Multiband Effect and Possible Dirac Fermions in Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$

TL;DR

This study reveals multiband effects and potential Dirac fermions in Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$, showing how annealing modifies its electronic transport properties and suggests the presence of Dirac cone states.

Contribution

It demonstrates that O$_2$ annealing induces significant changes in transport properties, indicating the emergence of Dirac fermions and multiband effects in the material.

Findings

Enhanced magnetoresistance after annealing

Observation of nonlinear Hall resistivity

Linear magnetoresistance suggesting Dirac cone states

Abstract

We investigated the transport properties of Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ single crystals with different amounts of excess Fe prepared by O$_2$ annealing. The O$_2$ annealing remarkably improves transport properties. In particular, a strongly nonlinear Hall resistivity was observed only in the fully-annealed crystal, and the magnetoresistance (MR) is drastically enhanced after annealing, reaching a value larger than 17% at 16 K and 14 T. The obvious change of transport properties after the annealing indicates that the band structure of Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ is affected by the excess Fe. The nonlinear Hall resistivity and violation of (modified) Kohler's scaling of the large MR prove the multiband effects in the Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ single crystal. The MR for the fully-annealed crystal develops linearly against magnetic field from intermediate field (e. g. 2 T at 16 K) to the measurement limit of 14 T. This phenomenon is interpreted by the existence of Dirac cone state, in which all the Dirac fermions occupy only the lowest Landau level in the quantum limit.