Structural, Electronic and Magnetic Properties of a Few Nanometer-Thick Superconducting NdBa₂Cu₃O₇ Films

Abstract: Epitaxial films of high critical temperature (Tc) cuprate superconductors preserve
their transport properties even when their thickness is reduced to a few nanometers. However,
when approaching the single crystalline unit cell (u.c.) of thickness, Tc decreases and eventually,
superconductivity is lost. Strain originating from the mismatch with the substrate, electronic
reconstruction at the interface and alteration of the chemical composition and of doping can be
the cause of such changes. Here, we use resonant inelastic x-ray scattering at the Cu L3 edge to study
the crystal field and spin excitations of NdBa2Cu3O7􀀀x ultrathin films grown on SrTiO3, comparing
1, 2 and 80 u.c.-thick samples. We find that even at extremely low thicknesses, the strength of the
in-plane superexchange interaction is mostly preserved, with just a slight decrease in the 1 u.c. with
respect to the 80 u.c.-thick sample. We also observe spectroscopic signatures for a decrease of the
hole-doping at low thickness, consistent with the expansion of the c-axis lattice parameter and oxygen
deficiency in the chains of the first unit cell, determined by high-resolution transmission microscopy
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and x-ray diffraction.