The very high neutral fractions measured in He${}^{+}$ scattered by graphitelike surfaces, at intermediate incoming energies (1 keV $<E_{\mathrm{in}}<$ 6 keV), cannot be explained only by the resonance of the He ionization level with the valence band states of the surface. Excited configurations $\left(1s2s\right)$ and $\left(1s2p\right)$ appear as possible resonant neutralization channels together with the ground state one $\left(1s^2\right)$. We develop, in this work, a time-dependent quantum-mechanical calculation of the charge-transfer process in He${}^{+}$/HOPG collision, where the resonant neutralization to the ground and first excited states of He is taken into account. We use an Anderson Hamiltonian projected on the electronic configurations of the projectile atom which are energetically favorable for the charge-exchange process. Thus, an exhaustive analysis of different possible approximations to the neutralization of He${}^{+}$ is performed: the typical neutralization to the ground state by either neglecting or not the electron spin and finally the one including excited configurations. Our results reproduce the observed experimental trends only when excited configurations $\left(1s2s\right)$ and $\left(1s2p\right)$ are involved in the charge exchange between the ion and the surface.

Scanning tunneling microscopy (STM) tips decorated with either a single carbon monoxide molecule or a single xenon atom are characterized by simultaneous force and conductance measurements using a combined low-temperature noncontact atomic force and scanning tunneling microscope (NC-AFM/STM). It is shown that in both cases the particle decorating the tip simultaneously performs the function of an atomic-scale force sensor and transducer which couples the short-range force acting on the tip to the tunneling conductance of the junction. On the basis of the experimental data, two distinct coupling regimes are identified; in one of them the force sensor-transducer function is calibrated quantitatively.

Plasmons dispersion and nonvertical interband transitions in Bi${}_2$Se${}_3$ single crystals were investigated by electron energy-loss spectroscopy in conjunction with (scanning) transmission electron microscopy [(S)TEM-EELS]. Both volume plasmons (π plasmon at 7 eV and π + σ plasmon at 17 eV) and surface plasmons (∼5.5 and 10 eV) were demonstrated in STEM-EELS spectra and the corresponding spectral imaging in real space. In further EELS experiments in reciprocal space, the momentum-dependent spectra reveal very different dispersion behavior between π and π + σ plasmons, with π + σ plasmons showing a typical quadratic dependence, whereas the π plasmon exhibited a linear dispersion analogous to what was reported for graphene. Furthermore, a low energy excitation 0.7–1.6 eV was also observed which is attributed to direct nonvertical interband transitions along the Γ$F$ direction.

Neutral excitons in strained axially symmetric In(Ga)As/GaAs quantum dots with a ringlike shape are investigated. Similar to experimental self-assembled quantum rings, the analyzed quantum dots have volcano-like shapes. The continuum mechanical model is employed to determine the strain distribution, and the single-band envelope function approach is adopted to compute the electron states. The hole states are determined by the axially symmetric multiband Luttinger-Kohn Hamiltonian, and the exciton states are obtained from an exact diagonalization. We found that the presence of the inner layer covering the ring opening enhances the excitonic Aharonov-Bohm (AB) oscillations. The reason is that the hole becomes mainly localized in the inner part of the quantum dot due to strain, whereas the electron resides mainly inside the ring-shaped rim. Interestingly, larger AB oscillations are found in the analyzed quantum dot than in a fully opened quantum ring of the same width. Comparison with the unstrained ringlike quantum dot shows that the amplitude of the excitonic Aharonov-Bohm oscillations are almost doubled in the presence of strain. The computed oscillations of the exciton energy levels are comparable in magnitude to the oscillations measured in recent experiments.

We present a microscopic theory of the magnetic field induced mixing of heavy-hole states $\pm 3/2$ in GaAs droplet dots grown on (111)A Ga-rich surfaces. The proposed theoretical model takes into account the striking dot shape with trigonal symmetry revealed in atomic force microscopy. Our calculations of the hole states are carried out within the Luttinger Hamiltonian formalism, supplemented with allowance for the triangularity of the confining potential. They are in quantitative agreement with the experimentally observed polarization selection rules, emission line intensities and energy splittings in both longitudinal and transverse magnetic fields for neutral and charged excitons in all measured single dots.

The structure and evolution of ultrashort shock waves generated by femtosecond laser pulses in single-crystal nickel films are investigated by molecular dynamics simulations. Ultrafast laser heating is isochoric, leading to pressurization of a 100-nm-thick layer below the irradiated surface. For low-intensity laser pulses, the highly pressurized subsurface layer breaks into a single elastic shock wave having a combined loading and unloading time $\approx$10–20 ps. Owing to the time-dependent nature of elastic-plastic transformations, an elastic response is maintained for shock amplitudes exceeding the Hugoniot elastic limit determined from simulations of steady shock waves. However, for high-intensity laser pulses (absorbed laser fluence $>$$0.6\mathrm{J}/{\mathrm{cm}}^2),$ both elastic and plastic shock waves are formed independently from the initial high-pressure state. Acoustic pulses emitted by the plastic front support the motion of the elastic precursor resulting in a fluence-independent elastic amplitude; whereas the unsupported plastic front undergoes significant attenuation during propagation and may fully decay within the metal film.

The magnetic properties of manganite bilayers composed of G-type antiferromagnetic (AFM) SrMnO${}_3$ and double-exchange ferromagnetic (FM) La${}_{0.7}$Sr${}_{0.3}$MnO${}_3$ are studied. A spin-glass state is observed as a result of competing magnetic orders and spin frustration at the La${}_{0.7}$Sr${}_{0.3}$MnO${}_3$/SrMnO${}_3$ interface. The dependence of the irreversible temperature on the cooling magnetic field follows the Almeida-Thouless line. Although an ideal G-type AFM SrMnO${}_3$ is featured with a compensated spin configuration, the bilayers exhibit exchange bias below the spin glass freezing temperature, which is much lower than the Néel temperature of SMO, indicating that the exchange bias is strongly correlated with the spin glass state. The results indicate that the spin frustration that originates from the competition between the AFM super-exchange and the FM double-exchange interactions can induce a strong magnetic anisotropy at the La${}_{0.7}$Sr${}_{0.3}$MnO${}_3$/SrMnO${}_3$ interface.

Inelastic neutron-scattering experiments have been performed on lightly doped La${}_{1.96}$Sr${}_{0.04}$CuO${}_4$, which shows diagonal incommensurate spin correlations at low temperatures. We previously reported that this crystal, with a single orthorhombic domain, exhibits the “hourglass” dispersion at low energies [Phys. Rev. Lett. 101, 197001 (2008)]. In this paper, we investigate in detail the energy evolution of the magnetic excitations up to 65 meV. It is found that the anisotropic excitations at low energies, dispersing only along the spin modulation direction, cross over to an isotropic, conical dispersion that resembles spin waves in the parent compound La${}_2$CuO${}_4$. The change from twofold to full symmetry on crossing the waist of the hourglass reproduces behavior first identified in studies of underdoped YBa${}_2$Cu${}_3$O${}_{6+x}$. We discuss the significance of these results.

Systematic experiments on irradiation damage in CaF${}_2$ were reported in a recent paper [M. Toulemonde et al., Phys. Rev. B 85, 054112 (2012)]. The measurements were performed in a broad range of ion energies and energy deposition and the inelastic thermal spike model was applied for the analysis. Here we demonstrate that contrary to what is stated in the paper, the experimental results do not prove that there is a velocity effect in CaF${}_2$ similar to that observed previously in Y${}_3$Fe${}_5$O${}_{12}$ and other track-forming insulators. We also show that the authors’ results do not support their hypothesis that tracks are formed by quenching of a boiling phase.

Experimental data presented in our earlier paper [Toulemonde et al., Phys. Rev. B 85, 054112 (2012)] indicate that there are two thresholds for damage creation by swift heavy ions in CaF${}_2$. Moreover, the comparison of the track sizes observed by transmission electron microscopy and generated by different cluster and monoatomic ion beams supports the occurrence of a velocity effect in this material.

At the mean-field level, on fully connected lattices, several disordered spin models have been shown to belong to the universality class of “structural glasses” with a “random first-order transition” (RFOT) characterized by a discontinuous jump of the order parameter and no latent heat. However, their behavior in finite dimensions is often drastically different, displaying either no glassiness at all or a conventional spin-glass transition. We clarify the physical reasons for this phenomenon and stress the unusual fragility of the RFOT to short-range fluctuations, associated, e.g., with the mere existence of a finite number of neighbors. Accordingly, the solution of fully connected models is only predictive in very high dimension, whereas despite being also mean-field in character, the Bethe approximation provides valuable information on the behavior of finite-dimensional systems. We suggest that before embarking on a full blown account of fluctuations on all scales through computer simulation or renormalization-group approach, models for structural glasses should first be tested for the effect of short-range fluctuations and we discuss ways to do it. Our results indicate that disordered spin models do not appear to pass the test and are therefore questionable models for investigating the glass transition in three dimensions. This also highlights how nontrivial is the first step of deriving an effective theory for the RFOT phenomenology from a rigorous integration over the short-range fluctuations.

The problem of spin diffusion is studied numerically in the one-dimensional classical Heisenberg model using a deterministic odd-even spin precession dynamics. We demonstrate that spin diffusion in this model is normal in the infinite-temperature limit and one obtains a long-time diffusive tail in the decay of the autocorrelation function. Some variations of the model with different coupling schemes and anisotropy are also studied and we find normal diffusion in all of them. A systematic finite-size analysis also suggests normal diffusive spreading of spin fluctuations, contrary to previous claims of anomalous diffusion. We also briefly discuss spin diffusion in this model at finite temperatures.

We study a two-species fermion mixture with different populations on a square lattice modeled by a Hubbard Hamiltonian with onsite interspecies repulsive interaction. Such a model can be realized in a cold-atom system with fermionic atoms in two different hyperfine states loaded on an optical lattice and with tunable interspecies interaction strength via external fields. For a two-dimensional square lattice, when at least one of the fermion species is close to half-filling, the system is highly affected by lattice effects. With the majority species near half-filling and varying densities for the minority species, we find that several correlated phases emerge as the ground state, including a spin density wave state, a charge density wave state with stripe structure, and various $p$-wave BCS pairing states for both species. We study this system using a functional renormalization group method, determine its phase diagram at weak coupling, discuss the origin and characteristics of each phase, and provide estimates for the critical temperatures.

We study the properties of a steplike defect on the surface of ultrathin topological insulator nanofilms. We calculate the reflectance of an electron from such a defect for different parameters of the nanofilm and different parameters of the defect. We show that an electron incident on a steplike defect not only produces reflected and transmitted waves but also generates the modes, which are localized at the steplike defect. Such modes result in an enhancement of electron density at the defect by $\approx$60$\%$. The magnitude of the enhancement depends on the parameters of the nanofilm and the height of the step and is the largest in the case of total electron reflection.

We study low-energy transport through a quantum dot coupled to one normal and two superconducting (SC) leads in a Y-shaped junction. In this geometry a crossover between Kondo-dominated and Cooper-pairing-dominated states occurs by tuning parameters such as the quantized energy level ${\epsilon }_d$ of the dot and the Josephson phase $\varphi$, which induces a supercurrent flowing between the two SC leads through the dot. Furthermore, Andreev scattering takes place at the interface between the dot and normal lead. The low-lying energy states of this system can be described by a local Fermi-liquid theory for interacting Bogoliubov particles. In a description based on an Anderson impurity model, we calculate transport coefficients, renormalized parameters, and spectral function, using Wilson's numerical renormalization group approach, in the limit of a large SC gap. Our results demonstrate how the Andreev resonance level approaches the Fermi level in the crossover region between the Cooper-pairing singlet state and the strong-coupling situation as ${\epsilon }_d$ or $\varphi$ are varied. The strong-coupling situation shows a Kondo effect with a significantly renormalized resonance width. The crossover is smeared when the coupling between the dot and the normal lead is large. Furthermore, asymmetry in the Josephson junction suppresses the cancellations of the SC proximity for finite $\varphi$, and it favors the SC singlet state rather than the Kondo singlet.

We study the electronic structure of bilayer graphene flakes in which the constituent layers are mutually rotated by some angle $\theta$. The large system sizes involved (up to ${10}^5$ carbon atoms) necessitate the use of a tight-binding approach in conjunction with Lanczos diagonalization. We find that a single moiré spot is sufficient for the low-energy density of states to resemble closely that of the periodic analog of such flakes, the graphene twist bilayer, implying that the low-energy physics in this system is well described as that of a “moiré quantum well” trapping low-energy graphene electrons. Furthermore, a graphene twist flake consisting of a single moiré unit cell leads already to electron localization on the AA “moiré spot,” in agreement with this moiré quantum well picture. The electron density fluctuations induced by the moiré lattice in twist graphene flakes are significant, being an order of magnitude greater than those generated by the rippling of suspended graphene. Finally, we determine the electronic properties of such flakes in the presence of an external magnetic field, finding a “zero-mode” structure and Landau states that exhibit an electron current well described as a charge flow on a torus situated at the AA regions of the moiré lattice.

We investigate the spontaneous formation of quantized vortices in microcavity polariton high density phases. Condensates formed in the optical parametric oscillator excitation scheme reveal single vortices in the case of a ring-shaped optical potential or in the presence of natural photonic disorder. We further investigate the dynamics of vortex formation for resonantly injected polaritons and observe the formation of vortex-antivortex pairs, but no single vortices. The observed effects are explained by the interplay between breaking of $y\mapsto -y$ reflection symmetry in the system and conservation of optical angular momentum.

We predict theoretically that when a normal metallic thin film (without bulk spin-orbit coupling, such as Cu or Al) is sandwiched by two insulators, two prominent effects arise due to the interfacial spin-orbit coupling: a giant spin-Hall conductivity due to the surface scattering and a transverse electric polarization due to the spin-dependent phase shift in the spinor wave functions.

When wurtzite ZnO is sliced perpendicular to the (0001) axis, two different polar surfaces, the (0001)-Zn and (000$\overline{1}$)-O terminated surfaces, are formed. In a simple ionic picture, both surfaces are electrostatically unstable due to a diverging electrostatic energy. Although the ionic picture is an oversimplification, the surfaces adopt a modified surface structure to compensate for the polarity. In close collaboration with experiment, a hexagonal honeycomblike reconstruction has been suggested [J. V. Lauritsen et al., ACS Nano 5, 5987 (2011)]. The remarkable observation is that the (000$\overline{1}$)-O surface behaves very differently than the (0001)-Zn surface. Here, we present a detailed density functional theory investigation of the (000$\overline{1}$)-O surface, including a systematic investigation of H and Zn coverage as well as an investigation of various surface reconstructions. The difference between the two polar surfaces is explained by the different bonding preferences of Zn and O atoms: as a $d$ element, Zn atoms are more flexible in their bond formation than O atoms.

We report the results of first-principles calculations for the atomic, electronic, and magnetic properties of the Co atoms embedded in divacancy defects in graphene nanoribbons with zigzag-shaped edges. We find that the edges play an important role in the stabilization of a Co-divacancy complex near the edge, where the edge C atoms undergo large relaxations, resulting in a tilted-edge structure. When Co is positioned in the middle of the ribbon, the edge C atoms generally remain on the sheet, forming a flat-edge structure due to the small edge effect. The spin polarization of the edges is determined by two bipartite lattices separated by the Co atom. When a Co-divacancy pair is formed near the same edge, the edge structure is significantly modified. The magnetic interaction between the Co atoms is either ferromagnetic or antiferromagnetic, depending on the relative positions of the Co atoms, and its magnetic ordering can be described by the combined effect of the bipartite lattices formed around individual Co atoms.