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Tunneling through magnetic molecules with arbitrary angle between easy axis and magnetic field
Inelastic tunneling through magnetically anisotropic molecules is studied
theoretically in the presence of a strong magnetic field. Since the molecular
orientation is not well controlled in tunneling experiments, we consider
arbitrary angles between easy axis and field. This destroys all conservation
laws except that of charge, leading to a rich fine structure in the
differential conductance. Besides single molecules we also study monolayers of
molecules with either aligned or random easy axes. We show that detailed
information on the molecular transitions and orientations can be obtained from
the differential conductance for varying magnetic field. For random easy axes,
averaging over orientations leads to van Hove singularities in the differential
conductance. Rate equations in the sequential-tunneling approximation are
employed. An efficient approximation for their solution for complex molecules
is presented. The results are applied to Mn12-based magnetic molecules.Comment: 10 pages, 10 figures include
Superconductivity in pure and electron doped MgB2: Transport properties and pressure effects
The normal state and superconducting properties of MgB2 and Mg1-xAlxB2 are
discussed based on structural, transport, and high pressure experiments. The
positive Seebeck coefficient and its linear temperature dependence for Tc<T<160
K provide evidence that the low-temperature transport in MgB2 is due to
hole-like metallic carriers. Structural and transport data show the important
role of defects as indicated by the correlation of Tc, the residual resistance
ratio, and the microstrain extracted from x-ray spectra. The decrease of Tc
with hydrostatic pressure is well explained by the strong-coupling BCS theory.
The large scatter of the pressure coefficients of Tc for different MgB2
samples, however, cannot be explained within this theory. We speculate that
pressure may increase the defect density, particularly in samples with large
initial defect concentration.Comment: Presented at NATO Advanced Research Workshop "New Trends in
Superconductivity", Yalta (Ukraine) 16-20 September, 200
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