Jahn-Teller Centers Research Articles

Photoelectric properties of Pb1?x?y Sn x Ge y Te:In epitaxial films

Pb1−x−ySnxGeyTe:In epitaxial films are examined in a wide temperature interval and at various background fluxes. These films have high sensitivity to infrared radiation in the spectral range λ<20μm. The lifetime depends exponentially on temperature and varies from several seconds at T=10 K to 10−2 s at T=20 K. The two-electron model of Jahn-Teller centers is proposed to explain the results. Multielement photoresistors based on these films are fabricated and D*=1.7×1013 cm Hz1/2 W−1 at T=25 K is achieved. Noise of the photoresistors is independent of background flux when it varies from 1012 cm−2 s−1 to 1018 cm−2 s−1. As compared with Si:Ga and Ge:Hg photoresistors, the responsitivity is several orders larger at the operating temperature 25–30 K.

Photoelectric characteristics of materials Pb1-xSnxTe〈In〉 subject to microwave and infrared emission

Current/voltage (CV) characteristics of solid solutions Pb1−xSnxTe〈ln〉 (with D. 22≤x≤3. 24 and 0.2≤NIn≤1.0 atomic percent) were studied at He temperatures, in presence of background at 300K. Also the photoconductivity of these materials was measured in conditions of background screening and illumination at wavelengths 100µm and 2mm. CV-characteristics are interpreted within a model of two-electron capturing by Jahn-Teller centers. Photoconductivity in millimeter and submillimeter wavebands, having relaxation times less than 10−5 sec, is related with heating of electronic gas in process of electromagnetic power absorption by the free carriers.

Photomagnetic effect in the CdCr2Se4 ferromagnetic semiconductor.

The influence of infrared irradiation on the ferromagnetic resonance (FMR) of CdCr 2 Se 4 single crystals has been investigated in the temperature range 4.2-130 K. It was found that the irradiation induced appreciable changes in the microwave conductivity, FMR line shape, FMR linewidth, and magnetocrystalline anisotropy of this material. These findings are discussed by taking into account three different mechanisms: the slow relaxation mechanism, the appearance of low-symmetry fields at the Cr 2+ Jahn-Teller centers, and the skin effect

Theoretical status of diamond and its defects, excited states and atomic motion

The key technical problems for diamond concern control: controlled growth (quality, composition, shape and texture), controlled doping (lasing species, electrically active species, redistribution and anneal of implanted species) and controlled interfaces (contacts, passivation, adhesion and friction). Theory has contributed to all these. Here I shall concentrate on electronic and motional aspects of defects. Several key phenomena involve features of the excited states, and I emphasize what can be obtained from the wealth of optical data. For diamond, optical data—involving a bound excited state—are especially fruitful; one feature to emerge is the strength and nature of the Jahn-Teller effect, which is both observed and measured through the satellite lines that it produces, so that the neutral vacancy is one of the best-understood Jahn-Teller centres. Three further features follow from examination of other properties. The first is the role of electron-electron correlation. The second is the question of whether all asymmetric defects are indeed examples of the Jahn-Teller effect. Isolated nitrogen is a key case, because of the families of nitrogen-related defects in diamond, and the important recent results of the Exeter group conclude, probably rightly, that wholly different mechanisms are involved. Thirdly, the nature and contributions to the energy in diamond and in ZnSe can be usefully contrasted, with again different reasons for the asymmetry of the singly charged cation vacancy.I shall also comment on some of the more general aspects of defect phenomena, including diffusion and reorientation behaviour. In particular, the motion of hydrogen and the muon studies need special comment, since it is likely that the diffusion mechanism is not simple classical diffusion (although this cannot yet be ruled out; further, other classical diffusion channels are available) and is perhaps closer to mechanisms now accepted for diffusion in of hydrogen in metals and indeed to reorientation of the V− centre in ZnSe.

Possible channel of carrier pairing around Jahn—Teller center

In CuO planes the p-band carriers change the initial tetragonal crystal field of undopant crystal and induce low symmetrical fields which change the energies of pseudodegenerate d states of Cu ions. The lowering of d-electron energy when two carriers are localized nearby the same Cu ion may appear to be greater than the stabilization energies of two isolated carriers. The resulting effective attraction between carriers is examined in the crystal field approximation. The situation in high- T c superconductors is shown to favor the pairing due to the dopant induced stabilization through renormalization of dd splitting.

Effects of vibrational intercentre interaction in a trigonal two-centre system with twofold electronic degeneracy at each centre

The effect of intercentre interaction (via vibrations and electron-electron coupling) on the magnetic and magnetic resonance characteristics of a pair of Jahn-Teller centres is investigated. In the limiting case of strong vibronic coupling when the splitting of degenerate electronic terms is sufficiently large the exchange Hamiltonian for the pair has the Heisenberg form. The energy spectrum of spin states of the pair system is obtained and the temperature dependence of the effective magnetic moment is investigated. It is shown that under some conditions this temperature dependence has an anomalous nonmonotonic character. The g factors of the two-centre Jahn-Teller system are obtained and the changes of the EPR spectrum with the exchange and vibrational coupling constants is analysed. With inclusion of only the exchange interaction, the pair spectrum possesses an additional isotropic resonance placed in the middle of the known broadened EPR spectrum of single centres with the Jahn-Teller effect for an orbital E term. The vibrational interaction between the centres results in a radical transformation of the spectrum, which in this case contains only an isotropic resonance line.

Phase transitions in systems with reorienting impurity centres

Phase transitions induced by the quenching of reorientations in a system of impurity centres with adiabatic potentials having several equivalent minima are discussed. In addition to inter-centre coupling the transition temperature depends on the interaction between the impurity and the crystal. The interaction of the impurity electronic states with the crystalline vibration causing the formation of the minima of the adiabatic potential also modifies the density of the crystal vibrations. The latter depends on the impurity reorientation which causes the essential change of the total energy of the crystalline vibrations in the phase transition. The system of Li+-KTaO3 and Jahn-Teller centres in twofold-degenerate electronic states are considered. The possibility of the formation of a Jahn-Teller glass phase is also discussed.

Magnetoelastic properties of crystals with Jahn-Teller centers

A theory describing the contribution of Jahn-Teller (JT) ions of the iron group to the magnetoelastic properties of crystals has been developed. It is shown that at low temperatures the magnitude of the effect is determined by the dispersion of random low-symmetry fields in the system. A comparison is made of theoretical results with experimental data (for ferrites with Fe2+, Mn3+, Cu2+, Co2+ and Ni2+ ions) and their qualitative correspondence is demonstrated.

Origin of the noncubic anisotropy in Mn∶ (Y, Gd)3Fe5O12 thin films

Thin Mn∶ YIG films exhibit a large noncubic anisotropy. We show that this anisotropy may be understood on the basis of a strong Jahn-Teller effect of octahedrally coordinated Mn3+ ions. The corresponding mechanism is simple — the strain caused by the film/substrate misfit orders the local Jahn-Teller distortions and, as a consequence, noncubic anisotropy appears. In particular, due to the nonparallel local coordinate systems of octahedral sites, an uniaxial anisotropy is predicted also for the (111) growth plane. It is pointed out that the interaction between the Jahn-Teller centres and/or their interaction with the lattice defects lower the magnitude of the anisotropy. The temperature dependence of the uniaxial anisotropy was measured on the (001) and (111) oriented films using the FMR method. The results are in qualitative agreement with the prediction of theory. Quantitative comparison of theory with the experiment is difficult, as little is known about the distribution of the strains due to the lattice defects and about the interaction between the Jahn-Teller centres.

Low-energy spectrum of the Jahn-Teller impurity centres. I. Light and sound scattering as a method of determination of the Jahn-Teller impurity parameters

A method is suggested for the determination of the reduction factors from the relative intensity of the Rayleigh contribution to the non-resonant Raman scattering. The possibility of determining the reduction factors from the integral intensities of magnetic circular dichroism is also considered. The expressions for the mean values of the symmetrised displacements of the number of virtual phonons coupled with the Jahn-Teller impurity centre and the ground-state energy are obtained in terms of the light scattering cross section for an arbitrary strength of vibronic coupling. It is shown that elastic sound scattering experiments give the possibility of determining the vibronic coupling constant and parameters characterising the variation of elastic constants of impurity. The general relations for the reduction factors are examined and the analytical expressions for the multimode reduction factors in limiting case of weak and strong coupling are investigated.

The methastable electronic states in Pb1−xSnxTe alloys

Abstract The long term relaxation processes (LTP) induced by magnetic field, infrared irradiation and strong electric field in PbSnTe(In) alloys have been investigated. The existence of the LTP at temperatures lower than 20K is due to the separation of localized states of charged carriers from band ones by a barrier W. The observed phenomena are interpreted in terms of Jahn-Teller center.

Ham reduction in a molecular-field theory for the cooperative Jahn-Teller effect of coupled E⊗ε centres

The influence of Ham reduction on the cooperative Jahn-Teller effect is investigated by studying a model system consisting of identical linear E(X) epsilon pseudo Jahn-Teller centres with antiferrodistortive interaction. For this model a molecular-field theory is worked out based upon the cluster model approach. In contrast with the displaced phonon method the present theory retains the local dynamics of each centre, while it leads to virtually the same effective interaction between pseudo-spins at different centres as that method, but without requiring self-energy corrections. The model exhibits a paradistortive/pseudo-spin flop phase transition if the electronic splitting of the E doublet is sufficiently small. Qualitatively, no differences from the predictions of the displaced phonon method are found, with the exception that the pseudo-spin does not saturate in the ordered phase at T=0. However, numerical calculations for weak to intermediate Jahn-Teller coupling show that the inclusion of local vibronic coupling has very significant quantitative effects. The critical temperature and critical electronic splitting are found to be reduced from their electronic value and to depend on both the Jahn-Teller coupling strength and the interaction strength.

Jahn-Teller effect in crystals: Optical response function and vibronic states

We present a method for evaluating the optical spectrum of a Jahn-Teller centre in a crystal. We are able to obtain an approximated, but not perturbative, expression for the optical response function of the system in the weak coupling case. All cases of electronic degeneracy are treated together. The general formulae giving the zero-phonon line and the one- and two-phonon bands are obtained in the two cases of Jahn-Teller interaction acting when the electron is in the ground or in an excited level. In particular, the contribution of the vibronic states as peaks in the one-phonon band is explicitly shown.

Absorption line shape of a dynamical Jahn-Teller Center. Strong-interaction limit

The dynamic Jahn-Teller effect, induced in a localized center by light absorption and due to a singlet-multiplet electronic transition, is studied in the strong-interaction limit (SIL) of the electron-phonon interaction. The absorption line shape $R(\ensuremath{\omega})$ is examined in the framework of the linear-response theory and the role of the host crystal in fading the correlations among the phonons associated to the electronic excitation is taken into account. $R(\ensuremath{\omega})$ is deduced by using the Feynman-paths method and the most significant contribution from each path, when working in the SIL, is identified. It is shown that the so-called semiclassical model (SCM) corresponds to considering only the Markovian many-phonon processes in the motion of the crystal phonons coupled to the Jahn-Teller excited electron. It is also shown why a refined version of the SCM leads to an effective Hamiltonian, where the adiabatic potential energy is related only to a few interaction modes and effective frequencies. In particular, the interaction modes are found to be stochastic, and not dynamical, variables. A correction to the well-known SCM Toyozawa-Inoue expression is found and discussed. Finally, the phonon quantum effects on $R(\ensuremath{\omega})$ in the SIL and beyond the SCM are introduced and commented on.

Lattice dynamics of KI perturbed by Jahn-Teller centres

Lattice dynamics of KI perturbed by Jahn-Teller centres

Optical response of ferroelectrics drive by jahn-teller effects

Abstract Homogeneous laser irradiation of ferroelectric perovskites produces the following effects simultaneously: the photo-domain effect, thermal effects and photovoltaic effects. For doped crystals (e.g. BaTiO3: Fe3+, Co2+, Cu2+), the analysis of some pyroelectric measurements on optically irradiated crystals is done on the basis of a true Jahn-Teller effect at 3d ions and of a pseudo Jahn-Teller effect at charge compensating vacancies. The concept of an optical Jahn-Teller centre is derived.

Lattice distortion induced by Jahn-Teller centres

An investigation is made of the strain field induced by Jahn-Teller (JT) centres in crystals. The approximation method which the authors employ in order to obtain the distortion pattern near the centre is based on variationally-constructed effective modes, which provide the best single-effective-frequency description of the coupling. A group-theoretical analysis is presented which classifies in some standard manner the phonon coordinates according to the irreducible representations of the point group at the impurity site. For each frequency, only one particular mode becomes coupled with the JT impurity; it is shown how this coupled mode, as well as the remaining uncoupled ones, can be obtained once a standard classification is available. The method is demonstrated for impurities with orbital doublet electronic states in cubic symmetry using simplified lattice models, and the possibility of experimental verification of the results by ENDOR is discussed.