Calculating Nonlocal Optical Properties of Structures with Arbitrary Shape

In a recent Letter [Phys. Rev. Lett. 103, 097403 (2009)], we outlined a computational method to calculate the optical properties of structures with a spatially nonlocal dielectric function. In this Article, we detail the full method, and verify it against analytical results for cylindrical nanowires. Then, as examples of our method, we calculate the optical properties of Au nanostructures in one, two, and three dimensions. We first calculate the transmission, reflection, and absorption spectra of thin films. Because of their simplicity, these systems demonstrate clearly the longitudinal (or volume) plasmons characteristic of nonlocal effects, which result in anomalous absorption and plasmon blueshifting. We then study the optical properties of spherical nanoparticles, which also exhibit such nonlocal effects. Finally, we compare the maximum and average electric field enhancements around nanowires of various shapes to local theory predictions. We demonstrate that when nonlocal effects are included, significant decreases in such properties can occur.Comment: 30 pages, 12 figures, 1 tabl

Microscopic theory of surface-enhanced Raman scattering in noble-metal nanoparticles

We present a microscopic model for surface-enhanced Raman scattering (SERS) from molecules adsorbed on small noble-metal nanoparticles. In the absence of direct overlap of molecular orbitals and electronic states in the metal, the main enhancement source is the strong electric field of the surface plasmon resonance in a nanoparticle acting on a molecule near the surface. In small particles, the electromagnetic enhancement is strongly modified by quantum-size effects. We show that, in nanometer-sized particles, SERS magnitude is determined by a competition between several quantum-size effects such as the Landau damping of surface plasmon resonance and reduced screening near the nanoparticle surface. Using time-dependent local density approximation, we calculate spatial distribution of local fields near the surface and enhancement factor for different nanoparticles sizes.Comment: 8 pages, 6 figures. Considerably extended final versio

Detection of the Second r-process Peak Element Tellurium in Metal-Poor Stars

Using near-ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope, we detect neutral tellurium in three metal-poor stars enriched by products of r-process nucleosynthesis, BD+17 3248, HD 108317, and HD 128279. Tellurium (Te, Z=52) is found at the second r-process peak (A=130) associated with the N=82 neutron shell closure, and it has not been detected previously in Galactic halo stars. The derived tellurium abundances match the scaled solar system r-process distribution within the uncertainties, confirming the predicted second peak r-process residuals. These results suggest that tellurium is predominantly produced in the main component of the r-process, along with the rare earth elements.Comment: Accepted for publication in the Astrophysical Journal Letters (5 pages, 2 figures

New Detections of Arsenic, Selenium, and Other Heavy Elements in Two Metal-Poor Stars

We use the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope to obtain new high-quality spectra covering the 1900 to 2360 Angstrom wavelength range for two metal-poor stars, HD 108317 and HD 128279. We derive abundances of Cu II, Zn II, As I, Se I, Mo II, and Cd II, which have not been detected previously in either star. Abundances derived for Ge I, Te I, Os II, and Pt I confirm those derived from lines at longer wavelengths. We also derive upper limits from the non-detection of W II, Hg II, Pb II, and Bi I. The mean [As/Fe] ratio derived from these two stars and five others in the literature is unchanged over the metallicity range -2.8 = +0.28 +/- 0.14 (std. dev. = 0.36 dex). The mean [Se/Fe] ratio derived from these two stars and six others in the literature is also constant, = +0.16 +/- 0.09 (std. dev. = 0.26 dex). The As and Se abundances are enhanced relative to a simple extrapolation of the iron-peak abundances to higher masses, suggesting that this mass region (75 < A < 82) may be the point at which a different nucleosynthetic mechanism begins to dominate the quasi-equilibrium alpha-rich freezeout of the iron peak. = +0.56 +/- 0.23 in HD 108317 and HD 128279, and we infer that lines of Cu I may not be formed in local thermodynamic equilibrium in these stars. The [Zn/Fe], [Mo/Fe], [Cd/Fe], and [Os/Fe] ratios are also derived from neutral and ionized species, and each ratio pair agrees within the mutual uncertainties, which range from 0.15 to 0.52 dex.Comment: Accepted for publication in the Astrophysical Journal. 13 pages, 10 figure

rp-Process weak-interaction mediated rates of waiting-point nuclei

Electron capture and positron decay rates are calculated for neutron-deficient Kr and Sr waiting point nuclei in stellar matter. The calculation is performed within the framework of pn-QRPA model for rp-process conditions. Fine tuning of particle-particle, particle-hole interaction parameters and a proper choice of the deformation parameter resulted in an accurate reproduction of the measured half-lives. The same model parameters were used to calculate stellar rates. Inclusion of measured Gamow-Teller strength distributions finally led to a reliable calculation of weak rates that reproduced the measured half-lives well under limiting conditions. For the rp-process conditions, electron capture and positron decay rates on $^{72}$Kr and $^{76}$Sr are of comparable magnitude whereas electron capture rates on $^{78}$Sr and $^{74}$Kr are 1--2 orders of magnitude bigger than the corresponding positron decay rates. The pn-QRPA calculated electron capture rates on $^{74}$Kr are bigger than previously calculated. The present calculation strongly suggests that, under rp-process conditions, electron capture rates form an integral part of weak-interaction mediated rates and should not be neglected in nuclear reaction network calculations as done previously.Comment: 13 pages, 4 figures, 4 tables; Astrophysics and Space Science (2012

Surface plasmons at single nanoholes in Au-films

The generation of surface plasmon polaritons (SPP's) at isolated nanoholes in 100 nm thick Au films is studied using near-field scanning optical microscopy (NSOM). Finite-difference time-domain calculations, some explicitly including a model of the NSOM tip, are used to interpret the results. We find the holes act as point-like sources of SPP's and demonstrate that interference between SPP's and a directly transmitted wave allows for determination of the wavelength, phase, and decay length of the SPP. The near-field intensity patterns can be manipulated by varying the angle and polarization of the incident beam.Comment: 12 pages, 3 figure

Hubble Space Telescope Near-Ultraviolet Spectroscopy of Bright CEMP-s Stars

We present an elemental-abundance analysis, in the near-ultraviolet (NUV) spectral range, for the bright carbon-enhanced metal-poor (CEMP) stars HD196944 (V = 8.40, [Fe/H] = -2.41) and HD201626 (V = 8.16, [Fe/H] = -1.51), based on data acquired with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope. Both of these stars belong to the sub-class CEMP-s, and exhibit clear over-abundances of heavy elements associated with production by the slow neutron-capture process. HD196944 has been well-studied in the optical region, but we are able to add abundance results for six species (Ge, Nb, Mo, Lu, Pt, and Au) that are only accessible in the NUV. In addition, we provide the first determination of its orbital period, P=1325 days. HD201626 has only a limited number of abundance results based on previous optical work -- here we add five new species from the NUV, including Pb. We compare these results with models of binary-system evolution and s-process element production in stars on the asymptotic giant branch, aiming to explain their origin and evolution. Our best-fitting models for HD 196944 (M1,i = 0.9Mo, M2,i = 0.86Mo, for [Fe/H]=-2.2), and HD 201626 (M1,i = 0.9Mo , M2,i = 0.76Mo , for [Fe/H]=-2.2; M1,i = 1.6Mo , M2,i = 0.59Mo, for [Fe/H]=-1.5) are consistent with the current accepted scenario for the formation of CEMP-s stars.Comment: 25 pages, 13 figures; accepted for publication in Ap