Insulator-metal transition in biased finite polyyne systems

A method for the study of the electronic transport in strongly coupled electron-phonon systems is formalized and applied to a model of polyyne chains biased through metallic Au leads. We derive a stationary non equilibrium polaronic theory in the general framework of a variational formulation. The numerical procedure we propose can be readily applied if the electron-phonon interaction in the device hamiltonian can be approximated as an effective single particle electron hamiltonian. Using this approach, we predict that finite polyyne chains should manifest an insulator-metal transition driven by the non-equilibrium charging which inhibits the Peierls instability characterizing the equilibrium state.     

Insulator-metal transition in thin films of ferrite Fe3O4

Insulator-metal transition in Fe₃O₄ is analysed on the basis of electron-electron correlation of nearest neighbour sites. A double time Zubarev Green function technique is used. It is shown that up to 120 °K system stays in a localised state and then goes into the band-state. The band-state completely disorders itself at 300 °K. Conductivity and thin film data are briefly analysed.     

Insulator-metal transition in the single-crystal high-T c superconductor Bi-2201

Variations in the temperature behavior of resistivity, ρ(T), in the ab plane of the anisotropic single-crystal high-T _c superconductor BiSrCuO (2201 phase) have been observed at the insulator-metal (IM) transition. At low temperatures, as one approaches the transition, the Mott relation for two dimensions, ln ρ ∝ T ^−1/3, changes to ln ρ ∝ T ^−1/2, which corresponds to hopping conduction with a Coulomb gap in the density of states. Negative temperature slopes were revealed in the samples near the transition. Estimates suggest that superconductivity in these samples sets in from the Anderson insulator state. The behavior of the width of the superconducting transition and of the temperature of its onset, T _con, at the IM transition has been studied from measurements of the ac magnetic susceptibility. It is shown that in the vicinity of the IM transition the superconducting transition becomes broader, and the onset of the transition T _con shifts toward higher temperatures. This behavior is attributed to nonuniform superconductivity resulting from formation in the crystal of superconducting droplets with different values of T _c , which is caused by fluctuations in the local density of states due to the inherent disorder in the crystal. In these conditions, superconductivity has a percolation character. Fiz. Tverd. Tela (St. Petersburg) 40, 1190–1194 (July 1998)     

Insulator-metal transition in V3O5

The results of a study of the insulator-metal phase transition in V₃O₅ occuring at T = 450 K are presented. Conductivity, thermopower, magnetic, structural and optical data are reported. On a basis of the data obtained we conclude that the high-temperature phase is a poor metal with strong electron correlations and localized magnetic moments. The metal-non-metal transition is presumably driven by the spatial ordering of V³⁺ and V⁴⁺ ions.     

Insulator-metal transition in a conservative system: An evidence for mobility coalescence in island silver films

Aging, which manifests itself as an irreversible increase in electrical resistance in island metal films is of considerable interest from both academic as well as applications point of view. Aging is attributed to various causes, oxidation of islands and mobility of islands followed by coalescence (mobility coalescence) being the main contenders. The effect of parameters like substrate temperature, substrate cleaning, residual gases in the vacuum chamber, ultrasonic vibration of the substrate, suggest that the mobility coalescence is responsible for the aging in island metal films. Electron microscopy studies show evidence for mobility of islands at high substrate temperatures. The comparison of aging data of island silver films deposited on glass substrates in ultra high vacuum and high vacuum suggests that the oxidation of islands, as being responsible for aging in these films, can be ruled out. Further, under certain conditions of deposition, island silver films exhibit a dramatic and drastic fall in electrical resistance, marking the insulator-metal transition. This interesting transition observed in a conservative system — after the stoppage of deposition of the film — is a clear evidence for mobility coalescence of islands even at room temperature. The sudden fall in resistance is preceded by fluctuations in resistance with time and the fluctuations are attributed to the making and breaking of the percolation path in the film.     

Spin dynamics in trans-polyacetylene

Detailed spin-echo and cw EPR measurements on trans-(CD)_x and trans-(CH)_x are reported. The magnitude of the on-chain diffusion rate, and its temperature dependence are evaluated. The results do not agree with predictions of the soliton model nor with results evaluated from recent NMR measurements.     

Electron-phonon coupling in intrinsic trans-polyacetylene

We consider a valence force field model for the phonon spectrum of (CH)_x and find that phonon coupling to extended π electron states must be included to describe the observed Raman active modes of the polymer. The calculated phonon spectra are shown to possess the zone center dispersion anomalies characteristic of a condensed Peierls ground state.     

Photoluminescence of cis- and trans-polyacetylene

The emission intensity in cis-(CH)_χ increases a little and the emission peak shifts a little to shorter wavelength with increasing temperature, which can be interpreted by the distribution of several emission centers in energy. Photoluminescence is also observed in trans-(CH)_χ at around 1.03 μm for the first time.     

电子关联对聚乙炔中激子类型的影响

在SSH哈密顿基础上引进弱电子关联,对反式聚乙炔链中光致激子的产生和演化过程实施分子动力学模拟.弱关联强度参数U取值0～1.250 eV,V=U/2取值0～0.625 eV.计算结果表明,关联强度的大小影响链中元激发类型,U＜0.555 eV时产生的元激发为孤子-反孤子对,U＞0.555 eV时产生的元激发为正负荷电极化子对.为进一步讨论该类型一维系统中不同类型激子的产生、输运、衰减等动态过程,关联强度参数U的选择提供参考.     

Thermal conductivity of undoped trans-polyacetylene films

The low temperature variation of the thermal conductivity of transpolyacetylene is reported. The thermal conductivity, which though it is comparable to that of a semi-crystalline polymer at 100 K, varies linearly with the temperature below this temperature. The temperature variation of the phonon mean free path is found to be similar to that of an oriented polyethylene.