Neutron spectroscopy on novel quantum materials

One of the most exciting pieces of recent news in materials science and condensed matter physics is the discovery of the first nickel oxide based superconductor Nd0.8Sr0.2NiO2. While superconductivity, which is a quantum state that carries electrical current without loss, was observed in conventional metals already more than 100 years ago, this is the first success of a decades-long quest to create a superconductor through targeted material design. The new nickelate superconductor was devised to mimic the properties of copper oxides (cuprates) – a material family famous for its exceptionally strong superconductivity. However, first x-ray spectroscopic data on NdNiO2 and LaNiO2 – the undoped parent compounds of Nd0.8Sr0.2NiO2 – suggest in conjunction with theoretical calculations that the magnetic and electronic structure of nickelates and cuprates could be profoundly different. Most notably, the Mott-insulating ground state, which is a hallmark of undoped cuprates, appears to have a different character in undoped nickelates, despite a nominally isoelectronic configuration with nine electrons in the 3d orbitals and a crystal structure closely similar to that of infinite-layer (IL) cuprates. In light of these initial findings, a clarification of the nature of the IL nickelates’ ground state and the mechanisms mediating their superconductivity upon doping is of outstanding importance.

Neutron spectroscopy techniques, such as time-of-flight spectroscopy, Larmor diffraction, and spin echo spectroscopy can provide essential insights on the ground state of a material. However, such techniques require relatively large single crystalline or powder samples, which do not exist yet in case of IL nickelates. Hence, we work on the preparation of RNiO2 powders with R = La, Pr, Nd and different Sr doping levels, which are obtained from the RNiO3 perovskite nickelate phase via a soft-chemistry treatment [Fig. 1].

In the scope of this undergraduate research project, the student will support the preparation of IL nickelate powder samples and learn to use state-of-the-art characterization methods, such as powder x-ray diffraction and electrical transport measurements at the Max Planck Institute for Solid State Research. After achieving high-quality powders, the student can possibly participate in our neutron spectroscopy experiments investigating the samples at major neutron facilities, such as FRM II (Garching, Germany) and ILL (Grenoble, France).

Fig. 1 | Infinite-layer nickelates RNiO2 (R = La, Pr, Nd) can be obtained from perovskite nickelates RNiO3 via a soft-chemistry treatment employing CaH2 as a reducing agent.

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