bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical nonuniformities and their impact on the electron mobility in semi-insulating SiC crystals
Authors: Li Qiang , Polyakov Alexander Y. , Skowronski Marek , Roth Matthew D. , Fanton Mark A. , Snyder David W.
Published: June 2004

Hall effect and van der Pauw measurements for temperatures in the range 300–1000 K were carried out on semi-insulating (SI) 4H-SiC wafers grown by physical vapor transport (PVT) and by high temperature chemical vapor deposition. It is shown that, for semi-insulating 4H-SiC samples the temperature dependence of both resistivity and electron mobility can be measured with some samples exhibiting mobilities close to the theoretical limit determined by intervalley and phonon scattering. However, most PVT-grown samples exhibit electron mobilities an order of magnitude lower than expected. The results are explained by the electrical macrononuniformities inherent in the PVT growth process. Measurements of the temperature dependence of the photocurrent indicate that microscale potential fluctuations characteristic of other high-resistivity semiconductors are less of an issue with SI–SiC.
Significance of Yb3+ concentration on the upconversion mechanisms in codoped Y2O3:Er3+, Yb3+ nanocrystals
Authors: Vetrone Fiorenzo , Boyer John-Christopher , Capobianco John A. , Speghini Adolfo , Bettinelli Marco
Published: June 2004

The optical properties of five different nanocrystalline Y2O3:Er3+, Yb3+ samples are presented and discussed. Green and red emission was observed following excitation with 488 nm and attributed to 2H11/2, 4S3/2→4I15/2, and 4F9/2→4I15/2 transitions, respectively. Striking red enhancement was observed in the upconversion spectra when exciting the Y2O3:Er3+, Yb3+ samples with 978 nm, and it became more pronounced with an increase in Yb3+ concentration. A cross relaxation mechanism (4F7/2→4F9/2 and 4F9/2←4I11/2) was responsible for directly populating the 4F9/2 state but did not explain the difference in the magnitude of red enhancement between identically doped bulk and nanocrystalline Y2O3:Er3+, Yb3+ samples. The 4F9/2 level was populated via a nonresonant mechanism that involved the 4F9/2←4I13/2 transition that is more prevalent in the nanocrystals, which is due to the high energy phonons inherent in this type of material. In nanocrystalline Y2O3:Er3+, Yb3+, we observe a change in the upconversion mechanism responsible for populating the 4S3/2 state, from a two-photon to a three-photon process with an increase in Yb3+ concentration. An explanation to account for this behavior is presented.
Modeling the washboard effect at the head/disk interface
Authors: Dai Qing , Hendriks Ferdi , Marchon Bruno
Published: June 2004

A model is presented that accounts for the observation of periodic lubricant ripples formed when a slider is flying on-track over a lubricated disk surface. It is shown that lubricant flow modulation from the air shear stress due to the low flying slider acting as a spring is responsible for the observed corrugation, as repeated flying over the same track enhances the effect. This is somewhat moderated by the relaxation of the lubricant perturbation, by surface diffusion. Depending on the relative magnitude of the shear and diffusion processes, the system can find itself into regions of stability where small perturbations decay away, or regions of instability where they increase exponentially. Instability is enhanced for lower fly height and higher disk rotational frequency, making it a challenge for future growth in disk drive performance and areal density.
Proton field-cycling nuclear magnetic resonance relaxometry in the smectic A mesophase of thermotropic cyanobiphenyls: Effects of sonication
Authors: Bonetto F. , Anoardo E.
Published: June 2004

Proton field-cycling nuclear magnetic resonance relaxometry is used to study the spin-lattice relaxation dispersion of selected standard smectic A liquid crystals at different temperatures. Relaxation features at both, in the presence and absence of a monochromatic ultrasonic field are considered. We show that the laboratory-frame spin-lattice relaxation time is mainly governed by traslational diffusion. Order director fluctuations (ODF) are less important while rotational difussion seems to be only relevant near the clearing point. Our study suggests that sonication enhances the ODF contribution in the SmA mesophase. Within the framework of the approach we have outlined, different features associated with the ODF mechanism can be investigated.
Study of nonsteady diffusional growth of a droplet in a supersaturated vapor: Treatment of the moving boundary and material balance
Authors: Grinin A. P. , Shchekin A. K. , Kuni F. M. , Grinina E. A. , Reiss H.
Published: June 2004

A new mathematical treatment of the problem of droplet growth via diffusion of molecules from a supersaturated vapor is presented. The theory is based on a semiquantitative analysis with good physical arguments and is justified by its reasonable predictions. For example it recovers the time honored growth law in which, to a high degree of approximation, the droplet radius increases with the square root of time. Also, to a high degree of approximation, it preserves material balance such that, at any time, the number of molecules lost from the vapor equals the number in the droplet. Estimates of the remaining approximational error are provided. On another issue, we show that, in contrast, the conventional treatment of droplet growth does not maintain material balance. This issue could be especially important for the nucleation of another droplet in the vicinity of the growing droplet where the rate of nucleation depends exponentially on supersaturation. Suggestions for further improvement of rigor are discussed.
The canonical ensemble via symplectic integrators using Nosé and Nosé–Poincaré chains
Authors: Leimkuhler Benedict J. , Sweet Christopher R.
Published: June 2004

Simulations that sample from the canonical ensemble can be generated by the addition of a single degree of freedom, provided that the system is ergodic, as described by Nosé with subsequent modifications by Hoover to allow sampling in real time. Nosé–Hoover dynamics is not ergodic for small or stiff systems and the addition of auxiliary thermostats is needed to overcome this deficiency. Nosé–Hoover dynamics, like its derivatives, does not have a Hamiltonian structure, precluding the use of symplectic integrators which are noted for their long term stability and structure preservation. As an alternative to Nosé–Hoover, the Hamiltonian Nosé–Poincaré method was proposed by Bond, Laird, and Leimkuhler [J. Comput. Phys. 151, 114 (1999)], but the straightforward addition of thermostatting chains does not sample from the canonical ensemble. In this paper a method is proposed whereby additional thermostats can be applied to a Hamiltonian system while retaining sampling from the canonical ensemble. This technique has been used to construct thermostatting chains for the Nosé and Nosé–Poincaré methods.
Zirconia grown by ultraviolet ozone oxidation on germanium (100) substrates
Authors: Chi David , Chui Chi On , Saraswat Krishna C. , Triplett Baylor B. , McIntyre Paul C.
Published: June 2004

Growth of zirconia (ZrO2)-based gate dielectrics on germanium (Ge) substrates by oxidation using activated oxygen species produced by ultraviolet radiation (UV/ozone) is reported here. In this technique, a thin layer of zirconium (Zr) metal (10–30 Å) is deposited by physical vapor deposition on Ge and subsequently oxidized in reactive oxygen. X-ray photoelectron spectroscopy (XPS) analysis indicates complete oxidation of the Zr metal. High resolution transmission electron microscopy (TEM) of UV-ozone oxidized ZrO2 on Ge indicates a sharp interface between the oxide and the substrate. However, conventional TEM is not well suited for identifying a Ge oxide layer in this system due to the closeness in atomic number of Zr and Ge. XPS spectra suggest the presence of a substoichiometric Ge oxide phase at the ZrO2/Ge interface. Depth profiling using angle-resolved XPS was performed on ZrO2/Ge gate stacks of varying oxide thickness. The results indicate that the amount of Ge oxide is dependent upon the ZrO2 overlayer thickness, suggesting that the interfacial layer can be controlled by the oxidation conditions.
Electrical properties of a two-dimensionally hexagonal C60 photopolymer
Authors: Onoe J. , Nakayama T. , Aono M. , Hara T.
Published: June 2004

We investigated the electrical properties of a two-dimensionally (2D) hexagonal C60 photopolymer, using four-probe measurements in air at room temperature. The current–voltage curve shows that the photopolymer exhibits nondoped semiconducting behavior. This behavior is consistent with the theoretical prediction that a 2D hexagonal C60 polymer with a crosslinkage of a [2+2] cycloadditional four-member ring between adjacent C60 molecules is semiconducting [S. Okada and S. Saito, Phys. Rev. B 59, 1930 (1999)]. The origin of these semiconducting properties is discussed on the basis of density-functional calculations for the valence molecular orbitals of the dumbbell C120, which is regarded as a basic unit of the [2+2] crosslinkage of the 2D photopolymer.
Calculations of long-range potential wells for highly excited homonuclear and heteronuclear alkali dimers
Authors: Normand Bradford , Stwalley William C.
Published: June 2004

The weakly bound long-range potential curves between a highly excited alkali atom M*(nes) and a ground state alkali atom M(ngs) are calculated using simple but reasonably accurate models for long-range dispersion and exchange interactions for all homonuclear and heteronuclear combinations. For K2, where experimental results are available, the agreement is quite good (binding energies of observed vibrational levels within ∼10%). We find that at least a zero-point vibrational level occurs for ne−ng⩽12.
Al:SiO thin films for organic light-emitting diodes
Authors: Han S. , Grozea D. , Huang C. , Lu Z. H. , Wood R. , Kim W. Y.
Published: June 2004

Al:SiO cermet thin films were synthesized by thermal coevaporation. Physical properties of these thin films were characterized by scanning electron microscope, transmission electron microscope, x-ray photoelectron spectroscopy, current–voltage, and optical absorption measurement. The data show that the Al:SiO films consist of crystalline Al islands embedded in an amorphous network of mixed Si, SiO2, and Al2O3 as Al weight percentage exceeds ∼50%. It is found that the size of Al islands increases with increasing Al concentration, which leads to a dramatic reduction in resisitivity and optical transmittance. A multilayered SiO:Al/Al/LiF structure has been utilized as a cathode for top-emission organic light-emitting diode (TOLED). A model based on Fabry–Pérot cavity has been used to simulate the TOLED light emission spectra. The results indicate that Al:SiO films can also be used as a semitransparent mirror for a half-wavelength planar microcavity light-emitting diode.
Effect of oxidation on charge localization and transport in a single layer of silicon nanocrystals
Authors: Krishnan R. , Xie Q. , Kulik J. , Wang X. D. , Lu S. , Molinari M. , Gao Y. , Krauss T. D. , Fauchet P. M.
Published: June 2004

The effect of oxidation on charge transport and retention within a sheet of silicon (Si) nanocrystals was investigated with an electrostatic force microscope. Single layers of nanocrystals with smooth and abrupt Si/SiO2 interfaces were prepared by thermal crystallization of thin amorphous Si layers, followed by an oxidation treatment for isolating the nanocrystals. Controlled amounts of charge were injected into the nanocrystals and their in-plane diffusion was monitored in real time. Rapid transport of the injected charge occurred for the nonoxidized nanocrystals. Oxidation of the nanocrystal layer resulted in suppression of lateral transport. The nanocrystals oxidized for 30 min retained the injected charge in a well-defined, localized region with retention times of the order of several days. These long-term charge retention characteristics indicate that nanocrystals prepared by this process could be attractive candidates for nonvolatile memory applications.
Pulsed plasmas as a method to improve uniformity during materials processing
Authors: Subramonium Pramod , Kushner Mark J.
Published: June 2004

Continuous wave operation of inductively coupled plasma (ICP) reactors as used for materials processing and which have geometrical or flow asymmetries may produce asymmetric species densities, temperatures, and fluxes. Flow asymmetries that produce nonuniformities in plasma conductivity initiate a positive feedback loop. In this feedback loop, asymmetries in conductivity are reinforced by the resulting nonuniform power deposition, which causes electron impact ionization to also be nonuniform. In this article, we discuss results from a computational investigation of long-term transients during pulsed operation of ICPs and their consequences on side-to-side asymmetries in plasma properties. During pulsed operation, diffusion of charged species during the afterglow between pulses smoothens these asymmetries prior to the next power pulse. The power deposition during subsequent pulses is more symmetric and this reduces the positive feedback. The improvement in uniformity afforded by pulsing is determined by the intrapulse plasma dynamics, and so is a function of the pulse repetition frequency, duty cycle, and feedstock gas. Improvements in the azimuthal uniformity of reactant densities were obtained in Ar and Cl2 plasmas in an asymmetrically pumped reactor using pulsed power. As dissociative attachment dominates in the afterglow of Cl2 pulsed plasmas, it provides a more uniform sink for electrons compared to ambipolar diffusion and different systematic behavior is obtained compared to argon.
Theoretical analysis of the synergism in the dielectric strength for SF6/CF4 mixtures
Authors: Larin A. V. , Meurice N. , Trubnikov D. N. , Vercauteren D. P.
Published: June 2004

Available cross section data of electron–molecule processes are scaled to simulate the behavior of the dielectric strength (DS) in SF6 and CF4 gases at the level of the two-term approximation of the Boltzmann equation solution corresponding to the homogeneous electric field model. Then, the DS of mixtures is evaluated and compared to experimental data. The reasons of the synergism and its “asymmetry” relative to both components in SF6/CF4 mixtures are analyzed in terms of rate constant variations with respect to their values for the pure gases as well as in terms of weighted rate constants for all channels of electron energy losses relevant under breakdown conditions.
Perturbation analysis for the rotational spectrum of the NiBr radical in the X 2Π3/2 and A 2Δ5/2 states
Authors: Yamazaki Emi , Okabayashi Toshiaki , Tanimoto Mitsutoshi
Published: June 2004

The millimeter- and submillimeter-wave spectra of the NiBr radical in the X 2Π3/2 and A 2Δ5/2 states were observed by a source-modulated microwave spectrometer. The NiBr radical was generated in a dc glow discharge through the mixture of Br2 vapor and Ar gas by the sputtering reaction with a Ni cathode. Observed transition frequencies were independently analyzed for both electronic states using a standard polynomial expression of a Hund’s case (c) approximation. Anomalous behavior of the effective molecular constants in the X 2Π3/2 state was interpreted as the result of the perturbation between the X 2Π3/2 and A 2Δ5/2 states. The deperturbed molecular constants were derived using a simplified supermultiplet Hamiltonian including the interaction terms between the two electronic states.
Thermal conductivity of the thermoelectric layered cobalt oxides measured by the Harman method
Authors: Satake A. , Tanaka H. , Ohkawa T. , Fujii T. , Terasaki I.
Published: June 2004

In-plane thermal conductivity of the thermoelectric layered cobalt oxides has been measured using the Harman method, in which thermal conductivity is obtained from temperature gradient induced by applied current. We have found that the charge reservoir block (the block other than the CoO2 block) dominates the thermal conduction, where a nano-block integration concept is effective for material design. We have further found that the thermal conductivity shows a small but finite in-plane anisotropy between a and b axes, which can be ascribed to the misfit structure.
Raman spectroscopy of B12As2 under high pressure
Authors: Pomeroy J. W. , Kuball M. , Hubel H. , van Uden N. W. A. , Dunstan D. J. , Nagarajan R. , Edgar J. H.
Published: June 2004

We report a Raman spectroscopy study of B12As2 under hydrostatic pressure up to 15 GPa. Mode Grüneisen parameters were determined for the B12As2 phonon modes. Phonon modes attributed to the As–As chain have a weak pressure dependence (0.6–1.9 cm−1 GPa−1) relative to inter- and intra-icosahedral vibrations (3.4–6.6 cm−1 GPa−1). The pressure dependence of B12As2 phonon frequencies is compared to those reported for α-boron and the origin of the mode at 505 cm−1 with its weak pressure dependence is discussed.
Population dynamics of Yb3+, Er3+ co-doped phosphate glass
Authors: Zhou Jing , Moshary Fred , Gross Barry M. , Arend Mark F. , Ahmed Samir A.
Published: June 2004

Excitation dynamics in Er3+–Yb3+ co-doped phosphate glasses were examined as a function of both erbium and ytterbium concentration by monitoring fluorescence on the 1.5 μm I13/24→I15/24 transition of Er3+ following pulsed laser excitation of the system at 977 nm under room temperature. For the range of donor (Yb3+) and acceptor (Er3+) concentration considered, both nonexponential and exponential relaxation dynamics were observed. Nonexponential acceptor fluorescence is characteristic of noninteracting donor (Yb3+) regime, while the exponential response is consistent with interacting donors. For the latter case, the linear acceptor dependence for the Yb3+–Er3+ transfer rate K implies energy super migration among donor ions.
Photoionization cross-section analysis for a deep trap contributing to current collapse in GaN field-effect transistors
Authors: Pässler Roland
Published: June 2004

We perform a more incisive numerical analysis of the photon energy dependence of the photoionization cross section of a prominent deep trap (conventionally labeled as Trap 1), which had been found [Klein et al., Appl. Phys. Lett. 75, 4016 (1999)] to act as a contributor to current collapse phenomena in GaN metal-semiconductor field-effect transistors. The analytical expression for the photoionization cross section of Trap 1 is taken in the form of a convolution of a temperature-independent electronic part with a thermally broadened Franck–Condon (FC) factor, which also applies to the relevant regime of large lattice relaxation. For a direct comparison with earlier results, we specialize the present analysis to an electronic cross-section part represented by the Lucovsky model in combination with the semiclassical (Gaussian) approximation for the FC factor. In qualitative accordance with an earlier estimation by Klein et al. we obtain a value of EO≈1.9 eV for the classical optical ionization energy in combination with a full width at half maximum of 0.64 eV. The latter implies, on the assumption for the average phonon energy, to be of order 50 meV, an apparently unusually large magnitude, D≈1.1 eV, for the Franck–Condon shift. This parameter constellation is equivalent to a thermal ionization (electron binding) energy, ET=EO−D, of about ET≈0.8 eV. Such a location of Trap 1 near the middle of the upper half of the fundamental gap of GaN, ET≈Eg/4, is at clear variance to the earlier suggestion by Klein et al. for Trap 1 to be a midgap level. The present estimation offers a chance for detecting the Trap 1 also by deep-level transient spectroscopy measurements. An eventual availability of photoionization cross-section data for different temperatures is seen to be the prerequisite for a decisive reduction of residual uncertainties concerning the configuration coordinate diagram.
Oxygen diffusion in atomic layer deposited ZrO2 and HfO2 thin films on Si (100)
Authors: Ferrari S. , Scarel G.
Published: June 2004

In this article, we investigate the oxygen diffusivity in ZrO2 and HfO2 thin films deposited on Silicon (100) by atomic layer deposition. In particular we study the kinetics of the SiO2 interfacial layer growth upon rapid thermal annealing in oxygen atmosphere and the oxygen diffusivity in ZrO2 and HfO2 at high temperature using isotopically enriched O218. The interfacial oxide growth is studied with time-of-flight secondary ion mass spectrometry and transmission electron microscopy. This technique allows us to measure the thickness of the SiO2 layer at the interface between Si and ZrO2 and HfO2 as well as the isotopic composition of oxygen in those films. The oxidation kinetics of silicon in the presence of ZrO2 and HfO2 is found to be totally different than the one occurring on bare silicon annealed in the same condition. During short annealings, a relatively thick SiO2 interfacial layer is formed, independently of O2 partial pressure, suggesting that ZrO2 and HfO2 are injecting oxygen into the silicon. For relatively long annealings Si oxidation is slower than bare silicon oxidation. The annealing in O218 allows us to understand the oxygen exchange mechanisms in ZrO2 and HfO2.
Stoner–Wohlfarth model applied to bicrystal magnetoresistance hysteresis
Authors: Gunnarsson R. , Hanson M. , Dubourdieu C.
Published: June 2004

We calculate numerically the magnetization direction as function of magnetic field in the Stoner–Wohlfarth theory and are able to reproduce the shape of the low-field magnetoresistance hysteresis observed in manganite grain boundary junctions. Moreover, we show that it is necessary to include biaxial magnetocrystalline anisotropy to fully describe the grain boundary magnetoresistance in La0.7Sr0.3MnO3/SrTiO3 bicrystal tunnel junctions.
Properties of arsenic antisite defects in Ga1−xMnxAs
Authors: Wolos A. , Kaminska M. , Palczewska M. , Twardowski A. , Liu X. , Wojtowicz T. , Furdyna J. K.
Published: June 2004

We report the results of optical absorption measurements on Ga1−xMnxAs layers grown by low-temperature molecular beam epitaxy. In the paramagnetic layers grown at very low temperatures (below 250 °C) the experiments reveal an absorption band at 1.2 eV arising from the presence of neutral arsenic antisites, AsGa. From the magnitude of the absorption we determine the concentration of AsGa to be between 4×1019 and 8×1019 cm−3 in these paramagnetic samples. These values are typical for GaAs specimens grown below 250 °C. Extrapolating the AsGa concentration from low-temperature-grown GaAs to Ga1−xMnxAs, we determine the concentration of this defect in ferromagnetic Ga1−xMnxAs layers grown at temperatures above 250 °C as 1×1019 down to 1×1018 cm−3. We conclude that the compensating role of arsenic antisites in Ga1−xMnxAs becomes gradually less important with increasing growth temperature.
Higher-order equation-of-motion coupled-cluster methods
Authors: Hirata So
Published: June 2004

The equation-of-motion coupled-cluster (EOM-CC) methods truncated after double, triple, or quadruple cluster and linear excitation operators (EOM-CCSD, EOM-CCSDT, and EOM-CCSDTQ) have been derived and implemented into parallel execution programs. They compute excitation energies, excited-state dipole moments, and transition moments of closed- and open-shell systems, taking advantage of spin, spatial (real Abelian), and permutation symmetries simultaneously and fully (within the spin–orbital formalisms). The related Λ equation solvers for coupled-cluster (CC) methods through and up to connected quadruple excitation (CCSD, CCSDT, and CCSDTQ) have also been developed. These developments have been achieved, by virtue of the algebraic and symbolic manipulation program that automated the formula derivation and implementation altogether. The EOM-CC methods and CC Λ equations introduce a class of second quantized ansatz with a de-excitation operator (Ŷ), a number of excitation operators (X̂), and a physical (e.g., the Hamiltonian) operator (Â), leading to the tensor contraction expressions that can be performed in the order of ((⋯((yx)x)⋯)x)a or ((⋯((ax)x)⋯)x)y at the minimal peak operation cost, where x, y, and a are basis-set representations (i.e., tensors) of the respective operators X̂, Ŷ, and Â. Any intermediate tensor resulting from either contraction order is shown to have at most six groups of permutable indices, and this knowledge is used to guide the computer-synthesized programs to fully exploit the permutation symmetry of any tensor to minimize the arithmetic and memory costs.
Plasma generation and plume expansion for a transmission-mode microlaser ablation plasma thruster
Authors: Keidar Michael , Boyd Iain D. , Luke James , Phipps Claude
Published: June 2004

An end-to-end model is presented of the transient plume created by a microlaser ablation plasma thruster. In this article, we describe a model of the plasma generation and expansion for a micro-laser plasma thruster operated in transmission-mode (T-mode). The laser ablation and plasma formation processes are modeled using a kinetic ablation model. This procedure provides boundary conditions at the target surface for the plume model that is based on a particle computational approach. The present study considers a 2.5–8 W diode-based laser irradiating a poly-vinyl chloride target for a pulse length of 3–10 ms. Laser beam shape full width at half maximum at the target is about 25×25 μm. The plume simulations reveal many details of the multicomponent plasma expansion. The results are compared with experimentally obtained plume signatures. Generally good agreement between experimental and calculated flux profiles is found.
Ionized physical vapor deposition of titanium nitride: A deposition model
Authors: Mao D. , Hopwood J.
Published: June 2004

A model for reactive ionized physical vapor deposition of TiN has been developed to predict film conformality and stoichiometry in a high aspect ratio trench. The two additional model components are a radio-frequency (rf) sheath model for the transport of ions to the wafer surface and a feature-scale deposition model to simulate the thickness and composition of TiNx films in a two-dimensional trench. The feature scale deposition model depends critically on the sticking coefficient for nitrogen atoms on TiNx surfaces since this is the primary method by which the Ti+ that is deposited inside deep trenches becomes nitrided. This sticking coefficient has been experimentally determined as a function of TiNx stoichiometry. The simulation results agree with both experimental observations and intuitive concepts for reactive sputter deposition. Namely, the deposition rates decrease slightly when rf bias is applied to the wafer. This decrease is due to resputtering. The model also predicts a decrease in the nitrogen content of deposited films along the sidewalls of trenches, and nitrogen deficient films at the bottom of trenches under metal-mode deposition conditions. The model’s accuracy is verified by measuring the thin film characteristics, especially deposition rate, step coverage, and composition. The deposition model gives an accurate prediction of composition ratio. The deposition model also predicts the deposition rate and step coverage accurately if no bias voltage is applied to the wafer. Although the bottom coverage and the deposition rate are accurately predicted, the sidewall coverage is 30% lower than experimental results for −50 V rf bias. This discrepancy is believed to be due to uncertainties in the angle-dependent sputter yield of TiNx at low ion energy, and the neglect of unthermalized fast neutral Ti atoms.
Exponentially localized magnetic fields for single-spin quantum logic gates
Authors: Lidar D. A. , Thywissen J. H.
Published: June 2004

An infinite array of parallel current-carrying wires is known, from the field of neutral particle optics, to produce an exponentially localized magnetic field when the current direction is antiparallel in adjacent wires. We show that a finite array of several tens of superconducting Nb nanowires can produce a peak magnetic field of 10 mT that decays by a factor of 104 over a length scale of 500 nm. Such an array is readily manufacturable with current technology, and is compatible with both semiconductor and superconducting quantum computer architectures. A series of such arrays can be used to individually address single single-spin or flux qubits spaced as little as 100 nm apart, and can lead to quantum logic gate times of 5 ns.
Dynamic screening effects on the electron-impact excitation in Lorentzian (kappa)-Maxwellian plasmas
Authors: Kim Chang-Geun , Jung Young-Dae
Published: June 2004

The electron-impact excitation process in the kappa-Maxwellian plasma is investigated by using the dynamically screened interaction potential. The scaled transition probability for the 1s→2p0 excitation is obtained by the semiclassical trajectory analysis with the plasma dielectric function as a function of impact parameter, collision energy, Debye length, and spectral index. The nonthermal and plasma screening effects on the excitation probability are more significant when the electron velocity related to the parallel temperature is greater than the projectile velocity, and the transition probability increases with decreasing nonthermal character of the plasma.
Weak ferromagnetism in the ferroelectric BiFeO3–ReFeO3–BaTiO3 solid solutions (Re=Dy,La)
Authors: Kim Jeong Seog , Cheon Chae Il , Lee Chang Hee , Jang Pyung Woo
Published: June 2004

The binary and ternary solid solutions, BiFeO3–BaTiO3, BiFeO3–ReFeO3–BaTiO3 (Re=Dy,Pr,La), and BiFeO3–BaFeO2.5–BaTiO3 have been explored for attaining ferromagnetic ferroelectrics in bulk ceramics and understanding the effect of rare earth orthoferrites ReFeO3 on the spontaneous magnetization. The coexistence of ferromagnetism and ferroelectricity has been observed over the composition range of 0.2⩽x⩽0.4 in the (1−x)BiFeO3–xBaTiO3 at room temperature. The introduction of DyFeO3 and LaFeO3 expands the composition range of the coexistence. The most superior ferromagnetic ferroelectrics obtained in this study are the 0.65BiFeO3–0.025DyFeO3–0.325BaTiO3 (Pr=5 μC/cm2,Mr=0.1 emu/g), 0.4875BiFeO3–0.025DyFeO3–0.4875BaTiO3 (Pr=7 μC/cm2,Mr=0.06 emu/g), and 0.475BiFeO3–0.05LaFeO3–0.475BaTiO3 (Pr=3.2 μC/cm2,Mr=0.2 emu/g). The spontaneous magnetization strongly depends on both the type and amount of the substitution components, DyFeO3, LaFeO3, PrFeO3, and BaFeO2.5 rather than the degree of G-type antiferromagnetic ordering. The origin of the spontaneous magnetization has been discussed in terms of antiferromagnetic ordering and charge carrier mediation.
Influences of biaxial strains on the vibrational and exciton energies in ZnO
Authors: Gruber Th. , Prinz G. M. , Kirchner C. , Kling R. , Reuss F. , Limmer W. , Waag A.
Published: June 2004

We have investigated the structural, optical, and vibrational properties of strained heteroepitaxial ZnO layers by high resolution x-ray diffraction, reflectivity, and Raman measurements. The ZnO layers were grown by metalorganic vapor phase epitaxy on sapphire substrates under varying growth conditions. A Poisson number of μ=0.303 and phonon deformation-potential parameters of a=−690 cm−1, b=−940 cm−1 for the high-energy E2 optical phonon mode have been determined. The shift of the excitonic resonances due to the strain in the layers agrees well with the experimentally determined Poisson ratio using the deformation-potentials D1–D4 determined by Wrzesinski and Fröhlich [Phys. Rev. B 56, 13087 (1997)].
Envelope-function analysis of wurtzite InGaN/GaN quantum well light emitting diodes
Authors: Xiao D. , Kim K. W. , Zavada J. M.
Published: June 2004

Fundamental electrical and optical properties of strained wurtzite InGaN/GaN-based quantum-well light-emitting diodes are calculated based on the Rashba–Sheka–Pikus Hamiltonian in the vicinity of the Γ point. It is found that the strain and the strain-induced piezoelectric field significantly alter the subband structure and determines the output intensity of the nitride quantum well light emitting diodes. For the case with high In composition (≳0.2), the calculation also supports the possibility of strain relaxation in the quantum well. Coupled with an optimized set of parameters, our theoretical model provides an excellent agreement with the available experimental data over a wide range of In composition (0–0.5).
Aging of piezoelectric properties in Pb(MgNb)O3-Pb(ZrTi)O3 multilayer ceramic actuators
Authors: Koh Jung-Hyuk , Jeong Soon-Jong , Ha Mun-Su , Song Jae-Sung
Published: June 2004

Aging characteristics of 0.2(PbMg1/3Nb2/3O3)-0.8(PbZr0.475Ti0.525O3) multilayer ceramic actuators have been investigated by applying rectified ac bias. Multilayer ceramic actuators 5×5×5 mm3 in size were fabricated by tape-casting methods. X-ray diffraction analyses were performed to compare crystalline structures and lattice parameters. Effective electromechanical coupling coefficient keff and pseudopiezoelectric constant d33 were calculated and discussed. Different rectified ac electric biases were applied to the actuators to investigate aging behavior in the multilayer ceramic actuators. The aging dependencies of cycles and electric fields were simulated and fitted to the linear logarithmic stretched exponential law.
Fabrication and characterization of metal-semiconductor field-effect-transistor-type quantum devices
Authors: Son S. H. , Cho K. H. , Hwang S. W. , Kim K. M. , Park Y. J. , Yu Y. S. , Ahn D.
Published: June 2004

Quantum dot transistors and nanowire transistors are fabricated from a metal-semiconductor field-effect-transistor-type wafer and are characterized at low temperatures. Clear single-electron tunneling and various quantum effects, such as transport through excited states and negative differential resistance, are observed in our wire device. Our data suggest that the potential fluctuation of the heavily doped GaAs layer has a much larger characteristic length than interimpurity spacing, and that this is due to the low ionization rate (approximately 10%) of the dopant atoms at 4.2 K.
Jahn-Teller effect for short-lived states: Study of the complex potential energy surfaces
Authors: Feuerbacher Sven , Cederbaum Lorenz S.
Published: June 2004

The Jahn-Teller effect for bound electronic states has been investigated for many decades. In contrast, nothing is known regarding its occurrence for short-lived electronic states. Here we investigate the linear and the quadratic E⊗e Jahn-Teller effect for degenerate resonance states with special regard to the complex potential energy surfaces. We find many new phenomena for both the real and imaginary parts of the potential energy surfaces including additional minima and intersections. Possible simplifications of the equations describing the adiabatic potential energy surfaces are discussed. We also briefly investigate other Jahn-Teller effects in linear approximation. The theoretical concepts are exemplified by calculating ab initio data for the degenerate Π*-type resonance states of the tris(boramethyl)amin anion along two different doubly degenerate vibrational modes.
Theory of covariance nuclear magnetic resonance spectroscopy
Authors: Brüschweiler Rafael
Published: June 2004

Covariance nuclear magnetic resonance (NMR) spectroscopy provides an effective way for establishing nuclear spin connectivities in molecular systems. The method, which identifies correlated spin dynamics in terms of covariances between 1D spectra, benefits from a high spectral resolution along the indirect dimension without requiring apodization and Fourier transformation along this dimension. The theoretical treatment of covariance NMR spectroscopy is given for NOESY and TOCSY experiments. It is shown that for a large class of 2D NMR experiments the covariance spectrum and the 2D Fourier transform spectrum can be related to each other by means of Parseval’s theorem. A general procedure is presented for the construction of a symmetric spectrum with improved resolution along the indirect frequency domain as compared to the 2D FT spectrum.
Ultrafast photoinduced reflectivity transients in doped manganite
Authors: Ren Y. H. , Zhao H. B. , Lüpke G. , Hong C. S. , Hur N. H. , Hu Y. F. , Li Qi
Published: June 2004

The temperature and magnetic field dependence of ultrafast photoinduced spin and quasiparticle relaxation dynamics is reported in La0.67Ca0.33MnO3 and LaMnO3 single crystals and thin films. Both manganites reveal an unusually slow (∼10 μs) carrier relaxation process attributed to the spin-lattice relaxation in localized states. The quasiparticle dynamics is governed by the temperature- and magnetic field-dependent pseudogap in La0.67Ca0.33MnO3, and by the temperature-independent Jahn–Teller gap in LaMnO3. The loss of spectral weight near the Fermi level in La0.67Ca0.33MnO3 strongy affects the quasiparticle relaxation dynamics as temperature increases from below TC. Our results show that the coupled dynamics of charge, spin and lattice is strongly correlated with the distinct gap structures in these manganites.
Thermal physics in carbon nanotube growth kinetics
Authors: Louchev Oleg A. , Kanda Hisao , Rosén Arne , Bolton Kim
Published: June 2004

The growth of single wall carbon nanotubes (SWNTs) mediated by metal nanoparticles is considered within (i) the surface diffusion growth kinetics model coupled with (ii) a thermal model taking into account heat release of carbon adsorption–desorption on nanotube surface and carbon incorporation into the nanotube wall and (iii) carbon nanotube–inert gas collisional heat exchange. Numerical simulations performed together with analytical estimates reveal various temperature regimes occurring during SWNT growth. During the initial stage, which is characterized by SWNT lengths that are shorter than the surface diffusion length of carbon atoms adsorbed on the SWNT wall, the SWNT temperature remains constant and is significantly higher than that of the ambient gas. After this stage the SWNT temperature decreases towards that of gas and becomes nonuniformly distributed over the length of the SWNT. The rate of SWNT cooling depends on the SWNT–gas collisional energy transfer that, from molecular dynamics simulations, is seen to be efficient only in the SWNT radial direction. The decreasing SWNT temperature may lead to solidification of the catalytic metal nanoparticle terminating SWNT growth or triggering nucleation of a new carbon layer and growth of multiwall carbon nanotubes.
Brownian dynamics simulations of simplified cytochrome c molecules in the presence of a charged surface
Authors: Gorba C. , Geyer T. , Helms V.
Published: June 2004

Simulations were performed for up to 150 simplified spherical horse heart cytochrome c molecules in the presence of a charged surface, which serves as an approximate model for a lipid membrane. Screened electrostatic and short-ranged attractive as well as repulsive van der Waals forces for interparticle and particle–membrane interactions are utilized in the simulations. At a distance from the membrane, where particle–membrane interactions are negligible, the simulation is coupled to a noninteraction continuum analogous to a heat bath [Geyer et al., J. Chem. Phys. 120, 4573 (2004)]. From the particles’ density profiles perpendicular to the planar surface binding isotherms are derived and compared to experimental results [Heimburg et al. (1999)]. Using a negatively charged structureless membrane surface a saturation effect was found for relatively large particle concentrations. Since biological membranes often contain membrane proteins, we also studied the influence of additional charges on our model membrane mimicking bacterial reaction centers. We find that the onset of the saturation occurs for much lower concentrations and is sensitive to the detailed implementation. Therefore we suggest that local distortion of membrane planarity (undulation), or lipid demixing, or the presence of charged integral membrane proteins create preferential binding sites on the membrane. Only then do we observe saturation at physiological concentrations.
Assignment of photoelectron spectra of AunO2− (n=2,4,6) clusters
Authors: Ding Xunlei , Dai Bing , Yang Jinlong , Hou J. G. , Zhu Qingshi
Published: June 2004

The low-lying excited states for AunO2 (n=2,4,6) clusters at their anionic equilibrium geometries are calculated using time-dependent density functional theory and used to assign the features in the experimental photoelectron spectra. Our results compare well with the experimental data.
Accurate energetics of small molecules containing third-row atoms Ga–Kr: A comparison of advanced ab initio and density functional theory
Authors: Yockel Scott , Mintz Benjamin , Wilson Angela K.
Published: June 2004

Advanced ab initio [coupled cluster theory through quasiperturbative triple excitations (CCSD(T))] and density functional (B3LYP) computational chemistry approaches were used in combination with the standard and augmented correlation consistent polarized valence basis sets [cc-pVnZ and aug-cc-pVnZ, where n=D(2), T(3), Q(4), and 5] to investigate the energetic and structural properties of small molecules containing third-row (Ga–Kr) atoms. These molecules were taken from the Gaussian-2 (G2) extended test set for third-row atoms. Several different schemes were used to extrapolate the calculated energies to the complete basis set (CBS) limit for CCSD(T) and the Kohn–Sham (KS) limit for B3LYP. Zero point energy and spin orbital corrections were included in the results. Overall, CCSD(T) atomization energies, ionization energies, proton affinities, and electron affinities are in good agreement with experiment, within 1.1 kcal/mol when the CBS limit has been determined using a series of two basis sets of at least triple zeta quality. For B3LYP, the overall mean absolute deviation from experiment for the three properties and the series of molecules is more significant at the KS limit, within 2.3 and 2.6 kcal/mol for the cc-pVnZ and aug-cc-pVnZ basis set series, respectively.
Valence electronic charge density of distorted C60− monomers in polymerized KC60 and RbC60
Authors: Verberck B. , Popov V. N. , Nikolaev A. V. , Lamoen D.
Published: June 2004

We investigate the valence electronic charge density of the C60− monomers in (C60−)n polymer chains in K- and RbC60 by means of a nonorthogonal tight-binding formalism using experimental data on the positions of the carbon atoms. Various configurations of the C60 cages are considered. Starting from the ideal icosahedral C60 structure and moving to the realistic, experimentally determined spatial configuration of the C60 cages in K- and RbC60, we observe a systematic increase of the electric quadrupole moments on the C60− monomers. We also confirm the validity of factorizing the charge density of a C60− monomer into an angular and a radial part.
Accuracy of photocarrier radiometric measurement of electronic transport properties of ion-implanted silicon wafers
Authors: Li Bincheng , Shaughnessy Derrick , Mandelis Andreas , Batista Jerias , Garcia Jose
Published: June 2004

The determination of the electronic transport properties of ion-implanted silicon wafers with the photocarrier radiometry (PCR) technique by fitting frequency scan data to a single layer model via a multiparameter fitting procedure is presented. A three-layer model is used to simulate the inhomogeneous structure of the ion-implanted wafers. The effects of the structural, electronic, and optical properties of the implanted layer, which are affected significantly by ion implantation, on the frequency behavior of the PCR signal of implanted wafers are discussed. Data simulated with the three-layer model are fitted to a single-layer model to extract the electronic transport properties of implanted wafers. The fitted carrier lifetime and diffusion coefficient are found to be close to that of the substrate layer which is assumed to remain intact after the ion implantation process. When self-normalized relative amplitude is used in the multiparameter fitting, the fitted surface recombination velocity is determined primarily by the level of electronic damage and is approximately independent of the level of optical damage. Experiments with boron implanted wafers were performed and the experimental results were in agreement with the simulations. These results show that the PCR technique is capable of measuring the bulk transport properties of ion-implanted silicon wafers.
Structures and properties of fluorinated amorphous carbon films
Authors: Huang K. P. , Lin P. , Shih H. C.
Published: June 2004

Fluorinated amorphous carbon (a-C:F) films were deposited by radio frequency bias assisted microwave plasma electron cyclotron resonance chemical vapor deposition with tetrafluoromethane (CF4) and acetylene (C2H2) as precursors. The deposition process was performed at two flow ratios R=0.90 and R=0.97, where R=CF4/(CF4+C2H2). The samples were annealed at 300 °C for 30 min. in a N2 atmosphere. Both Fourier transform infrared and electron spectroscopy for chemical analyzer were used to characterize the a-C:F film chemical bond and fluorine concentration, respectively. A high resolution electron energy loss spectrometer was applied to detect the electronic structure. The higher CF4 flow ratio (R=0.97) produced more sp3 linear structure, and it made the a-C:F film smoother and softer. A lifetime of around 0.34 μs and an energy gap of ∼2.75 eV were observed in both the as-deposited and after annealing conditions. The short carriers lifetime in the a-C:F film made the photoluminescence peak blueshift. The annealing changed both the structure and composition of the a-C:F film. The type of fluorocarbon bond and electronic structure characterized the mechanical and physical properties of a-C:F film.
Effects of a sheath boundary on electron energy distribution in Ar/He dc magnetron discharges
Authors: Seo Sang-Hun , In Jung-Hwan , Chang Hong-Young
Published: June 2004

In this study, the effects of a sheath boundary on electron energy distribution and discharge characteristics in an unbalanced and planar-type dc magnetron sputtering system are investigated. The anode sheath potential is changed by applying dc bias voltages to the substrate. The electron energy distribution functions (EEDFs) are measured in argon and helium discharges using a single Langmuir probe in conjunction with the ac superposition method. The evolutions of the EEDFs are first observed in argon at 3 mTorr and then in helium at 30 mTorr. The results show that, as the substrate bias voltage decreases to high negative voltage, the EEDF transition from the bi-Maxwellian to the Maxwellian in the downstream region occurs at a specific bias voltage that depends on the operating gas. The major factors that affect the EEDF formation are investigated. In particular, the concept of total electron bounce frequency is introduced to represent the change of the sheath boundary condition. The observed EEDF transition is explained by comparing it with the plasma characteristic frequencies calculated from the measured EEDFs. As a result, the bi-Maxwellian distribution observed at the small substrate bias voltage is attributed to the low electron–electron collision frequency and the different loss mechanisms of two electron groups: the ambipolar diffusion loss of low-energy electron group confined by low plasma potential and the direct thermal loss of high-energy electron group, providing the electron current that compensates for the discharge current in a steady state.