Photonic crystals are materials patterned with a periodicity in dielectric constant, which can create a range of 'forbidden' frequencies called a photonic bandgap. Photons with energies lying in the bandgap cannot propagate through the medium. This provides the opportunity to shape and mould the flow of light for photonic information technology.

Photonic crystals: putting a new twist on light

We present the first full band-structure calculations for periodic, elastic composites. For transverse polarization of the vibrations we obtain a ``phononic'' band gap which extends throughout the Brillouin zone. A complete acoustic gap or a low density of states should have important consequences for the suppression of zero-point motion and for the localization of phonons, and may lead to improvements in transducers and in the creation of a vibrationless environment.

Acoustic band structure of periodic elastic composites.

Gold is an element whose unique properties are strongly influenced by relativistic effects. A large body of appropriate calculations now exist and their main conclusions are summarized. The theoretical interpretation of the aurophilic attraction is discussed in detail.

Theoretical chemistry of gold.

http://myweb.rz.uni-augsburg.de/~eckern/WEH-287/ABSTRACTS/ruitenbeek.pdf

Quantum properties of atomic-sized conductors

Two procedures were developed to fit interatomic potentials of the embedded-atom method (EAM) form and applied to determine a potential which describes crystalline and liquid iron. While both procedures use perfect crystal and crystal defect data, the first procedure also employs the first-principles forces in a model liquid and the second procedure uses experimental liquid structure factor data. These additional types of information were incorporated to ensure more reasonable descriptions of atomic interactions at small separations than is provided using standard approaches, such as fitting to the universal binding energy relation. The new potentials (provided herein) are, on average, in better agreement with the experimental or first-principles lattice parameter, elastic constants, point-defect energies, bcc–fcc transformation energy, liquid density, liquid structure factor, melting temperature and other properties than other existing EAM iron potentials.

http://homepages.ed.ac.uk/graeme/Fe_PhilMag.pdf

Development of new interatomic potentials appropriate for crystalline and liquid iron

We have performed simulations of coexisting liquid and solid phases of aluminum as an efficient way of mapping out the coexistence line. This technique is convenient, as it does not require complicated free-energy calculations for the different phases, but simply allows the system to equilibrate to a co-existence point. By altering the simulation volume and/or energy, a new coexistence point is found. The calculated method temperature is lower than previous results for the identical model; we suspect that this difference is due to the difficulty of calculating the free energy of the liquid phase, leading to inaccuracies in the previous work. A thorough examination of several different system sizes, from 1024 to 65 536 particles, indicates that the results are only weakly dependent upon the system size.

/pdf/melting-line-of-aluminum-from-simulations-of-coexisting-17dp2dwzvm.pdf

Melting line of aluminum from simulations of coexisting phases.

Using a plane-wave-expansion method, we obtained expressions for the effective dielectric constants of composite periodic dielectric materials as the long-wavelength limit of the photonic-band-structure problem. The effective dielectric constants of several classes of photonic-band-gap structures are computed and we found that in the long-wavelength limit, they can be isotropic, uniaxial, or biaxial, depending on the symmetry of the structure under consideration. We also found that the scalar wave approximation gives poor results for effective dielectric constants while the Maxwell-Garnett theory offers very good approximations in the low-filling-ratio regimes.

/pdf/effective-dielectric-constant-of-periodic-composite-2dhr3yvfa3.pdf

Effective dielectric constant of periodic composite structures.

Using the transfer-matrix technique for the propagation of electromagnetic waves in dielectric structures, we calculate the transmission coefficient versus the frequency of the incident wave for different polarizations in two-dimensional periodic and/or random arrangements of dielectric cylinders. This technique has been applied to cases where the plane-wave method fails or becomes too time consuming, such as when the dielectric constant is frequency dependent or has a nonzero imaginary part, and when defects are present in an otherwise periodic system. For all the cases studied, the results compared well with experiment.

/pdf/photonic-band-gaps-and-defects-in-two-dimensions-studies-of-1rehy3eqca.pdf

Photonic band gaps and defects in two dimensions: Studies of the transmission coefficient

Pseudospin plays a central role in many novel physical properties of graphene and other artificial systems which have pseudospins of 1/2. Here we show that in certain photonic crystals (PCs) exhibiting conical dispersions at k = 0, the eigenmodes near the "Dirac-like point" can be described by an effective spin-orbit Hamiltonian with a pseudospin of 1, treating wave propagations in the upper cone, the lower cone and a flat band (corresponding to zero refractive index) within a unified framework. The 3-component spinor gives rise to boundary conditions distinct from those of pseudospin-1/2, leading to new wave transport behaviors as manifested in Klein tunneling and supercollimation. For example, collimation can be realized more easily with pseudospin-1 than pseudospin-1/2. The special wave scattering properties of pseudospin-1 photons, coupled with the discovery that the effective photonic "potential" can be varied by a simple change of length scale, may offer new ways to control photon transport.

Klein tunneling and supercollimation of pseudospin-1 photons

Vibrational potentials for hydrogen isotopes in PdH and ${\mathrm{Pd}}_{4}$H are determined from first-principles total-energy calculations. The strong anharmonicity of the potential for H around octahedral sites must be taken into account beyond perturbation theory. For PdH good quantitative agreement of our calculated vibration energies with inelastic-neutron-scattering (INS) results is achieved, allowing a reinterpretation of the INS spectra. For lower H concentrations (${\mathrm{Pd}}_{\mathit{n}}$H, n=4, 8, 16, and 32) the influence of local lattice relaxations on the vibration potentials and energies is important. Furthermore, metastable states confined to tetrahedral sites in Pd are found for the heavier isotopes $^{2}\mathrm{D}$ and $^{3}\mathrm{T}$.

/pdf/vibrational-states-for-hydrogen-in-palladium-16iaum4cfg.pdf

Vibrational states for hydrogen in palladium

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