The intrinsic fine structure splittings (FSSs) of the exciton states of semiconductor quantum dots (QDs) are known to be the major obstacle for realizing the QD-based entangled photon pair emitters. In this paper, we present a theoretical and computational investigation of the excitonic fine structures of droplet-epitaxial (DE) GaAs/AlGaAs QDs under the electromechanical control of micromachined piezoelectric actuators. From the group theory analysis with numerical confirmation based on the developed exciton theory, we reveal the general principle for the optimal design of micromachined actuators whose application onto an elongated QD can certainly suppress its FSS. We show that the use of two independent tuning stresses is sufficient to achieve the FSS elimination but is not always necessary as widely deemed. The use of a single tuning stress to eliminate the FSS of an elongated QD is possible as long as the crystal structure of the actuator material is in coincidence with that of the QD. As a feasible example, we show that a single symmetric biaxial stress naturally generated from the (001) $\left[{\mathrm{Pb}(\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}\right){\mathrm{O}}_3{]}_{0.72}{-\left[{\mathrm{PbTiO}}_3\right]}_{0.28}$ (PMN-PT) actuator can be used as a single tuning knob to make the full FSSelimination for elongated DE GaAs QDs.

The influence of long-range dipole interactions on two-dimensional magnetism has been studied extensively in the spin-reorientation transition of ferromagnetic ultrathin films. Although there is a great deal of experimental information on the perpendicular domain phase that is stabilized by dipole interactions, the transitions to or from the domain phase are subtle and difficult to characterize experimentally. Magnetic susceptibility measurements show no divergence in the vicinity of the spin-reorientation transition as a function of thickness—a null result that is difficult to interpret with confidence. This paper reports separate dynamical and equilibrium versions of the reorientation transition in Fe/2ML Ni/W(110) films, using measurements of the magnetic susceptibility as the films are grown. The dynamical version occurs when increasing the film thickness causes the domain walls to depin, and the system moves from a configuration that minimizes local energetics to one that minimizes global energetics. The dynamical transition $is$ marked by a divergent magnetic susceptibility measured with a field applied along the in-plane W[001] direction. A comparative study of the two versions of the same spin-reorientation transition aids in the experimental characterization of the effects of dipole interactions on the phase transitions. This comparison confirms the original null result found in magnetic susceptibility measurements of the equilibrium transition; despite its name, the spin-reorientation transition in ferromagnetic ultrathin films has no critical phase transition in either the magnetization or its orientation.

A formula for the Hall response of interacting multiband systems with arbitrary band topology and spin-orbit coupling is derived. The formula is valid at finite frequency, which is relevant for Faraday rotation, and it takes into account all particle-hole vertex corrections. The formula includes both three-leg (triangular) and four-leg (rectangular) diagrams. The latter diagrams are absent in the single-band case. We show that the rectangular diagrams are necessary to recover the semiclassical formula for the Hall effect from the Kubo formula in the DC limit. They also give the linear response of the anomalous Hall effect to an external magnetic field, an effect which goes beyond the semiclassical theory. Three- and four-particle scatterings are neglected.

We study the chiral electric separation effect (CESE) in Weyl semimetals (WSM). Within the model based on the kinetic equation we show that there is a nonzero chirality current in external electric and magnetic fields. We distinguish longitudinal, in-plane transverse, and Hall components of the CESE. It is shown that the first two components are quadratic in electric field and linear in magnetic field, while the Hall component is quadratic in both the electric and magnetic fields. All three are due to the chiral anomaly. In WSMs the chirality current can be associated with the bulk spin density.

Two-dimensional tight-binding models for quasicrystals made of plaquettes with commensurate areas are considered. Their energy spectrum is computed as a function of an applied perpendicular magnetic field. Landau levels are found to emerge near band edges in the zero-field limit. Their existence is related to an effective zero-field dispersion relation valid in the continuum limit. For quasicrystals studied here, an underlying periodic crystal exists and provides a natural interpretation to this dispersion relation. In addition to the slope (effective mass) of Landau levels, we also study their width as a function of the magnetic flux and identify two fundamental broadening mechanisms: (i) tunneling between closed cyclotron orbits and (ii) individual energy displacement of states within a Landau level. Interestingly, the typical broadening of the Landau levels is found to behave algebraically with the magnetic field with a nonuniversal exponent.

We propose a paradigm for the realization of nonreciprocal photonic devices based on time-modulated graphene capacitors coupled to photonic waveguides, without relying on magneto-optic effects. The resulting hybrid graphene-dielectric platform is low loss, silicon compatible, robust against graphene imperfections, scalable from terahertz to near-infrared frequencies, and it exhibits large nonreciprocal responses using realistic biasing schemes. We introduce an analytical framework based on solving the eigenstates of the modulated structure and on spatial coupled mode theory, unveiling the physical mechanisms that enable nonreciprocity and enabling a quick analysis and design of optimal isolator geometries based on synthetic linear and angular momentum bias. Our results, validated through harmonic-balance full-wave simulations, confirm the feasibility of the introduced low-loss ($<3$ dB) platform to realize large photonic isolation through various mechanisms, such as narrow-band asymmetric band gaps and interband photonic transitions that allow multiple isolation frequencies and large bandwidths. We envision that this technology may pave the wave to magnetic-free, fully integrated, and CMOS–compatible nonreciprocal components with wide applications in photonic networks and thermal management.

Precise and scalable defect engineering of two-dimensional (2D) nanomaterials is acutely sought after in contemporary materials science. Here, we present defect engineering in monolayer graphene and molybdenum disulfide (${\mathrm{MoS}}_2$) by irradiation with noble gas ions at 30 keV. Two ion species of different masses were used in a gas field ion source microscope: helium (${\mathrm{He}}^{+}$) and neon (${\mathrm{Ne}}^{+}$). A detailed Raman spectroscopy study was performed and a defect activation model applied with marked differences between the ion systems at a given dose. We propose that disparities between the ion systems are explained by different defect yields and defect sizes. Expanding on existing models, we suggest that the average defect size is smaller for supported than freestanding graphene and that the rate of defect production is larger. We infer that low-energy secondary atoms from the substrate play a significant role in defect production, creating smaller defects relative to those created by the primary ion beam. Furthermore, a similar model was also applied to supported ${\mathrm{MoS}}_2$, another promising member of the 2D material family. Defect yields for both ions were obtained for ${\mathrm{MoS}}_2$, demonstrating their different interaction with the material and facilitating comparison with other irradiation conditions in the literature.

Iridates abound with interesting magnetic behaviors because of their strong spin-orbit coupling. ${\mathrm{Sr}}_3{\mathrm{NiIrO}}_6$ brings together the spin-orbital entanglement of the ${\mathrm{Ir}}^{4+}$ ion with a $3d$ Ni cation and a one-dimensional crystal structure. It has a ferrimagnetic ground state with a 55-T coercive field. We perform a theoretical study of the magnetic interactions in this compound, and elucidate the role of anisotropic symmetric exchange as the source of its strong magnetic anisotropy. Our first-principles calculations reproduce the magnon spectra of this compound and predict a signature in the cross sections that can differentiate the anisotropic exchange from single-ion anisotropy.

Double perovskite $\mathrm{L}{\mathrm{a}}_2\mathrm{CoMn}{\mathrm{O}}_6$ (LCMO) thin films with the monoclinic $P{2}_1/n$ structure were grown on hexagonal $c$-cut sapphire substrates by metalorganic aerosol deposition technique. Epitaxial growth of $\mathrm{LCMO}\left(021\right)/\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3\left(0001\right)$ due to an almost perfect in-plane lattice matching between LCMO and $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ was evidenced by x-ray diffraction and transmission electron microscopy. A high degree of rock-salt-type Co/Mn ordering was directly visualized by means of high resolution transmission electron microscopy. The $B$-site ordering, influenced by the film growth rate and optimized by small compressive epitaxial strain, results in perfect magnetic properties and strong spin-phonon coupling, both necessary requisites for multiferroic performance.

We develop a self-consistent approach for calculating the local impedance at a rough surface of a chiral $p$-wave superconductor. Using the quasiclassical Eilenberger-Larkin-Ovchinnikov formalism, we numerically find the pair potential, pairing functions, and the surface density of states taking into account diffusive electronic scattering at the surface. The obtained solutions are then employed for studying the local complex conductivity and surface impedance in the broad range of microwave frequencies (ranging from subgap to above-gap values). We identify anomalous features of the surface impedance caused by generation of odd-frequency superconductivity at the surface. The results are compared with experimental data for ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ and provide a microscopic explanation of the phenomenological two-fluid model suggested earlier to explain anomalous features of the microwave response in this material.

Silica glass is an optically homogeneous material with many practical applications. Unlike crystalline materials, it remains optically homogeneous during transformations. However, high-pressure in situ small-angle x-ray-scattering measurements indicate an increase in the scattering intensity during the transformation in short-range order between fourfold-coordinated and sixfold-coordinated amorphous polymorphs, providing strong evidence for heterogeneity. Detailed analyses suggest that silica glass consists of optically invisible subnanometer-scale domains for the two amorphous polymorphs and boundary region between them. The boundary region, which likely has an intermediate structure, is comparable in size to the domains, and works as a buffer to prevent the glass from cracking during the transformation. The transformation is more drastic and the domains are more distinct upon decompression than compression.

Ultrafast laser irradiation of solids leads to a thermodynamic nonequilibrium within and between the electron and phonon subsystems of the material. Due to electron-electron and phonon-phonon collisions, both subsystems relax into respective new thermodynamic equilibria within a characteristic thermalization time, which is different for each one of them. Moreover, they equilibrate their temperatures by electron-phonon coupling. The relaxation of the electronic nonequilibrium and its effect on the electron-phonon coupling was the subject of a number of studies, and it is comparably well understood, while the nonequilibrium within the phononic subsystem is usually neglected and the influence of the nonequilibrium phonons on other relaxation processes is unclear. Our calculations show significant differences in the energy transfer rate between the electrons and the phonons depending on whether a nonequilibrium distribution is assumed for the phonons or not. Here, we present a model to study the relaxation of the nonequilibrium phonon subsystem. Collisions between phonons are described within the frame of Boltzmann integrals. From this, an energy-dependent relaxation time can be extracted and inserted into a relaxation-time approach. Within the frame of this model, we study the thermalization of a phonon distribution induced by ultrafast laser irradiation. We show that the thermalization time of such a distribution is of the order of some hundreds of picoseconds. Moreover, we discuss the energy transfer between Fermi-distributed electrons and nonequilibrium phonons and compare it to the energy transfer for equilibrium distributions in both subsystems.

In an ideal classical pyrochlore antiferromagnet without perturbations, an infinite degeneracy in a ground state leads to the absence of magnetic order and a spin-glass transition. Here we present ${\mathrm{Na}}_3\mathrm{Mn}{\left({\mathrm{CO}}_3\right)}_2\mathrm{Cl}$ as a new candidate compound where classical spins are coupled antiferromagnetically on the pyrochlore lattice and report its structural and magnetic properties. The temperature dependences of the magnetic susceptibility and heat capacity and the magnetization curve are consistent with those of an $S=5/2$ pyrochlore lattice antiferromagnet with nearest-neighbor interactions of 2 K. Neither an apparent signature of a spin-glass transition nor magnetic order is detected in magnetization and heat capacity measurements or powder neutron diffraction experiments. On the other hand, antiferromagnetic short-range order of the nearest neighbors is evidenced by the $Q$ dependence of the diffuse scattering which develops around $0.85{\AA }^{-1}$. A high degeneracy near the ground state in ${\mathrm{Na}}_3\mathrm{Mn}{\left({\mathrm{CO}}_3\right)}_2\mathrm{Cl}$ is supported by the magnetic entropy, estimated as almost $4\mathrm{J}{\mathrm{K}}^{-2}{\mathrm{mol}}^{-1}$ at 0.5 K.

The magnetic properties of (111)-oriented Rh/Co/Pt and Pd/Co/Pt multilayers are investigated by first-principles calculations. We focus on the interlayer exchange coupling, and identify thicknesses and compositions where a typical ferromagnet or a synthetic antiferromagnet across the spacer layer is formed. All systems under investigation show a collinear magnetic intralayer order, but the Dzyaloshinskii-Moriya interaction (DMI) is rather strong for Pd-based systems, so that single magnetic skyrmions can be expected. In general, we find a strong sensitivity of the magnetic parameters (especially the DMI) in Rh-based systems, but Pd-based multilayers are less sensitive to structural details.

Coupling double-quantum-dot circuits to microwave cavities provides a powerful means to control, couple, and manipulate qubits based on the charge or spin of individual electrons. Here, we revisit this standard configuration by adding superconductivity to the circuit. We combine theory and experiment to study a superconductor–double-quantum-dot circuit coupled to microwave cavity photons. First, we use the cavity as a spectroscopic probe. This allows us to determine the low-energy spectrum of the device and to reveal directly Cooper-pair-assisted tunneling between the two dots. Second, we observe a vacuum Rabi splitting which is a signature of strong charge photon coupling and a premiere with carbon-nanotube-based quantum-dot circuits. We show that our circuit design intrinsically combines a set of key features to achieve the strong coupling regime to the cavity. A low charging energy reduces the device sensitivity to charge noise, while sufficient coupling is provided by the shaping of the spectrum of the double quantum dot by the superconducting reservoir. Our findings could be adapted to many other circuit designs and shed light on the coupling of superconducting nanoscale devices to microwave fields.

We demonstrate that for a one-dimensional asymmetric polariton waveguide, coupling between the fundamental and the excited modes leads to the emergence of splitting in circular polarizations. This splitting scales linearly with the polariton momentum along the channel and the corresponding effective magnetic field is perpendicular to both the direction of propagation and the axis of asymmetry of the system. The effect thus represents a full optical analog of the Rashba spin-orbit interaction.

We propose a way to directly excite compact localized condensates in a nearly flat band of the exciton-polariton Lieb lattice by short Laguerre-Gaussian pulses and investigate the dynamics of these condensates in the presence of repulsive polariton-polariton interactions and distributed losses in the lattice. The evolution of a low-density compact polariton condensate shows fast Rabi oscillations between its excitonic and photonic components, with slow beatings of the Rabi oscillation amplitude. At higher polariton densities the Rabi oscillations are still present, but their beatings are smeared out due to the polariton-polariton repulsion and distributed losses in the lattice. We further show that an incoherent background pumping can be used to increase the lifetime and stability of compact localized states.

We develop the diagrammatic formulation of the many-body theory for the coupled collective modes in interacting electron systems of different dimensions. The formalism is then applied in detail to a two-dimensional (2D) system coupled to a three-dimensional (3D) electron gas. We find two dimensionally mixed plasmon modes which in the long-wavelength limit are respectively 3D-like and acoustic in nature, but are strongly coupled at larger wave vectors. The same formalism can be applied to any dimensional combinations, and we also present the results for 1D-2D and 1D-3D coupled systems.

We compute the valley/magnetic phase diagram of monolayers of transition-metal dichalcogenides in the hole-doped region where spin-orbit effects are particularly relevant. Taking into account the moderate to high local electron-electron interactions due to the presence of transition-metal atoms, we show that the system is unstable to an itinerant ferromagnetic phase where all charge carriers are spin and valley polarized. This phase shows an anomalous charge Hall and anomalous spin Hall response, and may thus be detected experimentally.