Index of content:
Volume 118, Issue 21, 07 December 2015
- Photonics, Plasmonics, Lasers, and Optical Phenomena
Goos-Hänchen shift at the reflection of light from the complex structures composed of superconducting and dielectric layers118(2015); http://dx.doi.org/10.1063/1.4936619View Description Hide Description
The Goos-Hänchen effect of light reflected from sandwich (three-layered) structures composed of a superconducting YBa2Cu3O7film and two different dielectric films is investigated theoretically. It has been shown that optical anisotropy of YBa2Cu3O7film, as well as its positions in the three-layer specimen, strongly effects on the lateral shift values. We have shown that, for all positions of the superconducting film in the three-layered structure, variation of temperature makes possible to control the values of the lateral shift of TE-polarized light at the incidence angles close to pseudo-Brewster angles, whereas for TM-polarized light the lateral shift is only significant at grazing incidence.
118(2015); http://dx.doi.org/10.1063/1.4936769View Description Hide Description
Finite-difference time-domain(FDTD) simulations showed that the reflectance spectra of crystalline Si nanopillar (NP) arrays with diameters of 40, 70, 100, and 130 nm differed depending on wetting state. The observed reflectance dips of the 40-nm-diameter NP array were in good agreement with those estimated from destructive interference conditions at the top and bottom of the NPs: the NP arrays were treated as a homogeneous medium with an effective permittivity according to the effective medium approximation model. In contrast, the dip positions of the FDTD-simulated spectra for 70-, 100-, and 130-nm-diameter NP arrays deviated from the results of interference calculations, particularly for short wavelengths. This suggested that Mie resonances in individual NPs significantly increased the absorption cross-section at the resonant wavelengths, which was sensitive to the refractive index of the surrounding medium (i.e., the wetting state). Optical reflectance measurements provide an easy and efficient means of inspecting the wetting behavior of non-flat surfaces.
- Magnetism, Spintronics, and Superconductivity
118(2015); http://dx.doi.org/10.1063/1.4936838View Description Hide Description
We report the results of comparative analysis of magnetic properties of the systems based on ε-Fe2O3, nanoparticles with different average sizes (from ∼3 to 9 nm) and dispersions. The experimental data for nanoparticles higher than 6–8 nm in size are consistent with the available data, specifically, the transition to the magnetically ordered state occurs at a temperature of ∼500 K and the anomalies of magnetic properties observed in the range of 80–150 K correspond to the magnetic transition. At the same time, Mőssbauer and ferromagnetic resonance spectroscopy data as well as the results of static magnetic measurements show that at room temperature all the investigated samples contain ε-Fe2O3 particles that exhibit the superparamagnetic behavior. It was established that the magnetic properties of nanoparticles significantly change with a decrease in their size to ∼6 nm. According to high-resolution electron microscopy and Mőssbauer spectroscopy data, the particle structure can be attributed to the ε–modification of trivalent iron oxide; meanwhile, the temperature of the magnetic order onset in these particles is increased, the well-known magnetic transition in the range of 80–150 K does not occur, the crystallographic magnetic anisotropy constant is significantly reduced, and the surfacemagnetic anisotropy plays a decisive role. This is apparently due to redistribution of cations over crystallographic positions with decreasing particle size, which was established using Mössbauer spectra. As the particle size is decreased and the fraction of surface atoms is increased, the contribution of an additional magnetic subsystem formed in a shell of particles smaller than ∼4 nm becomes significant, which manifests itself in the static magnetic measurements as paramagnetic contribution.
- Dielectrics, Ferroelectrics, and Multiferroics
Grains and grain boundaries contribution to dielectric relaxations and conduction of Bi5Ti3FeO15 ceramics118(2015); http://dx.doi.org/10.1063/1.4936782View Description Hide Description
Dielectric relaxation behaviors of Aurivillius Bi5Ti3FeO15ceramics were investigated in a wide range of frequency and temperature via dielectric and impedance spectroscopies. We distinguished two dielectric relaxations using the combination of impedance and modulus analysis. Resistance of the grain boundary was found to be much larger than grains, whereas capacitance was at the same level. The kinetic analysis of dielectric data was carried out to evaluate the contributions of microstructure and defects to the relaxation and conduction. The possible relaxation-conduction mechanism in the ceramics was discussed. The results enable deep understanding of microstructure-defect-relaxation behaviors in Bi5Ti3FeO15ceramics.
- Organic-Inorganic Systems, including Organic Electronics
118(2015); http://dx.doi.org/10.1063/1.4936367View Description Hide Description
We present a systematic device model that reproduces the important features of bulk heterojunction organic solar cells. While examining the model outputs we find that one of the limiting factors in organic solar cells is the reduced built-in potential due to effective pinning of the electrodes relative to the energy gap at the bulk of the device. Having identified this as a problem we suggest a device structure that can enhance the open circuit voltage. Our detailed modeling shows that such a structure can enhance the open circuit voltage as well as the short circuit current leading to above 40% improvement in power conversion efficiency of state of the art organic solar cells.
- Physics of Semiconductors
Charge movement in a GaN-based hetero-structure field effect transistor structure with carbon doped buffer under applied substrate bias118(2015); http://dx.doi.org/10.1063/1.4936780View Description Hide Description
Charge trapping and transport in the carbondopedGaN buffer of a GaN-based hetero-structure field effect transistor (HFET) has been investigated under both positive and negative substrate bias. Clear evidence of redistribution of charges in the carbondoped region by thermally generated holes is seen, with electron injection and capture observed during positive bias. Excellent agreement is found with simulations. It is shown that these effects are intrinsic to the carbondopedGaN and need to be controlled to provide reliable and efficient GaN-based power HFETs.
118(2015); http://dx.doi.org/10.1063/1.4936551View Description Hide Description
Alloys from ZnO and ZnS have been synthesized by radio-frequency magnetron sputtering over the entire alloying range. The ZnO1−xSxfilms are crystalline for all compositions. The optical absorption edge of these alloys decreases rapidly with small amount of added sulfur (x ∼ 0.02) and continues to red shift to a minimum of 2.6 eV at x = 0.45. At higher sulfur concentrations (x > 0.45), the absorption edge shows a continuous blue shift. The strong reduction in the band gap for O-rich alloys is the result of the upward shift of the valence-band edge with x as observed by x-ray photoelectron spectroscopy. As a result, the room temperature bandgap of ZnO1−xSx alloys can be tuned from 3.7 eV to 2.6 eV. The observed large bowing in the composition dependence of the energy bandgap arises from the anticrossing interactions between (1) the valence-band of ZnO and the localized sulfur level at 0.30 eV above the ZnO valence-band maximum for O-rich alloys and (2) the conduction-band of ZnS and the localized oxygen level at 0.20 eV below the ZnSconduction band minimum for the S-rich alloys. The ability to tune the bandgap and knowledge of the location of the valence and conduction-band can be advantageous in applications, such as heterojunction solar cells, where band alignment is crucial.