bluuebunny/crossref_metadata_2025 huggingface dataset:
Enhanced field sensitivity close to percolation in magnetoresistive La2/3Sr1/3MnO3/CeO2 composites
Authors: Balcells Ll. , Carrillo A. E. , Martínez B. , Fontcuberta J.
Published: July 2002

The magnetoresistance of La2/3Sr1/3MnO3/CeO2 composites is explored as a function of the metal/insulator composition, temperature, and magnetic field. An important enhancement of the low-field magnetoresistance is observed for samples close to metallic percolation threshold. The improved field sensitivity is still fairly large at room temperature. Manganese perovskites composites may constitute a new alternative for the development of magnetoresistive devices.
Valence-band structure of wurtzite GaN including the spin-orbit interaction
Authors: Ren G. B. , Liu Y. M. , Blood P.
Published: July 2002

We have investigated the effects of the spin-orbit interaction on the valence-band structure of wurtzite GaN in the k⋅p theory. The spin-orbit interaction is usually neglected in nitrides, which leads to three doubly degenerate bands: the heavy-hole, light-hole, and crystal-field split-off bands. Including the spin-orbit interaction, this degeneracy is removed to give six single bands. We obtained the Luttinger-like parameters in bulk wurtzite GaN by fitting data obtained from an empirical pseudopotential calculation obtaining a value for the parameter A7 of 93.7 meV/Å. We also used these results to calculate the valence-band structures of a GaN/AlGaN quantum well. Our result shows the spin-orbit interaction is important in GaN and associated quantum well structures.
Angular filtering of spatial modes in a vertical-cavity surface-emitting laser by a Fabry–Perot étalon
Authors: Chen Guoqiang , Leger James R , Gopinath Anand
Published: July 2002

A low-finesse Fabry–Perot étalon is employed as an angularly selective filter to discriminate against the high-order spatial modes of a vertical-cavity surface-emitting laser (VCSEL) in the near field. Single-transverse-mode operation is obtained over a wide region for a 15 μm diam VCSEL.
Plume-induced stress in pulsed-laser deposited CeO2 films
Authors: Norton D. P. , Park C. , Budai J. D. , Pennycook S. J. , Prouteau C.
Published: July 2002

Residual compressive stress due to plume-induced energetic particle bombardment in CeO2 films deposited by pulsed-laser deposition is reported. For laser ablation film growth in low pressures, stresses as high as 2 GPa were observed as determined by substrate curvature and four-circle x-ray diffraction. The amount of stress in the films could be manipulated by controlling the kinetic energies of the ablated species in the plume through gas-phase collisions with an inert background gas. The film stress decreased to near zero for argon background pressures greater than 50 mTorr. At these higher background pressures, the formation of nanoparticles in the deposited film was observed.
Temperature dependence of the reflectivity of silicon with surface oxide at wavelengths of 633 and 1047 nm
Authors: Heller J. , Bartha J. W. , Poon C. C. , Tam A. C.
Published: July 2002

We measure the temperature coefficient of the reflectivity of Si at a red wavelength of 633 nm that is much larger than the Si band gap, and at an infrared wavelength of 1047 nm that is close to the band gap. Our reflectivity measurement is done over a temperature range from room temperature to 200 °C, with an accuracy of better than 1 part in 105. Our results show that the temperature coefficient for the infrared reflection is over three times larger than that for the red reflection over the temperature range studied. Our results and technique can be useful for remote monitoring of temperatures of Si or other materials.
Polymer diodes with high rectification
Authors: Roman L. S. , Berggren M. , Inganäs O.
Published: July 2002

Polymer diodes made using a bilayer of doped poly(3,4-ethylenedioxythiophene) and a semiconducting polymer in a sandwich structure with two low-work-function metals are reported. The conducting polymer layer acted as a modifier of the injection properties of the low-work-function metal, allowing easy hole injection. Upon insertion of the conducting polymer layer, the contact-limited current flow became bulk limited. With this anode, the fabrication of diodes with a rectification ratio of seven orders of magnitude was possible. We present patterned microdiodes made with crossing of 10 μm lines, showing similar performance as the mm-size diode.
A high-speed photocurrent multiplication device based on an organic double-layered structure
Authors: Nakayama Ken-ichi , Hiramoto Masahiro , Yokoyama Masaaki
Published: July 2002

A high-speed photocurrent multiplication device that has an organic double-layered structure was fabricated. In this device, the improvement in the uniformity of the organic films, caused by inserting an additional amorphous-like organic underlayer, enabled high-electric-field operation of the device. Owing to efficient carrier photogeneration under the application of higher voltages, the response time of the multiplied photocurrent became shorter without a decrease in the multiplication rate, and showed clear saturation above 18 V. At a high applied voltage of 20 V, a large multiplication rate of 1.7×105-fold, with a high-speed response of 3.7 s, was achieved. A rise time of only 26 ms required for the 1000-fold multiplication suggests that the multiplication device could potentially be applied to a practical imaging device.
Determination of pore-size distribution in low-dielectric thin films
Authors: Gidley D. W. , Frieze W. E. , Dull T. L. , Sun J. , Yee A. F. , Nguyen C. V. , Yoon D. Y.
Published: July 2002

Positronium annihilation lifetime spectroscopy is used to determine the pore-size distribution in low-dielectric thin films of mesoporous methylsilsesquioxane. A physical model of positronium trapping and annihilating in isolated pores is presented. The systematic dependence of the deduced pore-size distribution on pore shape/dimensionality and sample temperature is predicted using a simple quantum mechanical calculation of positronium annihilation in a rectangular pore. A comparison with an electron microscope image is presented.
Magnetic domain and domain-wall imaging of submicron Co dots by probing the magnetostrictive response using atomic force microscopy
Authors: Wittborn J. , Rao K. V. , Nogués J. , Schuller Ivan K.
Published: July 2002

An approach to image the domains and domain walls of small ferromagnetic entities using atomic force microscopy (AFM), with a nonmagnetic AFM probe, has been developed. Exciting the sample in an external ac magnetic field, the distribution of magnetostrictive response at the surface is detected. By this technique, the domains and domain walls of submicron Co dots have been imaged with a 1 nm lateral resolution. In elliptical Co dots with a 350-nm-long axis on a triangular lattice array with 400 nm periodicity, we find evidence for two domains with opposite magnetization orientation across a wall. The domain-wall width in these dots is found to be about 35 nm. Furthermore, we observe a ferromagnetic alignment of the domains in the neighboring dots, which suggests a magnetostatic interaction among the dots.
Epitaxial growth of non-c-oriented SrBi2Nb2O9 on (111) SrTiO3
Authors: Lettieri J. , Zurbuchen M. A. , Jia Y. , Schlom D. G. , Streiffer S. K. , Hawley M. E.
Published: July 2002

Epitaxial SrBi2Nb2O9 thin films have been grown with a (103) orientation on (111) SrTiO3 substrates by pulsed-laser deposition. Four-circle x-ray diffraction and transmission electron microscopy reveal nearly phase-pure epitaxial films. Epitaxial (111) SrRuO3 electrodes enabled the electrical properties of these (103)-oriented SrBi2Nb2O9 films to be measured. The low-field relative permittivity was 185, the remanent polarization was 15.7 μC/cm2, and the dielectric loss was 2.5% for a 0.5-μm-thick film.
Spatially resolved band-edge emission from partially coalesced GaN pyramids prepared by epitaxial lateral overgrowth
Authors: Li X. , Bohn P. W. , Kim Jeongyong , White J. O. , Coleman J. J.
Published: July 2002

Partially coalesced GaN pyramidal structures are formed by metal-organic chemical vapor deposition using the epitaxial lateral overgrowth method. Spatially resolved optical characterization of these structures has been carried out using cathodoluminescence (CL) microscopy and spectroscopy. The coalesced region exhibits much stronger and more uniform luminescence than other regions of the sample. In addition, the emission from the coalesced region is blue-shifted, while that from the sidewalls is red-shifted, relative to broad area grown samples. The peak shift mechanism is discussed based on the CL temperature and power dependence and analysis of the confocal Raman scattering.
Mechanism of luminescence in InGaN/GaN multiple quantum wells
Authors: Yang H. C. , Kuo P. F. , Lin T. Y. , Chen Y. F. , Chen K. H. , Chen L. C. , Chyi Jen-Inn
Published: July 2002

We report a firm evidence of luminescence from InN clusters in InGaN/GaN multiple quantum wells. Photoluminescence, photoluminescence excitation, and Raman scattering measurements have been employed to study the optical properties of InGaN/GaN multiple quantum wells. A careful examination of the low energy shoulders of the main peak luminescence reveals the fact that their separation is in good agreement with the longitudinal optical phonon energy of pure InN film measured by Raman scattering. A large Stokes-like shift between the emission peak energy and the absorption edge is found; it increases with increasing indium content. All these observations can be explained in a consistent way by the effect of localization due to self-organized InN clusters within InGaN layers. Our results thus strongly suggest that the emission mechanism of InGaN/GaN quantum wells originates from radiation recombination within the localized states of self-organized InN clusters.
Explosively driven fast closing blast shutter
Authors: Ruden E. L. , Gale D. G.
Published: July 2002

The Air Force Research Laboratory’s Directed Energy Directorate has developed a fast closing shutter for the purpose of explosive debris mitigation. The shutter forms a high vaccum seal in 15 μs by the plastic deformation and pinching off of an 8 mm inside diameter profiled cartridge brass tube by two explosively propelled Ti slugs impacting from opposite directions. Two explosive detonators, the tube itself, and steel disks which back the Ti slugs are the only sacrificial parts. Design and performance details, the developmental history, scaling issues, and advantages over other shutter designs are discussed.
Mixed quantum-classical molecular dynamics: Aspects of the multithreads algorithm
Authors: Wan Chun-Cheng , Schofield Jeremy
Published: July 2002

The mixed quantum-classical Liouville equation is derived from a semiclassical perspective starting from the full quantum Schrödinger equation. An asymptotic numerical scheme for solving the equation is discussed which relies on propagating swarms of interacting “threads” which represent the density matrix or other observable. It is demonstrated that this “multithreads” method performs extremely well on simple one-dimensional model systems designed to test nonadiabatic molecular dynamic methods, yielding essentially exact results for a variety of models.
Color engineering by modified oligothiophene blends
Authors: Anni M. , Gigli G. , Paladini V. , Cingolani R. , Barbarella G. , Favaretto L. , Sotgiu G. , Zambianchi M.
Published: July 2002

Fully tunable light emission is demonstrated with combinations of binary blends of modified oligothiophenes of high efficiency, covering the entire spectrum of colors according to the standards of the Commission International de l’Eclairage. The emission spectrum of each blend is determined by the Forster transfer when the energy separation between the highest occupied molecular orbital–lowest unoccupied molecular orbital gap of the constituent molecules is smaller than 0.56 eV. For larger energy separation, the blend emission is just given by the superposition of the emission spectra of the constituent molecules.
Magnetochromatics resulted from optical gratings of magnetic fluid films subjected to perpendicular magnetic fields
Authors: Horng Herng-Er , Hong Chin-Yih , Lee S. L. , Ho C. H. , Yang S. Y. , Yang H. C.
Published: July 2002

A two-dimensional ordered structure can be formed in a thin film of homogeneous magnetic fluids by applying a magnetic field perpendicular to the plane of the film. The characteristic spacing of the ordered structure can be controlled by several parameters, such as the field strength, the sweep rate, the thickness of the film, and the volume concentration of homogeneous magnetic fluids. The length of the characteristic spacing ranges from several micrometers to submicrometers. Depending on the size of the spacing, magnetic fluid films are capable of diffracting an incoming white light. The diffracted light then interferes resulting in a chromatic effect. This effect is well illustrated by the chromatic rings produced by passing a white light through the magnetic fluid films possessing an ordered structure. Because of the changeability of the characteristic spacing, the magnetochromatic effect of magnetic fluid films can be controlled by altering these control parameters.
Large-scale numerical simulations of ion beam instabilities in unmagnetized astrophysical plasmas
Authors: Dieckmann M. E. , Ljung P. , Ynnerman A. , McClements K. G.
Published: July 2002

Collisionless quasiperpendicular shocks with magnetoacoustic Mach numbers exceeding a certain threshold are known to reflect a fraction of the upstream ion population. These reflected ions drive instabilities which, in a magnetized plasma, can give rise to electron acceleration. In the case of shocks associated with supernova remnants (SNRs), electrons energized in this way may provide a seed population for subsequent acceleration to highly relativistic energies. If the plasma is weakly magnetized, in the sense that the electron cyclotron frequency is much smaller than the electron plasma frequency ωp, a Buneman instability occurs at ωp. The nonlinear evolution of this instability is examined using particle-in-cell simulations, with initial parameters which are representative of SNR shocks. For simplicity, the magnetic field is taken to be strictly zero. It is shown that the instability saturates as a result of electrons being trapped by the wave potential. Subsequent evolution of the waves depends on the temperature of the background protons Ti and the size of the simulation box L. If Ti is comparable to the initial electron temperature Te, and L is equal to one Buneman wavelength λ0, the wave partially collapses into low frequency waves and backscattered waves at around ωp. If, on the other hand, Ti≫Te and L=λ0, two high frequency waves remain in the plasma. One of these waves, excited at a frequency slightly lower than ωp, may be a Bernstein–Greene–Kruskal mode. The other wave, excited at a frequency well above ωp, is driven by the relative streaming of trapped and untrapped electrons. In a simulation with L=4λ0, the Buneman wave collapses on a time scale consistent with the excitation of sideband instabilities. Highly energetic electrons were not observed in any of these simulations, suggesting that the Buneman instability can only produce strong electron acceleration in a magnetized plasma.
Optical evidence for compatibility of antiferromagnetism and superconductivity in YBa2Cu3O6+x
Authors: Eremenko V. V. , Samovarov V. N. , Vakula V. L. , Libin M. Yu. , Uyutnov S. A.
Published: July 2002

The evolution of the spectral composition of the absorption in the 1.25–2.6 eV region for metallic films of YBa2Cu3O6+x with superconducting transition temperatures of Tc=51 and 74 K is measured as the films are cooled from 180 to 20 K. Particular attention is paid to the temperature effects in two absorption bands: the A band (≃1.8 eV), which reflects the appearance of holes dressed in antiferromagnetic (AFM) fluctuations, and the (A+J) band (≃2.15 eV), which reflects an additional (magnon) excitation of the short-range AFM order. It is found that the changes of these bands begin in the normal phase at T<T* in the temperature region corresponding to the opening of the pseudogap state, and the (A+J) magnon band arises in the pseudogap state even in the case when it is absent at room temperatures. At the superconducting transition the parameters of the bands stop changing, and the (A+J) magnon band is preserved in the superconducting state. The results are interpreted as evidence of a magnetic nature of the pseudogap state and for the compatibility of AFM short-range order with superconductivity.
Optical and magneto-optical investigation on electronic structure of ordered ferromagnetic Fe3Pt
Authors: Kim K. J. , Lee S. J. , Wiener T. A. , Lynch D. W.
Published: July 2002

The optical and magneto-optical properties of ordered Fe3Pt have been investigated by spectroscopic ellipsometry and magneto-optical Kerr spectroscopy. The diagonal component of the optical conductivity tensor of the compound exhibits a broad absorption peak at about 2 eV, which is shifted by about 0.5 eV to lower energies from the corresponding one in pure bcc Fe. The Kerr angle spectrum of the compound disperses quite similarly in both spectral trend and magnitude to that of pure Fe below 3.5 eV but differently above it. The lower-energy shift of the 2-eV-absorption structure of the compound is interpreted as due to the shift of the minority-spin Fe-d states toward EF through the hybridization with Pt-d states. The Kerr effect of the compound is attributable to a large spin-orbit coupling in Pt as well as the well-hybridized spin-polarized bands.
Origin of the exponential distribution of traps in glass
Authors: Sakurai Takao , Shoji Kazuo , Itoh Kazuomi , Gartia Raj Kishor
Published: July 2002

A recent rigorous analysis of the broad thermoluminescence (TL) peak of a Brown microcline has shown it to be due to an exponential distribution of traps. Incidentally, from the statistical point of view one expects the traps to follow a Gaussian distribution. In order to elucidate the origin of the exponential trap distribution we have analyzed a set of glow peaks of a light green glass recorded under varying conditions of trap filling. The sample upon γ-rays irradiation (dose=7×103 Gy) exhibits a broad TL peak around 191 °C. The broad peak becomes narrower once a thermal cleaning technique is applied to the irradiated specimen. Application of computerized glow curve deconvolution reveals that the broad glow curve is due to the presence of a Gaussian distribution of traps. However, in thermally cleaned glow curves the distribution of traps is found to be exponential. This shows that the exponential distribution of traps results from the deeper tail portion of the Gaussian distribution.
Three-dimensional photonic crystals based on macroporous silicon with modulated pore diameter
Authors: Schilling J. , Müller F. , Matthias S. , Wehrspohn R. B. , Gösele U. , Busch K.
Published: July 2002

We report on the fabrication and optical characterization of a three-dimensional (3D) photonic crystal on the basis of macroporous silicon. The structure consists of a 2D array of air pores in silicon whose diameter is varied (modulated) periodically with depth. The bandstructure of the resulting 3D hexagonal photonic crystal is calculated and compared with transmission measurements. The described structure allows to adjust the dispersion relation along the pore axis almost independently from the dispersion relation in the plane perpendicular to the pore axis.
Pattern formation in non-Newtonian Hele–Shaw flow
Authors: Fast Petri , Kondic L. , Shelley Michael J. , Palffy-Muhoray Peter
Published: July 2002

We study theoretically the Saffman–Taylor instability of an air bubble expanding into a non-Newtonian fluid in a Hele–Shaw cell, with the motivation of understanding suppression of tip-splitting and the formation of dendritic structures observed in the flow of complex fluids, such as polymeric liquids or liquid crystals. A standard visco-elastic flow model is simplified in the case of flow in a thin gap, and it is found that there is a distinguished limit where shear thinning and normal stress differences are apparent, but elastic response is negligible. This observation allows formulation of a generalized Darcy’s law, where the pressure satisfies a nonlinear elliptic boundary value problem. Numerical simulation shows that shear-thinning alone modifies considerably the pattern formation and can produce fingers whose tip-splitting is suppressed, in agreement with experimental results. These fingers grow in an oscillating fashion, shedding “side-branches” from their tips, closely resembling solidification patterns. A careful analysis of the parametric dependencies of the system provides an understanding of the conditions required to suppress tip-splitting, and an interpretation of experimental observations, such as emerging length-scales.
Scaling and optimization of the radiation temperature in dynamic hohlraums
Authors: Slutz S. A. , Douglas M. R. , Lash J. S. , Vesey R. A. , Chandler G. A. , Nash T. J. , Derzon M. S.
Published: July 2002

A quasianalytic model of the dynamic hohlraum is presented. Results of the model are compared to both experiments and full numerical simulations with good agreement. The computational simplicity of the model allows one to find the behavior of the hohlraum radiation temperature as a function of the various parameters of the system and thus find optimum parameters as a function of the driving current. The model is used to investigate the benefits of ablative standoff and quasispherical Z pinches.
Zero-shear stress relaxation and long time dynamics of a linear polyethylene melt: A test of Rouse theory
Authors: Padding J. T. , Briels W. J.
Published: July 2002

Results of united atom molecular dynamics simulations of a n-C120H242 melt at 450 K are presented. It is shown that the results of mean square displacement, dynamic structure factor, end-to-end vector autocorrelation, and shear relaxation modulus can consistently be described by the Rouse model with a single set of fit parameters, provided the length scales involved are larger than the statistical segment length b≈1.2 nm. On smaller length scales the stiffness of the chain becomes prominent, and the results deviate increasingly from the Rouse predictions. The shear relaxation modulus G(t) is determined from the stress autocorrelation function from both atomic and molecular points of view. The integrals ∫G(t)dt are found to be identical after 1 ps and a Rouse description is shown to coincide for time scales larger than 0.4 ns. Compared to experimental values, the measured diffusion coefficient is overestimated by 63% and the viscosity is underestimated by 38%, consistent with molecular dynamics simulations of small molecules.
Multiple quantum coherence in HYSCORE spectra
Authors: Liesum Lorenz , Schweiger Arthur
Published: July 2002

The implementation of matched pulses in two-dimensional electron spin echo envelope modulation experiments results in a drastic increase in intensity of peaks representing nuclear multiple quantum coherence. For a spin system consisting of one electron spin S=12 and two nuclei with spin I=12 the nuclear coherence created by the sequence (π/2)−τ−(π/2) is calculated. The transfer amplitudes between different kinds of nuclear coherences of a microwave π pulse are given. A new matched HYSCORE pulse sequence, called SMART HYSCORE, is introduced with only one microwave pulse for the generation and detection of nuclear coherence, which does not suffer from blind spots. This sequence is applied to a single crystal and a disordered system, and it is demonstrated how the resolution of weakly coupled nuclei can be increased. Furthermore we show that information about the relative sign of hyperfine couplings and about the relative orientation of the hyperfine tensors can be derived from the ridge patterns in a HYSCORE experiment. The numerical simulations are confirmed experimentally.
Capillary condensation of periodic phases in self-assembling systems
Authors: Babin V. , Ciach A. , Tasinkevych M.
Published: July 2002

Self-assembling systems confined in slit-like pores of a width L are studied. We focus on phase transitions between uniform and ordered periodic phases. As shown by previous experimental and theoretical studies, the periodic phases respond elastically to the applied stress when the size λ of the unit cell is much larger than the size of molecules. For such phases a simple modification of the Kelvin equation for the phase coexistence in a slit is derived. The shift of the phase transition in confinement is given by two terms. The first term is the standard Kelvin equation, and the second one depends on the elastic modulus of the periodic phase. The modified Kelvin equation (MKE) is verified by explicit calculations in a lattice model for oil–water–surfactant mixtures. We show that the two terms can be comparable even for L∼10λ. While for L>5λ the MKE is obeyed very well in our model, for narrow slits we find significant deviations between actual transitions and the MKE, associated with an inelastic behavior of the periodic phase for L<5λ. We also show that in self-assembling systems a phase which in bulk is not stable for any thermodynamical state (except for a line of muliphase points) can become stable in a slit with particular external surfaces.
Physical picture for the laser hosing instability in a plasma
Authors: Ren C. , Mori W. B.
Published: July 2002

The hosing of a light beam centroid is caused by upward or downward tilting of the local wave fronts due to the transverse phase velocity difference across the wave fronts. The phase velocity gradient is caused by the plasma density perturbation, which in turn is driven by the ponderomotive force of the hosing light beam. The hosing equations in both long- and short-wavelength regimes can be heuristically derived from this physical picture.
Photodetectors fabricated from a self-assembly of a deoxyguanosine derivative
Authors: Rinaldi R. , Branca E. , Cingolani R. , Masiero S. , Spada G. P. , Gottarelli G.
Published: July 2002

A metal–semiconductor–metal (MSM) photodetector has been fabricated using as the semiconductor, a self-assembled layer of a DNA basis, namely a deoxyguanosine derivative, deposited between two gold electrodes. These were defined lithographically on a SiO2 substrate, separated by a distance of about 120 nm. The resulting self-assembled guanosine crystal has been deposited in such a way to achieve striking semiconducting properties. We show that with these conditions, the I–V characteristics are independent of the crystal orientation. The device shows a high current response (differential resistance at room temperature ranges in MΩ) which is symmetric with respect to bias sign and dependent on the illumination conditions. This behavior can be explained by taking into account the standard MSM theory and its applications as a photodetector.
Size dependence of III-nitride microdisk light-emitting diode characteristics
Authors: Jin S. X. , Shakya J. , Lin J. Y. , Jiang H. X.
Published: July 2002

Individual microdisk blue-light-emitting diodes (μ-LEDs) of varying diameters from 5 to 20 μm have been fabricated from InGaN/GaN quantum wells. Size effects on the μ-LED characteristics, including I–V and L–I characteristics, have been measured. The transient behavior of the μ-LEDs has also been studied. It was found that the turn-on time is on the order of our system response (30 ps) and the turn-off time is on the order of 0.2 ns and shows a strong size dependence. The ability of two-dimensional array integration with advantages of high speed, high resolution, low temperature sensitivity, and applicability under versatile conditions make III-nitride μ-LEDs a potential candidate for light sources in short-distance optical communications.
Magnetic and structural properties of Mn-implanted GaN
Authors: Theodoropoulou N. , Hebard A. F. , Overberg M. E. , Abernathy C. R. , Pearton S. J. , Chu S. N. G. , Wilson R. G.
Published: July 2002

High doses (1015–5×1016 cm−2) of Mn+ ions were implanted into p-GaN at ∼350 °C and annealed at 700–1000 °C. At the high end of this dose range, platelet structures of GaxMn1−xN were formed. The presence of these regions correlated with ferromagnetic behavior in the samples up to ∼250 K. At low doses, the implanted led to a buried band of defects at the end of the ion range.
On the effects of spin-orbit coupling on conical intersection seams in molecules with an odd number of electrons. I. Locating the seam
Authors: Matsika Spiridoula , Yarkony David R.
Published: July 2002

In molecules with an odd number of electrons when the spin-orbit interaction is added to the nonrelativistic Coulomb Hamiltonian the dimension of the seam of conical intersection is reduced from Nint−2 to Nint−3 or Nint−5. A generally applicable algorithm for locating points of conical intersection in such molecules is derived. The algorithm is based on a perturbative description of the vicinity of a point of conical intersection analogous to that used previously in the nonrelativistic case. It is tested using model Hamiltonians with quite promising results. An implementation of the algorithm based on ab initio wave functions is presented which treats the spin-orbit interaction within the Breit–Pauli approximation and incorporates it into the electronic Hamiltonian using the adiabatic states of the nonrelativistic Hamiltonian as a basis. An initial test of this implementation also yielded quite promising results.
Fluids of hard-spheres with two sticky thin layers, liquid–liquid transition for pure substances
Authors: Meng Shihong , Cai Jun , Liu Honglai , Hu Ying
Published: July 2002

The Ornstein–Zernike integral equation was solved with the Percus–Yevick approximation for systems containing hard spheres having two adhesive thin layers. One of them is on the surface as usual to account for the ordinary attraction between molecules. The other is located inside the hard core to mimic the collapse of structures under high pressures. Baxter’s factorization method was used to obtain the equation of state analytically. Besides the ordinary gas–liquid transition, liquid–liquid transition was also found in the region where the reduced densities are higher than one. The latter repeats qualitatively the recently discovered phenomena of high-density liquid and low-density liquid transition of pure water.
Anisotropy quantum beat in two-photon ionization of coherently excited hyperfine states of Na
Authors: Lee Young Jong , Cho Sung Haeng , Kim Nam Joon , Kim Seong Keun
Published: July 2002

A new method is proposed that utilizes the quantum beat of polarization anisotropy in two-photon ionization of atoms and molecules. It was shown that the frequency and the amplitude of the beating anisotropy contain information on, respectively, the hyperfine constants of the electronically excited state and the branching ratio of the photoionization channels leading to different photoelectron angular momentum states in the ionization continuum. The method was demonstrated for the case of the Na 3p P3/22 state by the time-resolved two-photon ionization with two pulses of parallel and perpendicular polarizations.
Real-time and in situ monitoring of ferroelectric domains during periodic electric field poling of KTiOPO4
Authors: Hellström J. , Clemens R. , Pasiskevicius V. , Karlsson H. , Laurell F.
Published: July 2002

We demonstrate a photographic method to view the inversion of ferroelectric domains during the periodic electric field poling of KTiOPO4 by taking advantage of the electro-optic effect and a high speed charge coupled device (CCD) camera. The method is noninvasive and allows monitoring in situ and in real time. The high speed CCD camera opens up the possibility to observe the dynamics of the domains during poling and to probe the samples afterwards to evaluate the grating formed in the material. This monitoring method should be applicable to other ferroelectric crystals, as well.
Electric field-induced carbon nanotube junction formation
Authors: Ho G. W. , Wee A. T. S. , Lin J.
Published: July 2002

We present experimental evidence of nanojunction structures explicitly observed after application of high electric fields on multiwall carbon nanotube arrays. The electric field as well as thermal effects result in carbon–carbon bond breaking and redeposition leading to nanojunction formation. The growth mechanism of the nanojunction is believed to be open-ended topological defect growth in which carbon atoms at two adjacent nanotube tips chemically react and fuse forming an array of nanojunctions.
Hybrid dielectric and iris-loaded periodic accelerating structure
Authors: Zou Peng , Xiao Liling , Sun Xiang , Gai Wei , Wong Thomas
Published: July 2002

One disadvantage of conventional iris-loaded accelerating structures is the high ratio of the peak surface electric field to the peak axial electric field useful for accelerating a beam. Typically this ratio Es/Ea⩾2. The high surface electric field relative to the accelerating gradient may prove to be a limitation for realizing technologies for very high gradient accelerators. In this article, we present a scheme that uses a hybrid dielectric and iris-loaded periodic structure to reduce Es/Ea to near unity, while the shunt impedance per unit length r and the quality factor Q compare favorably with conventional metallic structures. The analysis based on MAFIA simulations of such structures shows that we can lower the peak surface electric field close to the accelerating gradient while maintaining high acceleration efficiency as measured by r/Q. Numerical examples of X-band hybrid accelerating structures are given.
Small He clusters with aromatic molecules
Authors: Even U. , Al-Hroub I. , Jortner Joshua
Published: July 2002

Excitation spectra of ultracold (T=0.4 K) large aromatic molecules (naphthalene, anthracene, and tetracene) solvated by a few (1–16) He atoms are reported. Measured spectral shifts and vibronic excitations for a series of mass selected He clusters are presented. Clusters on aromatic molecules with an increasing number of rings show unexpected, and size-dependent, spectral effects. We suggest that the novel spectroscopic features are due to the anisotropic delocalization of the He atoms on the surface of the aromatic molecule.
Lithium–fluoride-modified indium tin oxide anode for enhanced carrier injection in phenyl-substituted polymer electroluminescent devices
Authors: Zhu Furong , Low Beeling , Zhang Keran , Chua Soojin
Published: July 2002

Phenyl-substituted polymer electroluminescent (EL) devices using an insulating lithium–fluoride (LiF) layer between indium tin oxide (ITO) and poly(styrene sulfonate)-doped poly(3,4-ethylene dioxythiophene) (PEDOT) hole transporting layer have been fabricated. By comparing the devices made without this layer, the results demonstrate that the former has a higher EL brightness operated at the same current density. At a given constant current density of 20 mA/cm2, the luminance and efficiency for devices with 1.5 nm LiF-coated ITO were 1600 cd/m2 and 7 cd/A. These values were 1170 cd/m2 and 5.7 cd/A, respectively, for the same devices made with only an ITO anode. The ultrathin LiF layer between ITO and PEDOT modifies the hole injection properties. A more balanced charge carrier injection due to the anode modification by an ultrathin LiF layer is used to explain this enhancement.
Detailed study of pyridine at the C 1s and N 1s ionization thresholds: The influence of the vibrational fine structure
Authors: Kolczewski C. , Püttner R. , Plashkevych O. , Ågren H. , Staemmler V. , Martins M. , Snell G. , Schlachter A. S. , Sant’Anna M. , Kaindl G. , Pettersson L. G. M.
Published: July 2002

High resolution, vibrationally resolved, near-edge x-ray absorption fine structure (NEXAFS) spectra at the C 1s and N 1s ionization thresholds of pyridine and deuterated d5-pyridine in the gas phase have been recorded. The high resolution of 65 meV (150 meV) at the C s (N 1s) ionization thresholds reveals vibrational structures in the spectra. Detailed ab initio and density functional theory (DFT) calculations were performed to interpret the experimental spectra and to assign the observed peaks. In particular we focused on the previously unexplained intensity ratio for the two components of the C 1s→1π* transition. For this transition the vibrational structure is included through a linear coupling model in the DFT calculations and leads to the experimentally observed ∼2:3 intensity ratio between the two π* components in the C 1s spectrum rather than the ∼3:2 ratio obtained without vibrational effects. After inclusion of relaxation effects in the excited states, in addition to the vibrational effects, both theoretical methods yield almost perfect agreement with experiment.
Evolution of anisotropic reflection gratings formed in holographic polymer-dispersed liquid crystals
Authors: Sutherland R. L. , Tondiglia V. P. , Natarajan L. V. , Bunning T. J.
Published: July 2002

The temporal evolution of an anisotropic reflection grating produced in a holographic polymer-dispersed liquid crystal film is investigated. We find that this type of grating is preceded in time by an isotropic concentration grating, and that the development of the anisotropic grating can be delayed until several seconds after laser exposure. The formation of an anisotropic grating is nearly coincident with the onset of phase separation of liquid crystal and implies a macroscopic ordering of liquid crystal droplet directors. Detailed knowledge of grating evolution may allow in situ control over the polarization sensitivity of the hologram.
Magnetization and magnetostriction oscillations in a superconducting 2H-NbSe2 single crystal
Authors: Eremenko V. , Sirenko V. , Shabakayeva Yu. , Schleser R. , Gammel P. L.
Published: July 2002

The results of a comparative study of the oscillating magnetostriction and absolute magnetization components in the single crystal of the layered superconducting compound 2H-NbSe2 are presented. The measurements were made in the temperature range 1.5–8 K in a magnetic field of 0–20 T with in-plane orientation (normal to the hexagonal axis). The mechanical stress derivative of the extremal cross-sectional area of the Fermi surface is derived from a thermodynamic analysis. The values obtained are considerably higher, by an order of magnitude, than those for other metals, which explains the observed enhancement of the magnetostriction oscillations in comparison with the magnetization oscillations.
Near-surface turbulence for evaporative convection at an air/water interface
Authors: Flack K. A. , Saylor J. R. , Smith G. B.
Published: July 2002

Turbulence measurements are reported for the flow beneath an air/water interface undergoing evaporative convection. Measurements were obtained using a two component laser Doppler velocimeter system. Two hydrodynamic boundary conditions were considered for the free surface: a shear free surface, which is the case when surfactants are absent, and a constant elasticity surface, created by depositing a monolayer of oleyl alcohol. The shear free boundary condition case results in significantly higher levels of near surface turbulence than the constant elasticity case. This difference between the two cases decreases with distance from the free surface. Profiles of the turbulent fluctuations were obtained for the horizontal and vertical velocity components and are compared with the somewhat analogous case of a heated solid wall.
Re-entrant ferroelectric phases in binary mixtures of ferroelectric and antiferroelectric homologues of a series with three chiral centers
Authors: Novotná Vladimı́ra , Glogarová Milada , Hamplová Věra , Kašpar Miroslav
Published: July 2002

In binary mixtures composed of homologues from a series derived from 4-alkoxybiphenyl- 4′-carboxylic acid exhibiting ferroelectric (FE) and antiferroelectric (AF) phases, a re-entrant FE phase may be induced in a certain concentration region, which is miscible with the SmC* phase. Dielectric spectroscopy study revealed a softening of the antiphase mode in the AF phase, which is responsible for the appearance of the re-entrant phase. The helix twist inversion occurs at the low temperature limit of the (AF) phase so that the helix handedness of the upper and lower temperature FE phases have opposite sense. The values of spontaneous polarization and the spontaneous tilt angle evolve continuously with the concentration, while dielectric strength exhibits a maximum for a certain concentration of homologues. All phase transitions existing in the binary mixtures are sharp and divide homogeneous phases in contrast to results reported in E. Gorecka et al. [Phys. Rev. Lett. 81 2946 (1998)], where a frustrated cluster phase exists. The reason is in different behavior of interlayer interactions at the FE-AF phase transition.
Real-space condensation in a dilute Bose gas at low temperature
Authors: Kulik I. O.
Published: July 2002

We show with a direct numerical analysis that a dilute Bose gas in an external potential—which is chosen for simplicity as a radial parabolic well—undergoes at a certain temperature Tc a phase transition to a state supporting a macroscopic fraction of particles at the origin of the phase space (r=0, p=0). Quantization of particle motion in a well wipes out the sharp transition but supports a distribution of a radial particle density ρ(r) peaked at r=0 (a real-space condensate) as well as a phase-space Wigner distribution density W(r,p) peaked at r=0 and p=0 below a crossover temperature Tc* of order of Tc. A fixed-particle-number canonical ensemble, which is a combination of the fixed-N condensate part and the fixed-μ excitation part, is suggested to resolve the difficulty of large fluctuation of the particle number (δN∼N) in the Bose-Einstein condensation problem treated within the orthodox grand canonical ensemble formalism.
Magnetocaloric effect in perovskite manganites La0.7−xNdxCa0.3MnO3 and La0.7Ca0.3MnO3
Authors: Wang Z. M. , Ni G. , Xu Q. Y. , Sang H. , Du Y. W.
Published: July 2002

In perovskite manganites La0.7−xNdxCa0.3MnO3 (x=0, 0.05, 0.1, 0.15, and 0.20) prepared by the sol–gel technique, large magnetocaloric effects have been observed. The maximum of the magnetic entropy change peaks at the magnetic ordering temperature. Compared to La0.7Ca0.3MnO3, an enhancement of the magnetocaloric effect has been obtained in the samples with Nd3+ dopant (for La0.5Nd0.2Ca0.3MnO3, the maximum of ΔSM is about −2.31 J/kg K).
Excited state polarizabilities of conjugated molecules calculated using time dependent density functional theory
Authors: Grozema F. C. , Telesca R. , Jonkman H. T. , Siebbeles L. D. A. , Snijders J. G.
Published: July 2002

In this paper, time-dependent density functional theory (TDDFT) calculations of excited state polarizabilities of conjugated molecules are presented. The increase in polarizability upon excitation was obtained by evaluating the dependence of the excitation energy on an applied static electric field. The excitation energy was found to vary quadratically with the field strength. The excess polarizabilities obtained for singlet excited states are in reasonable agreement with the experimental results for the shorter oligomers, particularly if the experimental uncertainties are considered. For longer oligomers the excess polarizability is considerably overestimated, similar to DFT calculations of ground state polarizabilities. Excess polarizabilities of triplet states were found to be smaller than those for the corresponding singlet state, which agrees with experimental results that are available for triplet polarizabilities. Negative polarizabilities are obtained for the lowest singlet Ag states of longer oligomers. The polarizability of the lowest Bu and Ag excited states of the conjugated molecules studied here are determined mainly by the interaction between these two states. Upon application of a static electric field a quadratic Stark effect is observed in which the lower Bu state has a positive excess polarizability and the upper Ag state exhibits a decrease in polarizability upon excitation. All results are explained in terms of a sum-over-states description for the polarizability.
Recombination activity of copper in silicon
Authors: Sachdeva R. , Istratov A. A. , Weber E. R.
Published: July 2002

The carrier recombination activity of copper in n-type and p-type silicon has been investigated. The minority carrier diffusion length has been found to decrease monotonically with increasing copper concentration in n Si and to exhibit a step-like behavior in p-type silicon at Cu concentrations above a certain critical level. It is suggested that the impact of copper on the minority carrier diffusion length is determined by the formation of copper precipitates. This process is retarded in perfect silicon due to the large lattice mismatch between Cu3Si and the silicon lattice and even more retarded in p Si, due to electrostatic repulsion effects between the positively charged copper precipitates and interstitial copper ions. Comparison of the impact of Cu on minority carrier diffusion length obtained with p-Si samples of different resistivity confirmed the electrostatic model. Studies of the impact of copper on minority carrier diffusion length in samples with internal gettering sites indicated that they provide heterogeneous nucleation sites for Cu precipitation at subcritical Cu concentration. Above a certain threshold of Cu concentration, the bulk recombination activity is dominated by quasihomogeneous formation of Cu precipitates, a process that is not detectably affected by the presence of oxide precipitates.
Site definition and characterization of La-substituted Bi4Ti3O12 thin films prepared by metalorganic chemical vapor deposition
Authors: Watanabe Takayuki , Funakubo Hiroshi , Mizuhira Manabu , Osada Minoru
Published: July 2002

The effects of La content on crystal structure and ferroelectric property of La-substituted Bi4−xLaxTi3O12 were investigated. We utilized as-deposited thin films prepared on (111)Pt/SiO2/Si substrates at 680 °C by metalorganic chemical vapor deposition. The films showed (117) preferred orientation, and the lattice spacing of the (117) plane increased with increasing La content. With the aid of Raman spectroscopy, we found a pronounced site selectivity of La ions for the Bi site (A site) in the pseudoperovskite layer. Even for a small content (x=0.2), an incorporation of La ion into the pseudoperovskite layer greatly enhanced both remanent polarization and coercive field, but improved properties were almost irrespective of La content for x>0.2. For (Bi3.27La0.73)Ti3O12 thin film, we confirmed a good fatigue endurance up to 2×1010 switching cycles.