Yi Tseng

We probe the spectrum of elementary excitations in SrIrO 3 by using heterostructured [ ( SrIrO 3 ) m / ( SrTiO 3 ) l ] samples to approach the bulk limit. Our resonant inelastic x-ray scattering (RIXS) measurements at the Ir L 3 edge reveal a robust low-lying collective magnetic mode with an antiferromagnetic (AFM) dispersion similar to the insulators Sr 2 IrO 4 and Sr 3 Ir 2 O 7 , albeit with a large gap and much larger linewidth. At higher energies we find the spin-orbit exciton, also strongly broadened, but with an inverted dispersion and doubled periodicity that are controlled by the charge hopping. These results demonstrate that the AFM paramagnon persists, somewhat counterintuitively, far into the metallic regime of the insulator-metal transition driven by the degree of confinement in the heterostructure. We conclude that these two excitations, which are contrasting but coexisting hallmarks of strong AFM pseudospin and charge fluctuations in a spin-orbit-coupled Mott-Slater material, are properties intrinsic to the ground state of semimetallic perovskite SrIrO 3 .

Bipolaronic superconductivity is an exotic pairing mechanism proposed for materials like Ba 1 − x K x BiO 3 (BKBO); however, conclusive experimental evidence for a (bi)polaron metallic state in this material remains elusive. Here, we combine resonant inelastic x-ray and neutron total scattering techniques with advanced modeling to study the local lattice distortions, electronic structure, and electron-phonon ( e − ph ) coupling in BKBO as a function of doping. Data for the parent compound ( x = 0 ) indicate that the electronic gap opens in predominantly oxygen-derived states strongly coupled to a long-range ordered breathing distortion of the oxygen sublattice. Upon doping, short-range breathing distortions and sizable e -ph coupling persist into the superconducting regime ( x = 0.4 ) . Comparisons with exact diagonalization and determinant quantum Monte Carlo calculations further support this conclusion. Our results provide compelling evidence that BKBO’s metallic phase hosts a liquid of small (bi)polarons derived from local breathing distortions of the lattice, with implications for understanding the low-temperature superconducting instability.

Type‐II multiferroicity from non‐collinear spin order is recently explored in the van der Waals material NiI ₂ . Despite the importance for improper ferroelectricity, the microscopic mechanism of the helimagnetic order remains poorly understood. Here, the magneto‐structural phases of NiI ₂ are investigated using resonant magnetic X‐ray scattering (RXS) and X‐ray diffraction. Two competing magnetic phases are identified. Below 60 K, an incommensurate magnetic reflection ( q ≈ [0.143,0,1.49] reciprocal lattice units) is observed which exhibits finite circular dichroism in RXS, signaling the inversion symmetry‐breaking helimagnetic ground state. At elevated temperature, in the non‐polar phase (60 K < T < 75 K), a distinct q  ≈  [0.087,0.087,1.5] magnetic order is observed, attributed to a collinear incommensurate (CI) state. The first‐order CI‐helix transition is concomitant with a structural transition characterized by a significant interlayer shear, which drives the helimagnetic ground state as evidenced by a mean‐field Heisenberg model including interlayer exchange and its coupling to the structural distortion. These findings identify interlayer magneto‐structural coupling as the key driver behind multiferroicity in NiI ₂ .

Honeycomb and kagome lattices can host propagating excitations with non-trivial topology as defined by their evolution along closed paths in momentum space. Excitations on such lattices can also be momentum-independent, and the associated flat bands are of interest due to strong interactions between heavy quasiparticles. Here, we report the discovery — using circularly polarized X-rays for the unambiguous isolation of magnetic signals — of a nearly flat spin-wave band and large (compared to elemental iron) orbital moment in the metallic ferromagnet Fe₃Sn₂ with compact AB-stacked kagome bilayers. As a function of out-of-plane momentum, the nearly flat optical mode and the global rotation symmetry-restoring acoustic mode are out of phase, consistent with a bilayer exchange coupling that is larger than the already large in-plane couplings. The defining units of this topological metal are therefore triangular lattices of octahedral iron clusters rather than weakly coupled kagome planes. The spin waves are strongly damped when compared to elemental iron, opening the topic of topological boson–fermion interactions for deeper exploration within this material platform.

In rare earth nickelates (RENiO₃), electron-lattice coupling drives a concurrent metal-to-insulator and bond disproportionation phase transition whose microscopic origin has long been the subject of active debate. Of several proposed mechanisms, here we test the hypothesis that pairs of self-doped ligand holes spatially condense to provide local spin moments that are antiferromagnetically coupled to Ni spins. These singlet-like states provide a basis for long-range bond and spiral spin order. Using magnetic resonant X-ray scattering on NdNiO₃ thin films, we observe the chiral nature of the spin-disproportionated state, with spin spirals propagating along the crystallographic (101)_ortho direction. These spin spirals are found to preferentially couple to X-ray helicity, establishing the presence of a hitherto-unobserved macroscopic chirality. The presence of this chiral magnetic configuration suggests a potential multiferroic coupling between the noncollinear magnetic arrangement and improper ferroelectric behavior as observed in prior studies on NdNiO₃ (101)_ortho films and RENiO₃ single crystals. Experimentally-constrained theoretical double-cluster calculations confirm the presence of an energetically stable spin-disproportionated state with Zhang-Rice singlet-like combinations of Ni and ligand moments.

Quantum fluctuations in low-dimensional systems and near quantum phase transitions have significant influences on material properties. Yet, it is difficult to experimentally gauge the strength and importance of quantum fluctuations. Here we provide a resonant inelastic x-ray scattering study of magnon excitations in Mott insulating cuprates. From the thin film of SrCuO₂, single- and bi-magnon dispersions are derived. Using an effective Heisenberg Hamiltonian generated from the Hubbard model, we show that the single-magnon dispersion is only described satisfactorily when including significant quantum corrections stemming from magnon-magnon interactions. Comparative results on La₂CuO₄ indicate that quantum fluctuations are much stronger in SrCuO₂ suggesting closer proximity to a magnetic quantum critical point. Monte Carlo calculations reveal that other magnetic orders may compete with the antiferromagnetic Néel order as the ground state. Our results indicate that SrCuO₂—due to strong quantum fluctuations—is a unique starting point for the exploration of novel magnetic ground states.