Effect of the growth orientation on the physical properties of Sr$_2$CoNbO$_6$ thin films

We study the effect of the growth orientation on the structural, electronic, and hence transport properties of Sr$_2$CoNbO$_6$ thin films grown on the orthorhombic NGO(100) and cubic MgO(100) substrates. The x-ray diffraction patterns show the growth of the thin film along $a$-axis resulting in the asymmetric ($b\neq c$) in-plane compressive strain in case of NGO(100), whereas along $c$-axis with tensile strain in case of MgO(100) substrate. The temperature dependent resistivity measurements indicate the lower electronic conductivity for the film grown on the NGO(100) substrate, which is found to be correlated with the higher degree of the oxygen deficiencies and hence larger concentration of the insulating Co$^{2+}$ in this sample. Further, the x-ray photoemission spectroscopy measurements show that Sr and Nb are present in the 2+ and 4+ valence state, whereas Co exist in the 2+, 3+ as well as 4+ states, fraction of which was found to vary with the growth orientation. Moreover, the analysis of leakage current using the sum exponent model indicate the presence of two different relaxation mechanisms in these samples.