Orientation-controlled epitaxy and anisotropic properties of La2−xSrxNiO4 with 0.5⩽x⩽1.5 covering the insulator–metal transition

The a- and c-axis-oriented epitaxial thin films of layered nickelate La2−xSrxNiO4 (0.5⩽x⩽1.5), of which bulk single crystals are not available, were grown on LaSrAlO4 (100) and (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (001) substrates, respectively. Due to the in-plane anisotropy of the lattice mismatch, the lattice parameters of the a-axis-oriented films vary with x in a different manner from those of the ceramics. In the a-axis-oriented films, resistivity, and optical spectra show large anisotropy between the b and c axes, indicating the quasi-two-dimensional nature of the electronic structure in La2−xSrxNiO4.