Effect of Cu$^{2+}$ substitution in Spin-Orbit Coupled Sr$_2$Ir$_{1-x}$Cu$_x$O$_4$: Structure, magnetism and electronic properties

Abstract: Sr$2$IrO$_4$ is an extensively studied spin-orbit coupling induced insulator with antiferromagnetic ground state. The delicate balance between competing energy scales plays 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 Sr$_2$Ir${1-x}$Cu$_x$O$_4$ ($x$ $\leq$ 0.2). The substitution of Cu$^{2+}$ (3$d^9$) for Ir$^{4+}$ (5$d^5$) acts for electron doping, though it tunes the related parameters such as, spin-orbit coupling, electron correlation and Ir charge state. Moreover, both Ir$^{4+}$ and Cu$^{2+}$ has single unpaired spin though it occupies different $d$-orbitals. With Cu substitution, system retains its original structural symmetry but the structural parameters show systematic changes. X-ray photoemission spectroscopy measurements show Ir$^{4+}$ equivalently converts to Ir$^{5+}$ and a significant enhancement in the density of states has been observed at the Fermi level due to the contribution from the Cu 3$d$ 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 paramagnetic-like behavior the overall system remains insulator. Analysis of resistivity data shows mode of charge conduction in whole series follows 2-dimensional variable-range-hopping model but the range of validity varies with temperature. Whole series of samples exhibit negative magnetoresistance at low temperature which is considered to be a signature of weak localization effect in spin-orbit coupled system, and its evolution with Cu appears to follow the variation of resistivity with $x$.