Depicting atomic-resolution orbital occupation in hole doped high-t_c superconducting superlattices

Abstract text (incl. figure legends and references)

La₂CuO₄ changes from an antiferromagnetic Mott-insulator into a superconductor upon hole doping. This phase transition can be observed via spectral features in the pre-edge region of the O-K edge by means of electron energy-loss spectroscopy (EELS). The O-K energy-loss near-edge structures (ELNES) allow the direct probing of this hole doping with atomic resolution.

We designed superlattices for the observation of evolving mobile carrier peaks (MCPs), the presence of the upper Hubbard band (UHB), and different Sr and Oxygen concentrations in direct proximity to each other. We link such electronic configurations and chemical distributions to the subsequent physical properties. Direct probing at the atomic-scale in order to differentiate between superconducting, metallic, and insulating parts in one single sample is unique, which to the best of our knowledge has not been shown before. This prospect, however, is of paramount interest for exploring future oxide-based quantum technologies, such as Josephson junctions.

The two heterostructures presented here consist of five repetitions of a bilayer with four and three unit cells La₂CuO₄ (LCO) and one unit cell Sr₂CuO₃ (SCO) with an additional capping layer of three and four unit cells La₂CuO₄, respectively. All superlattices have been grown via molecular-beam epitaxy (MBE) monitored by in-situ reflection high-energy electron diffraction (RHEED) under an ozone atmosphere on a LaSrAlO₄ (LSAO) (001)-oriented substrate. The samples have been characterized by resistance (R) versus temperature measurements, and scanning transmission electron microscopy (STEM); including EELS and high-angle annular dark-field (HAADF) imaging.

Transport properties of both heterostructures (Fig. 1) yield critical temperatures (T_c) of 32K for the 5x[4LCO + SCO] and 40K for the 5x[3LCO + SCO] superlattices, respectively. Due to the chemical potential between LCO and SCO, different amounts of intermixing between La and Sr can be observed at the top and bottom interfaces. This leads to a partial formation of insulating areas without charge carriers, superconducting areas with mobile carriers, and metallic areas with mobile carriers and oxygen vacancies. These phase-separated sample regions are further discussed with respect to their orbital occupation via a combination of atomically resolved EELS mappings and O-K ELNES analyses (Fig. 2). Further investigations of the blue-shaded area in Figure 2 highlight differences in the out-of-plane and in-plane orbital occupation by the introduced electron holes.

Here, we show for the first time that one can distinguish insulating, superconducting, and metallic areas within one single sample via O-K ELNES analyses and atomically resolved EELS. We want to highlight, that EELS experiments gave direct insights into the differences in orbital occupation of out-of-plane and in-plane orbitals, which open new possibilities for analyses of high-T_c superconductors answering open questions with respect to superconductivity.

Figure1: Resistance vs temperature of the two superlattices 5x[3LCO + SCO] (blue), and 5x[4LCO + SCO] (red).

Figure2: (a) 2D elemental EELS mapping (Sr: blue, Cu: green, La: red) of three different regions highlighting areas without Sr intermixing (red) and heavy Sr intermixing (blue). (b), (c) and (d) Gaussian fits with energy constraints for the MCP (blue) at 528 eV, the UHB (red) at 530.5 eV, and for the leading O-K edge-onset, a peak at 534 eV (turquoise).