Time-resolved imaging of spin transfer switching: beyond the macrospin concept

Time-resolved images of the magnetization switching process in a spin transfer structure, obtained by ultrafast x-ray microscopy, reveal the limitations of the macrospin model. Instead of a coherent magnetization reversal, we observe switching by lateral motion of a magnetic vortex across a nanoscale element. Our measurements reveal the fundamental roles played independently by the torques due to charge and spin currents in breaking the magnetic symmetry on picosecond time scales.     

The massless Thirring model: Positivity of Klaiber'sn-point functions

We present a simple solution to the problem of proving positivity of Klaiber'sn-point functions for the massless Thirring model. The corresponding fields are obtained as strong limits of explicitly given approximate fields, obviating reconstruction. By invoking recent results on the boson-fermion correspondence it is shown how the model can be formulated on the charged fermion Fock space. It is pointed out that the question of cyclicity of the vacuum is open, and that an affirmative answer is necessary to confirm the superselection sector picture of the model.     

Fluorescence and Rotational Dynamics of Dityrosine

We have examined the lifetimes and rotational correlation times of dityrosine emission by time-correlated single-photon counting. We first noticed dityrosine fluorescence in samples of tyrosine and tyrosine dipeptides by its characteristic red-shifted emission at 400 to 430 nm. The longer rotational correlation time relative to tyrosine proved that this fluorescence emanated from a distinct species. Comparison with the fluorescence properties of synthesized dityrosine established the identity of the emitting species. Fluorescence intensity decays of dityrosine are generally characterized by two decay components, one with a lifetime in the range of 150 to 800 ps and another between 2.5 and 4.5 ns. We found no evidence for an excited-state reaction, since a rising phase (negative-amplitude component) was not observed. In the pH range from 4 to 10, two ground-state species exist in equilibrium with pK _a 7. Both species exhibit two fluorescence decays. The average fluorescence lifetime increases gradually with pH over the pH range from 4 to 10 and decreases at pH 2. Anisotropy decays were measured for dityrosine and the alanine–dityrosine–alanine and leucine–dityrosine–leucine dipeptides. The rotational correlation times of dityrosine and dityrosine dipeptides increase linearly with van der Waals volumes. The slope indicates a stronger solute–solvent interaction than predicted with stick boundary conditions. It is suggested that these interactions result from the presence of two zwitterionic pairs.