Jahn-Teller Centers Research Articles

STRIPES IN MANGANITES

Topological aspects of the stripe structure in manganese oxides are discussed in terms of the "winding number" w associated with the Berry-phase conncection for e g orbitals acquired by the parallel transport through the periodic array of Jahn-Teller centers. For La 1-x Ca x MnO 3, w is shown to characterize both the three-dimensional spin-charge-orbital structures in the antiferromagnetic phase for x≥1/2 and the charge-orbital stripes in the two-dimensional ferromagnetic phase for <1/2.

Topological scenario for stripe formation in manganese oxides

The spin-charge-orbital complex structures of manganites are studied using topological concepts. The key quantity is the "winding number" w associated with the Berry-phase connection of an e(g) electron parallel transported through Jahn-Teller centers, along zigzag one-dimensional paths in an antiferromagnetic environment of t(2g) spins. From these concepts, it is shown that the "bi-stripe" and "Wigner-crystal" states observed experimentally have different w's. Predictions for the spin structure of the charge-ordered states for heavily doped manganites are discussed.

Effect of the fields of randomly spaced charged centers on the luminescence polarization

This paper considers the effect produced by randomly spaced charged centers in a crystal on the luminescence polarization associated with Jahn-Teller centers under conditions of uniaxial strain.

Non-collinear phase of a singlet–triplet magnet

The spin subsytem of copper oxides within the model of polar singlet–triplet Jahn–Teller centers is described by the Hamiltonian of a singlet–triplet magnet with multiparametric (non)collinear spin arrangements. Using the modified mean-field approximation we analyze the conditions of an appearance of the non-collinear phase. Certain static and dynamic properties of the new phase are considered.

Structural phase transformation and limited miscibility in Jahn–Teller systems

Immiscibility of crystal systems with Jahn–Teller (JT) ions is studied at low temperatures. Phase transformations under which concentrated JT systems are separated into phases with a high and low concentration of JT ions with structural JT transformations are analyzed. A dependence of the phase diagram topology on the ratio between the cooperative interaction energy in the JT subsystem and the intensity of random crystal fields acting on the JT ions is established. The possibility of low-temperature three-phase equilibrium of two cubic and a tetragonal phase with different concentrations of JT centers is demonstrated for a high degree of substitutional-type disorder.

Singlet-triplet model of cuprate magnetism

A model for describing the spin subsystem of cuprates within a model of polar singlet-triplet Jahn-Teller centers is proposed. In this model spin ordering is described by two vector order parameters S (the total spin of a CuO4 cluster) and V (the operator of the change in spin multiplicity per cluster). It is shown within a modified mean-field approximation that the formation of a noncollinear magnetic structure characterized by an antiferromagnetic wave vector, which, however, has a nontrivial temperature dependence of the order parameters, is possible along with the formation of ordinary collinear structures of the (anti)ferromagnetic type. The temperature dependences of the order parameters and the principal equilibrium thermodynamic characteristics, viz., the static susceptibility and the specific heat, are obtained.

Polar Jahn–Teller centers and magnetic neutron scattering cross-section in copper oxides

A cross-section for magnetic neutron scattering in high- T c cuprates is obtained in the framework of the novel model of the polar singlet–triplet Jahn–Teller centers. The multi-mode character of the CuO 4 cluster ground manifold in the new phase of polar centers determines the dependence of the magnetic form-factor on the local structure and charge state of the center. It is shown that magnetic inelastic neutron scattering in the system of the polar singlet–triplet Jahn–Teller centers permits to investigate the non-magnetic charge and structure excitations.

The singlet–triplet magnetism in high- TC cuprates

The unconventional properties of the copper oxides are connected with an active interplay of the charge, spin, orbital and local structural modes, taking into account of the strong charge inhomogeneity and multi-granularity. The spin subsystem of the copper oxides within the polar Jahn–Teller centers model corresponds to a singlet–triplet magnet where the local boson movement is accompanied by the induced longitudinal spin fluctuations. The model permits to explain the main features of the magnetic inelastic neutron scattering, spin-lattice relaxation and static susceptibility in the high- T c cuprates.

Bloch Electrons in a Jahn-Teller Crystal and an Orbital-Density-Wave State due to the Berry Phase

The effect of the Berry phase is included explicitly in the wave function describing conduction electrons in a crystal composed of periodically arrayed Jahn-Teller centers that have conically intersecting potential energy surfaces. The Berry phase can make a drastic change in the band structure, leading generally to the formation of an orbital-density-wave state. We discuss implications of our theory and possible relations to the orbital ordering observed in the manganese perovskites.

Microphase separation and magnetic Jahn-Teller polarons in LaSrAl1−xCuxO4−δ

An EPR study of the solid solutions LaSrAl1−xCuxO4, which are isostructural to La2CuO4, shows that microphase separation of the structure occurs already at small copper concentrations (x≈0.01). A phase enriched with Cu appears along with a phase of La2AlO4, which contains isolated CuO6 centers. It is established that the nature of the states and the deformations of the CuO6 centers is determined by internal Jahn-Teller factors. When x⩽0.1, EPR signals are detected for new dynamic centers, which are identified as CuO6 Jahn-Teller centers with a hole delocalized among the four in-plane oxygen ions (the total spin S=1). When 0.1⩽x<1, local CuO6− hole centers transform into magnetic Jahn-Teller polarons, which include five or more CuO6 fragments, in the copper phase of the structure. Their transformations and the conditions for observing them are discussed.

Jahn-Teller centers and pseudospin effects

The characteristics of the phenomenon of polar centers exhibiting Jahn-Teller behavior in copper oxides are investigated by the pseudospin approach. The emergence of relaxation pseudospin (dipole-quadrupole) modes and pseudospin-phonon interaction produces a number of anomalies in inelastic neutron scattering. The Jahn-Teller effect in polar centers brings about a change in the character of the tetra-ortho transition in the system La2−xMx-CuO4 as the concentration x is increased. The possibility of the onset of fluctuation domain nanostructures is suggested.

Origin of unconventional physical behaviour of copper oxide high-T c-superconductors

We introduce briefly the principal ideas of a new model approach to the copper oxides as to systems of the local bosons moving in a lattice of the singlet-triplet Jahn-Teller centers. The unconventional properties of the copper oxides are connected with an active interplay of the charge, spin, orbital and local structural modes with taking account of the strong charge inhomogeneity and multi-granularity.

Charge fluctuations and magnetic inelastic neutron scattering in copper-oxide high-T c superconductors

Spin subsystem of the copper oxides within the polar Jahn-Teller centers model corresponds to a singlet-triplet magnetic where the local boson movement accompanied by the induced longitudinal spin fluctuations. These fluctuations determine the features of the magnetic inelastic scattering in the high-T c cuprates.

Polar Jahn-Teller Centers and isotope effect in copper oxide high-T c-superconductors

The CuO 4-cluster based copper oxides are considered as generalized quantum lattice bose-gas or a system of the local singlet bosons moving in a lattice of the hole Jahn-Teller centers [CuO 4 5−] JT. The model is illustrated by the qualitative and quantitative description of the various peculiarities of an isotope shift (IS-) effect in a series of 123-like oxides.

The cooperative dynamical Jahn-Teller effect in alkali doped fullerides

The cooperative dynamical Jahn-Teller (JT) effect on a two-dimensional lattice of alkali doped fullerides is considered. It is shown that the C603− anions in the fulleride crystal may be subject locally to the E⊗e JT effect. The ground state of the layer composed of the JT centers is stabilized by coherent electronic and pseudorotational nuclear motion of molecules. The antiferromagnetic ordering of vibronic angular momenta on neighbouring JT centers causes “rotating-meshing-gears”-like dynamics of the lattice, where chains of dimers and a lattice of tetramers are successively formed. It is concluded that the cooperative dynamical JT effect of such a form may provide a fundamental mechanism for the theoretical description of superconductivity of these compounds.

Polar Jahn-Teller centers and the anomalous isotope effect in copper-oxygen high-T c superconductors

We examine the isotope effect in copper-oxygen high-T c superconductors using the model of polar Jahn-Teller centers, a variant of the local-boson model. The isotope-effect exponents for oxygen and copper depend on the structure of Jahn-Teller centers and its variation due to boson motion. A number of YBa2Cu3O7−δ -based compounds are used to illustrate the feasibility of quantitatively describing the experimentally observed values of α Cu and α O and their relationship to T c.

Disordered CuN(6) Jahn-Teller Centers in Hexakis(1-methyltetrazole)copper(II) Tetrafluoroborate: A Temperature-Dependent X-ray Diffraction and EPR Study.

The influence of the molecular crystalline arrangement upon the state of a Jahn-Teller-active center has been investigated in crystals of the complex Cu(mtz)(6)(BF(4))(2), where mtz = 1-methyltetrazole. Crystal structures at 293, 123, and 93 K were determined by X-ray diffraction for the copper complex and at 293 and 100 K also for the analogous zinc complex, Zn(mtz)(6)(BF(4))(2). The respective lattice parameters for the copper complex at 293, 123, and 93 K are as follows: a = 18.137(4), 17.597(4), 17.575(4) Å; b = 10.247(4), 10.131(4), 10.133(4); c = 18.446(5), 18.531(4), 18.535(4) Å; beta = 112.62(2), 113.55(2), 113.61(2) degrees. Those for the zinc complexes at 293 and 100 K, respectively, are as follows: a = 18.153(2), 17.663(2) Å; b = 10.289(1), 10.159(2) Å; c = 18.506(3), 18.578(3) Å; beta = 113.21(1), 114.15(2) degrees. The crystal system is monoclinic, space group P2(1)/n (Z = 4), for all crystals with two crystallographically inequivalent pairs of centrosymmetric molecules, M(mtz)(6)(BF(4))(2), in the unit cell. The two inequivalent Cu(mtz)(6)(2+) complexes, Cu(A) and Cu(B), both exhibit Jahn-Teller distortions, but in different ways, the Cu-N distances for the unit on site A being 2.015(4), 2.031(5), and 2.384(5) Å at 93 K, while those for the unit on site B are 2.053(5), 2.126(5), and 2.204(5) Å. However, the Jahn-Teller radii of the two complexes, as calculated from the metal-ligand distances and the U tensors of the two CuN(6) units, were both found to be 0.41(3) Å. EPR experiments at room temperature on polycrystalline samples of the pure copper compound and of the copper-doped zinc compound confirm the presence of two different Jahn-Teller centers; both complexes are rapidly pulsating, but the CuN(6) units on site A are confined predominantly to one potential well of the warped Mexican hat potential, whereas the CuN(6) units on site B have density in all three wells. At 78 K, however, the spectrum of the polycrystalline material is consistent with a single site having an axial g tensor with maximum anisotropy (g( parallel) = 2.300(5), g( perpendicular) = 2.068(5)). While the low-temperature X-ray results also indicate a structure in which the Cu(A) center is exclusively populated in one potential well, the U tensor and potential well population data for the Cu(B) centers indicate that at 93 K a nonpulsating averaged structure based on tetragonally elongated CuN(6) units is being observed. The more pronounced preference for the CuN(6) octahedron on site A to show elongation in one specific direction, in contrast to that on the B site, appears to be due to the differing impacts of the local-site strains at the two distinct centers of symmetry, and a simple model for evaluating a crystal "packing" strain from the bond length data for the isomorphous zinc complex is described.

The cooperative Jahn-Teller effect on a two-dimensional molecular layer

The cooperative dynamical Jahn-Teller (JT) effect on the two-dimensional layer is considered. The layer is constructed from model JT active centers (X 3 triangular molecules) ordered in a honeycomb-like lattice. Some electronic properties of the layer relevant to type-II superconductivity are discussed assuming a coherent pseudo-rotational motion of individual JT centers.

Jahn-Teller Center as Quasiparticle of Fractional Statistics

The Jahn-Teller (JT) effect [1] describes the interaction between the two components of a degenerate electronic state (E) through a, usually degenerate, pair of nuclear modes (e). An unusual aspect of the E®e-type JT effect is that the Born-Oppenheimer electronic wave functions, which diagonalize the electronic Hamiltonian in the fixed-nuclei approximation, undergo a sign change when the nuclear coordinates traverse a closed path around the conical intersection [2-4]. As it has been shown by Mead and Truhlar [5], the resulting double-valuedness of the electronic wave functions can be removed, but only at the expense of introducing a vector-potential-like term into the effective Hamiltonian for the nuclear motion, which has been termed the molecular Aharonov-Bohm effect [6]. The above-mentioned sign change is a special case of Berry's geometrical phase [7] which holds implications for topics as diverse as the theory of the fractional quantum Hall effect [8], the optical activity of a helical fiber [9], superconductivity [10], and gauge field theories [11]. A more general discussion in the context of molecular physics can be found in the review article by Mead [12]. In the present paper we argue that JT centers can, in some applications, be considered as quasiparticles of fractional statistics. Thus in Sec. 2 a brief re-derivation of the adia&atic analytical solution of the linear E®e-type Hamiltonian is presented and some of its properties are discussed. The arguments in favor of the fractional nature of the statistics of the JT quasiparticles and their relevance for description of the type-II superconductivity are discussed in Sec. 3. Some conclusions concerning high-T c superconductors are presented in Sec. 4.

The Jahn-Teller effect in icosahedral symmetry: extension of Ham factors in strongly coupled systems

We extend the standard definition of reduction factors (Ham factors) in strongly coupled Jahn-Teller (JT) systems. Our aim is to cover linear JT systems in which the vibronic ground state at strong coupling is in close proximity in energy to low-lying excited states belonging to singlet and non-trivial irreducible representations of the JT centre. Such a structure of low-lying vibronic states is present in the linear JT systems of the icosahedral orbital quarter and quintet, G and H. We calculate all the standard reduction factors as well as extended matrix elements, for the icosahedral systems G(X)g, G(X)h and H(X)g. We calculate the matrix of Ham factors needed to handle the extra multiplicity of an H operator in an H state. A direct group-theoretical approach which explains the origins of various features of our analysis is included.