  The electronic and magnetic properties of epitaxial LaNiO3/LaAlO3
superlattices can be tuned by layer thickness and substrate-induced strain.
Here, we report on direct measurements of the k-space-resolved electronic
structure of buried nickelate layers in superlattices under compressive strain
by soft x-ray photoemission. After disentangling strong extrinsic contributions
to the angle-dependent signal caused by photoelectron diffraction, we are able
to extract Fermi surface information from our data. We find that with
decreasing LaNiO3 thickness down to two unit cells (2 uc) quasiparticle
coherence becomes strongly reduced, in accord with the dimension-induced
metal-to-insulator transition seen in transport measurements. Nonetheless, on
top of a strongly incoherent background a residual Fermi surface can be
identified in the 2 uc superlattice whose nesting properties are consistent
with the spin-density wave (SDW) instability recently reported. The overall
behavior of the Ni 3d spectra and the absence of a complete gap opening
indicate that the SDW phase is dominated by strong order parameter
fluctuations.
