A theoretical investigation on slow light propagation based on electromagnetically induced transparency in a three-level quantum-dot system is performed including non-Markovian effects and correlated dephasing reservoirs. It is demonstrated that the non-Markovian nature of the process is quite essential even for conventional dephasing typical of quantum dots leading to significant enhancement or inhibition of the group velocity slow-down factor as well as to the shifting and distortion of the transmission window. Furthermore, the correlation between dephasing reservoirs may also either enhance or inhibit non-Markovian effects.

We examine the nature of topological currents in black phosphorus when its inversion symmetry is deliberately broken. Here, the conduction- and valence-band edges are located at the $\mathrm{\Gamma }$ point of the rectangular Brillouin zone, and they exhibit strong anisotropy along its two crystal axes. We will show below that these salient features lead to linear transverse neutral topological currents, accompanied also by nonlinear transverse charge currents at the Fermi surface. These topological currents are maximal when the in-plane electric field is applied along the zigzag crystal axes but zero along the armchair direction.

We propose a method for calculating Chern-Simons orbital magnetoelectric coupling, conventionally parametrized in terms of a phase angle $\theta$. According to previous theories, $\theta$ can be expressed as a three-dimensional (3D) Brillouin-zone (BZ) integral of the Chern-Simons 3-form defined in terms of the occupied Bloch functions. Such an expression is valid only if a smooth and periodic gauge has been chosen in the entire Brillouin zone, and even then, convergence with respect to the $\mathbf{k}$-space mesh density can be difficult to obtain. In order to solve this problem, we propose to relax the periodicity condition in one direction (say, the $k_z$ direction) so that a gauge discontinuity is introduced on a two-dimensional (2D) $\mathbf{k}$ plane normal to $k_z$. The total $\theta$ response then has contributions from both the integral of the Chern-Simons 3-form over the 3D bulk BZ and the gauge discontinuity expressed as a 2D integral over the $\mathbf{k}$ plane. Sometimes, the boundary plane may be further divided into subregions by 1D “vortex loops” which make a third kind of contribution to the total $\theta$, expressed as a combination of Berry phases around the vortex loops. The total $\theta$ thus consists of three terms which can be expressed as integrals over 3D, 2D, and 1D manifolds. When time-reversal symmetry is present and the gauge in the bulk BZ is chosen to respect this symmetry, both the 3D and 2D integrals vanish; the entire contribution then comes from the vortex-loop integral, which is either 0 or $\pi$ corresponding to the ${\mathbb{Z}}_2$ classification of 3D time-reversal-invariant insulators. We demonstrate our method by applying it to the Fu-Kane-Mele model with an applied staggered Zeeman field.

An analytical theory for spin diffusion in disordered organic semiconductors is derived. It is based on percolation theory and variable range hopping in a disordered energy landscape with a Gaussian density of states. It describes universally the dependence of the spin diffusion on temperature, carrier density, material disorder, magnetic field, and electric field at the arbitrary magnitude of the Hubbard energy of charge pairs. It is found that, compared to the spin transport carried by carriers hopping, the spin exchange will hinder the spin diffusion process at low carrier density, even under the condition of a weak electric field. Importantly, under the influence of a bias voltage, anomalous spreading of the spin packet will lead to an abnormal temperature dependence of the spin diffusion coefficient and diffusion length. This explains the recent experimental data for spin diffusion length observed in Alq3.

We demonstrate that the interfacial exchange coupling in ferromagnetic/antiferromagnetic (FM/AFM) systems induces symmetry breaking of the spin-orbit (SO) effects. This has been done by studying the field and angle dependencies of anisotropic magnetoresistance and vectorial-resolved magnetization hysteresis loops, measured simultaneously and reproduced with numerical simulations. We show how the induced unidirectional magnetic anisotropy at the FM/AFM interface results in strong asymmetric transport behaviors, which are chiral around the magnetization hard-axis direction. Similar asymmetric features are anticipated in other SO-driven phenomena.

The layered rare-earth diantimonides $R{\mathrm{Sb}}_2$ are anisotropic metals with generally low electronic densities whose properties can be modified by substituting the rare earth. ${\mathrm{LaSb}}_2$ is a nonmagnetic metal with a low residual resistivity presenting a low-temperature magnetoresistance that does not saturate with the magnetic field. It has been proposed that the latter can be associated to a charge density wave (CDW), but no CDW has yet been found. Here we find a kink in the resistivity above room temperature in ${\mathrm{LaSb}}_2$ (at 355 K) and show that the kink becomes much more pronounced with substitution of La by Ce along the ${\mathrm{La}}_{1-x}{\mathrm{Ce}}_x{\mathrm{Sb}}_2$ series. We find signatures of a CDW in x-ray scattering, specific heat, and scanning tunneling microscopy (STM) experiments in particular for $x\approx 0.5$. We observe a distortion of rare-earth–Sb bonds lying in-plane of the tetragonal crystal using x-ray scattering, an anomaly in the specific heat at the same temperature as the kink in resistivity and charge modulations in STM. We conclude that ${\mathrm{LaSb}}_2$ has a CDW which is stabilized in the ${\mathrm{La}}_{1-x}{\mathrm{Ce}}_x{\mathrm{Sb}}_2$ series due to substitutional disorder.

The ternary compounds $\mathrm{CsPb}{X}_3$ ($X=\mathrm{Br}$ or Cl) have perovskite structures that are being considered for optical and electronic applications such as lasing and gamma-ray detection. An above-band-gap excitonic photoluminescence (PL) band is seen in both $\mathrm{CsPb}{X}_3$ compounds. An excitonic emission peak centered at 2.98 eV, $\sim 0.1$ eV above the room-temperature band gap, is observed for $\mathrm{CsPbC}{\mathrm{l}}_3$. The thermal quenching of the excitonic luminescence is well described by a two-step quenching model, yielding activation energies of 0.057 and 0.0076 eV for high- and low-temperature regimes, respectively. $\mathrm{CsPbB}{\mathrm{r}}_3$ exhibits bound excitonic luminescence peaks located at 2.29 and 2.33 eV that are attributed to recombination involving Br vacancy centers. Activation energies for thermal quenching of the excitonic luminescence of 0.017 and 0.0007 eV were calculated for $\mathrm{CsPbB}{\mathrm{r}}_3$. Temperature-dependent PL experiments reveal unexpected blueshifts for all excitonic emission peaks in $\mathrm{CsPb}{X}_3$ compounds. A phonon-assisted step-up process leads to the blueshift in $\mathrm{CsPbB}{\mathrm{r}}_3$ emission, while there is a contribution from band-gap widening in $\mathrm{CsPbC}{\mathrm{l}}_3$. The absence of significant deep level defect luminescence in these compounds makes them attractive candidates for high-resolution, room-temperature radiation detection.

Many of the exotic properties proposed to occur in graphene rely on the possibility of increasing the spin-orbit coupling (SOC). By combining analytical and numerical tight-binding calculations, in this paper we study the SOC induced by heavy adatoms with active electrons living in $p$ orbitals. Depending on the position of the adatoms on graphene, different kinds of SOC appear. Adatoms located in a hollow position induce spin-conserving intrinsiclike SOC, whereas a random distribution of adatoms induces a spin-flipping Rashba-like SOC. The induced SOC is linearly proportional to the adatom concentration, indicating the nonexistent interference effects between different adatoms. By computing the Hall conductivity, we have proven the stability of the topological quantum Hall phases created by the adatoms against inhomogeneous spin-orbit coupling. For the case of Pb adatoms, we find that a concentration of 0.1 adatoms per carbon atom generates SOC's of the order of $\sim 40$ meV.

We study the electronic and transport properties of a topological insulator nanowire including selective magnetic doping of its surfaces. We use a model which is appropriate to describe materials like ${\text{Bi}}_2{\text{Se}}_3$ within a $\mathbf{k}·\mathbf{p}$ approximation and consider nanowires with a rectangular geometry. Within this model the magnetic doping at the (111) surfaces induces a Zeeman field which opens a gap at the Dirac cones corresponding to the surface states. For obtaining the transport properties in a two terminal configuration we use a recursive Green's function method based on a tight-binding model which is obtained by discretizing the original continuous model. For the case of uniform magnetization of two opposite nanowire (111) surfaces we show that the conductance can switch from a quantized value of $e^2/h$ (when the magnetizations are equal) to a very small value (when they are opposite). We also analyze the case of nonuniform magnetizations in which the Zeeman field on the two opposite surfaces change sign at the middle of the wire. For this case we find that conduction by resonant tunneling through a chiral state bound at the middle of the wire is possible. The resonant level position can be tuned by imposing an Aharonov-Bohm flux through the nanowire cross section.

Transport and gain properties of a resonant-phonon terahertz quantum cascade laser are calculated using nonequilibrium Green's functions. Impurity scattering is shown to be responsible for contrasting nonlinear effects in the transport and the gain properties. For typical doping concentrations, the current density is found to be weakly sensitive to the impurity scattering strength. In contrast, the calculated gain is found to be very sensitive to the impurity scattering strength. This difference is attributed to the strong momentum dependence of the long-range coupling to charged impurities. Small-momentum impurity scattering is shown to be responsible for an incoherent regime of resonant tunneling processes. These insights into the crucial role of impurity scattering open a route of improvement of terahertz quantum cascade lasers by engineering of the doping profile.

Using quantum Monte Carlo simulations along with higher-order spin-wave theory, bond-operator and strong-coupling expansions, we analyze the dynamical spin structure factor of the spin-half Heisenberg model on the square-lattice bilayer. We identify distinct contributions from the low-energy Goldstone modes in the magnetically ordered phase and the gapped triplon modes in the quantum disordered phase. In the antisymmetric (with respect to layer inversion) channel, the dynamical spin structure factor exhibits a continuous evolution of spectral features across the quantum phase transition, connecting the two types of modes. Instead, in the symmetric channel, we find a depletion of the spectral weight when moving from the ordered to the disordered phase. While the dynamical spin structure factor does not exhibit a well-defined distinct contribution from the amplitude (or Higgs) mode in the ordered phase, we identify an only marginally damped amplitude mode in the dynamical singlet structure factor, obtained from interlayer bond correlations, in the vicinity of the quantum critical point. These findings provide quantitative information in direct relation to possible neutron or light scattering experiments in a fundamental two-dimensional quantum-critical spin system.

Recently several experimental transmission electron microscopy (TEM) studies have reported the observation of nanoscale triangular defects in mono- and multilayer hexagonal boron nitride ($h$-BN). First-principles calculations are employed to study the thermodynamical stability and spectroscopic properties of these triangular defects and the chemical nature of their edge termination. Oxygen-terminated defects are found to be significantly more stable than defects with nitrogen-terminated edges. Simulated x-ray absorption spectra of the boron $K$ edge for oxygen-terminated defects show excellent agreement with experimental x-ray absorption near-edge spectroscopy (XANES) measurements on defective $h$-BN films with oxygen impurities. Finally, we show that the structural model for oxygen defects in $h$-BN as deduced from the simulated core-level spectroscopy is intrinsically linked to the equilateral triangle shape of defects as observed in many recent electron microscopy measurements.

We derive the boundary conditions for ${\mathrm{MoS}}_2$ and similar transition-metal dichalcogenide honeycomb (2H polytype) monolayers with the same type of $\mathbf{k}·\mathbf{p}$ Hamiltonian within the continuum model around the K points. In an effective two-band description, the electron-hole symmetry breaking quadratic terms are also taken into account. We model the effect of the edges with a linear edge constraint method that has been applied previously to graphene. Focusing mainly on zigzag edges, we find that different reconstruction geometries with different edge atoms can generally be described with one scalar parameter varying between 0 and $2\pi$. We analyze the edge states and their dispersion relation in ${\mathrm{MoS}}_2$ in particular, and we find good agreement with the results of previous density functional theory calculations for various edge types.

Iron-chromium alloys, the base components of various stainless steel grades, have numerous technologically and scientifically interesting properties. However, these features are not yet sufficiently understood to allow their full exploitation in technological applications. In this work, we investigate segregation, precipitation, and phase separation in Fe-Cr systems analyzing the physical mechanisms behind the observed phenomena. To get a comprehensive picture of Fe-Cr alloys as a function of composition, temperature, and time the present investigation combines Monte Carlo simulations using semiempirical interatomic potential, first-principles total energy calculations, and experimental spectroscopy. In order to obtain a general picture of the relation of the atomic interactions and properties of Fe-Cr alloys in bulk, surface, and interface regions several complementary methods have to be used. Using the exact muffin-tin orbitals method with the coherent potential approximation (CPA-EMTO) the effective chemical potential as a function of Cr content (0–15 at. % Cr) is calculated for a surface, second atomic layer, and bulk. At $\sim 10$ at. % Cr in the alloy the reversal of the driving force of a Cr atom to occupy either bulk or surface sites is obtained. The Cr-containing surfaces are expected when the Cr content exceeds $\sim 10$ at. %. The second atomic layer forms about a 0.3 eV barrier for the migration of Cr atoms between the bulk and surface atomic layer. To get information on Fe-Cr in larger scales we use semiempirical methods. However, for Cr concentration regions less than 10 at. %, the ab initio (CPA-EMTO) result of the important role of the second atomic layer to the surface is not reproducible from the large-scale Monte Carlo molecular dynamics (MCMD) simulation. On the other hand, for the nominal concentration of Cr larger than 10 at. % the MCMD simulations show the precipitation of Cr into isolated pockets in bulk Fe-Cr and the existence of the upper limit of the solubility of Cr into Fe layers in Fe/Cr layer systems. For high Cr concentration alloys the performed spectroscopic measurements support the MCMD simulations. Hard x-ray photoelectron spectroscopy and Auger electron spectroscopy investigations were carried out to explore Cr segregation and precipitation in the Fe/Cr double layer and ${\mathrm{Fe}}_{0.95}{\mathrm{Cr}}_{0.05}$ and ${\mathrm{Fe}}_{0.85}{\mathrm{Cr}}_{0.15}$ alloys. Initial oxidation of Fe-Cr was investigated experimentally at ${10}^{-8}$ Torr pressure of the spectrometers showing intense ${\mathrm{Cr}}_2{\mathrm{O}}_3$ signal. Cr segregation and the formation of Cr-rich precipitates were traced by analyzing the experimental atomic concentrations and chemical shifts with respect to annealing time, Cr content, and kinetic energy of the exited electron.

Barium cerate (${\mathrm{BaCeO}}_3$) is a well-known ionic conductor of both hydrogen and oxygen. In applications, it is frequently doped (for instance with Y) to increase stability and promote diffusion. However, the effects of doping and native defects are not fully understood. Computational studies have been stymied by the nature of the conduction band, which is made up of cerium $4f$ states. These states present a challenge to ab initio techniques based on density functional theory within the standard approximations for exchange and correlation. Using a hybrid functional, we investigate the effects of hydrogen impurities and native defects on the electrical and optical properties of ${\mathrm{BaCeO}}_3$. We discuss the tendency of excess electrons or holes to localize in the form of small polarons. We also explore the interactions of polarons with hydrogen impurities and oxygen vacancies, and their impact on luminescence properties.

Switching, i.e., polarization reversal, of ferroelectric P(VDF-TrFE) thin films has been investigated in a wide range of applied electric field and temperature. The measured polarization transients can be quantitatively described by a compressed exponential function as originally formulated by Kolmogorov, Avrami, and Ishibashi (the KAI model). The phenomenological parameters switching time and Avrami index are related to the velocity and morphology of the domain walls, respectively. We show that the switching time depends exponentially on the electric field as described by the Merz law. The experimentally obtained Avrami index is independent of temperature but decreases with applied electric field from 1.55 at low field to 1.0 at 300 MV/m, indicative of an out-of-equilibrium dynamics of the growing domains. Using a random walk model we demonstrate that the observed switching dynamics is in agreement with a domain growth mechanism where domains are circular at low electric field with few nucleation sites, whereas at high field irregular entangled domains are formed. The density of nucleation sites is extracted as $1.5\%$ of the monomeric units in this work. Switching transients on capacitors where the nuclei density has been artificially increased exhibit one-dimensional-like growth, as predicted by our model.