Measurements of the Raman scattering and extended x-ray-absorption fine-structure (EXAFS) spectroscopy are reported on a series of Bridgman-grown zinc-blende CdTe${}_{1-x}$Se${}_x$ (0.35 ≥ $x$ > 0.05) ternary alloys to empathize their lattice dynamical and structural properties. Low-temperature Raman spectra have revealed the classic CdTe-like (TO${}_1$, LO${}_1$) and CdSe-like (TO${}_2$, LO${}_2$) pairs of optical phonons. The composition-dependent peak positions of the LO${}_2$ modes exhibited shifts towards the higher-energy side, while those of the LO${}_1$ phonon frequencies have unveiled the slight redshifts. Detailed analyses of EXAFS data by using the first-principles bond orbital model have enabled us to estimate both the lattice relaxations and nearest-neighbor radial force constants around the Se/Te atoms in the CdTe/CdSe matrix. These results are methodically integrated in the “average $t$-matrix” formalism within the Green's-function theory for defining the impurity perturbations to comprehend the composition-dependent optical phonons in CdTe${}_{1-x}$Se${}_x$ alloys. Based on our comprehensive calculations of impurity modes in the low-composition regime $x\to$ 0, we have assigned the weak phonon feature observed near ∼175 cm${}^{-1}$ in the low-temperature infrared reflectivity spectroscopy study to a Se${}_{\mathrm{Te}}$ localized vibrational mode.

We investigate a strongly anisotropically strained $M$-plane GaN film by photoluminescence spectroscopy. The spectra are dominated by the lowest-energy free-exciton transition, but the second lowest one is also detectable even at low temperatures. Rotating the linear polarization of the excitation results in a systematic change of the intensity ratio between these two transitions. We analyze this observation with the help of $6\times 6$ $\mathbf{k}·\mathbf{p}$ calculations and associated rate equation models. This analysis shows that the pronounced excitation polarization anisotropy for this $M$-plane GaN film originates from a competition between hole relaxation and exciton recombination.

Back focal plane imaging microscopy of the autofluorescence of a colloidal photonic crystal at fixed wavelength is used to visualize photonic stop bands. Emission angles of up to ${74}^{\circ }$ against the crystals surface normal are detected. Structure and orientation of the crystal lattice, quality of the ordering, and the influence of defects on the optical properties can be probed at high spatial resolution. Exemplary results reveal domains with four-, six- and threefold symmetry, where the latter dominates, as expected for fcc lattices.

We consider the classical $O\left(N\right)$-symmetric models confined in a $d$-dimensional slablike geometry and subject to periodic boundary conditions. Applying the one-particle-irreducible variant of functional renormalization group (RG), we compute the critical Casimir forces acting between the slab boundaries. The applied truncation of the exact functional RG flow equation retains interaction vertices of arbitrary order. We evaluate the critical Casimir amplitudes ${\Delta }_f(d,N)$ for continuously varying dimensionality between two and three and $N=1,2$. Our findings are in very good agreement with exact results for $d=2$ and $N=1$. For $d=3$, our results are closer to Monte Carlo predictions than earlier field-theoretic RG calculations. Inclusion of the wave function renormalization and the corresponding anomalous dimension in the calculation has negligible impact on the computed Casimir forces.

We show that the nonequilibrium spin polarization of one-dimensional helical edge states at the boundary of a two-dimensional topological insulator can dynamically induce a polarization of nuclei via the hyperfine interaction. When combined with a spatially inhomogeneous Rashba coupling, the steady-state polarization of the nuclei produces backscattering between the topologically protected edge states leading to a reduction in the conductance which persists to zero temperature. We study these effects in both short and long edges, uncovering deviations from Ohmic transport at finite temperature and a current noise spectrum which may hold the fingerprints for experimental verification of the backscattering mechanism.

The possibility of controlling the optical transition probability between neighboring silicon nanoclusters (Si-NCs) constitutes nowadays an attractive prospect in nanophotonics and photovoltaics. In this work, by means of theoretical ab initio calculations, we investigate the effect of strain on the optoelectronic properties of Si-NCs pairs. We consider two sources of strain: the strain induced by an embedding SiO${}_2$ matrix and the strain generated by mutual NC-NC forces occurring at small distances. Independently on its source, we observe a fundamental impact of the strain on the orbital localization and, as a consequence, on the transition probability between energy states belonging or not to the same NC. The resulting picture allots to the structural strain a fundamental role in the NC-NC interaction mechanisms, suggesting the possibility of enabling a strain-controlled response in Si-NC ensembles.

Combined density functional theory and empirical-potential calculations are performed to investigate the lowest-energy sites and migration mechanisms of He in various $\alpha$-Fe grain boundaries (GBs). Before the defect calculations, we show that structural optimizations, including simulated annealing and atom removal, are crucial for locating the stable GB structure in a given temperature regime. Then, the He formation energies for all the substitutional and interstitial sites in two different GBs are evaluated, showing a strong He segregation tendency. At variance with the bulk Fe case, the formation energy of an interstitial He is either lower than or similar to that of a substitutional He in the GBs. Finally, both static and dynamic barriers for interstitial He diffusion in the GBs are determined. Although the diffusion details and precise paths are GB dependent, some common features are identified: (1) The He atom always remains confined to the GB region while diffusing; (2) the He diffusion is highly anisotropic along the GBs; (3) the GB diffusion of an interstitial He atom is found to be always slower than its bulk diffusion, but it can still be faster than the bulk diffusion of a substitutional He.

We present magnetic domain states in a material configuration with high (perpendicular) magnetic anisotropy and particularly low magnetic pinning. This material, a B-doped Co/Pt multilayer configuration, exhibits a strong magnetic contrast in x-ray transmission experiments, making it apt for dynamic imaging with modern synchrotron techniques, providing high spatial and high temporal resolution simultaneously. By analyzing the static spin structures in nanodisks at variable external fields, we show that CoB/Pt multilayers exhibit low enough domain wall pinning to manipulate the domain pattern with weak stimuli and in particular to move domains and domain walls. We demonstrate in a proof-of-principle experiment using pump-probe x-ray holographic imaging that moderate magnetic fields can induce elastic and deterministic and hence repeatable small variations of the domain configuration in CoB/Pt multilayers, which is the key to perform high-resolution imaging of the domain wall motion to gain insight to the details of the local magnetization dynamics.

The thermal conductivity of silicon-germanium superlattices is computed from density-functional perturbation theory using relaxation times that include both anharmonic and interface roughness effects. A decrease in the group velocity of low-frequency phonons in addition to the interface-disorder-induced scattering of high-frequency phonons drives the superlattice thermal conductivity to below the alloy limit. At short periods, interplay between decrease in group velocity and increase in phonon lifetimes with increase in superlattice period leads to a minimum in the cross-plane thermal conductivity. Increasing the mass mismatch between the constituent materials in the superlattice further lowers the thermal conductivity below the alloy limit, pointing to avenues for higher efficiency thermoelectric materials.

Neutron scattering experiments were performed to investigate magnetic order and magnetic excitations in ternary iron chalcogenide K${}_2$Fe${}_4$Se${}_5$. The formation of a superlattice structure below 580 K together with the decoupling between the Fe-vacancy order-disorder transition and the antiferromagnetic order transition appears to be a common feature in the $A$${}_2$Fe${}_4$Se${}_5$ family. The study of spin dynamics of K${}_2$Fe${}_4$Se${}_5$ reveals two distinct energy gaps at the magnetic Brillouin zone center, which indicates the presence of magnetic anisotropy and the decrease of local symmetry due to electronic and orbital anisotropy. The low-energy spin wave excitations of K${}_2$Fe${}_4$Se${}_5$ can be properly described by linear spin wave theory within a Heisenberg model. Compared to iron pnictides, K${}_2$Fe${}_4$Se${}_5$ exhibits a more two-dimensional magnetism as characterized by large differences not only between out-of-plane and in-plane spin wave velocities, but also between out-of-plane and in-plane exchange interactions.

Images of flux vortices in superconductors acquired by transmission electron microscopy should allow a quantitative determination of their magnetic structure but, so far, only visual comparisons have been made between experimental images and simulations. Here, we make a quantitative comparison between Fresnel images and simulations based on the modified London equation to investigate the magnetic structure of flux vortices in MgB${}_2$. This technique gives an absolute, low-field ($\sim 30$ Oe) measurement of the penetration depth from images of single vortices. We found that these simulations gave a good fit to the experimental images and that if all the other parameters in the fit were known, the penetration depth for individual vortices could be measured with an accuracy of $\pm 5$ nm. Averaging over 17 vortices gave a penetration depth of ${\Lambda }_{ab}=113\pm 2$ nm at 10.8 K assuming that the entire thickness of the sample was superconducting. The main uncertainty in this measurement was the proportion of the specimen which was superconducting. Allowing for a nonsuperconducting layer of up to 50-nm thickness on the specimen surfaces gave a penetration depth in the range ${\Lambda }_{ab}=100$–115 nm, close to values of $90\pm 2$ nm obtained by small-angle neutron scattering and 118–138 nm obtained by radio-frequency measurements. We also discuss the use of the transport of intensity equation which should, in principle, give a model-independent measure of the magnetic structure of flux vortices.

The effective barrier height between an electrode and a ferroelectric (FE) depends on both macroscopic electrical properties and microscopic chemical and electronic structure. The behavior of a prototypical electrode/FE/electrode structure, Pt/BaTiO${}_3$/Nb-doped SrTiO${}_3$, under in-situ bias voltage is investigated using x-ray photoelectron spectroscopy. The full band alignment is measured and is supported by transport measurements. Barrier heights depend on interface chemistry and on the FE polarization. A differential response of the core levels to applied bias as a function of the polarization state is observed, consistent with Callen charge variations near the interface.

SrVO${}_3$ is a prototypical strongly correlated metal. Here we interpret the signatures of electronic correlation measured in photoemission spectroscopy by combining the many-body $GW$ approximation of the self-energy with an exponential representation of the Green's function. We explain those correlation effects as the consequence of the dynamical screening of the Coulomb interaction and the coupling between elementary excitations in the solid. Moreover we address the issue of satellites for empty states and discuss the possibility of plasmon-satellite series above the Fermi energy $E_F$. In good agreement with experiment, we obtain from first principles the renormalization of the V $3d$ quasiparticle bands and the satellites close to $E_F$ that so far have been interpreted on the basis of the Hubbard model. Finally, we identify incoherent features due to dynamical correlation also at the bottom of the O $2p$ bands, beyond the traditional three-band Hubbard-model description.

In a recent paper [Nguyen et al., Phys. Rev. B 85, 081101(R) (2012)] we presented an approach to evaluate quasiparticle energies based on the spectral decomposition of the static dielectric matrix. This method does not require the calculation of unoccupied electronic states or the direct diagonalization of large dielectric matrices, and it avoids the use of plasmon-pole models. The numerical accuracy of the approach is controlled by a single parameter, i.e., the number of eigenvectors used in the spectral decomposition of the dielectric matrix. Here we present a comprehensive validation of the method, encompassing calculations of ionization potentials and electron affinities of various molecules and of band gaps for several crystalline and disordered semiconductors. We demonstrate the efficiency of our approach by carrying out $GW$ calculations for systems with several hundred valence electrons.

We study the electronic properties of rippled freestanding graphene membranes under central load from a sharp tip. To that end, we develop a gauge field theory on a honeycomb lattice valid beyond the continuum theory. Based on the proper phase conjugation of the tight-binding pseudospin Hamiltonian, we develop a method to determine conditions under which continuum elasticity can be used to extract gauge fields from strain. Along the way, we resolve a recent controversy on the theory of strain engineering in graphene: There are no $K$-point-dependent gauge fields. We combine this lattice gauge field theory with atomistic calculations and find that for moderate load, the rippled graphene membranes conform to the extruding tip without a significant increase in elastic energy. Mechanical strain is created on a membrane only after a certain amount of load is exerted. In addition, we find that the deformation potential—even when partially screened—induces qualitative changes on the electronic spectra, with Landau levels giving way to equally spaced peaks.

The kagome Heisenberg antiferromagnet is mapped onto an effective Hamiltonian on the star superlattice by contractor renormalization. A comparison of ground-state energies on large lattices to density matrix renormalization group justifies truncation of effective interactions at range 3 (36 sites). Within our accuracy, magnetic and translational symmetries are not broken (i.e., a spin liquid ground state). However, we discover doublet spectral degeneracies which signal the onset of $p6$ chirality symmetry breaking. This is understood by a simple mean field analysis. Experimentally, the $p6$ chiral order parameter should split the optical phonon degeneracy near the zone center. The addition of weak next to nearest neighbor coupling is discussed.

We study the problem of designing an artificial Mott insulator in a correlated oxide heterostructure. We consider the extreme limit of quantum confinement based on ionic discontinuity doping, and argue that a unique dimer Mott insulator can be achieved for the case of a single SrO layer in a GdTiO${}_3$ matrix. In the dimer Mott insulator, electrons are localized not to individual atoms but to bonding orbitals on molecular dimers formed across a bilayer of two TiO${}_2$ planes and are analogous to the Mott insulating state of Hubbard ladders, studied in the 1990s. We verify the existence of the dimer Mott insulator through both ab initio and model Hamiltonian studies, and find for reasonable values of Hubbard $U$ that it is stable and ferromagnetic with a clear bonding/antibonding splitting of order 0.65 eV and a significant smaller Mott gap whose size depends upon $U$. The combined effects of polar discontinuity, strong structural relaxation, and electron correlations all contribute to the realization of this unique ground state.

We have performed microwave photoresistance measurements in high mobility GaAs/AlGaAs quantum wells and investigated the value of the effective mass. Surprisingly, the effective mass, obtained from the period of microwave-induced resistance oscillations, is found to be about 12% lower than the band mass in GaAs, $m_{\mathrm{b}}^{☆}$. This finding provides strong evidence for electron-electron interactions which can be probed by microwave photoresistance in very high Landau levels. In contrast, the measured magnetoplasmon dispersion revealed an effective mass which is close to $m_{\mathrm{b}}^{☆}$, in accord with previous studies.