Paul M. Haney

Recent theoretical advances have highlighted the role of Bloch state intrinsic properties in enabling unconventional electro-optic (EO) phenomena in bulk metals, offering novel strategies for dynamic optical control in quantum materials. Here, we identify an alternative EO mechanism in bulk metallic systems that arises from the interplay between Berry curvature and the orbital magnetic moment of Bloch electrons. Focusing on twisted double-bilayer graphene (TDBG), we show that the enhanced intrinsic properties of moiré Bloch bands give rise to a sizable linear magnetoelectric EO response, a first-order, electric-field-induced non-Hermitian correction to the gyrotropic magnetic susceptibility. This mechanism dominates in C_3z-symmetric TDBG, where EO contributions originating from the Berry curvature dipole (BCD) are symmetry-forbidden. Our calculations reveal giant, gate-tunable linear and circular dichroism in the terahertz regime, establishing a robust and tunable platform for ultrafast EO modulation in two-dimensional materials beyond the BCD paradigm.

The quantum anomalous Hall effect shows great promise for realization of the ohm without the need for an external magnetic field. The most mature material platform is magnetically doped topological insulators. In these materials, precise quantization is limited to low temperatures, with the activation energy for dissipative transport typically in the range of 1 K. One potential source of dissipative transport is non-chiral edge states. These states are expected to be present in sufficiently thick samples. In this work, we perform extensive Hall and non-local resistance measurements in a Hall bar geometry at 2 K. By comparing 15 independent transport measurements to different transport models, we find that the system behavior is well-described by a simple continuum Ohm's law model. The addition of non-chiral edge states into the model does not significantly improve the fitting, and we conclude that there is not strong evidence for these states. We discuss the implications of our results for the prospect of high temperature quantized anomalous Hall effect in these materials.

Orbital angular momentum is a central quantity in the burgeoning field of orbitronics. Researchers have interpreted optical measurements of current-driven accumulation versus film thickness as determining the depth dependence of orbital angular momentum accumulation. The authors compute here both the current-induced orbital angular momentum spatial profile at the surface of a Pt thin film and the film-thickness dependence of the circular dichroism, and show that the latter is set by the mean free path while the former is considerably shorter.

Spin-accumulation and spin-current profiles are calculated for a disordered Pt film subjected to an in-plane electric current within the nonequilibrium Green's function approach. In the bulklike region of the sample, this approach captures the intrinsic spin Hall effect found in other calculations. Near the surfaces, the results reveal qualitative differences with the results of the widely used spin-diffusion model, even when the boundary conditions are modified to try to account for them. One difference is that the effective spin-diffusion length for transverse spin transport is significantly different from its longitudinal counterpart and is instead similar to the mean-free path. This feature may be generic for spin currents generated via the intrinsic spin Hall mechanism because of the differences in transport mechanisms compared to longitudinal spin transport. Orbital accumulation in the Pt film is only significant in the immediate vicinity of the surfaces and has a small component penetrating into the bulk only in the presence of spin-orbit coupling, as a secondary effect induced by the spin accumulation.

For almost a century, magnetic oscillations have been a powerful "quantum ruler" for measuring Fermi surface topology. In this study, we used Landau-level spectroscopy to unravel the energy-resolved valley-contrasting orbital magnetism and large orbital magnetic susceptibility that contribute to the energies of Landau levels of twisted double-bilayer graphene. These orbital magnetism effects led to substantial deviations from the standard Onsager relation, which manifested as a breakdown in scaling of Landau-level orbits. These substantial magnetic responses emerged from the nontrivial quantum geometry of the electronic structure and the large length scale of the moire lattice potential. Going beyond traditional measurements, Landau-level spectroscopy performed with a scanning tunneling microscope offers a complete quantum ruler that resolves the full energy dependence of orbital magnetic properties in moire quantum matter.

The lack of certain crystalline symmetries in strong spin-orbit-coupled nonmagnetic materials allows for the existence of unconventional spin Hall responses, with electrically generated transverse spin currents possessing collinear flow and spin directions. The injection of such spin currents into an adjacent ferromagnetic layer can excite magnetization dynamics via unconventional spin-orbit torques, leading to deterministic switching in ferromagnets with perpendicular magnetic anisotropy. We study the interplay between conventional and unconventional spin-orbit torques on the magnetization dynamics of a perpendicular ferromagnet in the small intrinsic damping limit, and identify a rich set of dynamical regimes that includes deterministic and probabilistic switching, precessional and pinning states. Contrary to common belief, we find that there exists a critical conventional spin Hall angle, beyond which deterministic magnetization switching transitions to a precessional or pinned state. Conversely, we show that larger unconventional spin Hall angle is generally beneficial for deterministic switching. We derive an approximate expression that qualitatively describes the state-diagram boundary between the full deterministic switching and precessional states and discuss a criterion for searching symmetry-broken spin Hall materials in order to maximize switching efficiency. Our work offers a roadmap towards energy-efficient spintronic devices, which might open the door for applications in advanced in-memory computing technologies.