1,055 research outputs found
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 MgB 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 . Application to MgB yields
good quantitative agreement with recently reported data of dHvA oscillation in
the superconducting state
Spectroscopic Evidence for Multiple Order Parameter Components in the Heavy Fermion Superconductor CeCoIn_5
Point-contact spectroscopy was performed on single crystals of the
heavy-fermion superconductor CeCoIn_5 between 150 mK and 2.5 K. A pulsed
measurement technique ensured minimal Joule heating over a wide voltage range.
The spectra show Andreev-reflection characteristics with multiple structures
which depend on junction impedance. Spectral analysis using the generalized
Blonder-Tinkham-Klapwijk formalism for d-wave pairing revealed two coexisting
order parameter components, with amplitudes Delta_1 = 0.95 +/- 0.15 meV and
Delta_2 = 2.4 +/- 0.3 meV, which evolve differently with temperature. Our
observations indicate a highly unconventional pairing mechanism, possibly
involving multiple bands.Comment: 4 pages, 3 figure
- …