Abstract:
Using first-principles density functional theory calculations, we investigated the effect of charge doping in a LaNiO$_3$/SrTiO$_3$ superlattice. The detailed analysis based on two different simulation methods for doping clearly shows that the electronic and structural properties change in a systematic way that the orbital polarization ({\it i.e.} relative occupation of two Ni-$e_g$ orbitals) is reduced and the Ni to apical oxygen distance enlarged as the number of doped electrons increases. Also, the rotation angles of the NiO$_6$/TiO$_6$ octahedra strongly and systematically depend on the doping so that the angle $\gamma$ gradually decreases whereas the $\alpha$ and $\beta$ increase as a function of electron doping. Further analysis shows that the electron (hole) doping can play a similar role with the compressive (tensile) strain for the octahedral rotations. Our results not only suggest a possible way to control the orbital and structural property by means of charge doping, but also provide useful information to understand the experiments under various doping situations such as oxygen vacancy.

Abstract:
In this paper, several schemes of soft X-ray and hard X-ray free electron lasers (XFEL) and their progress are reviewed. Self-amplified spontaneous emission (SASE) schemes, the high gain harmonic generation (HGHG) scheme and various enhancement schemes through seeding and beam manipulations are discussed, especially in view of the generation of attosecond X-ray pulses. Our recent work on the generation of attosecond hard X-ray pulses is also discussed. In our study, the enhanced SASE scheme is utilized, using electron beam parameters of an XFEL under construction at Pohang Accelerator Laboratory (PAL). Laser, chicane and electron beam parameters are optimized to generate an isolated attosecond hard X-ray pulse at 0.1 nm (12.4 keV). The simulations show that the manipulation of electron energy beam profile may lead to the generation of an isolated attosecond hard X-ray of 150 attosecond pulse at 0.1 nm.

Abstract:
There have been increasing efforts in realizing topological metallic phases with nontrivial surface states. It was suggested that orthorhombic perovskite iridates are classified as a topological crystalline metal (TCM) with flat surface states protected by lattice symmetries. Here we perform first-principles electronic structure calculations for epitaxially stabilized orthorhombic perovskite iridates. Remarkably, two different types of topological surface states are found depending on surface directions. On side surfaces, flat surface states protected by lattice symmetries emerge, manifesting the topological crystalline character. On the top surface, on the other hand, an unexpected Dirac cone appears, indicating surface states protected by a time-reversal symmetry, which is confirmed by the presence of a nontrivial topological $\mathbb{Z}_2$ index. These results suggest that the orthorhombic iridates are unique systems exhibiting both lattice- and global-symmetry-protected topological phases and surface states. Transitions to weak and strong topological insulators and implications of surface states in light of angle resolved photoemission spectroscopy are also discussed.

Abstract:
$\alpha$-RuCl$_3$ has been proposed recently as an excellent playground for exploring Kitaev physics on a two-dimensional (2D) honeycomb lattice. However, structural clarification of the compound has not been completed, which is crucial in understanding the physics of this system. Here, using {\it ab-initio} electronic structure calculations, we study a full three dimensional (3D) structure of $\alpha$-RuCl$_3$ including the effects of spin-orbit coupling (SOC) and electronic correlations. Two issues are addressed in this study; i) effect of SOC on a RuCl$_3$ layer structure and ii) the layer stacking order in the 3D structure and magnetism. First, in the absence of SOC, Ru dimerization occurs in the single honeycomb layer as found in other Ru compounds such as isostructural Li$_2$RuO$_3$. When SOC is introduced, however, dimerization is drastically suppressed making the honeycomb closer to an ideal one. Secondly, without electronic correlations the optimized 3D structure has $P{\bar 3}1m$ space group symmetry with and without SOC, but including electronic correlation changes the optimized 3D structure to either $C2/m$ or $Cmc2_1$ within 0.1 meV per formula unit (f.u.) energy difference. The reported $P3_112$ structure is also close in energy. The interlayer spin exchange coupling is a few percent of in-plane spin exchange terms, confirming $\alpha$-RuCl$_3$ is close to a 2D system. Significant effects of structural change on in-plane spin exchange couplings are also presented.

Abstract:
Ordered phases such as charge- and spin-density wave state accompany either full or partial gapping of Fermi surface (FS) leading a metal-insulator or metal-metal transition (MMT). However, there are examples of MMT without any signatures of symmetry breaking. One example is Na$_2$Ti$_2$Sb$_2$O, where a partial gapping of FS is observed but a density wave ordering has not been found. Here we propose a microscopic mechanism of such a MMT which occurs due to a momentum dependent spin-orbit coupled molecular orbital polarization. Since a molecular $d$ orbital polarization is present due to a small spin-orbit coupling of Ti, there is no spontaneous symmetry breaking involved. However, a sharp increase of polarization happens above a critical electron interaction which gaps out the $d$ orbtial FS and reduces the density of states significantly, while the rest of FS associated with Sb $p$ orbtials is almost intact across MMT. Experimental implications to test our proposal and applications to other systems are also discussed.

Abstract:
Collinear and non-collinear spin structures of wurtzite phase CoO often appearing in nano-sized samples are investigated using first-principles density functional theory calculations. We examined the total energy of several different spin configurations, electronic structure and the effective magnetic coupling strengths. It is shown that the AF3-type antiferromagnetic ordering is energetically most stable among possible collinear configurations. Further, we found that a novel spiral spin order can be stabilized by including the relativistic spin-orbit coupling and the non-collinearity of spin direction. Our result suggests that a non-collinear spin ground state can be observed in the transition-metal-oxide nanostructures which adds an interesting new aspect to the nano-magnetism study.

Abstract:
The recently discovered three-dimensional hyperhoneycomb iridate, $\beta$-Li$_2$IrO$_3$, has raised hopes for the realization of dominant Kitaev interaction between spin-orbit entangled local moments due to its near-ideal lattice structure. If true, this material may lie close to the sought-after quantum spin liquid phase in three dimensions. Utilizing ab-initio electronic structure calculations, we first show that the spin-orbit entangled basis, $j_{\rm eff}$=1/2, correctly captures the low energy electronic structure. The effective spin model derived in the strong coupling limit supplemented by the ab-initio results is shown to be dominated by the Kitaev interaction. We demonstrated that the possible range of parameters is consistent with a non-coplanar spiral magnetic order found in a recent experiment. All of these analyses suggest that $\beta$-Li$_2$IrO$_3$ may be the closest among known materials to the Kitaev spin liquid regime.

Abstract:
Motivated by an experimental report of iridate superlattices, we performed first-principle electronic structure calculations for SrIrO$_3$/SrTiO$_3$. Heterostructuring causes SrIrO$_3$ to become Sr$_2$IrO$_4$-like, and the system has the well-defined $j_{\rm eff}$=1/2 states near the Fermi level as well as canted antiferromagnetic order within the quasi-two-dimensional IrO$_2$ plane. In response to a larger tensile strain, the band gap is increased due to the resulting increase in bond length and the bandwidth reduction. The ground state magnetic properties are discussed in comparison to the metastable collinear antiferromagnetic state. Our work sheds new light for understanding the recent experimental results on the iridate heterostructures.

Abstract:
This paper investigated comparatively the characteristics of four types of artificial magnetic conductor (AMC) surface, including a mushroom-like (electromagnetic band gap) EBG, uniplanar compact EBG (UC-EBG), Peano curve, and Hilbert curve, as a ground plane for a low-profile antenna. The AMC surface structures are designed to have an in-phase reflection property for a plane wave of normal incidence in the vicinity of 2.45 GHz. The bandwidths of the in-phase reflection for the AMC surfaces and return losses, radiation patterns, and gains of the horizontal wire antennas on the AMC ground planes are all measured and compared with each other. The measured data show that all the AMC surfaces act as good ground planes for a low- profile antenna, yet the bandwidth and gain of the mushroom-like EBG structure are broader and larger, respectively, than those of the other structures.

Abstract:
The light transmission through a dispersive plasmonic circular hole is numerically investigated with an emphasis on its subwavelength guidance. For a better understanding of the effect of the hole diameter on the guided dispersion characteristics, the guided modes, including both the surface plasmon polariton mode and the circular waveguide mode, are studied for several hole diameters, especially when the metal cladding has a plasmonic frequency dependency. A brief comparison is also made with the guided dispersion characteristics of a dispersive plasmonic gap [K. Y. Kim, et al., Opt. Express 14, 320-330 (2006)], which is a planar version of the present structure, and a circular waveguide with perfect electric conductor cladding. Finally, the modal behavior of the first three TM-like principal modes with varied hole diameters is examined for the same operating mode.