bluuebunny/crossref_metadata_2025 huggingface dataset:
Interfacial resistance and spin-dependent scattering in the current-perpendicular-to-plane giant magnetoresistance using Co2Fe(Al0.5Si0.5) Heusler alloy and Ag
Authors: Nakatani T. M. , Furubayashi T. , Hono K.
Published: March 2011

We studied the interfacial spin-dependent scattering in Co2Fe(Al0.5Si0.5)/Ag/Co2Fe(Al0.5Si0.5) current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices. A large contribution of the interfacial scattering was evidenced by the small interfacial resistance of ARF/N ∼0.25 mΩμm2 and the large interfacial scattering spin asymmetries of γ ∼0.93 at 14 K and ∼0.77 at 290 K. Moreover, the insertions of 1 nm thick Co2Fe(Al0.5Si0.5) layers at Co50Fe50/Ag interfaces effectively improved the output of CPP-GMR. Thus, Ag is considered to be a suitable spacer material for the Co2Fe(Al0.5Si0.5) Heusler alloy with a good band matching.
Effect of Si and Ga substitutions on the magnetocaloric properties of NiCoMnSb quaternary Heusler alloys
Authors: Sahoo Roshnee , Nayak Ajaya K. , Suresh K. G. , Nigam A. K.
Published: March 2011

The effect of Si and Ga substitutions on the magnetic and the magnetocaloric properties in Heusler based system Ni46Co4Mn38Sb12−xZx (Z = Si and Ga) has been studied. From the M(T) plots it is found that Si substitution stabilizes the austenite phase, whereas Ga substitution stabilizes the martensite phase. Strong metamagnetic behavior is observed in the M(H) isotherms for Si = 0.75 and 1, whereas such a behavior is absent in the Ga substituted alloys. Associated with magneto-structural transition, a large magnetocaloric effect (MCE) of 58 and 70 J/kg K is observed for x = 0.75 and 1, respectively, in the case of Si. Though the MCE observed in x = 0.5 and 1 in the case of Ga is much lower, the MCE peak is found to be quite broad.
Hysteresis, d*33 and d33 of Fe81.6Ga18.4 textured polycrystals
Authors: Restorff J. B. , Wun-Fogle M. , Summers Eric
Published: March 2011

Measurements of d33* (major and minor loops) and d33 (major loops) were made on three Fe81.6Ga18.4 (Galfenol) highly textured polycrystalline rods, one of which had a small amount of 1002 steel substituted for the iron. Seven ΔT’s between ±25 and ± 0.5 MPa at 100 Oe were used for the minor loops. These measurements are useful for energy harvesting, which is the generation of energy by utilizing vibrations present in the environment. For magnetostrictive materials d33*=∂B/∂THis the relevant parameter, where B is the magnetic flux density, T is the stress and H is held constant. Since real energy harvesting devices will not have active H control, ∂B/∂Twas also measured where H was not held constant. The d33*’s and d33’s (d33=∂S/∂HT) for both binary samples with ΔT ≥ ±5 MPa were well fit by a Gaussian plus a constant. Peak amplitudes for d33 and d33*were ∼80 and ∼60 nm/A, respectively. At smaller ΔT’s, d33*vs. T was flat or showed a broad peak. The steel containing sample had a larger cubic anisotropy which resulted in a double peaked d33 curve with a maximum amplitude of ∼100 nm/A. At the points where the d33* and d33 measurements overlap, the two d-coefficients are within 5 to 50% of one another at 100 Oe. The hysteresis in the S-T and B-H loops was ∼2 and ∼0.9 kJ/m3, respectively.
Fermi level spin polarization of polycrystalline thulium by point contact Andreev reflection spectroscopy
Authors: Stamenov P. , Coey J. M. D.
Published: March 2011

The spin polarization near the Fermi level in bulk polycrystalline ferrimagnetic thulium is investigated by means of point contact Andreev reflection (PCAR), in the temperature interval (1.9–9.2 K). The highest polarization measured is P = 0.41(8), with barrier parameter Z = 0.39(8), proximity region gap of Δ1 = 1.26(5) meV and an elevation of the electronic temperature of ΔTe = 3.3(8) K, for a lattice temperature of T = 2.20(5) K, and a contact conductance of G = 330(5) G0. Both polarization and barrier power are found to be constant within the experimental uncertainties in the entire accessible temperature interval and linearly independent, while the proximity gap was found to obey the standard BCS temperature dependence with a TcGap = 10.2(2) K, higher than the TcGap = 9.2 K of bulk niobium. Despite its low ordering temperature, thulium could be suitable candidate for low critical current spin-transfer-torque demonstrations.
Power enhancement of angular polarizer spin torque oscillator in magnetic tunnel junction
Authors: Zhang Yisong , Zhao Hui , Lyle Andrew , Wang Jian-Ping
Published: March 2011

The power angular dependence for spin torque oscillators (STOs) is systematically investigated in magnetic tunnel junctions by varying the in-plane polarizer angle (IPPA) which refers to the angle between the free and fixed layer. The polarizer angle is adjusted by integrating a lithography process and a post-annealing process. An increase of power with increasing IPPA is experimentally demonstrated. STOs with a 30° and 60° IPPA show a 2–3 and 5–6 times higher efficiency in the power spectral density of the main peak than that with the traditional 0° IPPA, respectively. This increase of the power efficiency with IPPA for STOs can be explained by the power dependence on the precession angle at different IPPA.
Magnetic mirror effects on a collisionless plasma in a convergent geometry
Authors: Martínez-Sánchez Manuel , Ahedo Eduardo
Published: March 2011

Several plasma thruster concepts, as well as ion engine chambers, use magnetic cusps to protect walls and to throttle electron flow to anodes. We present a kinetic model of the plasma in the vicinity of one cusp. Electrons, strongly confined by the electrostatic presheath and sheath, are assumed isotropic. Collisionless ions are either magnetically guided or completely nonmagnetized, thus bracketing conditions of interest. For magnetized ions, electrostatic and magnetic mirror forces compete, and the resulting self-consistent potential is found by imposing quasineutrality. A similar competition occurs for nonmagnetized ions, this time as a result of the convergence of equipotential lines. Analytical solutions are found for monoenergetic ions, and these are generalized to the case of an initially Maxwellian population, for which some numerical iteration is required. The presheath potential drop is in all cases of the order of 0.6–0.75 times the electron temperature, and ions enter the sheath at a sonic velocity, according to Bohm’s criterion. Contrary to intuition, the cusp does not reduce the ion flux (per unit area) to the wall, only the size of the wall area section that carries this flux by virtue of its connection to the distant plasma. These kinetic results are verified by checking the conservation of relevant moments of the ion distribution, including two new quantities that generalize the average magnetic moment and the total ion enthalpy by accounting for the nonzero ion heat fluxes.
Extraction of physical parameters from photoacoustic spectroscopy using wavelet transform
Authors: Li Jianghua , Chen Changshui , Zheng Chujun , Liu Songhao , Tang Zhilie
Published: March 2011

Photoacoustic (PA) signals carry much information related to the physical parameters of a sample, such as its optical, thermal, acoustic, and elastic properties. These physical parameters synchronously make their own contributions to PA signals. It is difficult to distinguish the contributions of different physical parameters to the mixed PA signals. To separate these parameters and to extract the featured information, we report herein a means of dissecting the transient PA signal using time-frequency analysis in a forward detection model. The simulated and experimental results indicate that the features of different physical parameters related to the sample can be qualitatively extracted from the mixed PA signals.
The effect of impurity-induced lattice strain and Fermi level position on low temperature oxygen diffusion in silicon
Authors: Zeng Zhidan , Murphy J. D. , Falster R. J. , Ma Xiangyang , Yang Deren , Wilshaw P. R.
Published: March 2011

Oxygen diffusion in silicon is known to be affected by high concentrations of impurities, although the mechanism underpinning this is poorly understood. We have studied oxygen transport in Czochralski silicon by analyzing data on the locking of dislocations by oxygen as a function of time and temperature. In this paper, we present new data from crystals grown to contain high levels of germanium and arsenic. We analyze these new data, together with our previous data for silicon with a high boron concentration, to further the understanding of the mechanism by which high impurity concentrations affect oxygen transport at temperatures at which the oxygen dimer dominates transport (up to 550 °C). Our results show that a high level of boron doping (∼3 × 1018cm−3) enhances the effective diffusivity of oxygen by a factor of ∼8 to ∼25 relative to low doped material with the same oxygen concentration. High levels of germanium doping (∼8 × 1019cm−3) and arsenic doping (∼2 × 1019cm−3) can both have a slight retardation effect on oxygen transport. The magnitude of the reduction measured is less than a factor of ∼4 in the heavily germanium doped specimens and less than a factor of ∼5 in the heavily arsenic doped specimens, and in most cases is significantly less than this. Germanium doping introduces considerable strain into the silicon lattice without affecting the Fermi level position, so data from these samples show that lattice strain affects oxygen dimer transport. The arsenic and boron doping levels in the materials studied give rise to lattice strain with a smaller magnitude and opposite sign to that in the germanium doped samples. It is therefore suggested that the Fermi level position also affects the transport of oxygen dimers.
Regular network model for the sea ice-albedo feedback in the Arctic
Authors: Müller-Stoffels Marc , Wackerbauer Renate
Published: March 2011

The Arctic Ocean and sea ice form a feedback system that plays an important role in the global climate. The complexity of highly parameterized global circulation (climate) models makes it very difficult to assess feedback processes in climate without the concurrent use of simple models where the physics is understood. We introduce a two-dimensional energy-based regular network model to investigate feedback processes in an Arctic ice-ocean layer. The model includes the nonlinear aspect of the ice-water phase transition, a nonlinear diffusive energy transport within a heterogeneous ice-ocean lattice, and spatiotemporal atmospheric and oceanic forcing at the surfaces. First results for a horizontally homogeneous ice-ocean layer show bistability and related hysteresis between perennial ice and perennial open water for varying atmospheric heat influx. Seasonal ice cover exists as a transient phenomenon. We also find that ocean heat fluxes are more efficient than atmospheric heat fluxes to melt Arctic sea ice.
Viscoelastic characterization of the retracting cytoskeleton using subcellular detachment
Authors: Yoon Sang-Hee , Lee Chan , Mofrad Mohammad R. K.
Published: March 2011

Characterization of the cytoskeleton’s viscoelastic properties is essential for understanding cellular dynamics in many biological functions of the cell, especially cell migration. The viscoelastic properties of the retracting NIH 3T3 fibroblasts are investigated here to shed light on the mechanics and dynamics of cell migration. A part of the cell is detached by electrochemically desorbing a gold-thiol self-assembled monolayer. The viscoelastic properties of the detached and retracting cytoskeleton are then estimated using time-sequential imaging combined with atomic force microscopy. The retracting cytoskeleton exhibits a threefold decrease in stiffness and a 30-fold increase in viscous damping capacity as compared to those in the adhered (stationary) cytoskeleton.
Enhanced spin Hall accumulation with two charge current vortices in the Landauer setup with four terminals
Authors: Chen Chien-Liang , Su Yu-Hsin , Chen Kuo-Wei , Chang Ching-Ray
Published: March 2011

We investigate the enhancement of intrinsic spin Hall accumulation in a two-dimensional electron gas with Rashba and Dresselhaus (001) spin-orbit coupling. Employing the real-space Landauer–Keldysh Green’s function formalism, we show that intrinsic spin Hall accumulation can be strongly enhanced at the specific energy point where the system develops two charge current vortices with certain chirality. We find that the localized vortex state is accompanied by a conductance dip arising from the Fano–Rashba effect. The spin-orbit force picture is used to analyze the enhanced spin accumulation patterns.
Dynamic phase transition from localized to spatiotemporal chaos in coupled circle map with feedback
Authors: Sonawane Abhijeet R. , Gade Prashant M.
Published: March 2011

We investigate coupled circle maps in the presence of feedback and explore various dynamical phases observed in this system of coupled high dimensional maps. We observe an interesting transition from localized chaos to spatiotemporal chaos. We study this transition as a dynamic phase transition. We observe that persistence acts as an excellent quantifier to describe this transition. Taking the location of the fixed point of circle map (which does not change with feedback) as a reference point, we compute a number of sites which have been greater than (less than) the fixed point until time t. Though local dynamics is high dimensional in this case, this definition of persistence which tracks a single variable is an excellent quantifier for this transition. In most cases, we also obtain a well defined persistence exponent at the critical point and observe conventional scaling as seen in second order phase transitions. This indicates that persistence could work as a good order parameter for transitions from fully or partially arrested phase. We also give an explanation of gaps in eigenvalue spectrum of the Jacobian of localized state.
Force field parameter estimation of functional perfluoropolyether lubricants
Authors: Smith Robert , Seung Chung Pil , Steckel Janice A. , Jhon Myung S. , Biegler Lorenz T.
Published: March 2011

The head disk interface in a hard disk drive can be considered to be one of the hierarchical multiscale systems, which require the hybridization of multiscale modeling methods with coarse-graining procedure. However, the fundamental force field parameters are required to enable the coarse-graining procedure from atomistic/molecular scale to mesoscale models. In this paper, we investigate beyond molecular level and perform ab initio calculations to obtain the force field parameters. Intramolecular force field parameters for Zdol and Ztetraol were evaluated with truncated PFPE molecules to allow for feasible quantum calculations while still maintaining the characteristic chemical structure of the end groups. Using the harmonic approximation to the bond and angle potentials, the parameters were derived from the Hessian matrix, and the dihedral force constants are fit to the torsional energy profiles generated by a series of constrained molecular geometry optimization.
Bias voltage dependence of tunnel magnetoimpedance in AlOx-based magnetic tunnel junctions
Authors: Kuo Ming-Feng , Fu Chao-Ming , Han Xiu-Feng , Chang Chia-Ou , Chou Chan-Shin
Published: March 2011

This paper studies the bias voltage dependence of tunnel magnetoimpedance in two types of AlOx-based magnetic tunnel junctions (i.e., CoFeB/AlOx/CoFeB and CoFeB/AlOx). The impedance spectra methodology is applied to characterize the transport properties and barrier/interface behavior of the tunnel junctions. The bias voltage dependence of the impedance spectra shows different behaviors for each type of junction, thus contributing different physical parameters to the equivalent circuit model. The results indicate that the interplay of the spin dynamics and dielectric relaxation may be attributed to the magnetic tunnel dynamics. The physical parameters obtained from the equivalent circuit analysis provide information on the spin-dependence tunnel junctions for devices applications.
Layered transition-metal permanent-magnet structures
Authors: Kashyap A. , Skomski R. , Manchanda P. , Shield J. E. , Sellmyer D. J.
Published: March 2011

First-principle calculations based on the Vienna ab intio simulation package are used to determine the magnetic moment and anisotropy of Fe-containing layered permanent-magnet structures. Both artificial structures and less commonly considered natural magnets are considered. The former category includes multilayers that can or may be produced by different thin-film techniques, such as fcc(001) Pd–Fe, bcc(110) W–Fe, and bcc(001) Rh–Fe. The latter includes alloys of Fe with Rh, such as the C11b-ordered ferromagnet Fe2Rh. Our focus is on Pd2Fem, Rh2Fem, W2Fem multilayers (m ≤ 4) and Fe2Rh. In all structures, the spin magnetic moment of the Fe is enhanced at the interface. At the interface, the Pd and Rh atoms are ferromagnetically coupled to the Fe, but the Fe–W coupling is antiferromagnetic. For Fe2Rh, strong uniaxial anisotropy is predicted.
FePt:C/Fe graded and nongraded films with controllable coercivity fabricated by magnetron sputtering and postannealing
Authors: Wang Fang , Zhang Jing , Zhang Jun , Xu Xiao-Hong
Published: March 2011

Graded and nongraded FePt:C(hard)/Fe(soft) films with different structures are fabricated by magnetron sputtering and postannealing. Due to the interdiffusion between FePt and C layers during the annealing process, films with a continuous variation in anisotropy are realized by varying the C concentration in the FePt hard layer. In comparison to the FePt/Fe bilayer, the coercivities of graded and nongraded films are effectively reduced. However, the mechanisms of reduction are different. Moreover, adding a nonmagnetic C layer not only tailors the gradient of anisotropy, but also weakens the intergranular exchange interaction.
Magnetic properties and microstructure of bulk Nd–Fe–B magnets solidified in magnetic field
Authors: Wang C. , Lai Y. S. , Hsieh C. C. , Chang W. C. , Chang H. W. , Sun A. C.
Published: March 2011

The Nd–Fe–B bulk magnets with a slab shape of 0.9 × 4 × 15 mm3 were prepared by injection casting into a copper mold. The effects of applying a magnetic field during the casting process on the magnetic properties and microstructure of Nd9.5Fe71.5Ti2.5Zr0.5Cr1B14.5C0.5 alloy have been studied. The results show that the sample cast with magnetic field has a stronger (00L) texture of Nd2Fe14B phase with the c-axis perpendicular to the slab plane than the sample cast without magnetic field. The intensity of the texture weakens from surface to inner region of the bulk magnets. Applying a magnetic field during the casting process is helpful to refine the grain size effectively. As a result, the magnetic properties are improved from Br = 5.8 kG, iHc = 6.5 kOe, and (BH)max = 5.9 MGOe for thesample cast without magnetic field to Br = 6.1 kG, iHc = 10.3 kOe, and (BH)max = 7.3 MGOe for the sample cast with a 3.7 kOe magnetic field.
Magnetic states of BaFe2–xCoxAs2 single crystals: Magnetization and electron spin resonance study
Authors: Zhang Changjin , Ling Langsheng , Tong Wei , Ling Wei , Feng Guolin , Zhang Shile , Pi Li , Tan Shun , Zhang Yuheng
Published: March 2011

The magnetic states of the BaFe2-xCoxAs2 single crystal samples have been studied by magnetization and electron spin resonance measurements. We find a transformation of the Fe2+ spin state from high temperature high-spin state to low temperature low-spin state, indicating a small crystal field splitting energy. In the superconducting samples, both the magnetization and electron spin resonance results suggest an itinerant behavior of the system.
Modulation bandwidth of a spin laser
Authors: Banerjee D. , Adari R. , Murthy M. , Suggisetti P. , Ganguly S. , Saha D.
Published: March 2011

We have studied small signal frequency response of a spin laser. We have shown that the response is characterized by two distinct resonant peaks corresponding to the two polarization modes of the spin laser. It is observed that the modulation bandwidth of a spin laser can be smaller or larger than that of a conventional laser depending upon the current bias and spin relaxation time constant. A small value for spin relaxation constant may not be detrimental for modulation bandwidth. This anomalous observation is explained by considering both the amplitude and phase response of the two polarization modes. A spin laser can act as a combination of low-pass and bandpass filters. The passband frequency range is tunable by external bias. We have also studied the evolution of resonant peaks and modulation bandwidth as a function of spin relaxation time constant.
Preparation and characterization of GeTe4 thin films as a candidate for phase change memory applications
Authors: Jiang Hao , Guo Kang , Xu Hanni , Xia Yidong , Jiang Kun , Tang Fei , Yin Jiang , Liu Zhiguo
Published: March 2011

GeTe4 films derived by pulsed laser deposition technique show a higher crystallization temperature and a lower melting temperature as compared with GeTe and pure and doped Ge2Sb2Te5. The activation energy against transformation is determined to be 2.7 eV according to Kissinger’s formula. A good resistance ratio between the amorphous and the crystalline GeTe4 films is also demonstrated. These results reveal that GeTe4 can be a competitive candidate for phase change memory applications. In addition, the phase change in GeTe4 is accompanied by a transition in electric conduction behaviors, which is also discussed in the light of the Anderson transition.
Role of AlxGa1−xAs buffer layer in heterogeneous integration of GaAs/Ge
Authors: Chia C. K. , Dalapati G. K. , Chai Y. , Lu S. L. , He W. , Dong J. R. , Seng D. H. L. , Hui H. K. , Wong A. S. W. , Lau A. J. Y. , Cheng Y. B. , Chi D. Z. , Zhu Z. , Yeo Y. C. , Xu Z. , Yoon S. F.
Published: March 2011

The material and optical properties of the GaAs/AlxGa1−xAs/Ge structures grown by metalorganic chemical vapor deposition were examined and found to be dependent of the Al content x. SIMS and PL measurements show that the 10 nm AlxGa1−xAs buffer layer with x = 0.3 and 0.6 are equally effective in suppressing the outdiffusion of Ge, whereas x = 1.0 gives the most abrupt interface. The best morphology with surface rms of 0.3 nm is obtained in the structure with x = 0.3 buffer layer. Analysis on change of strain in the AlxGa1−xAs buffer layer suggests that the compressive strain at the AlxGa1−xAs-GaAs interface is compensated by the tensile strain at the Ge-AlxGa1−xAs interface when x ∼ 0.3. AlxGa1−xAs lattice matched to Ge is crucial for better result in surface morphology, but higher Al content is preferred for eliminating the interdiffusion of atoms at the heterointerface.
Oxygen effect on spin reorientation transition of CoNi alloy films on Cu(001)
Authors: Pan Wei , Shih Ying-Ta , Wu Zheng-Zhe
Published: March 2011

Combinations of CoNi alloying and oxygen exposure are used in order to study the spin reorientation transition (SRT) in Ni/Cu(001). These modifications on the volume layers or on the surface layers may result in an increase or decrease of the critical thickness (tc) of the SRT, respectively. We present the study on the magnetic properties of the oxygen exposure on Co5Ni95/Cu(001) ultrathin films. For films with a thickness below tc, a certain amount of oxygen exposure drives the SRT, whereas a large amount of oxygen demolishes the magnetization. For films with a thickness above tc, the out-of-plane magnetization persists under a large amount of oxygen exposure. Two-domain-like state and coercivity enhancement are found in the hysteresis loops. The results imply that oxygen adsorbates may drive the SRT and a large amount of oxygen may lead to the formation of antiferromagnetic oxide on the surface layers. Coercivity enhancement may refer to the exchange bias coupling, which provides the opportunity in applications in spintronics.
Piezoelectrics by design: A route through short-period Perovskite superlattices
Authors: Das Hena , Waghmare Umesh V. , Saha-Dasgupta Tanusri
Published: March 2011

Using first-principles density functional theory, we study piezoelectricity in short-period superlattices made with combinations of ferroelectric and paraelectric components that exhibit polar discontinuities. We show that the piezoelectric response of such a superlattice can be tuned in terms of sign and magnitude by the choice of components. As these superlattices with nonswitchable polarization do not undergo ferroelectric transitions, we predict them to exhibit a robust piezoelectric response with weaker temperature dependence compared to their bulk counterparts.
Ab initio study on the hydrogen desorption from $\rm {MH\text{–}NH}_3$MH–NH3 (M = Li, Na, K) hydrogen storage systems
Authors: Yamane A. , Shimojo F. , Hoshino K. , Ichikawa T. , Kojima Y.
Published: March 2011

The hydrogen storage system LiH + \documentclass[12pt]{minimal}\begin{document}$\rm {NH}_3$\end{document} NH 3 ↔ \documentclass[12pt]{minimal}\begin{document}$\rm {LiNH}_2$\end{document} LiNH 2 + \documentclass[12pt]{minimal}\begin{document}$\rm {H}_2$\end{document}H2 is one of the most promising hydrogen storage systems, where the reaction yield can be increased by replacing Li in LiH with other alkali metals (Na or K) in order of Li < Na < K. In this paper, we have studied the alkali metal M (M = Li, Na, K) dependence of the reactivity of MH with \documentclass[12pt]{minimal}\begin{document}$\rm {NH}_3$\end{document} NH 3 by calculating the potential barrier of the \documentclass[12pt]{minimal}\begin{document}$\rm {H}_2$\end{document}H2 desorption process from the reaction of an \documentclass[12pt]{minimal}\begin{document}$\rm {M}_2$\end{document}M2\documentclass[12pt]{minimal}\begin{document}$\rm {H}_2$\end{document}H2 cluster with an \documentclass[12pt]{minimal}\begin{document}$\rm {NH}_3$\end{document} NH 3 molecule based on the ab initio structure optimization method. We have shown that the height of the potential barrier becomes lower in order of Li, Na, and K, where the difference of the potential barrier in Li and Na is relatively smaller than that in Na and K, and this tendency is consistent with the recent experimental results. We have also shown that the H–H distance of the \documentclass[12pt]{minimal}\begin{document}$\rm {H}_2$\end{document}H2 dimer at the transition state takes larger distance and the change of the potential energy around the transition state becomes softer in order of Li, Na, and K. There are almost no M dependence in the charge of the H atom in \documentclass[12pt]{minimal}\begin{document}$\rm {NH}_3$\end{document} NH 3 before the reaction, while that of the H atom in \documentclass[12pt]{minimal}\begin{document}$\rm {M}_2$\end{document}M2\documentclass[12pt]{minimal}\begin{document}$\rm {H}_2$\end{document}H2 takes larger negative value in order of Li, Na, and K. We have also performed molecular dynamics simulations on the \documentclass[12pt]{minimal}\begin{document}$\rm {M}_2$\end{document}M2\documentclass[12pt]{minimal}\begin{document}$\rm {H}_2$\end{document}H2–\documentclass[12pt]{minimal}\begin{document}$\rm {NH}_3$\end{document} NH 3 system and succeeded to reproduce the \documentclass[12pt]{minimal}\begin{document}$\rm {H}_2$\end{document}H2 desorption from the reaction of \documentclass[12pt]{minimal}\begin{document}$\rm {Na}_2$\end{document} Na 2\documentclass[12pt]{minimal}\begin{document}$\rm {H}_2$\end{document}H2 with \documentclass[12pt]{minimal}\begin{document}$\rm {NH}_3$\end{document} NH 3.
Labeling of macrophage cell using biocompatible magnetic nanoparticles
Authors: Min Ji Hyun , Kim Sung Tae , Lee Ji Sung , Kim Kwanghee , Wu Jun Hua , Jeong Jaeho , Song Ah Young , Lee Kyung-Mi , Kim Young Keun
Published: March 2011

This work investigates the intrinsic cell labeling efficiency of the Fe3O4 nanoparticles prepared by a modified thermal decomposition method using nontoxic precursors and a biocompatible polymer surfactant. This method eliminates the current need for additional step of surface modification. The structural analysis reveals the highly crystalline feature of the nanoparticles, while the magnetic measurement shows their superparamagnetic behavior at room temperature. Fe3O4 nanoparticles were efficiently incorporated into the murine macrophage cells (RAW264.7) without visible cytotoxicity. Cell labeling efficiency was found to be over 90% as measured by magnetically activated cell sorting and physical property measurement system. Therefore, such Fe3O4 nanoparticles could provide a useful magnetic cell labeling tool for macrophage cells using their phagocytic/endocytic activity and further apply to the other relevant biomedical applications.
Transient absorption studies of vibrational relaxation and photophysics of Prussian blue and ruthenium purple nanoparticles
Authors: Weidinger D. , Brown D. J. , Owrutsky J. C.
Published: March 2011

Transient infrared and visible absorption studies have been used to characterize vibrational and electronic dynamics of Prussian blue (PB) and ruthenium purple (RP) nanoparticles produced and characterized in AOT reverse micelles. Studies include excitation and probing with both infrared (near 2000 cm−1) and visible (800 nm) pulses. From IR pump–IR probe measurements of the CN stretching bands, vibrational population lifetimes are determined to be 32 ± 4 ps for PB and 44 ± 14 ps for RP. These times are longer than those for ferrocyanide (4 ps) and ruthenocyanide (4 ps) in normal water and are closer to the times for these species in heavy water (25–30 ps) and for ferrocyanide in formamide (43 ps). The PB and RP lifetimes are also longer than those (<15 ps) previously measured for CN stretching bands following intervalence excitation and back-electron transfer (BET) for dinuclear mixed-valence compounds containing Fe, Ru, and Os in heavy water and formamide [A. V. Tivansky, C. F. Wang, and G. C. Walker, J. Phys. Chem. A 107, 9051 (2003)]. In 800 nm pump–IR probe experiments on RP and PB, transient IR spectra and decay curves are similar to those with IR excitation; a ground state bleach and a red shifted (by ∼40 cm−1) excited state decay are observed. These results for the visible pumping are consistent with rapid (<1 ps) BET resulting in population in the ground electronic state with vibrational excitation in the CN mode. In addition, transient absorption studies were performed for PB and RP probing with visible light using both visible and IR excitation. The early time response for the 800 nm pump–800 nm probe of PB exhibits an instrument-limited, subpicosecond bleach followed by an absorption, which is consistent with the previously reported results [D. C. Arnett, P. Vohringer, and N. F. Scherer, J. Am. Chem. Soc. 117, 12262 (1995)]. The absorption exhibits a biexponential decay with decay times of 9 and 185 ps, which could have been attributed to the CN band excitation indicated from 800 pump–IR probe results. However, IR pump–800 nm probe studies reveal that excitation of the CN band directly results in a decreased visible absorption that decays in 18 ps rather than an induced absorption at 800 nm. Characteristics of the IR pump–800 nm probe, especially that it induces a bleach instead of an absorption, unambiguously indicate that the transient absorption from the 800 nm pump–800 nm probe is dominated by states other than the CN bands and is attributed to population in other, probably lower frequency, vibrational modes excited following visible excitation and rapid BET.
Signature of a back contact barrier in DLTS spectra
Authors: Lauwaert J. , Khelifi S. , Decock K. , Burgelman M. , Vrielinck H.
Published: March 2011

The DLTS signal induced by a back contact barrier is studied both theoretically and through experiments on model circuits. A nonideal back contact is modeled either by a resistor and a capacitor, or by a germanium diode inversely polarized with respect to the junction diode. Depending on the back contact properties, this may result in a positive or negative capacitance transient. For these model circuits the capacitance transient time constants and amplitudes are studied as a function of voltage pulse height and compared with signals originating from emission and slow capture from a defect level. These two origins of DLTS signals present very different properties, which opens possibilities to distinguish between them.
Gradient induced spiral drift in heterogeneous excitable media
Authors: Sadeghi Parisa , Rotermund Harm H.
Published: March 2011

Nonlinear excitable systems far from equilibrium can exhibit pattern formation such as spirals, target patterns, etc. One such system is the heterogeneous catalytic reaction of CO with oxygen on platinum single crystals. It has been established that the resonant periodic forcing of spirals in such excitable systems can cause a spiral drift. Here, we investigate the effects of a linear thermal gradient on the spiral dynamics during CO oxidation on platinum (110) for the first time, both in simulations and with experiments. Our results suggest that a spatial thermal gradient established across the surface can act as an internal forcing drive and cause the spiral patterns to drift. This drift has components both parallel and perpendicular to the external gradient.
Focused ion beam patterned Fe thin films: A study by selective area Stokes polarimetry and soft x-ray microscopy
Authors: Cook P. J. , Shen T. H. , Grundy P. J. , Im M.-Y. , Fischer P. , Morton S. A. , Kilcoyne A. L. D.
Published: March 2011

A combination of analytical techniques, with special emphasis on selective area Stokes polarimetry, has been used to explore the structural properties and magnetic behavior of focused ion beam patterned Fe thin films under controlled Ga+ ion irradiation. Ion irradiation at doses ranging from 7.7 × 1015 to 5.2 × 1016 Ga ions cm−2 did not noticeably alter the chemical properties of the Fe, but changes to the film structure and increased coercivity were observed even after the lowest doses. Magnetic transmission x-ray microscopy provided detailed information about the magnetization reversal process occurring within a patterned area of film comprising both Fe and Fe-Ga regions, and clearly showed domain wall pinning around the magnetically harder Fe-Ga.
Phonon-wall interactions and frequency-dependent thermal conductivity in nanowires
Authors: Sellitto A. , Alvarez F. X. , Jou D.
Published: March 2011

Phonon-wall collisions (with smooth or rough walls) have a deep influence on the thermal conductivity of nanowires. Usually this influence is analyzed in the steady-state thermal conductivity. Here, by using a phonon-hydrodynamic model with slip heat flow along the walls, we explore the influence of phonon-wall collisions on frequency-dependent thermal conductivity, which imply a reduction of it with increasing frequency. Understanding of this dependence would allow one to obtain information on the collisions complementary to that of the steady states.
X-ray ablation rates in inertial confinement fusion capsule materials
Authors: Olson R. E. , Rochau G. A. , Landen O. L. , Leeper R. J.
Published: March 2011

X-ray ablation rates have been measured in beryllium, copper-doped beryllium, germanium-doped plastic (Ge-doped CH), and diamondlike high density carbon (HDC) for radiation temperatures T in the range of 160–260 eV. In beryllium, the measured ablation rates range from 3 to 12 mg/cm2/ns; in Ge-doped CH, the ablation rates range from 2 to 6 mg/cm2/ns; and for HDC, the rates range from 2 to 9 mg/cm2/ns. The ablation rates follow an approximate T3 dependence and, for T below 230 eV, the beryllium ablation rates are significantly higher than HDC and Ge-doped CH. The corresponding implied ablation pressures are in the range of 20–160 Mbar, scaling as T3.5. The results are found to be well predicted by computational simulations using the physics packages and computational techniques employed in the design of indirect-drive inertial confinement fusion capsules. An iterative rocket model has been developed and used to compare the ablation rate data set to spherical indirect-drive capsule implosion experiments and to confirm the validity of some aspects of proposed full-scale National Ignition Facility ignition capsule designs.
Comment on “Scalings for radiation from plasma bubbles” [Phys. Plasmas 17, 056708 (2010)]
Authors: Corde S. , Stordeur A. , Malka V.
Published: March 2011

Thomas has recently derived scaling laws for x-ray radiation from electrons accelerated in plasma bubbles, as well as a threshold for the self-injection of background electrons into the bubble [A. G. R. Thomas, Phys. Plasmas 17, 056708 (2010)]. To obtain this threshold, the equations of motion for a test electron are studied within the frame of the bubble model, where the bubble is described by prescribed electromagnetic fields and has a perfectly spherical shape. The author affirms that any elliptical trajectory of the form x′2/γp2+y′2=R2 is solution of the equations of motion (in the bubble frame), within the approximation py′2/px′2⪡1. In addition, he highlights that his result is different from the work of Kostyukov et al. [Phys. Rev. Lett. 103, 175003 (2009)], and explains the error committed by Kostyukov–Nerush–Pukhov–Seredov (KNPS). In this comment, we show that numerically integrated trajectories, based on the same equations than the analytical work of Thomas, lead to a completely different result for the self-injection threshold, the result published by KNPS [Phys. Rev. Lett. 103, 175003 (2009)]. We explain why the analytical analysis of Thomas fails and we provide a discussion based on numerical simulations which show exactly where the difference arises. We also show that the arguments of Thomas concerning the error of KNPS do not hold, and that their analysis is mathematically correct. Finally, we emphasize that if the KNPS threshold is found not to be verified in PIC (Particle In Cell) simulations or experiments, it is due to a deficiency of the model itself, and not to an error in the mathematical derivation.
Response to “Comment on ‘Scalings for radiation from plasma bubbles’ ” [Phys. Plasmas 18, 034701 (2011)]
Authors: Thomas A. G. R.
Published: March 2011

In the preceding Comment, Corde, Stordeur, and Malka claim that the trapping threshold derived in my recent paper is incorrect. Their principal argument is that the elliptical orbits I used are not exact solutions of the equation of motion in the fields of the bubble. The original paper never claimed this—rather I claimed that the use of elliptical orbits was a reasonable approximation, which I based on observations from particle-in-cell simulations. Integration of the equation of motion for analytical expressions for idealized bubble fields (either analytically [I. Kostyukov, E. Nerush, A. Pukhov, and V. Seredov, Phys. Rev. Lett. 103, 175003 (2009)] or numerically [S. Corde, A. Stordeur, and V. Malka, “Comment on ‘Scalings for radiation from plasma bubbles,’ ” Phys. Plasmas 18, 034701 (2011)]) produces a trapping threshold wholly inconsistent with experiments and full particle-in-cell (PIC) simulations (e.g., requiring an estimated laser intensity of a0∼30 for ne∼1019 cm−3). The inconsistency in the particle trajectories between PIC and the numeric model used by the comment authors arises due to the fact that the analytical fields are only approximately true for “real” plasma bubbles, and lack certain key features of the field structure. Two possible methods of resolution to this inconsistency are either to find ever more complicated but accurate models for the bubble fields or to find approximate solutions to the equations of motion that capture the essential features of the self-consistent electron trajectories. The latter, heuristic approach used in my recent paper produced a threshold that is better matched to experimental observations. In this reply, I will also revisit the problem and examine the relationship between bubble radius and electron momentum at the point of trapping without reference to a particular trajectory.
A new method for wideband characterization of resonator-based sensing platforms
Authors: Munir Farasat , Wathen Adam , Hunt William D.
Published: March 2011

A new approach to the electronic instrumentation for extracting data from resonator-based sensing devices (e.g., microelectromechanical, piezoelectric, electrochemical, and acoustic) is suggested and demonstrated here. Traditionally, oscillator-based circuitry is employed to monitor shift in the resonance frequency of the resonator. These circuits give a single point measurement at the frequency where the oscillation criterion is met. However, the resonator response itself is broadband and contains much more information than a single point measurement. Here, we present a method for the broadband characterization of a resonator using white noise as an excitation signal. The resonator is used in a two-port filter configuration, and the resonator output is subjected to frequency spectrum analysis. The result is a wideband spectral map analogous to the magnitude of the S21 parameters of a conventional filter. Compared to other sources for broadband excitation (e.g., frequency chirp, multisine, or narrow time domain pulse), the white noise source requires no design of the input signal and is readily available for very wide bandwidths (1 MHz–3 GHz). Moreover, it offers simplicity in circuit design as it does not require precise impedance matching; whereas such requirements are very strict for oscillator-based circuit systems, and can be difficult to fulfill. This results in a measurement system that does not require calibration, which is a significant advantage over oscillator circuits. Simulation results are first presented for verification of the proposed system, followed by measurement results with a prototype implementation. A 434 MHz surface acoustic wave (SAW) resonator and a 5 MHz quartz crystal microbalance (QCM) are measured using the proposed method, and the results are compared to measurements taken by a conventional bench-top network analyzer. Maximum relative differences in the measured resonance frequencies of the SAW and QCM resonators are 0.0004% and 0.002%, respectively. The ability to track a changing sensor response is demonstrated by inducing temperature variations and measuring resonance frequency simultaneously using the proposed technique in parallel with a network analyzer. The relative difference between the two measurements is about 5.53 ppm, highlighting the impressive accuracy of the proposed system. Using commercially available digital signal processors (DSPs), we believe that this technique can be implemented as a system-on-a-chip solution resulting in a very low cost, easy to use, portable, and customizable sensing system. In addition, given the simplicity of the signal and circuit design, and its immunity to other common interface concerns (injection locking, oscillator interference, and drift, etc.), this method is better suited to accommodating array-based systems.
Double-layer formation and propulsive assessment for a three-species plasma expanding in a magnetic nozzle
Authors: Ahedo Eduardo
Published: March 2011

The quasi one-dimensional expansion of a collisionless plasma with a hot-electron tail in a gentle convergent-divergent nozzle is studied. A parametric investigation of the plasma response is carried out in terms of the relative density and temperature of the hot-electron population. The formation of a steepened layer is shown to be due to the anomalous thermodynamic behavior of the plasma, which creates a local minimum of the Mach number. The change from a quasineutral to a non-neutral steepened layer occurs when this minimum goes below one and several sonic points appear. The non-neutral double layer does not introduce further changes in the plasma response. All gain in plasma momentum and thrust is related to the supersonic expansion in the divergent nozzle, with zero contribution of the double layer. A comparative analysis of thrust efficiency of plasmas with and without hot electrons does not find any gain in the presence of hot electrons; instead, a small penalty in the expansion efficiency seems to exist. The study is limited to Maxwellian electron populations and finite nozzles.
Information transfer dynamics in fixed-pathways networks
Authors: Moyano Luis G. , Cárdenas Juan P. , Salcedo Jorge , Mouronte Mary Luz , Benito Rosa M.
Published: March 2011

Most complex technological networks are defined in such a way that their global properties are manifested at a dynamical level. An example of this is when internal dynamical processes are constrained to predefined pathways, without the possibility of alternate routes. For instance, large corporation software networks, where several flow processes take place, are typically routed along specific paths. In this work, we propose a model to describe the global characteristics of this kind of processes, where the dynamics depends on the state of the nodes, represented by two possibilities: responsive or blocked. We present numerical simulations that show rich global behavior with unexpected emerging properties. In particular, we show that two different regimes appear as a function of the total network load. Each regime is characterized by developing either a unimodal or a bimodal distribution for the density of responsive nodes, directly related to global efficiency. We provide a detailed explanation for the main characteristics of our results as well as an analysis of the implications for real technological systems.
Contribution of nonlocal interactions to DNA elasticity
Authors: Eslami-Mossallam B. , Ejtehadi M. R.
Published: March 2011

A nonlocal harmonic elastic rod model is proposed to describe the elastic behavior of short DNA molecules. We show that the nonlocal interactions contribute to effective bending energy of the molecule and affect its apparent persistence length. It is also shown that the anomalous behavior which has been observed in all-atom molecular dynamic simulations [A. K. Mazur, Biophys. J. 134, 4507 (2006)] can be a consequence of both nonlocal interactions between DNA base pairs and the intrinsic curvature of DNA.
Theoretical investigations of candidate crystal structures for β-carbonic acid
Authors: Reddy Sandeep Kumar , Kulkarni Chidambar H. , Balasubramanian Sundaram
Published: March 2011

Using multiple computational tools, we examine five candidate crystal structures for β-carbonic acid, a molecular crystal of environmental and astrophysical significance. These crystals comprise of hydrogen bonded molecules in either sheetlike or chainlike topologies. Gas phase quantum calculations, empirical force field based crystal structure search, and periodic density functional theory based calculations and finite temperature simulations of these crystals have been carried out. The infrared spectrum calculated from density functional theory based molecular dynamics simulations compares well with experimental data. Results suggest crystals with one-dimensional hydrogen bonding topologies (chainlike) to be more stable than those with two-dimensional (sheetlike) hydrogen bonding networks. We predict that these structures can be distinguished on the basis of their far infrared spectra.
Hysteresis in a magnetic bead and its applications
Authors: Singh Vanchna , Banerjee Varsha
Published: March 2011

We study hysteresis in a micron-sized bead: a nonmagnetic matrix embedded with superparamagnetic nanoparticles. These hold tremendous promise in therapeutic applications as heat generating machines. In this letter, we provide a theoretical framework which enables manipulation of heat dissipation by a compatible selection of commercially available beads and the frequency f and amplitude ho of the oscillating field applied in the laboratory. We also introduce a protocol for gradual heating of a local region utilizing return point memory. Easy implementation makes it potentially useful in remedial procedures such as hyperthermia and chemotherapy.