bluuebunny/crossref_metadata_2025 huggingface dataset:
Surface diffusion of thin perfluoropolyalkylether films
Authors: O’Connor Teresa M. , Back Young R. , Jhon Myung S. , Min Byung G. , Yoon Do Y. , Karis Thomas E.
Published: July 2002

The spreading characteristics of thin polymeric liquid films of perfluoropolyalkylethers (PFPEs) Fomblin Z15 and Fomblin Zdol (hydroxyl terminated PFPE) on silica surfaces have been measured by scanning microellipsometry (SME). We estimated the surface diffusion coefficients and propose a modified diffusion equation to interpret the spreading phenomenon from film thickness profiles measured with SME. We investigated the spreading of Z15 as a function of binary blend ratio of monodisperse Z15 fractions and found that the surface diffusion coefficients of the blends do not obey a simple linear mixing rule. A summary of thin PFPE film spreading characteristics as a function of molecular weight, film thickness, chain-end functionality, temperature, and humidity is presented.
Micro/nanoscale studies of boundary layers of liquid lubricants for magnetic disks
Authors: Koinkar Vilas N. , Bhushan Bharat
Published: July 2002

The atomic force/friction force microscope is used to study the micro/nanotribiological properties of perfluoropolyether lubricants. Single-crystal silicon wafers were lubricated with nonpolar (Z-15) and polar (Z-DOL and Demnum S-100) lubricants. The nanowear tests show that the nonpolar (Z-15) lubricant depleted from the wear track within a few cycles, whereas polar (Z-DOL) lubricant exhibits excellent nanowear resistance with no degradation. The polar lubricant results in a lower value of microfriction as compared to the nonpolar lubricant and unlubricated silicon sample. The effect of thickness of polar lubricant is studied for the thermally bonded Z-DOL lubricant before and after wash. Unwashed polar lubricant film with unbonded fraction exhibited better resistance to wear than that of washed lubricant film. Thicker films are also more durable. Wear experiments with magnetic disks show that lubricant films on a super smooth disk is more effective in reduction of friction and wear than a smooth disk. Coefficient of friction on micro- and macroscales are compared. Variation in lubricant film thickness results in variation in coefficient of friction. Thus, friction force microscopy can be used to measure lubricant uniformity with spatial resolution on the order of tens of nm. Finally, adhesive experiments show that bonded film behaves as a soft polymeric solid.
Ferroelectric electron emission flat panel display
Authors: Rosenman G. , Shur D. , Skliar A.
Published: July 2002

Ferroelectric electron emission observed under periodic spontaneous polarization switching is the basis for the development of a new type of flat panel display. Electron emission occurs from individually operated ferroelectric domains. It is shown that ferroelectric displays may be fabricated in ‘‘plane-to-plane’’ geometry which is not possible for field emission displays.
Electrical transport properties of epitaxial BaTiO3 thin films
Authors: Gilbert S. R. , Wills L. A. , Wessels B. W. , Schindler J. L. , Thomas J. A. , Kannewurf C. R.
Published: July 2002

Measurements of the temperature dependent transport properties of epitaxial BaTiO3 are reported. Electrical resistivity and thermoelectric power were measured over the temperature range of 77–300 K. Room temperature resistivities of the as-deposited, undoped films range from 105 to 108 Ω cm, while values as low as 55 Ω cm are obtained for the La-doped films. The resistivity shows an activated temperature dependence with the measured activation energies ranging between 0.11 and 0.50 eV. The activation energy depends strongly upon the thin film carrier concentration. Thermoelectric power measurements indicate that the films are n-type. The Seebeck coefficient for La-doped BaTiO3 exhibits metallike behavior, with its magnitude directly proportional to temperature. Temperature dependent resistivity and thermopower measurements indicate that the carrier mobility is activated. A transport model is proposed whereby conduction occurs in the La-doped films via hopping between localized states within a pseudogap formed between a lower Hubbard band and the BaTiO3 conduction band edge.
The electromagneto-acoustic surface wave in a piezoelectric medium: The Bleustein–Gulyaev mode
Authors: Li Shaofan
Published: July 2002

By discarding the quasi-static approximation, this paper gives the exact solutions of a shear horizontal electromagneto-acoustic surface wave mode in a class of piezoelectric media. As the wave speed is much less than the speed of light, the solution degenerates to the well-known Bleustein–Gulyaev wave, or Maerfeld–Tournois wave. Taking into account both optical effect as well as the contribution from the rotational part of electric field, the solutions obtained here are not only valid for any wave speed range, but also provide accurate formulas to evaluate the acousto-optic interaction due to the piezoelectricity.
Effect of scan size and surface roughness on microscale friction measurements
Authors: Koinkar Vilas N. , Bhushan Bharat
Published: July 2002

The effect of scan size (scan length) and surface roughness on microscale friction was studied using atomic force/friction force microscopy. Three silicon specimens with different surface roughnesses were studied. Surface height and friction force plots were obtained simultaneously and friction mechanisms for the correlation between them were sought. The standard deviation of surface heights increases with the scan size initially and approaches a constant value at a scan length greater than the long wavelength limit of the roughness structure. Change in the value of the coefficient of friction at different scan lengths is random. In these measurements, the sampling interval is always lower than the correlation length which is believed to be responsible for the random variation in microscale friction with the scan size. The good correlation observed between the local change in friction force and the surface slope can be explained by a ratchet mechanism.
Artificial uniaxial and biaxial dielectrics with the use of photoinduced gratings
Authors: Yang Changxi , Yeh Pochi
Published: July 2002

We investigate the form birefringence due to photoinduced one- and two-dimensional volume index gratings in DuPont photopolymers. The effective medium theory has been applied to the continuous volume subwavelength gratings. With the presence of a one-dimensional volume subwavelength grating, the originally isotropic photopolymer exhibits a negative form birefringence and behaves like a uniaxial dielectric. A form birefringence of −1.2×10−3 at λ=632.8 nm is measured at a grating period of Λ=447 nm. Experimental results on the dispersion of the form birefringence versus the illumination wavelength and the grating period are presented. With the presence of a two-dimensional grating, in general, the originally isotropic photopolymer exhibits a biaxial form birefringence and behaves like a biaxial dielectric. Positive form birefringence can also be obtained by recording two volume gratings with different grating wave vectors. By recording two photoinduced volume gratings we can synthesize biaxial dielectrics with the principal axes along any desired directions. We also report the measured principal refractive indices of the medium due to the presence of two-dimensional volume gratings in DuPont photopolymer.
Stripe domains morphology versus layers thickness in CoPt multilayers
Authors: Belliard L. , Miltat J. , Kottler V. , Mathet V. , Chappert C. , Valet T.
Published: July 2002

The domain morphology in CoPt multilayers has been investigated by means of magnetic force microscopy for a broad range of thickness parameters. Following hard-axis saturation, complex closure domains, isolated reversed domains, and segmented and fully developed stripe patterns are clearly identified down to stripe widths of the order of 50 nm. Pending full numerical simulations, the stripe width behavior versus Co and Pt thicknesses has been confronted to the predictions of a purely magnetostatic model. The data are consistent with the existence of a single and physically realistic characteristic length, which, however, is viewed as mostly casual. Likely reasons are discussed.
Magnetic anomalies in NiO nanoparticles
Authors: Makhlouf Salah A. , Parker F. T. , Spada F. E. , Berkowitz A. E.
Published: July 2002

As first noted by Néel, antiferromagnetic nanoparticles could exhibit superparamagnetic relaxation of their spin lattices as well as permanent moments arising from uncompensated surface spins. Several samples of antiferromagnetic NiO nanoparticles with average sizes ranging from 50 to >800 Å were investigated in the present study. In addition to the inverse dependence on average particle size of the susceptibility predicted by Néel, and previously reported, some unusual behavior was observed. Above the blocking temperatures (TB) of the particles, the reversible magnetization could not be fit with a Langevin function that was consistent with the physically reasonable moment representing the uncompensated spins. For the 53 Å diameter particles, both zero-field-cooled (ZFC) and field-cooled (FC) loops below TB exhibit large coercive forces (several kOe) and the loops showed irreversibility up to 50 kOe. In addition, in the FC state below TB the hysteresis loops were strongly shifted. The latter behavior may be due to exchange coupling of the uncompensated spins with the antiferromagnetic core. However, the large coercive forces and high field irreversibility which accompany the shifted loops raise new questions about the nature of the uncompensated spins. The magnetization relaxation of these particles is also discussed.
Magnetic properties of the mineral, cylindrite (FePb3Sn4Sb2S14)
Authors: Salyer P. A. , ter Haar L. W.
Published: July 2002

Our interest in physical property characterization and possible applications of novel solid state structures has directed our research to the area of mineral sulfides and sulfosalts. The current focus is on the mineral cylindrite (FePb3Sn4Sb2S14), an incommensurate lattice material which uniquely forms cylinders up to several millimeters in diameter and length. The composition and complicated structure of cylindrite have been characterized in some detail separately, however the relationships between its structure, composition, and physical properties have not yet been investigated. This work describes the ferromagnetic-like behavior we have observed in cylindrite as well as possible magnetostructural correlations. Magnetic phase transitions near 8 and 23 K can be related to cylinder size and therefore to the distribution of Fe (II) ions in cylindrite’s lattice through a magnetic model that calls upon ferrimagnetism and spin glass behavior.
Theory of the acoustic signature of topological and morphological defects in SiC/porous SiC laminated ceramics
Authors: Esquivel-Sirvent R. , Noguez Cecilia
Published: July 2002

The sensitivity of acoustic techniques to topological and/or morphological variations in SiC/porous SiC laminated ceramics is studied within the transfer matrix formalism. To implement this theoretical formalism, the mechanical properties of the SiC porous layers are calculated using the effective medium approximation of Kuster and Toksöz [Geophys. 39, 587 (1974)]. We show that topological defects have a stronger acoustic signature than morphological defects. Also, we observe that there are particular frequencies at which the defects do not present any acoustic signature.
Self-aggregated InAs quantum dots in GaAs
Authors: Patanè A. , Grassi Alessi M. , Intonti F. , Polimeni A. , Capizzi M. , Martelli F. , Nasi L. , Lazzarini L. , Salviati G. , Bosacchi A. , Franchi S.
Published: July 2002

We present a detailed study, both structural and optical, of GaAs/InAs/GaAs heterostructures for InAs nominal coverages (L) ranging from 0.6 to 3 ML. Planar transmission electron microscopy (TEM) provides direct evidence of the presence of InAs quantum dots (QDs) for all values of L, with an increase in their density at high values of L. Transverse TEM shows also that those QDs have mostly small base angles. Accordingly, the evolution of the optical properties of InAs/GaAs is investigated by photoluminescence (PL) and PL excitation measurements (PLE). A broad PL band is observed in all samples, which is ascribed to the recombination of heavy-hole excitons in the InAs quantum dots, observed with TEM. For thin coverages (L⩽1.6ML), a narrow PL band is also observed, which is attributed to recombination of heavy-hole excitons in a two-dimensional (2D) InAs layer. The two bands shift to lower energy for increasing L. For L⩾1.6 ML, the QD band has a faster shift and exhibits a complex structure, while the exciton recombination in the 2D-InAs layer vanishes. Those features, as well as the PLE results, indicate that: (a) quantum dots are connected by a two-dimensional InAs layer, at least for thin InAs coverages, which allows an efficient carrier capture into the dots; (b) the dot size increases with L, the increase being faster for L⩾1.6 ML because above this thickness the growth becomes completely three dimensional. A simple model explains the PL data and results in a dot geometry in agreement with the TEM measurements.
Picotesla field sensor design using spin-dependent tunneling devices
Authors: Tondra Mark , Daughton James M. , Wang Dexin , Beech Russell S. , Fink Anita , Taylor John A.
Published: July 2002

Pinned spin-dependent tunneling devices were fabricated and tested in a mode suited for low-field sensing. The basic structure of the devices was NiFeCo125/Al2O325/CoFe70/Ru9/CoFe70/ FeMn125 (in Å). This structure had a tunneling resistivity of 110 MΩ μm2 and exhibited a 20% magnetoresistance when a field was swept along the easy direction of the soft electrode. High sensitivity, low hysteresis operation was achieved by applying a bias field orthogonal to the easy axis. A sensitivity of 3%/Oe with negligible hysteresis was observed using this mode of operation. A sensor using this type of material was designed to achieve a minimum resolvable field in the picotesla range. The sensor consists of a bridge with four elements, each having 16 tunnel junctions in series. A signal-to-noise ratio of 1:1 at 1 pT (10−8 Oe) is possible assuming achievable values for the tunneling resistivity, device size, bias level, and sensitivity.
Comparison of fluorescence-based temperature sensor schemes: Theoretical analysis and experimental validation
Authors: Collins S. F. , Baxter G. W. , Wade S. A. , Sun T. , Grattan K. T. V. , Zhang Z. Y. , Palmer A. W.
Published: July 2002

The performance of the two most promising fluorescence-based temperature sensing techniques, namely the fluorescence intensity ratio (FIR) and fluorescence lifetime (FL) schemes, have been compared. Theoretical calibration graphs for the two methods illustrate the useful monotonic change of the response with temperature variation. Comparison of the responses and the sensitivities of the two schemes show that at very low temperatures the FIR method exhibits a significant variation with temperature, while the response of the FL method becomes constant with its sensitivity approaching zero. With increasing temperature, the FIR and the FL methods (with short relaxation times and shorter intrinsic lifetimes of the upper energy levels) share a similar sensitivity over a wide temperature range. The presence of a long relaxation time or a longer intrinsic lifetime of the upper level in the use of the FL method gives a less satisfactory response. Experimental data obtained for a range of dopant ions in various host materials are found to be consistent with the theoretical expectation, with each material having a specific energy gap difference. The sensitivities of each material are compared graphically which would allow the most appropriate sensor for an intended application to be selected.
Diode-pumped continuous-wave, quasi-continuous-wave, and Q-switched laser operation of Yb3+,Tm3+: YLiF4 at 1.5 and 2.3 μm
Authors: Diening A. , Möbert P. E.-A. , Huber G.
Published: July 2002

We report continuous-wave (cw), quasi-cw and Q-switched laser oscillation of Yb3+,Tm3+-doped YLiF4 at 1.5 and 2.3 μm. Maximum cw output powers of 300 mW at 1.5 μm and 450 mW at 2.3 μm have been obtained. Quasi-cw excitation revealed 20 mJ pulses at 1.5 μm. The cw pumped 1.5 μm laser was passively Q-switched using a U:CaF2 crystal as saturable absorber with a repetition rate of 65 kHz and a pulsewidth of 800 ns. An average output power of 19 mW was achieved. The dynamics of the system was simulated using a rate equation model.
Electrodeposition of patterned magnetic nanostructures
Authors: Duvail J. L. , Dubois S. , Piraux L. , Vaurès A. , Fert A. , Adam D. , Champagne M. , Rousseaux F. , Decanini D.
Published: July 2002

We report on fabrication and characterization of two types of devices, both with submicronic dimensions, and fabricated by combining lithography and electrodeposition. The first device, obtained by combining electron-beam lithography and electrodeposition, was devised to measure the current perpendicular to the plane giant magnetoresistance (CPP-GMR) of a single permalloy/copper multilayered nanopillar (height ∼0.3 μm, diameter ∼0.1 μm). Besides the fundamental interest of the spin-dependent transport properties in such nanoscaled magnets, this system is a potential candidate as a CPP-GMR sensor used, for example, to read very high-density magnetic storage. The second device, relevant for high-density storage media, consists in large areas (4×4 mm2) of magnetic permalloy dots (diameter ∼0.26 μm, period ∼0.4 μm) electrodeposited in a x-ray patterned photoresist matrix. We study the magnetic behavior of such mesoscopic pillars as a function of their height. We emphasize that our processes are less damaging for the nanostructures, in comparison with samples prepared by high vacuum deposition followed by lithography. This is because our magnetic nanostructures are electrodeposited after the whole lithographic process.
Fe-based soft magnetic amorphous alloys with a wide supercooled liquid region
Authors: Koshiba H. , Inoue A. , Makino A.
Published: July 2002

Amorphous alloys with a wide supercooled liquid region (ΔTx) before crystallization were formed in the Fe56Co7Ni7Hf8M2B20 (M=Nb, Ta) alloys by melt spinning. The ΔTx which is a critical factor of glass-forming ability, increases to large values exceeding 90 from 82 K by the addition of 2 at. % M, and the largest ΔTx value is larger by about 10 K than the largest value for recently reported (Fe,Co,Ni)–Zr–M–B alloys. By the addition of the M metals, magnetic properties also changed. Good soft magnetic properties were obtained for the alloys annealed for 300 s at 800 K. The saturation magnetization (Is), coercive force (Hc), and effective permeability (μe) at 1 kHz are, respectively, 0.71 T, 1.0 A/m, and 16 700 for the M=Nb alloy and 0.76 T, 1.0 A/m, and 15 600 for the M=Ta alloy. The finding of the Fe-based amorphous alloys exhibiting simultaneously the wide supercooled liquid region before crystallization and the good soft magnetic properties seem to enable the future development of a new ferromagnetic bulk amorphous alloy.
The growth of nanoscale structured iron films by glancing angle deposition
Authors: Liu F. , Umlor M. T. , Shen L. , Weston J. , Eads W. , Barnard J. A. , Mankey G. J.
Published: July 2002

Chiral nanostructured thin films can be produced through precise control of the angle of incidence of a vapor flux concurrent with substrate rotation. The technique has been employed to create unique porous iron thin film structures on Si(100) with a columnar microstructure. Scanning electron microscopy images illustrate columnar iron films produced with azimuthal rotation during sample growth with the incident flux at an angle of 75° with respect to the surface normal. The columns were found to be well isolated with a narrow distribution of diameters, resulting in aspect ratios of approximately 8 to 1. Hysteresis loops reveal the columnar growth induced a large magnetic shape anisotropy relative to that observed for an iron film grown with normal incidence. The evolution of the columnar microstructure was followed from simple oblique deposition (no substrate rotation), giving a fibrous slanted microstructure, to high-speed rotation where a broad size distribution of highly faceted columnar structures was observed. The measured microstructure is related to the observed magnetic properties.
The influence of impurities within the barrier on tunneling magnetoresistance
Authors: Tsymbal E. Yu. , Pettifor D. G.
Published: July 2002

Tunneling magnetoresistance (TMR) of a ferromagnetic tunnel junction is calculated in the presence of nonmagnetic metal impurities and disorder in the barrier layer within a single-band tight-binding model. We found that the introduction of impurities which have onsite atomic energies close to the Fermi level leads to a rapid drop in TMR as a function of the impurity concentration. This results from the increasing number of localized electronic levels close to the Fermi energy which promote the formation of highly conducting channels within the insulator. With increasing atomic energy of the impurities, tunneling via the impurity band results in higher values of TMR due to a higher effective potential barrier associated with this mechanism of tunneling. In this case introduction of impurities within the barrier can provide enhanced values of conductance without a significant reduction in TMR, which is required for MRAM applications of tunnel junctions.
Permanent magnet bearings for horizontal- and vertical-shaft machines: A comparative study
Authors: Ohji T. , Mukhopadhyay S. C. , Iwahara M. , Yamada S.
Published: July 2002

Two types of magnetic bearing systems employing permanent magnets to be used for vertical-shaft and horizontal-shaft machines, respectively, have been designed and fabricated in our laboratory. In this article we report a comparative evaluation of (i) the permanent magnet configuration and its effect on radial disturbance attenuation, (ii) magnetic losses and their effect on energy storage, and (iii) the off-state position of the rotor magnet in two types of bearing systems. Experimental results are presented.
Characteristics of impact ionization rates in direct and indirect gap semiconductors
Authors: Harrison D. , Abram R. A. , Brand S.
Published: July 2002

Impact ionization rates for electrons and holes in three semiconductors with particular band structure characteristics are examined to determine underlying factors influencing their qualitative behavior. The applicability of the constant matrix element approximation is investigated, and found to be good for the indirect gap material studied, but overestimates threshold softness in the direct gap materials. The effect that final states in the Γ valley have in influencing characteristics of the rate in the direct gap materials is investigated, and it is found that they play a significantly greater role than the low density of Γ valley states would suggest. The role of threshold anisotropy in affecting threshold softness is examined, and it is concluded that it plays only a small part, and that softness is controlled mainly by the slow increase in available phase space as the threshold energy is exceeded.
X-ray diffraction and Rutherford backscattering spectrometry of Ba1NbxTi1−xO3 thin films synthesized by laser ablation
Authors: Nasir Khan M. , Kim Hyun-Tak , Kusawake T. , Kudo H. , Ohshima K. , Uwe H.
Published: July 2002

Niobium doped barium titanate Ba1NbxTi1−xO3 thin films have been successfully synthesized by laser ablation on sapphire (000̄1) and (100) SrTiO3 substrates, for the growth of hexagonal and cubic phases of this system. These films were synthesized with different concentrations of niobium under various conditions. X-ray diffraction measurements showed that the films were well aligned along the c-axis of the sapphire substrate for the hexagonal phase, while perpendicular to the SrTiO3 substrate surface for the cubic phase of this system. These films were deposited at substrate temperatures of 700 and 650 °C for the hexagonal and cubic phases, respectively, both in oxygen and nitrogen atmospheres. At room temperature these films have a very high resistance and large negative thermopower ≃300 mV/K. Rutherford backscattering spectrometry (RBS) was performed on very thin films of this system for different concentrations of niobium deposited in the N2 atmosphere. We have found a deficiency of Ti in these films by RBS analysis, which along with other antisite defects and oxygen deficiency, may be a cause of high resistance and large thermopower in these films.
Photoluminescence decay time measurements from self-organized InAs/GaAs quantum dots
Authors: Buckle P. D. , Dawson P. , Hall S. A. , Chen X. , Steer M. J. , Mowbray D. J. , Skolnick M. S. , Hopkinson M.
Published: July 2002

In this article we report the results of time integrated and time resolved photoluminescence spectroscopy and photoluminescence time decay measurements as a function of excitation density at 6 K on high quality self-organized InAs/GaAs quantum dots. To understand the form of the experimentally observed photoluminescence transients a Monte Carlo model has been developed that allows for the effects of random capture of photo-excited carriers. By comparison with the results of our model we are able to ascribe the excitation density dependence of the overall form of the decay of the emission from the quantum dot ground states and the biexponential nature of the decay of the first excited state emission as being due to the combined effects of radiative recombination, density dependent carrier scattering, and the restriction of carrier scattering due to state blocking caused by the effects of Pauli exclusion. To successfully model the form of the biexponential decay of the highest energy excited states we have to invoke the nonsequential scattering of carriers between the quantum dot states.
Numerical and experimental study on the thermal damage of thin Cr films induced by excimer laser irradiation
Authors: Lee S. K. , Chang W. S. , Na S. J.
Published: July 2002

Single-shot laser damage of thin Cr films on glass substrates has been studied to understand the cracking and peeling-off mechanism. A mathematical model is developed for the calculation of transient heat transfer and thermal stresses in Cr films during excimer laser irradiation and cooling, the transient temperature, and the stress–strain fields are analyzed by using a three-dimensional finite-element model of heat flow. The finite-element program ABAQUS, with user subroutines, is adopted to perform the numerical analysis. A KrF excimer laser is used in experiments as a source of UV radiation. Morphological inspection of damaged Cr films is carried out by using scanning electron microscopy and the threshold fluences for visible damages are investigated for various film thicknesses. According to the numerical analysis for the experimentally determined cracking and peeling-off conditions, cracking is found to be the result of the tensile brittle fracture due to the excessive thermal stresses formed during the cooling process, while peeling off is found to be the combined result of films bulging from the softened glass surface at higher temperature and the tensile brittle fracture during the cooling process.
Preparation and thermoelectric properties of some phosphide skutterudite compounds
Authors: Watcharapasorn A. , DeMattei R. C. , Feigelson R. S. , Caillat T. , Borshchevsky A. , Snyder G. J. , Fleurial J.-P.
Published: July 2002

Thermoelectric properties of CoP3 and CeFe4P12 have been measured. These compounds were synthesized by a flux technique using Sn as the solvent. The samples were characterized by x-ray diffractometry and electron microprobe analyses. The Seebeck coefficient, the electrical resistivity, the Hall effect, and the thermal conductivity were measured over a wide range of temperatures. The results indicate that CoP3 and CeFe4P12 are semiconductors, in agreement with theoretical predictions. The thermal conductivity of CeFe4P12 is about 10 times larger than that for CeFe4Sb12 which is primarily due to both reduced motion of the Ce ions in smaller voids and lower hole–phonon scattering. The results are analyzed and discussed to provide guidelines for optimization of the thermoelectric properties of these materials.
K
2
Te
photocathode growth: A photoemission study
Authors: Bisero Diego , di Bona Alessandro , Paradisi Paola , Valeri Sergio
Published: July 2002

X-ray photoemission spectroscopy (XPS) has been used to investigate the growth process of K–Te thin layers with high quantum efficiency (QE) for photoemission in the UV range, prepared by sequential deposition of Te and K on a Mo substrate under ultrahigh vacuum conditions. Film formation occurs through different steps, characterized by increasing QE up to a saturation value which correspond to K2Te stoichiometry. By quantitative, angle resolved XPS measurements, surface segregation of one monolayer of K has been detected. Films prepared starting from different amounts of Te exhibit the same K2Te stoichiometry. However, the QE values are significantly different. Charge transfer from K to Te has been detected during K deposition and monitored by the shift of the Te 3d5/2 photoemission line. At the end of the K evaporation a shift of both Te 3d and K 2p photoemission lines has been observed, due to a band bending mechanism. The degradation by oxygen exposure of the quantum efficiency at 254 nm of K2Te has also been studied and compared with that of Cs2Te, which is currently used as photocathode material in UV-laser driven photoinjectors. It has been shown that K2Te is more rugged than Cs2Te. QE degradation is associated to the formation of TeO2 on the K2Te surface.
Photoluminescence of silicon nanoclusters with reduced size dispersion produced by laser ablation
Authors: Patrone L. , Nelson D. , Safarov V. I. , Sentis M. , Marine W. , Giorgio S.
Published: July 2002

We report a photoluminescence study of silicon nanoclusters produced by laser ablation. It was found that by varying the preparation parameters it was possible to change the mean cluster size in the range 1–5 nm. Within this size variation, the photoluminescence band shifts in a wide spectral region from near ultraviolet to near infrared. This size-dependent photoluminescence of Si nanoclusters is consistent with a quantum confinement effect. The observed influence of cluster oxidation on the luminescence properties also supports the quantum confinement interpretation. We proposed a discrete size model which supposes that the spectral position of the luminescence band is essentially determined by the volume of clusters with a complete outer atomic layer. In the framework of this model, we were able to deconvolute the observed luminescence bands into a set of fixed Gaussian bands. The model is supported by the observation of a size selective doping of Si nanoclusters whose effect was well explained by Auger recombination. Finally, our model allowed us to obtain a dependence of the optical gap on the cluster size which is in good agreement with existing calculations of Si nanocrystal electronic structure.
Physically trapped oxygen in sputter-deposited MoO3 films
Authors: Vink T. J. , Verbeek R. G. F. A. , Snijders J. H. M. , Tamminga Y.
Published: July 2002

In the present study, we show that the stoichiometry of sputtered molybdenum oxide films can be changed from oxygen deficient to, more surprisingly, oxygen rich. O/Mo ratios as high as 3.2 were measured, depending on the oxygen partial pressure in the sputtering atmosphere. It is shown that a substantial part of the excess oxygen is physically trapped in the film while the remaining part is chemically bonded in the form of H2O groups. The presence of physically trapped oxygen is demonstrated through a hydration reaction of the film resulting in the controlled detrapping of both excess oxygen and argon. The quantities of released oxygen are electrochemically measured and compared with the elemental composition of the film as determined by means of Rutherford backscattering spectroscopy (Mo, O, and Ar) and elastic recoil detection (H).
Giant magnetostrictive thin films for applications in microelectromechanical systems (invited)
Authors: Ludwig Alfred , Quandt Eckhard
Published: July 2002

Sputter-deposited giant magnetostrictive thin films allow the realization of microactuators and sensors which can be addressed by remote control operation. Applications reviewed in this article are in the field of microelectromechanical systems (MEMS), e.g., microfluidic devices, micromotors, laser scanner mirrors etc. In general, all these MEMS applications of magnetostrictive thin films require a well defined uniaxial in-plane anisotropy and are operated at or above room temperature. In previous investigations it was found that (Tb40Fe60/Fe50Co50) multilayers represent the most promising thin film material with respect to their unique combination of soft magnetic and giant magnetostrictive properties. Consequently, this material was used to show the possibility to control the orientation of the magnetic easy axis by magnetic field annealing. Furthermore data about the temperature dependence of these magnetostrictive films are given.
Using the flowing afterglow of a plasma to inactivate Bacillus subtilis spores: Influence of the operating conditions
Authors: Moreau S. , Moisan M. , Tabrizian M. , Barbeau J. , Pelletier J. , Ricard A. , Yahia L’H.
Published: July 2002

The flowing afterglow of a microwave discharge can be used to efficiently inactivate bacterial spores. We have conducted a parametric study of the operating conditions of such a system, which shows that the species participating in the killing of spores are oxygen atoms and ultraviolet (UV) photons. The oxygen atoms and the excited atoms and molecules emitting the photons being carried by the flowing afterglow can be made available throughout the sterilization chamber. Typical operating conditions are: gas mixture 2%O2/98%N2, pressure range 1–7 Torr and gas flow 0.5–3 slm. Total inactivation of 106 B. subtilis spores is achieved within 40 min with 100 W absorbed microwave power, at afterglow gas temperatures not exceeding 50 °C, a feature of interest for heat sensitive medical devices. The present scheme depends on the gas flow reaching all parts of the objects to be sterilized and on the short-lived active species being transported there sufficiently rapid. Under our operating conditions, it is the UV emission intensity that sets the sterilization time as there are always more than sufficient oxygen atoms available for the process.
Hybrid electronic properties between the molecular and solid state limits: Lead sulfide and silver halide crystallites
Authors: Rossetti R. , Hull R. , Gibson J. M. , Brus L. E.
Published: July 2002

Tiny single PbS crystals of ∼25 Å diameter are synthesized and studied optically in low-temperature colloidal solutions. Electron microscopic examination shows a simple cubic rock salt structure with a lattice constant unchanged, within experimental error, from the bulk value. These crystallites lack the near infrared electronic absorption characteristic of bulk PbS. The small crystallite absorbance in the visible rises more steeply than does the bulk absorbance. These results reflect electron and hole localization if one considers the variation in effective mass across the band structure. A simple discussion of localization anywhere in the Brillouin zone is given. For the first time, crystallite syntheses are carried out in solvent mixtures that form transparent glasses upon cooling. The PbS spectra are independent of temperature (at current experimental resolution) down to 130 K, in contrast to earlier results for quantum size exciton peaks in ∼20 Å ZnS crystallites. Previously published observations of size dependence in the excited state electronic properties of AgI and AgBr are explained as consequences of electron and hole localization in the small crystallites. AgBr appears to be the first indirect gap semiconductor to be examined in the regime where bulk properties are not fully formed.
The observation of CH3O in the collision free multiphoton dissociation of CH3NO2
Authors: Wodtke Alec M. , Hintsa Eric J. , Lee Yuan T.
Published: July 2002

A continuous, seeded, molecular beam of nitromethane was passed through a heated (580 °K) nozzle, thus eliminating cluster formation. The CH3NO2 was dissociated by IR radiation from a CO2 laser. The time of flight spectrum was observed at masses 46, 30, 29, and 15. (AIP)
Molecular beam photoelectron spectroscopy and femtosecond intramolecular dynamics of H2O+ and D2O+
Authors: Reutt J. E. , Wang L. S. , Lee Y. T. , Shirley D. A.
Published: July 2002

The 584 Å photoelectron spectra of supersonic molecular beams of H2O and D2O have been obtained with improved resolution. The spectroscopic constants of the X̃ 2B1 and Ã 2A1 state ions, including ω01, x011, ω02, x022, and x012, are reported. For the first two electronic states of the ion, precise line splittings were evaluated with a least squares fitting procedure, employing sums of empirical instrument response functions and a linear background. A simulation of the vibrational manifolds of the B̃ 2B2 state ions with combination progressions in the symmetry-allowed modes ν1 and ν2 failed to reproduce the diffuse photoelectron bands observed for both H2O and D2O. Autocorrelation functions were calculated from the photoelectron bands of all three electronic states. The B̃ 2B2 state correlation functions exhibit ultrafast decay, occurring on a 10−14 s time scale. The ν2 motion appears to define the decay in the correlation function. This behavior supports a previously proposed B̃ 2B2–Ã 2A1 curve-crossing model for the nonradiative relaxation of the B̃ 2B2 state ions.
Effects of time delay in rate processes
Authors: Schell Mark , Ross John
Published: July 2002

Time delay processes in chemical kinetics occur naturally and can be imposed by means of a delayed feedback. We study the temporal evolution of homogeneous chemical reaction mechanisms (the autonomous system) describable by ordinary differential equations subjected to an imposed time delay, a delayed feedback. A natural delay process can be decomposed into analogous components. At short delays the delayed feedback is shown to stabilize different types of unstable stationary states of the autonomous systems, as well as induce bistability in a monostable system. At longer delays transitions into chaos are predicted. First we study the chaotic response of autonomous bistable systems which support only stationary states. A recipe for finding chaos is given. Chaos occurs for a small parameter range in which the delayed feedback repeatedly reinjects trajectories into the neighborhood of one of the marginally stable points of the autonomous system. Secondly, we consider the imposition of a delayed feedback to a system in which periodic oscillations occur and show this leads to both chaos and hyperchaos when memory effects in the delay cause the system to be reinjected into the neighborhood of a saddle-focus point. The examples studied in detail are thermo-illuminated chemical reactions, but we also consider the applicability of our results to reactions in continuously stirred tank reactors and biochemical processes. The results of this study show that natural time delays can regulate a wide range of dynamical behavior in biochemical processes.
Monte Carlo simulation of liquid–liquid benzene–water interface
Authors: Linse Per
Published: July 2002

A Monte Carlo simulation of liquid–liquid benzene–water interface is reported. Molecular pair potentials obtained from quantum mechanical calculations have been used. The two phases remain stable over 70 000 configurations/molecule and the interior of each phase is shown to have bulk properties. The intrinsic interface is molecularly sharp, but the interfacial region is broadened by capillary waves. The preferential water–benzene orientation at the interface is similar to that in a dilute aqueous solution of benzene. There is no longitudinal water ordering induced by the interface. The presence of the nonpolar phase gives an alignment of water dipoles parallel to the surface and a reduced probability of parallel orientation of adjacent dipoles. The hydrogen bonds between water neighbors are reinforced at the interface.
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o calculation of ν1 and ν3 for triplet methylene (X̃ 3B1 CH2) and the determination of the vibrationless singlet–triplet splitting T
e(ã 1A1)
Authors: McLean A. D. , Bunker P. R. , Escribano R. M. , Jensen Per
Published: July 2002

A b initio values of ν1, ν3 and the zero point energy for triplet methylene (X̃ 3B1 CH2) have been calculated. Complete second order configuration interaction calculations for the six valence electrons, with a Davidson-type unlinked cluster correction, were done at 37 selected nuclear geometries. We have fitted an analytic potential through these points and used the nonrigid bender Hamiltonian, and a variational procedure, to calculate the rotation–vibration energies. We have obtained ν1=2985±20 cm−1, ν3=3205±20 cm−1, and the zero point energy as 3710±20 cm−1. The uncertainties represent our best estimate of the full uncertainties in the calculations. From the experimental data for singlet methylene (ã 1A1) we have determined the zero point energy of that state to be 3620±20 cm−1. Combining these two zero point energies with the experimental value of T0(ã 1A1)=3156±5 cm−1 [Bunker et al. J. Chem. Phys. 85, 3724 (1986)] we obtain the vibrationless singlet–triplet splitting Te(ã 1A1) =3246±30 cm−1 (9.28±0.1 kcal/mol).
Ultraviolet photoelectron spectroscopy of copper clusters
Authors: Pettiette C. L. , Yang S. H. , Craycraft M. J. , Conceicao J. , Laaksonen R. T. , Cheshnovsky O. , Smalley R. E.
Published: July 2002

Using a new magnetically focused time-of-flight photoelectron spectrometer, the ultraviolet photoelectron spectra (UPS) of mass-selected negative copper clusters have been measured at photon energy of 4.66 eV for all clusters in the range from 6 through 41 copper atoms. These UPS data provide the first detailed probe of the 4s valence band structure of such medium size negative copper clusters, and extend previous approximate measures of the electron affinity of the corresponding neutral species. The results are in accord with the predictions of the ellipsoidal shell model for monovalent metal clusters. In particular, clusters 8, 20, and 40 (which correspond to spherical shell closings in this simple model) are found to have unusually low electron affinities and large HOMO–LUMO gaps. Subshell closings at 14 and 34 also appear special in this new UPS data.
Theory of microphase separation in block copolymer solutions
Authors: Olvera de la Cruz M.
Published: July 2002

Microphase separation in semidilute solutions of block copolymers in nonselective good solvent is analyzed. The swelling due to the presence of solvent is taken into account. The solvent effects on the thermodynamics in the disordered region and in the ordered region in the weak segregation limit are discussed. It is found that the concentration of block copolymer at the microphase separation transition scales as N−0.62 in good solvents and as N−1/2 in Φ solvents. In the asymptotic semidilute regime in good solvents, the coexistence curve at the microphase separation transition is negligible, and the transition scales as (χ′AB φ1.61N)t ∼constant, where the constant is determined by the relative composition of the blocks in the block copolymer and χAB is proportional to the usual Flory interaction parameter.
Equivalent combined systems of three-heat-source heat pumps
Authors: Chen Jincan , Yan Zijun
Published: July 2002

In this paper, the equivalent combined systems of three-heat-source heat pumps are studied. It is pointed out that there may be three combined ways for us to conceive a reversible three-heat-source heat pump as a combined system having a reversible Carnot heat pump driven by a reversible Carnot engine. Whereas for an endoreversible three-heat-source heat pump affected by the heat resistances, there is only one combined way for us to conceive it as a combined system having an endoreversible Carnot heat pump driven by an endoreversible Carnot engine. Then, the fundamental optimum formula of a three-heat-source heat pump is derived simply and directly by using those of two-heat-source cycles. The optimal peformance of an endoreversible three-heat-source heat pump is thus discussed. The inherent relation between the two- and the three-heat-source cycles as well as the fundamental distinction between the reversible and the endoreversible three-heat-source cycles is also revealed. These results may enrich the theory of finite time thermodynamics, and provide some new theoretical bases for the exploitation and application of three-heat-source apparatus such as chemical heat pumps, absorption heat pumps, and so on.
Generalized Mo/ller–Plesset perturbation theory: Second order results for two-configuration, open-shell excited singlet, and doublet wave functions
Authors: Wolinski Krzysztof , Pulay Peter
Published: July 2002

The recently proposed consistent generalization of the Mo/ller–Plesset perturbation theory to arbitrary reference states is reviewed and compared to previous methods. We have implemented the second order of this theory (GMP2) for systems which can be described by a two-configuration wave function. GMP2 is applied here to bond-breaking potential curves: HF, F2, CH3–H, and twisted ethylene, as well as to the geometry and dipole moment of ozone, to the electron affinity of the cyano radical, to the vertical ionization potentials of ethylene, and to the excited singlet states of formaldehyde. In general, the performance of GMP2 is very good and comparable to the two-configuration based doubles CI method. However, like all methods based on a single reference function, GMP2 is not applicable in nearly degenerate situations. We also discuss the continuity of potential surfaces in Monte Carlo self-consistent-field (MC-SCF) theory.
A theoretical study of multidimensional nuclear tunneling in malonaldehyde
Authors: Shida Norihiro , Barbara Paul F. , Almlöf Jan E.
Published: July 2002

Various aspects of the intramolecular proton transfer in malonaldehyde have been investigated theoretically within the reaction surface Hamiltonian framework, which was recently applied with a two-dimensional surface to this molecule by Carrington and Miller. The present calculation, which involves a three-dimensional reaction surface and a high level of ab initio accuracy, gives a tunneling splitting which is ∼50% smaller than experiment and a hydrogen/deuterium isotope effect that is within 40% of experiment with no adjustable parameter. The vibrational wave function has been analyzed to extract an effective curvilinear tunneling path on the hypersurface. The path calculations, and other analysis, clearly demonstrate the limitations of one-dimensional models for polyatomic tunneling systems like malonaldehyde. In addition, tunneling splittings have been calculated for excited vibrational states of malonaldehyde, leading to new insight into the multidimensional character of proton transfer.