Abstract:
We present three important extensions of the plasmon hybridization (PH) method: a generalization of the method to include realistic non-Drude dielectric permittivities for metals, the development of an algorithm for the calculation of plasmon-induced electric field enhancements, and the extension of the PH method to the modeling of plasmonic Fano resonances. We illustrate these developments with an application to a silver nanosphere dimer and a symmetric silver nanosphere heptamer.

Abstract:
Based on micromagnetic simulations, we report on a novel helical magnetic structure in a soft magnetic film that is sandwiched between and exchange-coupled to two hard magnetic layers. Confined between antiparallel hard magnetic moments, a helix with a turn of 180$^{\circ}$ is stable without the presence of an external magnetic field. The magnetic stability is determined by the energy minimization and is a result of an internal field created by exchange interaction and anisotropy. Since the internal field stores magnetic energy, the helix can serve as an energy-storing element in spin-based nanodevices. Due to the significantly different magnetic resonance frequencies, the ferromagnetic and helical ground states are easy to distinguish in a broadband ferromagnetic resonance experiment.

Abstract:
In the flurry of experiments looking for topological insulator materials, it has been recently discovered that some bulk metals very close to topological insulator electronic states, support the same topological surface states that are the defining characteristic of the topological insulator. First observed in spin-polarized ARPES in Sb (D. Hsieh et al. Science 323, 919 (2009)), the helical surface states in the metallic systems appear to be robust to at least mild disorder. We present here a theoretical investigation of the nature of these "helical metals" - bulk metals with helical surface states. We explore how the surface and bulk states can mix, in both clean and disordered systems. Using the Fano model, we discover that in a clean system, the helical surface states are \emph{not} simply absorbed by hybridization with a non-topological parasitic metallic band. Instead, they are pushed away from overlapping in momentum and energy with the bulk states, leaving behind a finite-lifetime surface resonance in the bulk energy band. Furthermore, the hybridization may lead in some cases to multiplied surface state bands, in all cases retaining the helical characteristic. Weak disorder leads to very similar effects - surface states are pushed away from the energy bandwidth of the bulk, leaving behind a finite-lifetime surface resonance in place of the original surface states.

Abstract:
Homogeneous, zero temperature scaling solutions with Bianchi VII spatial geometry are constructed in Einstein-Maxwell-Dilaton theory. They correspond to quantum critical saddle points with helical symmetry at finite density. Assuming $AdS_{5}$ UV asymptotics, the small frequency/(temperature) dependence of the AC/(DC) electric conductivity along the director of the helix are computed. A large class of insulating and conducting anisotropic phases is found, as well as isotropic, metallic phases. Conduction can be dominated by dissipation due to weak breaking of translation symmetry or by a quantum critical current.

Abstract:
We present a comprehensive study of plasmon dispersions in simple metals and Heusler compounds based on an accurate ab-initio evaluation of the momentum and frequency dependent dielectric function in the random-phase approximation. Using a momentum-dependent tetrahedron method for the computation of the dielectric function, we extract and analyze "full" and "intraband" plasmon dispersions: The "full" plasma dispersion is obtained by including all bands, the intraband plasma dispersion by including only intraband transitions. For the simple metals silver and alu- minum, we show that the intraband plasmon dispersion has an unexpected downward slope and is therefore markedly different from the results of an effective-mass electron-gas model and the full plasmon dispersion. For the two Heusler compounds Co2FeSi and Co2MnSi, we present spectra for the dielectric function, their loss functions and plasmon dispersions. The latter exhibit the same negative intraband plasmon dispersion as found in the simple metals. We also discuss the influence of spin-mixing on the plasmon dispersion.

Abstract:
Within the simplest model of metals, namely a gas of electrons with Coulomb interactions, in the presence of a uniform background of positive charge to enforce electric neutrality of the system, we have derived a mechanism, by which the Coulomb interaction between the electrons generates a new kind of magnetism. The ground state of the metal is represented by a magnetically ordered state described by a non-local magnetic field. This non-local magnetic field does not produce spin polarisation of electrons, but induces a special long range correlation between electrons of opposite spin. This mechanism results in a theoretical value for the binding energy per electron, which is more than twice the corresponding value for the unmagnetised state of the metal. The new magnetic order proposed and analysed theoretically here, can in principle be experimentally tested.

Abstract:
In this paper the variation of the propagation constant, the attenuation coefficient, penetration depth inside the metal and the dielectric has been evaluated. The propagation characteristics of Surface Plasmon Waves (SPWs) which exists on noble metals like gold (Au), silver (Ag) and aluminium (Al) due to the formation of Surface Plasmon Polaritons (SPPs), have been evaluated theoretically and simulated. It has been found that highly conducting metals Au and Ag provide a strong confinement to the SPWs than Al at optical frequencies. The comparative study reveals that metal having higher conductivity can support a more confined SPW, having a lower penetration depth than metals of lower conductivity at terahertz frequencies when its dielectric constant assumes a negative value.

Abstract:
The anomalous plasmon linewidth dispersion (PLD) measured in K by vom Felde, Sprosser-Prou, and Fink (Phys. Rev. B 40, 10181 (1989)), has been attributed to strong dynamical electron-electron correlations. On the basis of ab initio response calculations, and detailed comparison with experiment, we show that the PLD of K is, in fact, dominated by decay into particle-hole excitations involving empty states of d-symmetry. For Li, we shed new light on the physics of the PLD. Our all-electron results illustrate the importance of ab initio methods for the study of electronic excitations.

Abstract:
We study spin susceptibility and magnetic order at the edges/surfaces of two-dimensional and three-dimensional topological insulators when the Fermi surface is nested. We find that due to spin-momentum locking as well as time-reversal symmetry, spin susceptibility at the nesting wavevector has a strong {\em helical} feature. It follows then, a {\em helical} spin density wave (SDW) state emerges at low temperature due to Fermi surface nesting. The helical feature of spin susceptibility also has profound impact on the magnetic order in magnetically doped surface of three dimensional topological insulators. In such system, from the mean field Zener theory, we predict a {\em helical} magnetic order.

Abstract:
We report on first-principles calculations of multilayers of zinc-blende half-metallic ferromagnets CrAs and CrSb with III-V and II-VI semiconductors, in the [001] orientation. We examine the ideal and tetragonalised structures, as well as the case of an intermixed interface. We find that, as a rule, half-metallicity can be conserved throughout the heterostructures, provided that the character of the local coordination and bonding is not disturbed. At the interfaces with semiconductors, we describe a mechanism that can give also a non-integer spin moment per interface transition atom, and derive a simple rule to evaluate it.