We investigate the combined influence of structural defects and uniaxial longitudinal strain on the electronic transport properties of armchair graphene nanoribbons using the numerical approach based on the semiempirical tight-binding model, the Landauer formalism, and the recursion method for Green functions. We calculate the conductance of graphene nanoribbons in the quantum coherent regime with different types and concentrations of defects. Further, we apply uniform planar tension to the nonideal graphene ribbons with randomly distributed and oriented single and double vacancies and Stone-Wales defects. Since transport characteristics of graphene nanoribbons are found to be very sensitive to edge termination and aspect ratio, and it has been shown that energy gaps can emerge under critical strain; the interplay of both effects needs to be studied. We show that band gap engineering using strain is still possible for nonideal armchair ribbons with a small defect concentration, as the oscillatory behavior of the gap is preserved.

A neutron scattering investigation of the magnetoelectric coupling in PbFe${}_{1/2}$Nb${}_{1/2}$O${}_3$ has been undertaken. Ferroelectric order occurs below 400 K, as evidenced by the softening with temperature and subsequent recovery of the zone center transverse optic phonon mode energy ($\hslash {\Omega }_0$). Over the same temperature range, magnetic correlations become resolution limited on a terahertz energy scale. In contrast to the behavior of nonmagnetic disordered ferroelectrics [namely Pb(Mg,Zn)${}_{1/3}$Nb${}_{2/3}$O${}_3$], we report the observation of a strong deviation from linearity in the temperature dependence of ${\left(\hslash {\Omega }_0\right)}^2$. This deviation is compensated by a corresponding change in the energy scale of the magnetic excitations, as probed through the first moment of the inelastic response. The coupling between the short-range ferroelectric and antiferromagnetic correlations is consistent with calculations showing that the ferroelectricity is driven by the displacement of the body-centered iron site, illustrating the multiferroic nature of magnetic-lead-based relaxors in the dynamical regime.

An ab initio method for determining the dynamical structure function of an interacting many-body quantum system has been devised by combining a generalized integral transform method with quantum Monte Carlo (QMC) methods. A kernel has been found that (i) lets the transform be calculable with QMC methods and (ii) is a representation of the $\delta$ function, allowing an inversion of the transform with a much higher predictive power than the inverse Laplace transform. As a first application, the excitation spectrum of bulk atomic ${}^4$He has been computed, both in the low and intermediate momentum ranges. The peculiar form of the kernel allows us to predict, without using any model, both positions and widths of the collective excitations in the maxon-roton region, as well as the existence of the second collective peak. A prediction of the dispersion of the single-particle modes described by the incoherent part is also presented.

We propose a theoretical framework enabling the simulation of isothermal remanence magnetization (IRM) curves, based on the Stoner-Wohlfarth model combined with the Néel macrospin relaxation time description. We show how low temperature IRM curves, which have many advantages compared to hysteresis loops, can be efficiently computed for realistic assemblies of magnetic particles with both a size and anisotropy constant distribution, and a biaxial anisotropy. The IRM curves, which probe the irreversible switching provoked by an applied field, are shown to be complementary to other usual measurements (in particular low-field susceptibility curves where a thermal switching is involved). As an application, the experimental IRM curve of Co clusters embedded in a carbon matrix is analyzed. We demonstrate how powerful such an analysis can be, which in the present case allows us to put into evidence an anisotropy constant dispersion among the Co nanoparticles.

We investigated the magnetic properties of the antiferromagnetic (AFM) compound Gd$T$${}_2$Al${}_{10}$ ($T=\text{Ru}$ and Fe). Although GdRu${}_2$Al${}_{10}$ could be understood well as a simple AFM compound, the exchange interaction in GdFe${}_2$Al${}_{10}$ is found to be greatly varied with temperature. The magnitude of the AFM exchange interaction is reduced with decreasing temperature. We ascribed its origin to the exchange enhancement of the Fe ion with a decrease of temperature as was observed in YFe${}_2$Al${}_{10}$. The ordered moment is found to be along the [011] direction in the $bc$ plane in both compounds from the anisotropic magnetic susceptibility. The origin of the magnetic anisotropy below $T_{\mathrm{N}}$ could not be understood by the magnetic dipole interaction, which might come from the AFM order on the zigzag chain.

We present the results of magnetization, electron spin resonance (ESR), and nuclear magnetic resonance (NMR) measurements on single-crystal samples of the frustrated $S=1/2$ chain cuprate LiCu${}_2$O${}_2$ doped with nonmagnetic Zn${}^{2+}$. As shown by the x-ray techniques, the crystals of Li(Cu${}_{1-x}$Zn${}_x$)${}_2$O${}_2$ with $x<0.12$ are single-phase, whereas for higher Zn concentrations the samples were polyphase. ESR spectra for all monophase samples ($0\le x<0.12$) can be explained within the model of a planar spin structure with a uniaxial type anisotropy. The NMR spectra of the highly doped single-crystal sample Li(Cu${}_{0.9}$Zn${}_{0.1}$)${}_2$O${}_2$ can be described in the frame of a planar spin-glass-like magnetic structure with short-range spiral correlations in the crystal $ab$ planes with strongest exchange bonds. The value of magnetic moments of Cu${}^{2+}$ ions in this structure is close to the value obtained for undoped crystals: (0.8 $\pm$ 0.1) ${\mu }_B$.

We consider the classical magnetoresistance of a Weyl metal in which the electron Fermi surface possesses nonzero fluxes of the Berry curvature. Such a system may exhibit large negative magnetoresistance with unusual anisotropy as a function of the angle between the electric and magnetic fields. In this case the system can support an additional type of plasma wave. These phenomena are consequences of the chiral anomaly in electron transport theory.

We studied the low-lying electronic structure of the newly discovered iron-platinum-arsenide superconductor, Ca${}_{10}$(Pt${}_4$As${}_8$)(Fe${}_{2-x}$Pt${}_x$As${}_2$)${}_5$ ($T_c=22$ K) with angle-resolved photoemission spectroscopy. We found that the Pt${}_4$As${}_8$ layer contributes to a small electronlike Fermi surface, indicative of metallic charge reservoir layers that are rare for iron based superconductors. Moreover, the electronic structure of the FeAs layers resembles those of other prototype iron pnictides to a large extent. However, there is only $d_{xy}$-orbital originated holelike Fermi surface near the zone center, which is rather strange for an iron pnictide superconductor with relatively high superconducting transition temperature; and the $d_{xz}$ and $d_{yz}$ originated bands are not degenerate at the zone center. Furthermore, all bands near the Fermi energy show negligible $k_z$ dependence, indicating the strong two-dimensional nature of this material. Our results indicate this material possesses a rather interesting electronic structure, which enriches our current knowledge of iron based superconductors.

The magnetic ground state of the Rh-doped Kondo semiconductor CeRu${}_2$Al${}_{10}$ [Ce(Ru${}_{1-x}$Rh${}_x$)${}_2$Al${}_{10}$] is investigated with the muon spin relaxation method. Muon spin precession with two frequencies is observed in the $x=0$ sample, while only one frequency is present in the $x=0.05$ and 0.10 samples, which is attributed to the broad static field distribution at the muon site. The internal field at the muon site is enhanced from about 180 G in the $x=0$ sample to about 800 G in the Rh-doped samples, supporting the spin-flop transition as suggested by the magnetization measurement, and the boundary of different magnetic ground states is identified around $x=0.03$. The drastic change of magnetic ground state by a small amount of Rh doping (3%) indicates that the magnetic structure in CeRu${}_2$Al${}_{10}$ is not robust and can be easily tuned by external perturbations such as electron doping. The anomalous temperature dependence of the internal field in CeRu${}_2$Al${}_{10}$ is suggested to be attributed to the hyperfine interaction between muons and conduction electrons.

Using advanced first-principles calculations we predict that the nonpolar SrTiO${}_3$/SrZrO${}_3$ (001) interface, designed as either thin SrZrO${}_3$ film deposited on SrTiO${}_3$ or short-period (SrTiO${}_3$)${}_m$/(SrZrO${}_3$)${}_n$ superlattice, hosts a two-dimensionally confined electron gas. Mobile electron charge due to native impurities, field effect, or modulation doping remains tightly trapped at the interface. Key ingredients for this occurrence are (a) the peculiar chemistry of $3d$ orbitals and (b) the large band offset at the titanate-zirconate interface.

We study the relative intensity noise (RIN) per unit bandwidth in polariton laser diodes (LDs) for two pumping geometries, namely, the direct electrical and the intracavity ones as originally described in I. Iorsh et al. [Phys. Rev. B 86, 125308 (2012)], by using rate equations including Langevin noise sources that are adapted from equations employed to describe conventional semiconductor LDs. The obtained expressions for the RIN, which can be used for all inorganic semiconductor polariton LDs, are specifically applied to the case of III-nitride devices. It is highlighted that for frequencies larger than the relaxation resonance frequency the expected minimum RIN of polariton LDs—whatever the pumping geometry—is equal to the standard quantum limit ($\frac{2h\nu }{P_0}$). The general RIN line shape as a function of frequency and optical output power is discussed for the two geometries and simplified expressions for the RIN are given. Then a comprehensive account of the expected evolution of the linewidth of III-nitride polariton LDs upon increasing pumping strength is also given by considering the most advanced theories available to date. The modified Schawlow-Townes linewidth is estimated from the effective ground-state polariton lifetime at threshold, leading to a predicted linewidth as narrow as $\sim$15 MHz at room temperature for the two pumping geometries when using a consistent set of parameters for III-nitride polariton LDs.

We present a microscopic theory of the optical properties of self-assembled quantum dots doped with a single magnetic manganese (Mn) impurity and containing a controlled number of electrons. The single-particle electron and heavy-hole electronic shells are described by two-dimensional harmonic oscillators. The electron-electron, electron-hole Coulomb as well as the short-range electron spin-Mn spin and hole spin-Mn spin contact exchange interactions are included. The electronic states of the photoexcited electron-hole-Mn complex and of the final electron-Mn complex are expanded in a finite number of configurations and the full interacting Hamiltonian is diagonalized numerically. The emission spectrum is predicted as a function of photon energy for a given number of electrons and different number of confined electronic quantum dot shells. We show how emission spectra allow to identify the number of electronic shells, the number of electrons populating these shells and, most importantly, their spin. We show that electrons not interacting directly with the spin of Mn ion do so via electron-electron interactions. This indirect interaction is a strong effect even when Mn impurity is away from the quantum dot center.

The screening properties and collective excitations (plasmons) in AA-stacked bilayer graphene are studied within the random phase approximation. Whereas long-lived plasmons in single-layer graphene and in AB-stacked bilayer graphene can exist only in doped samples, we find that coherent plasmons can disperse in AA-stacked bilayer graphene even in the absence of doping. Moreover, we show that the characteristic low-energy dispersion relation is unaffected by changes in the number of carriers, unless the chemical potential of the doped sample exceeds the interlayer hopping energy. We further consider the effect of an external electric field applied perpendicular to the layers, and show how the dispersion of the modes can be tuned by the application of a gate voltage.

In this work we have calculated $U$ from first principles for the early lanthanides (Ce-Gd) using the constrained random phase approximation (cRPA). We compare the static limit of $U$ with experimental estimations from x-ray photoemission spectroscopy (XPS) and bremsstrahlung isochromat spectroscopy (BIS) spectra and also discuss the frequency dependence of $U$ across the series.The localized subspace is constructed using maximally localized Wannier functions (MLWFs). We discuss the choice of parameters used to construct the Wannier functions and show that in the case of Eu and Gd, where the local density approximation does not yield the correct position of the $4f$ bands, a physically motivated choice gives better agreement with experiment.

The slowdown of optical pulses due to quantum-coherence effects is investigated theoretically for an “active material” consisting of InGaAs-based double quantum dot molecules. These are designed to exhibit a long-lived coherence between two electronic levels, which is an essential part of a quantum-coherence scheme that makes use of electromagnetically induced transparency effects to achieve group-velocity slowdown. We apply a many-particle approach based on realistic semiconductor parameters that allows us to calculate the quantum dot material dynamics including microscopic carrier scattering and polarization dephasing dynamics. The group-velocity reduction is characterized in the frequency domain by a quasiequilibrium slowdown factor and in the time domain by the probe-pulse slowdown obtained from a calculation of the spatiotemporal material dynamics coupled to the propagating optical field. The group-velocity slowdown in the quantum dot molecule is shown to be substantially higher than what is achievable from similar transitions in typical InGaAs-based single quantum dots. The dependencies of slowdown and shape of the propagating probe pulses on lattice temperature and drive intensities are investigated.

The possibility of steady-state population inversion in a small sample of strongly driven two-level emitters such as quantum dots (QDs) in microcavities, and its utilization towards amplification of light and acoustic waves, is investigated theoretically. We find that inversion and absorption spectra of photons and phonons crucially depend on the interplay between the intrinsic vacuum and phonon environments. The absorption profiles of photons and phonons show marked novel features such as gain instead of transparency and absorption reversed to gain, respectively. Furthermore, we report collectivity-induced substantial enhancement of inversion and pronounced gain in the photon and phonon absorption spectra for a subwavelength-sized QD ensemble.

The efficient manipulation with optical spectra of colloidal photonic crystals was achieved using the metal mirror positioned either immediately or at a distance from the crystal surface. If the mirror is close to the surface, the changes are mostly quantitative and relate to the improved light confinement. If the spacer provides a $2\pi$ phase shift for round trip of the light at the wavelength of the diffraction resonance of the opal lattice, the defect band is formed in the photonic stop-band leading to 20–40 times increase of the transmission. The upper limit of the quality factor of the defect band was estimated by extrapolating the development of the defect transmission band as a function of the photonic crystal thickness. The specific features related to the photonic crystal dimensionality were revealed in the defect band spectra. The obtained results support the idea of achieving the control on the optical functionality of photonic crystals by recycling the transmitted light in the crystal volume.

We investigate the magnetic properties of a heterostructure comprised of alternating metallic and Mott insulating layers of fermions with varying interlayer hybridization. Results from large-scale quantum Monte Carlo simulations at half-filling show clear evidence of induced magnetism in the metallic layers due to coupling to the Mott insulating layers at small to intermediate values of interlayer hopping. The in-plane magnetism is completely suppressed via a Kondo proximity effect when the coupling between adjacent layers is increased beyond a critical strength. The nature of the phase in the Kondo-like insulating regime is investigated and is shown to be dominated by simultaneous in-plane and interplane short-range correlations.

We have studied the influence of epitaxial strain on magnetic orderings and the couplings between the spin and polar phonons in the 1:1 SrMnO${}_3$/LaMnO${}_3$ superlattice from first principles. Magnetic phase transitions of the superlattice induced by epitaxial strain are observed, consistent with previous reports. We find that oxygen octahedral rotations lower the ground state energy but do not destroy the magnetic phase transitions induced by strain. We compute zone center phonon frequencies and eigenvectors as functions of epitaxial strain and magnetic ordering. A substantial increase of the coupling strength between the spin and the lowest-frequency polar mode is observed for tensile strains. This increase can be attributed to a change of character of the lowest mode resulting from different relative couplings of the various polar modes to epitaxial strain. Finally, spin-phonon coupling strengths are computed in a Heisenberg formalism. This analysis directly reveals the changes in exchange couplings due to specific atomic displacements or phonon modes, as well as the nonequivalence of the out-of-plane exchange couplings across LaO layers and across SrO layers, the latter being the result of the artificial structuring in the superlattice.