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

Marco Moretti Sala,Yi Wang,Giacomo Ghiringhelli,Matteo Minola,Lucio Braicovich,Claudia Dallera,Matthieu Le Tacon,Bernhard Keimer,Greta Dellea,Vesna Srot,Peter A Van Aken,Marco Salluzzo,Gabriella Maria De Luca

doi:10.3390/nano10040817

Abstract

Epitaxial films of high critical temperature () 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, 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 edge to study the crystal field and spin excitations of NdBaCuO ultrathin films grown on SrTiO, 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 and x-ray diffraction.

Highlights

The quest for miniaturization of technological devices has pushed the development of nanotechnology, a field of applied research that deals with materials whose properties are intimately related to, and strongly dependent on their lateral size and shape
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 investigated the properties of unconventional high-temperature superconductors in the form of films, with variable thicknesses down to the single unit cell, in order to monitor the evolution of their physical properties as a function of thickness and possibly isolate new phenomena

Summary

Introduction

The quest for miniaturization of technological devices has pushed the development of nanotechnology, a field of applied research that deals with materials whose properties are intimately related to, and strongly dependent on their lateral size and shape. Resonant inelastic x-ray scattering is ideally suited for this purpose, as it probes the crystal field (dd) excitations that are related to local coordination of the Cu ions [8,9,10], and the spin excitations (paramagnons) [11,12,13,14,15], whose dispersion is linked to the in-plane superexchange interaction The latter is significant for ultrathin films: is the antiferromagnetic short and medium range order preserved in the single unit cell limit, where only 2 CuO2 planes are present, or is the quasi-perfect two-dimensionality of the system an obstacle to such order? On the contrary, we probe a single layer of cuprate material, opening the way to the study of 2D-atomic materials by RIXS down to the single elementary unit cell

Methods

Results

Conclusion