By means of density functional theory we have undertaken a structural, electronic, and magnetic survey of the adsorption of the ${\mathrm{Fe}}_x{\mathrm{Pt}}_y(x,y\le 4)$ clusters on the MgO(001) surface under the generalized gradient approximation. We have tested different atomic adsorption geometries with the aim of scanning a wider range of adsorption sites in order to determine the preferential surface covering. The intracluster (before and after the adsorption) and cluster-to-surface binding mechanisms were investigated via the adsorption energy, charge transfer, density of states, and hybridization analysis. The adsorption energy values increased for those geometries in which keeping the Fe or Pt atom $@top\text{−}\mathrm{O}$, and the outermost species was moved to cover the surface. In general the unsupported clusters present higher intracluster energies than the adsorbed ones with an average difference of 1.5 eV. In this regard there was a small reduction in the net magnetic moment of the supported clusters due to an internal and external rearrangement of the spin-up/-down charge. Furthermore, a complex and subtle charge transfer between different species takes place having an increase in the Pt and O population at the expense of the lost Fe charge.

Photonic crystal and split ring resonator (SRR) metamaterial waveguides with Kerr nonlinear dielectric impurities are studied. The transmission coefficients for two guided modes of different frequencies scattering from the Kerr impurities are computed. The systems are shown to exhibit multiple transmission coefficient solutions arising from the Kerr nonlinearity. Multiple transmission coefficients occur when different input intensities into a waveguide result in the same transmitted output intensities past its nonlinear impurities. (In the case of a single incident guided mode the multiplicity of transmission coefficients is known as optical bistability.) The analytical conditions under which the transmission coefficients are single and multiple valued are determined, and specific examples of both single and multiple valued transmission coefficient scattering are presented. Both photonic crystal and split ring resonator systems are studied as the Kerr nonlinearity enters the photonic crystal and SRR systems in different ways. This allows for an interesting comparison of the differences in behaviors of these two types of system which are described by distinctly different mathematical structures. Both the photonic crystal and SRR models used in the calculations are based on a difference equation approach to the system dynamics. The difference equation approach has been extensively employed in previous papers to model the basic properties of these systems. The paper is a continuation of work on the optical bistability of single guided modes interacting with Kerr impurities in photonic crystals originally considered by McGurn [Chaos 13, 754 (2003)] and work on the resonant scattering from Kerr impurities in photonic crystal waveguides considered by McGurn [J. Phys.: Condens. Matter 16, S5243 (2004)]. It generalizes this work making the extension to the more complex interaction of two guided modes at different frequencies. It extends the two guided mode treatment by McGurn [Organ. Electron. 8, 227 (2007)] which was limited to a special case of one of the photonic crystal systems considered here.

The renormalization group technique is applied to one-dimensional electron-phonon Hubbard models at half filling and zero temperature. For the Holstein-Hubbard model, the results of one-loop calculations are congruent with the phase diagram obtained by quantum Monte Carlo simulations in the $(U,g_{\mathrm{ph}})$ plane for the phonon-mediated interaction $g_{\mathrm{ph}}$ and the Coulomb interaction $U$. The incursion of an intermediate phase between a fully gapped charge-density-wave state and a Mott antiferromagnet is supported along with the growth of its size with the molecular phonon frequency ${\omega }_0$. We find additional phases enfolding the base boundary of the intermediate phase. A Luttinger liquid line is found below some critical $U^{*}\approx g_{\mathrm{ph}}^{*}$, followed at larger $U\sim g_{\mathrm{ph}}$ by a narrow region of bond-order-wave ordering which is either charge or spin gapped depending on $U$. For the Peierls-Hubbard model, the region of the $(U,g_{\mathrm{ph}})$ plane with a fully gapped Peierls-bond-order-wave state shows a growing domination over the Mott gapped antiferromagnet as the Debye frequency ${\omega }_D$ decreases. A power-law dependence $g_{\mathrm{ph}}\sim U^{2\eta }$ is found to map out the boundary between the two phases, whose exponent is in good agreement with the existing quantum Monte Carlo simulations performed when a finite nearest-neighbor repulsion term $V$ is added to the Hubbard interaction.

In this paper we report about linear and nonlinear optical properties of intersubband cavity polariton samples, where the resonant photonic mode is a defect state in a metallodielectric photonic crystal slab. By tuning a single geometric parameter of the resonator, the cavity $Q$ factor can reach values as large as 85, with a consequent large cooperativity for the light-matter interaction. We show that a device featuring large cooperativity leads to sharp saturation, or even bistability, of the polariton states. This nonlinear dynamics occurs at the crossover between the weak- and the strong-coupling regimes, where the weak critical coupling concept plays a fundamental role.

Quantum dots exhibit reproducible conductance fluctuations at low temperatures due to electron quantum interference. These fluctuations are not solely determined by dot geometry; they are also highly sensitive to the underlying disorder potential. Here we exploit this sensitivity to better understand the role that background impurities play in the electronic properties of undoped AlGaAs/GaAs heterostructures, and nanoscale devices based thereon. In particular, we report the remarkable ability to first alter the disorder potential in an undoped AlGaAs/GaAs heterostructure by optical illumination and then reset it back to its initial configuration by room temperature thermal cycling in the dark. We attribute this behavior to a mixture of C background impurities acting as shallow acceptors and deep trapping by Si background donor impurities, i.e., DX centers. This “alter and reset” capability is not possible in modulation-doped heterostructures and offers a route to new studies of how background impurities influence transport in nanoscale semiconductor devices.

Using the Landau theory we demonstrate the possibility of unusual, anomalously thick ferroelectric domain walls, where the order parameter rotates with a nearly constant modulus as in ferromagnets. The effect occurs in ${90}^{\circ }$ charged domain walls and is associated with the easy polarization rotation near the morphotropic phase boundary. This magneticlike profile is unique to charged domain walls due to the availability of free charges which screen the depolarizing field resulting from polarization rotation. In contrast to previously known thick ferroelectric domain walls, the large thickness of ${90}^{\circ }$ charged domain walls near the morphotropic boundary is predicted for a wide temperature interval. This property may be favorable for potential applications.

We consider the interplay between magnetic skyrmions in an insulating thin film and the Dirac surface states of a three-dimensional topological insulator (TI), coupled by the proximity effect. The magnetic texture of skyrmions can lead to confinement of Dirac states at the skyrmion radius, where out-of-plane magnetization vanishes. This confinement can result in charging of the skyrmion texture. The presence of bound states is robust in an external magnetic field, which is needed to stabilize skyrmions. It is expected that, for relevant experimental parameters, skyrmions will have a few bound states that can be tuned using an external magnetic field. We argue that these charged skyrmions can be manipulated directly by an electric field, with skyrmion mobility proportional to the number of bound states at the skyrmion radius. Coupling skyrmionic thin films to a TI surface can provide a more direct and efficient way of controlling skyrmion motion in insulating materials. This provides a different dimension in the study of skyrmion manipulation.

Structure and magnetic properties of high-quality polycrystlline CeMnAsO, a parent compound of the “1111”-type oxypnictides, have been investigated using neutron powder diffraction and magnetization measurements. We find that CeMnAsO undergoes a C-type antiferromagnetic order with ${\mathrm{Mn}}^{2+}(S=5/2)$ moments pointing along the $c$ axis below a relatively high Néel temperature of $T_{\mathrm{N}}=347\left(1\right)$ K. Below $T_{\mathrm{SR}}=35$ K, two simultaneous transitions occur where the Mn moments reorient from the $c$ axis to the $ab$ plane preserving the C-type magnetic order, and Ce moments undergo long-range AFM ordering with antiparallel moments pointing in the $ab$ plane. Another transition to a noncollinear magnetic structure occurs below 7 K. The ordered moments of Mn and Ce at 2 K are 3.32(4) ${\mu }_B$ and 0.81(4)${\mu }_B$, respectively. We find that CeMnAsO primarily falls into the category of a local-moment antiferromagnetic insulator in which the nearest-neighbor interaction $\left(J_1\right)$ is dominant with $J_2<J_1/2$ in the context of $J_1-J_2-J_c$ model. The spin reorientation transition driven by the coupling between Ce and the transition metal seems to be common to Mn, Fe, and Cr ions, but not to Co and Ni ions in the isostructural oxypnictides. A schematic illustration of magnetic structures in Mn and Ce sublattices in CeMnAsO is presented.

The one-dimensional free Fermi gas is a prototype conformally invariant system whose entanglement properties are well understood. In this paper, the effects of a single impurity on one-dimensional free fermion entanglement entropy are studied both analytically and numerically. Such an impurity represents an exactly marginal perturbation to the bulk conformally invariant fixed point. We find that the impurity leads to subleading contributions to the entanglement entropy that scale inversely with the subsystem size. The origins of such contributions are identified.

We present a study on the modification of the electronic structure and hole-doping effect for the layered dichalcogenide ${\mathrm{WSe}}_2$ with a multivalley band structure, where Ta is doped on the W site along with a partial substitution of Te for its lighter counterpart Se. By means of band-structure calculations and specific-heat measurements, the introduction of Te is theoretically and experimentally found to change the electronic states in ${\mathrm{WSe}}_2$. While in ${\mathrm{WSe}}_2$ the valence-band maximum is located at the $\Gamma$ point, the introduction of Te raises the bands at the $K$ point with respect to the $\Gamma$ point. In addition, thermal-transport measurements reveal a smaller thermal conductivity at room temperature of ${\mathrm{W}}_{1-x}{\mathrm{Ta}}_x{\mathrm{Se}}_{1.6}{\mathrm{Te}}_{0.4}$ than reported for ${\mathrm{W}}_{1-x}{\mathrm{Ta}}_x{\mathrm{Se}}_2$. However, when approaching 900 K, the thermal conductivities of both systems converge while the resistivity in ${\mathrm{W}}_{1-x}{\mathrm{Ta}}_x{\mathrm{Se}}_{1.6}{\mathrm{Te}}_{0.4}$ is larger than in ${\mathrm{W}}_{1-x}{\mathrm{Ta}}_x{\mathrm{Se}}_2$, leading to comparable but slightly smaller values of the figure of merit in ${\mathrm{W}}_{1-x}{\mathrm{Ta}}_x{\mathrm{Se}}_{1.6}{\mathrm{Te}}_{0.4}$.

We propose a pump-probe setup to analyze the properties of the collective excitation spectrum of a spinor polariton fluid. By using a linear response approximation scheme, we carry on a complete classification of all excitation spectra, as well as their intrinsic degree of polarization, in terms of two experimentally tunable parameters only: the mean-field polarization angle and a rescaled pump detuning. We evaluate the system response to the external probe, and show that the transmitted light can undergo a spin rotation along the dispersion for spectra that we classify as diffusivelike. We show that in this case, the spin flip predicted along the dispersion is enhanced when the system is close to a parametrically amplified instability.

We detail the influence of a magnetic field on exciton-polaritons inside a semiconductor microcavity. Magnetic field can be used as a tuning parameter for exciton and photon resonances. We discuss the change of the exciton energy, the oscillator strength, and redistribution of the polariton density along the dispersion curves due to the magnetically induced detuning. We have observed that field-induced shrinkage of the exciton wave function has a direct influence not only on the exciton oscillator strength, which is observed to increase with the magnetic field, but also on the polariton linewidth. We discuss the effect of the Zeeman splitting on polaritons the magnitude of which changes with the exciton Hopfield coefficient and can be modeled by independent coupling of the two spin components of excitons with cavity photons.

A monolayer of graphene irradiated with circularly polarized light suggests a unique platform for surface electromagnetic wave (plasmon-polariton) manipulation. In fact, the time periodicity of the Hamiltonian leads to a geometric Aharonov-Anandan phase and results in a photovoltaic Hall effect in graphene, creating off-diagonal components of the conductivity tensor. The latter drastically changes the dispersion relation of surface plasmon-polaritons, leading to hybrid wave generation. In this paper we present a systematic and self-contained analysis of the hybrid surface waves obtained from Maxwell equations based on a microscopic formula for the conductivity. We consider a practical example of graphene sandwiched between two dielectric media and show that in the one-photon approximation there is formation of propagating hybrid surface waves. From this analysis emerges the possibility of a reliable experimental realization to study Zitterbewegung of charge carriers of graphene.

HgTe is a band-inverted compound which forms a two-dimensional topological insulator if sandwiched between CdTe barriers for a HgTe layer thickness above the critical value. We describe the fine structure of Dirac states in the HgTe/CdTe quantum wells of critical and close-to-critical thicknesses and show that the necessary creation of interfaces brings in another important physical effect: the opening of a significant anticrossing gap between the tips of the Dirac cones. The level repulsion driven by the natural interface inversion asymmetry of zinc-blende heterostructures considerably modifies the electron states and dispersion but preserves the topological transition at the critical thickness. By combining symmetry analysis, atomistic calculations, and extended $\mathit{k}·\mathit{p}$ theory with interface terms, we obtain a quantitative description of the energy spectrum and extract the interface mixing coefficient. We discuss how the fingerprints of the predicted zero-magnetic-field splitting of the Dirac cones could be detected experimentally by studying magnetotransport phenomena, cyclotron resonance, Raman scattering, and THz radiation absorption.

We observe a bistability in an electrically driven polariton condensate, which is manifested by a memory dependent threshold characteristic. In contrast to the polariton bistabilities previously observed in resonantly optically pumped microcavities, our effect occurs under nonresonant electric pumping and is triggered by the current injection scheme. We explain the origin of the bistability by a dependence of the electron-hole tunneling lifetime on the carrier density in the embedded quantum wells. The field screening effect creates a positive feedback loop, which yields the bistable behavior of the condensate. We develop a rate-equation-based model which qualitatively explains the occurrence of the hysteresis under current injection, its reduction with increased magnetic field, and the absence of bistability under optical pumping.

An anomalously sharp ($\delta$-function-like) $n=0$ Landau level in the presence of disorder is usually considered to be a manifestation of the massless Dirac fermions in magnetic fields. This property persists even when the Dirac cone is tilted, which has been shown by Kawarabayashi et al. [Phys. Rev. B 83, 153414 (2011)] to be a consequence of a “generalized chiral symmetry.” Here we pose the question of whether this property will be washed out when the tilted Dirac fermion becomes massive. Surprisingly, the levels continue to be $\delta$-function-like, although the mass term that splits $n=0$ Landau levels may seem to degrade the anomalous sharpness. This has been shown both numerically for a tight-binding model and analytically in terms of the Aharonov-Casher argument extended to the massive tilted Dirac fermions. A key observation is that, while the generalized chiral symmetry is broken by the mass term, the $n=0$ Landau level continues to accommodate eigenstates of the generalized chiral operator, resulting in the robustness against chiral-symmetric disorders. Mathematically, the conventional and generalized chiral operators are related to each other via a nonunitary transformation, with which the split, nonzero-energy $n=0$ wave functions of the massive system are just gauge-transformed zero-mode wave functions of the massless system. The message is that the chiral symmetry, rather than a simpler notion of the sublattice symmetry, is essential for the robustness of the $n=0$ Landau level.

The emission spectrum of the trion state in very thick shell CdSe/CdS nanocrystals is characterized at 4 K by photon correlation Fourier spectroscopy. A value of $50 \mu \mathrm{eV}$ for the width of the zero phonon line is measured. The absence of blinking and the high photostability of these emitters offer the possibility to investigate the dynamics of the emission spectrum at a time scale as short as 250 ns. We show that the high value of the linewidth $\left(50 \mu \mathrm{eV}\right)$ is not due to spectral diffusion induced by the close environment of the emitter at time scales larger than 250 ns. The broadening is attributed to the additional third carrier when compared to the monoexcitonic state.