Structural phase transition induced by the Jahn-Teller effect. Antiferrodistortive ordering

We study the phase transitions induced by the Jahn-Teller effect ofE-doublet ions in a cubic crystal with antiferrodistortive interactions. AnS=1 pseudospin model is constructed which takes the three lowest vibronic levels of the Jahn-Teller complexes into account. We find a second-order phase transition to a tetragonal phase with two inequivalent sublattices. The transitions between the vibronic levels give rise to bands of collective vibronic excitations with strongly temperature-dependent frequencies. The nature of the various modes is analyzed in detail. We also study the coupling to the elastic displacement field of the crystal. For a sufficiently large coupling constant, this coupling stabilizes a different low-temperature tetragonal phase with two equivalent sublattices. In a certain region of coupling constants, a transition occurs between the two tetragonal phases by second-order transitions to an intermediate phase of lower symmetry. The influence of the coupling on the dynamic behaviour is discussed.     

Dynamics of cooperative Jahn-TellerT-systems

We consider a cubic crystal with a triply degenerate electronicT-state in each unit cell which is slightly split by an additional trigonal crystal field. A linear Jahn-Teller interaction ofT-e nature is assumed, involving predominantly acoustic phonons and the macroscopic strain. Below a critical temperatureT _c a tetragonal distortion is established. The phase transition is accompanied by a strong temperature dependence of the elastic constants. The latter are calculated under various thermodynamic conditions. Both static and dynamic approaches are discussed and related to each other.Supported by a grant from the Deutsche Forschungs-gemeinschaft     

Discontinuous freezing-out of local order

A soft impurity in a host crystal undergoing a second-order displacive phase transition may induce a freezing-out of local order at temperatures above the bulk transition temperature [1]. We show that in MFA this local phase transition is of first order, i.e. the local order appears discontinuously, and the stability limit of the disordered phase against local ordering lies below the stability limit of the locally ordered phase, if the representation of the impurity coordinate contains a third-order invariant, but the space-group representation of the soft optical phonon does not. This situation occurs for example for anE _g -type impurity coordinate coupling to a soft zone-edge or zone-corner phonon driving an antiferrodistortive transition. The theory is applied to the specific case of a Jahn-Teller impurity in a non-Jahn-Teller host crystal.     

New magnetic states in copper metaborate CuB<Subscript>2</Subscript>O<Subscript>4</Subscript>

The static and resonance properties of copper metaborate CuB₂O₄ were experimentally studied in a magnetic field applied in the crystal tetragonal plane. The field-induced second-order phase transition to a weakly ferromagnetic state was observed in the temperature range 10–20 K. The low-field state is characterized by the absence of spontaneous moment, and it represents, presumably, a long-period helicoid. At temperatures below 2 K, two sequential first-order phase transitions were observed. They were accompanied by jumps in resonance absorption with a hysteresis upon changing field-scan direction. These transitions can be caused by the transformation of the incommensurate spin structure into the helicoidal states with periods commensurate with the lattice translation period.     

Transformation of the acoustic mode near the structural and magnetic phase transitions in La1–xSrxMnO3 (x = 0.175) crystal

The peculiarities of the phase transitions in La_0.825Sr_0.175MnO₃ single crystal have been investigated by acoustic methods at the frequency f = 770 MHz. Generation of a magnetoelastic wave near the phase transition at 305 K is revealed, which is assigned to quasi-transverse modes. The most likely cause of the phase transition near T = 305 K and the formation of the quasi-transverse mode is the suppression of local Jahn-Teller distortions.     

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.     

Anomalies of transverse dielectric susceptibility in DyVO4 and DyAsO4 Jahn-Teller crystals,caused by quadratic vibronic coupling

The transverse dielectric susceptibility χ_⊥ in DyVO₄ and DyAsO₄ crystals is examined theoretically. The terms of quadratic vibronic coupling, which contain as the optical E_u modes so the soft acoustic branch or the electron order parameter, are taken into account. Such coupling is shown to lead to anomaly of χ_⊥ near the temperature of Jahn-Teller structural phase transition. From the experimental data the constants of the quadratic vibronic coupling are appreciated.     

Holographic superconductor models in the non-minimal derivative coupling theory

We study a general class of holographic superconductor models via the Stu¨ckelberg mechanism in the non-minimal derivative coupling theory in which the charged scalar field is kinetically coupling to Einstein’s tensor. We explore the effects of the coupling parameter on the critical temperature, the order of phase transitions and the critical expo- nents near the second-order phase transition point. Moreover, we compute the electrical conductivity using the probe approximation and check the ratios ω g /T c for the different coupling parameters.     

具有D4h对称性构型的C2+4分子的Jahn-Teller效应与能级分裂

依据量子理论与配位场理论,利用群论和对称性分析的方法探讨了C²⁺₄分子在具有D_4h对称性构型时,E×(b_1g+b_2g)系统的Jahn-Teller效应中的相关问题.研究了C²⁺₄分子的电子态与声子态的对称性及其活跃声子态,讨论了系统声子间的耦合与CG系数,构建了E×(b_1g+b_{2g 关键词： 2+4分子')" href="#">C²⁺4分子 对称性 能级分裂 Jahn-Teller畸变}     

EPR of two Cu2+ complexes showing dynamic Jahn-Teller effects

The powder electron paramagnetic resonance spectra of ∼1% Cu doped into [Zn(bipy)₂(NO₂)]NO₃ and [Zn(bipyam)₂(NO₂)]NO₃, bipy = 2,2′-bipyridyl, bipyam = 2,2′-bipyridylamine, over the temperature range 4.2–300 K are reported. For [Zn/Cu(bipy)₂(NO₂)]NO₃ a spectrum characteristic of a tetragonally elongated octahedral complex with a slight orthorhombic distortion is observed below ∼50 K, but above this temperature the signals associated with the two higherg-values gradually converge. At low temperature a spectrum similar to that of the bipy complex is observed for [Zn/Cu(bipyam)₂(NO₂)]NO₃, but between 40 and 60 K the spectrum gradually changes to one characteristic of a tetragonalg-tensor with g_∥ < g_⊥. The spectra may be interpreted satisfactorily with a model in which the temperature dependence of theg-vainlues is estimated from the vibronic wavefunctions of a copper(II) complex under the influence of second-order Jahn-Teller coupling and a ligand field of tetragonal symmetry perturbed by crystal lattice forces. The results are consistent with the crystal structures reported for the compounds at various temperatures.     

Rotational energy levels of symmetric top molecules in 2 E states

Expressions are derived for the rotational energy levels of a symmetric top molecule in a ² E state which take into account a spin-orbit interaction and vibronic Jahn-Teller interactions. Some group theoretical aspects of the Jahn-Teller problem for a general symmetric top molecule are discussed. In addition, the overall selection rules and formulae for the relative intensities of rotational lines in a ² A-² E transition are derived and the possibility of applying them to two recently published experimental examples is considered.     

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.     

Quantum phase transitions and conserved charges

The constraints on the scaling properties of conserved charge densities in the vicinity of a zero temperature (T), second-order quantum phase transition are studied. We introduce a generalized Wilson ratio, characterizing the nonlinear response to an external field,H, coupling to any conserved charge, and argue that it is a completely universal function ofH/T: this is illustrated by computations on model systems. We also note implications for transitions where the order parameter is a conserved charge (as in aT=0 ferromagnet-paramagnet transition).     

Magnetic phase transitions,soliton lattices,and electric polarization in <Emphasis Type="Italic">R</Emphasis>Mn<Subscript>2</Subscript>O<Subscript>5</Subscript> multiferroics

The magnetic phase transitions in RMn₂O₅ oxides were analyzed. It was shown that the transition from the paramagnetic phase to the incommensurate phase is described by not only the basic magnetic parameter but also an associated order parameter: electric polarization along the y axis of the crystal. It was established that the antiferromagnetic-incommensurate phase transformation is a second-order phase transition, accompanied by a decrease in electric polarization.     

Verwey ordering on magnetite as a co-operative Jahn-Teller transition

Verwey ordering of Fe²⁺ and Fe³⁺ on alternate planes along [001] direction occurring at 119° K is considered uniquely as a collective Jahn-Teller transition. The model predicts two successive second-order transitions, as a result of simultaneous coupling to both shear distortion and optical phonon mode. Expected elastic constant anomalies are calculated. The insulating state thus obtained at T = 0, does not invoke electron-electron correlation on near-neighbor sites.     

Structural transformations in Y-Ba-Cu-Ti-O metal oxides accompanying substitution of titanium for Y and Cu

Taking into account changes in the Cu charge states, general characteristics and distinctive features of structural transformations typical of high-temperature superconducting and ferroelectric phases in Y-Ba-Cu-Ti-O ceramic samples are studied. It is shown that with substitution of titanium for Y and Cu the concentration of Jahn-Teller ions Cu²⁺ detected by EPR technique does not rise higher than 10%. According to X-ray spectroscopy data, the Cu charge state lowers from Cu^2.2+ to Cu^0.5+ with increasing titanium content. It is found by X-ray diffraction studies that with increasing Ti content, the fraction of the layered structure containing (00l) planes of orthorhombic structure decreases and the growth of planes of the (11l) type pertaining to tetragonal structure is stimulated. The structural phase transition from the orthorhombic to tetragonal crystal structure mainly occurs in the range x=0.3–0.5.     

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.     

Strong exciton-photon coupling and exciton hybridization in a thermally evaporated polycrystalline film of an organic small molecule

We demonstrate strong exciton-photon coupling in an optical microcavity containing a thermally evaporated polycrystalline organic thin film. Microcavity polaritons result from coupling between the 0-0 excitonic transition of 3,4,7,8 napthalenetetracarboxylic dianhydride and a cavity photon. For thicker films, the 0-1 transition also couples to the cavity mode, as vibronic relaxation is overcome by the short Rabi period for strong coupling. To our knowledge, this is the first report of strong coupling between a cavity photon and multiple vibronic transitions in a single material, made possible by the pronounced vibronic absorption features characteristic of crystalline organic materials.     

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.