bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural, electronic, and luminescence investigation of strain-relaxation induced electrical conductivity type conversion in GeSi/Si heterostructures
Authors: Grillot P. N. , Ringel S. A. , Michel J. , Fitzgerald E. A.
Published: July 2002

Strain-relaxed, compositionally graded Ge0.3Si0.7/Si heterostructures grown by ultrahigh vacuum chemical vapor deposition at 650 °C are shown to display a consistent change from p-type to n-type conductivity as a function of rapid thermal annealing (RTA) temperature in the range 700–850 °C. Cross-sectional transmission electron microscopy, spreading resistance, and electron beam induced current (EBIC) studies eliminate the dislocations themselves as a possible source of this type conversion, by demonstrating that the spatially invariant hole concentration of 2×1014 cm−3 is not correlated to the dislocation density, which decreases from ∼108 cm−2 in the graded region to 7×105 cm−2 in the 30% Ge cap. To identify the source of type conversion, a systematic investigation was performed on 650 °C as-grown and annealed samples with deep-level transient spectroscopy (DLTS), photoluminescence (PL) and capacitance–temperature (C–T) measurements. DLTS measurements on as-grown samples reveal a complex spectrum of deep and shallow hole traps, while C–T studies reveal a prominent temperature dependence of the zero bias capacitance, indicating that the p-type background conductivity is associated with a high degree of compensation. Post-growth RTA at T≥800 °C eliminates this compensation, and yields background n-type films, consistent with the background n-type conductivity that is always observed in graded films grown at T≳800 °C in the same reactor. This change in conductivity type is accompanied by a strong increase in EBIC signal strength and a significant simplification of DLTS and PL spectra. These results are discussed in terms of dislocation interaction within the graded layers which generates nonequilibrium concentrations of intrinsic point defects that form extended complexes at growth temperatures ≤800 °C. These complexes are associated with thermally unstable acceptor-like defect states in the energy range H(0.05)–H(0.30) that convert low growth temperature, relaxed, graded GeSi/Si films from background n type to background p type.
Group-VI impurity-related DX centers in In0.18 Ga0.82As0.28 P0.72
Authors: Kwon Ho Ki , Choe Byung-Doo , Kwon S. D. , Lim H.
Published: July 2002

Properties of deep levels in S-, Se-, and Te-doped In0.18Ga0.82As0.2P0.72 grown on GaAs0.61P0.39 substrates by liquid phase epitaxy are studied by deep level transient spectroscopy and thermally stimulated capacitance measurements. The donor-related deep levels are observed and their activation energies are found to be 0.26, 0.23, and 0.14 eV for S-, Se-, and Te-doped In0.18Ga0.82As0.28P0.72, respectively. Persistent photoconductivity is observed in all the samples doped with these impurities. It is clear from these results that the S, Se, and Te donors form DX centers in In0.18Ga0.82As0.28P0.72.
Relaxation of the space charge created by Fowler–Nordheim injections in metal–oxide–semiconductor capacitors
Authors: Sagnes B. , Moragues J. M. , Yckache K. , Jérisian R. , Oualid J. , Vuillaume D.
Published: July 2002

A procedure to identify the different components of the oxide space charge created during a Fowler–Nordheim electron injection in metal–oxide–semiconductor capacitors is presented. This procedure is very simple and based on the study of the space-charge relaxation behavior depending on the conditions of polarization and temperature. A model, based on slow states, trapped holes, and fixed charges, is proposed to explain the reversible and irreversible behaviors observed. Their effective numbers per unit area can be evaluated. This procedure is used to compare the influence of different processes on the oxide resistance to electron injections.
Photoluminescence measurement of the facet temperature of 1 W gain-guided AlGaAs/GaAs laser diodes
Authors: Rommel J. M. , Gavrilovic P. , Dabkowski F. P.
Published: July 2002

The output facet temperature of high-power AlGaAs/GaAs single quantum well (SQW) laser diodes was measured during operation. The front output facets were passivated with Al2O3 coatings. The spectral shift of photoluminescence from the cladding layers was used to determine the temperature rise at the front facet with increasing output power. The spatial resolution of the technique allowed to look at each cladding layer individually and to study the correlation between the near-field pattern and the temperature profile along the active layer. The local temperature on the facet at 1 W total optical power (corresponding to an average linear power density of 10 mW/μm) was found to vary between 25 and 45 K above the average active layer temperature and to exceed the heat-sink temperature by up to 70 K. This represents a significant reduction of facet temperature in comparison to earlier reports and can be attributed to high-quality passivation coatings.
Field-assisted capture of electrons in semi-insulating GaAs
Authors: Cola A. , Reggiani L. , Vasanelli L.
Published: July 2002

We present a drift-diffusion modeling of the electric-field profile in semi-insulating n-GaAs detectors accounting for hot-carrier dynamics and the associated kinetics of electrical active traps. From the fitting of the detector active thickness we infer unambiguous evidence of a field-enhanced capture cross section from the two deepest electron traps we attribute to EL2 and EL3 centers.
Limitations of the process window for the bias enhanced nucleation of heteroepitaxial diamond films on silicon in the time domain
Authors: Schreck M. , Thürer K.-H. , Stritzker B.
Published: July 2002

The parameter space for the heteroepitaxial nucleation of diamond on Si(001) using the bias process was studied by x-ray diffraction texture measurements. It was found that heteroepitaxial orientation can be achieved over a wide range of different parameters provided that the bias time is within a definite time interval. It was observed that twidth, the width of the time window, and topt , the bias time for optimal azimuthal alignment, strongly decrease with the absolute value of the bias voltage. For high bias voltages an extremely low value of topt (20 s at −300 V) was found. Applying the bias conditions longer than topt resulted in a strong decrease of the pole density maxima of the heteroepitaxial grains accompanied by a significant broadening of their azimuthal distribution that is interpreted in terms of two different routes for the loss of epitaxy. The different time constants characterizing the process window for a fixed bias voltage are traced back to feedback of the growing film on the plasma and on the electrical field distribution above the substrate.
Orientational memory site in hydrogenation disproportionation desorption recombination process of anisotropic Nd2Fe14B-based magnets
Authors: Tomida T. , Sano N. , Uehara M.
Published: July 2002

To clarify the orientational memory site in the hydrogen disproportionation desorption recombination process of Nd2Fe14B-based anisotropic magnets, high resolution transmission electron microscopy (HRTEM) characterization of hydrogen disproportionated structure of Nd13.0Fe67.9Co11.0Ga1.0Zr0.1B7.0 has been performed. In particular, crystallographic orientations of disproportionated products relative to that of parent or original Nd2Fe14B have been carefully analyzed using partially disproportionated samples. No apparent orientational coherency exists between the disproportionated products of α-Fe, Fe2B, and NdH2 and the parent Nd2Fe14B. However, nanoscale Nd2Fe14B particles of 10 to 100 nm in diameter have been detected by HRTEM to be densely present within the disproportionated mixture. Lattice fringe observation has also revealed that the crystallographic axes of the Nd2Fe14B particles are nearly parallel to those of the original Nd2Fe14B. It is thus suggested that these Nd2Fe14B particles be the orientational memory site upon the recombination, which recovers the original orientation leading to the anisotropy formation.
Anomalous behaviour of leaky surface waves for stiffening layer near cutoff
Authors: Zinin P. , Lefeuvre O. , Briggs G. A. D. , Zeller B. D. , Cawley P. , Kinloch A. J. , Thompson G. E.
Published: July 2002

The propagation of surface acoustic waves has been investigated for a layered system (water-isotropic layer-isotropic substrate) where the layer is faster than the substrate. It has been found that when the velocity of the surface wave reaches the velocity of the shear wave of the substrate (cutoff) the behaviour of the attenuation has an anomalous character. Attenuation drops down towards zero below cutoff and increases abruptly beyond cutoff. Acoustic microscopy measurements on aluminum coated with oxalic oxide film were in coincidence with theory.
Modifying diffusion anisotropies: Cap layer induced changes in spreading anisotropies
Authors: Reuter D. , Gerth G. , Kirschner J.
Published: July 2002

We have studied the spreading of Fe, Co, and Ni from a dot deposited onto a W(110) substrate. A Cu cap layer covering dot and surface is found to change the fast spreading directions for all three metals to 〈100〉. This is in contrast to the spreading without a cap layer where the fast spreading direction is either along 〈110〉 in the case of Fe or influenced by the step directions on the substrate surface for Co and Ni. The cap layer induced spreading anisotropy is proposed to be caused by the structure in the alloy layer formed during the spreading process.
Angular spectroscopic analysis: An optical characterization technique for laterally oxidized AlGaAs layers
Authors: Heremans P. , Kuijk M. , Windisch R. , Vanderhaegen J. , De Neve H. , Vounckx R. , Borghs G.
Published: July 2002

We present an optical characterization technique to determine both the refractive index and the shrinkage of laterally oxidized AlAs and AlGaAs layers. The technique consists of measuring the angular dependence of the Fabry-Pérot dip wavelength in a simple cavity structure. Over standard ellipsometry, it has the advantage of measuring more realistic layer structures. Over transmission electron microscopy cross sections to determine the final aluminum-oxide layer thickness, it has the benefit of performing the measurement without elaborate sample preparation. We find that AlAs shrinks by approximately 3% during oxidation, and that the refractive index of oxidized AlAs is 1.52.
Localized magnetic polarons in diluted magnetic semiconductors
Authors: Hagston W. E. , Stirner T. , Miao J.
Published: July 2002

A theory of localized magnetic polarons in diluted magnetic semiconductors is developed. The theory involves below band gap states and alloy fluctuations utilizing the optimal fluctuation method. A comparison of the theoretical calculations with published experimental results is performed. The implications of the comparison are discussed and the need for extending the theory to include an additional impurity/native lattice defect potential described. The magnetic field and temperature dependence of the polaron energies are also evaluated.
Stopping and damage parameters for Monte Carlo simulation of MeV implants in crystalline Si
Authors: Lulli G. , Albertazzi E. , Bianconi M. , Nipoti R. , Cervera M. , Carnera A. , Cellini C.
Published: July 2002

Semiempirical models of electronic energy loss and damage formation for MeV ions (B, P, As) implanted in silicon at room temperature were investigated through the comparison of measurements with Monte Carlo simulations of both impurity and damage depth distributions. Accurate prediction of dopant profiles in an amorphous target and in a low-dose implanted crystal is achieved by a proper parametrization of well known analytic stopping models. Moreover, to accurately describe the dynamic effects of damage accumulation in medium dose implants, a dependence on ion energy of the efficiency parameter used in the Kinchin–Pease (KP) model must be introduced in the simulation. Such a factor, determined by the fit of the measured integral of defect profiles, is found to decrease for P and As ions with increasing the nuclear energy released to primary recoil atoms, apparently reaching a saturation value of about 0.25. Full cascade simulations show that the increasing fraction of the primary recoils energy spent in electronic processes, not considered in the simple KP approximation, cannot explain the observed trend. While the empirical adjustment of damage efficiency leads to a good agreement between simulated and experimental dopant profiles, a systematic underestimate in the depth position of the peaks of simulated damage distributions is observed, which cannot be accounted for by simple ballistic transport effects.
Local effects of interstitial versus substitutional atoms in Y2Fe17−xMxAy compounds, with M=Al or Si and A=C or N
Authors: Plugaru N. , Morariu M. , Galatanu A. , Lazar D. P. , Barb D.
Published: July 2002

Interstitial Y2Fe17−xMxCy compounds, with M=Al or Si and x⩽2, y⩽1, were investigated by Fe57 Mössbauer spectroscopy at room temperature. In the analysis of the spectra, the preferential occupation by Al or Si of the Fe (12k) sites in the hexagonal R2Fe17 phase was taken into account. The hyperfine parameters variations are discussed on the grounds of local crystallographic details at the inequivalent iron sites, altered by the composition modifications. The low values of the isomer shifts and quadrupole interactions suggest small electronic charge redistribution in Y2Fe17−xMxAy as compared to Y2Fe17. However, the isomer shift variations upon Wigner–Seitz cell volume changes evidence for screening and electron transfer effects. In order to provide a more consistent picture of the local effects of interstitial versus substitutional atoms in R2(Fe,M)17Ay compounds, the discussion of the present results is extended to Fe57 Mössbauer data obtained on the parent compounds and their nitrides.
Strain effects on InSb phonons in bulk and superlattice layers
Authors: Siakavellas M. , Raptis Y. S. , Anastassakis E. , Lockwood D. J.
Published: July 2002

We have studied the effect of uniaxial stress on the long-wavelength optical phonons of InSb at room temperature using Raman spectroscopy. From the observed frequency shifts and degeneracy splittings we obtain values for the phonon deformation potentials. The results are used to analyze previously reported Raman measurements of phonons in strained InSb/In1−xAlxSb superlattices.
Vertical exchange reaction in (Ga, In)As during migration-enhanced epitaxy
Authors: Harris S. , Dorsey D. L.
Published: July 2002

We formulate a kinetic description of the vertical exchange reaction in which Ga replaces In during migration enhanced epitaxy of GaAs on InAs. Considering the Ga deposition rate R to be finite rather than infinite, as was done previously, leads to a reinterpretation of experimental results. The kinetic coefficient k as found by comparison with the In concentration profile determined by secondary ion mass spectroscopy is found to be 25% greater at 500 °C, cycle time of 5 s and R=12 s−1 than that found earlier. The probability of an In atom being replaced by a newly deposited Ga atom, which depends on the details of the Ga deposition schedule, is determined and found to be identical, 98%, for these conditions to that found earlier.
Low-temperature cleaved-edge polarized-photoluminescence studies of spontaneously ordered GaInP2 alloys
Authors: Cheong Hyeonsik M. , Zhang Yong , Mascarenhas A. , Geisz J. F. , Olson J. M.
Published: July 2002

We report low-temperature polarized-photoluminescence (PL) measurements on (110) cleaved edges of ordered GaInP2 alloys. By comparing the intensity of the forbidden PL polarized along the ordering axis with that of the allowed PL polarized orthogonal to the ordering axis, we study the effect of coulomb interaction on the excitonic transition rate. The experimentally obtained polarization ratios of 13–20 are consistent with calculations based on an eight-band k⋅p model.
Double crystal x-ray diffraction simulations of diffusion in semiconductor microstructures
Authors: Fatah J. M. , Harrison P. , Stirner T. , Hogg J. H. C. , Hagston W. E.
Published: July 2002

Diffusion in group IV, III-V and II-VI semiconductors is an interesting problem not only from a fundamental physics viewpoint but also in practical terms, since it could determine the useful lifetime of a device. Any attempt to control the amount of diffusion in a semiconductor device, whether it be a quantum well structure or not, requires an accurate determination of the diffusion coefficient. The present theoretical study shows that this could be achieved via x-ray diffraction studies in quantum well structures. It is demonstrated that the rocking curves of single quantum wells are not sensitive to diffusion. However the intensity of the first order satellite, which is characteristic of superlattice rocking curves, is strongly dependent upon diffusion and it is proposed that this technique could be used to measure the diffusion coefficient D.
The influence of energetic bombardment on the structure and properties of epitaxial SrRuO3 thin films grown by pulsed laser deposition
Authors: Maria J-P. , Trolier-McKinstry S. , Schlom D. G. , Hawley M. E. , Brown G. W.
Published: July 2002

SrRuO 3 epitaxial thin films were prepared by pulsed laser deposition (PLD) under a range of growth conditions to study the impact of bombardment on properties. Growth conditions favoring energetic bombardment resulted in SrRuO3 films with expanded in-plane and out-of-plane lattice constants. In particular, SrRuO3 films with pseudocubic out-of-plane lattice constants as large as 4.08 Å were deposited (3.8% larger than the bulk value). Those films with expanded lattices had greater resistivities and depressed Curie transition temperatures. The relative lattice mismatch between film and substrate was found to temper the effect of bombardment such that as the mismatch increased, a higher degree of bombardment was required to produce extended lattice parameters. The pressure-dependent energetic species inherent to PLD and their interaction with the ambient are believed to be the source of the bombarding flux. Further experiments confirmed that in the range of 20–200 mTorr, oxygen/ozone partial pressure had a negligible effect on the film properties.
Valence-band splitting and band-gap reduction in ordered GaInAs/InP
Authors: Wirth R. , Seitz H. , Geiger M. , Porsche J. , Scholz F. , Hangleiter A.
Published: July 2002

Measurements of the linear absorption and the nonlinear absorption due to the bleaching of the excitonic resonance have been used to determine the band-gap reduction and valence-band splitting in spontaneously ordered GaInAs/InP. Tilts of the substrate ranging from 2° to 15° towards {111}B, different growth rates and temperatures have been used to produce a series of samples containing various degrees of ordering. Best sample quality including small x-ray and photoluminescence linewidth as well as low temperature luminescence from the band edge was obtained using a substrate tilted 6° towards {111}B. The ratio between ordering induced band-gap reduction and crystal-field splitting was found to be ζ=1.8±0.4.
Influence of pulse duration on mechanical effects after laser-induced breakdown in water
Authors: Noack Joachim , Hammer Daniel X. , Noojin Gary D. , Rockwell Benjamin A. , Vogel Alfred
Published: July 2002

The influence of the pulse duration on the mechanical effects following laser-induced breakdown in water was studied at pulse durations between 100 fs and 100 ns. Breakdown was generated by focusing laser pulses into a cuvette containing distilled water. The pulse energy corresponded to 6-times breakdown threshold energy. Plasma formation and shock wave emission were studied photographically. The plasma photographs show a strong influence of self-focusing on the plasma geometry for femtosecond pulses. Streak photographic recording of the shock propagation in the immediate vicinity of the breakdown region allowed the measurement of the near-field shock pressure. At the plasma rim, shock pressures between 3 and 9 GPa were observed for most pulse durations. The shock pressure rapidly decays proportionally to r−(2⋯3) with increasing distance r from the optical axis. At a 6 mm distance of the shock pressure has dropped to (8.5±0.6) MPa for 76 ns and to <0.1 MPa for femtosecond pulses. The radius of the cavitation bubble is reduced from 2.5 mm (76 ns pulses) to less than 50 μm for femtosecond pulses. Mechanical effects such as shock wave emission and cavitation bubble expansion are greatly reduced for shorter laser pulses, because the energy required to produce breakdown decreases with decreasing pulse duration, and because a larger fraction of energy is required to overcome the heat of vaporization with femtosecond pulses.
Polymer/metal interfaces and the performance of polymer light-emitting diodes
Authors: Bharathan Jayesh M. , Yang Yang
Published: July 2002

Conjugated polymers are often treated as semiconductors with low doping concentrations. Unlike the traditional semiconductors which have a high density of surface states (mainly due to the dangling bonds), the nature of the metal/polymer interface, including barrier height and charge injection efficiency, is quite sensitive to the work function of the contact metal. In this article, we present evidence to show that the pinning of the surface Fermi level effect commonly observed at the silicon/metal interface can also be observed at the metal/polymer interface. It is achieved by controlling the doping level at the metal/polymer [poly(2-methoxy-5(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) or MEH-PPV] interface. ITO/MEH-PPV/Al devices doped with 2 Å of calcium on the cathode side of the interfacial layer have the same device performance as the ITO/MEH-PPV/Ca devices. The heavily n-doped region pins the surface energy level, hence the polymer interface at the cathode side is no longer sensitive to the work function of the overcoated metal. It is believed that either the midgap bipolaron energy states created by the dopants or the sharp band bending at the interface is responsible for facilitating the electron injection. On the other hand, a p-doped region at the anode side, obtained by using a thin layer of an acid at the interface, pins the surface energy level and makes the contact insensitive to the work function of the anode. Therefore, an efficient polymer light-emitting diode with the p-i-n structure has been demonstrated without the matching of the work function of the metal electrodes.
Optical coupling in quantum well infrared photodetector by diffraction grating
Authors: Fu Y. , Willander M. , Lu W. , Xu Wenlan
Published: July 2002

Because of the isotropic energy band structure of the Γ electrons in a GaAs/AlGaAs quantum well infrared photodetector (QWIP), normal incident radiation absorption is not possible so that the optical grating and its optimization become key requirements for such QWIPs. In this work we study the optical grating structure on top of the QWIP based on the Kirchhoff formula. It has been shown that in general, optimal optical coupling is obtained when the wavelength of the incoming radiation is comparable to the geometric sizes of the grating structure and the photodetector. For our GaAs/AlGaAs QWIP with a wavelength response around 8.0 μm, a thin uniform reflective metal film is enough to reach optimal optical coupling. The theoretical conclusions are confirmed by the fabrication of the photoconductive QWIPs of a 1×128 focal plane array which works at 80 K.
Induced uniaxial magnetic anisotropy and preferred orientation in Co and Co–Ni alloy by magnetic annealing
Authors: Tanaka Toshiro , Takahashi Migaku , Kadowaki Shizuo
Published: July 2002

The preferred orientation of hexagonal close packed hcp grains in magnetically annealed Co and Co–20%Ni alloy was studied by both Schulz’s x-ray transmission and reflection methods to clarify the origin of the magnetic anisotropy induced by the magnetic annealing effect. The preferred orientation was first found at a low polar angle α<50° area for both Co and Co–20%Ni alloy. The mechanism of the magnetic anisotropy induced by the field cooling effect was successfully explained qualitatively and quantitatively by this preferred orientation of the grains. The formation of this preferred orientation was discussed in terms of field-induced martensitic transformation. The uniaxial magnetocrystalline anisotropy energy could be the motive energy of the field-induced martensitic transformation.
Nonlinear optical efficient LiNbO3 waveguides proton exchanged in benzoic acid vapor: Effect of the vapor pressure
Authors: Rams J. , Cabrera J. M.
Published: July 2002

The effect of the vapor pressure on the properties of LiNbO3 waveguides proton exchanged in benzoic acid vapor is reported. The vapor pressure has been controlled by buffering the acid melt with different lithium benzoate concentrations, 0%⩽ρ⩽10%. Exchanged layers have been characterized by measuring effective indices, OH− infrared absorption, propagation losses, x-ray rocking curves, and second-harmonic generation. At 300 °C (24 h), highly homogeneous layers of κ2 phase (index jump at 632.8 nm within 0.08<Δne<0.10) and β1 phase (0.10<Δne<0.11) are obtained respectively in z-cut and x-cut substrates for ρ⩽2%. For 4%⩽ρ⩽6%, in both z and x cuts, much less homogenous layers of κ1 phase (0.03<Δne<0.07) are obtained, and for ρ⩾8%, only highly homogeneous α-phase (Δne<0.03) layers are produced. Under none of our experimental conditions, κ2 phase could be obtained on x-cut substrates. All α, κ1, and κ2 layers exhibit good second-harmonic generation efficiency, while in β1 layers it is negligible. All homogeneous phases show very low propagation losses (<0.35 dB/cm), but losses are much greater in nonhomogeneous κ1 layers.
Capture radius of magnetic particles in random cylindrical matrices in high gradient magnetic separation
Authors: Natenapit Mayuree , Sanglek Wirat
Published: July 2002

An effective medium treatment (EMT) was used to model the magnetic field around randomly distributed magnetic cylindrical fine wires and applied to calculate the capture radius of paramagnetic particles in a filter operating either in the longitudinal or transverse design mode. This article reports capture radius as a function of the ratio of magnetic velocity to fluid entrance velocity with a magnetic parameter which determines the strength of the magnetic short-range force, as a parameter. Finally, comparisons of the results based on the EMT approach, with those obtained by using the single-wire model, are given along with discussion on the criteria for validity of the simple single-wire model.
Phase transition in a general continuous Ising model with long-range interactions
Authors: Bayong E. , Diep H. T. , Truong T. T.
Published: July 2002

Critical exponents for a one-dimensional general continuous Ising model with long-range ferromagnetic interactions decaying as 1/r1+σ are calculated using a histogram Monte Carlo technique. A continuous Ising model means that a spin can take any value between −1 and 1. The critical point behavior is investigated. It is found that the system exhibits a second-order phase transition with nonstandard critical exponents; the singularities in the specific heat and susceptibility depend on σ. For σ=1, there are extremely weak finite size effects: the first-order and the second-order cumulants of the order parameter yield ν=2.42(1). The susceptibility exponent γ=2.259(9). Results for σ=0.7 will be shown and discussed.
Calculations of current-perpendicular-to-plane giant magnetoresistance with a current-conserving method
Authors: Chen Jian , Choy Tat-Sang , Hershfield Selman
Published: July 2002

We present a current-conserving method to calculate the current-perpendicular-to-plane conductivity using Kubo’s formula in magnetic superlattices. We use this method to study the interface scattering in Co/Cu (111) superlattices with different sources of scattering: clean interfaces with Fermi surface mismatch, spin-independent, and spin-dependent impurities. We find significant changes to the density-of-states under moderate impurity scattering.
High frequency magnetic properties of polymer-bonded Terfenol-D composites
Authors: Pasquale M. , Lim S. H.
Published: July 2002

We measure the high frequency magnetic properties of polymer-bonded Terfenol-D rods compacted from powders of different size, in order to analyze the effect of structure and preparation conditions in sonic or ultrasonic frequency applications. Magnetic characterization at high fields and in the kilohertz range is made using a custom built amorphous metal yoke. Experimental data are analyzed using the theory of losses and the dynamic Preisach model. The electrical resistivity is estimated to be about 1.25 μΩ m, higher than in grain-oriented materials. The classical contribution to dynamic power loss, directly proportional to conductivity, is found to be dominant. The analysis establishes a direct connection between static and dynamic magnetic properties and structural characteristics, such as the density, conductivity, and the powder size, which can be modified for optimization purposes.
Magnetic nanoparticle array with perpendicular crystal magnetic anisotropy
Authors: Haginoya Chiseki , Heike Seiji , Ishibashi Masayoshi , Nakamura Kimio , Koike Kazuyuki , Yoshimura Toshiyuki , Yamamoto Jiro , Hirayama Yoshiyuki
Published: July 2002

By using electron beam lithography, a continuous CoCrPt film with a perpendicular crystal magnetic anisotropy has been patterned into a magnetic nanoparticle array of 29 Gdot/in.2 with a 150 nm period, an 80 nm diameter, and a 44 nm height. Studies of magnetic properties using a magnetic force microscope and a vibrating sample magnetometer show that this patterning increases the remanent-to-saturation magnetization ratio from 0.2 of the continuous film to 1 of the particles, and that each particle has a single magnetic domain with perpendicular anisotropy. The application of this array to future high density magnetic recording media is discussed.
First-principles calculation of the electronic structure of yttrium iron garnet (Y3Fe5O12)
Authors: Xu Yong-Nian , Gu Zong-quan , Ching W. Y.
Published: July 2002

The electronic structure of Y3Fe5O12 (YIG) crystal was calculated using the spin-polarized orthogonalized linear combination of atomic orbitals method in the local spin-density approximation. It is shown that YIG has a ferrimagnetic ordering with Fe spin magnetic moments of −0.62μB and +1.56μB at the octahedral 16(a) site and the tetrahedral 24(d) site, respectively. The origin of ferrimagnetism in the two Fe sublattices can be traced to the different ordering of the eg and t2g levels for the spin-up and spin-down electrons.
Which spin valve for next giant magnetoresistance head generation? (invited)
Authors: Araki Satoru , Sano Masashi , Li Shuxiang , Tsuchiya Yoshihiro , Redon Olivier , Sasaki Tetsuro , Ito Noriaki , Terunuma Kohichi , Morita Haruyuki , Matsuzaki Mikio
Published: July 2002

To maintain a challenging 100%/year growth rate of the areal density, new spin valve (SV) structures have recently emerged. This work reviews the transport properties of hybrid free, synthetic, and specular SV at film level and discusses their potential for read head application. High sensitivity can be achieved using hybrid free and synthetic free SV due to their small net magnetic moment, but the longitudinal biasing of synthetic free SV is a critical issue. Good thermal stability was obtained in both synthetic pinned and specular SV due to high effective exchange field or good protection from interdiffusion. Moreover, specular SV also shows large output. With structure having different benefits, the next generation of SV head should be a combination to optimize all properties. Nevertheless, it raises many technological issues, related to the processing of very thin layers, which should be solved to meet the requirements of reproducibility and high yield from mass production.
Quantum chemistry in Fock space. IV. The treatment of permutational symmetry. Spin-free diagrams with symmetrized vertices
Authors: Kutzelnigg W.
Published: July 2002

The theory of effective Hamiltonians in Fock space is extended by the use of spin-free basis operators as well as vertices (matrix elements) that are adapted to the permutation group. This reduces the number of the necessary operators considerably, especially for operators of high particle rank. An extension of the generalized Wick theorem of spin-free quantum chemistry to permutation symmetry adapted operators is presented and illustrated graphically. The second order diagrams for the energy and the fully contracted (vacuum) diagrams to third and fourth order are given explicitly. The diagram symmetry group GD that consists of line exchanges at the vertices and permutations of external lines is studied in detail and a recipe for the construction of permutation symmetry adapted diagrams with spin-free symmetrized vertices is derived. The use of permutation symmetry in nonperturbative approaches is discussed and the construction of the matrix elements for the final CI is illustrated. In this formalism there is no need for spin algebra.
Time evolution of individual rotational states after pulsed Doppler-free two-photon excitation: Influence of perturbations in S1 benzene
Authors: Schubert U. , Riedle E. , Neusser H. J.
Published: July 2002

Single rovibronic level excitation has been demonstrated for a molecule representing the statistical limit of a nonradiative electronic relaxation process. Resolution of the complex 1410 two-photon band of benzene was possible after elimination of the Doppler broadening in a Doppler-free two-photon absorption process with narrow bandwidth pulsed laser light and allowed for selective excitation of individual rotational states. Fluorescence decay measurements of these states under low pressure conditions display a pure exponential behavior and reveal that the nonradiative rate is independent of the rotational quantum numbers J, K for an electronic nonradiative relaxation process in the statistical limit. The decrease in lifetime found for perturbed states is attributed to the mixing with a background state containing quanta of an accepting mode.
Hydrodynamic screening and particle dynamics in porous media, semidilute polymer solutions and polymer gels
Authors: Altenberger Andrzej R. , Tirrell Matthew , Dahler John S.
Published: July 2002

The dynamics of particle interactions in porous media are studied using a newly formulated mean field theory. Statistical correlations in the gel network are analyzed in detail and found to have a crucial influence on the results. Self-diffusion and mutual diffusion coefficients of Brownian particles moving through a gel are determined. Preaveraged hydrodynamic interaction tensors are derived for two mobile Brownian particles a mobile particle and an obstacle.
Theoretical investigation of fluorescence concentration quenching in two-dimensional disordered systems. Application to chlorophyll a in monolayers of dioleylphosphatidylcholine
Authors: Boulu Laurent G. , Patterson Larry K. , Chauvet J. P. , Kozak John J.
Published: July 2002

A master equation approach is used for investigating energy transfer and trapping in two-dimensional disordered systems, where the traps are statistical pairs of pigment molecules closer than a critical distance Rc. Fluorescence decay curves are calculated over a range of concentrations as a function of Rc and the Förster transfer radius R̄0. The concentration dependence of the lifetimes is compared to the fluorescence self-quenching data that Chauvet et al. obtained from real-time measurements in monolayers of chlorophyll a and dioleylphosphatidylcholine (DOL). This dependence is found to be close to second order and for a choice of Rc =10 Å the experimental data are fit if R̄0=78±2 Å. This value is in close agreement with those found in the literature from depolarization measurements.
Low-frequency expansion for probability amplitudes: An alternative approach to certain intramolecular dynamics problems
Authors: Nadler Walter , Marcus R. A.
Published: July 2002

We present an algorithm to determine the averaged time evolution of the probability amplitude for a nonstationary state in a quantum mechanical system. The algorithm is based on a low-frequency expansion of the probability amplitude and is related to the generalized moment expansion method which has been applied successfully to the description of dynamic correlation functions in stochastic systems. It is shown that the proposed algorithm gives excellent results for the description of quantum beats in the time evolution of the occupation probability for a nonstationary state in model systems. The relation of the algorithm to other theoretical approaches and the relevance for the description of intramolecular energy transfer processes is discussed.
Critical properties of the Flory–Huggins lattice model of polymer mixtures
Authors: Sariban A. , Binder K.
Published: July 2002

Monte Carlo simulations are presented for a model of a symmetrical polymer mixture on the simple cubic lattice, modeling both polymers A, B by self-avoiding walks of NA=NB=N steps. If a pair of nearest-neighbor sites is taken by monomers of the same species, an energy ε is won. In the Monte Carlo algorithm local motions of the chains are considered (allowing for 20% vacancies to ensure enough chain mobility) as well as transformations of A chains into B chains and vice versa, since the simulation applies the grand-canonical ensemble where the chemical potential difference rather than the volume fraction is fixed. The phase diagram, the excess specific heat, and the structure factor in the long-wavelength limit are obtained for N=4, 8, 16, and 32 using finite L×L×L lattices with L ranging from 8 to 20. Analyzing these results with finite size scaling techniques, both critical exponents and critical amplitudes are estimated. Although the exponents are consistent with those of the three-dimensional Ising model, the variation of critical amplitudes with N is much closer to mean-field predictions. In contrast to qualitative expectations (‘‘Ginzburg criterion’’), the crossover scaling region from Ising to mean-field behavior has not yet been reached. Implications for experimental work are briefly discussed.
Time dependence of fluorescence polarization in jet-cooled diazabenzenes: Evidence for partial K mixing in the triplet manifold
Authors: Terazima M. , Lim E. C.
Published: July 2002

The time dependence of fluorescence polarization has been measured for several vibronic levels of S1 pyrimidine, 000 pyrazine, and 000 5-methylpyrimidine in the collision-free environment of a supersonic jet. In each case, the polarization decreases from the regular-rotor-limit value to a smaller constant value. The decay of the polarization occurs with a rate smaller than that of the fast-component fluorescence but larger than that of the slow-component emission. The decay rate of the fluorescence polarization increases with increasing density of triplet rovibronic levels coupled to the initially prepared S1 state. The results are interpreted as providing evidence for vibration–rotation interactions in the triplet manifold, leading to an extensive, but incomplete, K mixing. The time scale of the rotational dynamics is proposed to be in the tens of nanoseconds, based on the decay of the fluorescence polarization.
High resolution lifetime measurements of the perturbed J′=0 levels of the 1B3u state of pyrazine
Authors: van Herpen Willy M. , Meerts W. Leo , Drabe Karel E. , Kommandeur Jan
Published: July 2002

The lowest excited singlet 1B3u state of pyrazine is known to be coupled to a number of triplet 3B3u states. Using a strongly collimated molecular beam and a single frequency laser it is shown that the J′=0 of the 000 transition contains at least 36 states. We have individually excited eight of these states and studied its decay. The lifetimes found (typically 450 ns) do not scale with the intensities of the excitation spectrum. This deviation is caused by a nonradiative decay of the zero order 3B3u states. With a simple model it was possible to reconstruct the absorption spectrum, the energies of the zero order states and its coupling strengths. The zero order decay rates of the singlet and triplet states have been determined. The value obtained for the zero order singlet state is 5 MHz; the values for the triplet states range from 0.6 to about 5 MHz.
Relationship between free energy density functional, Born–Green–Yvon, and potential distribution approaches for inhomogeneous fluids
Authors: Fischer Johann , Heinbuch Uwe
Published: July 2002

For hard spheres in an external field it is shown that free energy density functionals of the type suggested by Percus [J. Chem. Phys. 75, 1316 (1981)] lead to certain approximations to the Born–Green–Yvon equation. For two functionals these approximations are worked out explicitly as well as are the expressions for the chemical potential and the direct correlation function of the homogeneous fluid. This formal relationship explains why results obtained from a Born–Green–Yvon approach and from coarse grained free energy density functionals are mostly rather similar. Moreover, a functional for the chemical potential derived by Robledo [J. Chem. Phys. 72, 1701 (1980)] from potential distribution theory is compared with the expressions from the Percus approach.
Red and near-infrared laser-induced emission of S2 in an Ar matrix
Authors: Chiang Su-Yu , Lee Yuan-Pern
Published: July 2002

The S2 emission in the red and near-infrared regions has been reinvestigated using the laser-induced emission technique. Four progressions of S2 in solid Ar were observed in the emission spectra following excitation in the UV region with a pulsed Nd-YAG laser system. 34S-isotopic shifts allowed the determination of spectroscopic parameters for these progressions. Among them, ν00=19 757 and 15 417 cm−1 for the two progressions with distinct zero-phonon lines (ZPL); ν00=19 384 and 15 003 cm−1 for the other two with no ZPL, respectively. The progressions are assigned as the c 1Σ−u→X 3Σ−g and the c 1Σ−u →a 1Δg transitions of S2 in two matrix sites, respectively. The data yield the T0 values for the c 1Σ−u and the a 1Δg states. The relative intensities of these four progressions varied with excitation wavelengths and isotopic species.
Theoretical rotational–vibrational spectrum of H2S
Authors: Senekowitsch Jörg , Carter Stuart , Zilch Andre , Werner Hans-Joachim , Handy Nicholas C. , Rosmus Pavel
Published: July 2002

The high resolution rovibrational spectrum of H2S has been evaluated from three-dimensional ab initio potential energy and electric dipole moment functions and variational rovibrational eigenfunctions, which took full account of anharmonicity effects and rotation–vibration coupling. The quality of the near equilibrium theoretical potential energy function has been checked by comparisons with experimental equilibrium structure, empirical quartic force fields, vibrational band origins, centrifugal distortion constants, and rotational energy levels. All parameters agree well with the available experimental data. Vibrational band intensities for the ν2, 2ν2, ν1, and ν3 bands have been calculated from empirical and ab initio dipole moment functions and compared with experimental and theoretical integrated band intensities. The difficulties arising by the derivation of such data from the experimental intensities of H2S are discussed. The theoretical results strongly suggest that higher than first derivatives are needed for a proper description of the dipole moment function. The room temperature absorption spectra have been evaluated ab initio for the pure rotational and the ν2, 2ν2, ν1, and ν3 transitions. The unusual intensity pattern of the P, Q, and R branches attributed to the rotational–vibrational coupling has been well reproduced. Absolute line intensities calculated previously by perturbation theory are compared with variational results. The purely theoretical line intensities agree satisfactorily with experiment for the bending transitions, however, the extremely flat regions of the dipole moment functions along the bond stretching displacements make the transition intensities very sensitive to the values of the dipole moment derivatives.
Dissociation lifetimes and level mixing in overtone-excited HN3(X̃ 1A′)
Authors: Foy B. R. , Casassa M. P. , Stephenson J. C. , King D. S.
Published: July 2002

Vibrational overtone photodissociation is used to examine the spectroscopy and vibrational predissociation lifetimes of HN3 in its ground electronic state. Direct overtone pumping of the N–H stretching levels 5νNH and 6νNH prepares molecules in selected states (v,J,K) near 15 100 and 17 700 cm−1 of vibrational energy; spin-forbidden NH(X 3 Σ−) dissociation fragments are detected by laser-induced fluorescence. Photodissociation spectra of beam-cooled HN3 display mixing of individual rotational levels of the nνNH vibrations with several background states, with derived coupling matrix elements in the range 0.01–0.1 cm−1. Vibrational predissociation lifetimes of mixed components of 5νNH are state specific, with variations of a factor of 2 for only 0.1 cm−1 energy differences. Average lifetimes for low J, K are 210 ns for 5νNH and 0.95 ns for 6νNH. The ratio of decay rates for the two overtone levels, k(6νNH)/k(5νNH)=220, is much greater than predicted by statistical theory, which gives a ratio of 4.
The adsorption of hydrogen on a ruthenium (101̄0) surface
Authors: Lauth G. , Schwarz E. , Christmann K.
Published: July 2002

Video-LEED, HREELS, TDS, and Δφ measurements were used to investigate the adsorptive, structural, and vibrational properties of the Ru(101̄0)/H system between 100 and 500 K. At all temperatures investigated hydrogen adsorbs dissociatively with very high initial sticking probability (s0≈1.0) with apparent precursor mechanism. The saturation coverage at 100 K is extraordinarily high (Θmax =2≂1.728×1015 H atoms cm−2), up to this coverage four H binding states α, β1, β2, and β3 can be distinguished having desorption energies between 56 KJ/mol (α) and 80 KJ/mol (β3). The H binding states are intimately correlated with the four observable ordered H phases: At Θ=1 a c(8×2) or ‘‘1×2’’ structure with weak split spots appears which transforms at higher coverages into a clear 1×2 phase with likewise weak spots and with maximum intensity at Θ=1.2. It follows a c(2×2)-3H phase (I maximum at Θ=1.5) with rather more intense ‘extra’ spots which fade away with increasing coverage until at Θ=2.0 a (1×1)-2H pattern is reached. The (positive) H-induced work function change Δφ runs through two maxima and saturates at ∼250 mV. The vibrational loss spectra which were measured in two perpendicular azimuths exhibit a variety of bands which can be correlated with the ordered H phases and point to H species bound in two different kinds of threefold coordinated sites. Our data suggest several structural similarities with the neighboring system in the periodic table, Rh(110)/H, but also interesting differences.