Costel R. Rotundu

Metallic systems with delocalized valence electrons host collective charge density oscillations known as plasmons. On the other hand, conventional insulators do not have free electrons and the low-energy charge degrees of freedom are pinned to the ions. The fate of the collective charge excitations in the intermediate regime is an outstanding question. This problem is especially important for strongly correlated systems such as the layered cuprates, where unconventional superconductivity and other emergent phenomena arise from valence electrons on the border between Mott localization and itinerancy. Using resonant inelastic xray scattering, we track this evolution in the prototypical electron-doped cuprate Nd 2 − x Ce x CuO 4 . We find a continuous transformation of the low-energy charge response: from an acoustic plasmon in the metallic regime, to a gapped mode with a dispersion anomaly at intermediate doping, and finally to a nearly dispersionless 139 meV excitation at half filling. The 139 meV excitation at zero doping has approximately twice the energy of the oxygen phonons responsible for the dispersion anomaly observed in angle-resolved photoemission spectroscopy. To explain this evolution, we introduce an inhomogeneous microscopic framework where plasmons in metallic regions couple with two-phonon excitations in insulating regions. These results establish a unified picture of collective charge excitations across the phase diagram of electron-doped cuprates and reveal a missing link in the charge dynamics of carrier doped Mott insulators.

The ability to rapidly manipulate domain walls in magnetic materials is key to developing novel high-speed spintronic memory and computing devices. Antiferromagnetic materials present a particularly promising platform due to their robustness against stray fields and their potential for exceptional domain wall velocities. Among various proposed driving mechanisms, coherent spin waves could potentially propel antiferromagnetic domain walls to the magnon group velocity while minimizing dissipation from Joule heating. However, experimental realization has remained elusive due to the dual challenges of generating coherent antiferromagnetic spin waves near isolated mobile antiferromagnetic domain walls and simultaneously measuring high-speed domain wall dynamics. Here we experimentally realize an approach where ultrafast laser pulses generate coherent spin waves that drive antiferromagnetic domain walls and develop a technique to directly map the spatiotemporal domain wall dynamics. Using the room-temperature antiferromagnetic insulator Sr₂Cu₃O₄Cl₂, we observe antiferromagnetic domain wall motion with record-high velocities up to ~50 km s⁻¹. Remarkably, the direction of domain wall propagation is controllable through both the pump laser helicity and the sign of the domain wall winding number. This bidirectional control can be theoretically explained, and numerically reproduced, by the domain wall dynamics induced by coherent spin waves of the in-plane magnon mode—a phenomenon unique to magnets with an easy-plane anisotropy. Our work uncovers a novel domain wall propulsion mechanism that is generalizable to a wide range of antiferromagnetic materials, unlocking new opportunities for ultrafast coherent antiferromagnetic spintronics.

The orbital angular momentum (OAM) of electron states is an essential ingredient for topological and quantum geometric quantities in solids. For example, Dirac surface states with helical spin- and orbital-angular momenta are a hallmark of a 3D topological insulator. Angle-resolved photoemission spectroscopy (ARPES) with variable circular light polarization, known as circular dichroism (CD), has been assumed to be a direct probe of OAM and, by proxy, of the Berry curvature of electronic bands in energy- and momentum-space. Indeed, topological surface states have been shown to exhibit angle-dependent CD (CDAD), and more broadly, CD is often interpreted as evidence of spin-orbit coupling. Meanwhile, it is well-established that CD originates from the photoemission matrix elements, which can have extrinsic contributions related to the experimental geometry and the inherently broken inversion symmetry at the sample surface. Therefore, it is important to broadly examine CD-ARPES to determine the scenarios in which it provides a robust probe of intrinsic material physics. We performed CD-ARPES on the canonical topological insulator Bi 2 Se 3 over a wide range of incident photon energies. Not only do we observe angle-dependent CD in the surface states, as expected, but we also find CD of a similar magnitude in virtually all bulk bands. Since OAM is forbidden by inversion symmetry in the bulk, we conclude this originates from symmetry-breaking in the photoemission process. Comparison with theoretical calculations supports this view and suggests that “hidden” OAM—localized to atomic sites within each unit cell—contributes significantly. Additional effects, including inter-atomic interference and final-state resonances, are responsible for the rapid variation of the CDAD signal with photon energy.

Spin- and angle-resolved photoemission spectroscopy (“spin-ARPES”) is a powerful technique for probing the spin degree-of-freedom in materials with nontrivial topology, magnetism, and strong correlations. Spin-ARPES faces severe experimental challenges compared to conventional ARPES attributed to the dramatically lower efficiency of its detection mechanism, making it crucial for instrumentation developments that improve the overall performance of the technique. In this paper, we demonstrate the functionality of our spin-ARPES setup based on time-of-flight spectroscopy and introduce our recent development of an electrostatic deflector mode to map out spin-resolved band structures without sample rotation. We demonstrate the functionality by presenting the spin-resolved spectra of the topological insulator Bi2Te3 and describe in detail the spectrum calibrations based on numerical simulations. By implementing the deflector mode, we minimize the need for sample rotation during measurements, hence improving the overall efficiency of experiments on small or inhomogeneous samples.

Superconductivity in the cuprates is found to be intertwined with charge and spin density waves. Determining the interactions between the different types of order is crucial for understanding these important materials. Here, we elucidate the role of the charge density wave (CDW) in the prototypical cuprate La_1.885Sr_0.115CuO₄, by studying the effects of large magnetic fields (H) up to 24 Tesla. At low temperatures (T), the observed CDW peaks reveal two distinct regions in the material: a majority phase with short-range CDW coexisting with superconductivity, and a minority phase with longer-range CDW coexisting with static spin density wave (SDW). With increasing magnetic field, the CDW first grows smoothly in a manner similar to the SDW. However, at high fields we discover a sudden increase in the CDW amplitude upon entering the vortex-liquid state. Our results signify strong coupling of the CDW to mobile superconducting vortices and link enhanced CDW amplitude with local superconducting pairing across the H − T phase diagram.