Victor Z. Polinger

On the origin of dynamic instability of molecular systems

The vibronic origin of dynamic instability of molecular systems considered earlier, is here given a more complete and rigorous treatment. It is shown that the nonvibronic contribution to the curvature of the adiabatic potential arising due to nuclear displacements under fixed electronic density distribution, is always positive, and hence the only reason for dynamic instability is the pseudo Jahn-Teller effect. For some examples of special interest (planar equilateral NH3, planar square CH4 and linear H3+) the molecular excited states, responsible for the instability of the ground state, are revealed by means of ab initio calculations.

MO LCAO analysis of the vibronic instability of the CuCl53− trigonal bipyramidal configuration. Critical view on the angular overlap model in vibronic problems

The vibronic origin of the instability of the CuCl53− trigonal-bipyramidal configuration (resulting in three equivalent square pyramidal stable configurations) is revealed by solving the three-mode (A′1 + E′)⊗(e′ + e′ + e′) problem, the electronic functions, vibronic constants and bare force constants being calculated in the semiempirical MO LCAO scheme. The three-mode problem is reduced to the one-mode one by means of the “interaction mode” presentation. The semiempirical calculations are based on the extended Huckel approach including self-consistency with respect to atomic charges and configuration (the SCCC MO LCAO method). It is shown that the main contribution to the instability is due to the pseudo Jahn-Teller mixing to three excited E′ states formed by one-electron excitations 1e′ → 5a1, 1e′ → 6a′1 and 1a′1 → 6e′ (in the frozen MO approximation excited states). By comparison with the results obtained earlier in the angular overlap model (AOM) and additional analysis, it is shown that the only (A′1 + E′)⊗e′ mixing considered in the AOM has a minor contribution to the instability; in general, the AOM is inapplicable to pseudo Jahn-Teller problems.

Multiple lines of conical intersections and nondegenerate ground state in T⊗t2 Jahn–Teller systems

We have investigated the T⊗t2 Jahn–Teller problem with linear and quadratic vibronic coupling including a fourth-order term. First, numerical calculations of the lowest vibronic states were carried out by direct diagonalization of the Hamiltonian. The results show that the energy level of the ground vibronic state, which is triply degenerate T for small quadratic coupling g values, intersects the next A level with increasing g, thus realizing the nondegenerate ground state at sufficiently large g values. This result reverses the long-standing belief that the ground vibronic state for the T⊗t2 system has the same degeneracy and symmetry T as the initial electronic state. To explain these results in terms of Berry phase requirements and conical intersections, the adiabatic potential energy surface of the system is analyzed, and the relationships among the type and number of minima, conical intersections, and relevant tunneling paths are revealed. Depending on the vibronic coupling parameter values, there ar...

Where do the doped holes go in La2−xSrxCuO4? A close look by XAFS

Polarized x-ray absorption fine structure (XAFS) measurements at the La and Sr sites in La2−xSrxCuO4 (0.075≲x≲0.35) indicate that doped holes introduced with Sr are not uniformly distributed in the CuO2 planes but reside in impurity states with the majority of charge located on CuO6 octahedra coupled to the Sr dopants by an apical oxygen (denoted as Sr octahedra). A model based on doped hole-induced Jahn Teller (JT) distortions of these Sr octahedra indicates the impurity states are overlapping singlet and triplet ones resulting from pairings of intrinsic and extrinsic holes. The more mobile singlet pairs reside in the CuO2 planes, are bound by more than 0.1 eV and could Bose-condense into a superconducting state. The predominant appearance of (mostly) out-of plane triplet states at high x intimately related to the 2D-3D crossover in transport properties and could relate to the observed loss of high Tc in the overdoped regime, as confinement of carriers to the CuO2 planes is reduced.

Ferroelectric phase transitions in cubic perovskites

In low-temperature phase, due to the pseudo Jahn-Teller effect in unit cells, atoms are coherently localized in one of eight trigonal wells of the adiabatic potential energy surface. With temperature, nuclear motion delocalizes over symmetry-equivalent wells. Depending on the size of potential barriers, the two limiting cases are considered: (a) shallow wells, and (b) deep wells. They correspond to two different mechanisms of delocalization. When potential barriers are shallow compared to kTC, the delocalization is due to over-the-barrier activation of Arrhenius type. Alternatively, when the over-the-barrier activation is thermally inaccessible, the delocalization is due to tunnelling. Temperature dependence of spontaneous polarization is treated in terms of the mean-field approximation. Using experimental data for BaTiO3, numerical values of the corresponding parameters are estimated. The qualitative description is followed by a numeric evaluation of the temperature sequence of ferroelectric phase transitions for the two cases of shallow and deep wells. The theory agrees with the experimental data.

Orbital Ordering Versus the Traditional Approach in the Cooperative Jahn–Teller Effect: A Comparative Study

In Jahn–Teller crystals, at low temperatures, symmetry-breaking lattice distortion creates an ordering of otherwise degenerate atomic orbitals. In the theory of cooperative Jahn–Teller effect, the traditional approach includes solving the complex problem of coupled dynamics of the respective electron-vibrational system. During last two decades, an alternative way of treating the cooperative Jahn–Teller effect attracts increasing attention. It replaces ligand-mediated intercell coupling by an effective intersite orbital exchange of electron-degenerate atoms. Known as the orbital ordering approach, it explores stable ordered patterns due to the exchange coupling of orbital pseudo spins. The respective symmetry break of crystal lattice structure is treated as a secondary effect resulting from the orbital ordering. This paper examines some approximations implicitly included in the orbital ordering approach and compares it to the traditional theory of the Jahn–Teller effect. As the orbital ordering approach replaces ligand-mediated intersite coupling by an orbital exchange, the fundamental effect of dynamic strengthening chemical bonds with low-symmetry lattice distortions is lost. This may bring to a wrong conclusion about possible bond-ordered structures. On the other hand, a number of cases are outlined when both approaches yield close results. Based on these estimates, conclusions regarding applicability of the orbital ordering approach are derived and some general recommendations are given.

Solution of Vibronic Equations. Tunneling Splitting

The evaluation of the energy spectrum and wave functions is the central problem of the theory of vibronic interactions. In the limiting cases of weak and strong vibronic coupling, this problem can approximately be solved analytically (Sects. 4.1–4.3), while in the general case, numerical solutions can be obtained (Sect. 4.4). Of special interest is the determination of the lowest vibronic levels, which, in the case of strong vibronic coupling, result from the tunneling of the system between the equivalent minima of the adiabatic potential (tunneling splitting). In some cases physical magnitudes determined by the electronic subsystem in only its ground state can be calculated by means of the so-called vibronic reduction factors without solving the vibronic problem (Sect. 4.7).

Suppression of superconductivity in La1.85Sr0.15Cu1−yNiyO4: The relevance of local lattice distortions

The effect of Ni substitution upon the local structure of La1.85Sr0.15Cu1−yNiyO4 is commonly neglected when addressing the Ni-induced destruction of the superconducting state at y≈0.03 and a metal-insulator transition at y≈0.05. It is also sometimes assumed that direct substitution of a dopant into the CuO2 planes has a detrimental effect on superconductivity due to in-plane lattice distortions around the dopants. We present here results from angular-dependent x-ray absorption fine structure (XAFS) measurements at the Ni, La and Sr K-edges of oriented powders of La1.85Sr0.15Cu1−yNiyO4 with y=0.01, 0.03, 0.06. A special magnetic alignment geometry allowed us to measure pure ĉ and ab oriented XAFS at the Ni K-edge in identical fluorescence geometries. Both the near-edge absorption spectra (XANES) and the XAFS unequivocally show that the NiO6 octahedra are largely contracted along the c-axis, by ≈ 0.16 A. Surprisingly, the Ni-O planar bonds and the Ni-O-Cu/Ni planar buckling angle are nearly identical to th...

Spectroscopic Manifestations of Vibronic Effects

The essential changes of the energy spectrum and wave functions of molecules and crystals caused by vibronic interactions directly affect all the spectroscopic properties of the system. Even when the vibronic coupling is weak, the resulting changes in the spectra can be very significant, especially when the vibronic interaction leads to the appearance of additional spectral lines (for instance, in the rotational absorption spectrum of spherical-top molecules).

Cooperative Phenomena. Structural Phase Transitions

In this chapter the ordering of Jahn-Teller and pseudo-Jahn-Teller local distortions in crystals is considered. In the case of Jahn-Teller distortions the cooperative Jahn-Teller effect considered in Sect. 6.1 takes place. The ordering of pseudo-Jahn-Teller distortions as well as Jahn-Teller distortions in some noncentrosymmetrical crystals may lead to spontaneous polarization of the lattice and to ferroelectricity (Sect. 6.2). In Sect. 6.3 the vibronic model of structural phase transformations in condensed media is discussed.