Spin-peierls systems in magnetic field: Phase transition from dimerized to soliton lattice state

The phase diagram for the spin-Peierls system in high magnetic field is studied. The line of transition from dimerized (D) to soliton lattice (SL) phase is found, and it is shown that the SL phase is stable in fields h;h_c. Magnetization vs h is calculated as well as the sound velocity u_s of the domain walls oscillations. The possible experimental verification of the proposed theory is discussed also.     

Unified derivation of soliton, polaron and soliton lattice solutions in polyacetylene

The trans-(CH)_x polyacetylene system is studied in the framework of mean field approximation. In this paper it is shown that the phonon field satisfies a differential equation, which admits as solutions, besides the well-known solutions. found by other authors, many others.     

Cis-trans isomerization of iodine-doped polyacetylene

A quantitative analysis of the cis—trans isomerization of the lightly doped polyacetylene with iodine at room temperature is reported. The cis—trans ratio was determined on the basis of the infrared absorption measurement. The result clearly demonstrates the promotion of the isomerization with increase in the dopant concentration. The degree of the amount of the isomerization, however, is rather small compared with the previous study based on the nuclear magnetic resonance measurement.     

Non-ohmic behaviour of iodine-doped polyacetylene

We report on the temperature and electric field strength dependence of the conductivity of iodine-doped polyacetylene in the metallic regime. The experimental results are discussed in terms of Sheng model of fluctuation induced tunneling and characteristic dimensions of the tunneling zones between fibres are deduced from a fit between this theory and our experiments.     

Soliton lattice to metal: A first order phase transition

$\text{In-situ}$ measurements of the electron spin resonance of [Na⁺_y (CH)^?y]_x during electrochemical doping are reported. The susceptibility and the esr linewidth exhibit step-like increases, with hysteresis, as functions of the chemical potential. These magnetic data, when combined with electrochemical and structural results on the same system, imply a first order phase transition from soliton lattice to metal.     

Phase transition spectroscopy of Eu-chalcogenides

The thermo-reflectance spectra are investigated on EuO, EuS, EuSe, and EuTe, as the spin-order modulation spectroscopy of magnetic semiconductors. The spectral shapes, intensities, and relative energy shifts of the exciton-related fine structures are discussed in terms of the spin-polarization and spin-disorder-scattering effects of the non-local Bloch electrons.     

Phase transition of the compressible ising lattice

It is shown that the presence of harmonic lattice vibrations in a spin-$\text{1}\text{2}$ Ising system leads, in the mean field approximation, to transitions of first or second order, according to whether the pressure lies below or above a critical value. The results are being discussed in relation to experiments on the order-disorder transition in NH₄Cl.     

129I-Mössbauer study of iodine-doped polyacetylene

Iodine-vapour doped cis- and trans-polyacetylene films, (CHI_y)_x with 0.022 ? y ? 0.103, were studied at 4.2 K using the 27.7-keV Mössbauer resonance of ¹²⁹I. Both I^?₃ and I^?₅ linear molecules are identified, and the I^?₃ content (?69% in cis-(CHI_0.022)_x) is found to decrease with increasing doping level. For both polyiodide molecules a total charge transfer of about 0.8 electrons from the polymer is derived.     

Phase transition in one-dimensional lattice gauge theories

Considering one-dimensional nonminimally coupled lattice gauge theories, a class of nonlocal one-dimensional systems is presented which exhibits a phase transition. It is shown that the transition has a latent heat, and therefore is a first-order phase transition.     

Phase transition in the lattice gross-neveu model

The confining properties of the leading logarithm approximation to the effective lagrangian $\text{L}\text{= F}{}^2\text{/2g}{}^2\text{(t)}$ [ with g(t) a running coupling function of t = log(F²/μ⁴)] are seen to disappear when the second and the third approximations of the β-function power series expansion are considered.     

Shell model lattice dynamics of transition metal oxides

The lattice dynamics of transition metal oxides has been investigated in detail with threebody force shell model which is a successful extension of the shell model and includes the effect of electron shell displacements as well as deformations. The phonon dispersion relations along three principal symmetry directions obtained from this model have been compared with neutron data and found to give better agreement as compared to other models. The complete phonon spectra have also been computed and used to derive the temperature dependence of Debye temperature, and the second-order Raman and infrared spectra. The derived data agree fairly well with the observed ones wherever available.     

Nonlinear phenomenology from quantum mechanics: soliton in a lattice

We study a soliton in an optical lattice holding bosonic atoms quantum mechanically using both an exact numerical solution and quantum Monte Carlo simulations. The computation of the state is combined with an explicit account of the measurements of the numbers of the atoms at the lattice sites. In particular, importance sampling in the quantum Monte Carlo method arguably produces faithful simulations of individual experiments. Even though the quantum state is invariant under lattice translations, an experiment may show a noisy version of the localized classical soliton.     

Phenomenological theory of soliton formation in lightly-doped polyacetylene

A phenomenological theory is introduced to investigate the influence on soliton formation of the Coulomb repulsion associated with the double occupation of the soliton's localized mid-gap state and the soliton's interaction with the iodized dopant molecule. Markedly different equilibrium parameters result for the neutral, charged-free and charged-pinned soliton forms. The frequency of the i.r. active pinning mode is predicted to be of the order of 300 cm^-1. A large zero-point motion is found for shape deformations of the soliton.     

Far-Infrared spectroscopy of isolated transition metal clusters

The vibrational far-infrared (IR) spectra of isolated metal clusters in the gas phase can be measured by performing photo dissociation spectroscopy of their rare gas complexes. For these experiments an intense and widely tunable source of far-IR radiation is required, and the Free Electron Laser for Infrared eXperiments (FELIX) is ideally suited for this. Vibrational spectra are obtained for vanadium cluster cations as well as for neutral and cationic niobium clusters. The comparison of the experimental vibrational spectra with theoretically calculated spectra allows for the determination of the structure of the metal clusters.     

Phase transition of hard hexagons on a triangular lattice

Systems of hard hexagons on a triangular lattice are investigated. The orientation of the hexagons is kept fixed, while the size of the hexagons is varied. The existence of a phase transition is proved for all sizes by means of the Peierls'argument. The proof does not imply a phase transition in the continuous limit.Work supported by the U. S. Air Force under Contract No. F 44620-71-C-0013.