The interplay of $s{p}^2$- and $s{p}^3$-type bonding defines silicon allotropes in two- and three-dimensional forms. A two-dimensional phase bearing structural resemblance to a single ${\mathrm{MoS}}_2$ layer is found to possess a lower total energy than low-buckled silicene and to be stable in terms of its phonon dispersion relations. A set of cigar-shaped nematic orbitals originating from the Si $s{p}^2$ orbitals realizes bonding with a sixfold coordination of the inner Si atoms of the layer. The identification of these nematic orbitals advocates diverse Si bonding configurations different from those of C atoms.

We model rutile titanium dioxide nanocrystals (NCs) up to $\sim 1.5$ nm in size to study the effects of quantum confinement on their electronic and optical properties. Ionization potentials (IPs) and electron affinities (EAs) are obtained via the perturbative $GW$ approximation ($G_0{W}_0$) and $\Delta \mathrm{SCF}$ method for NCs up to 24 and 64 ${\mathrm{TiO}}_2$ formula units, respectively. These demanding $GW$ computations are made feasible by using a real-space framework that exploits quantum confinement to reduce the number of empty states needed in $GW$ summations. Time-dependent density functional theory (TDDFT) is used to predict the optical properties of NCs up to 64 ${\mathrm{TiO}}_2$ units. For a NC containing only 2 ${\mathrm{TiO}}_2$ units, the offsets of the IP and the EA from the corresponding bulk limits are of similar magnitude. However, as NC size increases, the EA is found to converge more slowly to the bulk limit than the IP. The EA values computed at the $G_0{W}_0$ and $\Delta \mathrm{SCF}$ levels of theory are found to agree fairly well with each other, while the IPs computed with $\Delta \mathrm{SCF}$ are consistently smaller than those computed with $G_0{W}_0$ by a roughly constant amount. TDDFT optical gaps exhibit weaker size dependence than $GW$ quasiparticle gaps, and result in exciton binding energies on the order of eV. Altering the dimensions of a fixed-size NC can change electronic and optical excitations up to several tenths of an eV. The largest NCs modeled are still quantum confined and do not yet have quasiparticle levels or optical gaps at bulk values. Nevertheless, we find that classical Mie-Gans theory can quite accurately reproduce the line shape of TDDFT absorption spectra, even for (anisotropic) ${\mathrm{TiO}}_2$ NCs of subnanometer size.

Exact ground states of a spin-1/2 Ising-Heisenberg model on the Shastry-Sutherland lattice with Heisenberg intradimer and Ising interdimer couplings are found by two independent rigorous procedures. The first method uses a unitary transformation to establish a mapping correspondence with an effective classical spin model, while the second method relies on the derivation of an effective hard-core boson model by continuous unitary transformations. Both methods lead to equivalent effective Hamiltonians providing a convincing proof that the spin-1/2 Ising-Heisenberg model on the Shastry-Sutherland lattice exhibits a zero-temperature magnetization curve with just two intermediate plateaus at one-third and one-half of the saturation magnetization, which correspond to stripe and checkerboard orderings of singlets and polarized triplets, respectively. The nature of the remarkable stripe order relevant to the one-third plateau is thoroughly investigated with the help of the corresponding exact eigenvector. The rigorous results for the spin-1/2 Ising-Heisenberg model on the Shastry-Sutherland lattice are compared with the analogous results for the purely classical Ising and fully quantum Heisenberg models. Finally, we discuss to what extent the critical fields of ${\mathrm{SrCu}}_2{(\mathrm{BO}}_3){}_2$ and ${\left(\mathrm{CuCl}\right)\mathrm{Ca}}_2{\mathrm{Nb}}_3{\mathrm{O}}_{10}$ can be described within the suggested Ising-Heisenberg model.

The thermomechanical effect in superfluid helium is used to create an initial chemical potential difference $\Delta {\mu }_0$ across a solid ${}^4\mathrm{He}$ sample. This $\Delta {\mu }_0$ causes a flow of helium atoms from one reservoir filled with superfluid helium, through a sample cell filled with solid helium, to another superfluid-filled reservoir until chemical potential equilibrium between the reservoirs is restored. The solid helium sample is separated from each of the reservoirs by Vycor rods that allow only the superfluid component to flow. With an improved technique, measurements of the flow $F$ at several fixed solid helium temperatures $T$ have been made as a function of $\Delta \mu$ in the pressure range 25.5–26.1 bar, and measurements of $F$ have been made as a function of temperature in the range $180<T<545$ mK for several fixed values of $\Delta \mu$. The temperature dependence of the flow above 100 mK shows a reduction of the flux with an increase in temperature that is well described by $F=F_0^{*}[1-Kexp(-E/T)]$. The nonlinear functional dependence $F\sim {\left(\Delta \mu \right)}^b$, with $b<0.5$ independent of temperature but dependent on pressure, documents in some detail the dissipative nature of the flow and suggests that this system demonstrates Luttinger liquid-like one-dimensional behavior. The mechanism that causes this flow behavior is not certain, but is consistent with superflow on the cores of edge dislocations.

First-principles determination of the vacancy formation enthalpies has been long-term believed to be highly successful for metals. However, a widely known fact is that the various conventional density functional theory (DFT) calculations with the typical semilocal approximations show apparent failures to yield accurate enthalpies of Ag and Au. Recently, the previously commonly assumed linear Arrhenius extrapolation to determine the vacancy formation enthalpies at $T=0$ K from the high-temperature measured concentration of thermally created vacancies has been demonstrated to have to be replaced by the non-Arrhenius local Grüneisen theory (LGT) [A. Glensk, B. Grabowski, T. Hickel, and J. Neugebauer, Phys. Rev. X 4, 011018 (2014)]. The large discrepancies between the conventional DFT-PBE data and the unrevised experimental vacancy formation enthalpies disappear for Cu and Al. Even by following the same LGT revisions for Ag, the large discrepancies still remain substantial at $T=0$ K. Here, we show that the hybrid functional (HSE), by including nonlocal exchange interactions to extend the conventional DFT method, can further correct these substantial failures. Upon a comparison of the experimental valence-band spectra for Cu, Ag, and Au, we have determined the HSE exchange-correlated mixing parameters $\alpha$ of 0.1, 0.25, and 0.4, and further derived the HSE enthalpies of vacancy formation of 1.09, 0.94, and 0.72 eV, respectively; in nice agreement with available LGT-revised experimental data. Our HSE results shed light on how to improve the theoretical predictions to accurately determine the defect formation energies and related thermodynamical properties.

Exciting a ferromagnetic sample with an ultrashort laser pulse leads to a quenching of the magnetization on a subpicosecond time scale. On the basis of the equilibration of intensive thermodynamic variables, we establish a powerful model to describe the demagnetization process. We demonstrate that the magnetization dynamics is mainly driven by the equilibration of chemical potentials. The minimum of magnetization is revealed as a transient electron equilibrium state. Our method identifies the slowing down of ultrafast magnetization dynamics by a critical region within a magnetic phase diagram.

We have performed neutron diffraction measurements at 12 K on a single crystal of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$, which serves as the parent compound of iron-based superconductors. To investigate the in-plane anisotropy of the magnetic form factor in the antiferromagnetic phase, the single crystal was detwinned. The magnetic structure factor and magnetic form factor are well explained by the spin densities comprising $3d_{yz}$ electrons with a fraction of approximately 40% and electrons in the other four $3d$ orbitals each with a fraction of approximately 15%. It is a direct observation of the largely anisotropic spin density relative to the small orthorhombic lattice distortion, $(a-b)/(a+b)\sim 0.3\%$.

We have obtained the equilibrium volumes, bulk moduli, and equations of state of the ferromagnetic cubic $\alpha$ and paramagnetic hexagonal $\epsilon$ phases of iron in close agreement with experiment using an ab initio dynamical mean-field-theory approach. The local dynamical correlations are shown to be crucial for a successful description of the ground-state properties of paramagnetic $\epsilon$-Fe. Moreover, they enhance the effective mass of the quasiparticles and reduce their lifetimes across the $\alpha \to \epsilon$ transition, leading to a stepwise increase of the resistivity, as observed in experiment. The calculated magnitude of the jump is significantly underestimated, which points to nonlocal correlations. The implications of our results for the superconductivity and non-Fermi-liquid behavior of $\epsilon$-Fe are discussed.

Using the conventional $T$-matrix approach, we discuss gapped phases in one-, two-, and three-dimensional (3D) spin systems (both with and without a long-range magnetic order) with bond disorder and with weakly interacting bosonic elementary excitations. This work is motivated by recent experimental and theoretical activity in spin-liquid-like systems with disorder and in the disordered interacting boson problem. In particular, we apply our theory to both paramagnetic low-field and fully polarized high-field phases in dimerized spin-$\frac{1}{2}$ systems and in integer-spin magnets with large single-ion easy-plane anisotropy $\mathcal{D}$ with disorder in exchange coupling constants (and/or $\mathcal{D}$). The elementary excitation spectrum and the density of states are calculated in the first order in defects concentration $c\ll 1$. In 2D and 3D systems, the scattering on defects leads to a finite damping of all propagating excitations in the band except for states lying near its edges. We demonstrate that the analytical approach is inapplicable for states near the band edges and our numerical calculations reveal their localized nature. We find that the damping of propagating excitations can be much more pronounced in considered systems than in magnetically ordered gapless magnets with impurities. In 1D systems, the disorder leads to localization of all states in the band, while those lying far from the band edges (short-wavelength excitations) can look like conventional wave packets.

A density matrix based method is introduced for computation of steady-state dynamics in quantum cascade systems of arbitrary size, which incorporates an optical field coherently. The method is applied to a model terahertz quantum dot cascade laser system, where a means of treating coherent electron-optical-phonon coupling is also introduced. Results predict a strong increase in the upper state lifetime and operating temperature as compared to traditional well-based terahertz quantum cascade lasers. However, new complications also arise, including multiple peaks in the gain spectrum due to strong electron-phonon coupling, and strong parasitic subthreshold current channels that arise due to reduced dephasing. It is anticipated that novel design schemes will be necessary for such lasers to become a reality.

The coverage-dependent intercalation of oxygen, CO, hydrogen, alkali metals, and halogens between graphene and Ir(111) is investigated using density functional theory with van der Waals corrections. By comparing adsorption on clean Ir to intercalation we show that the presence of the graphene layer shifts the stability of the adsorption structures towards higher coverages, with oxygen as the only exception preferring low-coverage intercalation structures. In general, we find that the preferred adsorption site of the intercalant is important for the stability of intercalation structures, where an atop adsorption site favors higher-coverage structures compared to a hollow adsorption site. Overall, the predicted stable intercalation structures are in good agreement with experimentally observed intercalation structures. We calculate doping levels of intercalated graphene and show that there is a correlation between the amount of charge transfer to or from the graphene sheet and the graphene binding energy, which is an indication for ionic bonding between the graphene sheet and the intercalants. We show that the graphene doping level can be tuned almost continuously between strong $n$-type doping for the alkali metals and strong $p$-type doping for F. Further, we calculate C $1s$ core-level shifts for intercalated graphene and show that these are correlated with the calculated doping level. The obtained values for graphene doping levels and core-level shifts are qualitatively in good agreement with available experimental values, but we find quantitative disagreements of up to 0.3 eV.

We theoretically investigate general existence conditions for broadband bulk large-wave-vector (high-$k)$ propagating waves (such as volume plasmon polaritons in hyperbolic metamaterials) in subwavelength periodic multilayer structures. Describing the elementary excitation in the unit cell of the structure by a generalized resonance pole of a reflection coefficient and using Bloch's theorem, we derive analytical expressions for the band of large-wave-vector propagating solutions. We apply our formalism to determine the high-$k$ band existence in two important cases: the well-known metal-dielectric and recently introduced graphene-dielectric stacks. We confirm that short-range surface plasmons in thin metal layers can give rise to hyperbolic metamaterial properties and demonstrate that long-range surface plasmons cannot. We also show that graphene-dielectric multilayers tend to support high-$k$ waves and explore the range of parameteres for which this is possible, confirming the prospects of using graphene for materials with hyperbolic dispersion. The approach is applicable to a large variety of structures, such as continuous or structured microwave, terahertz, and optical metamaterials.

With angle-resolved photoemission experiments and ab initio electronic structure calculations, the pentacene monolayers on Ag(110) and Cu(110) are compared and contrasted, allowing the molecular orientation to be determined and an unambiguous assignment of emissions to specific orbitals to be made. On Ag(110), the orbitals remain essentially isolated-molecule-like, while strong substrate-enhanced dispersion and orbital modification are observed upon adsorption on Cu(110). We show how the photoemission intensity of extended systems can be simulated and that it behaves essentially like that of the isolated molecule modulated by the band dispersion due to intermolecular interactions.

Ge-based spin-photodiodes have been employed to investigate the spectral dependence of optical spin orientation in germanium, in the range 400–1550 nm. We found the expected maximum in the spin polarization of photocarriers for excitation at the direct gap in $\Gamma$ (1550 nm) and a second sizable peak due to photogeneration in the $L$ valleys (530 nm). Data suggest distinct spin depolarization mechanisms for excitation at $\Gamma$ and $L$, with shorter spin relaxation times whether the $X$ point is involved. These devices can be used as integrated photon-helicity detectors over a wide spectral range.

We examine the influence of the excess of interstitial Fe on the magnetic properties of ${\mathrm{Fe}}_{1+y}\mathrm{Te}$ compounds. Because in iron chalcogenides the correlations are stronger than in the iron arsenides, we assume in our model that some of the Fe orbitals give rise to localized magnetic moments. These moments interact with each other via exchange interactions as well as phonon-mediated biquadratic interactions that favor a collinear double-stripe state, corresponding to the ordering vectors $(\pm \pi /2,\pm \pi /2)$. The remaining Fe orbitals are assumed to be itinerant, giving rise to the first-principles derived Fermi surface displaying nesting features at momenta $(\pi ,0)/(0,\pi )$. Increasing the amount of itinerant electrons due to excess Fe, $y$, leads to changes in the Fermi surface and to the suppression of its nesting properties. As a result, due to the Hund's coupling between the itinerant and localized moments, increasing $y$ leads to modifications in the local moments' exchange interactions via the multiorbital generalization of the long-range Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction. By numerically computing the RKKY corrections and minimizing the resulting effective exchange Hamiltonian, we find, in general, that the excess electrons introduced in the system change the classical magnetic ground state from a double-stripe state to an incommensurate spiral, consistent with the experimental observations. We show that these results can be understood as a result of the suppression of magnetic spectral weight of the itinerant electrons at momenta $(\pi ,0)/(0,\pi )$, combined with the transfer of broad magnetic spectral weight from large to small momenta, promoted by the introduction of excess Fe.

It has been theoretically predicted that isolated Bi forms a resonant state in the valence band of the dilute bismide alloy, ${\mathrm{GaAs}}_{1-x}{\mathrm{Bi}}_x$. We present ultrafast pump-probe reflectivity measurements of this interesting alloy system, which provide experimental evidence for the resonant state. The reflectivity transients for pump/probe wavelengths λ ∼ 860–900 nm have negative amplitude, which we attribute to the absorption of the probe pulse by the pump induced carriers that are localized at the Bi-resonant state. Our measurements show that the lifetime of carriers localized at the resonant state is ∼200 ps at 10 K.

Optical-pump terahertz-probe differential transmission measurements of as-prepared single layer graphene (AG) (unintentionally hole doped with Fermi energy $E_F$ at $\sim -180\mathrm{meV}$), nitrogen doping compensated graphene (NDG) with $E_F\sim -10$ meV, and thermally annealed doped graphene (TAG) are examined quantitatively to understand the opposite signs of photoinduced dynamic terahertz conductivity $\Delta \sigma$. It is negative for AG and TAG but positive for NDG. We show that the recently proposed mechanism of multiple generations of secondary hot carriers due to Coulomb interaction of photoexcited carriers with the existing carriers together with the intraband scattering can explain the change of photoinduced conductivity sign and its magnitude. We give a quantitative estimate of $\Delta \sigma$ in terms of controlling parameters—the Fermi energy $E_F$ and momentum relaxation time $\tau$. Furthermore, the cooling of photoexcited carriers is analyzed using a supercollision model which involves a defect mediated collision of the hot carriers with the acoustic phonons, thus giving an estimate of the deformation potential.

A recently developed method of nanoclusters growth in a pulsed plasma is studied by means of molecular dynamics. A model that allows one to consider high-energy charged particles in classical molecular dynamics is suggested, and applied for studies of single impact events in nanoclusters growth. In particular, we provide a comparative analysis of the well-studied inert gas aggregation method and the growth from ions in a plasma. The importance to consider of the angular distribution of incoming ions in the simulations of the nanocluster growth is underlined. A detailed study of the energy transfer from the incoming ions to a nanocluster, as well as the diffusion of incoming ions on the cluster surface, is carried out. Our results are important for understanding and control of the nanocluster growth process.

The spin-resolved nonequilibrium real-time electron transport through a double-quantum-dot (DQD) Aharonov-Bohm (AB) interferometer with spin-orbit interaction (SOI) is explored. The SOI and AB interference in the real-time dynamics of spin transport is expressed by effective magnetic fluxes. Analytical formulas for the time-dependent currents, for initially unpolarized spins, are presented. In many cases, there appear spin currents in the electrodes, for which the spins in each electrode are polarized along characteristic directions, predetermined by the SOI parameters and by the geometry of the system. Special choices of the system parameters yield steady-state currents in which the spins are fully polarized along these characteristic directions. The time required to reach this steady state depends on the couplings of the DQD to the leads. The magnitudes of the currents depend strongly on the SOI-induced effective fluxes. Without the magnetic flux, the spin-polarized current cannot be sustained to the steady states, due to the phase rigidity for this system. For a nondegenerate DQD, transient spin transport can be produced by the sole effects of SOI. We also show that one can extract the spin-resolved currents from measurements of the total charge current.

We study theoretically subwavelength physical phenomena, such as resonant transmission and broadband sound shielding for Lamb waves propagating in an acoustic metamaterial made of a thin plate drilled with one or two row(s) of rectangular holes. The resonances and antiresonances of periodically arranged rectangular junctions separated by holes are investigated as a function of the geometrical parameters of the junctions. With one and two row(s) of holes, high frequency specific features in the transmission coefficient are explained in terms of a coupling of incident waves with both Fabry-Perot oscillations inside the junctions and induced surface acoustic waves between the homogeneous part of the plate and the row of holes. With two rows of holes, low frequency peaks and dips appear in the transmission spectrum. The choice of the distance between the two rows of holes allows the realization of a broadband low frequency acoustic shielding with attenuation over 99% for symmetric waves in a wide low frequency range and over 90% for antisymmetric ones. The origin of the transmission gap is discussed in terms of localized modes of the “H” element made by the junctions, connecting the two homogeneous parts of the plate.