V. K. Vlasko-Vlasov

Surface plasmons at single nanoholes in Au films

The generation of surface plasmon polaritons (SPPs) at isolated nanoholes in 100nm thick Au films is studied using near-field scanning optical microscopy. Finite-difference time-domain calculations, some explicitly including a model of the scanning tip, are used to interpret the results. We find the holes act as point-like sources of SPPs and demonstrate that the interference between the SPP and a directly transmitted wave allows 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.

Nickel antidot arrays on anodic alumina substrates.

Large-area nickel antidot arrays with a density up to 1010/cm2 have been fabricated by depositing nickel onto anodic aluminum oxide membranes that contain lattices of nanopores. Electron microscopy images show a high degree of order of the antidot arrays. Various sizes and shapes of the antidots were observed with increasing thickness of the deposited nickel. New features appear in the antidot arrays in both magnetization and transport measurements when the external magnetic field is parallel to the current direction, including an enhancement and a nonmonotonous field dependence of the magnetoresistance, larger values of the coercive field and remanence moment, and smaller saturation field.

Uniaxial in-plane magnetic anisotropy of Ga1−xMnxAs

The anisotropic magnetic properties of a series of epitaxial Ga1−xMnxAs films with thicknesses ranging from 0.2to6.8μm were investigated using magnetometry and x-ray diffraction. The films all show a distinct uniaxial contribution to the magnetic anisotropy along the in-plane [110] direction, whose relative importance increases with the temperature. The uniaxial anisotropy field is found to be essentially thickness independent, ruling out the possibility that this is an effect produced by a single surface or interface due to well-known surface reconstruction. Furthermore, even the very thick samples are found to be tetragonal and coherently strained to the GaAs substrate. Our results suggest that the uniaxial anisotropy arises from surface reconstruction induced preferential Mn incorporation occurring at every step of layer-by-layer growth and thus uniformly penetrates the entire sample.

Superconducting transition and vortex pinning in Nb films patterned with nanoscale hole arrays

Nb films containing extended arrays of holes with 45-nm diameter and 100-nm spacing have been fabricated using anodized aluminum oxide as substrate. Pronounced matching effects in the magnetization and Little-Parks oscillations of the superconducting critical temperature have been observed in fields up to 9 kOe. Flux pinning in the patterned samples is significantly enhanced as compared to unpatterned reference samples in applied fields exceeding 5 kOe. Matching effects are a dominant contribution to vortex pinning at temperatures as low as 4.2 K due to the extremely small spacing of the holes.

Modification of vortex behavior through heavy ion lithography

Columnar defects induced by high-energy heavy ion irradiation constitute one of the most effective vortex pinning sites in high temperature superconductors. We demonstrate a novel method of effectively patterning vortex pinning sites in single crystals of the high temperature superconductors YBa 2 Cu 3 O 7-d and Bi 2 Sr 2 CaCu 2 O x using heavy ion irradiation. These patterns include linear channels, a lattice of squares, and asymmetric pinning gradient sites where vortices can either flow unimpeded or are trapped in spatially periodic arrangements.

Enhanced Raman scattering from focused surface plasmons

Surface plasmon polaritons launched at concentric arcs can be focused into a subwavelength wide focal spot of high near-field light intensity. The focused plasmons give rise to enhanced Raman scattering from R6G molecules placed in the focal area. By exploiting the polarization dependence of the focusing the authors establish an enhancement of the Raman signal by a factor of ∼6. The results show that focusing of propagating surface plasmons on flat metal surfaces may be an alternative to localized plasmons on metal nanostructures for achieving enhanced Raman scattering. In particular, a flat metal substrate enables better control over the local electric fields and the placement of analyte molecules, and, therefore, ultimately better fidelity of Raman spectra.

Decoration of Josephson vortices by pancake vortices in Bi 2 Sr 2 CaCu 2 O 8 + d

Josephson vortices are imaged magneto-optically due to their decoration with pancake vortices in

Magneto‐optical imaging of flux patterns in multifilamentary (BiPb)2Sr2Ca2Cu3Ox composite conductors

Bi_{2}Sr_{2}CaCu_{2}O_{8+d}

Macroturbulence in high-Tc superconductors

single crystals. Peculiarities of interaction between the pancake and Josephson vortices (JV) depending on the values of crossing fields and temperature are studied based on the observations of these decoration patterns. Evidences of the period-doubling in the decoration patterns compared to the JV stack period, migration of JV lines between neighboring stacks, and transitions between different JV configurations are reported. Imaging of the pancake/Josephson vortex decoration patterns over large areas is shown to be a sensitive tool for detecting local variations of the anisotropy and mapping imperfections in layered HTS samples.

Magnetization reversal in arrays of individual and coupled Co-rings

We present a study of the superconducting morphology of the transport current carrying cross section of a 19‐filament (BiPb)2Ca2Cu3Ox (Bi‐2223) composite conductor using magneto‐optical imaging of magnetic flux patterns. In conjunction with electron microscopy on the same sample this technique allows a unique correlation of superconducting and microstructural properties. Direct evidence for enhanced superconducting properties in platelike regions along the silver/Bi‐2223 interface and for weak properties near the core of the filaments is obtained. Misaligned grain colonies are found to cause an interruption of the superconducting continuity in the filaments.