Observation of Possible Nonlinear Anomalous Hall Effect in Organic Two-Dimensional Dirac Fermion System

TL;DR

This paper reports the first observation of nonlinear anomalous Hall effect in an organic 2D Dirac fermion system, revealing topological transport and spontaneous symmetry breaking in organic conductors.

Contribution

It demonstrates the existence of NLAHE in an organic conductor near the Dirac fermion phase transition, a novel topological transport phenomenon in such materials.

Findings

Detected nonlinear Hall voltage proportional to square current at zero magnetic field.

Observed current direction dependence and correlation with charge order.

First evidence of topological transport in organic conductors.

Abstract

We report the observation of nonlinear anomalous Hall effect (NLAHE) in the multilayered organic conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ in the charge order (CO) insulating phase just under the critical pressure for transition into two-dimensional (2D) massless Dirac fermion (DF) phase. We successfully extracted the finite nonlinear Hall voltage proportional to square current at zero magnetic field. The observed NLAHE features, current direction dependence and correlation with CO, are consistent with the previous estimation assuming 2D massive DF with a pair of tilted Dirac cones. This is the first observation of topological transport in organic conductors, and also the first example of NLAHE in the electronic phase with spontaneous symmetry breaking.