Effect of spin-orbit coupling on magnetoresistance in organic semiconductors

We study the recently discovered organic magnetoresistance (OMAR) effect in a pair of materials that have similar chemical structures except that one contains a heavy atom to enhance spin-orbit coupling. We use photoluminescence spectroscopy to estimate the spin-orbit coupling strength. In the material with weak spin-orbit coupling the characteristic magnetic field scale is comparable to the hyperfine coupling strength. In the material with strong spin-orbit coupling we find that the OMAR is strongly reduced in size and the OMAR traces clearly exhibit a second, higher field scale which we identify with the spin-orbit coupling strength. We model our results using the standard spin-dynamics Hamiltonian.