Atom transfer reactions of high-valent chromium and manganese porphyrins

The terminally bound nitrido ligand of manganese(V) porphyrin complexes serves as a good bridging species in electron transfer processes. Thus, the nitrogen atom of a porphyrinatomanganese nitrido complex was reversibly transferred to porphyrinatomanganese(II), in a formal three-electron redox process. Evidence is presented that supports an inner-sphere mechanism for this process. The coordinating solvent, tetrahydrofuran, decreased the rate constant for this reaction by three orders of magnitude compared to that observed in the noncoordinating solvent, toluene. The number of redox equivalents exchanged in nitrogen atom transfer processes can be varied from three to two by choosing the appropriate reductant such as a porphyrinatomanganese(III) chloride complex. Evidence is presented to support a mechanism for the reaction between porphyrinatomanganese nitride and porphyrinatomanganese chloride which involves prior dissociation of the chloride ion before the formation of a [mu]-nitrido species. Thus when given a choice of potential bridging ligands (eg. N vs Cl or pivalate) the multi-electron process studied here preferentially selects the nitrido ligand;The terminally bound oxo ligand of chromium porphyrin complexes has also been shown to also serve as a good bridging ligand in electron transfer processes. It was demonstrated that a one-electron transfer involving complete atom transfer was possible by choosing the correct redox partner, a porphyrinatochromium(III) complex. The mechanism for this reaction has been shown to proceed through a [mu]-oxo species. Unlike the mechanism for porphyrinatomanganese chloride, the porphyrinatochromium chloride complex does not require prior dissociation of the chloride ion. The rate constant was not decreased by addition of a ten fold excess of chloride ion

Point-contact spectroscopy in heavy-fermion superconductors

We develop a minimal model to calculate point-contact spectra between a metallic tip and a superconducting heavy-fermion system. We apply our tunneling model to the heavy fermion CeCoIn5, both in the normal and superconducting state. In point-contact and scanning tunneling spectroscopy many heavy-fermion materials, like CeCoIn5, exhibit an asymmetric differential conductance, dI/dV, combined with a strongly suppressed Andreev reflection signal in the superconducting state. We argue that both features may be explained in terms of a multichannel tunneling model in the presence of localized states near the interface. We find that it is not sufficient to tunnel into two itinerant bands of light and heavy electrons to explain the Fano line shape of the differential conductance. Localized states in the bulk or near the interface are an essential component for quantum interference to occur when an electron tunnels from the metallic tip of the point contact into the heavy-fermion system.Comment: 13 pages, 9 figures. Accepted for publication in Physical Review

Andreev experiments on superconductor/ferromagnet point contacts

Andreev reflection is a smart tool to investigate the spin polarisation P of the current through point contacts between a superconductor and a ferromagnet. We compare different models to extract P from experimental data and investigate the dependence of P on different contact parameters.Comment: 14 pages, 5 figures, accepted for publication in Fizika Nizkikh Temperatu

Hall conductance of a pinned vortex lattice in a high magnetic field

We calculate the quasiparticle contribution to the zero temperature Hall conductance of two-dimensional extreme type-II superconductors in a high magnetic field, using the Landau basis. As one enters the superconducting phase the Hall conductance is renormalized to smaller values, with respect to the normal state result, until a quantum level-crossing transition is reached. At high values of the order parameter, where the quasiparticles are bound to the vortex cores, the Hall conductance is expected to tend to zero due to a theorem of Thouless.Comment: To appear in Journ. Phys. : Cond. Matte

Large Broadening of the Superconducting Transition by Fluctuations in a 3D Metal at High Magnetic Fields: The MgB2_{2} case

It is shown that the transition to the low temperature superconducting state in a 3D metal at high magnetic field is smeared dramatically by thermal fluctuation of the superconducting order parameter. The resulting superconducting-to-normal crossover occurs in a vortex liquid state which is extended well below the mean-field $H_{c2}$. Application to MgB$_{2}$ yields good quantitative agreement with recently reported data of dHvA oscillation in the superconducting state