bluuebunny/crossref_metadata_2025 huggingface dataset:
Understanding field angle for heat assisted magnetic recording via dynamic modeling
Authors: Wang Yiming , Zhu Jian-Gang
Published: March 2011

Heat assisted magnetic recording (HAMR) is one of the potential candidates for future hard disk drives. In order to identify critical factors in HAMR recording, a combined micromagnetic-thermal-FEM modeling is used to simulate the dynamic recording process. Based on simulation results, the recording field angle is one of the critical parameters directly affecting recording at high linear densities and high data rates. The higher field angle is necessary to ensure sufficient magnetization switching speed since the switching needs to occur before medium anisotropy field increases to beyond the recording field strength during cooling. The modeling study found the medium Gilbert damping constant is also a critical parameter for the same physical reason.
Current driven oscillation and switching in Co/Pd perpendicular giant magnetoresistance multilayer
Authors: Sim C. H. , Lua S. Y. H. , Liew T. , Zhu J.-G.
Published: March 2011

Spin torque transfer (STT) induced magnetization oscillation and switching in metallic spin valves with Co/Pd electrodes of perpendicular magnetic anisotropy are demonstrated. The bottom Co/Pd multilayer, acting as a perpendicular spin-polarizing/reference layer, is relatively thick with a strong perpendicular anisotropy and a perpendicular switching field of 8 kOe. An in-plane spin valve is placed on the top for reading back magnetization oscillation of the middle Co layer, whose thickness is varied from 6 to 30 Å. When the middle Co layer is thin, current driven magnetization switching is observed. When the middle Co layer is relatively thick, perpendicular spin torque oscillation is clearly observed with oscillating frequency at 4 GHz. STT-included micromagnetic modeling has been performed which predicts the exact observed behavior and illustrates the significance of magnetization configuration of the Co layer on determining STT-induced dynamics.
Giant negative thermal expansion in antiperovskite manganese nitrides
Authors: Hamada T. , Takenaka K.
Published: March 2011

Giant negative thermal expansion (NTE), over α = −30 × 10−6 K−1 (α: coefficient of linear thermal expansion), at room temperature can be achieved in Mn3ZnN-based antiperovskite manganese nitrides by simultaneous substitution of C and B for N as well as Sn for Zn. The developed NTE nitrides show larger negative α, although the width of the operating-temperature window is comparable to that of Mn3CuN-based materials developed to date. Such a large, isotropic, and high-stiffness NTE material can compensate for the large positive thermal expansion of, for example, even aluminum or plastic materials.
Nanomechanical torsional resonator torque magnetometry (invited)
Authors: Davis J. P. , Vick D. , Li P. , Portillo S. K. N. , Fraser A. E. , Burgess J. A. J. , Fortin D. C. , Hiebert W. K. , Freeman M. R.
Published: March 2011

Micromechanical resonators are very useful for detection of magnetic torque. We have developed nanoscale torsional resonators fabricated within silicon nitride membranes, as a platform for magnetometry of nanoscale magnetic elements. We describe the rotational magnetic hysteresis of a 10 nm thick film deposited on a resonator, and a study of magnetic hysteresis in a single, 1 μm diameter permalloy disk. The torsional resonator is patterned using a dual beam scanning electron/focused ion system. For the 1 μm diameter disk, it is found to be possible to tune the conditions such that an apparent magnetic supercooling of vortex nucleation is observed, as would be suggested by the modified Landau theory of the C- to vortex-state switch as a first-order phase transition. Complementary transmission electron and Lorentz microscopy of the same structures have also been performed.
Magnetothermal effect in Gd3Rh
Authors: Kumar Pramod , Suresh K. G. , Nigam A. K.
Published: March 2011

In this paper, we report on the magnetic, heat capacity, and magnetocaloric effect (MCE) of the Gd3Rh compound. MCE has been calculated in terms of isothermal entropy change (ΔSM) and adiabatic temperature change (ΔTad) using the heat capacity data in different fields. The maximum values of ΔSM and ΔTad are 9.2 and ∼ 4.4 J/kg K, respectively, for a field change of 50 kOe,whereas the relative cooling power is ∼ 473 J/kg for the same field change.
Neutral beam dump with cathodic arc titanium gettering
Authors: Smirnov A. , Krivenko A. S. , Murakhtin S. V. , Savkin V. Ya. , Korepanov S. A. , Putvinski S.
Published: March 2011

An incomplete neutral beam capture can degrade the plasma performance in neutral beam driven plasma machines. The beam dumps mitigating the shine-through beam recycling must entrap and retain large particle loads while maintaining the beam-exposed surfaces clean of the residual impurities. The cathodic arc gettering, which provides high evaporation rate coupled with a fast time response, is a powerful and versatile technique for depositing clean getter films in vacuum. A compact neutral beam dump utilizing the titanium arc gettering was developed for a field-reversed configuration plasma sustained by 1 MW, 20–40 keV neutral hydrogen beams. The titanium evaporator features a new improved design. The beam dump is capable of handling large pulsed gas loads, has a high sorption capacity, and is robust and reliable. With the beam particle flux density of 5 × 1017 H/(cm2s) sustained for 3–10 ms, the beam recycling coefficient, defined as twice the ratio of the hydrogen molecular flux leaving the beam dump to the incident flux of high-energy neutral atoms, is ∼0.7. The use of the beam dump allows us to significantly reduce the recycling of the shine-through neutral beam as well as to improve the vacuum conditions in the machine.
Thermal stability characterization of magnetic tunnel junctions using hard-axis magnetoresistance measurements
Authors: Upadhyaya Pramey , Amiri Pedram Khalili , Kovalev Alexey A. , Tserkovnyak Yaroslav , Rowlands Graham , Zeng Zhongming , Krivorotov Ilya , Jiang Hongwen , Wang Kang L.
Published: March 2011

The use of hard-axis magnetoresistance (MR) measurements for characterization of the device-level anisotropy field and thermal stability in CoFeB/MgO/CoFeB magnetic tunnel junctions is proposed and evaluated. We develop functional forms describing the hard-axis MR curves using a Stoner–Wohlfarth particle model, which are then used for fitting to the experimental curves to extract the free layer magnetic anisotropy field. The model accounts for nonidealities observed in the experimental MR curves in the form of asymmetry (with respect to applied fields), and linear drop in resistance at high fields. Micromagnetic simulations are used to identify the cause for these deviations and verify the presented model.
Rutherford backscattering oscillation in scanning helium-ion microscopy
Authors: Kostinski Sarah , Yao Nan
Published: March 2011

Scanning helium-ion microscopy (SHIM) yields high-resolution imaging and is capable of surface elemental analysis at the nanometer scale. Here we examine recently discovered SHIM backscattered intensity oscillations versus the target atomic number. These oscillations are contrary to the expected monotonic increase of the backscattered helium-ion (He+) rate with the atomic number of elemental samples. We explore the ion-sample interaction via numerical simulations for a variety of scattering geometries and confirm the presence of oscillations. The oscillations are attributed to the atomic rather than the nuclear structure of the target. To that end, we study the link (near anticorrelation) between backscatter rate and He+ beam stopping power, both versus the target atomic number. This leads us to ascribe the origin of the backscatter oscillation to the “Z2-oscillations” of the stopping power in ion-beam physics, with the latter being rooted in the valence electron configuration of elemental targets.
Transport and magnetic properties of Cr-, Fe-, Cu-doped topological insulators
Authors: Choi Y. H. , Jo N. H. , Lee K. J. , Yoon J. B. , You C. Y. , Jung M. H.
Published: March 2011

We report a new class of three-dimensional topological insulators of transition metal doped Bi2Se3, TrxBi2Se3, and Bi2-xTrxSe3 (Tr = Cr, Fe, Cu) with x = 0.15, for the intercalation and substitution cases. With varying the doping atoms, a great variety of properties are observed in a single crystal form. The Cu-intercalated crystal shows superconducting behavior below 3 K, where the diamagnetic signal is found. The increase of carrier density at low temperature is likely to be responsible for the superconductivity. The magnetically doped case of the Fe-substituted Bi2Se3 exhibits dominance of ferromagnetic interactions, whereas the Cr-substituted Bi2Se3 favors antiferromagnetic interactions. From these results, we learn that the peculiar transport and magnetic properties of topological insulator Bi2Se3 are easily tunable by chemical doping elements, which change the Fermi energy and manipulate the charge carriers.
Structure and applied-field Mössbauer studies of NiCuZn ferrite
Authors: Hui Li Yong , Kim Woochul , Sung Kim Chul
Published: March 2011

Ni0.6Cu0.2Zn0.2Fe2O4 powder was synthesized by the solid-state reaction method. The crystal structure was found to be a cubic spinel with the lattice constant a0 of 8.373 Å and internal structure parameter (x) of the oxygen was 0.254. Ni and Cu ions in NiCuZn ferrite prefer octahedral sites B and Zn ions prefer tetrahedral sites A. Based on the distribution probability, we have analyzed Mössbauer spectra measured at 4.2 K as 5 sets with six-lines. Hyperfine fields of A and B sites at 4.2 K in zero magnetic field were Hhf(B0) = 547 kOe, Hhf(B1) = 532 kOe, Hhf(B2) = 519 kOe, Hhf(B3) = 491 kOe, and Hhf(A) = 507 kOe. Applied-field Mössbauer spectra of the Ni0.6Cu0.2Zn0.2Fe2O4 were measured with external field under 50 kOe, parallel to the incident γ-ray at 4.2 K. The hyperfine field Hhf(A) of A sites under 50 kOe was 521 kOe, larger than that under no applied-field. At B site the average value of hyperfine field 〈Hhf(B)〉 was 447 kOe, smaller than 〈Hhf(B)〉 = 522 kOe under no applied-field. Also, we noticed that the second and fifth absorption lines of Mössbauer spectra completely disappeared above 10 kOe, indicating that the spins of Fe ions at A and B sites were collinear to the applied-field. The Fe valence state was determined to be ferric from the isomer shift values.
Hall sign reversal and electrical transport properties of single crystalline SrPd2Ge2
Authors: Sung N. H. , Jo Y. J. , Cho B. K.
Published: March 2011

In this study, Hall effects, magnetoresistance, and low temperature resistivity of single crystalline SrPd2Ge2 were measured. The sign change of the Hall coefficient from negative to positive with increasing temperature in the normal state and the violation of Kohler’s rule in magnetoresistance data were observed. These results suggest that electrons and holes would contribute together to the formation of multi bands and that the scattering on the Fermi surface is anisotropic, which should be taken into account for the transport properties of SrPd2Ge2. In addition, we estimated the upper critical field at T = 0 K, Hc2(0) = 6.1 kOe, from the temperature-dependent electrical resistivity measurement down to 0.35 K. Including this, we discuss the nature of SrPd2Ge2 single crystals.
Precise control of domain wall injection and pinning using helium and gallium focused ion beams
Authors: Franken J. H. , Hoeijmakers M. , Lavrijsen R. , Kohlhepp J. T. , Swagten H. J. M. , Koopmans B. , van Veldhoven E. , Maas D. J.
Published: March 2011

In experiments on current-driven domain wall (DW) motion in nanostrips with perpendicular magnetic anisotropy (PMA), the initial DW preparation is usually not well controlled. We demonstrate precise control of DW injection using Ga and novel He focused ion beam (FIB) irradiation to locally reduce the anisotropy in part of a Pt/Co/Pt strip. DWs experience pinning at the boundary of the irradiated area. This DW pinning is more pronounced at the He irradiation boundary compared to Ga. This is attributed to a better He beam resolution, causing an anisotropy gradient over a much smaller length scale and hence, a steeper energy barrier for the DW. The results indicate that He FIB is a useful tool for anisotropy engineering of magnetic devices in the nanometer range.
Thermoelectric power of Gd4(Co1-xCux)3 compounds
Authors: Seixas T. M. , Salgueiro da Silva M. A. , Braun H. F. , Eska G.
Published: March 2011

We report results of thermoelectric power measurements (S(T)) in ferrimagnetic Gd4(Co1-xCux)3 compounds with x = 0.05, 0.1, 0.2, and 0.3, in the temperature range 8–300 K. Whereas in Gd4Co3, S(T) is always negative, for x > 0 the substitution of Co for Cu gives rise to the appearance of a low temperature positive maximum in S(T) at around 30 K. Based on our previous study of these compounds, we argue that this maximum in S(T) originates from electron-magnon scattering and is sensitive to electron band structure changes resulting from the substitution of Co for Cu and the accompanying reduction in the ratio between the electron-magnon and the electron-phonon scattering strengths. The decreasing role of Co 3d electrons with the progressive substitution of Co for Cu, evidenced by a strong reduction in the spin disorder resistivity and the Co magnetic moment, is seen to be crucial for the existence of such a low temperature maximum in S(T) for x > 0.
Electron mediated/enhanced ferromagnetism in a hydrogen-annealed Mn:Ge magnetic semiconductor
Authors: Dung Dang Duc , Yun Won Seok , Hwang Younghun , Feng Wuwei , Hong Soon Cheol , Cho Sunglae
Published: March 2011

We report on the carrier type changes of the p-type for as-grown Mn:Ge films to n-type for post-annealed samples in a hydrogen ambient. The hydrogen-annealed samples exhibit the increased Curie temperature, from 165 to 198 K, and the enhanced magnetic moment, from 0.78 to 1.10 μB/Mn. The first principles calculation using the all-electron full-potential linearized augmented plane wave method indicates that the addition of an electron carrier strengthens the ferromagnetic coupling between the Mn atoms, while the hole carrier caused it to weaken.
Tunable, continuous-wave Terahertz photomixer sources and applications
Authors: Preu S. , Döhler G. H. , Malzer S. , Wang L. J. , Gossard A. C.
Published: March 2011

This review is focused on the latest developments in continuous-wave (CW) photomixing for Terahertz (THz) generation. The first part of the paper explains the limiting factors for operation at high frequencies ∼ 1 THz, namely transit time or lifetime roll-off, antenna (R)-device (C) RC roll-off, current screening and blocking, and heat dissipation. We will present various realizations of both photoconductive and p-i-n diode–based photomixers to overcome these limitations, including perspectives on novel materials for high-power photomixers operating at telecom wavelengths (1550 nm). In addition to the classical approach of feeding current originating from a small semiconductor photomixer device to an antenna (antenna-based emitter, AE), an antennaless approach in which the active area itself radiates (large area emitter, LAE) is discussed in detail. Although we focus on CW photomixing, we briefly discuss recent results for LAEs under pulsed conditions. Record power levels of 1.5 mW average power and conversion efficiencies as high as 2 × 10−3 have been reached, about 2 orders of magnitude higher than those obtained with CW antenna-based emitters. The second part of the paper is devoted to applications for CW photomixers. We begin with a discussion of the development of novel THz optics. Special attention is paid to experiments exploiting the long coherence length of CW photomixers for coherent emission and detection of THz arrays. The long coherence length comes with an unprecedented narrow linewidth. This is of particular interest for spectroscopic applications, the field in which THz research has perhaps the highest impact. We point out that CW spectroscopy systems may potentially be more compact, cheaper, and more accurate than conventional pulsed systems. These features are attributed to telecom-wavelength compatibility, to excellent frequency resolution, and to their huge spectral density. The paper concludes with prototype experiments of THz wireless LAN applications. For future telecommunication systems, the limited bandwidth of photodiodes is inadequate for further upshifting carrier frequencies. This, however, will soon be required for increased data throughput. The implementation of telecom-wavelength compatible photomixing diodes for down-conversion of an optical carrier signal to a (sub-)THz RF signal will be required.
Bias dependence of spin injection into GaAs from Fe, FeCo, and (Ga,Mn)As contacts
Authors: Endres B. , Hoffmann F. , Wolf C. , Einwanger A. , Utz M. , Schuh D. , Woltersdorf G. , Ciorga M. , Weiss D. , Back C. H. , Bayreuther G.
Published: March 2011

Spin injection from Fe(001) and (Ga,Mn)As(001) into n-GaAs(001) was investigated using a method which provides two-dimensional cross-sectional images of the spin polarization in GaAs. While the distribution of the spin polarization below the injecting contact is nearly uniform for (Ga,Mn)As, a strong confinement near the contact edge is observed for Fe and FeCo. The spin polarization in GaAs changes sign when the injected current is reversed. Multiple sign reversals as a function of bias voltage as reported previously for Fe injectors are not observed with (Ga,Mn)As and Fe contacts grown on clean n++−GaAs in agreement with earlier results for an epitaxial FeCo injector.
Sound Generation Using a Magnetostrictive Microactuator
Authors: S. Albach Thorsten , Horn Peter , Sutor Alexander , Lerch Reinhard
Published: March 2011

In this paper, we present the design and performance of a MEMS-device based on the magnetostrictive effect, which can be used as a micro-loudspeaker. The device basically consists of a comb structure of monomorph bending cantilevers with an active area up to 3.0×2.5 mm2. It produces a sound-pressure-level up to 101 dB at 400 Hz in a standard 2 ccm measurement volume. We show our measurement setup as well as a mechanic-acoustic-coupled lumped element model to calculate sound pressure. The model incorporates finite element results for mechanical behavior. Measurement results validate our model assumptions.
Electron states and magnetic moments in Co/Ni(111) multilayers and overlayers
Authors: Gimbert F. , Calmels L.
Published: March 2011

Single crystal Co/Ni(111) superlattices with perpendicular magnetic anisotropy could give access to interesting magnetic memories switched with a rather small spin-torque current. The magnetic behavior of Co/Ni(111) superlattices is not perfectly understood in terms of the electronic structure. We report the existence of minority spin quantum-well and surface states in Ni/Co/Ni(111) sandwiches and Co/Ni(111) overlayers, and we show that the number of these states depends on the Co layer thickness. In particular, we find quantum-well states with dxz + dyz symmetry above Fermi level that can be measured by inverse photoemission spectroscopy and states with dxy + dx2−y2 symmetry below the Fermi level. The spin magnetic moment increases at the interfaces or at the surface.
Transient (000)-order attenuation effects in ultrafast transmission electron diffraction
Authors: Ligges Manuel , Rajković Ivan , Streubühr Carla , Brazda Thorsten , Zhou Ping , Posth Oliver , Hassel Christoph , Dumpich Günter , Linde Dietrich von der
Published: March 2011

We discuss the observation of a transient (000)-order attenuation in time-resolved transmission electron diffraction experiments. It is shown that this effect causes a decrease of the diffraction intensity of all higher diffraction orders. This effect is not unique to specific materials as it was observed in thin Au, Ag and Cu films.
Influence of high-frequency vibration on the Rayleigh–Marangoni instability in a two-layer system
Authors: Liu Qiusheng , Liu Rong
Published: March 2011

The influences of high-frequency vibrations on the Rayleigh–Marangoni instability in a two-layer system are investigated theoretically in the framework of the averaging method. We focus on the effects of vertical and horizontal vibrations on the stability of different convection modes. The results show that vertical vibrations significantly stabilize the system, while horizontal vibrations significantly destabilize it. In the presence of vertical vibrations, instability only occurs in a system heated from below. However, in the presence of horizontal vibrations, instability can also occur in a system cooled from below. When Marangoni effect is dominant at the interface, it is found that there are four types of coupling modes. The oscillatory convection is the result of the competition between different modes. In the presence of Marangoni effect at the interface, the structure of the interfacial flow is complicated. In some cases, small counter-rolls may develop to preserve the nonslip condition of fluids in either the upper layer or the lower layer.
Neutron diffraction study of the coupling between spin, lattice, and structural degrees of freedom in 0.8BiFeO3-0.2PbTiO3
Authors: Ranjan Rajeev , Kothai V. , Senyshyn Anatoliy , Boysen Hans
Published: March 2011

A powder neutron diffraction study was carried out on 0.8BiFeO3-0.2PbTiO3 in the temperature range 27–1000°C. The system exhibits magnetic transition at ∼300°C and a rhombohedral (R3c)-cubic (Pm3m) ferroelectric phase transition at ∼650°C. Anomalous variation in the lattice parameters and the octahedral tilt angle is observed across the magnetic transition temperature. In the magnetic phase, the c parameter is contracted and the octahedral tilt angle is slightly increased. The results suggest coupling between the spin, lattice and structural degrees of freedom.
Electron energy loss spectroscopy of ZnO nanocrystals with different oxygen vacancy concentrations
Authors: Dileep K. , Panchakarla L. S. , Balasubramanian K. , Waghmare U. V. , Datta R.
Published: March 2011

ZnO nanocrystals with different oxygen vacancy concentrations were characterized by high-resolution electron energy loss spectroscopy (HREELS). ZnO nanocrystals show a decrease in green emission with increasing annealing temperature in an oxygen environment and which significantly quenches in a sample annealed at 400 °C. O K and Zn L3 pre-edge absorption structures of ZnO nanocrystals were studied by HREELS as a function of annealing temperature. Absorption edge peak broadening and variation in pre-edge absorption edge structure of Zn L3 were observed in experimental electron energy loss spectroscopy spectra with different oxygen defect concentrations. All electron density functional theory (DFT) based (WIEN2k) calculation of electronic density of states and electron energy loss near edge structure were carried out with different oxygen vacancy concentrations and compared with experimental observations. Appearance of additional peaks in pre-edge electron energy loss structure with increasing oxygen vacancy is shown to be due to the oxygen defect states in the bandgap just below the conduction band.
Use of trapezoidal waves and complementary static fields incident on magnetic nanoparticles to induce magnetic hyperthermia for therapeutic cancer treatment
Authors: Morgan Sean M. , Sohn Hweerin , Victora R. H.
Published: March 2011

Magnetic hyperthermia for treatment of tumors would benefit greatly from increased heating of the superparamagnetic particles, coupled with reduced incident electromagnetic wave power. Previous micromagnetic simulations based on the Landau–Lifshitz–Gilbert equation with thermal fluctuations showed that the use of incident square waves greatly increases the normalized heat. Experimentally generating a square waveform may produce a trapezoidal waveform due to an inherent rise and fall time. It is found that the normalized heating power given by this wave shows a 30% improvement over the sinusoidal waveform for an anisotropy distribution of 20%. A static magnetic field can increase the effectiveness of a sinusoidal waveform, but gives little added benefit to the effectiveness of a square waveform.
Selol-loaded magnetic nanocapsules: A new approach for hyperthermia cancer therapy
Authors: Falqueiro A. M. , Primo F. L. , Morais P. C. , Mosiniewicz-Szablewska E. , Suchocki P. , Tedesco A. C.
Published: March 2011

Polylactic-co-glycolic nanocapsules, loaded with nanosized magnetic particles and Selol (a selenium-based anticancer drug), were successfully prepared by the precipitation method. Maghemite (γ-Fe2O3) nanoparticles were incorporated into the nanocapsules using a highly stable ionic magnetic fluid sample. The obtained nanocapsules presented no agglomeration, negative surface charge while revealing a narrow monomodal size distribution. All the nanocapsule formulations exhibited a good physical stability at 4 °C during 3 month storage period. The in vitro antitumoral activity of Selol-magnetic nanocapsules was assessed using a murine melanoma cell line. The influence of nanocapsules on cell viability was investigated by spectrophotometric assay. The results demonstrated that Selol-loaded magnetic nanocapsules (at 100 μg/ml/5 × 109 particle/ml) showed antitumoral activity of 50% on melanoma cells (absence of magnetic field). These results clearly indicate that the loaded nanocapsules represent a novel and promising magnetic drug delivery system suitable for cancer treatment via the active drug and magnetohyperthermia.
Note: An easy way to enable total internal reflection-fluorescence correlation spectroscopy (TIR-FCS) by combining commercial devices for FCS and TIR microscopy
Authors: Yordanov Stoyan , Best Andreas , Weisshart Klaus , Koynov Kaloian
Published: March 2011

Total internal reflection-fluorescence correlation spectroscopy (TIR-FCS) is a powerful method for studying dynamic processes at liquid–solid interfaces that may have numerous applications in biology, physics, and material science. Despite of its power and versatility, however, the use of TIR-FCS is still rather limited. The main reason for this is the need of a complex, in-house constructed optical setup whose assembly and adjustment is a quite difficult task. Clearly, the availability of ready to use, commercial TIR-FCS setups will strongly boost the application of this important method in many research areas. In this note we show that although such setups are still not available in the market, a proper combination of commercial devices for confocal fluorescence correlation spectroscopy and for total internal reflection microscopy may enable TIR-FCS in a way that do not require any special optical alignments. Furthermore, we demonstrate the capabilities of the setup by measuring the diffusion coefficient of single dye molecule and quantum dots in the very proximity of a water–glass interface.
Bandgap shift by quantum confinement effect in 〈100〉 Si-nanowires derived from threshold-voltage shift of fabricated metal-oxide-semiconductor field effect transistors and theoretical calculations
Authors: Yoshioka Hironori , Morioka Naoya , Suda Jun , Kimoto Tsunenobu
Published: March 2011

Si-nanowire (Si-NW) MOSFETs, the cross-sectional size (square root of the cross-sectional area of NWs) of which was changed from 18 to 4 nm, were fabricated and characterized. Both n- and p-channel MOSFETs have shown a nearly ideal subthreshold swing of 63 mV/decade. The threshold voltage of n-/p-channel MOSFETs has gradually increased/decreased with decreasing the cross-sectional size. The bandgap shift from bulk Si has been derived from the threshold-voltage shift. The bandgap of Si-NWs was calculated by a density functional theory, tight binding method, and effective mass approximation. The calculated bandgap shows good agreement with that derived from threshold voltage. The theoretical calculation indicates that the bandgap is dominated by the cross-sectional size (area) and is not very sensitive to the shape within the aspect-ratio range of 1.0-2.5.
Doping type and thickness dependence of band offsets at the amorphous/crystalline silicon heterojunction
Authors: Korte L. , Schmidt M.
Published: March 2011

We conduct a systematic investigation of the valence band offset ΔEv for amorphous/crystalline silicon heterojunctions (a-Si:H/c-Si) using low-energy photoelectron spectroscopy in the constant final state mode. The dependence of ΔEv on a-Si:H thickness as well as on the possible combinations of c-Si substrate and a-Si:H film doping types are explored. ΔEv is found to be independent of both substrate and film doping and amounts to ΔEv¯=0.458(6) eV, averaged over all doping combinations and thicknesses, with a systematic error of 50–60 meV. A slight but statistically significant dependency of ΔEv on the a-Si:H film thickness may be explained by a changing interface dipole due to variations in dangling bond saturation during a-Si:H growth.
Poloidally and radially resolved parallel D+ velocity measurements in the DIII-D boundary and comparison to neoclassical computations
Authors: Boedo J. A. , Belli E. A. , Hollmann E. , Solomon W. M. , Rudakov D. L. , Watkins J. G. , Prater R. , Candy J. , Groebner R. J. , Burrell K. H. , deGrassie J. S. , Lasnier C. J. , Leonard A. W. , Moyer R. A. , Porter G. D. , Brooks N. H. , Muller S. , Tynan G. , Unterberg E. A.
Published: March 2011

First measurements of the D+ parallel velocity, V∥D+, in L-mode discharges in the DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] tokamak boundary region at two poloidal locations, θ∼0° and θ∼255°, made using Mach probes, feature a peak with velocities of up to 80 km/s at the midplane last closed flux surface (LCFS), as high as ten times the charge exchange recombination C6+ toroidal velocity, VϕC6+, in the same location. The V∥D+ profiles are very asymmetric poloidally, by a factor of 8–10, and feature a local peak at the midplane. This peak, 1–2 cm wide, is located at or just inside the LCFS, and it suggests a large source of momentum in that location. This momentum source is quantified at ∼0.31 N m by using a simple momentum transport model. This is the most accurate measurement of the effects of so called “intrinsic” edge momentum source to date. The V∥D+ measurements are quantitatively consistent with a purely neoclassical computational modeling of V∥D+ by the code NEO [E. A. Belli and J. Candy, Plasma Phys. Controlled Fusion 50, 095010 (2008)], using VϕC6+ as input, for ρ∼0.7–0.95 at the two poloidal locations, where V∥D+ measurements exist. The midplane NEO-calculated V∥D+ grows larger than V∥C6+ in the steeper edge gradient region and trends to agreement with the probe-measured V∥D+ data near ρ∼1, where the local V∥D+ velocity peak exists. The measurements and computations were made in OH and L-mode discharges on an upper single null, with ion ∇BT drift away from the divertor. The rotating layer finding is similar in auxiliary heated discharges with and without external momentum input, except that at higher density the edge velocity weakens.
Determination of bandgap states in p-type In0.49Ga0.51P grown on SiGe/Si and GaAs by deep level optical spectroscopy and deep level transient spectroscopy
Authors: González M. , Carlin A. M. , Dohrman C. L. , Fitzgerald E. A. , Ringel S. A.
Published: March 2011

The presence and properties of traps in p-type In0.49Ga0.51P grown on low dislocation density, metamorphic Ge/SiGe/Si substrates and GaAs substrates were determined using deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) leading to the quantification of trap behavior throughout the entire 1.9 eV bandgap of the In0.49Ga0.51P material as a function of substrate. Thermal emission-based DLTS revealed a single hole trap at Ev + 0.71 eV for growth on both lattice matched and mismatched substrates with similar concentrations. Complementary, optical emission-based DLOS measurements detected bandgap states at Ev + 1.18 eV, Ev + 1.36 eV, and Ev + 1.78 eV for p-type In0.49Ga0.51P grown on both substrate types. The total concentration of the DLOS-detected states was found to comprise approximately 80% of the entire trap concentration in p-type In0.49Ga0.51P bandgap. This relatively high concentration of above midgap levels may be of great significance for minority carrier devices that utilize p-type In0.49Ga0.51P (such as high efficiency III–V multijunction solar cells) since their position in the bandgap and high concentrations suggest that strong minority carrier electron trapping behavior can be expected. The primary effect of substituting the GaAs substrate by Ge/SiGe/Si is to increase the concentration of these states by a factor of 2–3, with no additional levels detected due to the replacement by the Si-based substrates, indicating that all detected traps are native to the epitaxial In0.49Ga0.51P material (regardless of the substrate), but whose concentrations appear to be influenced by dislocation density.
Tuning the electrical conductivity of Pt-containing granular metals by postgrowth electron irradiation
Authors: Porrati F. , Sachser R. , Schwalb C. H. , Frangakis A. S. , Huth M.
Published: March 2011

We fabricated Pt-containing granular metals by focused electron beam–induced deposition from the (CH3)3CH3C5H4Pt precursor gas. The granular metals are made of platinum nanocrystallites embedded in a carbonaceous matrix. We exposed the as-grown nanocomposites to low-energy electron beam irradiation and measured the electrical conductivity as a function of irradiation dose. Postgrowth electron beam irradiation transforms the matrix microstructure and thus the strength of the tunneling coupling between Pt nanocrystallites. For as-grown samples (weak tunnel coupling regime) we find that the temperature dependence of the electrical conductivity follows the stretched exponential behavior characteristic of the correlated variable-range hopping transport regime. For briefly irradiated samples (strong tunnel coupling regime) the electrical conductivity is tuned across the metal-insulator transition. For long-time irradiated samples the electrical conductivity behaves like that of a metal. In order to further analyze changes of the microstructure as a function of the electron irradiation dose, we carried out transmission electron microscope (TEM), micro-Raman spectroscopy, and atomic force microscopy (AFM) investigations. TEM pictures reveal that crystallite size in long-time irradiated samples is larger than that in as-grown samples. Furthermore, we do not have evidence of microstructural changes in briefly irradiated samples. By means of micro-Raman spectroscopy we find that by increasing the irradiation dose the matrix changes, following a graphitization trajectory between amorphous carbon and nanocrystalline graphite. Finally, by means of AFM measurements we observe a reduction of the volume of the samples with increasing irradiation time, which we attribute to the removal of carbon molecules.
Correlation of magnetic domains and magnetostrictive strains in Terfenol-D via magnetic force microscopy
Authors: Xie S. H. , Liu X. Y. , Zhou Y. C. , Li J. Y.
Published: March 2011

Local evolution of magnetic domain structures in Terfenol-D under variable magnetic fields is characterized using magnetic force microscopy and the resulting magnetostrictive strains parallel and perpendicular to the applied magnetic field are simultaneously measured. It is found that both magnetic domain structure and magnetostrictive response depend upon the loading history and a substantial change in magnetic domains is observed for Terfenol-D under opposite magnetic field, resulting in notable asymmetry in magnetostrictive strain. The technique will be useful in characterizing the evolution of magnetic domains for a wide range of ferromagnetic materials under an external field.
Linear and nonlinear optical absorption coefficients and refractive index changes in a two-electron quantum dot
Authors: Lu Liangliang , Xie Wenfang , Hassanabadi Hassan
Published: March 2011

Analytical expressions of the optical absorption coefficients and refractive index changes in a two-electron quantum dot with oscillating and linear confining terms are obtained by using compact-density matrix approach and exact analytical method. Numerical results on typical GaAs/AlGaAs materials show that, an increase of the confinements blueshifts the peak positions of absorption coefficients and refractive index changes. Additionally, an increase of the optical intensity and relaxation time considerably changes the absorption coefficients as well as the refractive index changes.
Interpolation by fast Wigner transform for rapid calculations of magnetic resonance spectra from powders
Authors: Stevensson Baltzar , Edén Mattias
Published: March 2011

We introduce a novel interpolation strategy, based on nonequispaced fast transforms involving spherical harmonics or Wigner functions, for efficient calculations of powder spectra in (nuclear) magnetic resonance spectroscopy. The fast Wigner transform (FWT) interpolation operates by minimizing the time-consuming calculation stages, by sampling over a small number of Gaussian spherical quadrature (GSQ) orientations that are exploited to determine the spectral frequencies and amplitudes from a 10–70 times larger GSQ set. This results in almost the same orientational averaging accuracy as if the expanded grid was utilized explicitly in an order of magnitude slower computation. FWT interpolation is applicable to spectral simulations involving any time-independent or time-dependent and noncommuting spin Hamiltonian. We further show that the merging of FWT interpolation with the well-established ASG procedure of Alderman, Solum and Grant [J. Chem. Phys. 134, 3717 (1986)] speeds up simulations by 2–7 times relative to using ASG alone (besides greatly extending its scope of application), and between 1–2 orders of magnitude compared to direct orientational averaging in the absence of interpolation. Demonstrations of efficient spectral simulations are given for several magic-angle spinning scenarios in NMR, encompassing half-integer quadrupolar spins and homonuclear dipolar-coupled 13C systems.
Elasticity of cubic boron nitride under ambient conditions
Authors: Zhang Jin S. , Bass Jay D. , Taniguchi Takashi , Goncharov Alexander F. , Chang Yun-Yuan , Jacobsen Steven D.
Published: March 2011

As a superhard material with properties similar to diamond including chemical inertness, cubic boron nitride (cBN) is an excellent candidate as a pressure calibration standard for high-temperature high-pressure research using its pressure-volume-temperature (P-V-T) equation of state. However, the elastic properties of cBN at ambient conditions reported in the literature vary by up to 8%, which can likely be attributed in part to variability in the cBN composition and defect structure and measurement uncertainties. We have measured the single-crystal elastic moduli of high-purity cBN with high precision by Brillouin scattering measurements, making an effort to minimize experimental uncertainties. We obtain values of C11 = 798.4 ± 1.7 GPa, C44 = 469.0 ± 1.0 GPa, and C12 = 172.4 ± 1.1 GPa, from which the isotropic aggregate bulk modulus Ks = 381.1 ± 1.3 GPa and shear modulus G = 398.8 ± 1.2 GPa (the Hill average) were calculated. Our results improve the precision and reduce the uncertainties in the elastic moduli of high-purity cBN as a reference for future high P-T pressure scales.
Propagation of terahertz waves in an atmospheric pressure microplasma with Epstein electron density profile
Authors: Yuan Chengxun , Zhou Zhongxiang , W. Zhang Jingwen , Xiang Xiaoli , Sun Hongguo , Wang He , Du Yanwei
Published: March 2011

Propagation properties of terahertz (THz) waves in a bounded atmospheric-pressure microplasma (AMP) are analyzed in this study. A modified Epstein profile model is used to simulate the electron density distribution caused by the plasma sheaths. By introducing the dielectric constant of a Drude–Lorentz model and using the method of dividing the plasma into a series of subslabs with uniform electron density, the coefficients of power reflection, transmission, and absorption are derived for a bounded microplasma structure. The effects of size of microplasma, electron density profile, and collision frequency on the propagation of THz waves are analyzed numerically. The results indicate that the propagation of THz waves in AMPs depend greatly on the above three parameters. It is demonstrated that the THz wave can play an important role in AMPs diagnostics; meanwhile, the AMP can be used as a novel potential tool to control THz wave propagation.
Broadband and omnidirectional absorption in heterostructures with a highly absorptive metallic film and a dielectric Bragg reflector
Authors: Du Gui-qiang , Zhang Li-wei , Jiang Hai-tao
Published: March 2011

We have found theoretically that nearly total absorption in a wideband of frequencies and a large range of angles of incidence for both polarizations can be realized in a heterostructure consisting of a highly absorptive metal film and a truncated photonic crystal. Furthermore, the width of the absorption band is broadened with increase of the incident angle for the transverse electric polarization. Such properties are significant for designing near-perfect broadband and wide-angle absorbers in the visible and near infrared regions.
Nonlinear density waves in the single-wave model
Authors: Marinov Kiril B. , Tzenov Stephan I.
Published: March 2011

The single-wave model equations are transformed to an exact hydrodynamic closure by using a class of solutions to the Vlasov equation corresponding to the waterbag model. The warm fluid dynamic equations are then manipulated by means of the renormalization group method. As a result, amplitude equations for the slowly varying wave amplitudes are derived. Since the characteristic equation for waves has in general three roots, two cases are examined. If all the three roots of the characteristic equation are real, the amplitude equations for the eigenmodes represent a system of three coupled nonlinear equations. In the case where the dispersion equation possesses one real and two complex conjugate roots, the amplitude equations take the form of two coupled equations with complex coefficients. The analytical results are then compared to the exact system dynamics obtained by solving the hydrodynamic equations numerically.
Neoclassical electron transport calculation by using δf Monte Carlo method
Authors: Matsuoka Seikichi , Satake Shinsuke , Yokoyama Masayuki , Wakasa Arimitsu , Murakami Sadayoshi
Published: March 2011

High electron temperature plasmas with steep temperature gradient in the core are obtained in recent experiments in the Large Helical Device [A. Komori et al., Fusion Sci. Technol. 58, 1 (2010)]. Such plasmas are called core electron-root confinement (CERC) and have attracted much attention. In typical CERC plasmas, the radial electric field shows a transition phenomenon from a small negative value (ion root) to a large positive value (electron root) and the radial electric field in helical plasmas are determined dominantly by the ambipolar condition of neoclassical particle flux. To investigate such plasmas’ neoclassical transport precisely, the numerical neoclassical transport code, FORTEC-3D [S. Satake et al., J. Plasma Fusion Res. 1, 002 (2006)], which solves drift kinetic equation based on δf Monte Carlo method and has been applied for ion species so far, is extended to treat electron neoclassical transport. To check the validity of our new FORTEC-3D code, benchmark calculations are carried out with GSRAKE [C. D. Beidler et al., Plasma Phys. Controlled Fusion 43, 1131 (2001)] and DCOM/NNW [A. Wakasa et al., Jpn. J. Appl. Phys. 46, 1157 (2007)] codes which calculate neoclassical transport using certain approximations. The benchmark calculation shows a good agreement among FORTEC-3D, GSRAKE and DCOM/NNW codes for a low temperature (Te(0)=1.0 keV) plasma. It is also confirmed that finite orbit width effect included in FORTEC-3D affects little neoclassical transport even for the low collisionality plasma if the plasma is at the low temperature. However, for a higher temperature (5 keV at the core) plasma, significant difference arises among FORTEC-3D, GSRAKE, and DCOM/NNW. These results show an importance to evaluate electron neoclassical transport by solving the kinetic equation rigorously including effect of finite radial drift for high electron temperature plasmas.
Eu/RG absorption and excitation spectroscopy in the solid rare gases: State dependence of crystal field splitting and Jahn–Teller coupling
Authors: Byrne Owen , McCaffrey John G.
Published: March 2011

Absorption spectroscopy recorded for annealed samples of matrix-isolated atomic europium reveals a pair of thermally stable sites in Ar and Kr while a single site exists in Xe. Plots of the matrix shifts of the visible s → p bands versus host polarizability, allowed the association of the single site in Xe and the blue sites in Ar and Kr. On the basis of the similar ground state bond lengths expected for the Eu–rare gas (RG) diatomics and the known Na–RG molecules, the blue sites are attributed to Eu occupancy in the smaller tetra-vacancy while the red sites are proposed to arise from hexa-vacancy sites. Both sites are of cubic symmetry, consistent with the pronounced Jahn–Teller structure present on the y8P ← a8S7/2 transition for these bands in the three hosts studied. Site-selective excitation spectroscopy has been used to reanalyze complex absorption spectra previously published by Jakob et al. [Phys. Lett. A 57, 67 (1976)] for the near-UV f → d transitions. On the basis that a pair of thermally stable sites exist in solid argon, the occurrence of crystal field splitting has been identified to occur for the J ≥ 5/2 level of the 8P state when isolated in these two sites with cubic symmetry. From a detailed lineshape analysis, the magnitude of the crystal field splittings on the J = 5/2 level in Ar is found to be 105 and 123 cm−1 for the red and blue sites, respectively.
Plasmonic excitations in ZnO/Ag/ZnO multilayer systems: Insight into interface and bulk electronic properties
Authors: Philipp Martin , Knupfer Martin , Büchner Bernd , Gerardin Hadia
Published: March 2011

Electron energy-loss spectroscopy experiments in transmission were carried out on silver-based multi-layer systems, consisting of a silver layer of various thicknesses (8, 10 and 50 nm) sandwiched between two Al-doped ZnO layers. The films were produced by magnetron sputtering using potassium bromide single crystals as substrates. The electronic structure of these systems was probed and analyzed with respect to their plasmonic excitations, which can be basically split up into excitations of the electrons in the bulk silver and excitations at the ZnO:Al/Ag interface. A detailed examination of the momentum dependence of the plasmon peaks revealed a positive dispersion for both, the volume and the interface plasmon, where only for the first one a quadratic behavior (as expected for a free electron gas) could be observed. Furthermore, the peak width was analyzed and set into relation to electrical conductivity measurements by calculating the plasmon lifetime and the electron scattering rate. Here, a good agreement between these different methods was obtained.
Instability analysis of a programmed hydrogel plate under swelling
Authors: Zhang X. X. , Guo T. F. , Zhang Y. W.
Published: March 2011

We investigate the bifurcation instabilities of swelling circular hydrogel plates programed with a space-varying cross-linking density along the radial direction. Depending on the distribution of the cross-linking density and the aspect ratio of the plate, different buckling patterns are observed. When the cross-linking density is lower at the center and increases monotonically outwards, a global buckling occurs, and the hydrogel plate forms an axisymmetric bowl-shaped container. When the cross-linking density is higher at the center and decreases monotonically outwards, edge wrinkling occurs. These wrinkles can be either localized near the edge or penetrate deep into the plate, depending on the detailed distribution of the cross-linking density. The rich post-buckling morphologies exhibited by the programed hydrogel plate provide a powerful route to fabricate smart hydrogel components, such as, self-adjusting containers and microlenses.