Asymmetrically doped polyacetylene

Doped one-dimensional (1D) conjugated polymers, such as polyacetylene, have a conductivity of some metals, like copper. We show that when this polymer is asymmetrically doped, unexpected properties are revealed, when compared to the behavior of the symmetrically standard doped systems (SDS). Depending on the level of imbalance between the chemical potentials of the two involved fermionic species, the polymer can be converted into a 1D partially or fully polarized organic conductor.     

Micro-ring cavity mode of dye-doped thin film: Dependence on the excitation energy

We have prepared π-conjugated organic dye specimens embedded in a transparent polymer film and observed a characteristic photoluminescence peak, having very narrow peak width as compared with the fluorescence in solution, when we prepared the film around the perimeter of a multimode silica fiber. The peak position showed no dependence on the excitation wavelength, indicating that it could not be explained by a whispering gallery mode. The peaks can be explained by amplified spontaneous emissions (ASE) because the intensity depended linearly on the excitation intensity with some threshold. When the excitation laser was directly coupled to the fiber core, we observed ASE peaks of the optical fiber itself. These fiber ASE peaks shifted to longer wavelength when we varied the excitation to shorter wavelength, which clearly ruled out the possibility of silicate Raman scattering as the origin.     

Speciation of actinide ions in aqueous solution by laser-induced pulsed photoacoustic spectroscopy

The oxidation-state specific detection of actinide ions, U(VI), Pu(IV), Pu(VI), and Am(III), was investigated by pulsed laser photoacoustic spectroscopy. With a single-beam differential technique, the solvent effect is compensated and thus the detection sensitivity substantially increased. The observed detection sensitivities are8×10^–7M/L for U(VI)7×10^–8M/L for Pu(IV)3×10^–8M/L for PU(VI)2×10^–8M/L for Am(III)The present method facilitates the study of complexation and polymerization of actinide ions in the submicromole concentration range.     

Raman characterization of H:LiNbO3 waveguides

In this work we present polarized Raman measurements of z-cut multimode H:LiNbO₃ waveguides in the κ_i- and β_i-phase states. A spectrum with unexpected form and behaviour, originating from the layer at the very surface, was observed in samples with various degrees of Li–H substitution. Possible reasons for the presence of this strange spectrum are considered. Received: 16 May 2001 / Revised version: 7 August 2001 / Published online: 23 October 2001     

Phase diagram of electron-hole systems: Interplay between exciton Mott transition and quantum pair condensation

Quasi-thermal-equilibrium states of electron-hole (e-h) systems in photoexcited insulators are studied from a theoretical viewpoint, stressing the exciton Bose-Einstein condensation (BEC), the e-h BCS-type pair-condensed state, and the exciton Mott transition between an insulating exciton/biexciton gas phase and a metallic e-h plasma phase. We determine the quasi-equilibrium phase diagram of the e-h system at zero and finite temperatures with applying the dynamical mean-field theory (DMFT) to the e-h Hubbard model with both repulsive and attractive on-site interactions. Effects of inter-site interactions on the exciton Mott transition are also clarified with applying the extended DMFT to the extended e-h Hubbard model.     

Initial oxidation of Mn surface studied by ultraviolet photoelectron spectroscopy and metastable induced electron spectroscopy

Metastable induced electron spectroscopy (MIES) was combined to ultraviolet photoelectron spectroscopy (UPS) to study the initial steps of manganese oxidation. Oxygen exposure directly led to the formation of MnO with no intermediate states. The MnO feature saturation observed by MIES and UPS techniques showed noticeable differences and proved the formation of several oxide layers. The oxidation kinetics was studied by measuring MnO features by UPS, which depend on the surface coverage by oxygen. We observe a decrease of oxygen adsorption probability with oxygen exposure. Oxidation proceeds by oxygen dissolution into the first layers to form a three-dimension MnO. This hypothesis was confirmed by our work function measurements.     

Temperature dependence of infrared and Raman modes in polymeric RbC60

Temperature dependence of the intra-molecular vibrational modes of C₆₀ in the quasi-1D polymeric RbC₆₀, across the low temperature transition at ∼50 K, has been probed through infrared (IR) and Raman spectroscopies. With the lowering of temperature, the split IR modes of RbC₆₀ are seen to harden but below 50 K a small but definitive signature of an anomalous softening is observed. In addition, the background IR transmission shows an increase below 50 K with the opening of a well defined gap in the electronic spectrum. The implications of these results, along with those of Raman measurements, are discussed in terms of the interaction of intra-molecular phonons with electrons and spin excitations in the system.     

Study of photo-induced phenomena in VO2 by terahertz pump-probe spectroscopy

The ultrafast terahertz response to the photoexcitation in vanadium dioxide was investigated using the optical-pump terahertz-probe technique at room temperature. The optical excitation at 790 nm induced an ultrafast decrease of the transmittance of the terahertz pulses corresponding to the increase of the electronic conductivity within 0.7 ps, and then the transmittance decreases gradually up to 100 ps. This two-step behavior is very similar to the previous reports of the time resolved X-ray and electron diffractions. This fact indicates that the increase of the electronic conductivity and the change of the lattice structure proceed in parallel, and the photo-induced insulator-metal phase transition is of the Peierls type.     

Effects of low frequency near metal-insulator transition temperatures on polycrystalline La0.65Ca0.35Mn1−yFeyO3 (where y=0.05-0.10) ceramic oxides

The ac electrical properties of 5-10% Fe doped polycrystalline sample have been investigated by complex impedance analysis over the frequency and temperature ranges of 1-100 kHz and 77-300 K, respectively. The average normalized change (ΔZ′/Δf)/Z₀ has been deduced for these Fe doped CMR samples which shows an increasing trend with iron doping. The most pronounced effect of frequencies is at T_c, with the increase of Fe doping it is observed that not only T_c is lowered substantially but also the height of the peaks of real part of impedance (Z′) is increased which in turn decreases considerably with the increase of the ac field. An equivalent circuit model, R_g(R_gbC_gb), i.e. a resistor-capacitor network, has been proposed to explain the impedance results at different temperatures. The plot between τ and 1/T gives a straight line from where relaxation time (τ₀) has been deduced. The correlated barrier hopping (CBH) model has been employed and the binding energy of the defect states is estimated to be between 0.39 and 0.25 eV while the minimum hoping distance varies within the range of 2.93-5.21 Å for these 5-10% Fe doped LCM samples.     

Effects of doping Na on the structure and physical properties of La2/3Ca1/3MnO3

Effects of doping Na on the structure, electrical and magnetic properties of La_2/3Ca_1/3MnO₃ are investigated. A structural phase transition from orthorhombic to rhombohedral structure takes place at y=0.375. All samples show metal-insulator (M-I) transition at the transition temperature and undergo the transition from paramagnetism to ferromagnetism at the Curie temperature T_C. and T_C increase monotonically with increasing Na content. However the Na-doped samples have a shoulder in their electrical transport curves found below and shows a widened magnetic transition process. On the other hand, intrinsic colossal magnetoresistance (CMR) peaks are observed in all the samples, but samples with y around 0.25 show two MR peaks which can be attributed to magnetic inhomogeneity induced by the doped Na⁺ ions. Here we propose a method to broaden the CMR peak of perovskite manganite, which is beneficial for practical applications.     

Two kinds of phase transition in 1-D backward scattering

The one-dimensional Fermion system with backward scattering has been analyzed by use of the methods of bosonization and Gaussian wave functional. We find that there exist two kinds of phase transitions in the spin density degree according to the interaction parameters: one is a Kosterlitz-Thouless type transition and the other is a first order phase transition when backward scattering becomes sufficiently strong.     

Cr-doping effect on the structural, magnetic, transport properties and Raman spectroscopy of La(2+x)/3Sr(1−x)/3Mn1−xCrxO3 perovskites

A series of the double-doping samples La_(2+x)/3Sr_(1−4x)/3Mn_1−xCr_xO₃ (0?x?0.25) with the Mn³⁺/Mn⁴⁺ ratio fixed at 2:1 have been fabricated. The structural, magnetic, transport properties and Raman spectroscopy have been investigated, and no apparent crystal structure change is introduced by Cr doping up to x=0.25. But the Curie temperature T_C and metal-insulator transition temperature T_MI are strongly affected by Cr substitution. The room temperature Raman spectra start exhibiting some new features following the increasing concentration of Cr substitutions. Moreover, it is worth noting that the frequency of the A_1g phonon mode can also be well correlated with the A-site mismatch effect (σ²), which is influenced mainly by the variety of the Sr content.     

Crystal structure and structural transition caused by charge-transfer phase transition for iron mixed-valence complex (n-C3H7)4N[FeFe(dto)3] (dto=C2O2S2)

(n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] shows a new type of first order phase transition called charge-transfer phase transition around 120 K, where the charge transfer between Fe^II and Fe^III occurs reversibly. Recently, we have succeeded in obtaining single crystals of the title complex and determined the crystal structure at room temperature. Crystal data: space group P6₃, Z=2. Moreover, we have investigated the structural transition caused by the charge-transfer phase transition by means of powder X-ray diffraction measurement. When the temperature is decreased, the a-axis, which corresponds to the hexagonal ring size in two-dimensional honeycomb network structure of [Fe^IIFe^III(dto)₃]_∞, contracts by 0.1 Å at the charge-transfer transition temperature (T_CT), while the c-axis, perpendicular to the honeycomb network layer, elongates by 0.1 Å at T_CT. Consequently, when the temperature is decreased, the unit cell volume decreases without noticeable anomaly around T_CT, which is responsible for the quite small vibrational contribution to the entropy change, compared with usual spin crossover transition. Thus, the charge-transfer phase transition around 120 K for (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] is regarded as spin entropy driven phase transition.     

Metastability in the resistance of polycrystalline La0.8Li0.2MnO3

Bulk polycrystalline La_0.8Li_0.2MnO₃ is found to switch between a low‐resistance state and a high‐resistance state on thermal cycling. The low‐temperature, high‐resistance state exhibits strong electroresistance whereas the high‐temperature, low‐resistance state does not. The change in resistance between the two distinct states is of two orders of magnitude. It is proposed that the observed metastability may serve as the basis for resistive thermal‐switching devices.

     

Insulator–semiconductor–metallic state transition induced by electric fields in Mn‐doped NaNbO3

The electro‐forming procedure was applied to NaNbO₃:Mn and NaNbO₃ insulator crystals. The electric current flow induced a transition to the metallic‐type temperature dependence of the resistance. The Mn dopant shortened the time needed for the transition. The LC‐AFM measurement showed a non‐homogeneous distribution in local resistance resulting from the electric field via the AFM tip. We ascribe this effect to percolation in the network of the highly conducting filaments, whose formation is facilitated by the Mn ions. We conclude that the insulator–metal transition is induced within a subsystem of extended defects already existing in the NaNbO₃:Mn crystal lattice host. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spin-state transition, magnetic, electrical and thermal transport properties of the perovskite cobalt oxide Gd0.7Sr0.3CoO3

The magnetization, resistivity ρ, thermoelectric power (TEP) S, and thermal conductivity κ in perovskite cobalt oxide Gd_0.7Sr_0.3CoO₃ have been investigated systematically. Based on the temperature dependence of susceptibility χ_g(T) and Seebeck coefficient S(T), a combination of the intermediate-spin (IS) state for Co³⁺ and the low-spin (LS) state for Co⁴⁺ can be suggested. A metal-insulator transition (MIT) caused by the hopping of σ* electrons (localized or delocalized e_g electrons) from the IS Co³⁺ to the LS Co⁴⁺ is observed. Meanwhile, S(T) curve also displays an obvious phonon drag effect. In addition, based on the analysis of the temperature dependence of S(T) and ρ(T), the high-temperature small polaron conduction and the low-temperature variable-range-hopping conduction are suggested, respectively. As to thermal conduction κ(T), rather low κ values in the whole measured temperature range is attributed to unusually large local Jahn-Teller (JT) distortion of Co³⁺O₆ octahedra with IS state.     

Dielectric critical behaviour at a metal-insulator transition under lattice compression

We study dielectric critical behaviour around a continuous metal-insulator transition in crystalline Cesium Iodide induced by changing the lattice parameter. The ab initio calculations of band structure and various quantities related to the dielectric response are performed in the transition region, within the local density approximation of the density functional theory. These calculations allow us to establish the power-law singularities of various quantities on two sides of the transition. The exponents obtained here are mean-field like due to the approximation in which interactions and disorder are treated. The critical behaviour is discussed by applying the scaling principle to the wavevector and frequency dependent dielectric function. We further investigate the effect of dielectric anomalies on optical properties by calculating the reflectance around transition region taking the ionic contribution to the dielectric function also into account. We find that the reflectance as a function of frequency shows very different kind of behaviour on both sides of the metal-insulator transition.     

Lanthanum deficiency effect in magnetic transition and transport property of La0.8−δCa0.2MnO3

The evolution of magnetic and electrical phases in La_0.8−δCa_0.2MnO₃ was investigated in terms of La deficiency. We found that the increase of the La deficiency tends to raise the Curie temperature (T_C) in La_0.8−δCa_0.2MnO₃. The FM clusters formed in compounds with large La deficiency provide percolation paths above T_C. With increasing the La defect, the transport property changes from insulating to metallic state, which is in association with the crossover from a second order to a first order magnetic phase transition in the vicinity of T_C.     

Magnetoresistance and transport properties of different impurity doped La0.67Ca0.33MnO3 composite

An enhanced magnetoresistance and a two-fold effect result from impurity dopant were observed in composites of La_0.67Ca_0.33MnO₃/YSZ and La_0.67Ca_0.33MnO₃/Fe₃O₄. Where YSZ represents yttria-stabilized zirconia and the doping level of both YSZ and Fe₃O₄ is 1 mol%. Different electrical and magnetic transport properties, in particular a lower field magnetization behavior, were observed between pure La_0.67Ca_0.33MnO₃ and the impurity doped La_0.67Ca_0.33MnO₃ composites. Compared with pure La_0.67Ca_0.33MnO₃, a possible interpretation is presented by considering the influences of YSZ and Fe₃O₄ on the structure of grain boundaries and/or surfaces of La_0.67Ca_0.33MnO₃grains.     

Valence transition behavior of the doped Falicov-Kimball model at nonzero temperatures

The extrapolation of small-cluster exact-diagonalization calculations is used to study the influence of doping on valence transitions in the spinless Falicov-Kimball model at nonzero temperatures. Two types of doping are examined, and namely, the substitution of rare-earth ions by nonmagnetic ions that introduce (i) one or (ii) none additional electron (per nonmagnetic ion) into the conduction band. It is found that the first type of substitution increases the average f-state occupancy of rare-earth ions, whereas the second type of substitution has the opposite effect. The results obtained are used to describe valence transition behavior of samarium in the hexaboride solid solutions Sm_1−xM_xB₆ (M=Y³⁺, La³⁺, Sr²⁺, Yb²⁺) and a very good agreement of theoretical and experimental results is found.