An electron correlation originated negative magnetoresistance in a system having a partly flat band

TL;DR

This paper proposes a new mechanism for negative magnetoresistance driven by electron correlations in systems with flat band regions, confirmed through theoretical calculations on a Hubbard model.

Contribution

It introduces a novel correlation-based explanation for negative magnetoresistance in flat-band systems, supported by detailed fluctuation exchange approximation calculations.

Findings

Negative magnetoresistance occurs when the Fermi level is in the flat band region.

Magnetic fields shift the majority-spin Fermi level to dispersive regions, reducing resistance.

The mechanism is confirmed in a Hubbard model with significant second-nearest neighbor hopping.

Abstract

Inspired from an experimentally examined organic conductor, a novel mechanism for negative magnetoresistance is proposed for repulsively interacting electrons on a lattice whose band dispersion contains a flat portion (a flat bottom below a dispersive part here). When the Fermi level lies in the flat part, the electron correlation should cause ferromagnetic spin fluctuations to develop with an enhanced susceptibility. A relatively small magnetic field will then shift the majority-spin Fermi level to the dispersive part, resulting in a negative magnetoresistance. We have actually confirmed the idea by calculating the conductivity in magnetic fields, with the fluctuation exchange approximation, for the repulsive Hubbard model on a square lattice having a large second nearest-neighbor hopping.