G. Y. Guo

High-Efficiency Broadband Anomalous Reflection by Gradient Meta-Surfaces

We combine theory and experiment to demonstrate that a carefully designed gradient meta-surface supports high-efficiency anomalous reflections for near-infrared light following the generalized Snells law, and the reflected wave becomes a bounded surface wave as the incident angle exceeds a critical value. Compared to previously fabricated gradient meta-surfaces in infrared regime, our samples work in a shorter wavelength regime with a broad bandwidth (750-900 nm), exhibit a much higher conversion efficiency (∼80%) to the anomalous reflection mode at normal incidence, and keep light polarization unchanged after the anomalous reflection. Finite-difference-time-domain (FDTD) simulations are in excellent agreement with experiments. Our findings may lead to many interesting applications, such as antireflection coating, polarization and spectral beam splitters, high-efficiency light absorbers, and surface plasmon couplers.

Intrinsic spin Hall effect in platinum: first-principles calculations.

G. Y. Guo, S. Murakami, T.-W. Chen, and N. Nagaosa Department of Physics and Center for Theoretical Sciences, National Taiwan University, Taipei 106, Taiwan Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan CREST, Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan Correlated Electron Research Center, National Institute of Advanced Industrial Science and Technology, 1-1-1, Higashi, Tsukuba, Ibaraki 305-8562, Japan (Dated: February 11, 2008)

Systematic ab initio study of the magnetic and electronic properties of all 3d transition metal linear and zigzag nanowires

It is found that all the zigzag chains except the nonmagnetic (NM) Ni and antiferromagnetic (AF) Fe chains which form a twisted two-legger ladder, look like a corner-sharing triangle ribbon, and have a lower total energy than the corresponding linear chains. All the 3d transition metals in both linear and zigzag structures have a stable or metastable ferromagnetic (FM) state. The electronic spin-polarization at the Fermi level in the FM Sc, V, Mn, Fe, Co and Ni linear chains is close to 90% or above. In the zigzag structure, the AF state is more stable than the FM state only in the Cr chain. It is found that the shape anisotropy energy may be comparable to the electronic one and always prefers the axial magnetization in both the linear and zigzag structures. In the zigzag chains, there is also a pronounced shape anisotropy in the plane perpendicular to the chain axis. Remarkably, the axial magnetic anisotropy in the FM Ni linear chain is gigantic, being ~12 meV/atom. Interestingly, there is a spin-reorientation transition in the FM Fe and Co linear chains when the chains are compressed or elongated. Large orbital magnetic moment is found in the FM Fe, Co and Ni linear chains.

Excitonic effects in the optical properties of a SiC sheet and nanotubes

Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA(Dated: May 30, 2011)The quasiparticle band structure and optical properties of single-walled zigzag and armchair SiCnanotubes (SiC-NTs) as well as single SiC sheet are investigated by abinitiomany-body calculationsusing the GW and the GW plus Bethe-Salpeter equation (GW+BSE) approaches, respectively.Significant GW quasiparticle corrections of more than 1.0 eV to the Kohn-Sham band gaps fromthe local density approximation (LDA) calculations are found. The GW self-energy correctionstransform the SiC sheet from a indirect LDA band gap to a direct band gap material. Furthermore,the quasiparticle band gaps of SiC-NTs with different chiralities behave very differently as a functionof tube diameter, and this can be attributed to the difference in the curvature-induced orbitalrehybridization between the different chiral nanotubes. The calculated optical absorption spectraare dominated by discrete exciton peaks due to exciton states with large binding energy up to2.0 eV in the SiC sheet and SiC-NTs. The formation of strongly bound excitons is attributed tothe enhanced electron-hole interaction in these low dimensional systems. Remarkably, the excitedelectron amplitude of the exciton wavefunction is found to peak on the Siatoms near the hole position(which is on the C site) in the zigzag SiC-NTs, indicating a charge transfer from an anion (hole) toits neighboring cations by photoexcitation. In contrast, this pronounced peak structure disappearin the exciton wavefunction in the armchair SiC-NTs. Furthermore, in the armchair SiC-NTs, thebound exciton wavefunctions are more localized and also strongly cylindrically asymmetric. Thelarge excitation energy of ∼ 3.0 eV of the first bright exciton with no dark exciton below it, suggeststhat the small-radius armchair SiC-NTs be useful for optical devices working in the UV regime.On the other hand, the zigzag SiC-NTs have many dark excitons below the first bright exciton andhence may have potential applications in tunable optoelectric devices ranging from infrared to UVfrequencies by external perturbations.

Quantum oscillations in the mixed state of the type II superconductor 2H-NbSe2

Quantum oscillations have been investigated far into the mixed state of the type II superconductor 2H-NbSe2. The de Haas-van Alphen effect is found to persist to fields as low as approximately 0.3BC2 and to temperatures of approximately 0.003TC. A self-consistent band structure calculation, performed for 2H-NbSe2 in the normal non-charge-density-wave state using a full potential linearized augmented plane wave (FLAPW) method, accounts well for the experimental angular dependence of the quantum oscillations in terms of a small flat hole Fermi surface around the Gamma point. This feature derives mainly from chalcogen p bands and has a mass-enhancement factor much smaller than expected from the specific heat. Quantum oscillations in the presence of the vortex lattice are observed to experience an additional damping, from which the order parameter is estimated to be Delta (0)=0.6+or-0.1 meV.

Optical magnetic response in three-dimensional metamaterial of upright plasmonic meta-molecules

We report the first three-dimensional photonic metamaterial, an array of erected U-shape plasmonic gold meta-molecules, that exhibits a profound response to the magnetic field of light incident normal to the array. The metamaterial was fabricated using a double exposure e-beam lithographic process. It was investigated by optical measurements and finite-element simulations, and showed that the magnetic field solely depends on the plasmonic resonance mode showing either enhanced in the centre of the erected U-shape meta-molecule (16 times enhancement) or enhanced around two prongs of erected U-shape meta-molecule (4 times enhancement).

Electronic structure of possible 3d `heavy-fermion' compound

Recent transport, specific heat and magnetization measurements indicated that the Heusler-type compound is a candidate for a 3d heavy-fermion system. As a first step towards a detailed theoretical understanding of the observed anomalous electronic properties of this compound, we have performed first-principles electronic structure and total energy calculations for and . The calculated lattice constants and magnetic moments are in good agreement with experiments. Remarkably, we find that, unlike , Heusler-type is a nonmagnetic semimetal with a narrow pseudogap at the Fermi level. Furthermore, our calculations suggest that the observed large enhancement of electronic specific heat coefficient is largely caused by the mechanisms such as spin-fluctuations rather than electron - phonon coupling in . The relations between the existence of the Fermi surface and negative temperature dependence of the low-temperature electrical resistivity in are discussed and further experimental and theoretical work is suggested.

Chemical composition tuning of the anomalous Hall effect in isoelectronic L10FePd(1-x)Pt(x) alloy films.

The anomalous Hall effect (AHE) in L1(0)FePd(1-x)Pt(x) alloy films is studied both experimentally and theoretically. We find that the intrinsic contribution (σ(AH)(int)) to the AHE can be significantly increased, whereas the extrinsic side-jump contribution (σ(AH)(sj)) can be continuously reduced from being slightly larger than σ(AH)(int) in L1(0) FePd to being much smaller than σ(AH)(int) in L1(0) FePt, by increasing the Pt composition x. We show that this chemical composition tuning of the intrinsic contribution is afforded by the stronger spin-orbit coupling strength on the Pd/Pt site when the lighter Pd atoms are replaced by the heavier Pt atoms. Our results provide a means of manipulating the competing AHE mechanisms in ferromagnetic alloys for fully understanding the AHE and also for technological applications of ferromagnetic alloys.

Second-harmonic generation and linear electro-optical coefficients of BN nanotubes

A systematic ab initio study of the second-order nonlinear optical properties of BN nanotubes within density functional theory in the local density approximation has been performed. Highly accurate full-potential projector augmented-wave method was used. Specifically, the second-harmonic generation

Fabrication of three dimensional split ring resonators by stress-driven assembly method

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