Soft X-Ray Spectroscopy of Low-Valence Nickelates

Abstract

Low-valence nickelates—including infinite-layer (IL) and trilayer (TL) compounds—are longstanding candidates for mimicking the high-temperature superconductivity of cuprates. A recent breakthrough in the field came with the discovery of superconductivity in hole-doped IL nickelates. Yet, the degree of similarity between low-valence nickelates and cuprates is the subject of a profound debate for which soft x-ray spectroscopy experiments at the Ni L- and O K-edge provided critical input. In this review, we will discuss the essential elements of the electronic structure of low-valance nickelates revealed by x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS). Furthermore, we will review magnetic excitations observed in the RIXS spectra of IL and TL nickelates, which exhibit characteristics that are partly reminiscent of those of cuprates.

Resonant inelastic x-ray scattering data for Ruddlesden-Popper and reduced Ruddlesden-Popper nickelates

“…Resonant x-ray spectroscopy offers several advantages for probing low valence nickelates 11 . It can probe small sample volumes, which is useful since these materials are challenging to prepare as large single crystals.…”

“…Another unique feature is the ability of RIXS to selectively study different atomic species through the resonant process and its access to magnetic and orbital excitations, which are optically forbidden 12 . Efforts to understand the detailed electronic properties of Ruddlesden-Popper and reduced Ruddlesden-Popper nickelates have consequently relied heavily on resonant x-ray spectroscopy 11 , 13 – 23 .…”

Effects of rare-earth magnetism on the superconducting upper critical field in infinite-layer nickelates

“…One exception is in PrBa 2 Cu 3 O 7−δ , where coincidental hybridization between the Pr 4f and oxygen 2p states is believed to compete with the emergence of superconductivity (11). In contrast, in the nickelate superconductors, there is consistent transport, spectroscopic, and theoretical evidence for hybridization between a broad range of R 5d orbitals and the Ni 3d orbitals (2)(3)(4)(5)(6)(7)(8)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24). An open question currently under debate is the potential role of the R 4f electrons (25)(26)(27)(28).…”

High pO2 Flux Growth and Characterization of NdNiO3 Crystals

“…The discovery of superconductivity in the thin film of Nd 0.8 Sr 0.2 NiO 2 by Li et al [1] has attracted intense attention in the fields of condensed matter physics and materials science and launched the Nickel Age of Superconductivity [2][3][4][5][6][7][8][9][10][11][12][13][14]. Until today, superconductivity with a critical temperature (T c ) in the range of 9-15 K has been observed in the infinite-layer R 1-x M x NiO 2 (R = La, M = Ca, Sr; R = Pr, Nd, M = Sr) [15][16][17][18][19] and the quintuple-layer Nd 6 Ni 5 O 12 [20].…”

“…Under high static pressure, T c has been increased to 30 K in Pr 0.8 Sr 0.2 NiO 2 films [21]. After 3 years of intense research, several fundamental issues still exist [2][3][4][5][6][7][8][9][10][11][12][13][14], including the mechanism of superconductivity, the similarity and difference between nickelate and cuprate superconductivity, whether the ground state is magnetic, and whether competing phases such as pseudogaps and strange metals exist. Single crystals are the ideal platform to solve the abovementioned open questions.…”