Investigating the charge-ordered phase in layered nickelates

All these suggest we may have found the right recipe for producing a high-temperature superconductor. However, significant challenges are to be urgently solved. For instance, so far, superconductivity only exists in thin films of layered nickelates, and less than a dozen research groups could successfully synthesise them, prompting a rather unstable state of the material. Moreover, scientists are still striving to understand why their superconducting critical temperature is about an order of magnitude lower than that of cuprates (Tc ~ 130 K).

a, The experimental geometry of XAS and RIXS experiment. b, Ni L3 XAS of the parent NdNiO2 film. c,d, Integrated quasi-elastic peak intensity as a function of momentum transfer along the (±H, 0) direction by excitation at the Ni 3d resonance (A peak in b). e, The resonant energy profile of the charge-order at (+0.333, 0). f, Integrated quasi-elastic peaks as a function of momentum transfer along the (H, 0) direction by probing the Ni 3d–Nd 5d hybridized state (A' peak in b) and Nd 4f states at the Nd M5 edge. g,h, RIXS maps on layered nickelate at 20 K at H = −0.35 r.l.u. with σ (g) or π incident X-ray polarisation (h).

In cuprates, doped electron or hole charges, besides forming Cooper pairs in the superconducting state, may segregate into regions with periodically variable density, i.e. forming charge-ordered phase. So, naturally, we may wonder whether such charge-ordered phases exist in their close cousin, layered nickelates.   

An international team of researchers addressed this question using Diamond's I21-RIXS beamline. Resonant Inelastic X-ray Scattering (RIXS) is one of the few techniques capable of probing the charge-ordered state. In particular, owing to the sub-100 nm probing depth, RIXS is particularly suited for the nm-thick nickelate films. Moreover, as I21 is equipped with high energy resolution and high photon flux, it is an ideal facility for this type of study.

The research team found the charge-ordered states exist in the parent layered nickelate NdNiO₂. They show strong resonance at both Ni 3d and Nd 5d states, illustrating the coupled electronic structure between the two. The charge-ordered state also shows clear temperature dependence, a hallmark of an ordered state with an electronic origin. Using the continuous tunability of the RIXS spectrometer, they found the charge-ordered state has non-negligible L-dependence, hinting it's a three-dimensional object.

Despite the differences to cuprates, the existence of the charge-ordered state demonstrates that the layered nickelates are remarkably similar to cuprate superconductors. It is clear that the combination of the MO2 (M=Cu, Ni) building block, an active dx2-y2 orbital near Fermi level, and spin S=1/2 antiferromagnetic (AFM) correlation, are crucial for achieving unconventional superconductivity. Understanding the common collective excitations of the AFM correlation and the charge-order may provide essential knowledge to eventually unfold the superconducting pairing mechanism.

Many materials in condensed matter physics exhibit remarkable properties such as zero-resistivity, colossal magneto-resistance and the magneto-optical Kerr effect. These novel properties lie in the competition of electronic and magnetic interactions under the angstrom and nanometre scale. A spectroscopy technique such as RIXS is imperative to understand and eventually make newer materials with richer functionality, due to its remarkable sensitivity.

Tam, CC. et al. Charge density waves in infinite-layer NdNiO2 nickelates. Nature Materials 21, 1116–1120 (2022). DOI: 10.1038/s41563-022-01330-1

Diamond Light Source and the University of Bristol under joint doctoral studentship no. STU0372.

NSFC (grant nos. 11774044, 52072059 and 11822411) SPRP-B of CAS (grant no. XDB25000000).

NSF of Beijing (grant no. JQ19002).

Ke-Jin Zhou, Diamond Light Source [email protected]