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.     

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.     

The generalized pseudospectral approach to the bound states of the Hulthén and the Yukawa potentials

The generalized pseudospectral (GPS) method is employed to calculate the bound states of the Hulthén and the Yukawa potentials in quantum mechanics, with special emphasis onhigher excited states andstronger couplings. Accurate energy eigenvalues, expectation values and radial probability densities are obtained through a non-uniform and optimal spatial discretization of the radial Schrödinger equation. Results accurate up to thirteen to fourteen significant figures are reported for all the 55 eigenstates of both these potentials withn <- 10 for arbitrary values of the screening parameters covering a wide range of interaction. Furthermore, excited states as high asn = 17 have been computed with good accuracy for both these potentials. Excellent agreement with the available literature data has been observed in all cases. Then > 6 states of the Yukawa potential has been considerably improved over all other existing results currently available, while the same for Hulthén potential are reported here for the first time. Excepting the 1s and 2s states of the Yukawa potential, the present method surpasses the accuracy of all other existing results in the stronger coupling region for all other states of both these systems. This offers a simple and efficient scheme for the accurate calculation of these and other screened Coulomb potentials.     

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.     

The electronic Raman spectrum of cobalt doped MgO

The energies of the ⁴T_1g states of Co²⁺ as a dilute substitutional impurity in MgO relative to the ground Λ_6g doublet have been found by low temperature Raman spectroscopy to be two Λ_8g states at 305 ± 3 cm^?1 and 930 ± 3 cm^?1; the remaining Λ_7g energy is predicted to be in the 980–1010 cm^?1 range the corresponding Λ_6g → Λ_7g Raman transition being weak and buried in the extensive two-phonon background. A second-order perturbation calculation which couples the spin-orbit states to both E_g and T_2g modes of vibration gives a weak but important Jahn-Teller stabilization energy for the Λ_8g states.     

An electron-paramagnetic-resonance study of the dynamic Jahn-Teller effect for Sc2+ in ZnS

Electron-paramagnetic-resonance spectra for ZnS:Sc²⁺ were observed from 1.3 K to 4.2 K. The spectra are characteristic of the dynamic Jahn-Teller effect for orbital E states.     

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.     

Reverse polarized highly intense Raman transitions in Jahn-Teller and non-Jahn-Teller modes

Intensity enhancement effects in vibrational Raman spectra of electronically degenerate states are investigated. It is shown that the fundamental and overtone Raman transitions in Jahn-Teller active modes are strongly intensified via enhancement effects involving the symmetric and anti-symmetric Raman polarizability tensors. Such enhancements ensue via intensity borrowing from the Rayleigh scattering process. It is explicitly shown that the isotropic Raman polarizability tensor associated with these transitions is not subjected to enhancement effects. Thus the intense fundamental and overtone Raman transitions in the Jahn-Teller active modes will be reverse polarized (or depolarized) rather than polarized. Moreover, fundamental and overtone Raman transitions associated with vibrations which couple with Jahn-Teller active modes via mixed cross quadratic nuclear potential interactions may also be intensified and reverse polarized. The present work modifies and extends the selection rules which were established long ago by M. S. Child and H. C. Longuet-Higgins [Philos. Trans. R. Soc. London A 254, 259–294 (1961)].     

Comparison of the molecular cluster model with the phonon model for Jahn—Teller active impurities in crystals

For comparing the molecular cluster model with the phonon model in describing the Jahn-Teller interaction for magnetic impurities in crystals, the Jahn-Teller energy and the Ham reduction factors are expressed in terms of the phonon Green's function of the host crystal for an orbital triplet coupled to E_g modes of vibrations. Numerical results for the case of MgO lattice have been obtained using the phonon Green's function derived from the breathing shell model.     

Symmetry analysis of the possible low-temperature phases in KDy(MoO4)2

The possible asymmetric phases in the Jahn-Teller crystal of KDy(MoO₄)₂ are analyzed. Phases appearing upon a second-order phase transition from an orthorhombic D _2h ¹⁴ group with the unit cell volume doubling are described. It is shown that KDy(MoO₄)₂ crystals are capable of featuring the Jahn-Teller pseudoeffect caused by the interaction between the electron states of dysprosium ions and the compressive (tensile) or shear deformations.     

Assignment of the HCOOH CW-submillimeter laser

Twenty one laser lines of the 250–1000 GHz range have been assigned in the v₆ and v₈ excited states of the H¹²COOH molecule. The microwave study of these two states has allowed us to determine the molecular constants and to calculate the energy levels up to J = 50. The values of the energy levels of the ground state are well known and allow the frequency calculation of infrared rovibrational transitions near the 9.6–10.6 μm region and the comparison with the frequencies of the CO₂ laser lines. A microwave infrared double-resonance experiment has also been performed to confirm assignment based on the calculation of the energy levels. The value of the two band centers has been determined.     

Low-energy excited states of 3d transition metal ions in zinc selenide

The results of studying the impurity heat capacity of Zn_1?x M _xSe (M = Cr²⁺, Fe²⁺, Ni²⁺, Mn²⁺) solid solutions in the temperature range 1.8–20.0 K are presented. A heat-capacity method is described and applied for the measurement of the intracenter-transition energy in these systems. The role of the Jahn-Teller effect in the formation of low-energy excited states of 3d ions in ZnSe is discussed.     

Spectroscopic effects of conical intersections of molecular potential energy surfaces

A simple vibronic coupling model involving two electronic states and two vibrational modes is considered. The model is based on harmonic diabatic potentials and linear coupling of the diabatic electronic states. It is shown that the adiabatic electronic potential energy surfaces exhibit, in general, a conical intersection. The well known E × E and E × B Jahn-Teller problems are contained as special cases. Using numerical methods the optical absorption spectrum is calculated exactly. Extremely complex vibronic spectra are obtained when the conical intersection occurs within the Franck-Condon (FC) zone. The exact vibronic spectra are compared with spectra calculated in the adiabatic and FC approximation. The genuine spectroscopic effects of conical intersections are revealed by a comparison with the results of standard one-dimensional vibronic coupling calculations. The presence of a conical intersection limits the applicability of the adiabatic and FC approximations much more strongly than in the one-dimensional case. The upper adiabatic electronic state is strongly affected by non-adiabatic coupling even when the point of intersection lies outside the FC zone. The relevance of these results for the calculation of molecular electronic spectra is briefly discussed.     

Electronic density distorsion of NiO2 due to intercalation by Li

High resolution measurements of Compton profiles of Li_xNiO₂ have been carried out using 60 keV photons at ESRF (Grenoble, France). Theoretical profiles are obtained using the plane wave expansion of wave functions calculated using ab-initio SCF method. Theory and experiment are in good agreement. We have further decomposed the charge density modification into a rigid charge transfer from lithium to e_g¹ states of NiO₂ and the distorsion of NiO₂ charge density mainly due to Ni–O bond changes resulting from lattice parameter change and Jahn-Teller effect.     

Structural deformations of the cubic lattice of the Zn1 ? x Fe x Se (x = 0.001) crystal

The structural state of a Zn_{1 ? x} Fe _x Se (x = 0.001) crystal has been studied using thermal neutron diffraction. The diffraction patterns of the cubic crystal have been found to contain diffuse scattering regions concentrated in the vicinity of the strong Bragg reflections. It has been shown that the diffuse scattering effects are due to local transverse displacements of the crystal lattice atoms, and these displacements are induced by iron ions that demonstrate the static Jahn-Teller effect of the tetragonal type in the ZnSe compound.     

Jahn-teller effects in (3d)2 ions in Al2O3

APR experiments have been carried out on V³⁺ and Cr⁴⁺ ions in Al₂O₃ partly to remove discrepancies in earlier measurements, and partly to provide additional information necessary for the theoretical analysis which follows. We find |G_-| ～ 45± 10 cm^?1 and ～ 160±30 cm^?1 for V³⁺ and Cr⁴⁺ respectively. It is shown that all the data for both ions can only be explained satisfactorily if a moderately strong Jahn-Teller effect operates in the cubic ³T₁, ground state of the ion. Full details are given, γ is found to be 0.14 and 0.36 for V³⁺ and Cr⁴⁺ respectively and important contributions from second-order Jahn-Teller effects are also present. The results also show that it is necessary to use a multimode full-lattice model of the Jahn-Teller effect as the constraints of the cluster model are broken. The implications of the proposed model on thermal conductivity, phonon spectroscopy and optical measurements are also briefly discussed. It is also pointed out that some important errors exist in the literature concerning the interpretation of experimental data for these systems.     

Local and pseudolocal Jahn-Teller states of a crystal with impurity Γ8 term

The multimode Jahn-Teller effect for an impurity in the Γ₈ ground state of a double point group is considered. The vibronic coupling with the trigonal (t₂) crystal vibrations is taken into account. It is shown that the vibronic coupling leads to the appearance of local and pseudolocal electron-phonon states. The spectral and symmetry characteristics of these states are obtained.     

Jahn-Teller effects and hyperfine parameters of V4+ on tetragonally distorted tetrahedral sites

The g values and hyperfine parameters of V⁴⁺ on tetragonal sites in various host lattices are analysed in terms of a theory which includes Jahn-Teller interactions. For garnet hosts Jahn-Teller effects are weak, unlike the case for SiO₂ and synthetic zircon where Jahn-Teller quenching is significantly strong.     

Jahn-Teller phase transitions in TbcGd1−cVO4

The temperatures at which the cooperative Jahn-Teller phase transitions occur in Tb_cGd_1?cVO₄ have been determined from Raman and optical birefringence measurements. Over a narrow range of c there are two transition temperatures; between these temperatures the crystal is distorted, but a higher and lower temperatures the crystal has the same undistorted structure. This unusual behaviour was predicted earlier, and is accurately described by molecular field theory.