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S C I E N T I F I C H I G H L I G H T S
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Single-layer T -type nickelates: novel opportunities for high-temperature unconventional superconductivity
Unconventional high-temperature superconductivity remains one of the most important problems in physics. Establishing parallels between previous superconducting materials and new counterparts may be crucial to understanding this phenomenon. The synthesis of unprecedented single-layer T nickelates and characterisation of their exceptional electronic properties using X-ray spectroscopy brings new opportunities in this respect.
Unconventional superconductivity, broadly defined as superconductivity beyond the electron-phonon paradigm, remains a conceptual challenge that is outstandingly materialised in the high-Tc cuprates. To unlock the field, access to a broader palette of related materials may be a crucial step.
Among other systems, the infinite-layer nickelates RNiO2 (R = rare-earth element) have long been discussed as potential cuprate-like superconductors . This is because of the nominal 3d9 electronic configuration that should be
realised in these materials if the Ni atom is in a +1 oxidation state. Thus, the recent discovery of superconductivity in Sr-doped NdNiO2  has sparked a renewed interest in these systems . However, the actual electronic structure of these nickelates seems to fall outside the cuprate-like paradigm. In particular, they are placed in the Mott-Hubbard regime of the Zaanen-Sawatzky-Allen classification even if they remain metallic according to the experiments while the cuprates are charge-transfer insulators. Besides this, there is a self-doping effect associated with the rare-earth that makes them multiband systems with a potentially different low-energy physics. Also, in practice, there is another important limitation resulting from the thermodynamic fragility of these nickelates, since this hampers the crystal growth and has prevented the observation of superconductivity in bulk samples so far.
In this context, single-layer T -type nickelates emerge as particularly promising materials. The first members of this new series, La2NiO3F and Pr2NiO3F, have recently been synthesised by means of an original two-step topochemistry method [3 and principal publication]. From a structural point of view, these mixed-anion compounds represent nickelate analogues of T -type cuprates like Nd2CuO4 (see Figure 96).
In order to probe their electronic structure, Ka1 high-energy- resolution fluorescence-detected X-ray absorption spectra (HERFD-XAS) at the Ni -K-edge were measured at beamline ID26. In addition, X-ray absorption spectroscopy (XAS) experiments at the Ni K-edge were carried out at the DESY beamline P65. These experiments were complemented with first-principles calculations to analyse the trends across the R2NiO3F series (R = La - Lu) (see also ).
The combination of these experimental methods and theory revealed intriguing features. The measured XAS spectrum clearly indicated a metallic character (see Figure 97a). This is consistent with the calculated electronic
Fig. 96: Ball-and-stick model of the single-layer T -type nickelates.
Fig. 97: a) XAS spectra of the novel single-layer T nickelates. Compared with the insulating precursors, the intensity of the pre-edge feature is much higher and overlaps with the main edge, thereby revealing their metallic character.
b) Calculated band structure of Pr2NiO3F and (c) corresponding Fermi surface.