J. W. Flocken

Structural phase transitions, ferroelectricity and high temperature superconductivity

Abstract Calculations based on a local double well potential show the near coincidence of a ferroelectric transition and a high temperature superconducting phase transition. Using double well parameters consistent with the Abrahams-Kurtz-Jamieson (AKJ) relation for ferroelectric phase transitions in oxide perovskite ferroelectrics we find that the superconducting transition temperature falls above or below the ferroelectrics transition temperature depending on modest changes in the height of the unrenormalized double well barrier. For the ReBa2Cu3O7-δ superconductors, this ferroelectric transition appears to fall above the superconducting transition temperature. The shallowness of the double-well in this case is consistent with our previous suggestion that the ReBa2Cu3O7-δ are relaxor ferroelectrics with a ferroelectric transition temperature that depends on the specific rare earth and a more or less constant superconducting transition temperature in the 90–100°K region. This result has important implicat...

Ab Initio Calculations of the Effect of Non-Rigid Ions on the Statics and Dynamics of Oxides

The potential induced breathing of oxygen ions in a crystal, which plays a major role in explaining the violation of the Cauchy relations and the splitting of the longitudinal and transverse optic frequencies in alkaline earth oxides, is analyzed for BaTiO3 in the perovskite structure and the distorted structure corresponding to the tetragonal phase. The results indicate that nonspherical ions are required to describe the ferroelectric transition.

Ferroelectric phase transitions in NaCa-halide perovskites

Abstract We have used lattice statics and dynamics, as well as molecular dynamics to investigate the possibility of structural phase transitions in NaCaX3 compounds in which X is one of the halides; F, Cl, Br, I. These simulations were done using 320-ion clusters and ab initio potentials with no adjustable parameters. Our computations lead to the prediction that these compounds all have high temperature phases which are distorted perovskites and that each of them undergoes a phase transition to a polar state, although the structure of the ground state and the direction of the polar axis varies with the size of the halide ion.

A double well oscillator model for the ferroelectric phase transition in SBSi

Abstract The dielectric response spectrum for antimony sulfoiodide has been computed using a model in which the material is simulated by a damped double well oscillator. It is found that with the use of simple damping and relaxation terms and realistic well parameters the experimental dielectric response, including the central peak, can be reproduced quite closely. The choice of well depth, in particular, plays a major role in determining the frequency of the first peak above the soft mode.

Vibronic origins of high Tc

Abstract In this paper we present a detailed development of our vibronic mechanism for the explanation of high temperature superconductivity. We first review the evidence for believing that some unified mechanism for “low” and “high” T c is required. We then develop the case for the existence of the double-well motions, required by our theory, in these systems, and proceed to develop our ideas further to the point of unifying, the T c ∼ 100 K, and the T c ∼ 40 K systems. We also advance a possible explanation of the anomalous isotope effects in these systems, and conclude with an extended discussion of non-cuprate systems and general criteria for high T c theories, including the gap to kT c ratio.

The crossover of phase transitions from NaCaF3 to KCaF3

We have performed molecular dynamics simulations on a series of 320 ion samples of NaCaF3 with specific amounts of randomly distributed K substituted for Na. Our computations used ab initio potentials with no adjustable parameters to calculate the short range interactions between ion pairs in these samples. Our results indicate the gradual crossover of the phase transitions from NaCaF3 to KCaF3. The polar state seems to disappear at a K to Na ratio of three.

Ferroelectric phase transition in K-DOPED NACAF3

Abstract We have used lattice statics and molecular dynamics to investigate the effect on the structural phase transitions in NaCaF3 in which two adjacent Na ions near the center of the sample are replaced by K ions. Computations were done using ab initio potentials with no adjustable parameters. Our results indicate that the structural properties and the ferroelectric transition temperature depend strongly on the orientation of the K dipole.

Possible origin of high Tc superconductivity

Abstract A mechanism to explain high-Tc superconductivity, based on the double-well motion of the oxygen ions in such systems, is extended to predict Tc values of for (La, Ba)2CuO4 and Ba2Cu3O7. This gives lower bounds on Tc of 53K for the former and 44K for the latter. Since the second number, in particular, could be markedly enhanced by full allowance for the large amplitudes of the oxygen motions we argue that large Tc values have a purely vibronic origin.

Competition between ferroelectricity and ferroelasticity in RbCdF3

Abstract We have made initial theoretical studies of lattice instabilities in RbCdF3. These have revealed clear evidence of crystal field effects on the free ion charge densities. This is contrary to findings of earlier studies on isomorphous RbCaF3.

Molecular dynamics simulation of the phase transition in LaAlO3

Abstract We have used molecular dynamics and lattice statics to study LaAlO3 in the temperature range 0–1500 K. Short range pair potentials were obtained using the Gordon-Kim1 approach. The O−2 ion charge density was stabilized using Watson Sphere potentials ranging from 0.7 to 0.8 Har./e. within this range, a phase transition is found to occur at the temperature reported experimentally (800K), however, we find the ground state to have a tetragonal (Pmnb) structure rather than the observed R3C configuration. It is found that the R3C structure can be stabilized by transferring charge from the O−2 ion to the La+3 ion in the long range potential sums.