Spatial mapping of mobility and density of the conduction electrons in (FA)2PF6

Novel implementation of the Fourier imaging technique on conduction electron spins in the one-dimensional organic conductors (FA)2PF6 (FA: fluoranthene) is reported. Two-dimensional spatial imaging of resolution 30 &mgr;m(2) is combined with the pulsed-gradient spin-echo technique, to derive maps revealing the local properties of the electron spin density and mobility. The maps generally show pronounced inhomogeneity of both density and mobility on the scale of approximately 30-300 &mgr;m. Highly mobile regions were identified to exist, and the mobility in these was quantitatively evaluated by a basic theoretical model of restricted diffusion.

2-d ESR diffusion coefficient imaging with the projection reconstruction method.

The static gradient spin echo (SGSE) of the conduction electron spins in a quasi-one dimensional organic conductor is analyzed by simulation and for a microstructured (fluoranthene)(2)PF(6) single crystal. It is shown that useful two-dimensional magnetic resonance images of the diffusion coefficient (mobility) D can be obtained from pixel-by-pixel analysis of the signal decay with pulse separation tau, adopting the projection reconstruction (PR) method for signal generation.

Combined k-space q-space pulsed ESR imaging: mapping of restricted diffusion in (FA)(2)PF(6).

A (FA)(2)PF(6) crystal from the family of the quasi-one-dimensional organic conductors was selectively damaged by a beam of Helium ions with a slitted mask placed in the beam's trajectory. Pulsed ESR density weighted imaging of the damaged crystal revealed the appearance of regions where the ESR signal was absent. The one-dimensional motion of the charge carriers was thus restricted to the undamaged sections. The local charge carrier spin dynamics in these restricted areas was probed by combined k-space q-space pulsed ESR imaging. The local expected appearance of the restricted pulsed gradient spin echo (PGSE) "diffusive diffraction" effect is shown. The position of the diffraction minima is compatible with the density imaging results.

Conduction-electron drift velocity measurement via electron spin resonance.

In analogy with NMR, motion induced phase shift of pulsed ESR signals enables in principle the direct detection of electron drift velocity or electronic current, respectively. Overcoming the difficulties with additional magnetic field gradients induced by the current itself, we succeeded in demonstrating the detection of electron flow via ESR. Measuring the electron drift velocity in the organic conductor (fluoranthene)2PF6 the microscopic Ohmic law could be observed in a current range of more than +/-0.25 A.

Zebralike patterned organic conductor with periodic modulation of mobility and Peierls transition.

We manipulated the defect concentration in a (fluoranthene)2PF6 crystal by proton irradiation through a periodic grid, resulting in a striped defect pattern. Spatially resolved pulsed X-band ESR analysis was used to quantify the resulting local defect concentrations, spin diffusion coefficients, and electron spin concentrations. The temperature dependence of the data proves that spin diffusion coefficient and Peierls transition can be tailored in a controlled way via the defect concentrations.

Diffusive diffraction of the local ESR pulse-gradient spin-echo signal in a restricted one-dimensional conductor.

The diffusive motion of the conducting electrons in the one-dimensional organic conductor FA2PF6 (FA, Fluoranthene) is studied with the ESR pulse-gradient spin-echo (PGSE) signal combined with spatial density ESR imaging. A local measurement of the short restricted regions reveals diffraction patterns in the local PGSE data. A model calculation adapted to the local measurements provides a highly accurate quantitative description of the results. The appearance of these patterns provides information about the diffusive motion of the charge at the crystal boundaries.