Anisotropic giant magnetoresistance near the Mott transition in pressurizedCa2RuO4

Abstract

We have observed an anisotropic and giant magnetoresistance (MR) in the $4d$-electron Mott transition system of ${\text{Ca}}_{2}{\text{RuO}}_{4}$. On the border of the Mott transition $(\ensuremath{\sim}2\text{ }\text{GPa})$, the MR effect at $\ensuremath{\sim}10\text{ }\text{T}$ reaches $\ensuremath{\sim}\ensuremath{-}55%$ at ${T}_{\text{C}}$ for longitudinal and $\ensuremath{\sim}+120%$ at low temperatures for the transverse effects. The negative MR is most likely interpreted as a reduction in a ferromagnetic (FM) fluctuation at ${T}_{\text{C}}$. In contrast, the large positive effect is actually rare and is characteristic of the mixed state where the FM metallic islands are flecked with the insulating phases. We discuss the reason of the peculiar MR from the viewpoint of the ``anisotropic magnetism,'' the ``tunnel MR,'' and the ``orbital physics.''