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We have measured the specific heat of the antiferromagnetic spin-1 chain compounds NENP and ${\mathrm{Y}}_{2}$${\mathrm{BaNiO}}_{5}$ diluted with Zn, at temperatures between 0.1 and 15 K and magnetic fields up to 9 T. We find that the defect-related Schottky anomaly in ${\mathrm{Y}}_{2}$${\mathrm{BaNi}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$${\mathrm{O}}_{5}$ is better described by assuming the defects are S=1, rather than free S=1/2, the latter proposed to exist within the valence-bond-solid model of finite S=1 chains. The relationship of these results to electron-spin-resonance data for NENP, which imply free S=1/2 defects is discussed.

The muon-spin-relaxation (\ensuremath{\mu}SR) technique has been used to study the internal field modulation dynamics of fluxons in crystals of \ensuremath{\kappa}-[BEDT-TTF${]}_{2}$Cu[NCS${]}_{2}$ (${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}10.5 K). The data exhibit a strong field dependence for applied fields in the range 129 Oe\ensuremath{\le}\ensuremath{\Vert}${\mathbf{H}}_{\mathrm{ext}}$\ensuremath{\Vert}\ensuremath{\le}4 kOe, which is attributed to a combination of vortex motion, finite core size effects, and pinning. In low fields (\ensuremath{\Vert}${\mathbf{H}}_{\mathrm{ext}}$\ensuremath{\Vert}=129 Oe), the data exhibit a flux-pinning transition at ${\mathit{T}}_{\mathit{x}}$\ensuremath{\sim}5 K, as evidenced by a peak in the second moment of the local field distribution at \ensuremath{\sim}5 K accompanied by an enhanced minimum in the first moment at the same temperature. In higher fields (\ensuremath{\Vert}${\mathbf{H}}_{\mathrm{ext}}$\ensuremath{\Vert}=750 Oe, 3 kOe, and 4 kOe), where the effects of motion and pinning are minimized, the symmetry of the underlying gap function is confirmed as s wave. Corrections for finite fluxon core, finite mean free path, and flux-lattice disorder were applied in determining the London penetration depth, ${\ensuremath{\lambda}}_{\mathit{L}}^{(\mathit{b}\mathit{c})}$(0)=7680\ifmmode\pm\else\textpm\fi{}700 \AA{}.

We report the first direct measurement of the effective magnetic penetration depth in oriented single crystals of {kappa}-(BEDT-TTF){sub 2}Cu(NCS){sub 2}, with {ital T}{sub {ital c}}(5 G){approx}9 K. Results yield an effective in-plane value of {lambda}{sub {ital e}{ital f}{ital f}}{sup {ital b}{ital c}}(0){approx}9800 A (for {ital H}{sub ext}{approx}3 kG), and a temperature dependence consistent with conventional {ital s}-wave pairing. Comparison with the London penetration depth, {lambda}{sub {ital L}}(0) (estimated to be {approx}5100 A), indicates a tendency toward dirty-limit superconductivity, with the ratio of coherence length over mean free path of {xi}{sub 0}{sup {ital b}{ital c}}/{ital l}{sub {ital b}{ital c}}{approx}2.7. From our results, it appears unnecessary to invoke any unconventional pairing schemes to explain the superconductivity in this material.

Dans la phase metallique, la magnetoconductivite est proportionnelle a Hα ou α est inferieur a 2, et α decroit lorsque le champ magnetique tourne autour de l'axe c * . Dans les etats d'onde de densite de spin induite par champ, il y a une augmentation des anomalies aux transitions de phase lorsque le champ tourne

The usefulness of a simple three-chamber apparatus for the growth of organic donor-acceptor salts has been demonstrated. Relatively large, well-formed crystals of one such salt, TTF-TCNQ, have been grown in this manner and exemplify the method.

Studies of multiline ESR spectra in ten organic charge-transfer salts give g-tensors which are characteristic of individual molecular ions containing sulphur, nitrogen and smaller atoms. Such nearly universal g-tensors are not found in three salts with molecules containing selenium. These conclusions are supported by some results at variable temperature and hydrostatic pressure.

comparaison entre le potentiel d'ionisation obtenu a partir du spectre de photoelectron UV et celui calcule par des methodes ab initio STO-3G

The preparation of high T/sub c/ superconductors, in most cases, proceeds by ceramic methodology from the metal oxides and carbonates. In this paper the authors show that the same materials can easily be derived, as well, from nitrates. Nitrates are a potentially useful class of starting materials because of their solubility and thus possible use in technologies that require the deposition of thin films of precursors to the high T/sub c/ superconductors. When the appropriate mixture of nitrate salts is prepared and ground together, then fired, first in air, and then in pure oxygen, the superconducting material results. Both the La/sub 1.85/Sr/sub 0.15/CuO/sub 4/ and Ba/sub 2/YCu/sub 3/O/sub 7/ materials have been prepared in this way. Their authenticities have been confirmed by comparison of their x-ray powder patterns and resistivity properties with those published for known materials. In addition the authors demonstrated that solutions of nitrate salts can be analyzed for purity by a simple spectroscopic method.

Poly(pyrrole) may be prepared by electrochemical or chemical systhesis, and under appropriate conditions, free-standing films of the oxidized material are produced, which exhibit relatively high conductivity and stability. This material provides an interesting contrast with the phosphazene polymers. Although the polyphosphazene are completely conjugated, all of the currently known materials are insulators; nevertheless the polyphosphazenes exhibit excellent mechanical and chemical stability properties. In the present study we report the synthesis of polybis(pyrrolyl)phosphazene (PBPP), a new polymer which incorporates certain features of both classes of materials. Subsequent electrochemical oxidation of PBPP leads to the deposition of electrically conducting fibrous black films at the anode.

The non-polymeric organic compound: 3,4,9,10 - perylenetetracarboxylic dianhydride (PTCDA) has recently been observed to undergo a large increase in conductivity on irradiation with energetic particle beams. In this paper we discuss the unique properties of both the as-deposited and irradiated forms of PTCDA and related compounds. When the organic compounds are irradiated with 2 MeV Ar+; beams, their resistivity can be reduced over 14 orders of magnitude from ρ (300K) =; 1010 Ω-cm (as-deposited) to ρ (300K) = 5 × 10-4 Ω-cm using a maximum dose of 1017 ions/cm.2 The temperature dependence of the resistivity for doses 1016cm−2 follows ρ(T) ∝ exp (-const./ T1/2) consistent with conduction due to hopping of carriers between isolated, conducting islands formed furing the irradiation process. In addition, we report on studies of structural changes induced by particle irradiation. The unirradiated PTCDA was vacuum sublimed onto (100), 10 Ω-cm p-Si substrates yielding a rectifying contact barrier with a height of ϕB = 0.74 eV. The diodes thus formed undergo avalanche breakdown at Vg = 230V, and exhibit current densities at VB/2 of ≤50 μ A/cm.2 In addition, the forward current-voltage (IF-V) characteristics are strongly dependent on the contact metal used on the top PTCDA surface. The rectifying characteristics reported for the as-deposited material, coupled with the electronic and optical properties of irradiated PTCDA and related compounds suggest that these are interesting electronic materials whose potential device applications appear worthy of further study.

Utilizing the empirical principle that the g-tensor of unpaired spins on molecular ions is nearly independent of the crystalline environment and degree of charge transfer, we discuss the electron spin resonance of a series of conducting salts of tetrathiofulvalene with halogens and pseudohalogens. Comparison is made with other TTF charge-transfer salts.