Tunneling investigation of superconducting (SN) x

Tunneling contacts have been prepared (i) between (SN) _x and bulk indium and (ii) between different (SN) _x crystals. The differential tunneling resistance was measured for temperatures down to 50mK and information on the energy gap of (SN) _x has been derived. The measured gap turns out to depend critically on the way the tunneling contact is prepared.This suggests a strong anisotropy of the gap with a value of 2 /k _B T _c =6.5±1 for tunneling parallel to the polymer chains, being unobserved.     

Electrical properties of (SN)x films

The temperature dependence of the thermopower S and the conductivity σ of (SN)_x films has been investigated as a function of the substrate temperature during film deposition T_s. In contrast to the transport properties of SN_x films previously reported, films made with high T_s have metallic-like transport properties approaching those of (SN)_x crystals. For low substrate temperatures σ and S are found to increase strongly with rising temperature and thermopower values up to 0.9 mV/K are observed. This conduction behavior is explained by a heterogeneous model with areas having different electronic gaps. Annealing as well as decomposition effects suggest that the gap width is correlated to the (SN)_x chain length. No evidence was seen for superconductivity in these films.     

Raman scattering in (SN)x crystals

Polarized Raman spectra are reported for the high frequency intrachain modes in crystals of the metallic polymer (SN)_x. The principal features observed (at 454, 621, 658 and 782 cm^?1) are strong, very anisotropic, and sharper than in thin films. The strongest feature, at 658 cm^?1, narrows but does not shift with decreasing temperature or increasing pressure. Some of the peaks vary in relative intensity as the laser frequency is varied from 4579 to 6005 Å through the plasma resonance.     

Theory of plasmon excitations in (SN)x

The anomalous anisotropy and dispersion of plasmons in polymeric sulfur nitride (SN)_x are explained within a model of an electron gas with an anisotropic effective mass ratio. Calculations in the random phase approximation yield very good agreement with the electron loss data. Exchange corrections for the model are shown to be small for systems of the (SN)_x type which exhibit reduced dimensionality.     

Observation of the Meissner effect in polysulphur nitride, (SN)x

The superconductivity of (SN)_x is definitely established by the observation of a Meissner effect with the magnetic field applied perpendicular and parallel to the (SN)_x fibre axis. Considerable flux trapping is observed and the magnetization curves are similar to those expected for coupled filamentary type II superconductors.     

Mass spectrometric and thermodynamic studies of vapor-phase growth of In(1−x)GaxP

A time-of-flight mass spectrometer was coupled directly to an open tube vapor growth system. The vapor composition and the processes occurring during the deposition of In_(1?x)Ga_xP alloys were studied. It was found that the vapor species present in this system are InCl, GaCl, HCl, PH₃, P₂, P₄ and H₂. The deviations from the chemical equilibrium in the system were determined and measured.A chemical equilibrium model was set and used to calculate the equilibrium partial pressures of all species under a wide range of experimental conditions. These calculations were used to predict successfully the proper conditions for growth of desired In_(1?x)Ga_xP alloys.     

A.C. susceptibility and magnetisation measurements on polysulphur nitride, (SN)x

Measurements of the complex A.C. susceptibility and magnetization of crystals of polysulphur nitride, (SN)_x, are reported. At temperatures near 0.2K both the in-phase and quadrature components of the A.C. susceptibility show a marked change which can be related to the onset of superconductivity. However, changes in the sample magnetization could not be detected.     

High-frequency dielectric study of the dynamics of (KBr)1−x (KCN) x mixed crystals

Dielectric measurements have been performed at 1 and 10 GHz in (KBr)_1–x (KCN) _x mixed crystals for cyanide concentrationsx=0.25 andx=0.50 in the temperature rangeT=1.7–150 K. The maxima of the real and imaginary parts of the dielectric constant falls into the Arrhenius straight lines obtained at lower frequencies. These dipolar lines cross over the so-called quadrupolar (acoustic) lines, in contradiction with theoretical predictions on the nature of the freezing. We also present a qualitative model which reinterprets the dipolar and quadrupolar freezings in terms of an individual and a collective processes.Associated with the Centre National de la Recherche Scientifique     

Angle-resolved photoemission study of the cobalt oxide superconductor Na(x)CoO(2) x yH(2)O: observation of the Fermi surface

The cobalt oxide superconductor Na(x)CoO(2) x yH(2)O is studied by angle-resolved photoemission spectroscopy. We report the Fermi surface (FS) topology and electronic structure near the Fermi level (E(F)) in the normal state of Na(x)CoO(2) x yH(2)O. Our result indicates the presence of the hexagonal FS centered at the Gamma point, while the small pocket FSs along Gamma-K direction are absent, similar to Na(x)CoO(2). The top of the e(g)(') band, which is expected in band calculations to form the small pocket FSs, extends to within approximately 30 meV below E(F), closer to E(F) than in Na(x)CoO(2). We discuss its possible role in superconductivity, comparing with other experimental and theoretical results.     

MuSR study of weakly magnetic Y(Co1−x Al x )2

All previous experiments have suggested that Y(Co_1−x Al _x )₂ is a weak itinerant ferromagnet for 0.12<x<0.20. The muon transverse damping rate (λ) increases rapidly near the Curie point forx≥0.155 and for one sample withx=0.145. The value of λ increases with decreasing temperature for other samples withx<0.155 but shows no feature at the Curie point.     

The orientational glass instability and localization in (KCN) x (KBr)1−x and (KCN) x (NaCN)1−x

Substitutional disorder in mixed crystals with orientational and translational degrees of freedom leads to the appearance of random strain fields. The strains act as static scattering centers for dynamic modes. Near a ferroelastic phase transition where the crystal is very soft, internal friction processes dominate the dynamic restoring forces. The resulting nonergodic instability marks the onset of the orientational glass state, which is characterized by a freezing-in of orientational and translational modes without long range order. The method is based on the study of dynamic equations of mode-coupling type and was originally developed by Götze for the Anderson localization.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

Dielectric properties of (KBr)1−x (KCN) x

The temperature dependence of the dielectric constant of (KBr)_1–x (KCN) _x was measured at frequencies of 87.5 Hz, 1, 10 and 100 kHz for concentrations ofx=0, 0.01, 0.04, 0.14, 0.25, and 0.50. Both the real and the imaginary part showed maxima at characteristic temperatures which depend on the measuring frequency and the concentration. The results are analogous to the behaviour of the magnetic susceptibility in spin glasses and support the existence of an orientational glass state.     

Optical Properties of the (CuInSe2) x ·(2ZnSe)1−x and (CuInTe2) x (·2ZnTe)1−x Solid Solutions

Crystals of the compounds CuInSe₂, CuInTe₂, ZnSe, and ZnTe, and the solid solutions (CuInSe₂) _x ·(2ZnSe)_1–x and (CuInTe₂) _x ·(2ZnTe)_1–x were grown by the Bridgman and chemical transport reactions methods. Their transmission and reflection spectra in the region of the main absorption line edge were studied. The forbidden band gap of the indicated materials was determined and its concentration dependences were built for the solid solutions. It is established that the forbidden band gap changes linearly with the x composition and is satisfactorily described by the square-law dependence.     

Electron spin resonance study of the LaIn(3-x)Sn(x) superconducting system

The LaIn(3-x)Sn(x) alloy system is composed of superconducting Pauli paramagnets. For LaIn3 the superconducting critical temperature T(c) is approximately 0.7 K and it shows an oscillatory dependence as a function of Sn substitution, presenting its highest value T(c) ≈ 6.4 K for the LaSn3 end member. The superconducting state of these materials was characterized as being of the conventional type. We report our results for Gd3+ electron spin resonance measurements in the LaIn(3-x)Sn(x) compounds as a function of x. We show that the effective exchange interaction parameter J(fs) between the Gd3+ 4f local moment and the s-like conduction electrons is almost unchanged by Sn substitution and observe microscopically that LaSn3 is a conventional superconductor.     

Non-ergodic instability and the quadrupolar glass state in K(CN) x Br1−x and (NaCN) x (KCN)1−x

Random strain fields, due to substitutional disorder, couple to translational and orientational dynamic modes in plastic crystals. One distinguishes the cases of weak and strong random fields. In weak fields, the crystal comes close to a structural phase transition. Then elastic restoring forces are weak while internal friction dominates. The resulting non ergodic instability marks the onset of a structural glass state. The evolution of the glass state is discussed within a selfconsistent theory. The results of an extensive numerical study are compared with experiment. In presence of strong random fields no instability occurs, and orientational freezing is a continuous process.     

A study of the microstructure of La1 − x Ca x MnO3 (x = 0.5, 0.8) solid solutions

HREM and XPD methods are used to study the microstructure of La_{1 − x} Ca _x MnO₃ perovskite-like oxides synthesized by pyrolysis of polymer-salt compositions. The XPD data show that the samples studied are single-phase solid solutions. Morphological transitions are observed in series of samples depending on the substitution parameter x. At 0 ≤ x ≤ 0.4 the samples can be characterized by rhombic symmetry (space group Pnmb); at 0.5 ≤ x ≤ 0.8 the symmetry increases to the tetragonal space group I4/mmm; at 0.9 ≤ x ≤ 1 the symmetry lowers to the monoclinic space group P1121. Heating of samples with x = 0.5 and 0.8 up to 1200°C in air does not lead to noticeable changes in the sample structure, which indicates their high thermal stability. When the La_0.2Ca_0.8MnO₃ sample is heated to 1100°C in vacuum nanocrystalline states with particle disintegration into microphases of different structures form because of a partial decomposition of the solid solution. As a result, a system of Mn₃O₄ nanoparticles appears, which is coherently bound with the perovskite phase of a defective structure.     

Crystallization study of amorphous system Fe84−x W x B16 (x=3; 5)

A complex study of the crystallization process of the amorphous system was carried out by Mössbauer spectroscopy, X-ray diffraction and DTA. Alloy samples crystallized at 600 °C contain the following phases: α-Fe, Fe₃B and solid solution (Fe, W)₃B.