Absorption spectra of neutral, negatively, and positively charged semiconductor quantum dots are studied theoretically. We provide an overview of the main energetic structure around the $p$-shell transitions, including the influence of nearby nominally dark states. Based on the envelope function approximation, we treat the four-band Luttinger theory as well as the direct and short-range exchange Coulomb interactions within a configuration interaction approach. The quantum dot confinement is approximated by an anisotropic harmonic potential. We present a detailed investigation of state mixing and correlations mediated by the individual interactions. Differences and similarities between the differently charged quantum dots are highlighted. Especially large differences between negatively and positively charged quantum dots become evident. We present a visualization of energetic shifts and state mixtures due to changes in size, in-plane asymmetry, and aspect ratio. Thereby we provide a better understanding of the experimentally hard to access question of quantum dot geometry effects. Our findings show a method to determine the in-plane asymmetry from photoluminescence excitation spectra. Furthermore, we supply basic knowledge for tailoring the strength of certain state mixtures or the energetic order of particular excited states via changes of the shape of the quantum dot. Such knowledge builds the basis to find the optimal QD geometry for possible applications and experiments using excited states.

We consider light scattering by a coated magnetoplasmonic nanoparticle with a Kerr-type nonlinear plasmonic shell and a magneto-optic core. Such a structure features two plasmon dipole modes, associated with electronic oscillations on the inner and outer surfaces of the shell. Driven in a nonlinear regime, each mode exhibits a bistable response. Bistability of an inner plasmon leads to switching between this state and a Fano resonance (Fano switching). Once the external light intensity exceeds a critical value, the bistability zones of both eigenmodes overlap, yielding optical tristability characterized by three stable steady states for a given wavelength and light intensity. We develop a dynamic theory of transitions between nonlinear steady states and estimate the characteristic switching time to be as short as 0.5 ps. We also show that the magneto-optical effect allows red and blue spectral shifts of the Fano profile for right and left circular polarizations of the external light, rendering Fano switching sensitive to light polarization. Specifically, one can reach Fano switching for the right circular polarization while cancelling it for the left circular polarization. The results point to a class of ultrafast Fano switchers tunable by a magnetic field for applications in nanophotonics.

Single-layered tin selenide that shares the same structure with phosphorene and possesses intriguing optoelectronic properties has received great interest as a two-dimensional material beyond graphene and phosphorene. Herein, we explore the optoelectronic response of the newly discovered stable honeycomb derivatives (such as $\alpha$, $\beta$, $\gamma$, $\delta$, and $\varepsilon$) of single-layered SnSe in the framework of density functional theory. The $\alpha$, $\beta$, $\gamma$, and $\delta$ derivatives of a SnSe monolayer have been found to exhibit an indirect band gap, however, the dispersion of their band-gap edges demonstrates multiple direct band gaps at a relatively high energy. The $\varepsilon$-SnSe, however, features an intrinsic direct band gap at the high-symmetry $\mathrm{\Gamma }$ point. Their energy band gaps (0.53, 2.32, 1.52, 1.56, and 1.76 eV for $\alpha$-, $\beta$-, $\gamma$-, $\delta$-, and $\varepsilon$-SnSe, respectively), calculated at the level of the Tran-Blaha modified Becke-Johnson approach, mostly fall right in the visible range of the electromagnetic spectrum and are in good agreement with the available literature. The optical spectra of these two-dimensional (2D) SnSe polymorphs (besides $\beta$-SnSe) are highly anisotropic and possess strictly different optical band gaps along independent diagonal components. They show high absorption in the visible and UV ranges. Similarly, the reflectivity, refraction, and optical conductivities inherit strong anisotropy from the dielectric functions as well and are highly visible-UV polarized along the cartesian coordinates, showing them to be suitable for optical filters, polarizers, and shields against UV radiation. Our investigations suggest these single-layered SnSe allotropes as a promising 2D material for next-generation nanoscale optoelectronic and photovoltaic applications beyond graphene and phosphorene.

In a recent article [Phys. Rev. B 91, 205315 (2015)] Shi, Xu, and Wang presented a comparison of several formalisms to calculate nonradiative recombination rates and concluded the “one-dimensional (1D) quantum formula” that was used by Alkauskas et al. [Phys. Rev. B 90, 075202 (2014)] is insufficient to accurately describe nonradiative capture rates. Our analysis of the results of Shi, Xu, and Wang indicates that their conclusions about the 1D quantum formula are unfounded and stem from an error in their calculations. Our own calculations demonstrate that the 1D quantum formula approach yields reliable and accurate results for nonradiative recombination rates.

As pointed out by the comments of Wickramaratne et al., there was indeed an algebra error in our original publication [Phys. Rev. B 91, 205315 (2015)] in calculating the 1D model. We have corrected this error, and did a renewed calculation and comparison of the 1D model and other approximated models. The revised comparison provides some insights for the origin of the difference between the full phonon spectrum electron-phonon coupling static formula results and other approximated results, especially for how to evaluate the amplitude of electron-phonon coupling.

Silicon carbide (SiC) is nowadays a major material for applications in high power electronics, quantum optics, or nitride semiconductors growth. Mastering the surface of SiC substrate is crucial to obtain reproducible results. Previous studies on the $6H$-SiC(0001) surface have determined several reconstructions, including the $(\sqrt{3}\times \sqrt{3})\text{−}R{30}^{\circ }$ and the $(3\times 3)$. Here, we introduce a process of progressive Si enrichment that leads to the formation of two reconstructions, the giant $(12\times 12)$ and the $(4\times 8)$. From electron diffraction and tunneling microscopy completed by molecular dynamics simulations, we build models introducing a type of Si adatom bridging two Si surface atoms. Using these Si bridges, we also propose a structure for two other reconstructions, the $(2\sqrt{3}\times 2\sqrt{3})\text{−}R{30}^{\circ }$ and the $(2\sqrt{3}\times 2\sqrt{13}$). We show that five reconstructions follow each other with Si coverage ranging from 1 and 1.444 monolayer. This result opens the way to greatly improve the control of $6H$-SiC(0001) at the atomic scale.

We apply a large-scale computational technique, known as topology optimization, to the inverse design of photonic Dirac cones. In particular, we report on a variety of photonic crystal geometries, realizable in simple isotropic dielectric materials, which exhibit dual-polarization Dirac cones. We present photonic crystals of different symmetry types, such as fourfold and sixfold rotational symmetries, with Dirac cones at different points within the Brillouin zone. The demonstrated and related optimization techniques open avenues to band-structure engineering and manipulating the propagation of light in periodic media, with possible applications to exotic optical phenomena such as effective zero-index media and topological photonics.

We report the magnetotransport properties and the electronic structure of TmSb. TmSb exhibits extremely large transverse magnetoresistance and Shubnikov-de Haas (SdH) oscillation at low temperature and high magnetic field. Interestingly, the split of Fermi surfaces induced by the nonsymmetric spin-orbit interaction has been observed from SdH oscillation. The analysis of the angle-dependent SdH oscillation illustrates the contribution of each Fermi surface to the conductivity. The electronic structure revealed by angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations demonstrates a gap at the $X$ point and the absence of band inversion. Combined with the trivial Berry phase extracted from SdH oscillation and the nearly equal concentrations of electron and hole from Hall measurements, it is suggested that TmSb is a topologically trivial semimetal and the observed XMR originates from the electron-hole compensation and high mobility.

It is generally believed that in first-order phase transitions in materials with imperfections, the formation of phase domains must be affected to some extent by stochastic (probabilistic) processes. The stochasticity would lead to unreliable performance in nanoscale devices that have the potential to exploit the transformation of physical properties in a phase transition. Here we show that stochasticity at nanometer length scales is completely suppressed in the thermally driven metal-insulator transition (MIT) in sputtered vanadium dioxide ($\mathrm{V}{\mathrm{O}}_2$) films. The nucleation and growth of domain patterns of metallic and insulating phases occur in a strikingly reproducible way. The completely deterministic nature of domain formation and growth in films with imperfections is a fundamental and unexpected finding about the kinetics of this material. Moreover, it opens the door for realizing reliable nanoscale devices based on the MIT in $\mathrm{V}{\mathrm{O}}_2$ and similar phase-change materials.

The present paper theoretically investigates features of quantum dynamics for localized plasmons in three-particle or four-particle spaser systems consisting of metal nanoparticles and semiconductor quantum dots. In the framework of the mean field approximation, the conditions for the observation of stable stationary regimes for single-particle plasmons in spaser systems are revealed, and realization of these regimes is discussed. The strong dipole-dipole interaction between adjacent nanoparticles for the four-particle spaser system is investigated. We show that this interaction can lead to the decreasing of the autocorrelation function values for plasmons. The generation of entangled plasmons in a three-particle spaser system with nonlinear plasmon-exciton interaction is predicted. The use of an external magnetic field is proposed for control of the cross correlations between plasmons in the three-particle spaser system.

We present the analytical and numerical details behind our recently published article [Phys. Rev. Lett. 118, 253901 (2017)], describing the impact of disorder-induced multiple scattering on counterpropagating solitons in photonic crystal waveguides. Unlike current nonlinear approaches using the coupled mode formalism, we account for the effects of intraunit cell multiple scattering. To solve the resulting system of coupled semilinear partial differential equations, we introduce a modified Crank-Nicolson-type norm-preserving implicit finite difference scheme inspired by the transfer matrix method. We provide estimates of the numerical dispersion characteristics of our scheme so that optimal step sizes can be chosen to either minimize numerical dispersion or to mimic the exact dispersion. We then show numerical results of a fundamental soliton propagating in the presence of multiple scattering to demonstrate that choosing a subunit cell spatial step size is critical in accurately capturing the effects of multiple scattering, and illustrate the stochastic nature of disorder by simulating soliton propagation in various instances of disordered photonic crystal waveguides. Our approach is easily extended to include a wide range of optical nonlinearities and is applicable to various photonic nanostructures where power propagation is bidirectional, either by choice, or as a result of multiple scattering.

The spin texture of the spin-split surface states on Bi(111) has been comprehensively investigated by high energy and angular resolution spin- and angle-resolved photoelectron spectroscopy. A large out-of-plane spin component that alternately changes its sign is clearly observed. There is no evidence for peculiar polarization oscillation of the in-plane spin component, which was suggested to be due to breaking of the time-reversal symmetry in a previous report [A. Takayama et al., Phys. Rev. Lett. 106, 166401 (2011)]. The observed band structure and spin polarization are well reproduced by the $kp$ model Hamiltonian considering the $C_{3v}$ crystal symmetry, and the in-plane polarization variation at the different $k$ points can be understood by the photoemission final-state effect.

We investigate two prototypical dissipative bosonic systems under slow driving and arbitrary system-bath coupling strength, recovering their dynamic evolution as well as the heat and work rates, and we verify that thermodynamic laws are respected. Specifically, we look at the damped harmonic oscillator and the damped two-level system. For the former, we study independently the slow time-dependent perturbation in the oscillator frequency and in the coupling strength. For the latter, we concentrate on the slow modulation of the energy gap between the two levels. Importantly, we are able to find the entropy production rates for each case without explicitly defining nonequilibrium extensions for the entropy functional. This analysis also permits the definition of phenomenological friction coefficients in terms of structural properties of the system-bath composite.

We derive viscous forces for vortices in a thin-film ferromagnet. The viscous force acting on vortex $i$ is a linear superposition ${\mathbf{F}}_i=-{\sum }_j{\widehat{D}}_{ij}{\mathbf{V}}_j$, where ${\mathbf{V}}_j$ is the velocity of vortex $j$. Thanks to the long-range nature of vortices, the mutual drag tensor ${\widehat{D}}_{ij}$ is comparable in magnitude to the coefficient of self-drag $D_{ii}$.

$\mathrm{C}{\mathrm{o}}_2\mathrm{FeAl}$ (CFA) ultrathin films, of various thicknesses $\left(0.9\mathrm{nm}\le t_{\mathrm{CFA}}\le 1.8\mathrm{nm}\right)$, have been grown by sputtering on Si substrates, using Ir as a buffer layer. The magnetic properties of these structures have been studied by vibrating sample magnetometry (VSM), miscrostrip ferromagnetic resonance (MS-FMR), and Brillouin light scattering (BLS) in the Damon-Eshbach geometry. VSM characterizations show that films are mostly in-plane magnetized and the saturating field perpendicular to the film plane increases with decreasing CFA thickness suggesting the existence of a perpendicular interface anisotropy. The presence of a magnetic dead layer of 0.44 nm has been detected by VSM. The MS-FMR with the magnetic field applied perpendicularly to the film plane has been used to determine the gyromagnetic factor. The BLS measurements reveal a pronounced nonreciprocal spin wave propagation, due to the interfacial Dzyaloshinskii-Moriya interaction (DMI) induced by the Ir interface with CFA, which increases with decreasing CFA thickness. The DMI sign has been found to be the same (negative) as that of Pt/Co, in contrast to the ab initio calculation on Ir/Co, where it is found to be positive. The thickness dependence of the effective DMI constant shows the existence of two regimes similarly to that of the perpendicular anisotropy constant. The surface DMI constant $D_s$ was estimated to be $-0.37\mathrm{pJ}/\mathrm{m}$ for the thickest samples, where a linear thickness dependence of the effective DMI constant has been observed.

Granular crystals are periodic structures of elastic beads arranged in crystal lattices. One important feature of granular crystals is that the interactions between beads can take place via noncentral contact forces, leading to the propagation of rotational and coupled rotational-translational waves. Here, we theoretically demonstrate the topological properties of these mechanical rotational waves in a granular graphene, a two-dimensional monolayer honeycomb granular crystal with Dirac dispersion at the center of the Brillouin zone. Around the Dirac point, effective spin, helicity, and effective spin-orbit coupling are illustrated in the mechanical granular system. Finally, quasitopological transport, where the rotational edge waves are nearly topologically protected, is observed on the interface.

We have investigated the topological Hall effect (THE) in MnGa/Pt and MnGa/Ta bilayers induced by the inter- facial Dzyaloshinskii-Moriya interaction (DMI). By varying the growth parameters, we can modulate the domain wall energy, and the largest THE signals are found when the domain wall energy is the smallest. The large topological portion of the Hall signal from the total Hall signal has been extracted in the whole temperature range from 5 to 300 K. These results open up the exploration of the DMI induced magnetic behavior based on the bulk perpendicular magnetic anisotropy materials for fundamental physics and magnetic storage technologies.

We find evidence that the newly discovered Fe-based superconductor ${\mathrm{KCa}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2(T_c=33.36\left(7\right)$ K) displays multigap superconductivity with line nodes. Transverse field muon spin rotation $\left(\mu \mathrm{SR}\right)$ measurements show that the temperature dependence of the superfluid density does not have the expected behavior of a fully gapped superconductor, due to the lack of saturation at low temperatures. Moreover, the data cannot be well fitted using either single band models or a multiband $s$-wave model, yet are well described by two-gap models with line nodes on either one or both of the gaps. Meanwhile the zero-field $\mu \mathrm{SR}$ results indicate a lack of time reversal symmetry breaking in the superconducting state, but suggest the presence of magnetic fluctuations. These results demonstrate a different route for realizing nodal superconductivity in iron-based superconductors. Here the gap structure is drastically altered upon replacing one of the spacer layers, indicating the need to understand how the pairing state is tuned by changes of the asymmetry between the pnictogens located either side of the Fe planes.

${\mathrm{URu}}_2{\mathrm{Si}}_2$ exhibits an anomalous peak in the nonlinear magnetic susceptibility at the hidden order transition. In order to investigate this anomaly, we conducted direct magnetization measurements and investigated the detailed angular dependence of the ${}^{29}\mathrm{Si}$ nuclear magnetic resonance Knight shift tensor. We find that the nonlinear magnetization is smaller than previously reported, and the analogous nonlinear Knight shift tensor is below the detection limit. Our results suggest that the magnitude of the anomalous peak is sample dependent.