Effect of Cu2+ substitution in spin-orbit coupled Sr2Ir1-xCuxO4: Structure, magnetism, and electronic properties

Sr2IrO4 is an extensively studied spin-orbit coupling induced insulator with antiferromagnetic ground state. The delicate balance between competing energy scales plays a crucial role for its low-temperature phase and the route of chemical substitution has often been used to tune these different energy scales. Here, we report an evolution of structural, magnetic, and electronic properties in doped Sr2Ir1-xCuxO4(x <= 0.2). The substitution of Cu2+ (3d(9)) for Ir4+ (5d(5)) acts for electron doping, though it tunes the related parameters such as spin-orbit coupling, electron correlation, and Ir charge state. Moreover, both Ir4+ and Cu2+ has single unpaired spin, though it occupies different d orbitals. With Cu substitution, the system retains its original structural symmetry but the structural parameters show systematic changes. X-ray photoemission spectroscopy measurements show that Ir4+ equivalently converts to Ir5+ and a significant enhancement in the density of states has been observed at the Fermi level due to the contribution from the Cu 3d orbitals, which supports the observed decrease in the resistivity with Cu substitution. While the long-range magnetic ordering is much weakened and the highest-doped sample shows almost paramagneticlike behavior, the overall system remains insulator. Analysis of the resistivity data shows a mode of charge conduction in the whole series follows a two-dimensional variable-range-hopping model, but the range of validity varies with temperature. The whole series of samples exhibits negative magnetoresistance at low temperature, which is considered to be a signature of a weak-localization effect in a spin-orbit coupled system, and its evolution with Cu appears to follow the variation of resistivity with x.