bluuebunny/crossref_metadata_2025 huggingface dataset:
Low-frequency noise in n-channel metal-oxide-semiconductor field-effect transistors undergoing soft breakdown
Authors: Chen Ming-Jer , Kang Ting-Kuo , Lee Yuan-Hwa , Liu Chuan-Hsi , Chang Yih J. , Fu Kuan-Yu
Published: July 2002

For 3.3-nm thick gate oxide n-channel metal-oxide-semiconductor field-effect transistors subject to a stress gate voltage of 5.5 V, three distinct events are encountered in the time evolution of the gate current: stress-induced leakage current (SILC), soft breakdown (SBD), and hard breakdown (HBD). The localization of SBD and HBD paths, as well as their developments with the time, is determined electrically, showing random distribution in nature. At several stress times, we interrupt the stressing to measure the drain current low-frequency noise power Sid. As expected, Sid follows up the spontaneous changes at the onset of SBD and HBD. The Sid spectra measured in fresh and SILC mode are reproduced by a literature model accounting for the carrier number and surface mobility fluctuations in the channel, and, as a result, both preexisting and newly generated trap densities are assessed. The post-SBD Sid does originate from current fluctuations in the SBD percolation paths, which can couple indirectly to drain via underlying channel in series, or directly to drain if the SBD path is formed close to drain extension. In particular, a fluctuation in Sid itself in the whole SBD duration is observed. This phenomenon is very striking since it indeed evidences the dynamic percolation origin concerning the trapping–detrapping processes in and around the SBD paths. The subsequent HBD duration remarkably features a flat Sid, indicating the set-up of a complete conductive path prevailing over the trapping–detrapping processes.
A low-loss, ultrahigh vacuum compatible helium cryostat without liquid nitrogen shield
Authors: Okamoto Hiroshi , Chen Dongmin
Published: July 2002

An ultrahigh vacuum compatible compact helium cryostat with a 48 h hold time for a 2.5 l of liquid helium capacity has been realized. The design exploits the large enthalpy of the gas phase helium, by using two layers of vapor-cooled radiation shield in place of the conventional liquid nitrogen jacket. The mechanical rigidity, the absence of liquid nitrogen bubbling, and a low helium boil-off rate make this cryostat particularly suitable for applications that require a low vibration environment such as scanning probe microscopy.
Frictional properties of thin chain alcohol films
Authors: Mugele F. , Salmeron M.
Published: July 2002

We used the surface forces apparatus (SFA) to measure the viscosity and friction of confined films of octanol and undecanol as a function of film thickness. In contrast to structurally similar alkanes, the alcohols retain their bulk-like lubrication properties down to a thickness of only one (bi)layer. Only the self-assembled monolayers of alcohol molecules that form spontaneously on each mica surface display solid-like stick–slip motion upon shearing. During stick–slip motion, microscopic creep is found to precede macroscopic slip events at high loads. Apart from this creep process, we find a well-defined critical shear stress during stick–slip motion, which decreases weakly with increasing sliding speed.
Amorphous diamond-like carbon film prepared by pulsed laser deposition with application of pulsed negative bias voltage
Authors: Matsuyama Nobuyuki , Yukimura Ken , Maruyama Toshiro
Published: July 2002

Amorphous diamond-like carbon films were prepared by pulsed laser deposition with a synchronized application of pulsed negative bias voltage to the substrate. A beam from a Nd:yttrium–aluminum–garnet laser (1064 nm) was radiated to a graphite target at a laser fluence of 10 J/cm2. The negative bias voltage was changed from 0 to 10 kV at a repetition rate of 10 Hz. The film was deposited on a Si (111) substrate at a chamber pressure of 5×10−5 Torr for 120 min. The effects of bias voltage on the structure of the film were discussed on the basis of the measured deposition rate, Raman spectra, refractive index, dynamic hardness, and surface roughness. The effect of self-sputtering on deposition rate was observed at a negative bias voltage above 0.6 kV. The application of bias voltage increased the fraction of sp3 configuration in the film and made the film surface smoother. In particular, the application of 3 kV gave the largest fraction of sp3 bonding, and formed the hardest and smoothest film. Bias voltage above 5 kV, however, drastically reduced the hardness of the film.
Manipulation and immobilization of alkane-coated gold nanocrystals using scanning tunneling microscopy
Authors: Rolandi M. , Scott K. , Wilson E. G. , Meldrum F. C.
Published: July 2002

Colloidal nanoparticles, comprised of gold nanocrystals, of mean diameter 2.8 nm, coated with an insulating chemically stable self-assembled monolayer of dodecanethiol, have been prepared. Monolayers of nanoparticles have been physisorbed on highly oriented pyrolitic graphite, first by self-assembly, and second by assembly as Langmuir films and subsequent deposition. Nanoparticles have been self-assembled on gold, and immobilized by chemisorption, using decanedithiol during assembly as a linking molecule. Scanning tunneling microscope images of the monolayers are obtained. At high substrate–tip voltages, >0.6 V, the tip is able to climb above the nanoparticles. The tunneling is then a two-step event, tunneling from the substrate to the gold nanocrystal, and subsequently from the gold nanocrystal to the tip. At low voltage, 0.25 V, the Coulomb blockade prevents one extra electron occupying the gold nanocrystal. The tip cannot then climb above the nanoparticles. The theoretical threshold of the blockade is estimated from the nanoparticle size, and shown to be consistent with the observations. At low substrate–tip voltages, rastering of the tip sweeps the nanoparticles from the raster area (but not at high tip voltage). This result has not been described previously, and it is envisaged that it could be used to separate nanoparticles of differing size. However, immobilized isolated nanoparticles are not removed, but only pushed temporarily aside by the scanning tip.
The reactions CHnD4−n+OH→P and CH4+OD→CH3+HOD as a test of current direct dynamics computational methods to determine variational transition-state rate constants. I.
Authors: Masgrau Laura , González-Lafont Àngels , Lluch José M.
Published: July 2002

In the present work, we have theoretically calculated the rate constants and their temperature dependence for the reactions CHnD4−n+OH→P, and for the reaction of methane with OD, by means of variational transition-state theory plus multidimensional tunneling corrections, at the MP-SAC2//MP2/cc-pVTZ/// and CCSD(T)//MP2/cc-pVTZ/// electronic levels. Also, the newly developed single-point energy interpolation algorithm has been used at the CCSD(T)/aug-cc-pVTZ//MP2/cc-pVTZ and CCSD(T)-SAC//MP2/cc-pVTZ levels. For reactions with n=1, 2 or 3, the competitive canonical unified statistical theory has been applied as they involve more than one nonequivalent reaction channel. Variational effects and tunneling have been found to be very important. The proton shift classical energy barrier turns out to be 5.83 and 4.97 kcal/mol at the CCSD(T)/aug-cc-pVTZ//MP2/cc-pVTZ and CCSD(T)-SAC//MP2/cc-pVTZ levels, respectively. Even though we have used the highest ab initio electronic level reported up to now for dynamics calculations on these reactions, and although our results are quite good, we still do not match exactly the available experimental data. From our results it can be inferred that, probably, an adiabatic energy maximum between the CCSD(T)-SAC//MP2/cc-pVTZ and CCSD(T)/aug-cc-pVTZ//MP2/cc-pVTZ values (5.6 and 6.2 kcal/mol, respectively, for the perprotio reaction) could be the most feasible, and that the description of the adiabatic profile fails especially in that region away from the transition-state location but crucial for tunneling corrections.
Mathematical analysis of soft baking in photolithography
Authors: Hsu Jyh-Ping , Lin Sung-Hwa , Chen Wen-Chang , Tseng Shiojenn
Published: July 2002

The soft baking step of a photolithography process is analyzed theoretically, taking the effect of the temperature dependence of the diffusivity of solvent into account. A coordinates-transform technique is chosen to solve the moving boundary problem under consideration. The temporal variation of the thickness of a film is predicted, and the result obtained justified by fitting experimental data reported in the literature for both poly(methylmethacrylate) film and Shipley UVIII photoresist. We show that, depending upon the types of photoresist film and the operating conditions, the transport of solvent may be controlled by the diffusion of solvent in a film or the convective transport of solvent from the gas–film interface to the bulk gas phase.
Control of vortex breakdown by a transversely oriented wire
Authors: Akilli H. , Sahin B. , Rockwell D.
Published: July 2002

A small wire oriented orthogonally to the axis of the leading-edge vortex on a delta wing at high angle of attack generates substantial changes in the vortex structure, which is characterized using a technique of high-image-density particle image velocimetry. A wire having a diameter two orders of magnitude smaller than the diameter of the leading-edge vortex prior to the onset of vortex breakdown can substantially advance the onset of breakdown by as much as 15 vortex diameters. Depending upon the dimensionless diameter of the wire and wire location along the axis of the vortex, the onset of vortex breakdown can occur either upstream or downstream of the wire. Contours of constant velocity indicate that the rate of decrease of streamwise velocity along the centerline of the vortex is substantially enhanced, even for locations well upstream of the wire, relative to the case of vortex breakdown in the absence of a wire. Patterns of instantaneous vorticity in the presence of the wire typically exhibit a form characteristic of either a spiral- or bubble-like mode of breakdown that occurs in the absence of the wire.
Incorporation kinetics of indium and gallium in indium gallium nitride: A phenomenological model
Authors: Storm D. F.
Published: July 2002

A phenomenological model of indium and gallium incorporation in InxGa1−xN thin films grown by molecular beam epitaxy is presented. The group III incorporation rates are hypothesized to vary as different powers of the respective metal surface concentrations. A self-blocking process by indium, such as indium droplet formation, is easily inserted into the model in a meaningful way. A two-parameter expression for x is derived and is in excellent quantitative agreement with experimental observations. Finally, there is evidence that suggests the surface lifetime of indium adatoms varies with substrate temperature as (665 °C−Ts)1/2 for 600°<Ts<665 °C.
Symmetry properties and explicit solutions of the generalized Weierstrass system
Authors: Bracken P. , Grundland A. M.
Published: July 2002

The method of symmetry reduction is systematically applied to derive several classes of invariant solutions for the generalized Weierstrass system inducing constant mean curvature surfaces and to the associated two-dimensional nonlinear sigma model. A classification of subgroups with generic orbits of codimension one of the Lie point symmetry group for these systems provides a tool for introducing symmetry variables and reduces the initial systems to different nonequivalent systems of ordinary differential equations. We perform a singularity analysis for them in order to establish whether these ordinary differential equations have the Painlevé property. These ordinary differential equations can then be transformed to standard forms and next solved in terms of elementary and Jacobi elliptic functions. This results in a large number of new solutions and in some cases new interesting constant mean curvature surfaces are found. Furthermore, this symmetry analysis is extended to include conditional symmetries by subjecting the original system to certain differential constraints. In this case, several new types of nonsplitting algebraic, trigonometric, and hyperbolic multisoliton solutions have been obtained in explicit form. Some physical interpretation of these results in the areas of fluid membranes, string theory, two-dimensionl gravity, and cosmology are given.
Construction of highly conductive nanowires on a DNA template
Authors: Richter Jan , Mertig Michael , Pompe Wolfgang , Mönch Ingolf , Schackert Hans K.
Published: July 2002

We present measurements of the electrical conductivity of metallic nanowires which have been fabricated by chemical deposition of a thin continuous palladium film onto single DNA molecules to install electrical functionality. The DNA molecules have been positioned between macroscopic Au electrodes and are metallized afterwards. Low-resistance electrical interfacing was obtained by pinning the nanowires at the electrodes with electron-beam-induced carbon lines. The investigated nanowires exhibit ohmic transport behavior at room temperature. Their specific conductivity is only one order of magnitude below that of bulk palladium, confirming that DNA is an ideal template for the production of electric wires, which can be utilized for the bottom-up construction of miniaturized electrical circuits.
The compressible evolution of the super-Alfvénic magnetized wake
Authors: Dahlburg R. B. , Keppens R. , Einaudi G.
Published: July 2002

The effects of compressibility on the linear and nonlinear properties of the magnetized wake are examined, with an emphasis on the high speed flow situation. It is found that compressibility can modify properties of this system previously identified for the incompressible case. Of particular interest is an investigation of how the properties of the magnetized wake vary with the sonic Mach number. It is found that, in general, the growth rates of the unstable sinuous and varicose modes decrease with increasing Mach number and with increasing Alfvén number. However, at high sonic Mach numbers the varicose modes can have a growth rate which increases as the spanwise wave number increases, a significant difference from the incompressible case. The linear compressible equations are solved by a Chebyshev collocation technique. Nonlinear computations based on a finite volume method are also presented. Growth rates computed by both codes in the linear regime are in excellent agreement. At long times the system relaminarizes to an overall accelerated and broadened wake channel. It is found that variations in the Mach and Alfvén numbers have a strong affect on the evolution of the magnetized wake, e.g., for high M fast magnetosonic shocks are observed to develop.
Aging of piezoelectric composite transducers
Authors: Barzegar A. , Bagheri R. , Karimi Taheri A.
Published: July 2002

The effect of aging on some piezoelectric properties, including d33, K33, and d33g33 in 1–3 composite transducers has been evaluated in this article. The spontaneous changes of properties of piezocomposites (lead zirconate titanate/epoxy) and piezoceramic under zero external stress was measured at 5 days and 500 days after poling. The results show that the aging behavior in piezoelectric composites is very much dependent on the ceramic content. The composite material behaves similar to a piezoceramic at high volume fraction of ceramic, i.e., greater than 25 vol. %. At low volume fraction of ceramic, however, the rate of aging is much higher in piezoelectric composite compared to that of the piezoceramic. At lower than 25 vol. % of ceramic, it was seen that the aging rate increases with decreasing the ceramic content.
Transfer time optimization of a rapid cyclic instrumental neutron activation analysis for trace element detection
Authors: Khamis I. , Othman I. , Nasri M. , Bakkour M.
Published: July 2002

A rapid pneumatic transfer system for the cyclic instrumental neutron activation has been indigenously designed and installed. The system enables the detection of short-lived thermal and epithermal neutron activated nuclides. Interfacing between gamma spectrometry and other components of the system is achieved through two programmable logic controllers. Optimization of the transfer time for trace element detection was thoroughly studied. Different factors affecting the reduction of transfer time were analyzed. Transfer time of the irradiating capsule under current operating conditions is about 700 ms. The setup will be used for both cyclic and pseudo-cyclic as well as instrumental neutron activation analysis.
Size selectivity in a confined polydisperse hard-disk fluid
Authors: Kim Soon-Chul , Suh Soong-Hyuck
Published: July 2002

The Rosenfeld density functional approximation has been considered to investigate the local size segregation and selectivity in the adsorption of a polydisperse hard-disk fluid, which has a continuous distribution of the particle diameter. The calculated results have shown that the preferred species in a circular cavity depend strongly on the size ratio of a polydisperse hard-disk fluid as well as the cavity size and bulk packing fraction. The pore average size distribution for small particles decreases roughly linearly with increasing the cavity size, and the pore average size distribution for an average diameter of particle 〈σ〉 is almost the same as the bulk packing fraction. The local relative concentration oscillates with a spatial period close to the diameter of the large particle as well as the equilibrium density distribution does. These results show that a polydisperse hard-disk fluid confined in a circular cavity rivals similar structural properties compared with those of a multicomponent hard-disk fluid.
Large and abrupt optical band gap variation in In-doped ZnO
Authors: Kim Kwang Joo , Park Young Ran
Published: July 2002

Optical absorption properties of n-type In-doped ZnO films were investigated by spectroscopic ellipsometry for varying carrier concentration. The fundamental optical band gap (E0) edge of the compound showed a blueshift below the carrier concentration n0=5×1019 cm−3, which can be explained in terms of the Burstein–Moss band-filling effect. An abrupt jump of the E0 edge from blue- to redshift was observed as the carrier concentration increased beyond n0. It is interpreted as due to a merging of the donor and conduction bands of the compound near n0. The redshift increases quite linearly with the carrier concentration, reaching 600 meV for n=1.2×1020 cm−3. Such linear increase is interpreted as mainly due to a band gap narrowing caused by impurity-induced potential fluctuations.
Enhancing the active lifetime of luminescent semiconducting polymers via doping with metal nanoshells
Authors: Hale G. D. , Jackson J. B. , Shmakova O. E. , Lee T. R. , Halas N. J.
Published: July 2002

We report a dramatic, concentration-dependent decrease in the rate of photo-oxidation of semiconducting polymers due to the addition of small amounts of metal nanoshells to the polymer. In each case, the nanoshell resonances are tuned to the triplet exciton-ground state energy of the polymer. The nanoshell dopants slow the oxidation rate yet do not affect the photoluminescent properties of the polymers to which they have been added.
Observation of Fe-related defects in neutron irradiated semi-insulating InP
Authors: Marı́ B. , Hernández-Fenollosa M. A. , Navarro F. J.
Published: July 2002

Optical absorption and positron lifetime measurements have been performed on Fe-doped semi-insulating InP single crystals irradiated with thermal neutrons in a wide dose range from 0.1 to 2.7×1017 n cm−2. Two lifetimes were found: τ1=210 ps is constant in all the irradiation range; and τ2=340 ps reaches an intensity of almost 40% at the higher fluence used. When comparing these results with those obtained on unintentionally doped InP, a large increase of the longest lifetime is observed, from 300 ps in the nondoped InP to 340 ps in the semi-insulating InP. The increase of the second lifetime in InP:Fe means that the positron traps are less attractive to positrons. These positron traps have been associated to a complex defect generated by the main neutron-originated defect, the indium vacancy, and the clusters or interstitial atoms of Fe. The optical absorption spectra show a background absorption related to Fe precipitates in as-grown InP:Fe. This background absorption disappears after neutron irradiation, suggesting the destruction of Fe precipitates by the energetic particles generated in the transmutation process of In115.
Interaction of ultrashort high-intensity laser pulses with atomic clusters
Authors: Eloy M. , Azambuja R. , Mendonça J. T. , Bingham R.
Published: July 2002

A fully relativistic particle-in-cell code is used to simulate atomic cluster explosions following the interaction with ultrashort high-intensity laser pulses, investigating the dynamics of a 1 μm hydrogen cluster explosion and providing information about the time-resolved position, momentum, and energy of electrons and ions, for different laser intensities. The results indicate that the mechanism responsible for cluster expansion is Coulombic explosion producing MeV ions and electrons.
Numerical study of spin relaxation by thermal fluctuation: Effect of shape anisotropy
Authors: Lee K. J. , Park N. Y. , Lee T. D.
Published: July 2002

Effects of the shape anisotropy on the thermally activated spin relaxation have been investigated using the stochastic Landau–Lifshitz–Gilbert equation. The relaxation times of a noninteracting particle and a thin film were compared with each other. In a noninteracting particle, the relaxation time largely increased with the shape anisotropy when the damping constant was smaller than a certain critical value. In this study, the critical damping constant was 0.02. However, the effect of the shape anisotropy on the energy barrier was negligible in a thin film. All of these results can be explained from the effect of magnetostatic interaction that is enhanced by precession motion at low damping constant.
Magnetism of Fe–Al disordered alloys: An Ising–Monte Carlo approach
Authors: Restrepo J. , Pérez Alcázar G. A. , Landau D. P.
Published: July 2002

Using a model of atoms randomly distributed on a cubic lattice, we have simulated the atomic disorder induced in quenched binary alloys. Our study has been developed within the framework of a random site-diluted Ising model with nearest-neighbor interactions and used a Monte Carlo algorithm implemented with Metropolis kinetics for sampling states. After equilibration, ensemble and configurational averages for magnetization, magnetic susceptibility, and heat capacity were computed. We conclude that, in agreement with previous experimental Mössbauer data for which a comparison is carried out, Fe–Al disordered alloys exhibit room-temperature ferromagnetic behavior up to around 43 at. % Al, beyond which the system becomes paramagnetic. This result contrasts with that for alloys with atomic order, which exhibit ferromagnetism only up to 30 at. % Al.
Electron and trap dynamics in As-ion-implanted and annealed GaAs
Authors: Giniũnas L. , Danielius R. , Tan H. H. , Jagadish C. , Adomavičius R. , Krotkus A.
Published: July 2002

The ultrafast dynamics of As-ion-implanted and annealed GaAs is investigated using transmission pump–probe measurements. Carrier recombination time was found to increase from 4 to 40 ps with increasing annealing temperature. At lower annealing temperatures, the transmitted optical signal is dominated by induced absorption and at higher annealing temperatures this effect is replaced by induced transparency.
Finite-size effects in giant magnetoresistance multilayers
Authors: Perez M. , Marrows C. H. , Hickey B. J.
Published: July 2002

It is of interest to determine the characteristic length scale that determines giant magnetoresistance (GMR). In order to understand this behavior, GMR multilayers of Co/Ru and Co/Cu have been studied at a temperature of 4.2 K. The total thickness of Co/Ru multilayers has been varied from 96 to 1654 Å and Co/Cu from 77 to 2712 Å by increasing the number of bilayers (N). It has been observed that GMR increases with the number of bilayers and more than 20 bilayers for Co/Ru and 50 for Co/Cu are needed to reach the saturation value.
Domain-induced anomaly of the spin-wave frequency in perpendicularly magnetized Ni(001) ultrathin films
Authors: Gubbiotti G. , Albini L. , Carlotti G. , Pini M. G. , Politi P. , Rettori A. , Vavassori P. , Ciria M. , Ha K. , O’Handley R. C.
Published: July 2002

Epitaxial Cu/Ni/Cu/Si(001) magnetic films have been studied in the thickness range (17–120 Å) where a uniaxial anisotropy favoring the direction perpendicular to the film plane is dominant over the easy-plane dipolar interaction. Using magneto-optic magnetometry and Brillouin light scattering we show that on decreasing an in-plane magnetic field the perpendicular component of the magnetization breaks into domains.
Spin transport dynamics of excitons in CdTe/Cd1−xMnxTe quantum wells
Authors: Kayanuma Kentaro , Shirado Eiji , Debnath Mukul C. , Souma Izuru , Chen Zhanghai , Oka Yasuo
Published: July 2002

Transport properties of spin-polarized excitons were studied in the double quantum well system composed of Cd0.95Mn0.05Te and CdTe wells. Circular polarization degrees of the time resolved exciton photoluminescence in magnetic field showed that the spin-polarized excitons diffused from the magnetic quantum well and injected to the non-magnetic quantum well by conserving their spins. The spin-polarized excitons injected into the nonmagnetic well reaches 18% of the nonmagnetic well excitons. From the circular polarization degree and the lifetime of the magnetic quantum well excitons, the spin relaxation time of the excitons in the Cd0.95Mn0.05Te well was determined as 275–10 ps depending on the magnetic field strength.
Magnetic phase diagram of the charge ordered manganite Pr0.8Na0.2MnO3
Authors: Hejtmánek J. , Jirák Z. , Šebek J. , Strejc A. , Hervieu M.
Published: July 2002

Pr 1−x Na x MnO 3 solid solutions up to x=0.25 were prepared. Irrespective of magnetic order all samples are electrically insulating. The Curie temperature is maximum at TC=125 K for x=0.15; the samples with x⩾0.2 are charge-ordered antiferromagnets. The x=0.2 antiferromagnetic charge-ordered insulator (AFCO) can be switched to the ferromagnetic metallic state (FMM) by application of a relatively weak external magnetic field. This is accompanied by a huge CMR effect, irreversible at low temperatures. The neutron diffraction data, which show a decoupling of charge order (TCO=215 K) and long range AF order of the “pseudo” CE (TN=175 K), are supported by specific heat measurement. The specific heat at 14 T confirmed the transformation of the x=0.2 sample to FMM state and the nature of the AFCO–FMM transition was probed by magnetic moment and electrical resistivity relaxation measurements. This comprehensive characterization of the sample led to a complete determination of the magnetic phase diagram which demonstrated the near degeneracy of the ferro- and antiferromagnetic ground states in Pr0.8Na0.2MnO3.
Electron paramagnetic resonance and magnetization in Co doped La2/3Ca1/3MnO3
Authors: Rivadulla F. , Sande P. , Causa M. T. , Hueso L. E. , López-Quintela M. A. , Rivas J.
Published: July 2002

Spin dynamics of the Mn-site substituted colossal magnetoresistance system were studied by means of electron paramagnetic resonance (EPR) and dc magnetic susceptibility in La2/3Ca1/3Mn1−xCoxO3 (0⩽x⩽0.1,Δx=0.025). A dramatic increase in the high temperature limiting value for the EPR linewidth (ΔHpp(∞)) was observed with doping. Structural analysis revealed that only negligible changes are produced in bond distances and angles at these low doping levels. On the other hand, dipolar contribution to ΔHpp(∞) was estimated in a few Gauss. To explain the effect of doping on ΔHpp(∞), we propose here a scenario including fast Mn-spin lattice relaxation through paramagnetic Co ions.
Exploring the microscopic origin of exchange bias with photoelectron emission microscopy (invited)
Authors: Scholl A. , Nolting F. , Stöhr J. , Regan T. , Lüning J. , Seo J. W. , Locquet J.-P. , Fompeyrine J. , Anders S. , Ohldag H. , Padmore H. A.
Published: July 2002

It is well known that magnetic exchange coupling across the ferromagnet–antiferromagnet interface results in an unidirectional magnetic anisotropy of the ferromagnetic layer, called exchange bias. Despite large experimental and theoretical efforts, the origin of exchange bias is still controversial, mainly because detection of the interfacial magnetic structure is difficult. We have applied photoelectron emission microscopy (PEEM) on several ferromagnet–antiferromagnet thin-film structures and microscopically imaged the ferromagnetic and the antiferromagnetic structure with high spatial resolution. Taking advantage of the surface sensitivity and elemental specificity of PEEM, the magnetic configuration and critical properties such as the Néel temperature were determined on LaFeO3 and NiO thin films and single crystals. On samples coated with a ferromagnetic layer, we microscopically observe exchange coupling across the interface, causing a clear correspondence of the domain structures in the adjacent ferromagnet and antiferromagnet. Field dependent measurements reveal a strong uniaxial anisotropy in individual ferromagnetic domains. A local exchange bias was observed even in not explicitly field-annealed samples, caused by interfacial uncompensated magnetic spins. These experiments provide highly desired information on the relative orientation of electron spins at the interface between ferromagnets and antiferromagnets.
Formulation of radiation temperature in hohlraums with large radiation energy loss
Authors: Chang Tieqiang , Wang Guangyu
Published: July 2002

This paper describes the radiation temperature of a hohlraum with large radiation energy loss. The hohlraum radiation temperature is crucial for indirect drive laser fusion and the radiation energy loss induced by the laser entrance hole or in other ways that can seriously reduce the radiation temperature. An analytic, concise formula is given in this paper, which needs not assume a special form of laser pulse. The corresponding numerical results are given for some typical laser pulses. It is shown that the temperature correction due to the radiation energy loss is sensitive to its area fraction, but relatively less sensitive to laser pulse shape, and weakly dependent of the incident laser energy.
High frequency write head measurement with the phase detection magnetic force microscope
Authors: Abe M. , Tanaka Y.
Published: July 2002

We demonstrated the measurement of the high frequency (HF) magnetic field of a write head with the phase detection magnetic force microscope. An amplitude-modulated current was applied to the head coil to detect the force gradient induced by the HF magnetic field. Spatial resolution of this method was higher than that of the deflection detection method previously proposed. By the phase detection method, dynamic HF magnetic fields at the poles of the write heads were clearly imaged. HF magnetic field leakage was observed along the P2 pole shape on the air-bearing surface. The frequency dependence of the write head dynamics up to 350 MHz was also investigated.
Fabrication of Ge1−xMnxTe ferromagnetic fine structure using phase change technology
Authors: Fukuma Y. , Asada H. , Nishimura N. , Koyanagi T.
Published: July 2002

We present a novel fabrication method of desirable ferromagnetic fine structure patterns in an amorphous GeMnTe film by using phase change optical disk technology. The a-Ge1−xMnxTe films are prepared by the rf sputtering and ionized-cluster beam (ICB) methods. The characteristics of crystalline wire fabricated by irradiating a laser beam in a-Ge1−xMnxTe films are reported. The ferromagnetic crystalline wire pattern of 1 μm in width is successfully formed in this experiment. The magnetic property of crystalline wire for the sputtering methods is similar to that for the ICB method. Magnetoresistance exhibits the negative in both cases of the magnetic fields applied along the longitudinal and transverse directions of crystalline wires. Through the measurement of the magnetic field angular dependence of magnetroresistance, anisotropic magnetoresistance-like behavior is observed in a diluted magnetic semiconductor system.
Spin-dependent tunneling junctions with parallel hard bias for read heads
Authors: Sin Kyusik , Gibbons Matthew R. , Funada Shin , Mao Ming , Rao Dean , Chien Chester , Tong Hua-Ching
Published: July 2002

We investigated the feasibility of a spin-dependent tunneling (SDT) read head with a parallel hard bias. In this scheme, the longitudinal biasing to the free layer is provided by fringe fields from a hard magnet which is fabricated over or under the free layer. A linear response to the applied field is achieved for a SDT junction biased with 400 Å CoCrPt underneath. Thinner CoCrPt layers yield Barkhausen jumps in the free layer. Micromagnetic simulation indicates the bias field at the edge of the free layer is smaller than that which would result from an abutted magnet. The simulation results are similar to experimental data, and indicate that shielded devices with 400 Å permanent magnet will provide stable transfer curves.
Deep centers in a free-standing GaN layer
Authors: Fang Z.-Q. , Look D. C. , Visconti P. , Wang D.-F. , Lu C.-Z. , Yun F. , Morkoç H. , Park S. S. , Lee K. Y.
Published: July 2002

Schottky barrier diodes, on both Ga and N faces of a ∼300-μm-thick free-standing GaN layer, grown by hydride vapor phase epitaxy (HVPE) on Al2O3 followed by laser separation, were studied by capacitance–voltage and deep level transient spectroscopy (DLTS) measurements. From a 1/C2 vs V analysis, the barrier heights of Ni/Au Schottky contacts were determined to be different for the two polar faces: 1.27 eV for the Ga face, and 0.75 eV for the N face. In addition to the four common DLTS traps observed previously in other epitaxial GaN including HVPE-grown GaN a new trap B′ with activation energy ET=0.53 eV was found in the Ga-face sample. Also, trap E1 (ET=0.18 eV), believed to be related to the N vacancy, was found in the N-face sample, and trap C (ET=0.35 eV) was in the Ga-face sample. Trap C may have arisen from reactive-ion-etching damage.
Temperature- and field-dependent quantum efficiency in tris-(8-hydroxy) quinoline aluminum light-emitting diodes
Authors: Saha S. K. , Su Y. K. , Juang F. S.
Published: July 2002

Temperature- and field-dependent electroluminescence and quantum efficiency have been investigated in tris-(8-hydroxy) quinoline aluminum (Alq3) light-emitting diodes over the temperature range from 10 to 300 K. At lower applied voltage, two peaks have been observed in the quantum efficiency with temperature. The two peaks are attributed to the deep trap levels (high-temperature regime) and shallow trap levels (low-temperature regime) in Alq3. With increasing voltage, the high-temperature peak shifts toward lower temperature but no significant shift of the low-temperature peak is observed. At voltage around 10 V, superposition of two peaks causes the apparent saturation in the low-temperature regime of the quantum efficiency.
Electron transport across fractal-like nanocrystalline clusters in N+ ion-beam induced poly(phenylene oxide)
Authors: Das A. , Dhara S. , Patnaik A.
Published: July 2002

Nanocrystalline carbonaceous cluster evolution and electron transport in the N+ beam induced spin coated poly(2,6-dimethyl-1,4-phenylene oxide) thin films as a function of ion fluence has been investigated. Following Robertson’s model and electron diffraction, the narrow optical band gaps were explained in terms of polyaromatic, single crystalline graphitelike clusters. With a threshold fluence of 1×1015 ions/cm2 for cluster growth, the size of the clusters ranged from 2 to 50 nm with the number of aromatic rings varying between 20 and 170 over the entire fluence range upto 8×1016 ions/cm2. A molecular reconstruction/self organization has been envisaged as a possible clue to the above structure evolution upon a critical energy density transferred to the 53 nm implanted layer. Transmission electron microscopy study of fractal scaling in the nanoparticle aggregates revealed a fractal dimension of 1.37±0.02 with the growth process to follow a diffusion limited aggregation model. Electrical conductivity data are explained in terms of a phase transition from an insulating state to a trap controlled hopping conduction of charge carriers between localized states on the backbone cluster with a backbone fractal exponent ∼3.
Electrical isolation of n-type and p-type InP layers by proton bombardment
Authors: Boudinov H. , Tan H. H. , Jagadish C.
Published: July 2002

The evolution of the sheet resistance (Rs) of n-type and p-type conductive InP layers during proton irradiation and the stability of the formed isolation during postirradiation annealing were investigated. It was found that the threshold dose (Dth) to convert the conductive layer to a highly resistive one is different for n- and p-type samples with similar initial free carrier concentrations. From our results, one infers that the antisite defects and/or related defect complexes formed by the replacement collisions are the carrier trapping centers, where InP is responsible for electron trapping and PIn for the hole trapping. A time dependence of the Rs was observed after each irradiation step to doses of ≅Dth and higher. This time variation is related to metastable processes involving free carriers. The thermal stability of the isolation of n-type samples is limited to temperatures lower than 200 °C, irrespectively of the irradiated dose. For p-type samples the thermal stability of electrical isolation is extended to 450–500 °C.
Low-loss optical planar waveguides in Li2B4O7 crystal formed by He+ implantation
Authors: Boudrioua A. , Bakhouya Ch. , Loulergue J. C. , Moretti P. , Polgár K.
Published: July 2002

Formation of planar waveguide in lithium tetraborate (Li2B4O7) crystal by 2 MeV helium (He+) implantation is reported. Investigation of the induced defects is performed by using a modified m-lines setup. Results indicate that the optical properties of the guiding core are not affected by the implantation process. Moreover the produced defects are shown to be isotropic and localized at the optical barrier position without any extension towards the surface. Optical losses measured by using a new approach are found to be less than 2 dB cm−1. Finally the influence of postannealing procedure applied to the sample showed a stability of the optical properties of the guide with 10% improvement of optical losses.
Role of correlation in the operation of quantum-dot cellular automata
Authors: Tóth Géza , Lent Craig S.
Published: July 2002

Quantum-dot cellular automata (QCA) may offer a viable alternative of traditional transistor-based technology at the nanoscale. When modeling a QCA circuit, the number of degrees of freedom necessary to describe the quantum mechanical state increases exponentially making modeling even modest size cell arrays difficult. The intercellular Hartree approximation largely reduces the number of state variables and still gives good results especially when the system remains near ground state. This suggests that a large part of the correlation degrees of freedom are not essential from the point of view of the dynamics. In certain cases, however, such as, for example, the majority gate with unequal input legs, the Hartree approximation gives qualitatively wrong results. An intermediate model is constructed between the Hartree approximation and the exact model, based on the coherence vector formalism. By including correlation effects to a desired degree, it improves the results of the Hartree method and gives the approximate dynamics of the correlation terms. It also models the majority gate correctly. Beside QCA cell arrays, our findings are valid for Ising spin chains in transverse magnetic field, and can be straightforwardly generalized for coupled two-level systems with a more complicated Hamiltonian.
High-transmittance surface textures formed by plasma etching of metallophthalocyanine films
Authors: Sakata Hajime
Published: July 2002

The effect of rf-induced plasma etching on thermally evaporated metallophthalocyanine films is investigated. Etching by a gas mixture of nitrogen and CF4 results in a transparent microstructuring residue with a grain size in the 100 nm range. The residue-covered surface increases visible transmittance up to 3%–4% over the glass substrate. The high-transmittance effect, which is nearly insensitive to wavelength, is characterized by modeling a gradient refractive-index profile bounded on discrete interfaces with surrounding media.
High peak-to-valley ratios observed in InAs/InP resonant tunneling quantum dot stacks
Authors: Borgstrom M. , Bryllert T. , Sass T. , Gustafson B. , Wernersson L.-E. , Seifert W. , Samuelson L.
Published: July 2002

Resonant tunneling was observed through single InAs quantum dot (QD) stacks embedded in InP barriers with peak-to-valley ratios as high as 85 at 7 K. Negative differential resistance in the current–voltage [I(V)] characteristics was obtained up to a point above the temperature of liquid nitrogen. These features were observed in measurements on low-density QD stacks, in which a macroscopic ohmic contact covered less than 150 QD stacks. Due to the design of the structure, the upper QD in the stack has the function of a zero-dimensional emitter. Electrons easily fill the upper dot, whereas tunneling through the entire structure is only allowed when two states in the dots align energetically, resulting in sharp resonant tunneling peaks with high peak-to-valley ratios.
Processing of amorphous carbon films by ultrafast temperature treatment in a confined geometry
Authors: Lenz J. A. , Perottoni C. A. , Balzaretti N. M. , da Jornada J. A. H.
Published: July 2002

A pressure cell with an anvil made of sapphire and the other made of tungsten carbide, was constructed to process thin film samples using a high power Nd:YAG pulsed laser, in a regime of ultrafast quenching rate and confined geometry. The sapphire anvil worked as the optical window to the laser beam and also as a good thermal conductor substrate. Thin films of amorphous carbon deposited over copper substrate were processed under pressure by Nd:YAG laser pulses. This process induces the formation of a high temperature region at the sample surface during a very short time interval of the order of the 8 ns laser pulse duration. To avoid the complete evaporation of the film, an external pressure of about 0.5 to 1.0 GPa was applied, confining the sample. With the aid of the nanosecond pulsed laser, absorbed on a very thin film sample, this specially designed apparatus provides the means to produce ultrafast quenching as the formation of a plume is suppressed and heat dissipation is accelerated by the high thermal conductivity of the copper substrate and sapphire anvil. The processed samples were analyzed by microRaman spectroscopy and the results indicated the formation of polyynic carbyne structures, as revealed by the presence of a characteristic Raman peak at about 2150 cm−1. Another set of Raman peaks observed at 996, 1116, and 1498 cm−1, also appeared when the amorphous carbon film was processed with a sequence of more than three consecutive laser pulses. These peaks, whose general aspect is very similar to that of polyacethylene (CnHn), could be ascribed to the cumulenic carbyne structure, stabilized by some dispersed copper atoms.
Continuous compositional-spread technique based on pulsed-laser deposition and applied to the growth of epitaxial films
Authors: Christen H. M. , Silliman S. D. , Harshavardhan K. S.
Published: July 2002

A novel continuous-compositional-spread (CCS) technique based on the nonuniformity of the deposition rate typically observed in pulsed-laser deposition (PLD) is introduced. Using rapid (submonolayer) sequential deposition of the phase spread’s constituents, intermixing of the constituents occurs on the atomic scale during the growth process. Therefore, a pseudobinary or pseudoternary phase diagram is deposited without the requirement of a postanneal. The approach uses the spatial variations in the deposition rate naturally occurring in PLD; therefore, there is no need for the masks typically used in combinatorial techniques. Consequently, combinatorial materials synthesis can be carried out under optimized film growth conditions (for example, complex oxides can be grown at high temperature). Additionally, lifting the need for postannealing renders this method applicable to heat-sensitive materials and substrates (e.g., films of transparent oxides on polymer substrates). PLD CCS thus offers an interesting alternative to traditional “combi” for situations where the number of constituents is limited, but the process variables are of critical importance. Additionally, the approach benefits from all the advantages of PLD, particularly the flexibility and the possibility to work with targets of relatively small size. Composition determination across the sample and mapping of physical properties onto the ternary phase diagram is achieved via a simple algorithm using the parameters that describe the deposition-rate profiles. Experimental verification using energy-dispersive x-ray spectroscopy and Rutherford backscattering spectroscopy measurements demonstrates the excellent agreement between the predicted and the calculated composition values. Results are shown for the high-temperature growth of crystalline perovskites [including (Ba,Sr)TiO3 and the formation of a metastable alloy between SrRuO3 and SrSnO3] and the room-temperature growth of transparent conducting oxides.
Measurement and theoretical simulation of the HCCO− anion photoelectron spectrum
Authors: Schäfer-Bung Boris , Engels Bernd , Taylor Travis R. , Neumark Daniel M. , Botschwina Peter , Perić Miljenko
Published: July 2002

The photoelectron spectrum of HCCO− at the photodetachment wavelength of 355 nm is reported. A theoretical model for the simulation of the photodetachment process is described and the influence of various parameters is discussed. The experimental spectrum is compared with the simulation and an assignment of the spectrum is given.
Optical characterization and laser gain modeling of a NdAl3(BO3)4 (NAB) microchip laser crystal
Authors: Jaque D. , Enguita O. , Caldiño G. U. , Ramı́rez M. O. , Garcı́a Solé J. , Zaldo C. , Muñoz-Santiuste J. E. , Jiang A. D. , Luo Z. D.
Published: July 2002

In this work a systematic optical characterization of the NdAl3(BO3)4 (NAB) crystal has been carried out by means of spectroscopic measurements. The energy levels (118) of the Nd3+ constituent ions have been identified and labeled by appropriate crystal field quantum numbers (μ=1/2 or μ=3/2). Laser site selective experiments revealed the presence of two nonequivalent environments for the Nd3+ ions. From room temperature absorption and emission spectra and using the Judd–Ofelt formalism the emission cross sections and quantum efficiency (η=0.1) of the metastable state F3/24 have been determined. By further modeling we have estimated the crystal length and output coupler transmittance which optimize laser efficiency of a diode pumped NAB laser. The optimum crystal length found in all cases is around 100 μm. Finally, the high laser efficiencies predicted (up to 61% and 45% for laser oscillation at 1.06 and 1.34 μm, respectively) make NAB an excellent candidate to be incorporated in a diode pumped microchip laser device.