Decoupling of magnetism and electric transport in single-crystal (Sr1-xAx)2IrO4 (A = Ca or Ba)

TL;DR

This study shows that chemical pressure via Ca or Ba doping in Sr2IrO4 significantly reduces resistivity and induces an insulator-metal transition without affecting the magnetic ordering temperature, highlighting the decoupling of magnetism and charge transport.

Contribution

It demonstrates that lattice modifications through isoelectronic doping can drastically alter electrical transport properties independently of magnetic order in iridates.

Findings

Resistivity decreases by up to five orders of magnitude with doping.

Insulator-to-metal transition occurs without changing the Néel temperature.

Chemical pressure effects differ from applied pressure in magnetic suppression.

Abstract

We report a systematical structural, transport and magnetic study of Ca or Ba doped Sr2IrO4single crystals. Isoelectronically substituting Ca2+ (up to 15%) or Ba2+ (up to 4%) ion for the Sr2+ ion provides no additional charge carriers but effectively changes the lattice parameters in Sr2IrO4. In particular, 15% Ca doping considerably reduces the c-axis and the unit cell by nearly 0.45% and 1.00 %, respectively. These significant, anisotropic compressions in the lattice parameters conspicuously cause no change in the N\'eel temperature which remains at 240 K, but drastically reduces the electrical resistivity by up to five orders of magnitude or even precipitates a sharp insulator-to-metal transition at lower temperatures, i.e. the vanishing insulating state accompanies an unchanged N\'eel temperature in (Sr1-xAx)2IrO4. This observation brings to light an intriguing difference between chemical pressure and applied pressure, the latter of which does suppress the long-range magnetic order in Sr2IrO4. This difference reveals the importance of the Ir1-O2-Ir1 bond angle and homogenous volume compression in determining the magnetic ground state. All results, along with a comparison drawn with results of Tb and La doped Sr2IrO4, underscore that the magnetic transition plays a nonessential role in the formation of the charge gap in the spin-orbit-tuned iridate.