bluuebunny/crossref_metadata_2025 huggingface dataset:
Electromechanical properties and self-polarization in relaxor Pb(Mg1/3Nb2/3)O3 thin films
Authors: Kighelman Zian , Damjanovic Dragan , Setter Nava
Published: July 2002

Pyrochlore free Pb(Mg1/3Nb2/3)O3 (PMN) thin films were prepared from alkoxide-based solution precursors. The influence of different seeding layers and chemical solution on the microstructures is shown. Dielectric, electrostrictive, and piezoelectric properties of the films were characterized in detail. Films show relaxor-like behavior, but with dielectric permittivity which is low (around 4000 at peak) compared to bulk ceramics and single crystals. Several parameters which might be responsible for this lower permittivity are suggested. Electrostrictive coefficients, M and Q, were determined by measuring strain S and polarization P as a function of the electric field (Eac). At large fields (>2.6×106 V/m), S vs P2 appears to deviate from linear behavior possibly suggesting that the electrostrictive coefficient Q becomes nonlinear in this field range. Investigated as-prepared PMN films exhibit piezoelectric response in the absence of a dc electric field (d33=8–20 pm/V). The value of the associated self-polarization in the films is estimated and its presence confirmed by zero-field pyroelectric measurements. The self-polarization and the piezoelectric coefficient are strong functions of the ac field amplitude. Asymmetry of S vs Eac and d33 vs Edc loops are related to the self-polarization. An ac field induces shift in P vs E loops along the field axis. This increase in the coercive field is associated, through a simple model, with the presence of the self-polarization.
Poling and relaxation of dipoles in dispersive media
Authors: Binks D. J. , Khand K. , West D. P.
Published: July 2002

The poling of dipoles in dispersive media under the influence of an applied electric field and the subsequent relaxation after the removal of the applied field have been analyzed. Both the poling and relaxation processes are described by simple and exact analytical solutions to the rotational diffusion equation. The resulting expressions for the order parameters are power laws with respect to time and hence have no characteristic lifetimes. The poling and relaxation rates are, in fact, determined by a single material parameter: the diffusion constant, D0. These theoretical results have been experimentally verified for dipolar molecules doped into an amorphous polymer host. An excellent agreement between theory and measurement was found.
Quantifying the efficiency of electrodes for positive carrier injection into poly(9,9-dioctylfluorene) and representative copolymers
Authors: Campbell Alasdair J. , Bradley Donal D. C. , Antoniadis Homer
Published: July 2002

The perfect injecting contact for any semiconductor device is, by definition, an ohmic contact. When such a contact is made to an organic semiconductor the current density is limited by bulk space-charge effects. In the absence of charge carrier traps, J reaches the ultimate, trap-free, space-charge-limited value, JTFSCLC=(9/8)εμV2/d3. Knowledge of the mobility μ, permittivity ε, applied bias V, and film thickness d, thus allows the maximum possible current density to be calculated. The absolute injection efficiency of any specific contact can then be quantified via a figure of merit, χ=J/JTFSCLC, namely the ratio of the actual current density to that expected for the ideal trap-free, space-charge-limited current. In this article we report on the injection efficiency of positive carriers into poly(9,9-dioctylfluorene) (PFO) and two representative copolymers, poly(9,9-diocytlfluorene-co-bis-N,N′-(4-methoxyphenyl)-bis-N, N′-phenyl-1,4-phenylenediamine) (PFMO) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (BT). Time-of-flight photocurrent, dark injection transient current, and current density–voltage (J–V) measurements were each performed on indium tin oxide (ITO)/polymer/Au or Al diode structures. The hole injection efficiency of various pretreated ITO electrodes and of the top Au and Al contacts was investigated. ITO coated glass substrates were cleaned by washing with solvents and then either not subjected to further treatment (untreated), exposed to an oxygen plasma (O2 plasma), or coated in a poly(ethylenedioxythiophene)/polystyrenesulphonic acid (PEDOT/PSS) blend. The steady-state J–V characteristics for the different device structures were compared to the expected JTFSCLC and the figure of merit χ was calculated. At an applied field of 5×105 V/cm, the absolute injection efficiencies of holes into PFMO (ionization potential, Ip=4.98 eV) from untreated, O2 plasma treated, and PEDOT/PSS treated ITO were found to be χ=10−3, 1, and 1, respectively. For PFO (Ip=5.8 eV) the same contacts gave χ=10−7, 10−6, and 10−3, respectively. For BT (Ip=5.9 eV) hole transport is highly dispersive, precluding determination of the hole mobility and hence an analysis of the injection efficiency. The injected current densities are, however, comparable to (within a factor of 5 of) those for PFO. The top Au and Al contacts exhibited injection limited behavior (10−8<χ<10−2). Heat treatment of the Au top contacts did not improve injection, contrary to previous observations for a molecularly doped xerographic polymer system.
Observations of fast protons above 1 MeV produced in direct-drive laser-fusion experiments
Authors: Hicks D. G. , Li C. K. , Séguin F. H. , Schnittman J. D. , Ram A. K. , Frenje J. A. , Petrasso R. D. , Soures J. M. , Meyerhofer D. D. , Roberts S. , Sorce C. , Stöckl C. , Sangster T. C. , Phillips T. W.
Published: July 2002

Fast protons ≳1 MeV have been observed on the 60-beam, 30 kJ OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at an intensity I≃1015 W/cm2 and a wavelength λ=0.35 μm. These energies are more than 5 times greater than those observed on previous, single-beam experiments at the same Iλ2. The total energy in the proton spectrum above 0.2 MeV is ∼0.1% of the laser energy. Some of the proton spectra display intense, regular lines which may be related to ion acoustic perturbations in the expanding plasma.
Dielectric relaxation in flux grown KTiOPO4 and isomorphic crystals
Authors: Urenski P. , Gorbatov N. , Rosenman G.
Published: July 2002

Dielectric spectroscopy and dc conductivity have been studied in KTiOPO4 and isomorphic crystals in a temperature range 150–400 K. The experimental data indicate two temperature regions related to different types of relaxation processes. Alternation of the activation energy, suppression of the dielectric dispersion, and decreasing dissipation factor by three orders of magnitude has shown that the superionic state in KTiOPO4 and its isomorphs transit gradually to the dielectric state with temperature lowering. The transition leads to variation of transport properties from ionic to electronic conductivity. It was found that the parameters of the relaxation process (activation energy in the superionic state, temperature range of transition from ionic to electron conductivity) are determined by the sort of mobile cations. The temperature dependence of conductivity relaxation time was obtained from the electric modulus plots.
Internally shunted sputtered NbN Josephson junctions with a TaNx barrier for nonlatching logic applications
Authors: Kaul Anupama B. , Whiteley Stephen R. , Van Duzer Theodore , Yu Lei , Newman Nathan , Rowell John M.
Published: July 2002

We report on the growth, fabrication, and device characterization of NbN internally shunted Josephson junctions with a TaNx barrier. The resistivity of TaNx films could be varied from a few hundred micro-ohms to a few hundred milliohms by increasing the N2 pressure during reactive sputtering. The temperature dependence of IcRn of the junctions with ∼13 mΩ cm barrier resistivity was measured for various barrier thicknesses. The coherence length of the barrier was determined to be 5 nm. By adjusting the barrier thickness, IcRn values >500 μV were observed up to 8.3 K, with Ic and Rn of magnitudes that are suitable for single-flux-quantum digital circuits.
Hydrogen induced tunnel emission in Pt/(BaxSr1−x)Ti1+yO3+z/Pt thin film capacitors
Authors: Baniecki J. D. , Laibowitz R. B. , Shaw T. M. , Parks C. , Lian J. , Xu H. , Ma Q. Y.
Published: July 2002

The leakage current density–applied field (J−EA) characteristics of (BaxSr1−x)Ti1+yO3+z (BSTO) thin film capacitors with Pt electrodes that have been annealed in forming gas (95% Ar 5% H2 or D2) were investigated over the temperature range from −60 to +60 °C. Forming gas annealing significantly increased the leakage current density. The J–EA characteristics exhibited features that could not be fully explained by either a simple thermionic emission or tunneling (Fowler–Nordeim) formalism. Using the general charge transport theory of Murphy and Good, we show that the J–EA characteristics can be successfully interpreted in terms of tunneling of electrons through the interfacial Schottky barrier with the peak in energy distribution of the incident carriers strongly dependent on applied field. At high applied fields the energy distribution of incident carriers is peaked near the Fermi level in the electron injecting metal electrode at all temperatures considered in this study, leading to almost temperature independent J–EA characteristics. At lower applied fields the peak in energy distribution shifts towards the conduction band edge where thermally assisted tunneling occurs and a more pronounced temperature dependence of the current density is observed. Good agreement between experiment and theory is demonstrated for a reasonable parameter set for BSTO thin films strongly suggesting that the high leakage current density often observed after forming gas annealing results from tunneling of electrons through the interfacial Schottky barrier.
Effect of spin-polarized injection on the mixed state of YBa2Cu3O7−δ
Authors: Mikheenko P. , Colclough M. S. , Severac C. , Chakalov R. , Welhoffer F. , Muirhead C. M.
Published: July 2002

We report measurements of the magnetic moment of YBa2Cu3O7−δ films when subjected to injection of short pulses of spin-polarized current from the colossal magnetoresistance (CMR) manganites La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3, and unpolarized current from the nonmagnetic perovskite LaNiO3(LNO). We study relaxation from the Bean critical state, with and without current pulses. For pulses longer than 100 ms, the effect of injection is similar for all samples and is due to heating. Below 1 ms, we see a clear difference between the CMR and LNO samples and attribute this to the spin polarization of the current injected from the CMR materials.
Normal and defective perylene substitution sites in alkane crystals
Authors: Leontidis Epameinondas , Heinz Hendrik , Palewska Krystyna , Wallenborn Ernst-Udo , Suter Ulrich W.
Published: July 2002

We examine experimentally and computationally the nature of substitution of perylene in polycrystalline solid alkane matrices (Shpol’skii systems). The technique of low temperature excitation-emission matrix spectroscopy is used to determine all substitution sites in alkane matrices from hexane to decane. A theoretical method from the group of Jortner [Shalev et al., J. Chem. Phys. 95, 3147 (1991)], which was extended and applied by us to this problem in the past [Wallenborn et al., J. Chem. Phys. 112, 1995 (2000)], allows one to separate the perylene sites in all alkanes into normal and defective sites. Normal sites are obtained by direct substitution of two alkane molecules by a perylene molecule, while defective sites are derived from normal sites by eliminating one of the four nearest neighbors of perylene in the lattice planes parallel to the chromophore. We discuss the strengths and limitations of the present theoretical treatment, which can serve as a valuable supplement and guide to line-narrowing and single-molecule spectroscopic investigations of impurity centers in low-temperature solids.
Effect of condensation and evaporation on the viscous-convective subrange
Authors: Jeffery Christopher A.
Published: July 2002

The effect of condensation and evaporation on the viscous-convective subrange is investigated using a general mean-field approximation that is consistent with the nonhomogeneous vertical structure of the condensate’s first and second moments and experimental observations of mean vertical flux in a condensation cloud. Expressions for the scalar density in the Batchelor limit are derived and used to reproduce the spectral behavior of new atmospheric measurements that exhibit anomalous scaling of cloud liquid water in the near inertial-convective regime. Good agreement between the model and data are obtained when axisymmetric Kraichnan transfer of scalar variance is balanced by axisymmetric production by condensation/evaporation resulting in an isotropic contribution to the real (homogeneous) part of the spectrum. The model also assumes a significant imaginary (nonhomogeneous) component to the spectrum that is indicative of a strong vertical coherence in condensation clouds. A “production subrange” is predicted in which the scalar dissipation rate increases with increasing wave number and the usual −1 viscous-convective scaling evolves into an anomalous −3 regime. The strongly nonhomogeneous (anisotropic) character of the predicted scalar spectrum is in stark contrast with atmospheric models of inertial-convective regime cloud inhomogeneity that are used in radiative transfer calculations and are typically isotropic.
Low turn-on voltage GaAs heterojunction bipolar transistors with a pseudomorphic GaAsSb base
Authors: Oka Tohru , Mishima Tomoyoshi , Kudo Makoto
Published: July 2002

We have developed GaAs heterojunction bipolar transistors (HBTs) with low turn-on voltage by using pseudomorphic GaAsSb as the base layer. The turn-on voltage of GaAs/GaAs0.91Sb0.09 HBT is 0.10 V lower than that of InGaP/GaAs HBT. The lower turn-on voltage is attributed to the smaller band gap of the GaAsSb base layer, indicating that GaAsSb is useful material for reducing turn-on voltage of GaAs HBTs. The current gain of 20 is obtained for GaAs/GaAs0.91Sb0.09 HBT, which is larger than those of previously reported GaAs/GaAsSb HBTs owing to the pseudomorphic, fully strained GaAsSb with no misfit dislocations. The knee voltage of 0.47 V is attained at the collector current density of 5×104 A/cm2. These results indicate that GaAs/GaAsSb HBTs have a great potential for reducing operating voltage and power dissipation.
Calculations using generalized valence bond based Møller–Plesset perturbation theory
Authors: Sejpal Mihir , Messmer Richard P.
Published: July 2002

This work presents calculations on small molecules using second-order Møller–Plesset perturbation theory with a generalized valence bond reference wave function. Møller–Plesset perturbation theory applied to a generalized valence bond reference (GVB-MP2) currently provides the best tradeoff between accuracy and computational feasibility among the methodologies of electronic structure. Frienser and co-workers have shown that the computational effort required for the GVB-MP2 methodology scales as no more than the third power of the size of the system, while that for the coupled-cluster and complete-active space methods scales as the seventh or worse power of system size. The GVB wave function is a qualitative wave function. Spectroscopic parameters and energetics at the GVB level are in qualitative agreement with experimentally determined values. The calculations presented in this work demonstrate that spectroscopic parameters computed using GVB-MP2 are in better agreement with experiment than those computed at the GVB level, and in close agreement with those obtained from the coupled-cluster plus singles and doubles with triples substitutions, which requires significantly more effort.
All Ya–Ba–Cu–O c-axis trilayer interface-engineered Josephson junctions
Authors: Moeckly B. H.
Published: July 2002

We demonstrate the fabrication of c-axis YBCO trilayer Josephson junctions incorporating an engineered interface as the barrier layer. These devices display resistively shunted junction-like I–V characteristics with IcRn products >1 mV at 4.2 K and ∼100 μV at 60 K.
Single-electron inverter
Authors: Heij C. P. , Hadley P. , Mooij J. E.
Published: July 2002

A single-electron inverter was fabricated that switches from a high output to a low output when a fraction of an electron is added to the input. For the proper operation of the inverter, the two single-electron transistors that make up the inverter must exhibit voltage gain. Voltage gain was achieved by fabricating a combination of parallel-plate gate capacitors and small tunnel junctions in a two-layer circuit. Voltage gain of 2.6 was attained at 25 mK and remained larger than one for temperatures up to 140 mK. The temperature dependence of the gain agrees with the orthodox theory of single-electron tunneling.
2.5% efficient organic plastic solar cells
Authors: Shaheen Sean E. , Brabec Christoph J. , Sariciftci N. Serdar , Padinger Franz , Fromherz Thomas , Hummelen Jan C.
Published: July 2002

We show that the power conversion efficiency of organic photovoltaic devices based on a conjugated polymer/methanofullerene blend is dramatically affected by molecular morphology. By structuring the blend to be a more intimate mixture that contains less phase segregation of methanofullerenes, and simultaneously increasing the degree of interactions between conjugated polymer chains, we have fabricated a device with a power conversion efficiency of 2.5% under AM1.5 illumination. This is a nearly threefold enhancement over previously reported values for such a device, and it approaches what is needed for the practical use of these devices for harvesting energy from sunlight.
Switching of a ferroelectric polymer Langmuir–Blodgett film studied by electrostatic force microscopy
Authors: Blinov L. M. , Barberi R. , Palto S. P. , De Santo M. P. , Yudin S. G.
Published: July 2002

Ultrathin ferroelectric Langmuir–Blodgett films are studied using an electrostatic force microscope (EFM). The films were deposited by a subsequent transfer of monolayers of a ferroelectric polymer from the water surface onto Si substrates covered with Al electrodes. A film was switched locally by a voltage applied between a tip and the Al electrode. The dynamics of the electric field relief above a switched ferroelectric domain is studied by EFM imaging with simultaneous control of the topographic relief. In order to explain an image contrast a simple model for a thin ferroelectric film is discussed which takes into account the remanent polarization of the film, the surface charge, and interface work functions.
Photoluminescence method for detecting trace levels of iron in ultrapure silicon
Authors: Broussell I. , Karasyuk V. A. , Thewalt M. L. W.
Published: July 2002

A nondestructive technique is presented for the determination of trace levels of interstitial iron contamination in ultrapure silicon. This approach is based on the well-known ability of iron to undergo a reversible pairing reaction with boron near room temperature. A variety of float-zoned silicon samples with low concentrations of boron (∼1011 cm−3) were subjected to thermal annealing treatments to study changes in the apparent boron concentration as determined by the standard method of comparing the photoluminescence intensity of the boron bound exciton to that of the free exciton. Changes in the apparent boron concentration were attributed to the formation or dissociation of iron–boron pairs, allowing us to estimate the interstitial iron concentration in these samples. Remarkably, relatively mild thermal treatments can change the apparent boron concentration in some of these samples by up to a factor of ten.
Reflectivity of crystalline Ge and Si at the melting temperature measured in real time with subnanosecond temporal resolution
Authors: Chaoui N. , Siegel J. , Solis J. , Afonso C. N.
Published: July 2002

Real time reflectivity measurements with subnanosecond time resolution have been used to determine the reflectivity at the melting temperature RS(Tm) of single crystalline Ge and Si at 514.5 nm. Due to the excellent time resolution and sensitivity achieved in a single exposure experiment, the reflectivity of the solid just before melting could be measured. Values of RS(Tm)=0.470±0.006 and RS(Tm)=0.440±0.008 for c-Ge and c-Si have, respectively, been determined. These values, together with those determined by heating in vacuum in the range 300–800 K, are compared to those reported earlier in the literature and the differences are discussed.
Low-field magnetoresistance in nanosized La0.7Sr0.3MnO3/Pr0.5Sr0.5MnO3 composites
Authors: Liu J.-M. , Yuan G. L. , Sang H. , Wu Z. C. , Chen X. Y. , Liu Z. G. , Du Y. W. , Huang Q. , Ong C. K.
Published: July 2002

Nanosized La0.7Sr0.3MnO3/Pr0.5Sr0.5MnO3 (LSMO1−xPSMOx) ceramic composites are prepared using solid-sate sintering. Their microstructural, electro- and magnetotransport properties are characterized by means of various techniques. It is found that the antiferromagnetic/ferromagnetic coupling between PSMO/LSMO at low temperature and the weak ferromagnetic order of PSMO at high temperature results in enhanced low-field magnetoresistance (LFMR) of the composites. With increasing temperature up to 250 K the observed LFMR decays more slowly than that for pure LSMO and this behavior may be explained by the spin coupling near boundaries between LSMO and PSMO grains.
InAs self-assembled quantum dots grown on an InP (311)B substrate by molecular beam epitaxy
Authors: Li Y. F. , Wang J. Z. , Ye X. L. , Xu B. , Liu F. Q. , Ding D. , Zhang J. F. , Wang Z. G.
Published: July 2002

Self-assembled InAs quantum dots (QDs) have been grown by solid-source molecular beam epitaxy on a (311)B InP substrate. Transmission electron microscopy clearly shows that a high density of smaller InAs islands can be obtained by using such a high index substrate. After introducing a lattice-matched underlying In0.52Al0.24Ga0.24As layer, the InAs QDs are much more uniform in size and form two-dimensional well ordered arrays. The photoluminescence (PL) spectra also confirm that the InAs QDs grown on underlying In0.52Al0.24Ga0.24As have a better quality than those grown in the In0.52Al0.48As matrix. A simple calculation indicates that the redshift of the PL peak energy mainly results from InAs QDs on underlying In0.52Al0.24Ga0.24As of large size.
Model for the incorporation of excess arsenic into interstitial positions during the low-temperature growth of GaAs(001) layers
Authors: Marek T. , Kunsági-Máté S. , Strunk H. P.
Published: July 2002

Using the GAUSSIAN 94 program package on a Ga16As17H36 cluster, the interaction of an As2 molecule approaching an As-terminated nonreconstructed GaAs(001) surface along the symmetry axis C2 of the cluster is studied. To describe the interaction between As2 molecule and cluster, the total energy of the system was calculated for various cluster As2 distances. The results show that the system (cluster plus molecule) has a stable conformation where one As atom resides at a tetrahedral interstitial site below the surface, i.e., inside the crystal. This result indicates the possibility of As incorporation by the particular process presented here.
Resistivity analysis on n-semi-insulating-n and p-semi-insulating-p structures exemplified with semi-insulating InP
Authors: Söderström D. , Lourdudoss S.
Published: July 2002

Current–voltage characteristic in semi-insulating (SI) InP layers sandwiched between n- or p-type layers have been simulated. Deep acceptor and deep donor levels were considered. A one-dimensional two-carrier numerical model was used, which takes into account carrier diffusion and Shockley–Read–Hall recombination through the deep traps. The current–voltage behavior is studied in the linear ohmic regime in order to deduce the resistivity, which is analyzed and compared to values obtained from equilibrium calculations. The simulations predict an increased resistivity for n-SI-n and a decreased resistivity for p-SI-p structures, while increasing the concentration of deep acceptors. Furthermore, a maximum in the resistivity as a function of the trap concentration is found for an n-SI-n structure, which accommodates electron injection. In addition, the influence of the position of the deep acceptor in the band gap on the resistivity is analyzed, and it is shown that it significantly affects the behavior when comparing n-SI-n and p-SI-p structures. Incorporating a deep donor level along with a deep acceptor level will improve the SI behavior of the p-SI-p structure, whereas a slight drop in resistivity occurs for the n-SI-n structure. Our predicted electrical behavior from simulation is found to be in accordance with the previously published experimental results in InP:Fe and InP:Fe,Ti.
Electro-optical response of hybrid twisted nematic liquid crystal displays
Authors: Kubono Atsushi , Kyokane Yozo , Kasajima Yasushi , Akiyama Ryuichi , Tanaka Katsufumi
Published: July 2002

A novel operating mode for a liquid crystal display device has been developed. The devices involved both hybrid alignment and twisted director configuration. The operating mode was thus termed a hybrid twisted nematic (HTN) mode. In comparison with the conventional twisted nematic mode, faster response and smaller operation voltage have been obtained by using a proper amount of chiral dopant and a homeotropically aligning surfactant having an appropriate length of alkyl chains. The characteristics of the HTN cells were strongly dependent on the chain length and the applied bias voltage. It was also found that a chiral dopant played an important role in the director configuration associated with hybrid twisted alignment.
A study of vacancy-type defects introduced by the carburization of Si by monoenergetic positron beams
Authors: Uedono Akira , Muramatsu Makoto , Ubukata Tomohiro , Watanabe Masahito , Ichihashi Toshinari , Suzuki Ryoichi , Ohdaira Toshiyuki , Mikado Tomohisa , Takasu Seiichi
Published: July 2002

Vacancy-type defects introduced by the carburization of Si were studied by means of monoenergetic positron beams. Doppler broadening spectra of the annihilation radiation were measured for Si substrates with carbon films at temperatures between 298 and 1473 K. The line-shape parameter S, which corresponds to the annihilation of positrons near the interface between the carbon film and the Si substrate, started to increase above 1173 K. This was attributed to the trapping of positrons by vacancy-type defects introduced by carburization. The major species of the defects detected by positron annihilation was identified to be vacancy clusters in the Si substrate.
Raman spectroscopy and x-ray diffraction studies of (Ti,Al)N films deposited by filtered cathodic vacuum arc at room temperature
Authors: Cheng Y. H. , Tay B. K. , Lau S. P. , Shi X.
Published: July 2002

(Ti,Al)N films were deposited by an off-plane double bend filtered cathodic vacuum arc technique in N2 ambient at room temperature. X-ray diffraction (XRD) and Raman spectroscopy were used to characterize the film structure. The influence of deposition pressure and the substrate bias on the XRD patterns and Raman spectra were systematically studied. As deposition pressure is increased, the film structure evolves from metallic, to metal rich (Ti,Al)2N, and finally to a single face-centered cubic (Ti,Al)N. As substrate bias is increased, the structure evolves from amorphous to crystalline (Ti,Al)N for bias at 200 V. Further increase of substrate bias results in the decrease of the crystalline size and increase of disorder phase. Four peaks at 238, 326, 442, and 679 cm−1, arisen from the transverse acoustic, longitudinal acoustic, transverse acoustic optical, and longitudinal acoustic modes of (Ti,Al)N phase, respectively, can be observed in the Raman spectra. The variation of the Raman spectra with the deposition pressure and substrate bias are in good agreement with that of the XRD measurements.
Flux pinning and dissipation in textured multifilamentary (Bi–Pb)2Sr2Ca2Cu3Ox/Ag tapes
Authors: Pradhan A. K. , Feng Y. , Wu Y. , Nakao K. , Koshizuka N. , Zhang P. X. , Zhou L. , Li C. S.
Published: July 2002

We have studied the magnetotransport, magnetization properties, and microstructure of multifilamentary (Bi–Pb)2Sr2Ca2Cu3Ox (Bi-2223)/Ag tapes with varying filaments from 1 to 37 to explore the energy dissipation, reversible flux motion, and flux pinning related to the microstructure. Our results show that the dissipation is temperature, current, and magnetic field induced. The dissipation is thermally activated and the vortex liquid state is plastically deformed. The dissociation of vortex–antivortex pairs by current, magnetic field, and temperature plays a major role in enhancing the dissipation process, resulting in large broadening in the transition. We observed a reversible fluxoid motion of vortices. The critical current density follows an exponential dependence on the magnetic field. The weak links seem to be broken even for a field as low as 5 G even 12 K below the transition temperature. However, at low temperature, the weak links behave as superconducting. The microscopic characterizations show evidence for the dislocations, stacking faults, and misorientation of grains and grain boundaries that control the critical current in tapes.
The polaron and bipolaron states of poly(phenylene vinylene)
Authors: Yao K. L. , Han S. E. , Zhao L.
Published: July 2002

We utilize the Pariser–Parr–Pople model for poly (phenylene vinylene) (PPV) and take into account the realistic lattice structures of PPV considering the electron–electron and the electron–phonon interactions so that we can expose the subtle electronic and the lattice structures of PPV. We find that the lattice deformation is mainly in the vinylene group, and we also discuss the spin density waves of polaron and the charge density waves of polaron and bipolaron. The spin density wave is modulated by charge density wave. The competition between polaron and bipolaron are studied. We find that when the electron–phonon interaction λ<0.21 and the intersite electron–electron interaction v is comparably small, v favors the polaron. When λ>0.21, v favors the bipolaron.
CH-stretching overtone spectra of a fast rotating methyl group. I. Toluene C6D5CH3
Authors: Cavagnat D. , Lespade L.
Published: July 2002

The CH-stretching overtone spectra of the methyl group in gaseous toluene C6D5CH3 are recorded with conventional Fourier transform near infrared spectroscopy in the ΔvCH=1–4 regions and by intracavity laser photoacoustic spectroscopy in the ΔvCH=5 and 6 regions. All spectra exhibit a complex structure. They are analyzed with a theoretical model which takes into account, within the adiabatic approximation, the coupling of the anharmonic CH stretch vibrations, described by Morse potentials, with the quasifree internal rotation of the methyl group and with isoenergetic combination states involving the six angle deformation modes of the methyl group. Using uniquely determined canonical potential-energy and kinetic-energy matrices allows us to solve the problem of indeterminacy caused by the description of molecular vibrations in such redundant coordinates. A set of Fermi resonance parameters and their variation with the internal rotation coordinate are determined from the fitting of the ΔvCH=1–6 overtone spectra. At Δv=1 and 2, the vibrational energy is expressed in a normal mode basis. Fermi resonance phenomena involving mainly the HCH bending modes lead to strong intramolecular vibrational energy redistribution. At this energy, a Coriolis coupling through internal rotation, which gives rise to a characteristic asymmetric top vibration-rotation profile, further perturbs the vibrational modes perpendicular to the rotation axis. From the second overtone (Δv=3), the vibrational energy is described in a local mode basis and perturbations due to Fermi resonance vanish progressively up to Δv=5. At Δv=6, a strong spectral perturbation is again observed. But, the efficient Fermi resonance phenomena are now essentially related to combination states involving HCH and HCC bending combination modes. This simple calculation successfully describes the relative intensity and frequency of each peak within a given overtone.
Stiffness analysis for the micromagnetic standard problem No. 4
Authors: Tsiantos Vassilios D. , Suess Dieter , Schrefl Thomas , Fidler Josef
Published: July 2002

In this article solutions to micromagnetic standard problem No. 4, a 500-nm×125-nm-wide NiFe film, are presented. A three-dimensional-finite element simulation based on the solution of the Gilbert equation has been used. The simulations show that two different reversal mechanisms occur for the two different applied fields. For a field at 170° counterclockwise from the saturation direction there is a nonuniform rotation of magnetization towards the direction of the applied field, with the magnetization at the ends rotating faster than the magnetization in the center. For a field at 190° counterclockwise from the saturation direction the magnetization at the ends and in the center rotate in opposite directions leading to the formation of a 360° wall after 0.22 ns associated with a peak in the exchange energy. Moreover, the time for the magnetization component parallel to the long axis to cross the zero is 0.136 and 0.135 ns for field 1 and field 2, respectively. The stiffness of the problem has been investigated solving the system of ordinary differential equations with a nonstiff method (Adams) and a stiff one (backward differentiation formula, BDF). For the measure of stiffness the ratio of the total number of time steps (nst) taken by the two solvers, that is nst(Adams)/nst(BDF), has been used. This ratio is 0.784 for field 1 and 0.593 for field 2, which means that the nonstiff method (Adams) uses larger time steps than the stiff method (BDF) and consequently the systems are not stiff. The average time step for the Adams method was 0.2 ps for both fields.
Sb-doped SrTiO3 transparent semiconductor thin films
Authors: Wang H. H. , Chen F. , Dai S. Y. , Zhao T. , Lu H. B. , Cui D. F. , Zhou Y. L. , Chen Z. H. , Yang G. Z.
Published: July 2002

Optically transparent Sb-doped SrTiO3 thin films with a transmittance higher than 95% in most of the visible region have been grown on SrTiO3 (001) substrate by pulsed laser deposition. The films behave as an n-type semiconductor between 10 K and room temperature. The carrier concentration and mobility of the films at room temperature are ∼5.8×1017 cm−3 and ∼6.4 cm2/V s, respectively. X-ray photoelectron spectroscopy measurement reveals that the delocalized electrons from the Sb dopants give rise to deep impurity levels within the band gap of the parent compound, which are responsible for the electrical conduction observed. The wide band gap and low density of states in the conduction band account for transparency of the films.
Critical current in a spin injection device
Authors: Sarkar S. , Raychaudhuri P. , Mal P. K. , Bhangale A. R. , Pinto R.
Published: July 2002

We report on direct evidence of the suppression of critical current due to pair-breaking in a superconducting microbridge when the measurement is carried out by injecting spin-polarized carriers instead of normal electrons. A thin layer of La0.7Ca0.3MnO3 was used as the source of spin-polarized carriers. The microbridge was formed on the DyBa2Cu3O7−δ thin film by photolithographic techniques. The design of our spin-injection device allowed us to inject spin-polarized carriers from the La0.7Ca0.3MnO3 layer directly to the DyBa2Cu3O7−δ microbridge (without any insulating buffer layer) making it possible to measure the critical current when polarized electrons alone are injected into the superconductor. Our results confirm the role of polarized carriers in breaking the Cooper pairs in the superconductor.
Magnetic resonance in RuSr2RECu2O8 (RE=Eu, Gd) ferromagnetic superconductor
Authors: Butera A. , Fainstein A. , Winkler E. , van Tol J. , Oseroff S. B. , Tallon J.
Published: July 2002

Magnetic resonance techniques at frequencies of 9.4, 34, 110, and 219 GHz were used to investigate the magnetism of the recently discovered ferromagnetic superconducting system RuSr2RECu2O8 (RE=Eu, Gd). One absorption is observed in both compounds below the magnetic ordering temperature, TM∼135 K, and can be ascribed to the ferromagnetic resonance of the ordered Ru ions. The resonance field of this absorption is frequency dependent and is well described assuming an in-plane ferromagnetic component with strong out-of-plane (Hz) and weak in-plane (Hx) anisotropy fields. We have derived almost the same anisotropy fields for both compounds at 70 K (〈Hz〉∼250 kOe and 〈Hx〉∼165 Oe). Another absorption is observed only in the Gd compound and arises from the paramagnetic resonance of the Gd3+ ions. Below TM the Gd3+ signal shifts from g=2 evidencing the appearance of a homogeneous internal field, Hi,Gd, that is frequency (and hence field) dependent. This variation is probably due to the nonsaturated nature of the Ru lattice. Below 10 K there is an additional shift of the Gd3+ line arising from the Gd dipole-dipole interaction.
Mean-field approach for the B phase of (La,Ca)MnO3
Authors: Schlottmann P.
Published: July 2002

I consider a simple cubic lattice of mixed valent Mn ions (Mn4+ and Mn3+) and calculate the ground state energy for the ferromagnetic B phase using a slave-boson mean-field approach. Each Mn ion has three localized t2g electrons with their spins ferromagnetically coupled to form a spin S=3/2. Ions in the Mn3+ configuration have an additional eg electron to form a total spin of (S+1/2). The eg electrons are allowed to hop between the Mn sites (giving rise to the double exchange), but the multiple occupancy of the eg levels is excluded at each site. Five slave bosons per site are introduced to take into account the correlations between the eg electrons.
Modification of exchange interaction parameters by wire width in Cd0.95Mn0.05Te/Cd0.90Mg0.10Te quantum wires
Authors: Chen Z. H. , Debnath M. C. , Shibata K. , Saitou T. , Sato T. , Oka Y.
Published: July 2002

Magneto-photoluminescence from mesa-shaped Cd0.95Mn0.05Te/Cd0.90Mg0.10Te quantum wires were studied. The Zeeman shift of the excitonic photoluminescence from the quantum wires in a magnetic field is significantly decreased, compared to that from quantum wells. This decrease results from the possible reduction of the exchange interaction of electrons and holes with Mn ions in low dimensional structures. In the excitonic photoluminescence of the quantum wires with the width of 50–100 nm, the transient relaxation of the luminescence peak energy due to the excitonic magnetic polaron formation is remarkably less than that in the quantum well. This result shows the suppressed excitonic magnetic polaron effect in the narrower quantum wires.
Fabrication and properties of spin dependent tunneling junctions with CoFeHfO as free layers
Authors: Wang Dexin , Qian Zhenghong , Daughton James M. , Nordman Cathy , Tondra Mark , Reed Daniel , Brownell David
Published: July 2002

Spin-dependent tunneling (SDT) structures of Ta–Cu–Ta–CoFeHfO–Al2O3–FeCo–CrPtMn have been deposited by rf diode sputtering. The junctions have been fabricated using photolithographic techniques. A junction magnetoresistive ratio as high as 34% has been obtained after annealing the junctions at 250 °C for 1 h. The junctions have a typical bias voltage of 475 mV at half-maximum magnetoresistance values. The resistance–area–product is about 1 MΩ μm2, and the dc breakdown voltage is about 1.5 V. AlN has also been investigated as a barrier for the junctions. CoFeHfO layers have a high in-plane induced anisotropy field of 65 Oe and a high 4πMs value of 1.2 T, leading to a ferromagnetic resonance frequency higher than 2 GHz. This material has a high bulk resistivity of 1000 μΩ cm, resulting in a small eddy current effect. Therefore, a SDT device with CoFeHfO as the free layer is an attractive candidate for high-speed applications.
Spin tunneling in Co/Au/I/BiSrCaCuO tunnel junctions
Authors: Ishibashi Takayuki , Sato Kazushige , Yonemitsu Koutarou , Sato Katsuaki
Published: July 2002

Conductance spectra of Co/Au/I/Bi2Sr2CaCu2Ox+8(BSCCO) tunnel junctions have been measured to investigate the tunneling of spin polarized quasiparticles. BSCCO thin films prepared by molecular beam epitaxy are used for the Co/Au/I/BSCCO tunnel junctions because those have flat surface without any large steps as observed on the cleaved surface of single crystals. The superconducting gap is clearly observed in the conductance spectra at the temperature below the critical temperature Tc. In addition, a splitting of the zero bias conductance peak and the imbalance of the peak heights are observed. This feature is induced by the spin polarization of quasiparticles injected into the superconductors.
Theory of the reentrant charge-order transition in the manganites
Authors: Hellberg C. Stephen
Published: July 2002

A theoretical model for the reentrant charge-order transition in the manganites is examined. This transition is studied with a purely electronic model for the eg Mn electrons: the extended Hubbard model. The electron–phonon coupling results in a large nearest-neighbor repulsion between eg electrons. Using a finite-temperature Lanczos technique, the model is diagonalized on a 16-site periodic cluster to calculate the temperature-dependent phase boundary between the charge-ordered and homogeneous phases. A reentrant transition is found. The results are discussed with respect to the specific topology of the 16-site cluster.
Transition between thermally assisted relaxation and quantum tunneling in a molecular magnet
Authors: Mertes K. M. , Zhong Y. , Sarachik M. P. , Paltiel Y. , Shtrikman H. , Zeldov E. , Rumberger E. , Hendrickson D. N.
Published: July 2002

We report Hall sensor measurements of the magnetic relaxation of Mn12–acetate as a function of magnetic field applied along the easy axis of magnetization. Data taken at a series of closely spaced temperatures between 0.24 and 1.9 K provide new clues for understanding the physics of quantum tunneling of magnetization in Mn12–acetate.
Effect of flow profile on low frequency drift-type waves in a dusty plasma
Authors: Chakraborty M. , Sarmah D. , Sen S. , Saikia B. K.
Published: July 2002

A nonlocal theory has been developed to study the effect of a flow profile on the stability of low frequency drift-type instability in an inhomogeneous, dusty plasma. The representative experimental values for the plasma parameters show that, due to the presence of dust particles, the well-known stability of the drift wave in a sheared slab geometry does not hold. However, a radially varying transverse (to the equilibrium magnetic field) velocity field is found to play a key role in stabilizing this instability.
Temperature dependence of interlayer exchange coupling in manganite-based superlattices
Authors: Nikolaev K. R. , Dobin A. Yu. , Krivorotov I. N. , Dahlberg E. Dan , Goldman A. M.
Published: July 2002

We present a study of the temperature variation of the interlayer coupling strength of antiferromagnetically coupled La2/3Ba1/3MnO3/LaNiO3 heterostructures. No drastic decrease in the coupling strength was observed up to the Curie point of the manganite. This behavior suggests that the spin polarization of the carriers persists to high temperatures.
Probing diffusion barrier integrity on porous silica low-k thin films using positron annihilation lifetime spectroscopy
Authors: Sun Jia-Ning , Gidley David W. , Dull Terry L. , Frieze William E. , Yee Albert F. , Ryan E. Todd , Lin Simon , Wetzel Jeff
Published: July 2002

The technique of positron annihilation lifetime spectroscopy (PALS) has been used to investigate the continuity and thermal stability of thin barrier layers designed to prevent Cu atom diffusion into porous silica, low-dielectric constant (k) films. Nanoglass™ K2.2-A10C (A10C), a porous organosilicate film, is determined to have interconnected pores with an average tubular-pore diameter of (6.9 ± 0.4) nm. Cu deposited directly on the A10C films is observed to diffuse into the porous structure. The minimum necessary barrier thickness for stable continuity of Ta and TaN layers deposited on A10C is determined by detecting the signal of positronium (Ps) escaping into vacuum. It is found that the 25 nm thick layers do not form continuous barriers. This is confirmed by the presence of holes observed in such films using a transmission electron microscope. Although 35 nm and 45 nm Ta and TaN layers perform effectively at room temperature as Ps barriers, only the Ta-capped samples are able to withstand heat treatments up to 500 °C without breakdown or penetration into the porous film. TaN interdiffusion into the silica pores is indicated by the reduction of the Ps lifetime after high annealing temperatures. The validity of using Ps diffusion to test barrier layers designed to inhibit Cu diffusion is discussed. The procedures to standardize the testing of barrier layer integrity and thermal stability using PALS are proposed. Extension to probing barrier layers in realistic vias and trenches should be straightforward.
Design and fabrication of organic complementary circuits
Authors: Crone B. K. , Dodabalapur A. , Sarpeshkar R. , Filas R. W. , Lin Y.-Y. , Bao Z. , O’Neill J. H. , Li W. , Katz H. E.
Published: July 2002

We have used a simple model description of single field effect transistor characteristics to design organic complementary circuits ranging in complexity from simple inverters through 48-stage shift registers and three-bit row decoders. The circuits were fabricated using standard silicon photolithographic techniques to define the metal, insulator, and interconnect levels. The ohmic source and drain contacts and part of the interconnect metallization were formed by electroless/immersion deposition of Ni-P/Au on prepatterned TiN. The n-type and p-type organic semicondcutors were evaporated onto these substrates to complete the circuits. Measured circuit characteristics were in reasonable agreement with simulations based on the simple device model.
Comment on “Uncertainty principle for proper time and mass” [J. Math. Phys 40, 1237 (1999)]
Authors: Krikorian R. A.
Published: July 2002

Kudaka and Matsumoto derive the uncertainty relation c2ΔmΔτ≈h between the proper time τ and rest mass m of a material particle by adopting for the description of the particle the Lagrangian L=M(τ̇−c−1−gμνẋμẋν)+eAμ(x)ẋμ, defined on the enlarged space (xμ,τ,M), where M is an additional dynamical variable. A key step in the derivation of their uncertainty relation is the identification of the variable M with the rest mass m. Adopting the point of view of variational analysis, we show that such an identification goes against the relativistic requirement of the path dependence of proper time.
Nonlinear photoluminescence of fullerene-doped optical glasses
Authors: Zeng Heping , Sun Zhenrong , Segawa Yusaburo , Lin Fucheng , Mao Sen , Xu Zhizhan
Published: July 2002

Strong broadband white photoluminescence was observed in fullerene-doped phosphate and fluorophosphate optical glasses irradiated by an ultraviolet laser. Microphotoluminescence measurements demonstrated the existence of microislands in those amorphous glasses, where fullerene dopants provided high photosensitivity and optical nonlinearity. Nonlinear photoluminescence was observed under ultralow continuous-wave laser excitations. The photoluminescence peak wavelengths were demonstrated to depend nonlinearly on the laser excitation power.
An application of solvable structures to classical and nonclassical similarity solutions
Authors: Barco M. A.
Published: July 2002

Using exterior differential systems, we extend work by Harrison and Estabrook for deriving similarity solutions of hyperbolic and parabolic partial differential equations (PDEs). We use exterior calculus results to show that a symmetry (isovector) of the differential ideal corresponding to some hyperbolic or parabolic PDE can be used to generate a Cauchy characteristic vector field of a restricted exterior differential system defined on some four-dimensional regular submanifold of the first jet bundle. We then show that this restricted differential ideal has a Frobenius integrable annihilating space, which can be used to yield a similarity solution of the PDE by applying results from Lie and Cartan on integrating Frobenius integrable vector field distributions via symmetry. We also give an extension to conditional symmetries.