Extraordinary magnetoresistance in graphite: experimental evidence for the time-reversal symmetry breaking

TL;DR

This paper reports extraordinary, anisotropic magnetoresistance in graphite, providing experimental evidence for broken time-reversal symmetry possibly linked to superconducting clusters and unconventional order parameters.

Contribution

It presents the first experimental observation of linear orbital NMR as a signature of broken TRS in graphite, suggesting a novel mechanism involving superconducting clusters.

Findings

Anisotropic magnetoresistance observed in graphite.

Linear and negative MR in certain directions at low fields.

Giant super-linear positive MR at higher fields.

Abstract

The ordinary magnetoresistance (MR) of doped semiconductors is positive and quadratic in a low magnetic field, B, as it should be in the framework of the Boltzmann kinetic theory or in the conventional hopping regime. We observe an unusual highly-anisotropic in-plane MR in graphite, which is neither quadratic nor always positive. In a certain current direction MR is negative and linear in B in fields below a few tens of mT with a crossover to a positive MR at higher fields, while in a perpendicular current direction we observe a giant super-linear and positive MR. These extraordinary MRs are respectively explained by a hopping magneto-conductance via non-zero angular momentum orbitals, and by the magneto-conductance of inhomogeneous media. The linear orbital NMR is a unique signature of the broken time-reversal symmetry (TRS) in graphite. While some local paramagnetic centers could be responsible for the broken TRS, the observed large diamagnetism suggests a more intriguing mechanism of this breaking, involving superconducting clusters with unconventional (chiral) order parameters and spontaneously generated normal-state current loops in graphite.