We develop the theory of single-electron silicon spin qubit relaxation in the presence of a magnetic field gradient. Such field gradients are routinely generated by on-chip micromagnets to allow for electrically controlled quantum gates on spin qubits. We build on a valley-dependent envelope function theory that enables the analysis of the electron wave function in a silicon quantum dot with an arbitrary roughness at the interface. We assume the presence of single-layer atomic steps at a Si/SiGe interface and study how the presence of a gradient field modifies the spin-mixing mechanisms. We show that our theoretical modeling can quantitatively reproduce the results of experimental measurements of qubit relaxation in silicon in the presence of a micromagnet. We further study how a field gradient can modify the EDSR Rabi frequency as well as the quality factor of a silicon spin qubit. We show that this strongly depends on the details of the interface roughness. Interestingly, for a quantum dot with an ideally flat interface, adding a micromagnet can give rise to the reduction of the EDSR frequency within some interval of the external magnetic field strength.

We present a theory for the differential shot noise, dS/dV, as measured via shot-noise scanning tunneling spectroscopy, and the differential conductance, dI/dV, for tunneling into Majorana zero modes (MZMs) in the putative topological superconductor FeSe0.45Te0.55. We demonstrate that for tunneling into chiral Majorana edge modes near domain walls, as well as MZMs localized in vortex cores and at the end of defect lines, dS/dV vanishes whenever dI/dV reaches a quantized value being equal to the quantum of conductance. These results are independent of the particular orbital tunneling path, thus establishing a vanishing dS/dV concomitant with a quantized dI/dV, as universal signatures for Majorana modes in two-dimensional topological superconductors, irrespective of the material's specific complex electronic band structure.

In easy-plane ferromagnets, we show that the interplay between a domain wall and a spin wave packet can be formulated as the collision of two massive particles with a gravitylike attraction. In the presence of magnetic dissipation, the domain wall mimics a particle subject to viscous friction, while the spin wave packet resembles a particle of variable mass. Due to the attractive nature of the interaction, the domain wall acquires a backward displacement as a spin wave packet penetrating the domain wall, even though there is no change in momentum of the wave packet before and after penetration.

Due to its linear dispersion, monolayer graphene is expected to generate a third harmonic response at terahertz frequencies. There have been a variety of different models of this effect and recently it has been experimentally observed. However, there is still considerable uncertainty as to the role of scattering on harmonic generation in graphene. In this paper, we model third harmonic generation in doped monolayer graphene at THz frequencies by employing a nearest-neighbour tight-binding model in the length gauge. We include optical phonon and neutral impurity scattering at the microscopic level, and examine the effects of scattering on the third harmonic response. We also compare the results of a phenomenological semiclassical theory, using a field-dependent scattering time extracted from the simulation, and find a significantly lower third-harmonic field than that found from the microscopic model. This demonstrates that third harmonic generation is much more sensitive to the nature of the scattering than is the linear response. We also compare the results of our full simulation to recent experimental results and find qualitative agreement.

The recent discovery of higher-order topology has largely enriched the classification of topological materials. Theoretical and experimental studies have unveiled various higher-order topological insulators that exhibit topologically protected corner or hinge states. More recently, higher-order topology has been introduced to topological semimetals. Thus far, realistic models and experimental verifications on higher-order topological semimetals are still very limited. Here we design and demonstrate a three-dimensional photonic crystal that realizes a higher-order Dirac semimetal phase. Numerical results on the band structure show that the designed three-dimensional photonic crystal is able to host two fourfold Dirac points, which are connected in the momentum-space projections via higher-order hinge states localized at the hinge. The higher-order topology can be characterized by the topological invariant chi((6)) at different values of k(z). An experiment at microwave frequencies is also presented to measure the hinge state dispersion. Our work demonstrates the physical realization of a higher-order Dirac semimetal phase and paves the way to explore higher-order topological semimetal phases in three-dimensional photonic systems.

Recently, evidence has emerged for a field-induced even- to odd-parity superconducting phase transition in CeRh2As2 [S. Khim et al., Science 373, 1012 (2021)]. Here we argue that the P4/nmm nonsymmorphic crystal structure of CeRh2As2 plays a key role in enabling this transition by ensuring large spin-orbit interactions near the Brillouin zone boundaries, which naturally leads to the required near-degeneracy of the even- and odd-parity channels. We further comment on the relevance of our theory to FeSe, which crystallizes in the same structure.