Phototransport effects in polyacetylene, (CH)x

We have made the first comprehensive measurements of the photovoltaic and photoconductivity effects in polyacetylene in that we have extended the spectral range originally covered by Matsui and Nakamura [17] to include the visible region as well. The photovoltaic experiments were done on Schottky barrier junctions formed between AsF₅ lightly-doped p-type trans-(CH)_x and a low work function metal, the first junctions of this type to be produced in semiconducting polyacetylene. The observation of a photovoltaic response threshold at 1.48 eV provides the first definitive measurement of the single-particle band gap in trans-(CH)_x. In addition, we have found the existence of a peak at 1.35 eV in the photoconductivity spectrum of undoped trans-(CH)_x which may be due either to extrinsic sources or to thermal dissociation of a weakly bound Wannier exciton.     

Proton NMR studies of the structure of trans-(CH)x

The proton NMR line width in undoped and AsF₅ doped polyacetylene shows an abrupt change as a function of temperature, indicative of motional narrowing. The characteristic temperature for the onset of narrowing decreases with increasing AsF₅ concentration. The results are discussed in terms of large amplitude librational or translational motions of the trans-(CH)_x chains. The magnitudes obtained for the second moment at temperatures below and above the narrowing transition are used to obtain information on the chain packing of trans-(CH)_x within the three-dimensional crystal structure.     

Frequency dependence of the large,electronic χ(3) in polyacetylene

The third order optical susceptibilities (χ⁽³⁾) of trans- and cis- (CH)_x have been determined in the spectral region below the gap by measuring the third harmonic generation efficiency. The magnitude of χ⁽³⁾ parallel to the polymer chains in trans-(CH)_x is in excess of 10⁻⁹ esu for wavelengths larger than 1.3 μm, and the spectrum of χ⁽³⁾ in trans-(CH)_x has a sharp two-photon resonance at an energy corresponding to half the semiconducting gap. Both the off resonance and the two-phonon resonance enhanced values of χ⁽³⁾ are explained by a simple band picture.     

Doping effect on photoluminescence of polyacetylene

Luminescence peaks of both cis-and trans-(CH)x shift to lower energy and their intensities decrease with decreasing temperature. On the other hand, the luminescence peak of cis-(CH)x shifts to higher energy and its intensity decreases with I₂ doping into (CH)x. These results are discussed, tentatively, in terms of luminescence originated from self localized state in (CH)x.     

Solitons and solar cells: The non-equilibrium thermodynamics of the photovoltaic effect in polyacetylene

The production of photovoltages by trans-(CH)_x based devices is analyzed in terms of the non-equilibrium thermodynamics of the photovoltaic effect and compared with the model of photogenerated charged soliton pairs. It is concluded that the source of (CH)_x photovoltages is a photo-reaction channel in which photoexcitations remain as band electrons and holes for appreciable lifetimes.     

PMR investigation of the structure of native and iodine doped polyacetylene films

The second moment of the proton magnetic resonance line of native cis- polyacetylene, thermally isomerized trans-polyacetylene, and iodine doped initially cis-polyacetylene has been measured by a pulse technique. The results differ qualitatively and quantitatively from previous reports. They are discussed in terms of the proposed crystalline structures of (CH)_x.     

Hyperfine magnetic fields on Cd impurity in the Pd2MnIn1-xSnx and Pd2MnSn1-ySby heusler alloys

The TDPAC technique was used to measure the magnetic hyperfine field (mhf) acting on Cd impurity in the Heusler alloys Pd₂MnIn_1-xSn_x and Pd₂MnSn_1-ySb_y for various values of x and y in the range 0 ? x, y ? 1. The alloys of Pd₂MnIn_1-xSn_x are antiferromagnetic at the In-rich end and ferromagnetic at the Sn-rich end, with a transition region x ≈ 0.5?0.7 where both phases coexist; the alloys containing Sn/Sb are ferromagnetic for all values of y. The mhf on the Cd impurity in the antiferromagnetic, transition and ferromagnetic regions of Pd₂MnIn_1-xSn_x are respectively zero, -150 and -200 kOe. For the Sn/Sb alloys the field changes from -200 at the Sn-rich end to -235 kOe at the Sb-rich end. The values of the field very closely follow the trend of the ferromagnetic Curie temperatures for the same alloys as a function of the s-p electron concentration. The observed large distribution of field intensities (～20%) and the lower values of the field in the region x = 0.5?0.6 are attributed mainly to the effect of antiferromagnetic domains. The results are compared with previous Mössbauer mhf measurements at the sites of Sn and Sb in the same alloys as well as with measurements in other Heusler alloys.     

Reconciling charged polarons with the anomalously small Curie-law susceptibility in lightly doped trans-polyacetylene

Results of restricted and unrestricted Hartee-Fock calculations on finite (C₂H₂)_x clusters and of Koster-Slater calculations on the infinite trans-polyacetylene chain suggest the possibility of eliminating the objection that the formation of a charged polaron due to doping is not consistent with susceptibility measurements. Proper self-consistent inclusion of electron-electron interaction leads to localized electronic states in the gap which are spin paired.     

Variation of the magnetic inhomogeneity in glassy FexNi80-xB18Si2 alloys

A careful analysis of the magnetization measurements of amorphous Fe_xNi_80-xB₁₈Si₂ alloys (3 ⩽ x ⩽ 15) above the percolation threshold (x_c = 2.55) has been performed. The non-monotonic variation of the effective critical exponent of the initial magnetic susceptibility, γ¹, with temperature is found to become less pronounced with increasing x - x_c, and probably disappears around x = 20. The tendency towards a homogeneous ferromagnetism around that concentration is also obtained from the analysis of the Arrott-Belov-Kouvel plots. The results are compared with those for similar glassy ferromagnetic systems and a possible origin of the magnetic inhomogeneity is discussed.     

Magnetic phase diagram of diluted spinel Zn1−xCuxCr2Se4 system

By using mean field theory, we have evaluated the nearest-neighbour and the next-neighbour super-exchange J₁(x) and J₂(x), respectively, for Zn_1−xCu_xCr₂Se₄ in the range 0?x?1. The intraplanar and the interplanar interactions are deduced. High-temperature series expansions are derived for the magnetic susceptibility and two-spin correlation functions for a Heisenberg ferromagnetic model on the B-spinel lattice. The calculations are developed in the framework of the random phase approximation. The magnetic phase diagram is deduced. A spin glass phase is predicted for intermediate range of concentration. The results are comparable with those obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility (γ) and the correlation lengths (ν) have been deduced. The values are comparable to those of the 3D Heisenberg model, and are insensitive to the dilution x.     

Magnetic phase diagram of the Ca1−xMnxO systems

The magnetic properties of the Ca_1−xMn_xO systems in the range 0?x?1 have been studied by mean field theory and high-temperature series expansions (HTSEs). By using the first theory, we have evaluated the nearest neighbour and the next-neighbour super-exchange interaction J₁(x) and J₂(x) respectively, in the range 0.45?x?1. The corresponding classical exchange energy for magnetic structure is obtained for the Ca_1−xMn_xO systems. The HTSEs combined with the Padé approximants (PA) method is applied to the Ca_1−xMn_xO systems; we have obtained the magnetic phase diagrams (TN or TSG versus dilution x) in the range 0?x?1. The obtained theoretical results are in agreement with experimental ones obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility (γ) and the correlation lengths (ν) are deduced in the range 0?x?1.     

Vibrational mode of polaron in trans-(CH)x

The vibrational frequency of polaron configuration in trans-(CH)_x is calculated, and we find the obtained wavenumber v_p = 578 cm^?1 coincides with the observed feature of the infrared absorption spectra at the region of 500–600 cm^-1.     

Existence of a Cu3Au-type ordered structure in CdxZn1−xTe epilayers grown on (100) GaAs substrates

Selected area electron diffraction pattern (SADP) and transmission electron microscopy (TEM) measurements were carried out to investigate the spontaneously ordered structure in Cd_xZn_1−xTe epilayers grown on GaAs (100) substrates. The SADP showed superstructure reflections with symmetrical intensity, and the high-resolution TEM (HRTEM) micrographs showed doublet periodicity in the contrast of the {100} lattice planes. The results of the SADP and the HRTEM measurements showed a Cu₃Au-type ordered structure was formed in the Cd_xZn_1−xTe epilayer. The present results can help improve understanding of the Cu₃Au-type ordered structures in Cd_xZn_1−xTe epilayers grown on GaAs substrates.     

Photoinduced absorption in Trans-(CD)x; isotope energy shifts

Steady state photoinduced absorption in trans (CD)_x is reported for energies of 0.1–2.1 eV. All induced infrared active vibrations as well as the photoinduced electronic bands show an energy down shift compared to those of (CH)_x. The change in the induced vibrations spectrum is fully understood and it results from different bare-phonons in the two isotopes. The origin of the isotope shift observed in the electronic bands is discussed.     

Ionic conductivity in tysonite-type solid solutions La1−xBaxF3−x

The ionic conductivity of single crystals of tysonite-type solid solutions La_1?xBa_xF_3?x(0?x?0.095) has been studied parallel and perpendicular to the crystallographic c axis in the temperature range 293–1300 K. Three regions can be discerned in the compositional dependence of the ionic conductivity: (i) the “pure” crystal, in which at room temperature no exchange occurs between different types of anion sites in the tysonite structure; (ii) an intermediate region(0 < x < 7 × 10^-2) which reveals changes in both the conductivity activation enthalpy and the magnitude of the conductivity; (iii) a concentrated solid-solution region (x > 7 × 10^-2), where fluoride ions interchange easily among the different anion sublattices. Diffusion coefficients calculated from ionic conductivity results, are in good agreement with those calculated from ¹⁹F NMR measurements. Using the present data, along with ¹⁹F NMR data, dielectric relaxation data and structural considerations, mechanisms governing the ionic conductivity are proposed.     

On the miscibility gap of MnxCr3−xO4 spinels

Annealing experiments on Mn_xCr_3?xO₄ system were carried out in order to determine the miscibility gap boundaries. The coexisting phases were determined by X-ray analysis. An influence of the oxygen non-stiochiometry was investigated and the boundaries of three phase regions (Cr₂O₃-cubic spinel-bixbyite and cubic spinel-tetragonal spinel-bixbyite) at 1235 K were found.The experimentally determined miscibility gap is compared with that calculated on the basis of the simple thermodynamic model using ΔH_trans and ΔS_trans dependences on compositions recently published.     

Local surrounding of Mn in LaMn1−xCoxO3 compounds by means of EXAFS on Mn–K

A systematic study of LaMn_1?xCo_xO₃ perovskite series by means of X-ray absorption spectroscopy in the extended X-ray absorption fine structure (EXAFS) range of the K-absorption edge of Mn is reported. The Mn–K edge absorption measurements in the EXAFS region were performed to study the local surrounding of Mn ions. Polycrystalline powder samples of LaMn_1?xCo_xO₃ (x=0, 0.02; 0.2; 0.4; 0.5; 0.6; 0.8) prepared by solid-state reaction were used. The EXAFS spectra were analyzed with the FEFF8 computer program. The Mn–O distances of Mn to the nearest oxygen surroundings were evaluated for the samples in the series and compared with the Co–O distances obtained by EXAFS in V. Procházka et al., JMMM 310 (2007) 197 and with results of X-ray powder diffraction in C. Autret, J. Phys. Condens. Matter 17 (2005) 1601.     

Magnetic phase transitions in PrMn2−xCrxGe2

The structural and magnetic properties of PrMn_2−xCr_xGe₂ (0⩽x⩽1.0) were studied by X-ray diffraction and magnetization measurements. The powder samples crystallize in the ThCr₂Si₂-type structure, and the lattice constants at room temperature show almost no variation as Cr substitutes Mn. The observed phase transitions are summarized in a proposed magnetic x−T phase diagram and compared with previous Moessbauer spectroscopy and neutron diffraction results for x=0.     

Spectroscopic and laser properties of (Pr,La)P5O14 and (Pr,Y)P5O14 crystals

Systematic emission and absorption spectroscopic studies of Pr_xLa_1?xP₅O₁₄ (for x=1, 0.9, 0.8, 0.7, 0.5, 0.1) and Pr_xY_1?xP₅O₁₄ (for x=0.75 and 0.5) were performed at 300 and 77 K. The structure of Pr³⁺ energetic levels in pentaphosphates was determined for each concentration considered. Numerical values of the most important spectroscopic parameters — the emission cross section, fluorescence quantum yield and branching ratio — determining the laser properties of the compounds investigated, were evaluated. Also, the radiative lifetime and normalized quenching rate values were obtained. The results of the measurements complete the information about the laser properties of praseodymium pentaphosphates, which were reported in previous papers.