Extraordinary magnetoresistance of organic semiconductors : Hopping conductance via non-zero angular momentum orbitals

TL;DR

This paper links extraordinary magnetoresistance in graphite and organic insulators to hopping conductance via non-zero angular momentum orbitals, suggesting broken time-reversal symmetry as a key factor.

Contribution

It introduces a theoretical framework connecting magnetoresistance behavior to non-zero angular momentum orbitals and TRS breaking in organic and graphite materials.

Findings

Linear negative MR indicates broken TRS.

Quadratic negative MR occurs with preserved TRS.

Broken TRS signatures observed in organic insulators and graphite.

Abstract

Highly-anisotropic in-plane magneto-resistance (MR) in graphite (HOPG) samples has been recently observed (Y. Kopelevich et al., arXiv:1202.5642) which is negative and linear in low fields in some current direction while it is giant, super-linear and positive in the perpendicular direction. In the framework of the hopping conductance theory via non-zero angular momentum orbitals we link extraordinary MRs in graphite and in organic insulators (OMAR) observed in about the same magnetic fields. The theory predicts quadratic negative MR (NMR) when there is a time-reversal symmetry (TRS), and linear NMR if TRS is broken. We argue that the observed linear NMR could be a unique signature of the broken TRS both in graphite and organic compounds. While some local paramagnetic centers are responsible for the broken TRS in organic insulators, a large diamagnetism of our HOPG samples may involve a more intriguing scenario of TRS breaking.