Organic magnetoresistance near saturation: mesoscopic effects in small samples.

TYPECondensed Matter Seminar
Speaker:Prof. M. E. Raikh
Affiliation:Department of Physics and Astronomy, University of Utah
Location:Lidow Nathan Rosen (300)
Abstract:In organic light emitting diodes with small area the current may be dominated by a finite number, N of sites in
which the electron-hole recombination occurs. As a result, averaging over the hyperfine magnetic fields, b_h , that
are generated in these sites by the environment nuclei is incomplete. This creates a random (mesoscopic) current
component, δI(B), at field B having relative magnitude ~N^1/2. To quantify the statistical properties of δI(B)
we calculate the correlator K(B, ∆B) = ⟨δI(B−∆B/2)δI(B+∆B/2)⟩ for parallel, ∆B∥B, and perpendicular, ∆B⊥ B orientations of ∆B.
We demonstrate that mesoscopic fluctuations develop at fields |B| ≫ |b_h|, where the average
magnetoresistance is near saturation. These fluctuations originate from the slow beating between S and T_0 states of
the recombining e-h spin pair-partners. We identify the most relevant processes responsible for the current fluctuations
as due to anomalously slow beatings that develop in sparse e-h polaron pairs at sites for which the b_h pro jections on
the external field direction almost coincide∗ .
∗ based on R. C. Roundy, Z. V. Vardeny, and M. E. Raikh, arXiv:1210.3443