New superconducting T'-(La3+)2-x(RE3+)xCuO4 with isovalent doping (RE = Sm, Eu, Tb, Lu, and Y)

TL;DR

This paper reports the synthesis of new superconducting cuprates with isovalent doping, achieving superconductivity without effective doping and highlighting the importance of high-quality thin film preparation and impurity removal.

Contribution

The study introduces a novel method to stabilize T'-phase La-based cuprates with isovalent doping, leading to superconductivity without traditional doping.

Findings

Maximum Tconset ~25 K and Tczero ~21 K.

High-quality T' films were prepared at low temperature (~650°C).

Impurity oxygen removal and larger lattice constants are key to superconductivity.

Abstract

We report the synthesis of new superconducting cuprates T'-La2-xRExCuO4 (RE = Sm, Eu, Tb, Lu, and Y) using molecular beam epitaxy. The new superconductors have no effective dopant, at least nominally. The substitution of isovalent RE for La was essentially performed to stabilize the T' phase of La2CuO4 instead of the T phase. The maximum Tconset is ~ 25 K and Tczero is ~ 21 K. The keys to our discovery are (1) the preparation of high-crystalline-quality La-based T' films by low-temperature (~650C) thin film processes, and (2) more thorough removal of impurity oxygen at the apical site, which is achieved by the larger in-plane lattice constant (a0) of T'-La2-xRExCuO4 than other T'-Ln2CuO4 (Ln = Pr, Nd, Sm, Eu, Gd) with the aid of large surface-to-volume ratio of thin films. The photoemission spectra suggest that the end member compounds of the T' system may not be a Mott insulator.