Magnetoresistance of Sr$_2$IrO$_4$ Epitaxial Thin Films

Recent studies on Sr$_2$IrO$_4$ single crystals and thin films have revealed an intriguing insulating ground state, even though there are continued debates whether to classify this material as a Mott or Slater insulator. We have recently synthesized epitaxial Sr$_2$IrO$_4$ thin films on various substrates, which allow for the deposition of films under either tensile or compressive strain. The measurements of temperature-dependent magnetoresistance (MR), $\Delta R/R = [R(H)-R(0)]/R(0)$ on these samples reveal a negative linear MR near the room temperature, which is well above the antiferromagnetic ordering temperature ($T_{N} \approx 240$~K). However, as the temperature decreases, the {MR} becomes larger with a positive parabolic response. This behavior is very robust showing no noticeable dependence on magnetic field direction, strain, or film thickness and is remarkably different from the {MR} observed on Sr$_2$IrO$_4$ single crystals. This intriguing effect can be potentially explained by the presence of multiple conducting channels within the sample. We will discuss this model as well as other possible mechanisms for this unique phenomenon.