We study the static susceptibilities for charge and spin sectors in paramagnetic states for Ca${}_{2-x}$Sr${}_x$RuO${}_4$ in $0.5\le x\le 2$ within random phase approximation on the basis of an effective Ru $t_{2g}$ orbital Hubbard model. We find that several modes of spin fluctuation around $\mathit{q}=(0,0)$ and $\mathit{q}\sim (0.797\pi ,0)$ are strongly enhanced for the model of $x=0.5$. This enhancement arises from the increase of the corresponding susceptibilities for the $d_{xy}$ orbital due to the rotation-induced modifications of the electronic structure for this orbital (i.e., the flattening of the bandwidth and the increase of the density of states near the Fermi level). We also find that the ferromagnetic spin fluctuation becomes stronger for a special model than for the model of $x=0.5$, while the competition between the modes of spin fluctuation at $\mathit{q}=(0,0)$ and around $\mathit{q}\sim (\pi ,0)$ is weaker for the special model; in this special model, the van Hove singularity (vHs) for the $d_{xy}$ orbital is located on the Fermi level. These results indicate that the location of the vHs for the $d_{xy}$ orbital, which is controlled by substitution of Ca for Sr, is a parameter to control this competition. We propose that the spin fluctuations for the $d_{xy}$ orbital around $\mathit{q}=(0,0)$ and $\mathit{q}\sim (\pi ,0)$ play an important role in the electronic states around $x=0.5$ other than the criticality approaching the usual Mott transition where all electrons are localized.

The Si(111)-($3\times 1$)Li and Ag surface structures consist of Si honeycomb chains separated by channels occupied by Li or Ag atoms. The dimerization of neighboring Ag atoms results in a larger $c(12\times 2)$Ag superstructure. We report calculations of the electronic surface states, dielectric function, and reflectance anisotropy spectrum of these surfaces using hybrid density functional theory (DFT) methods. The positions of surface states and the reflectance anisotropy spectrum calculated using DFT are in very good agreement with experimental results, where they have been reported. The surface states which contribute to the reflectance anisotropy below the band gap are identified for each orientation of the incident electromagnetic field.

Atomistic simulations give new insights into the properties of carbon nanofoams, a low-density nanoporous and nanostructured material. It is shown that agglomeration, crosslinking, and deformation, processes that are often ignored in theoretical descriptions of nanomaterials, have a dramatic effect on their properties. A most striking finding is that nanofoams composed exclusively of semiconducting nanostructures turn out to be metallic with high conductivity and optical absorptance. The underlying mechanism may explain relevant observations in other nanoporous materials. The simulated structures contain trivalent carbon atoms, suggested earlier to be a major source of magnetism in these materials.

Accommodation of nonstoichiometry in SrTiO${}_3$ pulsed laser deposited (PLD) films was investigated using positron annihilation lifetime spectroscopy and (scanning) transmission electron microscopy. Increasing PLD laser fluence changed the stoichiometry from Ti to Sr deficient. Cation vacancy defects were detected, and the concentration ratio of Sr to Ti vacancies, [$V_{\mathrm{Sr}}$]/[$V_{\mathrm{Ti}}$], was observed to increase systematically in the Sr-deficient region, although no change in the electron microscopy lattice images was detected. Increasing Ti deficiency resulted in the accommodation of SrO layers in planar defects, and in the formation of vacancy cluster defects. A change from $V_{\mathrm{Ti}}$ to $V_{\mathrm{Sr}}$ defect positron trapping was also detected.

Reduced terminations of the Fe${}_3$O${}_4$(001) surface were studied using scanning tunneling microscopy, x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Fe atoms, deposited onto the thermodynamically stable, distorted B-layer termination at room temperature (RT), occupy one of two available tetrahedrally coordinated sites per ($\sqrt{2}\times \sqrt{2}$)$R$45° unit cell. Further RT deposition results in Fe clusters. With mild annealing, a second Fe adatom per unit cell is accommodated, though not in the second tetrahedral site. Rather both Fe atoms reside in octahedral coordinated sites, leading to a “Fe-dimer” termination. At four additional Fe atoms per unit cell, all surface octahedral sites are occupied, resulting in a FeO(001)-like phase. The observed configurations are consistent with the calculated surface phase diagram. Both XPS and DFT+$U$ results indicate a progressive reduction of surface iron from Fe${}^{3+}$ to Fe${}^{2+}$ upon Fe deposition. The antiferromagnetic FeO layer on top of ferromagnetic Fe${}_3$O${}_4$(001) suggests possible exchange bias in this system.

The surface barrier is a key issue for understanding and describing the results of surface-sensitive electron spectroscopies, such as photoemission and inverse photoemission. We present a feasibility study showing that the shape of the surface barrier can be derived from very-low-energy electron diffraction (VLEED) fine-structure measurements. In particular, we focus on the spin dependence of the barrier probed by a spin-polarized experiment. We show model calculations of spin-dependent fine structures for specific ferromagnetic surfaces, where spin-dependent barrier models were successfully employed to describe spectroscopic results. We come to the conclusion that spin-polarized VLEED experiments with state-of-the-art energy and angular resolution are able to reveal details of the spin-dependent shape of the surface barrier.

Motivated by the nickel valence controversy in the perovskite nickelate $R$NiO${}_3$, we consider a one-dimensional underscreened Kondo chain consisting of alternating spin-1 (“nickel”) and electron (“oxygen”) sites, which in addition to the usual electron hopping and spin-spin interaction between the $S=1$ spin and the electron also contains a spin-mediated electron hopping term. Using the density-matrix renormalization group (DMRG), we obtained the zero-temperature phase diagram of the model, as well as various correlation functions in each phase. Importantly, for a certain range of parameters the model exhibits a quasi-long-range spiral (QS) order. To understand the DMRG results, we construct a mean-field theory based on a Schwinger fermion decomposition of the $S=1$ spins, from which we argue that the QS phase corresponds to a phase in proximity to the spin Bose metal state proposed by Sheng, Motrunich, and Fisher [Phys. Rev. B 79, 205112 (2009)]. Notably, we find no evidence for a phase with the symmetry of “nickel”-centered charge order, which has been argued to arise due to site-selective Kondo screening of half the $S=1$ spins, and suggest that order of this type occurs only due to an additional energy gain from spontaneous lattice distortions, not present in this model.

We exploit exact inequalities that refer to the entropy of a distribution to derive a simple variational principle at finite temperature for trial density matrices of Gutzwiller and Jastrow type. We use the result to extend at finite temperature the Gutzwiller approximation, which we apply to study a two-orbital model that we believe captures some essential features of V${}_2$O${}_3$. We indeed find that the phase diagram of the model bears many similarities to that of real vanadium sesquioxide. In addition, we show that in a Bethe lattice, where the finite-temperature Gutzwiller approximation provides a rigorous upper bound of the actual free energy, the results compare well with the exact phase diagram obtained by dynamical mean-field theory.

We study the photoluminescence (PL) of a two-dimensional liquid of oriented dipolar excitons in ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ coupled double quantum wells confined to a microtrap. Generating excitons outside the trap and transferring them at lattice temperatures down to $T=240$ mK into the trap we create cold quasiequilibrium bosonic ensembles of some 1000 excitons with thermal de Broglie wavelengths exceeding the excitonic separation. With decreasing temperature and increasing density $n\lesssim 5\times {10}^{10}\frac{1}{{\mathrm{cm}}^2}$ we find an increasingly asymmetric PL line shape with a sharpening blue edge and a broad red tail which we interpret to reflect correlated behavior mediated by dipolar interactions. From the PL intensity $I\left(E\right)$ below the PL maximum at $E_0$ we extract at $T<5$ K a distinct power law $I\left(E\right)\sim {(E_0-E)}^{-\left|\alpha \right|}$ with $-\left|\alpha \right|\approx -0.8$ in the range $E_0-E$ of 1.5–4 meV, comparable to the dipolar interaction energy.

We present a study of free carrier photogeneration and multicarrier bound states, such as biexcitons and trions (charged excitons), in semiconducting single-walled carbon nanotubes. Pump-and-probe measurements performed with fs pulses reveal the effects of strong Coulomb interactions between carriers on their dynamics. Biexciton formation by optical transition from exciton population results in an induced absorption line (binding energy 130 meV). Exciton-exciton annihilation process is shown to evolve at high densities towards an Auger process that can expel carriers from nanotubes. The remaining carriers give rise to an induced absorption due to trion formation (binding energy 190 meV). These features show the dynamics of exciton and free carriers populations.

A theoretical control of the electromagnetic coupling between localized surface plasmons and pointlike sources of radiation is a relevant topic in nanoscience and nanophotonics. In this paper a numerical approach based on the discrete dipole approximation is presented as a practical and reliable computational tool to study the decay dynamics of a dipole when it is located in the near proximities of metallic nanoparticles whose shapes do not allow a fully analytical treatment. The method is first applied to Ag nanospheres and nanoshells, which represent two analytically solvable cases, and it is shown to lead to a very good agreement with exact results. The approach is then used to consider the response, in terms of perturbations induced on the radiative and nonradiative decay rates, of elongated nanoparticles, like Ag prolate spheroids and nanocones. Results demonstrate how the optical response of conically shaped nanoparticles can be affected by the distance and the orientation of the emitter of radiation, as well as by other geometrical parameters. The particular symmetry of these plasmonic objects results in peculiar features: the absorption efficiencies of the modes depend on the distance of the source of radiation in a counterintuitive way, and this is explained in terms of the excited charge density distributions. The possibility to simulate arbitrary-shaped nanostructures and several dipole-metal configurations presented here, could thus open new avenues for an aware use of surface plasmons in fluorescence spectroscopy applications or single photon emission studies.

We further investigate a class of time-reversal-invariant two-band $s$-wave topological superconductors introduced earlier [Deng, Viola, and Ortiz, Phys. Rev. Lett. 108, 036803 (2012)]. Provided that a sign reversal between the two superconducting pairing gaps is realized, the topological phase diagram can be determined exactly (within mean field) in one and two dimensions as well as in three dimensions upon restricting to the excitation spectrum of time-reversal-invariant momentum modes. We show how, in the presence of time-reversal symmetry, ${\mathbb{Z}}_2$ invariants that distinguish between trivial and nontrivial quantum phases can be constructed by considering only one of the Kramers’ sectors in which the Hamiltonian decouples into. We find that the main features identified in our original two-dimensional setting remain qualitatively unchanged, with nontrivial topological superconducting phases supporting an odd number of Kramers’ pairs of helical Majorana modes on each boundary, as long as the required $\pi$-phase difference between gaps is maintained. We also analyze the consequences of time-reversal-symmetry breaking either due to the presence of an applied or impurity magnetic field or to a deviation from the intended phase matching between the superconducting gaps. We demonstrate how the relevant notion of topological invariance must be modified when time-reversal symmetry is broken, and how both the persistence of gapless Majorana modes and their robustness properties depend in general upon the way in which the original Hamiltonian is perturbed. Interestingly, a topological quantum phase transition between helical and chiral superconducting phases can be induced by suitably tuning a Zeeman field in conjunction with a phase mismatch between the gaps. Recent experiments in doped semiconducting crystals, of potential relevance to the proposed model, and possible candidate material realizations in superconductors with $s_{\pm }$ pairing symmetry are discussed.

ZnAl${}_2$O${}_4$ (gahnite) is a ceramic which is considered a possible transparent conducting oxide (TCO) due to its wide band gap and transparency for UV. Defects play an important role in controlling the conductivity of a TCO material along with the dopant, which is the main source of conductivity in an otherwise insulating oxide. A comprehensive first-principles density functional theory study for point defects in ZnAl${}_2$O${}_4$ spinel is presented using the Heyd, Scuseria, and Ernzerhof hybrid functional (HSE06) to overcome the band gap problem. We have investigated the formation energies of intrinsic defects which include the Zn, Al, and O vacancy and the antisite defects: Zn at the Al site (Zn${}_{\mathrm{Al}}$) and Al at the Zn site (Al${}_{\mathrm{Zn}}$). The antisite defect Al${}_{\mathrm{Zn}}$ has the lowest formation energy and acts as a shallow donor, indicating possible $n$-type conductivity in ZnAl${}_2$O${}_4$ spinel by Al doping.

Magnetotransport properties of an Fe${}_{48}$Mn${}_{24}$Ga${}_{28}$ Heusler-based ferromagnetic shape memory alloy are tracked in a temperature interval that covers both martensitic and austenitic phases. A large temperature hysteresis indicative of a coupled magnetostructural transition from ferromagnetic martensite to paramagnetic austenite is observed on the temperature dependencies of magnetization and electrical resistivity. The temperature dependency of the anomalous Hall-effect coefficient in Fe${}_{48}$Mn${}_{24}$Ga${}_{28}$ cannot be described in terms of skew scattering, side-jump, and intrinsic mechanisms of the anomalous Hall-effect theory. The Hall-effect resistivity in the martensitic state is smaller, but is of the same order of magnitude as in the case of the giant Hall effect in a half-metallic Co${}_2$MnAl. Specific features of the temperature dependencies of magnetization, resistivity, magnetoresistance, and ordinary and anomalous Hall-effect coefficients are discussed and the possible routes for increasing Hall-effect resistivity are suggested.

In bilayer nanowires consisting of a ferromagnetic layer and a nonmagnetic layer with strong spin-orbit coupling, currents create torques on the magnetization beyond those found in simple ferromagnetic nanowires. The resulting magnetic dynamics appear to require torques that can be separated into two terms, dampinglike and fieldlike. The dampinglike torque is typically derived from models describing the bulk spin Hall effect and the spin transfer torque, and the fieldlike torque is typically derived from a Rashba model describing interfacial spin-orbit coupling. We derive a model based on the Boltzmann equation that unifies these approaches. We also consider an approximation to the Boltzmann equation, the drift-diffusion model, that qualitatively reproduces the behavior, but quantitatively differs in some regimes. We show that the Boltzmann equation with physically reasonable parameters can match the torques for any particular sample, but in some cases, it fails to describe the experimentally observed thickness dependencies.

Using atomistic simulations we investigate the thermodynamical properties of a single atomic layer of hexagonal boron nitride (h-BN). The thermal induced ripples, heat capacity, and thermal lattice expansion of large scale h-BN sheets are determined and compared to those found for graphene (GE) for temperatures up to 1000 K. By analyzing the mean-square height fluctuations $\langle h^2\rangle$ and the height-height correlation function $H\left(q\right)$ we found that the h-BN sheet is a less stiff material as compared to graphene. The bending rigidity of h-BN (i) is about $16\%$ smaller than the one of GE at room temperature (300 K), and (ii) increases with temperature as in GE. The difference in stiffness between h-BN and GE results in unequal responses to external uniaxial and shear stress and different buckling transitions. In contrast to a GE sheet, the buckling transition of a h-BN sheet depends strongly on the direction of the applied compression. The molar heat capacity, thermal-expansion coefficient, and Gruneisen parameter are estimated to be 25.2 J mol${}^{-1}$ K${}^{-1}$, $7.2\times {10}^{-6}$ K${}^{-1}$, and 0.89, respectively.

Ab initio molecular dynamics simulations based on density functional theory have been performed to generate a model of amorphous InSb by quenching from the melt. The resulting network is mostly tetrahedral with a minor fraction ($10\%$) of atoms in a fivefold coordination. The structural properties are in good agreement with available x-ray diffraction and extended x-ray-absorption fine structure data and confirm the proposed presence of a sizable fraction of homopolar In-In and Sb-Sb bonds whose concentration in our model amounts to about $20\%$ of the total number of bonds.

We synthesize sub-THz longitudinal quasimonochromatic acoustic phonons in a SrTiO${}_3$ single crystal using a SrRuO${}_3$/SrTiO${}_3$ superlattice as an optical-acoustic transducer. The generated acoustic phonon spectrum is determined using ultrafast x-ray diffraction. The analysis of the generated phonon spectrum in the time domain reveals a $k$-vector dependent phonon lifetime. It is observed that even at sub-THz frequencies the phonon lifetime agrees with the 1/${\omega }^2$ power law known from Akhiezer's model for hyper sound attenuation. The observed shift of the synthesized spectrum to the higher $q$ is discussed in the framework of nonlinear effects appearing due to the high amplitude of the synthesized phonons.

The preparation of quantum states with a defined spin is analyzed in a hybrid system consisting of a $p$-doped semiconductor quantum dot (QD) coupled to a metallic nanoparticle. The quantum dot is described as a four-level atom-like system using the density matrix formalism. The lower levels are Zeeman-split hole spin states and the upper levels correspond to positively charged excitons containing a spin-up, spin-down hole pair and a spin electron. A metallic nanoparticle with spheroidal geometry is placed in close proximity to the quantum dot, and its effects are considered in the quasistatic approximation. A linearly polarized laser field drives two of the optical transitions of the QD and produces localized surface plasmons in the nanoparticle which act back upon the QD. The frequencies of these localized plasmons are very different along the two principal axes of the nanoparticle, thus producing an anisotropic modification of the spontaneous emission rates of the allowed optical transitions which is accompanied by local-field corrections. This effect translates into a preferential acceleration of some of the optical pathways and therefore into a fast initialization of the QD by excitation with a short optical pulse. The population transfer between the lower levels of the QD and the fidelity is analyzed as a function of the nanoparticle's aspect ratio, the external magnetic field, and the Rabi frequency of the driving field. It is also shown that the main effect of the local-field corrections is a lengthening of the time elapsed to reach the steady-state. The hole spin is predicted to be successfully cooled from 5 to $0.04$ K at a magnetic field of $4.6$ T applied in the Voigt geometry.

We investigate current-current correlations of adiabatic charge pumping through interacting quantum dots weakly coupled to reservoirs. To calculate the zero-frequency noise for a time-dependently driven system, possibly in the presence of an additional dc bias, we perform within a real-time diagrammatic approach a perturbative expansion in the tunnel coupling to the reservoirs in leading and next-to-leading orders. We apply this formalism to study the adiabatic correction to the zero-frequency noise, i.e., the pumping noise, in the case of a single-level quantum dot charge pump. If no stationary bias is applied, the adiabatic correction shows Coulomb-interaction-induced deviations from the fluctuation-dissipation theorem. Furthermore, we show that the adiabatic correction to the Fano factor carries information about the coupling asymmetry and is independent of the choice of the pumping parameters. When including a time-dependent finite bias, we find that there can be pumping noise even if there is zero adiabatically pumped charge. The pumping noise also indicates the respective direction of the bias-induced current and the pumping current.