bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron bubbles in helium clusters. I. Structure and energetics
Authors: Rosenblit Michael , Jortner Joshua
Published: May 2006

In this paper we present a theoretical study of the structure, energetics, potential energy surfaces, and energetic stability of excess electron bubbles in (He4)N (N=6500–106) clusters. The subsystem of the helium atoms was treated by the density functional method. The density profile was specified by a void (i.e., an empty bubble) at the cluster center, a rising profile towards a constant interior value (described by a power exponential), and a decreasing profile near the cluster surface (described in terms of a Gudermannian function). The cluster surface density profile width (∼6Å) weakly depends on the bubble radius Rb, while the interior surface profile widths (∼4–8Å) increase with increasing Rb. The cluster deformation energy Ed accompanying the bubble formation originates from the bubble surface energy, the exterior cluster surface energy change, and the energy increase due to intracluster density changes, with the latter term providing the dominant contribution for N=6500–2×105. The excess electron energy Ee was calculated at a fixed nuclear configuration using a pseudopotential method, with an effective (nonlocal) potential, which incorporates repulsion and polarization effects. Concurrently, the energy V0 of the quasi-free-electron within the deformed cluster was calculated. The total electron bubble energies Et=Ee+Ed, which represent the energetic configurational diagrams of Et vs Rb (at fixed N), provide the equilibrium bubble radii Rbc and the corresponding total equilibrium energies Ete, with Ete(Re) decreasing (increasing) with increasing N (i.e., at N=6500, Re=13.5Å and Ete=0.86eV, while at N=1.8×105, Re=16.6Å and Ete=0.39eV). The cluster size dependence of the energy gap (V0−Ete) allows for the estimate of the minimal (He4)N cluster size of N≃5200 for which the electron bubble is energetically stable.
Electron bubbles in helium clusters. II. Probing superfluidity
Authors: Rosenblit Michael , Jortner Joshua
Published: May 2006

In this paper we present calculations of electron tunneling times from the ground electronic state of excess electron bubbles in (He4)N clusters (N=6500–107, cluster radius R=41.5–478Å), where the equilibrium bubble radius varies in the range Rb=13.5–17.0Å. For the bubble center located at a radial distance d from the cluster surface, the tunneling transition probability was expressed as A0φ(d,R)exp(−βd), where β≃1Å−1 is the exponential parameter, A0 is the preexponential factor for the bubble located at the cluster center, and φ(d,R) is a correction factor which accounts for cluster curvature effects. Electron tunneling dynamics is grossly affected by the distinct mode of motion of the electron bubble in the image potential within the cluster, which is dissipative (i.e., τD<τ0) in normal fluid (He4)N and (He3)N clusters, while it is undamped (i.e., τD⪢τ0) in superfluid (He4)N clusters, where τD is the bubble motional damping time (τD≃4×10−12s for normal fluid clusters and τD≃10s for superfluid clusters), while τ0≃10−9–10−10s is the bubble oscillatory time. Exceedingly long tunneling lifetimes, which cannot be experimentally observed, are manifested from bubbles damped to the center of the normal fluid cluster, while for superfluid clusters electron tunneling occurs from bubbles located in the vicinity of the initial distance d near the cluster boundary. Model calculations of the cluster size dependence of the electron tunneling time (for a fixed value of d=38–39Å), with lifetimes increasing in the range of 10−3–0.3s for N=104–107, account well for the experimental data [M. Farnik and J. P. Toennies, J. Chem. Phys. 118, 4176 (2003)], manifesting cluster curvature effects on electron tunneling dynamics. The minimal cluster size for the dynamic stability of the bubble was estimated to be N=3800, which represents the threshold cluster size for which the excess electron bubble in (He4)N− clusters is amenable to experimental observation.
Observation of hot luminescence and slow inter-sub-band relaxation in Si-doped GaN∕AlxGa1−xN (x=0.11, 0.25) multi-quantum-well structures
Authors: Monroy E. , Guillot F. , Gayral B. , Bellet-Amalric E. , Jalabert D. , Gérard J.-M. , Si Dang Le , Tchernycheva M. , Julien F. H.
Published: May 2006

We report on the growth, structural, electronic, and optical properties of Si-doped GaN∕AlxGa1−xN (x=0.11, 0.25) multiple-quantum-well structures grown on SiC by plasma-assisted molecular-beam epitaxy. We have demonstrated that the use of In as a surfactant during growth improves the structural and optical properties of these layers. Photoluminescence studies have made possible the identification of the fundamental and excited electronic levels by comparison with simulations of the electronic structure. Temperature dependence studies reveal an anomalous behavior of the photoluminescence intensity, which is the quenching of the e1−hh1 line, while the e2−hh1 and e3−hh1 transitions become dominant at room temperature in the samples with 11% and 25% Al in the barrier, respectively. This behavior can be explained by the population of the e2 and e3 electronic states by thermally excited carriers, and by the higher oscillator strength of e2−hh1 and e3−hh1 transitions compared with e1−hh1, due to the intense electric field in the quantum wells. Long inter-sub-band (ISB) relaxation times (40–100ps) have been measured in GaN∕AlGaN multiple quantum wells, as a consequence of the quenching of the longitudinal optical (LO)-phonon emission when the ISB transition energy is below the LO-phonon energy.
Conductivity analysis of n-GaAs molecular beam epitaxial layers using multicarrier fitting
Authors: Wolkenberg A. , Przeslawski T. , Kaniewski J. , Kowalczyk E. , Reginski K.
Published: May 2006

We have set up a model for the energy-dependent lifetime, including the effects of the charge carrier collisions with ionized impurities, polar optical phonons, and space charge. The model is then used to compute the mobility spectrum at each temperature, which used to compute the pertinent magnetic-field-dependent Hall parameters. The computed Hall parameters compared can then be measured values to estimate the validity of the lifetime model and parameters. The method has been applied to two n-GaAs∕SI–GaAs epitaxial layers containing two types of carriers. The magnetic field dependence of the Hall voltage has been observed in the studied GaAs layers. We present the numerical solution of the neutrality equation, which contains all sources of charges present in the sample, and numerical integration of the total relaxation times ⟨τ⟩ so combined that is possible to obtain the mobility of the partial charges and their part in the whole conductivity of the layer.
Reversible tuning of surface plasmon resonance of silver nanoparticles using a thermochromic matrix
Authors: Jin P. , Tazawa M. , Xu G.
Published: May 2006

Reversible tuning of surface plasmon resonance (SPR) of Ag nanoparticles by temperature was achieved using a thermochromic VO2 as matrix. The SPR wavelength (λSPR) was significantly shifted from 506nm for Ag on sapphire to 1070nm for Ag embedded in VO2 at room temperature, giving a large sensitivity factor (ΔλSPR∕Δn) of approximate 352nm per refractive index (n) unit. The λSPR of Ag in VO2 can be changed reversibly and easily by heating-cooling circles between λSPR=1070nm at 20°C and λSPR=844nm at 90°C, a tunable wavelength range of as large as 226nm only by changing the temperature. The reversible change of refractive index of thermochromic VO2 matrix upon temperature is responsible for this λSPR tuning.
Nonadiabatic response of molecules to strong fields of picosecond, femtosecond, and subfemtosecond duration: An experimental study of the methane dication
Authors: Mathur D. , Rajgara F. A.
Published: May 2006

The double ionization of methane has been accomplished using strong optical fields that are generated using moderately intense lasers, and by strong fields that are induced by fast-moving, highly charged ions. In the former case laser intensities in the range 1014Wcm−2 generate fields whose durations are of 35ps and 36fs while in the latter case equivalent fields last for only 200–300as. The dynamics of the field-ionized electrons are different in the two temporal regimes, fast (picoseconds), and ultrafast (few tens of femtoseconds and subfemtoseconds). Our experiments show that nonadiabatic effects come into play in the ultrafast regime; we directly monitor such effects by measuring the kinetic energy that is released when a specific bond in the doubly charged methane molecular ion breaks.
Environmentally induced quantum dynamical phase transition in the spin swapping operation
Authors: Álvarez Gonzalo A. , Danieli Ernesto P. , Levstein Patricia R. , Pastawski Horacio M.
Published: May 2006

Quantum information processing relies on coherent quantum dynamics for a precise control of its basic operations. A swapping gate in a two-spin system exchanges the degenerate states ∣↑, ↓⟩ and ∣↓, ↑⟩. In NMR, this is achieved turning on and off the spin-spin interaction b=ΔE that splits the energy levels and induces an oscillation with a natural frequency ΔE∕ℏ. Interaction of strength ℏ∕τSE, with an environment of neighboring spins, degrades this oscillation within a decoherence time scale τϕ. While the experimental frequency ω and decoherence time τϕ were expected to be roughly proportional to b∕ℏ and τSE, respectively, we present here experiments that show drastic deviations in both ω and τϕ. By solving the many spin dynamics, we prove that the swapping regime is restricted to ΔEτSE≳ℏ. Beyond a critical interaction with the environment the swapping freezes and the decoherence rate drops as 1∕τϕ∝(b∕ℏ)2τSE. The transition between quantum dynamical phases occurs when ω∝(b∕ℏ)2−(k∕τSE)2 becomes imaginary, resembling an overdamped classical oscillator. Here, 0⩽k2⩽1 depends only on the anisotropy of the system-environment interaction, being 0 for isotropic and 1 for XY interactions. This critical onset of a phase dominated by the quantum Zeno effect opens up new opportunities for controlling quantum dynamics.
A radio-frequency nonequilibrium atmospheric pressure plasma operating with argon and oxygen
Authors: Moravej M. , Yang X. , Hicks R. F. , Penelon J. , Babayan S. E.
Published: May 2006

A capacitively coupled, atmospheric pressure plasma has been developed that produces a high concentration of reactive species at a gas temperature below 300°C. The concentration of ground-state oxygen atoms produced by the discharge was measured by NO titration, and found to equal 1.2vol%, or 1.2±0.4×1017cm−3, using 6.0vol% O2 in argon at 150W∕cm3. The ozone concentration determined at the same conditions was 4.3±0.5×1014cm−3. A model of the gas phase reactions was developed and yielded O atom and O3 concentrations in agreement with experiment. This plasma source etched Kapton® at 5.0μm∕s at 280°C and an electrode-to-sample spacing of 1.5cm. This fast etch rate is attributed to the high O atom flux generated by the plasma source.
Experimental studies of the two-ion species flow in the plasma presheath
Authors: Wang X. , Hershkowitz N.
Published: May 2006

The ion flow created in the presheath of weakly ionized He+Ar plasma is studied experimentally. Ion acoustic wave measurements combined with previous LIF measurements [Phys. Rev. Lett. 90, 145001 (2003)] suggest that the Ar ion and He ion drift velocities approach each other near the sheath-presheath boundary. Their common velocity is the ion sound velocity of the system. The Ar ion and He ion drift velocities are also calculated assuming mobility-limited flow, including symmetric and asymmetric ion-neutral collisions, combined with the measured plasma potential profile. Comparing these calculations to the wave and LIF data shows that both ion species drift in the presheath with mobility-limited flow until very close to the sheath edge.
Tuning of the metal-insulator transition in iodine incorporated amorphous carbon
Authors: Kumari Latha , Subramanyam S. V.
Published: May 2006

We present the synthesis and properties of iodine incorporated amorphous carbon films. Optical studies depict a decrease in band gap with variation in iodine content and pyrolysis temperature. Tuning of the metal-insulator transition is achieved by varying the pyrolysis temperature and iodine concentration. Appreciable decrease in magnetoresistance is observed with iodine incorporation, but negative magnetoresistance typical behavior of metallic samples is not witnessed.
Low-temperature photoluminescence study of thin epitaxial GaAs films on Ge substrates
Authors: Brammertz Guy , Mols Yves , Degroote Stefan , Motsnyi Vasyl , Leys Maarten , Borghs Gustaaf , Caymax Matty
Published: May 2006

Thin epitaxial GaAs films, with thickness varying from 140to1000nm and different Si doping levels, were grown at 650°C by organometallic vapor phase epitaxy on Ge substrates and analyzed by low-temperature photoluminescence (PL) spectroscopy. All spectra of thin GaAs on Ge show two different structures, one narrow band-to-band (B2B) structure at an energy of ∼1.5eV and a broad inner-band-gap (IB) structure at an energy of ∼1.1eV. Small strain in the thin GaAs films causes the B2B structure to be separated into a light-hole and a heavy-hole peak. At 2.5K the good structural quality of the thin GaAs films on Ge can be observed from the narrow excitonic peaks. Peak widths of less than 1meV are measured. GaAs films with thickness smaller than 200nm show B2B PL spectra with characteristics of an n-type doping level of approximately 1018at.∕cm3. This is caused by heavy Ge diffusion from the substrate into the GaAs at the heterointerface between the two materials. The IB structure observed in all films consists of two Gaussian peaks with energies of 1.04 and 1.17eV. These deep trapping states arise from Ge-based complexes formed within the GaAs at the Ge–GaAs heterointerface, due to strong diffusion of Ge atoms into the GaAs. Because of similarities with Si-based complexes, the peak at 1.04eV was identified to be due to a GeGa–GeAs complex, whereas the peak at 1.17eV was attributed to the GeGa-VGa complex. The intensity of the IB structure decreases strongly as the GaAs film thickness is increased. PL intensity of undoped GaAs films containing antiphase domains (APDs) is four orders of magnitude lower than for similar films without APDs. This reduction in intensity is due to the electrically active Ga–Ga and As–As bonds at the boundaries between the different APDs. When the Si doping level is increased, the PL intensity of the APD-containing films is increased again as well. A film containing APDs with a Si doping level of ∼1018at.∕cm3 has only a factor 10 reduced intensity. We tentatively explain this observation by Si or Ge clustering at antiphase boundaries, which eliminates the effects of the Ga–Ga and As–As bonds. This assumption is confirmed by the fact that, at 77K, the ratio between the intensity of the IB peak at 1.17eV to the intensity of the peak at 1.04eV is smaller than 1.4 for all films containing APDs, whereas it is larger than 1.4 for all films without APDs. This shows stronger clustering of Si or Ge in the material with APDs. For future electronic applications, Ge diffusion into the GaAs will have to be reduced. PL analysis will be a rapid tool for studying the Ge diffusion into the GaAs thin films.
Effects of dielectric charging on fundamental forces and reliability in capacitive microelectromechanical systems radio frequency switch contacts
Authors: Patton Steven T. , Zabinski Jeffrey S.
Published: May 2006

Microelectromechanical systems (MEMS) radio frequency switches hold great promise in a myriad of commercial, aerospace, and military applications. In particular, capacitive-type switches with metal-to-dielectric contacts (typically gold-on-silicon nitride) are suitable for high frequency (⩾10GHz) applications. However, the devices are known to be unstable in their performance due to parasitic dielectric charging. Although several authors have previously reported the switch failure along with shifts in pull-down and release voltages due to charging, there is some disagreement and lack of understanding among the various reports. This study uses a switch simulator capable of measuring microscale electrostatic and adhesive forces to investigate charging and its effect on reliability and fundamental forces acting within MEMS capacitive switches. An important advantage of the switch simulator is that it can be actuated with or without a bias voltage. Electrostatic force and dielectric charging increased as surfaces were worn smooth by cycling. This is because the surface smoothening decreases separation and increases the electric field strength inside the dielectric. A simple analytical model was developed using electromagnetic theory for the electrostatic force in terms of bias voltage and the areal density of parasitic charge. Using the model and experimental data, it was determined that “charging” (net charge is zero) with the same polarity as the bias voltage resulted in reduced electrostatic force (under bias voltage) when a worn-in switch was actuated repeatedly at constant bias voltage ⩾40V. Small electrostatic force under bias voltage can explain failure in the “up” position (failure to actuate and self-release). Reversing the polarity of the bias voltage between actuations prevented charge buildup and doubled the electrostatic force, which can help explain the effectiveness of bipolar actuation. The charging time constant for parasitic dielectric charge is about 30s under typical MEMS contact conditions. In the specific case of parasitic charge being present, high electrostatic and adhesion forces were measured at zero volts bias. This can explain failure in the down position under zero bias voltage due to self-actuation or adhesion.
Fundamental reactions controlling anion exchange during mixed anion heterojunction formation: Chemistry and kinetics of P-for-As exchange reaction
Authors: Brown April S. , Losurdo Maria , Capezzuto Pio , Bruno Giovanni , Brown Terence , May Gary
Published: May 2006

The fundamental chemical and kinetic surface processes governing the P-for-As exchange reaction during epitaxial layer synthesis are investigated. Exposure of a GaAs surface to phosphorus molecular beams (P2) is carried out to create superlattice structures realized by surface reactions. The impact of the GaAs surface reconstruction, the P-soak time, and the surface temperature on the extent of intermixing and on the mechanism governing the anion exchange has been studied using x-ray diffraction, spectroscopic ellipsometry, x-ray photoelectron spectroscopy, and atomic force microscopy. It is found that As-rich GaAs surface reconstructions inhibit P-for-As exchange. The extent of the anion exchange increases with temperature. Furthermore, the P-for-As exchange is not controlled by P diffusion into the GaAs. We propose a chemical model that includes P chemisorption and indiffusion, and the competition between P-for-As anion exchange and the formation of AsP isoelectronic compounds.
Electrical resistivity of Cu films deposited by ion beam deposition: Effects of grain size, impurities, and morphological defect
Authors: Lim J.-W. , Isshiki M.
Published: May 2006

Cu films deposited by ion beam deposition with or without a negative substrate bias voltage were found to have different states of dependence of electrical resistivity on film thickness. We have attempted to evaluate the effects of the film thickness, average grain size, impurities, and morphological defect on the resistivity increase of Cu films in addition to the surface scattering of the Fuch-Sondheimer model and the grain boundary scattering of the Mayadas-Shatzkes model. When the theoretical model was fit to the experimental data, the Cu films deposited at a substrate bias voltage of −50V showed that the electrical resistivity was in good agreement with the theoretical curve under the condition that the film thickness was 2.3 times larger than the average grain size and when p=0 and R=0.24. For the Cu films deposited without a substrate bias voltage, however, there was a slight deviation between the theoretical curve and the measured resistivity below a 100nm thickness, even at the condition of the film thickness that was six times larger than the average grain size, which is considered to result from the effect of solute impurities in the Cu films. Therefore it was found that the impurity effect on the electrical resistivity could not be neglected. It was also confirmed that the morphological defect, such as a columnar structure with noticeable gaps in the Cu films deposited without the substrate bias voltage, caused a great increase in resistivity even above the 100nm thickness.
Gettering of transition metal impurities during phosphorus emitter diffusion in multicrystalline silicon solar cell processing
Authors: Bentzen A. , Holt A. , Kopecek R. , Stokkan G. , Christensen J. S. , Svensson B. G.
Published: May 2006

We have investigated the gettering of transition metals in multicrystalline silicon wafers during a phosphorus emitter diffusion for solar cell processing. The results show that mainly regions of high initial recombination lifetime exhibit a significant lifetime enhancement upon phosphorus diffusion gettering. Nevertheless, transition metal profiles extracted by secondary ion mass spectrometry in a region of low initial lifetime reveal significant gradients in Cr, Fe, and Cu concentrations towards the surface after the emitter diffusion, without exhibiting a significant enhancement in the lifetime. In a region of higher initial lifetime, however, diminutive concentration gradients of the transition metal impurities are revealed, indicating a significantly lower initial concentration in these regions. From spatial maps of the dislocation density in the wafers, we find that lifetime enhancements mainly occur in regions of low dislocation density. Thus, it is believed that a generally higher concentration of transition metals combined with an impurity decoration of dislocations in regions of high dislocation density limit the initial lifetime and the lifetime after the phosphorus diffusion, in spite of the notable gettering of transition metal impurities towards the surface in these regions. Furthermore, after a hydrogen release from overlying silicon nitride layers, we observe that only regions of low dislocation density experience a significant lifetime enhancement. This is attributed to impurity decoration of the dislocations in the regions of both high dislocation density and high transition metal impurity concentration, reducing the ability of hydrogen to passivate dislocations in these regions.
Self-assembly and conformation of tetrapyridyl-porphyrin molecules on Ag(111)
Authors: Auwärter W. , Weber-Bargioni A. , Riemann A. , Schiffrin A. , Gröning O. , Fasel R. , Barth J. V.
Published: May 2006

We present a low-temperature scanning tunneling microscopy (STM) study on the supramolecular ordering of tetrapyridyl-porphyrin (TPyP) molecules on Ag(111). Vapor deposition in a wide substrate temperature range reveals that TPyP molecules easily diffuse and self-assemble into large, highly ordered chiral domains. We identify two mirror-symmetric unit cells, each containing two differently oriented molecules. From an analysis of the respective arrangement it is concluded that lateral intermolecular interactions control the packing of the layer, while its orientation is induced by the coupling to the substrate. This finding is corroborated by molecular mechanics calculations. High-resolution STM images recorded at 15K allow a direct identification of intramolecular features. This makes it possible to determine the molecular conformation of TPyP on Ag(111). The pyridyl groups are alternately rotated out of the porphyrin plane by an angle of 60°.
A line integral reaction path approximation for large systems via nonlinear constrained optimization: Application to alanine dipeptide and the β hairpin of protein G
Authors: Khavrutskii Ilja V. , Byrd Richard H. , Brooks Charles L.
Published: May 2006

A variation of the line integral method of Elber with self-avoiding walk has been implemented using a state of the art nonlinear constrained optimization procedure. The new implementation appears to be robust in finding approximate reaction paths for small and large systems. Exact transition states and intermediates for the resulting paths can easily be pinpointed with subsequent application of the conjugate peak refinement method [S. Fischer and M. Karplus, Chem. Phys. Lett. 194, 252 (1992)] and unconstrained minimization, respectively. Unlike previous implementations utilizing a penalty function approach, the present implementation generates an exact solution of the underlying problem. Most importantly, this formulation does not require an initial guess for the path, which makes it particularly useful for studying complex molecular rearrangements. The method has been applied to conformational rearrangements of the alanine dipeptide in the gas phase and in water, and folding of the β hairpin of protein G in water. In the latter case a procedure was developed to systematically sample the potential energy surface underlying folding and reconstruct folding pathways within the nearest-neighbor hopping approximation.
Primary photoluminescence in as-neutron (electron) -irradiated n-type 6H-SiC
Authors: Zhong Z. Q. , Wu D. X. , Gong M. , Wang O. , Shi S. L. , Xu S. J. , Chen X. D. , Ling C. C. , Fung S. , Beling C. D. , Brauer G. , Anwand W. , Skorupa W.
Published: May 2006

Low-temperature photoluminescence spectroscopy has revealed a series of features labeled S1, S2, S3 in n-type 6H-SiC after neutron and electron irradiation. Thermal annealing studies showed that the defects S1, S2, S3 disappeared at 500°C. However, the well-known D1 center was only detected for annealing temperatures over 700°C. This experimental observation not only indicated that the defects S1, S2, S3 were a set of primary defects and the D1 center was a kind of secondary defect, but also showed that the D1 center and the E1, E2 observed using deep level transient spectroscopy might not be the same type of defects arising from the same physical origin.
90° switching of polarization in La3+-doped SrBi2Ta2O9 thin films
Authors: Liu J. S. , Zhang S. R. , Zeng H. Z. , Fei W. D. , Du S. Y.
Published: May 2006

The crystal structure and polarization switching behavior of SrBi1.4La0.6Ta2O9 (SBLT) thin films have been studied by x-ray diffraction and piezoresponse force microscopy (PFM), respectively. Compared with SrBi2Ta2O9 (SBT), SBLT thin films show a reduced orthorhombic distortion. The polarization rotation of SBLT thin film, which is driven by negative and positive direct current (dc) biases, has been investigated by a combination of vertical and lateral PFM (VPFM and LPFM, respectively). After dc bias applications, the VPFM image is hardly changed, whereas the LPFM image experiences an obvious variation. It is believed that such difference is caused by 90° polarization switching. However, this kind of switching can be only realized by the exchange of a axis and b axis. By virtue of the reduced orthorhombic distortion, the a-b exchange in SBLT is easier than that in SBT. Unfortunately, stress is created due to the 90° polarization switching in SBLT thin films. The internal stress is found to increase with the repeated switching cycles, and so the polarization reorientation in SBLT is constrained. Thus, the fatigue resistance of SBLT thin films is not thought to be as good as that of SBT.
Fiber-optic evanescent field modulator using a magnetic fluid as the cladding
Authors: Pu Shengli , Chen Xianfeng , Chen Yuping , Xu Yonghao , Liao Weijun , Chen Lijun , Xia Yuxing
Published: May 2006

A kind of fiber-optic modulation-depth-tunable modulator is developed in this paper. In this modulator, a magnetic fluid is used as the cladding of the drawn fiber, which attenuates the evanescent field when the light is guided in the fiber. Because the attenuation depends on the strength of the applied external magnetic field, the emergent light intensity from the fiber is modulated with the change of the magnetic field strength. The response times, i.e., the span for the light intensity to reach the final steady value from the time when the magnetic field is turned on or off, are evaluated quantitatively.
Coexistence of a phase separation and an ordered structure in CdxZn1−xTe epilayers grown on GaAs(001) substrates
Authors: Lee H. S. , Sohn H. S. , Lee J. Y. , Lee K. H. , Kim Y. H. , Kim T. W. , Kwon M. S. , Park H. L.
Published: May 2006

The coexistence of a phase separation and an ordered structure in CdxZn1−xTe epilayers grown on GaAs(001) substrates by using molecular beam epitaxy was investigated. The results of selected-area electron diffraction pattern and transmission electron microscopy measurements showed that ordered structures, such as CuPt-type and CuAu-I-type ordered structures, together with a spinodal-like phase separation were formed in the CdxZn1−xTe∕GaAs heteroepitaxial layers. The coexistence of the phase separation and the ordered structures is discussed.
Coupling of cantilever lateral bending and torsion in torsional resonance and lateral excitation modes of atomic force microscopy
Authors: Song Yaxin , Bhushan Bharat
Published: May 2006

Torsional resonance (TR) mode and lateral excitation (LE) mode of atomic force microscopy (AFM) have been increasingly used in atomic and nanoscale surface imaging. In TR and LE modes, it is the changes of the cantilever dynamic characteristics caused by tip-sample interaction that reveal surface properties. For quantitative explanation of measured results, researchers have established dynamic models of TR and LE modes in which the AFM cantilever was modeled as a shaft in pure torsional vibration. However, the cantilever in TR and LE modes actually undergoes both torsion and lateral bending since due to lateral tip-sample interaction, not only a torque but also a lateral force are exerted on the cantilever. In this paper, a dynamic analysis is performed for TR and LE modes with the coupling between the cantilever torsion and lateral bending taken into account. Analytical solutions of cantilever responses are obtained considering linear viscoelastic tip-sample interaction. Through parametric analyses, we demonstrate that in TR and LE modes, the pure torsional approximation can provide accurate cantilever responses if the lateral tip-sample interaction is relatively weak compared to the cantilever torsional (or lateral bending) stiffness. Otherwise, lateral bending of the cantilever has to be considered in any endeavors for quantitative extraction of surface properties. The differences between the two modes due to the different excitation mechanisms are also discussed.
Enhanced electron field emission from oriented AlN films
Authors: Yue S. L. , Shi C. Y. , Li J. J. , Gu C. Z.
Published: May 2006

(002) and (100) oriented AlN films were deposited on silicon substrates and tungsten tips by radio frequency magnetron reactive sputtering. The electron field emission (FE) properties of (002) and (100) oriented AlN films were investigated and found to be significantly different in emission threshold and current density. The threshold electric field was only 0.14V∕μm for (002) oriented AlN film—far lower than the threshold of 1.13V∕μm for (100) oriented AlN film on tungsten tips. Maximum FE currents of 183 and 27μA were obtained for (002) and (100) oriented AlN films on tungsten tips, respectively. Comparative analysis showed that the absence of linear relation in Fowler-Nordheim plots could be attributed to the high current density integrated over the emitting areas. The excellent FE property of (002) oriented AlN film can be attributed to its vertically oriented grains and the broad distribution of defect-related subbands within its band gap as analyzed by photoluminescence spectra from (002) and (100) oriented AlN films.
Crystalline properties of wide band gap BeZnO films
Authors: Kim W. J. , Leem J. H. , Han M. S. , Park I.-W. , Ryu Y. R. , Lee T. S.
Published: May 2006

The crystalline properties of Be substituted ZnO(BeZnO) films deposited on c-Al2O3 substrates are studied. As Be concentration in BeZnO increases, the lattice parameters of BeZnO increases; the c-axis lattice is more sensitively shifted than the a-axis lattice. The energy band gap of the BeZnO films has been efficiently modulated from 3.38eV to 3.81eV by adding Be into ZnO. A linear dependency between band gap and lattice parameters is observed from BeZnO films. BeZnO alloys can be used for new ZnO-based devices based on double heterostructures, quantum wells, or superlattices made with ZnO and BeZnO.
Effect of nanocrystallization on the structure and the magnetic properties of Nd–Fe–Co–Al–B glassy alloy
Authors: Bessais L. , Djéga-Mariadassou C. , Beaunier P.
Published: May 2006

Nd 40 Fe 30 Co 15 Al 10 B 5 bulk amorphous prepared by high energy milling shows a coercivity of 650kA∕m with a Curie temperature of 645K. The controlled nanocrystallization enhances the coercivity to 1600kA∕m and the remanence ratio is equal to 0.59. The coexistence of two crystalline magnetic phases, ferromagnetic Nd2(Fe,Co,Al)14B, and antiferromagnetic Nd6(Fe,Co,Al)14 are revealed by x-ray diffraction, high-resolution transmission electron microscopy, magnetization measurements, and Mössbauer spectrometry. The grain size for optimal magnetic properties is around 30nm. The nucleation process may play a leading role in the high magnetic behavior.
Preparation of La-doped BiFeO3 thin films with Fe2+ ions on Si substrates
Authors: Gao F. , Cai C. , Wang Y. , Dong S. , Qiu X. Y. , Yuan G. L. , Liu Z. G. , Liu J.-M.
Published: May 2006

La-doped BiFeO3 thin films with Fe2+ ions have been prepared on Pt∕TiO2∕SiO2∕Si substrates by pulsed laser deposition in order to enhance the ferroelectric and magnetic properties. The targets for the film deposition were synthesized using a rapid liquid phase sintering technique to ensure the low leakage. The dielectric properties at room temperature and above were investigated. It was observed that the La doping greatly enhances the ferroelectric polarization at room temperature by modifying the film structure from rhombohedral to monoclinic. The saturation magnetization was enhanced about two times due to the Fe2+ ions in the thin films.
Sub-meV photoluminescence linewidth and >106cm2∕Vs electron mobility in AlGaAs∕GaAs quantum wells grown by metalorganic vapor phase epitaxy on slightly misoriented substrates
Authors: Pelucchi E. , Moret N. , Dwir B. , Oberli D. Y. , Rudra A. , Gogneau N. , Kumar A. , Kapon E. , Levy E. , Palevski A.
Published: May 2006

We report sub-meV (as low as 0.6meV) low-temperature photoluminescence linewidth and high low-temperature electron mobility (μ∼1−1.5×106cm2∕Vs) of GaAs quantum wells in AlGaAs barriers grown by standard metalorganic vapor phase epitaxy. These records values are achieved by epitaxial growth on (100) slightly misoriented substrates [⩽0.6 degrees off-(100) GaAs substrates] in combination with a high V/III ratio for AlGaAs growth. Such small misorientations are sufficient to drastically modify the optical and transport properties as well as the growth mode and surface morphologies of both GaAs and AlGaAs epitaxial layers, allowing greater interface quality and reduced impurity incorporation. The quantum wells so obtained show optical properties comparable to high-quality samples grown by molecular beam epitaxy. In addition, the slight misorientation considerably reduces the impact of substrate temperature on electron mobility, which allows achieving high values of μ within a much broader range of growth temperatures.
Polarization dependent loss in III-nitride optical waveguides for telecommunication devices
Authors: Iizuka Norio , Kaneko Kei , Suzuki Nobuo
Published: May 2006

Excess polarization dependent loss (PDL) was investigated for GaN waveguide devices grown by molecular beam epitaxy (MBE). The loss for transverse magnetic polarization strongly depended on the edge dislocation density in the crystal, because the dislocations capture electrons and act like a wire-grid polarizer. By means of MBE regrowth on GaN grown with metal-organic chemical vapor deposition (MOCVD), the PDL was reduced to 1∼2dB∕mm with an edge dislocation density of 3×109cm−2, whereas it was approximately 10dB∕mm for an all-MBE-grown sample. An ultrafast all-optical switch utilizing the intersubband transition was fabricated with a multiple quantum well structure that was regrown with MBE on MOCVD-grown GaN. An extinction ratio of as high as 11.5dB was achieved with a control pulse energy of 150pJ, which is attributable to the reduction of the excess PDL.
Magnetic reversal field map combined with medium noise analysis in CoCrPt–SiO2 granular perpendicular recording medium
Authors: Ishio S. , Wasiya T. , Saito H. , Bai J. , Pei W.
Published: May 2006

The spatial distribution of the magnetic reversal field (MRF) in a CoCrPt–SiO2 granular perpendicular recording medium was measured with in-field magnetic force microscopy and combined with the recorded pattern and the medium noise observed at the same location. The areas having low MRF tend to make reverse domains at bit or zigzag transition boundaries, while regions with high MRF do not correlate well with medium noise. The MRF map measured at 20Oe intervals shows that magnetic reversal occurs mainly in regions adjacent to an already reversed area, suggesting the existence of moderate intergrain exchange couplings. It is concluded that control of the spatial fluctuation of physical parameters such as intergrain exchange interactions, the anisotropy field, grain size, and chemical composition is essential to reduce the medium noise.
Observables of angular momentum as observables on the Fedosov quantized sphere
Authors: Tillman Philip , Sparling George
Published: May 2006

In this paper we construct quantum mechanical observables of a single free particle that lives on the surface of the two-sphere S2 by implementing the Fedosov ∗-formalism. The Fedosov ∗ is a generalization of the Moyal star product on an arbitrary symplectic manifold. After their construction we show that they obey the standard angular momentum commutation relations in ordinary nonrelativistic quantum mechanics. The purpose of this paper is threefold. One is to find an exact, nonperturbative solution of these observables. The other is to verify that the commutation relations of these observables correspond to angular momentum commutation relations. The last is to show a more general computation of the observables in Fedosov ∗-formalism; essentially an undeformation of Fedosov’s algorithm.
Extremely high quantum efficiency of donor-acceptor-pair emission in N-and-B-doped 6H-SiC
Authors: Kamiyama S. , Maeda T. , Nakamura Y. , Iwaya M. , Amano H. , Akasaki I. , Kinoshita H. , Furusho T. , Yoshimoto M. , Kimoto T. , Suda J. , Henry A. , Ivanov I. G. , Bergman J. P. , Monemar B. , Onuma T. , Chichibu S. F.
Published: May 2006

High-efficiency visible light emission in N-and-B-doped 6H-SiC epilayers was observed in photoluminescence measurements at room temperature. The orange-yellow light emission due to the recombination of donor-acceptor pairs (DAPs) has a broad spectrum with a peak wavelength of 576nm and a full width at half maximum of 110nm at 250K. The high B concentration of more than 1018cm−3 improves the emission efficiency of the DAP recombination at a high temperature. Compared with the photoluminescence spectrum of GaN at 10K, a high quantum efficiency of 95% was estimated for the highly B-doped sample. From time-resolved photoluminescence measurements, a DAP recombination time of 5.0ms was obtained, which is in good agreement with the calculated value by the rate equation with the assumption of a 95% internal quantum efficiency. This is quite promising as a light-emitting medium by optical pumping, as well as monolithic light sources combined with nitride-based light-emitting diodes grown on the DA-doped SiC epilayer.
Resonance energy transfer from organic chromophores to fullerene molecules
Authors: Liu Yu-Xiang , Summers Melissa A. , Scully Shawn R. , McGehee Michael D.
Published: May 2006

The mechanism of charge separation in polymeric bulk heterojunction photovoltaic cells is usually described as electron transfer from the absorbing polymer to an electron acceptor material such as (6,6)-phenyl C61 butyric acid methyl ester (PCBM). We consider the possibility of energy transfer to PCBM as another potential mechanism for charge separation. We demonstrate resonance energy transfer from a red-emitting organic chromophore (Nile red) to PCBM and measure a Förster radius of 3.1nm. Using standard Förster energy transfer theory, we calculate a Förster radius (R0) of around 2.7nm for this donor-acceptor pair in polystyrene. Nile red has a similar emission spectrum to commonly used conjugated polymers used in polymer/PCBM photovoltaic cells. We consider the implications of an energy transfer mechanism on the design requirements for future photovoltaic cells.
Terahertz optical constants of ytterbium-doped yttrium aluminum garnet crystals
Authors: Tsai Tsong-Ru , Chang Chih-Fu , Chen Shi-Jie , Tani Masahiko , Yamaguchi Mariko , Sumikura Hisashi , Chiang Hai-Pang , Chen Yuan-Fan , Tse Wan-Sun
Published: May 2006

Terahertz time-domain spectroscopy has been employed to measure the optical constants of ytterbium-doped yttrium aluminum garnet (YbxY1−x)3Al5O12 (Yb:YAG) crystals for nominal x values of 0.0, 0.1, 0.2, 0.5, 0.8, and 1.0 in the frequency range from 0.2to1.8THz. The real refractive indices for Yb:YAG crystals increase linearly with the concentration of Yb3+. The experimental results can be fitted by the Sellmeier equation. The results imply that the phonon modes of the Yb:YAG crystals shift to lower frequencies with substitution of Yb for Y. The extinction coefficients of the Yb:YAG crystals are smaller than 0.05.
Electrically switchable holographic parabolic mirrors
Authors: Ermold Michael L. , Fontecchio Adam K.
Published: May 2006

This paper reports on electrically controlled holographic optical elements (HOEs) in the form of parabolic mirrors that have the ability to collect light. These mirrors were written in a material known as holographic polymer dispersed liquid crystal, where the liquid crystal embedded within the device allows for electronic control of the optical properties of the mirrors. Essentially, we have replicated the function of a focusing mirror in a lightweight, thin film, electro-optic device. In this work, we have experimentally characterized the optical and electro-optical properties of the HOEs and provided a qualitative explanation of the operation of these structures. This and further investigations of the properties of electro-optic HOEs will open avenues in bulk HOEs and integrated photonic device applications.
Space charge limited transient currents and oxygen vacancy mobility in amorphous BaTiO3 thin films
Authors: El Kamel F. , Gonon P. , Ortega L. , Jomni F. , Yangui B.
Published: May 2006

Time-dependent transient currents were studied at elevated temperatures (200–220°C) in amorphous barium titanate thin film capacitors. Current transients display a peak whose time position varies with applied voltage and temperature. The response is analyzed through space charge limited current transient theories. Extracted drift mobilities are in the 10−11–10−12cm2V−1s−1 range and show an activation energy of 1eV. The phenomena are associated with oxygen vacancies migration in BaTiO3.
Lasing in whispering gallery mode in ZnO nanonails
Authors: Wang D. , Seo H. W. , Tin C.-C. , Bozack M. J. , Williams J. R. , Park M. , Tzeng Y.
Published: May 2006

We report on the observation of ultraviolet lasing in optically pumped ZnO nanonails synthesized by thermal chemical vapor deposition method. The lasing threshold was found to be 17MWcm−2. Very sharp emission peaks (full width at half maximum of 0.08nm) were observed in the emission spectrum, indicating a high Q factor of the cavity formed by the hexagonally shaped nanonail head. The analysis of the lasing spectra strongly suggests the whispering gallery mode lasing from a hexagonally shaped head of the single ZnO nanonail.
Composition dependence of the microspectroscopy of Cr ions in double-clad Cr:YAG crystal fiber
Authors: Chen Jian-Cheng , Huang Kuang-Yao , Tsai Cheng-Nan , Lin Yen-Sheng , Lai Chien-Chih , Liu Geng-Yu , Kao Fu-Jen , Huang Sheng-Lung , Lo Chia-Yao , Lin Yen-Sheng , Shen Pouyen
Published: May 2006

We have demonstrated the use of microspectroscopy for measuring the Cr3+ and Cr4+ fluorescence spectra in double-clad Cr:YAG crystal fiber. The emission spectra of Cr3+ and Cr4+ are detected from core and inner cladding. The Cr3+ spectrum in the inner cladding shows a broad-band emission from 650to950nm, while the emission of Cr4+ occurs in the range of 1.15–1.55μm with a peak around 1.22μm. The characteristic of Cr ion at high-field sites shows a narrow-band emission (E2→A24 for Cr3+; E1→A23 for Cr4+), whereas that at low-field sites shows a broad-band emission (T24→A24 for Cr3+; T23→A23 for Cr4+). The emission intensity ratio of high-field sites to low-field sites in the inner cladding with different compositions has been investigated. It varies from 20% to 29% for Cr3+ and from 7.1% to 11.3% for Cr4+ when the concentration of SiO2 increases from 26.9to43.0wt%.
Pb
(
Zr
0.3
Ti
0.7
)
O
3
∕
PbTiO
3
multilayer thin films for pyroelectric infrared sensor application
Authors: Sun L. L. , Tan O. K. , Zhu W. G.
Published: May 2006

We report on the properties of Pb(Zr0.3Ti0.7)O3∕PbTiO3 (PZT/PT) multilayer thin films for pyroelectric infrared sensor application. The effect of various PZT and PT stacking sequences has been systematically studied. The optimal structure for pyroelectric infrared sensor application is found to be the multilayer 5PZT/4PT thin film which has the PZT and the PT layers deposited alternately. It is characterized that the 5PZT/4PT multilayer thin film has reduced dielectric constant and comparable pyroelectric coefficient and dielectric loss when compared with the pure PZT thin film. The interface induced space charge polarization and “pinched” ferroelectric hysteresis loop of the multilayer thin film have been discussed. A detectivity figure of merit of 2.1×10−5Pa−1∕2 at 30Hz has been characterized for the 5PZT/4PT thin film, which is better than that of the pure PZT thin film.
The feature of the Breit-Wigner-Fano Raman line in DNA-wrapped single-wall carbon nanotubes
Authors: Kawamoto H. , Uchida T. , Kojima K. , Tachibana M.
Published: May 2006

Breit-Wigner-Fano (BWF) Raman line in DNA-wrapped single-wall carbon nanotubes (SWNTs) was investigated. For as-produced HiPco SWNT bundles, the asymmetric profile of BWF line is clearly observed with an excitation energy of 2.33eV. The asymmetric feature almost disappears in isolated HiPco SWNTs by DNA wrapping in aqueous solution. Such a significant change provides evidence that the asymmetric feature of BWF line is attributed to the bundling effect of SWNTs. In addition, the asymmetric feature of BWF line strongly appears even when the DNA-wrapped SWNTs are exposed to air. This means that the rebundling in the DNA-wrapped SWNTs can occur due to drying.
Self-assembled InAs quantum dots and wires grown on a cleaved-edge GaAs(110) surface
Authors: Blumin M. , Ruda H. E. , Savelyev I. G. , Shik A. , Wang H.
Published: May 2006

We studied the conditions for the Stranski-Krastanov mode of molecular beam epitaxial growth of InAs on a cleaved GaAs(110) surface. Temperature distributions on a subholder with cleaved facets were revealed using thermophotography. Combining these data with a theoretical model enabled a determination of the real temperature on the cleaved-edge surfaces (110), which differed markedly from the temperature on a planar wafer (100). Based on these results, we proposed an approach that combines different growth conditions in one technological process. As a result, appropriate growth conditions were established for InAs quantum dots grown on the cleaved GaAs(110) surface. Control over the dot nucleation process was shown to permit growth of both linear arrays of quantum dots and planar quantum wires on these (110) surfaces.