Origin of superconducting carriers in "non-doped" T'- (La,RE)2CuO4 (RE = Sm, Eu, Gd, Tb, Lu, and Y) prepared by molecular beam epitaxy

TL;DR

This study investigates the lattice constant variations in nominally undoped T'-RE2CuO4 superconductors prepared by MBE, revealing that charge carriers are not primarily introduced by electron doping through oxygen deficiencies.

Contribution

It provides evidence that the superconducting carriers in these materials are not mainly due to electron doping via oxygen deficiencies, challenging previous assumptions.

Findings

Lattice constants decrease linearly with RE substitution.

Extrapolated lattice constants match reported values for RE2CuO4.

No Cu-O bond stretching observed, indicating lack of electron doping effects.

Abstract

We have performed a systematic investigation of the variations of the lattice constants with substituent rare-earth element concentration x in the nominally undoped superconductors T'-(La3+)2-x(RE3+)xCuO4 (RE = Sm, Eu, Gd, Tb, Lu, and Y), which we have recently discovered using MBE. The results show both the in-plane and out-of-plane lattice constants (a0 and c0) linearly decrease with x, whose extrapolation to x = 2 agrees well with the reported a0 and c0 values for each T'-RE2CuO4. This behavior is what one would expect simply from the ionic size difference between La3+ and RE3+. The absence of the Cu-O bond stretching due to electron-doping, which is commonly observed in electron-doped T' and infinite-layer superconductors, implies that electron doping via oxygen deficiencies is, at least, not a main source of charge carriers.