Berit H. Goodge

We present a comparative Ni core level photoemission study of La 3 Ni 2 O 7 , Nd 3 Ni 2 O 7 , and LaNiO 3 using both the Ni 2 p and the Ni 1 s . We address the challenges in analyzing the widely investigated Ni 2 p spectra arising from the substantial overlap in energy of the Ni 2 p with the La 3 d . We show that, on the other hand, the deep Ni 1 s core level does provide a clean view on the intrinsic electronic excitations and we highlight its potential to resolve detailed differences in the electronic structure within the strongly correlated Ruddlesden-Popper series La n + 1 Ni n O 3 n + 1 .

The discovery of superconductivity in square-planar nickelates has offered a rich materials platform to explore the origins of high-temperature superconductivity. However, experimental investigations have largely been limited to the infinite-layer R NiO ₂ ( R , rare earth) nickelates. We constructed a phase diagram of multilayer square-planar Nd _{n +1} Ni _n O _{2 n +2} compounds and found signatures of superconductivity for dimensionality n = 4 to 8. Upon decreasing n , the superconducting anisotropy evolves owing to 4 f electron effects, and electronic structure characteristics approach cuprate-like behavior. Magnetic fluctuations persist from within the superconducting regime and into the overdoped, nonsuperconducting regime. The superconducting regime overlaps with that of chemically doped infinite-layer nickelates, demonstrating underlying commonalities as well as differences across varying structural realizations of square-planar nickelates. Our work establishes this layered template for creating new nickel-based superconductors.

The discovery of high-temperature superconductivity in bulk La ₃ Ni ₂ O ₇ under high hydrostatic pressure ^1–4 and biaxial compression in epitaxial thin films ^5–8 has generated substantial interest in understanding the interplay between atomic and electronic structure in these compounds. Subtle changes in the nickel–oxygen bonding environment are thought to be key drivers for stabilizing superconductivity, but specific details of which bonds and which modifications are most relevant remain unresolved so far. Although direct, atomic-scale structural characterization under hydrostatic pressure is beyond present experimental capabilities, static stabilization of strained La ₃ Ni ₂ O ₇ films provides a platform well suited to investigation with new picometre-resolution electron microscopy methods. Here we use multislice electron ptychography (MEP) ^9,10 to directly measure the atomic-scale structural evolution of La ₃ Ni ₂ O ₇ thin films across a wide range of biaxial strains tuned by substrate choice. By resolving both the cation and oxygen sublattices, we study the strain-dependent evolution of atomic bonds, providing the opportunity to isolate and disentangle the effects of specific structural motifs for stabilizing superconductivity. We identify the lifting of crystalline symmetry through modification of the nickel–oxygen octahedral distortions under compressive strain as a key structural ingredient for superconductivity and identify in-plane lattice compression as a common attribute between bulk and thin-film superconductivity. Building on the detailed structures obtained by MEP, we introduce a theoretical framework to disentangle coupled structural distortions in corner-sharing octahedra ¹¹ , which suggest that both known superconducting geometries of La ₃ Ni ₂ O ₇ (hydrostatic pressure and compressive strain) suppress local t _{2 g} orbital mixing in the low-energy Ni bands by raising the octahedral symmetry.

ZUSAMMENFASSUNG: Magnetostriktive Materialien sind nicht nur aus rein wissenschaftlicher Sicht von Interesse, sondern auch im Hinblick auf mögliche Anwendungen, die von Spintronik bis zur Energiegewinnung reichen. Ein neues magnetostriktives Material – SmFe ₅ As ₃ – zeigt ein komplexes Zusammenspiel zwischen Magnetostriktion, magnetischen Eigenschaften und der Kristallstruktur. Der Grundzustand von SmFe ₅ As ₃ ist ferrimagnetisch, was durch magnetische Suszeptibilitätsdaten, Bandstrukturrechnungen und XANES‐Messungen belegt wird. Bei = 28 4 K ordnet sich ein Teil des magnetischen Fe‐Untergitters neu an und geht in einen ferromagnetischen Zustand über. Darauf folgt bei = 76 4 K der Übergang in den paramagnetischen Zustand. Die in den magnetischen Messungen beobachteten Effekte werden von strukturellen Phasenübergängen begleitet. Alle drei Phasen – unterhalb von , zwischen und sowie oberhalb von – werden durch denselben strukturellen Motif vom UCr ₅ P ₃ ‐Typ (monokline Raumgruppe ) beschrieben und unterscheiden sich lediglich im Grad der Verformung der Fe–As Teilstruktur. Das neue Material zeigt eine riesige Magnetostriktion von . Dilatometrische Messungen an einem SmFe ₅ As ₃ ‐Einkristall weisen auf ein stark anisotropes Verhalten hin, das sowohl negatives, konstantes sowie positives thermoelastisches Verhalten zeigt.

Magnetostrictive materials are of interest not only from a fundamental perspective but also for their potential applications, spanning spintronics to energy harvesting. A new magnetostrictive material—SmFe ₅ As ₃ —reveals a complex interplay between magnetostriction, magnetic properties, and the crystal structure behavior. The ground state of SmFe ₅ As ₃ is ferrimagnetic, as evidenced by magnetic susceptibility data, band‐structure calculations, and XANES measurements. At K, part of the Fe sublattice reorients, transitioning into a ferromagnetic state. This is followed by an entrance into the paramagnetic state at K. The effects observed in the magnetic measurements are accompanied by structural phase transitions. All three phases—below , between and , and above —are described by the same structural motif of the UCr ₅ P ₃ type (monoclinic space group ), differing only in the degree of deformation of the Fe–As framework. The new material exhibits giant magnetostriction of . Dilatometry measurements on a SmFe ₅ As ₃ single crystal indicate strongly diverse behavior, exhibiting not only negative, but also zero, as well as positive thermo‐elastic effects.

Strong magnetic fields applied to metals confine electrons into Landau orbits, except at the boundaries at which frequent surface collisions disrupt their cyclotron motion. In two-dimensional systems, these boundary states form dissipationless chiral edge channels in the quantum Hall regime. By contrast, the quantum limit of three-dimensional (3D) metals is traditionally thought to differ fundamentally and instead contains gapless Landau bands, lacking quantized Hall conductance or dissipationless transport. Here we demonstrate enhanced surface conduction in the quantum limit of the 3D semimetal bismuth, characterized by the counterintuitive increase in conductivity as material is removed by micropatterning. The conductance of the 3D chiral boundary states—3D analogues of quantum Hall states in two dimensions—naturally accounts for this behaviour and for the highly non-local transport observed in micrometre-sized crystalline bismuth structures. These findings introduce an approach for engineering and exploiting chiral conduction on the surfaces of 3D materials, offering a design space for geometries beyond the simple one-dimensional boundary modes of two-dimensional systems.

Common belief is that the large band shifts observed in incommensurate misfit compounds, e.g., (LaSe)_{1.14}(NbSe_{2})_{2}, are due to interlayer charge transfer. By contrast, our analysis, based on both angle-resolved photoemission spectroscopy (ARPES) measurements and a specialized ab initio framework employing only quantities well defined in incommensurate materials, demonstrates that the large band shifts instead reflect changes in valence band hybridization and interlayer bonding. The strong alignment of our ab initio predictions and ARPES measurements confirms our understanding of the incommensurate electronic structure and charge transfer.