cu_{1 ? x} Ni _x Cr₂O₄, such as the presence of one orthorhombic and two anti-isostructural tetragonal phases, are associated with the competition between orderings of the subsystems of tetrahedra distorted due to the Jahn-Teller effect. The degree and character of the ordering of each subsystem of the solid solution have been determined. The main factors responsible for the thermodynamic properties and phase diagrams, in particular, the conditions for splitting of an isolated critical point into two triple points, have been investigated.     

Direct observation of high-temperature polaronic behavior in colossal magnetoresistive manganites

The temperature dependence of the electronic and atomic structure of the colossal magnetoresistive oxides La1-xSrxMnO3 (x=0.3, 0.4) has been studied using core and valence level photoemission, x-ray absorption and emission, and extended x-ray absorption fine structure spectroscopy. A dramatic and reversible change of the electronic structure is observed on crossing the Curie temperature, including charge localization on and spin-moment increase of Mn, together with Jahn-Teller distortions, both signatures of polaron formation. Our data are also consistent with a phase-separation scenario.     

Thermodynamic design data for absorption heat pump systems operating on ammonia-sodium thiocyanate—I. Cooling

The free choice of operating temperatures in absorption systems is limited by the Gibbs phase rule and the thermodynamic properties of the working pair. Tables of possible combinations of operating temperatures and concentrations, including flow ratios, Carnot coefficients of performance and enthalpy based coefficients of performance have been presented for ammonia-sodium thiocyanate absorption systems for cooling. The interaction of operating temperatures has been illustrated graphically.     

Thermodynamic design data for absorption heat pump systems operating on ammonia-sodium thiocyanate—II. Heating

The free choice of operating temperatures in absorption systems is limited by the Gibbs phase rule and the thermodynamic properties of the working pair. Tables of possible combinations of operating temperatures and concentrations, including flow ratios, Carnot coefficients of performance and enthalpy based coefficients of performance have been presented for ammonia-sodium thiocyanate absorption systems for heating. The interaction of operating temperatures has been illustrated graphically.     

Polaron effects on the optical absorption coefficients and refractive index changes in a square quantum well

The linear and nonlinear optical absorption coefficients and refractive index changes are obtained by using the compact density-matrix approach and an iterative procedure. With typical semiconducting GaAs materials, the linear, third-order nonlinear, total optical absorption coefficients and the optical refractive index have been examined. We find that the polaron effect has an important influence on the linear, third-order nonlinear, and total absorption coefficients as well as the refractive index changes.     

p型GaAs的远红外波段光学特性

砷化镓(GaAs)是太赫兹波段半导体异质结构激光器的重要材料之一,为了获得p型GaAs材料在远红外波段的光学特性,采用气态源分子束外延(GSMBE)技术在半绝缘GaAs(100)衬底上生长了掺Be的p型GaAs薄膜材料,其载流子浓度从1.54×1015~1.85×1019cm-3。用远红外变换傅里叶光谱仪测量了其远红外反射光谱,并对反射光谱进行了理论模拟和分析,计算得出了不同空穴浓度的p型GaAs在远红外波段的折射率、消光系数和吸收系数。发现在这一波段消光系数和吸收系数均随着载流子浓度的增加而增大,吸收系数最大值可达到4.0×104cm-1。     

Thermodynamic design data for absorption heat pump systems operating on ammonia-sodium thiocyanate—III. Simultaneous cooling and heating

The free choice of operating temperatures in absorption systems is limited by the Gibbs phase rule and the thermodynamics propertie of the working pair. Tables of possible combinations of operating temperatures and concentrations, including flow ratios, Carnot coefficients of performance and enthalpy based coefficients of performance have been presented for ammonia-sodium thiocyanate absorption systems for simultaneous cooling and heating. The interaction of operating temperatures has been illustrated graphically.     

Extended perturbation formulae for Jahn-Teller molecules

The effect of the Jahn-Teller interaction on a symmetric top molecule with a threefold axis of symmetry in a ^2S + 1E state has been investigated by perturbation theory. Contributions up to sixth order are included. Explicit formulae for various quantities have been derived on the assumption that there is only one Jahn-Teller active mode of vibration; both linear and quadratic Jahn-Teller interactions are considered. The quantities concerned are (i) the vibronic energy levels, (ii) the orbital quenching factor d_t, (iii) the correction to the A-rotational constant, (iv) the correction to the spin-spin dipolar coupling term, and (v) the correction to the spin-rotation coupling constant ε_aa. Because the perturbation expansion converges slowly, the results are only applicable to molecules subject to a weak Jahn-Teller effect. There are several examples of this type of molecule which have been studied experimentally.     

Luminescence depolarization mechanisms for color centers with the luminescence peak at 670 nm in gamma-irradiated LiF:Mg crystals

Processes of tunneling and thermal depolarization of luminescence of color centers with two absorption bands at 385 and 560 nm and the emission band peaked at 670 nm were studied in gamma-irradiated LiF:Mg crystals. As was previously concluded, relaxed excited states of the center have trigonal symmetry and are characterized by four equivalent trigonal Jahn-Teller minima corresponding to the C ₃ crystallographic axes. The luminescence depolarization observed at 110–120 K results both from the thermally stimulated transition and the tunnel transition of the system between the Jahn-Teller minima. Temperature dependence of the polarization degree was calculated and compared with experimental data.     

Positron transmission and effective mass absorption coefficient in nickel

The penetration of positrons from a²²Na source into Ni foils sandwiched between different backscattering materials (Kapton, NaCl, Ni, and Mo) has been studied for the geometry commonly used in PAT experiments. Transmitted intensities and effective mass absorption coefficients have been found and the dependence on the backscattering material is discussed. General formulas for the effective mass absorption coefficients are derived. The large geometrical effects are stressed. Suggestions for source corrections are supplied.     

Magnetic phase diagram of a random mixture of Jahn-Teller ions

A magnetic model of a random mixture containing Jahn-Teller ions, in which several kinds of exchange interactions between the Jahn-Teller ions and between the Jahn-Teller and non-Jahn-Teller ions are assumed, is proposed and studied by using the distribution function method. It is shown that a spin glass phase occurs when these exchange interactions have different signs. Our model would be applicable to a mixture Rb₂Cr_xMn_1-xCl₄ in which the spin glass phase has been observed.     

Data analysis of z-scan experiment using Fresnel–Kirchoff integral method in colloidal TiO2 nanoparticles

In this study, a precise method to evaluate nonlinear optical absorption and refraction of materials using z-scan method based on Fresnel–Kirchhoff integral method (FK method) has been offered. The real electric field of a Gaussian beam passing through a nonlinear sample having both nonlinear absorption and refraction has been investigated using FK method. Subsequently, the z-scan curves have been studied. This is the first time that FK method has been used for calculating the nonlinear absorption coefficient. Additionally, an appropriate numerical curve-fitting method for calculating the nonlinear optical coefficients based on z-scan method has been suggested. Z-scan curves and nonlinear optical coefficients have been obtained for TiO₂ nanoparticles in CW irradiation regime with the particle size ranges from 70 to 90 nm. This is the first experimental study which uses this analytical numerical method. Finally, all calculated results extracted from this new method have been compared with those of the previous methods.     

Static and Dynamic Jahn-Teller Distortions in CMR Manganites: A Raman Spectrometric Study

Jahn-Teller distortions in CMR manganites cause large changes of the scattering efficiencies and line frequencies of various phonon modes in Raman spectra. The effect can be separated from other structural changes, such as variations of the tolerance factor. The Jahn-Teller distortions can be quantitatively correlated with structural distortions as measured from diffraction data. The behaviour of both, static and dynamic distortions has been investigated. It is shown that Raman spectrometry allows to measure variations of the Jahn-Teller distortion on a local scale with a remarkable accuracy.     

Energy Spectrum and Its Pressure-Induced Shift and J-T Effect of ZnTe:Mn2+

By using strong-field scheme, the complete d⁵ energy matrix with symmetry has been constructed. Then, by diagonalization of this matrix at normal and various pressures,the whole energy spectrum [including the ground-state zero-field splitting (GSZFS)] and its pressure-induced shift (PS) of Znlb:Mn²⁺ have uniformly been calculated. The results are in very good agreement with experimental data. According to the eigenfimctions and PS, the assignments of four absorption bands have been given. By taking into account the dect of different deformations of t₂ and e radial wavefunctions on t₂³(⁴A₂)e²(³A₂)⁴ A₁ and t₂³(²E)e²(³A₂)⁴E, the position of the third absorption band at normal pressure has been estimated.The tetragonal field is important for GSZFS of ZnTe:Mn²⁺ and its PS, which supports the existence of tetragonal Jahn-Teller distortion in ZnTe:Mn²⁺ The physical essentials of typical levels, GSZFS and their PS have been revealed.     

Lattice,electrical, and magnetic effects in lanthanum manganites La1−x Sr x MnO3 (x = 0.125, 0.150, 0.175)

The temperature dependences of the velocities of transverse and longitudinal hypersound in lanthanum strontium manganites of the composition La_1−x Sr _x MnO₃ (x = 0.125, 0.150, 0.175) have been measured at frequencies of 0.5–0.7 GHz. The structural phase transitions have been revealed, and their positions have been confirmed by data on the electrical resistance and magnetic measurements performed using the same samples. The results obtained have been analyzed in the framework of the model of competing Jahn-Teller distortions and magnetic ordering. The anomalies observed in the behavior of the velocities of longitudinal hypersound have been attributed to the local Jahn-Teller distortions, and their suppression due to the magnetic ordering has been considered a possible factor responsible for the colossal magnetoresistance.     

A method of moments for the optical Jahn-Teller problem

A Method of moments is presented for optical transitions to Jahn-Teller states. The Condon and a modified Born-Oppenheimer approximation have been adopted. On the basis of these 2 principles, the method is exact. Specific orthonormality and closure properties are derived for Jahn-Teller states, which are essential for performing the calculation. As illustrative examples the pure Jahn-Teller casesE-e andT-t have been calculated in the strong coupling limit. Each of these cases leads to specific combinatorial problems and to an application of Wick's theorem. The latter is common to both cases. Comparison with the semi-classical calculation reveals the identity of the results for theE- e system.