We present experimental data for the Raman intensity in the spin-Peierls compound CuGeO3 and theoretical calculations from a one-dimensional frustrated spin model. The theory is based on (a) exact diagonalization and (b) a recently developed solitonic mean field theory. We find good agreement between the 1D-theory in the homogeneous phase and evidence for a novel dimerization of the Raman operator in the spin-Peierls state. Finally we present evidence for a coupling between the interchain exchange, the spin-Peierls order parameter and the magnetic excitations along the chains. 78.20.Ls Low-dimensional spin systems exhibit many unusual properties resulting from quantum and dimensionality effects. An example is the continuum of spin-wave excitations in quantum one-dimensional (1D) spin systems which has been predicted for a long time [1] and has recently been confirmed by neutron scattering experiments [2] on KCuF 3 .Quantum 1D spin systems also show a variety of instabilities. Of particular interest is the spin-Peierls (SP) phase due to residual magnetoelastic couplings [3], which leads to the opening of a gap in the spin excitation spectrum. The discovery [4] of the spin-Peierls transition below T SP = 14 K in an inorganic compound, CuGeO 3 , has attracted widespread attention. This compound consists of chains of spin-1/2 Cu 2+ ions coupled by antiferromagnetic superexchange via the oxygen orbitals [4,5]. The Cu ions lie along the crystallographic c-axis and the exchange along the chains can be modeled by the 1D Hamiltonianwhere δ is the dimerization parameter that vanishes above T SP [6,7]. The special geometry [6,8] of the superexchange path in CuGeO 3 leads to a small value of the exchange integral J ≈ 150K and a substantial n.n.n. frustration term ∼ α which competes with the n.n. antiferromagnetic exchange. The interchain couplings have been estimated to be small, J b ≈ 0.1J and J a ≈ −0.01J for the interchain exchange constants along a-and b-directions, respectively [5]. The phase diagram of H in Eq.(1) has been calculated using the density-matrix renormalization-group method [9]. For δ = 0 and α < α c ≈ 0.2411, the ground state is gapless and renormalizes to the Heisenberg fixed point. For α = 0.5 and δ = 0, the ground state is given by a valence-bond state and a gap of order J/2 induced by frustration is present. While the evaluation of the dynamical properties of (1) is a challenge to theory, the rich phase diagram can be explored by a variety of interesting experiments. In this context, the substantial value of the n.n.n. exchange integral in CuGeO 3 allows the experimental investigation of the effects of competing interactions in a low dimensional magnet, both in the uniform and in the spin-Peierls state.An experimental method particularly suited for the study of magnetic excitations in an antiferromagnet is two magnon Raman scattering. For CuGeO 3 , the Raman operator in A 1g symmetry [10]is proportional toIn the homogeneous state (δ = γ = 0) the interaction Hamiltonian commutes with the Heisenberg...

Passive films formed in 1N HCI at different potentials on mechanically polished samples of the alloy Fe,,Cr,,P,,C, were studied by XPS surface analysis. The thickness of the films increases with the applied potential; at 0.4 V/SCE (saturated calomel electrode) a discontinuity is observed. The passive Alms are enriched in chromium oxyhydroxide; phosphates are incorporated in notable amounts. The substrate underneath the film is enriched in phosphorus The outstanding corrosion resistance of this alloy may thus be attributed to this enrichment of phosphorus underneath the film, reducing to a great extent the ionic conductivity of the interface, and to the incorporation of phosphates in the passive film.

Raman experiments on the spin-Peierls compound CuGeO3 and the substituted (Cu1−x,Znx)GeO3 and Cu(Ge1−x,Gax)O3 compounds were performed in order to investigate the response of specific magnetic excitations of the one-dimensional spin-1/2 chain to spin anisotropies and substitution-induced disorder. In pure CuGeO3, in addition to normal phonon scattering which is not affected at all by the spin-Peierls transition, four types of magnetic scattering features were observed. Below TSP =14 K a singlet-triplet excitation at 30 cm −1 , two-magnon scattering from 30 to 227 cm −1 and folded phonon modes at 369 and 819 cm −1 were identified. They were assigned by their temperature dependence and lineshape. For temperatures between the spin-Peierls transition TSP and approximately 100 K a broad intensity maximum centered at 300 cm −1 is observed. The temperature dependence of this intensity is similar to the behavior of lattice fluctuations recently observed in an electron diffraction study. This scattering is attributed to dynamical spin-Peierls fluctuations of the weakly or non-static dimerized spin chain. 1 1

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