In this work, we propose an approach for the design of a waveguide structure that allows for efficient and highly asymmetric coupling of the quantum sources with circularly polarized transition dipole moments to the guided mode of the structure. The approach is based on the mixing of the two quasidegenerate modes of a periodic waveguide with an auxiliary single-mode waveguide leading to the formation of the dispersion with a stationary inflection point and consequently to the high coupling efficiency of this mode with a dipole source. We show that the distribution of the field polarization inside the waveguide is relatively homogeneous, maintaining the circular polarization in a large area. Consequently, this leads to a high degree of tolerance of the coupling asymmetry and strength to the position of the quantum emitter. We believe that our results will extend the variety of designs of the efficient chiral nanophotonic interfaces based on planar semiconductor nanostructures.

Topological superconductivity (TSC) has received great theoretical and experimental attention recently. Type-I Rashba nodal point (RNP) with isotropic band dispersions and point Fermi surface (FS) induced by the Rashba spin-orbit coupling (SOC) provides a promising route to the artificial TSC, because the inherent interspin coupling (ISC) shares identical form as the $p$-wave pairing $(k_x{\sigma }_y-k_y{\sigma }_x)$ exactly. Here we discuss the potential TSC of other types of RNPs with different ISC forms. By constructing a generic tight-binding model with Rashba SOC, we demonstrate type-IV, -III, -II, and -I′ RNPs can be achieved on two-dimensional (2D) Bravais lattices, whose FS consists of only a hole (electron) pocket, two contacted hole (electron) pockets, contacted hole and electron pockets, and point of tangency, respectively. With the coorpration of $s$-wave pairing and Zeeman gaps, these new types of RNP will evoke TSC phases with chiral Majorana edge modes (MEMs), where the Chern number will be larger than 1 for multiple symmetry-equivalent RNPs. The Chern number can be further composited when the energies of unequivalent RNPs are equal, leading to edge-dependent MEMs. Moreover, by using first-principles calculations, we demonstrate the BiSb monolayer is an ideal platform for realizing TSC with Chern number 6 from type-II, -I, or -IV RNP. This work enriches the types of nodal point induced by Rashba SOC and offers a generic guidance on realizing multiple and edge-dependent MEMs from the abundantly synthesized 2D surface metal layers.

We demonstrate that dislocations in two-dimensional non-Hermitian systems can give rise to density accumulation or depletion through the localization of an extensive number of states. These effects are shown by numerical simulations in a prototype lattice model and expose a different face of the non-Hermitian skin effect, by disentangling it from the need for boundaries. We identify a topological invariant responsible for the dislocation skin effect, which takes the form of a ${\mathbb{Z}}_2$ Hopf index that depends on the Burgers vector characterizing the dislocations. Remarkably, we find that this effect and its corresponding signature for defects in Hermitian systems fall outside of the known topological classification based on bulk-defect correspondence.

We investigate the statistics of photons emitted by tunneling electrons in a single electronic level plasmonic nanojunction. We compute the waiting-time distribution of successive emitted photons $w\left(\tau \right)$. When the cavity damping rate $\kappa$ is larger than the electronic tunneling rate $\mathrm{\Gamma }$, we show that in the photon-antibunching regime, $w\left(\tau \right)$ indicates that the average delay time between two successive photon-emission events is given by $1/\mathrm{\Gamma }$. This is in contrast with the usually considered second-order correlation function of emitted photons, $g^{\left(2\right)}\left(\tau \right)$, which displays the single timescale $1/\kappa$. Our analysis shows a relevant example for which $w\left(\tau \right)$ gives independent information on the photon-emission statistics with respect to $g^{\left(2\right)}\left(\tau \right)$, leading to physical insight into the problem. We discuss how this information can be extracted from experiments even in the presence of a nonperfect photon-detection yield.

Comparing with the widely known transitional metal based van der Waals (vdW) materials, rare-earth based ones are rarely explored in the research of intrinsic two-dimensional (2D) magnetism. In this work, we report the physical properties of DyOCl, a rare-earth based vdW magnetic insulator with a direct band gap of $\sim 5.72\text{eV}$. The magnetic order of bulk DyOCl is determined by neutron scattering as the $A$-type antiferromagnetic structure below the Néel temperature $T_N=10\mathrm{K}$. The large magnetic moment near 10.1 ${\mu }_B$/Dy lies parallel to the $a$-axis with strong uniaxial magnetic anisotropy. At $2\text{K}$, a moderate magnetic field ($\sim 2T$) applied along the easy axis generates spin-flip transitions and polarizes DyOCl to a ferromagnetic state. Density functional theory calculations reveal an extremely large magnetic anisotropy energy ($-5850\mu \text{eV}/Dy$) for DyOCl, indicating the great potential to realize magnetism in the 2D limit. Furthermore, the mechanical exfoliation of bulk DyOCl single crystals down to seven layers is demonstrated. Our findings suggest DyOCl is a promising material playground to investigate 2D $f$-electron magnetism and spintronic applications at the nanoscale.

We present a minimal model that provides a description of the magnetic and thermodynamic properties of $\mathrm{Eu}{\mathrm{Ir}}_2{\mathrm{P}}_2$ . The model contains two exchange coupling parameters, which are calculated using density functional theory, and a local easy-axis magnetic anisotropy term. The classical ground state of the system is a generalization of the well known ${120}^{\circ }$ structure observed in triangular antiferromagnets. Monte Carlo simulations show two phase transitions as a function of the temperature. With increasing temperature, the system transitions from the ground state into a high-entropy collinear antiferromagnet, which in turn at higher temperatures presents a second-order transition to a paramagnetic state. A high enough external magnetic field parallel to the anisotropy axis produces a spin-flop transition at low temperatures. The field also reduces the temperature range of stability of the collinear antiferromagnet phase and leads to a single-phase transition as a function of the temperature. The reported behavior of the specific heat, the magnetization, and the magnetic susceptibility is in agreement with the available experimental data. Finally, we present the magnetic phase diagrams for magnetic fields parallel and perpendicular to the easy axis.

Magnetism of the spin-$\frac{1}{2}\alpha -\mathrm{K}\mathrm{V}\mathrm{O}\mathrm{P}{\mathrm{O}}_4$ is studied by thermodynamic measurements, ${}^{31}\mathrm{P}$ nuclear magnetic resonance, neutron diffraction, and density-functional band-structure calculations. Ferromagnetic Curie-Weiss temperature of ${\theta }_{\mathrm{CW}}\simeq 15.9$ K and the saturation field of ${\mu }_0{H}_s\simeq 11.3$ T suggest the predominant ferromagnetic coupling augmented by a weaker antiferromagnetic exchange that leads to a short-range order below 5 K and the long-range antiferromagnetic order below $T_{\mathrm{N}}\simeq 2.7$ K in zero field. Magnetic structure with the propagation vector $\mathbf{k}=(0,\frac{1}{2},0)$ and the ordered magnetic moment of 0.58 ${\mu }_B$ at 1.5 K exposes a nontrivial spin lattice where strong ferromagnetic dimers are coupled antiferromagnetically. The reduction in the ordered magnetic moment with respect to the classical value $\left(1{\mu }_{\mathrm{B}}\right)$ indicates sizable quantum fluctuations in this setting, despite the predominance of ferromagnetic exchange. We interpret this tendency toward ferromagnetism as arising from the effective orbital order in the folded chains of the $\mathrm{V}{\mathrm{O}}_6$ octahedra.

By means of diffuse x-ray scattering (DS) and inelastic x-ray scattering (IXS), we probe directly the charge-ordering (CO) dynamics in the Verwey system $\left({\mathrm{NaMn}}_3\right){\mathrm{Mn}}_4{\mathrm{O}}_{12}$, where a peculiar quadruple perovskite structure with no oxygen disorder stabilizes a nearly full ${\mathrm{Mn}}^{3+}$/${\mathrm{Mn}}^{4+}$ static charge order at $T_{\mathrm{CO}}=175$ K concomitant to a commensurate structural modulation with propagation vector ${\mathbf{q}}_{\mathrm{CO}}=(\frac{1}{2},\frac{1}{2},0)$. At $T_{\mathrm{CO}}$, the IXS spectra unveil a softening of a 35.3-meV phonon at ${\mathbf{q}}_{\mathrm{CO}}$. Lattice dynamical calculations enable us to attribute this soft phonon to an ${\mathrm{A}}_g$ mode whose polarization matches the Jahn-Teller-like distortion pattern of the structural modulation. This result demonstrates that the Jahn-Teller instability is the driving force of the CO Verwey transition in $\left({\mathrm{NaMn}}_3\right){\mathrm{Mn}}_4{\mathrm{O}}_{12}$, thus elucidating a long-standing controversy regarding the mechanism of this transition observed in other mixed-valence systems like magnetite.

We report the results of a Hartree-Fock study applied to interacting electrons moving in two different bipartite lattices: the dice and the Lieb lattices, at half-filling. Both lattices develop ferrimagnetic order in the phase diagram $U\text{−}\lambda$, where $U$ is the Hubbard on-site repulsion and $\lambda$ is the Rashba spin-orbit coupling strength. Our main result is the observation of an unexpected multitude of topological phases for both lattices. All these phases are ferrimagnetic, but they differ among themselves in their set of six Chern numbers (six numbers because the unit cells have three atoms). The Chern numbers $\left|C\right|$ observed in our study range from 0 to 3, showing that large Chern numbers can be obtained by the effect of electronic correlations, adding to the recently discussed methodologies to increase $\left|C\right|$ based on extending the hopping range in tight-binding models, using sudden quenches, or photonic crystals, all without including electronic interactions.

By means of the kernel polynomial method (KPM), a numerically exact theoretical approach, we calculate the density of states (DOS) and diagonal conductivity of a two-dimensional Dirac nodal system in the presence of a high magnetic field. The effect of Dirac cone tilt on the profiles of disorder-broadened Landau level (LL) peaks in both the DOS and diagonal conductivity spectra is our main concern. Our numerical results show that the profile of an isolated LL peak in DOS, especially the $n=0$ one, is tilt independent. On the other hand, the Dirac cone tilt enhances/reduces the diagonal conductivity peaks in the direction perpendicular/parallel to the cone tilt direction ($y$ direction). In particular, at the Dirac point, i.e., zero energy, the ratio of the former to the latter is ${\sigma }_{xx}\left(0\right)/{\sigma }_{yy}\left(0\right)=1/(1-{\beta }^2)$, with $\beta$ being the tilt parameter. In addition, in comparison with the results obtained by KPM, we check the validity of the self-consistent Born approximation (SCBA) which has thus far been widely exploited for studying the DOS and quantum transport properties of Dirac or Weyl systems with disorder. We find that the SCBA fails to describe the detail of the LL peak around zero energy. At zero energy, the KPM result of the diagonal conductivity increases with the disorder strength before saturation, rather than a disorder-independent constant as reported previously by SCBA.

Field localization boosts nonlinear optical processes at the hot spots of metal nanostructures. Fano resonances can further enhance these “local” processes taking place at the hot spots. However, in conventional nonlinear materials, the frequency conversion takes place along the entire crystal body. That is, the conversion process is “unlocalized.” The path interference (Fano resonance) schemes developed for localized processes become useless in such materials. Here, we develop Fano enhancement schemes for unlocalized nonlinear optical processes. We show that three-orders-of-magnitude Fano enhancement multiplies the enhancements achieved via field-trapping techniques, e.g., in epsilon-near-zero materials. We demonstrate the phenomenon both analytically and using numerical solutions of Maxwell's equations. The agreement between the two solutions is impressive. We observe that the interference scheme for unlocalized processes is richer than that for the local processes. The method can be employed for any kind of nonlinear optical conversion. Moreover, Fano enhancement can be continuously controlled by an applied voltage.

Topological superconductors are materials in which superconductivity is closely entangled with topological electronic states. This provides a promising route to create Majorana bound states that are of fundamental importance to fault-tolerant quantum computation. In this paper, the recently discovered superconductor ${\mathrm{CaBi}}_2$ has been identified as a three-dimensional topological material. Spin-split topological surface states intersecting the Fermi energy are experimentally observed via spin- and angle-resolved photoemission spectroscopy. They are highly consistent with theoretical calculations. Furthermore, the layer-locked spin structure of ${\mathrm{CaBi}}_2$ is revealed via the layer-resolved spin texture analysis. These results unambiguously reveal the topological properties of the spin-polarized electronic structure in ${\mathrm{CaBi}}_2$ and provide important clues in the search for Majorana fermions in Bi-based topological superconductors.

Co-doping is an important method to improve doping properties in semiconductors, which is expected to lower the defect formation energy through the Coulomb interaction or covalent coupling between co-dopants and reduce the transition energy level through the level repulsions. However, the effectiveness of co-doping is sensitive to the type of co-dopants, and in some cases, negative effects can exist. Here, using $p$-type doping in CdTe as an example, we systematically investigate different dopant combinations based on first-principles hybrid functional calculations. We show that the complex of one donor + one double acceptor, e.g., ${\mathrm{Cl}}_{\mathrm{Te}}+{\mathrm{V}}_{\mathrm{Cd}}$, can cause a decrease in defect formation energy and a reduction in acceptor level, thus improving the dopability, especially when the Cl treatment is done at low temperature. The combination of two acceptors + one donor, e.g., ${\mathrm{Cl}}_{\mathrm{Te}}+2{\mathrm{Cu}}_{\mathrm{Cd}}$, has little or even negative effects because of the coexistence of the acceptor-acceptor and donor-acceptor coupling. More importantly, we show that the combination of one deep acceptor + one shallow acceptor, e.g., ${\mathrm{Sb}}_{\mathrm{Te}}+{\mathrm{P}}_{\mathrm{Te}}$, produces a much deeper acceptor level because the nominal deep acceptor ${\mathrm{Sb}}_{\mathrm{Te}}$ acts as a donor with respect to the shallow one, and then the deep acceptor state will be pushed even deeper due to the level repulsion. Thus, we should avoid this type of acceptor-acceptor co-doping to achieve high hole concentration. Our general understanding of the co-doping approach, therefore, provides a guideline for the future design of shallow defect complexes in CdTe and other semiconductors.

The disorder effects on higher-order topological phases in periodic systems have attracted much attention. However, in aperiodic systems, such as quasicrystalline systems, the interplay between disorder and higher-order topology is still unclear. In this paper, we investigate the effects of disorder on two types of second-order topological insulators, including a quasicrystalline quadrupole insulator and a modified quantum spin Hall insulator, in a two-dimensional Amman-Beenker tiling quasicrystalline lattice. We demonstrate that the higher-order topological insulators are robust against weak disorder in both models. More striking, the disorder-induced higher-order topological insulators called higher-order topological Anderson insulators are found at a certain region of disorder strength in both models. Our paper extends the study of the interplay between disorder and higher-order topology to quasicrystalline systems.

An exceptional point (EP) is exclusive for non-Hermitian systems and distinct from that at a degeneracy point (DP), supporting intriguing dynamics which can be utilized to probe quantum phase transitions and prepare eigenstates in a Hermitian many-body system. In this paper, we investigate the transition from a DP for a Hermitian system to an EP driven by non-Hermitian terms. We present a theorem on the existence of a transition between a DP and EP for a general system. Specifically, one of twofold degenerate eigenstates of a Hermitian system becomes a coalescing state when a selected non-Hermitian term is added. The obtained EP is robust to the strength of non-Hermitian terms. We illustrate the theorem by an exactly solvable quasi-one-dimensional model, which allows for the existence of a transition between fully degenerate and exceptional spectra driven by non-Hermitian tunnelings in real and $k$ spaces, respectively. We also study the EP dynamics for generating coalescing edge modes in Su-Schrieffer-Heeger-like models. This finding reveals the ubiquitous connection between DPs and EPs.

We theoretically investigate the topological band structures of a non-Hermitian Dirac Hamiltonian in two dimensions that effectively describes a non-Hermitian Chern insulator. The optical conductivity in various phases has been calculated. Five distinct phases with different topological features are shown explicitly. We redefine the point-gapped phase for the non-Hermitian counterparts and the gapless phases with the eigenenergies coalescing into a Dirac point or bulk Fermi arc, where the exceptional points (EPs) are formed at the ends of arc, as the non-Hermitian Dirac semiconductor (NHDSC), non-Hermitian Dirac semimetal (NHDSM), and non-Hermitian Fermi arc (NHFA) phases, respectively. It has been found that the evolution of topological phases can be achieved by coordinating the relative strengths of band-dependent scattering. By extending the optical response theory into the realm of non-Hermitian quantum systems, we analyze the spectral functions and the optical conductivity tensors in the respective phases in detail. We show that the mass parameter is critical for the anomalous Hall effect in the non-Hermitian system. The influence of temperature on the real and imaginary parts of frequency dependent conductivities is also presented. Our results indicate that the non-Hermiticity modifies the electron structure, so that the bulk topology is embodied in the dynamic response. We suggest an optical approach to probe new physics accompanied by non-Hermiticity in the massive Dirac systems.

The magnetic topological insulator ${\mathrm{EuSn}}_2{\mathrm{As}}_2$ is known to crystallize in a ${\mathrm{Bi}}_2{\mathrm{Te}}_3$-type layered structure in rhombohedral symmetry at ambient conditions and undergo a structural phase transition toward a monoclinic $\beta$ phase under increasing pressure. Here, we identify by ab initio calculations a new high-pressure rhombohedral structure (termed the $\gamma$ phase) in $R\overline{3}m$ symmetry comprising Sn-As-As-Sn linear chains along the $z$ axis via the formation of As-As bonds in the AsSn/SnAs bilayers with strong anisotropic lattice distortion along the [001] orientation. This pressure-densified new rhombohedral phase has a metallic A-type antiferromagnetic ground state and is energetically more favorable than the original rhombohedral structure and the monoclinic $\beta$ phase above 11 GPa. Simulated x-ray diffraction patterns provide an excellent match to the previously unexplained distinct diffraction peak around $14.7^{\circ }$ found in experiments for ${\mathrm{EuSn}}_2{\mathrm{As}}_2$ above 13 GPa. These findings offer key insights into a new type of high-pressure phase of layered magnetic topological insulators, laying a foundation for further exploration of this intriguing class of materials.