Nicholas Clark Plumb

Interfaces between correlated oxides and strong spin–orbit coupled materials provide a fertile platform for emergent quantum phases, yet controlling these phases through interfacial magnetic reconstruction remains a major challenge. Here, we observe emergent metallicity in ultrathin SrIrO ₃ /SrRuO ₃ heterostructures formed from insulating constituent layers, and this transition is closely associated with interfacial magnetic reconstruction. Combining transport, magnetometry, and angle-resolved photoemission spectroscopy, supported by first-principles calculations, we show that finite spectral weight at the Fermi level and ferromagnetic signatures emerge in the heterostructures. Our calculations further suggest that a staggered Dzyaloshinskii–Moriya interaction at the interface favors ferromagnetic order in SrRuO₃ and collinear antiferromagnetism in SrIrO₃, relative to the collinear and noncollinear antiferromagnetism in isolated ultrathin SrRuO₃ and SrIrO₃, respectively, providing a possible microscopic framework for the coupled magnetic and electronic reconstruction. Our findings identify interfacial magnetic interactions as a powerful tuning parameter for controlling correlated electronic phases in spin-orbit-coupled oxides, opening pathways toward the design of emergent quantum and topological states.

Excitons in semiconductors with negligible group velocity have been extensively investigated using optical methodologies. However, one significant characteristic of excitons for information transmission applications is their mobility, which poses a challenge in band-gapped systems. Meanwhile, three-dimensional dispersive excitons formed with electrons in a partially occupied conduction band are rare due to electron screening effect, excitonic instability, and the technical challenge of realizing them. Here, using angle-resolved photoemission spectroscopy and theoretical simulation, we observed dispersive excitonic bound states that are manifested as replica valence bands across a large portion of the three-dimensional Brillouin in the metallic material TbNiC2. Furthermore, our research indicates that the excitonic bound states are promoted by low density of states of free charges induced by electron-phonon coupling, and the resonance between topological surface and bulk states. Our findings provide a new thread to generate and stabilize three dimensional dispersive excitons in metals, which paves the way for the exploration of novel materials potentially suitable as metallic optoelectronic media which could conduct both electronic and excitonic signals simultaneously.

Kagome materials offer a versatile platform for exploring correlated and topological quantum states, where Van Hove singularities (VHSs) play a pivotal role in driving electronic instabilities, exhibiting distinct behaviors depending on electron filling and interaction settings. In the recently discovered kagome superconductors AV_{3}Sb_{5} (A=K, Rb, Cs), unconventional charge density wave order, superconductivity, and electronic chirality emerge, yet the nature of VHSs near the Fermi level (E_{F}) and their connection to these exotic orders remain elusive. Here, using high-resolution polarization-dependent angle-resolved photoemission spectroscopy, we uncover a universal electronic structure across AV_{3}Sb_{5} that is distinct from density-functional theory predictions that show noticeable discrepancies. We identify multiple common sublattice-pure VHSs near E_{F}, arising from strong V-d/Sb-p hybridization, which significantly promote bond-order fluctuations and likely drive the observed charge density wave order. These findings provide direct spectroscopic evidence for hybridization-driven VHS formation in kagome metals and establish a unified framework for understanding the intertwined electronic instabilities in AV_{3}Sb_{5}.

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.

Angle-resolved photoemission spectroscopy (ARPES) is a powerful probe for elucidating the electronic structures of quantum materials. As the technique's throughput continues to increase, there is a pressing need for specialized tools that enable rapid and intuitive data inspection and analysis. The PIVA package introduced in this work addresses this need by providing an interactive interface specifically designed for the efficient visualization and examination of large volumes of multidimensional datasets. It integrates image-processing methods, measurement tools, and a comprehensive suite of analysis routines within an intuitive environment. Together, these capabilities offer a robust framework for conducting detailed ARPES investigations, significantly enhancing the efficiency and depth of data exploration.

The fate of the Fermi surface in bulk electron-doped Sr₂IrO₄ remains elusive, as does the origin and extension of its pseudogap phase. Here, we use high-resolution angle-resolved photoelectron spectroscopy (ARPES) to investigate the electronic structure of Sr_2−x La _x IrO₄ up to x = 0.2, a factor of two higher than in previous work. We find that the antinodal pseudogap persists up to the highest doping level, and thus beyond the sharp increase in Hall carrier density to ≃ 1 + x recently observed above x* ≃ 0.16¹. This suggests that doped iridates host a unique phase of matter in which a large Hall density coexists with an anisotropic pseudogap, breaking up the Fermi surface into disconnected arcs. The temperature boundary of the pseudogap is T* ≃ 200 K for x = 0.2, comparable to cuprates and to the energy scale of short range antiferromagnetic correlations in cuprates and iridates.

Crystalline solids can become band insulators due to fully filled bands, or Mott insulators due to strong electronic correlations. While Mott insulators can theoretically occur in systems with an even number of electrons per unit cell, distinguishing them from band insulators experimentally has remained a longstanding challenge. In this work, we present a unique momentum-resolved signature of a dimerized Mott-insulating phase in the experimental spectral function of Nb₃Br₈: the top of the highest occupied band along the out-of-plane direction k_z has a momentum-space separation Δk _z = 2π/d, whereas that of a band insulator is less than π/d, where d is the average interlayer spacing. Identifying Nb₃Br₈ as a Mott insulator is crucial to understand its role in the field-free Josephson diode effect. Moreover, our method could be extended to other van der Waals systems where tuning interlayer coupling and Coulomb interactions can drive a band- to Mott-insulating transition.