bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantum magnetotransport in an n-InxGa1−xAs∕GaAs double quantum well in tilted magnetic fields
Authors: Yakunin M. V. , Arapov Yu. G. , Neverov V. N. , Podgornykh S. M. , Kharus G. I. , Shelushinina N. G. , Zvonkov B. N. , Uskova E. A.
Published: April 2007

Precision scanning of the (B⊥,B‖) plane between projections of the magnetic field perpendicular and parallel to the layers of an n-InxGa1−xAs∕GaAs (x≈0.2) double quantum well in measurements of its longitudinal magnetoresistance can reveal a number of features due to the complex energy spectrum of the double quantum well against the background of structure due to magnetic breakdown. Trajectories describing the features of the magnetoresistance on the (B⊥,B‖) plane can be described semiquantitatively on the basis of quasiclassical calculations of the quantization of the energy spectrum of the double quantum well under the influence of the perpendicular component of the field. Peaks due to magnetic breakdown are enhanced as the total value of the magnetic field increases. Their observed spin splitting corresponds to an effective value of the Landé factor ∣g*∣≈3.
Features of quantum effects in two-dimensional GaAs∕n-InGaAs∕GaAs structures with double quantum wells
Authors: Arapov Yu. G. , Yakunin M. V. , Gudina S. V. , Karskanov I. V. , Neverov V. N. , Harus G. I. , Shelushinina N. G. , Podgornykh S. M. , Zvonkov V. N. , Uskova E. A.
Published: April 2007

In an n-InxGa1−xAs∕GaAs double quantum well (x≈0.2) the temperature dependence of the longitudinal resistance ρxx(T) of a 2D electron gas with low mobility and with an electron density close to the B=0 metal–insulator transition is of an “insulator” character in the temperature interval T=1.8–70K(kBTτ∕ℏ=0.1–3.8). Anomalous temperature dependence of σxy(B,T) in the region ωcτ=1 leads to a number of features of the transition from the regime of weak localization and electron–electron interaction to the quantum Hall effect regime at low magnetic fields.
Diluted ferromagnetic semiconductors as spintronic materials
Authors: Lashkarev G. V. , Radchenko M. V. , Karpina V. A. , Sichkovskyi V. I.
Published: April 2007

A brief review of research papers on some diluted magnetic semiconductors is given. Experimental results on the study of the ferromagnetic state in the most promising materials for use in spintronics are presented.
Transport, magnetotransport, and ferromagnetism in diluted magnetic semiconductors
Authors: Kulbachinskii V. A. , Gurin P. V. , Tarasov P. M. , Davydov A. B. , Danilov Yu. A. , Vikhrova O. V.
Published: April 2007

The magnetic properties, Hall effect, and magnetoresistance are discussed for various diluted magnetic semiconductors: In1−xGaxAs quantum well structures, delta-doped with Mn; layers of InAs quantum dots in GaAs, delta-doped with Mn; GaAs structures ion-implanted with Mn and Mg; the thermoelectrics Bi2Te3⟨Fe⟩ and Sb2−xCrxTe3. We investigate the influence of the ferromagnetism that arises in all of those systems on the transport properties, Hall effect, and low-temperature negative magnetoresistance. In Bi2Te3⟨Fe⟩ and Sb2−xCrxTe3 we investigate the influence of Fe and Cr magnetic impurities on the Seebeck coefficient and thermal conductivity.
Resistive stability of 2∕1 modes near 1∕1 resonance
Authors: Brennan D. P. , Turnbull A. D. , Chu M. S. , La Haye R. J. , Lao L. L. , Osborne T. H. , Galkin S. A.
Published: April 2007

The stability of resistive modes is examined using reconstructions of experimental equilibria in the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)], revealing the important physics in mode onset as discharges evolve to instability. Experimental attempts to access the highest β in tokamak discharges, including “hybrid” discharges, are typically terminated by the growth of a large 2∕1 tearing mode. Model equilibria, based on experimental reconstructions from one of these discharges with steady state axial q0≈1, are generated varying q0 and pressure. For each equilibrium, the PEST-III code [A. Pletzer, A. Bondeson, and R. L. Dewar, J. Comput. Phys. 115, 530 (1994)] is used to determine the ideal magnetohydrodynamic solution including both tearing and interchange parities. This outer region solution must be matched to the resistive inner layer solutions at the rational surface to determine resistive mode stability. From this analysis it is found that the approach to q=1 simultaneously causes the 2∕1 mode to become unstable and the nonresonant 1∕1 displacement to become large, as the ideal β limit rapidly decreases toward the experimental value. However, the 2∕2 harmonic on axis, which is also large and is coupled to the saturated steady state 3∕2 mode, is thought to contribute to the current drive sustaining q0 above 1 in these hybrid discharges. Thus, the approach to the q=1 resonance is self-limiting in this context. This work suggests that sustaining q0 slightly above 1 will avoid the 2∕1 instability and will allow access to significantly higher β values in these discharges.
A study on spin wave resonance in patterned trilayer films
Authors: Zhai Y. , Zhang D. , Zhang Y. , Shi J. , Wong P. K. J. , Niu D. X. , Li G. D. , Xu Y. B. , Zhai H. R.
Published: April 2007

Patterned magnetic thin films of NiFeCo(3nm)∕Cu(3nm)∕NiFeCo(6nm) spin valve structure with arrays of rectangular elements of micron and submicron sizes have been studied by ferromagnetic resonance. All the rectangular elements have the same aspect ratio of 12 but with different sizes. A set of evenly spaced peaks was observed when the magnetic field was applied near the film normal. The resonant fields of these peaks are in linear proportion with the peak number, and the separation of resonant fields between peaks are approximately inversely proportional to the width of the elements, which suggests that spin waves are excited in the film plane and along the short edge of the elements.
Powder neutron diffraction evidence for enhanced inter plane magnetic coupling in La1.2Sr1.8Mn2−xRuxO7 layered manganites
Authors: Sundar Manoharan S. , Singh Brajendra , Sahu R. K.
Published: April 2007

Rotation of Mn spin moments along the c axis enhances interplane magnetic coupling, leading to a steep increase in the Curie temperature (TC) and metal to insulator transition (TMIT) in Ru substituted two dimensional layered manganites, La1.2Sr1.8Mn2−xRuxO7. The temperature dependent neutron diffraction data show the absence of (004) reflection, indicating that the Mn spin moments are not aligned in the a-b plane, and rather the increase in intensity of (110) reflection at 12K shows that the MnO2 layers are stacked ferromagnetically along the c axis. The presence of Ru at the Mn site in pentavalent state assures the required hole carrier density for a sustained double-exchange interaction. The cumulative effect of rotation of Mn spin moments and the variable valence states of Ru lead to a pronounced increase in TC up to ∼50K, which is not observed hitherto in any of the B-site substituted manganites.
Local investigation of magnetism at R and In sites in RNiIn (R=Gd, Tb, Dy, Ho) compounds
Authors: Lapolli A. L. , Saxena R. N. , Mestnik-Filho J. , Leite D. M. T. , Carbonari A. W.
Published: April 2007

The local magnetism at R and In sites in RNiIn (R=Gd, Tb, Dy, Ho) compounds was investigated by measuring hyperfine interactions via perturbed gamma-gamma angular correlation (PAC) technique using La140–Ce140 and In111–Cd111 probe nuclei. PAC measurements carried out below respective TC for each compound showed a combined electric quadrupole plus magnetic dipole interaction for Cd111 probe at In sites and a pure magnetic interaction for Ce140 at R sites. The temperature dependence of the magnetic hyperfine field (Bhf) shows that, whereas the Bhf values at rare-earth sites measured with Ce140 drop to zero at temperatures around the expected TC for each compound, the respective fields at In sites measured with Cd111 drop to zero at lower temperatures. The difference between the temperatures at which Bhf is zero for Ce140 and Cd111 probes when compared with TC of each compound shows that this difference decreases with decreasing TC values. The results are discussed in terms of the Ruderman-Kittel-Kasuya-Yosida model for magnetic interactions and the crystalline electric field.
Stress dependence and effect of plastic deformation on magnetic hysteresis and anhysteretic magnetization of FeNi32% films
Authors: Finkel P. , Lofland S.
Published: April 2007

The magnetic hysteresis and anhysteretic magnetization of FeNi32% films were investigated as a function of isotropic stress. The magnetostriction contribution to dc magnetization under elastic stress and the effect of the plastic strain on the hysteresis loops are discussed. Also, a role of the plastic deformation interrelated with the elastic stress in the magnetization process is established. An experimental system based on a conventional vibrating sample magnetometer equipped with a specially designed loading fixture and optical resonant spectroscopy tension monitoring technique are used to measure anhysteretic permeability and magnetization curve as a function of stress. Measurements of magnetostriction as a function of magnetic field were shown to be also possible using this fixture. Stresses are deduced from the characteristic resonant frequency of the sample in the fixture and verified via pulse propagation velocity measurement. Both indirect stress measurements are contactless, relying on remote vibration measurement using a laser Doppler vibrometer. Uniaxial stresses up to 1GPa can be applied for samples down to 50μm specimens. Anhysteretic permeability was extracted from the anhysteretic B-H curves constructed by degaussing the sample at the given longitudinal (parallel to the stresses) dc field. The large positive magnetostriction constant leads to higher susceptibility and lower coercivity with tensile stress while the large volume magnetostriction results in reduced saturation magnetization. Large stresses imposed on the sample result in plastic strain of the sample which induces increase in dislocation density and domain wall pinning. This causes the gain in hysteresis loss and coercivity to increase at the highest stresses.
Converse magnetoelectric effect in three-phase composites of piezoceramic, metal cap, and magnet
Authors: Wong Wai Yin , Or Siu Wing , Chan Helen Lai Wa , Jia Yanmin , Luo Haosu
Published: April 2007

We report experimentally and theoretically the converse magnetoelectric (CME) effect in a three-phase piezoceramic-metal-cap-magnet composite made by sandwiching a thickness-polarized Pb(ZrxTi1−x)O3 (PZT) disk between two truncated conical brass caps and two thickness-magnetized SmCo disks. The reported CME effect originates from the product of the converse piezoelectric effect in the PZT disk, the mechanical transformation/amplification effect in the brass caps, and the magnetic induction effect in the SmCo disks. The composite exhibits a large CME coefficient (αB) in excess of 2mG∕V with a flat response in the broad frequency range of 0.1–100kHz. The measured magnetic flux density shows an extremely linear relationship to the applied voltage with amplitude varying from 10to100V over a wide range of detection distance of 0.7–6.5mm. This electromechanomagnetically coupled effect enables applications of the composite in coil-free magnetic flux control devices, featuring smaller Joule heating loss, wider operational bandwidth, and greater property-tailorable flexibility compared to conventional electromagnet-based devices.
Nucleation and propagation of domains walls in a Co∕Pt multilayer wire
Authors: San Emeterio Alvarez L. , Burnell G. , Marrows C. H. , Wang K.-Y. , Blackburn A. M. , Williams D. A.
Published: April 2007

The domain wall processes in Hall bar devices patterned from Co∕Pt multilayers with perpendicular magnetic anisotropy have been studied by Kerr microscopy and extraordinary Hall effect measurements. The samples are extremely thin (<2nm) so that they show full remanence and a square hysteresis loop with a coercive field of ∼25Oe. The Kerr microscope observations of the as-patterned Hall bars have shown an uncontrolled domain wall nucleation followed by rapid propagation, without significant pinning. This shows that the nucleation field exceeds any propagation and pinning fields in these samples. Controlled domain wall nucleation by irradiation of a selected area of the Co∕Pt multilayer structure with different doses of Ga+ ions on the multilayers has been studied proving the decrease of coercivity in such irradiated areas with respect to the rest of the sample. This method can be used to lower the nucleation field below any pinning fields that exist in the sample, enabling controlled wall positioning within the Hall bar in the future.
General orbital invariant MP2-F12 theory
Authors: Werner Hans-Joachim , Adler Thomas B. , Manby Frederick R.
Published: April 2007

A general form of orbital invariant explicitly correlated second-order closed-shell Møller-Plesset perturbation theory (MP2-F12) is derived, and compact working equations are presented. Many-electron integrals are avoided by resolution of the identity (RI) approximations using the complementary auxiliary basis set approach. A hierarchy of well defined levels of approximation is introduced, differing from the exact theory by the neglect of terms involving matrix elements over the Fock operator. The most accurate method is denoted as MP2-F12/3B. This assumes only that Fock matrix elements between occupied orbitals and orbitals outside the auxiliary basis set are negligible. For the chosen ansatz for the first-order wave function this is exact if the auxiliary basis is complete. In the next lower approximation it is assumed that the occupied orbital space is closed under action of the Fock operator [generalized Brillouin condition (GBC)]; this is equivalent to approximation 2B of Klopper and Samson [J. Chem. Phys. 116, 6397 (2002)]. Further approximations can be introduced by assuming the extended Brillouin condition (EBC) or by neglecting certain terms involving the exchange operator. A new approximation MP2-F12/3C, which is closely related to the MP2-R12/C method recently proposed by Kedžuch et al. [Int. J. Quantum Chem. 105, 929 (2005)] is described. In the limit of a complete RI basis this method is equivalent to MP2-F12/3B. The effect of the various approximations (GBC, EBC, and exchange) is tested by studying the convergence of the correlation energies with respect to the atomic orbital and auxiliary basis sets for 21molecules. The accuracy of relative energies is demonstrated for 16 chemical reactions. Approximation 3C is found to perform equally well as the computationally more demanding approximation 3B. The reaction energies obtained with smaller basis sets are found to be most accurate if the orbital-variant diagonal Ansatz combined with localized orbitals is used for the first-order wave function. This unexpected result is attributed to geminal basis set superposition errors present in the formally more rigorous orbital invariant methods.
Behavior of implanted strontium in yttria-stabilized zirconia
Authors: Zhu S. , Wang S. X. , Wang L. M. , Ewing R. C. , Zu X. T.
Published: April 2007

Yttria-stabilized zirconia (YSZ) is one of the promising candidates for application in the inert matrix fuel. This letter presents the effects of fission product strontium incorporation in the YSZ microstructure. The strontium ions were implanted at room temperature to a dose of 1×1017ions∕cm2 and that was followed by subsequently annealing at 1000°C for 1–3h. Transmission electron microscopy (TEM) was utilized to characterize the microstructure evolution after the implantation and/or annealing. No amorphization was observed in the YSZ matrix with a damage level of 200dpa (displacements per atom). A layer of nanoscale strontium zirconate perovskite (SrZrO3) precipitates formed in strontium-ion implanted YSZ single crystals after the annealing. Cross-sectional TEM revealed the crystalline particles of ∼15nm in size and they are crystallographically highly oriented with the YSZ matrix. The orientation relationship between the matrix and the precipitates has been determined as ⟨011⟩YSZ‖⟨111⟩SrZrO3, {200}YSZ‖{110}SrZrO3. The interface is composed of misfit dislocations with b=1∕2⟨200⟩YSZ.
A note on the effective slip properties for microchannel flows with ultrahydrophobic surfaces
Authors: Sbragaglia M. , Prosperetti A.
Published: April 2007

A type of superhydrophobic surface consists of a solid plane boundary with an array of grooves which, due to the effect of surface tension, prevent a complete wetting of the wall. The effect is greatest when the grooves are aligned with the flow. The pressure difference between the liquid and the gas in the grooves causes a curvature of the liquid surface resisted by surface tension. The effects of this surface deformation are studied in this paper. The corrections to the effective slip length produced by the curvature are analyzed theoretically and a comparison with available data and related mathematical models is presented.
Surface microstructure of GeSbSe chalcogenide thin films grown at oblique angle
Authors: Martín-Palma R. J. , Ryan J. V. , Pantano C. G.
Published: April 2007

Surface analysis of GeSbSe chalcogenide thin films grown at normal incidence and at four different oblique evaporation angles (45°, 75°, 80°, and 85°) was performed by a combination of scanning electron microscopy and atomic force microscopy. Additionally, quantitative roughness and microstructural analyses of the GeSbSe chalcogenide thin films were performed. Increasing roughness and surface area for increasingly oblique evaporation angle were observed, following an exponential relationship in both cases. Two-dimensional power spectral density analysis further supported the exponential behavior of the surface characteristic feature size. The notable increase of roughness and surface area with increasing evaporation oblique angle can have a significant effect on the further development of optical sensors.
Mask-edge defects in hybrid orientation direct-Si-bonded substrates recrystallized by solid phase epitaxy after patterned amorphization
Authors: Saenger K. L. , Fogel K. E. , Ott J. A. , Sung C. Y. , Sadana D. K. , Yin H.
Published: April 2007

Solid phase epitaxy (SPE) of patterned amorphized Si regions in direct-Si-bonded (DSB) hybrid orientation substrates is complicated by the fact that the amorphized Si regions being recrystallized have sides and bases formed from different Si crystals. In DSB wafers with a Si (011) DSB layer on a Si (001) handle wafer, the competition between lateral and vertical SPE produces distinctively angled mask-edge defects and recrystallization fronts. Some mask edges exhibit triangular bands of defective Si bounded by DSB and handle wafer {111} planes meeting at 90° angles, while other mask edges are relatively defect free, but can have downward-pointing facets of Si (011) growing below the DSB interface. A simple model recently developed to explain trench-edge defect formation and faceted recrystallization in bulk single-orientation Si appears to explain all of these observations.
Stark and Rashba effects in GaN nanowires
Authors: Zhang X. W. , Xia J. B.
Published: April 2007

The effects of an external electric field on the electronic structure of GaN nanowires, as well as GaAs nanowires for comparison, are investigated theoretically. It is found that there is an anti-crossing effect in GaN nanowires caused by a small electric field, the hole energy levels, hole wave functions, and optical oscillator strengths change dramatically when the radius (R) is around a critical radius (Rc), while this effect is absent in GaAs nanowires. When R is slightly smaller than Rc, the highest hole states are optically dark in the absence of the electric field, and a small electric field can change them to be optically bright, due to the coupling of hole states brought by the field. The Rashba spin-orbit effect is also studied. The electron Rashba coefficient α increases linearly with the electric field. While the hole Rashba coefficients β do not increase linearly, but have complicated relationships with the electric field.
Wave propagation in orthotropic microtubules
Authors: Qian X. S. , Zhang J. Q. , Ru C. Q.
Published: April 2007

For various cellular functions of microtubules, wave propagation along microtubules is one of the issues of major concern. In this article, general behavior of wave propagation in microtubules is examined based on an orthotropic elastic shell model, with particular emphasis on the role of strongly anisotropic elastic properties of microtubules. Strong anisotropy of microtubules is found to substantially lower both torsional and radial wave velocities, although it does not affect longitudinal wave velocity. In many cases, it is found that one of three wave velocities in orthotropic microtubules depends on wave vector nonmonotonically, and reaches a minimum velocity around a specific value of the wave vector. In particular, this interesting phenomenon would not exist if microtubules were isotropic. In addition, transverse bending waves of orthotropic microtubules always correspond to the lowest wave velocity, and can be determined by the (isotropic) elastic beam model provided the wavelength is long enough. Many of the results obtained in the present article have been absent from the literature on wave propagation in microtubules.
Fluctuation-induced tunneling in TiO2-derived nanotube pellets
Authors: Filipič Cene , Levstik Adrijan , Kutnjak Zdravko , Umek Polona , Arčon Denis
Published: April 2007

The frequency-dependent conductivity of TiO2-derived nanotubes pressed to a pellet was measured over the temperature range 100 to 390 K. The temperature dependence of the electrical conductivity measured at 100 Hz indicates a three-dimensional variable range hopping mechanism at higher temperatures and fluctuation-induced tunneling conduction below 300 K. From the frequency dependence of the conductivity it is possible to conclude that the conductivity is governed by two parallel channels. The first channel, dominating at higher temperatures, is characterized by the three-dimensional variable range hopping mechanism and the second channel, which takes over at lower temperatures, by the fluctuation-induced tunneling mechanism. Such a two-channel mechanism may be responsible for the similar temperature dependence of the electrical conductivity observed in some other three-dimensional systems.
Thermoelectric properties of layered perovskite-type (Sr1−xCax)3(Ti1−yNby)2O7
Authors: Lee Kyu Hyoung , Kim Sung Wng , Ohta Hiromichi , Koumoto Kunihito
Published: April 2007

The thermoelectric properties and crystallographic features of layered perovskite-type Ca-substituted (Sr1−xCax)3(Ti1−yNby)2O7 (y=0–0.2), Ruddlesden-Popper phases were investigated. The influence of crystal structure and carrier concentration on the carrier effective mass for Ti-based metal oxides based on TiO6 octahedra and having a significant Ca substitution effect are discussed. It was found from structural analysis that the large Seebeck coefficients of Ca-substituted compounds can be attributed to the high values of carrier effective mass, which originates from the enhancement of the symmetry of the TiO6 octahedra. Low thermal conductivities, 3.6–3.8 W m−1 K−1 at 300 K, were obtained for Ca-substituted compositions due to phonon scattering at SrO∕(SrTiO3)2 interfaces of the inherent superlattice structure and to mass-defect phonon scattering between Ca (MCa=40) and Sr (MSr=88). The highest ZT value obtained was 0.15 at 1000 K, for 10 at % Ca-substituted Sr3(Ti0.95Nb0.05)2O7.
Crystal-to-crystal phase transition in α-quaterthiophene: An optical and structural study
Authors: Campione M. , Tavazzi S. , Moret M. , Porzio W.
Published: April 2007

At 191°C the organic semiconductor α-quaterthiophene is demonstrated to undergo a phase transition from the low temperature to the high temperature polymorph. Calorimetry, x-ray diffraction, optical reflectivity, and ellipsometry measurements were employed to analyze this transition and to show that this can be thermally induced on single crystals preserving the single crystalline nature of the sample over large domains.
Emission of acoustic waves by nonlinear drop oscillations
Authors: Alabuzhev A. A. , Shklyaev S.
Published: April 2007

The paper deals with the calculation of the emission of monopole and dipole modes of acoustic waves in a gas generated by nonlinear drop oscillations. Assuming that the density of the gas is much smaller than that of the liquid, we show that the amplitudes of these two modes strongly depend on the wave number. It is demonstrated that no monopole radiation exists in the long-wave limit. A comparison with a series of recent studies is provided.
Ultrasonic investigation of the relaxation time and the dynamic, relaxed, and unrelaxed elastic moduli in ZnSe:Cr
Authors: Gudkov V. V. , Lonchakov A. T. , Sokolov V. I. , Zhevstovskikh I. V.
Published: April 2007

The temperature dependence of the phase velocity and absorption of ultrasound propagating along the [110] axis in a ZnSe crystal doped with Cr2+ ions is investigated in the frequency interval 32–158MHz. It is found that the absorption peak and anomalies of the phase velocity observed in the temperature interval 10–20K are of a relaxation nature. The temperature dependence of the relaxation time and the relaxed (isothermal) and unrelaxed (adiabatic) elastic moduli Cl=12(C11+C12+2C44) are recovered from the data. Oscillations of the amplitude and phase of the signal are observed at 32MHz. An interpretation of this phenomenon is proposed.
Statistical model analysis of local structure of quaternary sphalerite crystals
Authors: Robouch B. V. , Marcelli A. , Guidi M. Cestelli , Kisiel A. , Sheregii E. M. , Polit J. , Cebulski J. , Piccinini M. , Mycielski A. , Ivanov-Omskii V. I. , Sciesiñska E. , Sciesiñski J. , Burattini E.
Published: April 2007

At the 2004 Ural International Winter School, we introduced the statistical strained tetrahedron model and discussed ternary tetrahedron structured crystals. The model allows one to interpret x-ray absorption fine structure (EXAFS) data and extract quantitative information on ion site occupation preferences and on the size and shape of each elemental constituent of the configuration tetrahedra. Here we extend the model to cover quaternary sphalerite crystal structures. We discuss the two topologically different quaternary sphalerite systems: the pseudo-balanced A1−xBxYyZ1−y (2:2 cation:anion ratio), and the unbalanced AxBx′C1−x−x′Z or AXyYy′Z1−y−y′ (3:1 or 1:3 cation:anion ratios) truly quaternary alloy systems. These structural differences cause preference values to vary with the relative contents in pseudo-quaternaries but to remain constant in truly quaternary compounds. We give equations to determine preference coefficient values from EXAFS or phonon spectra and to extract nearest-neighbor inter-ion distances by EXAFS spectroscopy. The procedure is illustrated and tested on CdMnSeTe, GaInAsSb, and ZnCdHgTe quaternary alloys.
Hyperfine interactions at the lanthanum ion nucleus in LaMnO3
Authors: Agzamova P. A. , Leskova Yu. V. , Nikiforov A. E. , Gonchar’ L. E. , Popov S. E.
Published: April 2007

The influence of the orbital and magnetic structures on the hyperfine interaction of the nuclear moment of the lanthanum ions with the magnetic subsystem of manganese ions in LaMnO3 is investigated. The temperature dependence of the nuclear magnetic resonance spectrum in a polycrystalline sample in an external magnetic field is constructed. It is shown that the governing role is played by the anisotropic hyperfine interaction due to the polarization of the outer p-shell electrons of lanthanum. The proposed model can give a correct description of the experimental data.
Antiferromagnetic resonance in LaMnO3
Authors: Mozhegorov A. A. , Gonchar’ L. E. , Nikiforov A. E.
Published: April 2007

The magnetic structure of LaMnO3 and the antiferromagnetic resonance frequencies in it are investigated as functions of magnetic field H and temperature T. A previously developed model that includes the orbitally dependent superexchange and single-ion anisotropy is used. The temperature dependences described in the framework of the mean field approximation are in good agreement with experiment. The H–T phase diagrams are constructed for magnetic field directed along the principal crystallographic directions (a,b,c). The problem of the origin of the weak noncollinearity of the Mn3+ spins is solved. An optimal method of obtaining the expression for the magnetic free energy is proposed.
Electrical resistance and magnetic properties of double-doped ceramics La1.85−4∕3xSr0.15+4∕3xCu1−xMnxO4
Authors: Tkach A. V. , Ponomarev A. I. , Charikova T. B. , Tashlykov A. O. , Kozhevnikov V. L.
Published: April 2007

Resistive and magnetic measurements are made on La1.85−4∕3xSr0.15+4∕3xCu1−xMnxO4 ceramics with x=0.02, 0.04, 0.06, and 0.08. It is found that at these concentrations the temperature of the beginning and end of the superconducting transition depend weakly on the concentration x. The resistive properties exhibit a combination of metallic conductivity (or superconductivity) along the grains with hopping transport between grains. At low temperatures the superconductivity apparently coexists with two different magnetic phases.
Electron–electron interaction effects in multiwall carbon nanotubes
Authors: Tkachev E. N. , Romanenko A. I. , Anikeeva O. V. , Kuznetsov V. L. , Usol’tseva A. N.
Published: April 2007

The temperature dependence of the electrical conductivity σ and the field dependence of the magnetoresistivity ρ of samples of multiwall catalytic carbon nanotubes containing a small amount of amorphous carbon impurity. Below a temperature of 20K the observed temperature dependence of the conductivity of the nanotubes shows a contribution from two-dimensional quantum corrections to the conductivity. At a temperature of 4.2K one observes negative magnetoresistance on the ρ(B) curves in the field interval 0–10kG, due to the dominance of the contribution to the magnetoresistance from the quantum corrections for interacting electrons. A special synthesis technique is used to obtain multiwall catalytic carbon nanotubes that are practically free of amorphous carbon impurities, making it possible to observe electron–electron interaction effects unnoticeable in previously synthesized nanotubes of this kind.
Impedance of Sn24P19.3BrxI8−x semiconducting clathrates
Authors: Yakimchuk A. V. , Zaikina J. V. , Reshetova L. N. , Ryabova L. I. , Khokhlov D. R. , Shevelkov A. V.
Published: April 2007

The temperature and frequency dependences of the real and imaginary parts of the complex impedance of ceramic samples of the clathrates of variable composition Sn24P19.3BrxI8−x are investigated in the frequency range from 20 to 106Hz at temperatures from 4.2 to 200K. The conductivity of the samples is measured in the dc regime at temperatures from 4.2 to 300K; it is characterized by the presence of an activation part, with energy increasing monotonically from 18 to 77meV as the bromine concentration increases. The impedance spectra are investigated by a graphoanalytical method. As an approximating equivalent circuit, an RC circuit with a frequency-dependent capacitance is considered. It is shown that the capacitive contribution to the admittance falls off sharply with decreasing temperature below 75K and with increasing frequency above 150kHz. The observed dielectric anomalies are attributed to a substantial contribution to the complex impedance from the grain boundaries.
Electrical properties of the chalcogenides AgGeAsS3xSe3(1−x) (0.1≤x≤0.9)
Authors: Kheifets O. L. , Babushkin A. N. , Shabashova O. A. , Melnikova N. V.
Published: April 2007

The chalcogenides AgGeAsS3xSe3(1−x) (x=0.9–0.9) are synthesized and their electrical properties are investigated at low temperatures. The synthesized materials, which are ionic conductors, are studied by the method of impedance spectroscopy. Temperature intervals are found in which the conductivity and dielectric constant of the materials exhibit singular behavior.
Physical working principles of semiconductor disk lasers
Authors: Averkiev N. S. , Sherstnev V. V. , Monakhov A. M. , Grebenshikova E. A. , Kislyakova A. Yu. , Yakovlev Yu. P. , Krier A. , Wright D. A.
Published: April 2007

The physical working principles of whispering gallery mode semiconductor disk lasers are examined. A new method is proposed for measuring the single-pass gain. The theory of cylindrical waveguides is considered. The spectral characteristics of whispering gallery mode lasers is investigated experimentally in the temperature range 4–300K. It is shown that lasers of this kind are very promising as potential laser radiation sources operating at room temperature.
Plasma wave resonant detection of terahertz radiations by nanometric transistors
Authors: Knap W. , El Fatimy A. , Torres J. , Teppe F. , Orlov M. , Gavrilenko V.
Published: April 2007

We report on resonant terahertz detection by the two-dimensional electron plasma in nanometric InGaAs and GaN transistors. Up to now, the majority of research has been devoted to GaAs-based devices as the most promising from the point of view of the electron mobility. However, resonant detection has been reported only in the sub-THz range. According to the predictions of the Dyakonov–Shur plasma wave detection theory, an increase of the detection frequency can be achieved by reducing the length or increasing the carrier density in the gated region. We demonstrate that the 1THz limit can be overcome by using ultimately short-gate InGaAs and GaN nanotransistors. For the first time the tunability of the resonant signal by the applied gate voltage is demonstrated. We show that the physical mechanism of the detection is related to the plasma waves excited in the transistor channel (Dyakonov–Shur theory). We also show that increasing of the drain-to-source current leads to a transformation of the broadband detection to a resonant and tuneable one. We can get resonant detection at room temperature. We finally discuss the possible application of detection by nanotransistors in different types of THz spectroscopy research.
Non-Hamiltonian molecular dynamics implementation of the Gibbs ensemble method. II. Molecular liquid-vapor results for carbon dioxide
Authors: Bratschi Christoph , Huber Hanspeter , Searles Debra J.
Published: April 2007

The Gibbs ensemble molecular dynamics algorithm introduced in the preceding paper (paper I) [C. Bratschi and H. Huber, J. Chem. Phys. v126, 164104 (2007)] is applied to two recently published CO2 ab initio pair potentials, the Bock-Bich-Vogel and symmetry-adapted perturbation theory site-site potentials. The critical properties of these potentials are calculated for the first time. Critical values and points in the single and two-phase zones are compared with Monte Carlo results to demonstrate the accuracy of the molecular dynamics algorithm, and are compared with experiment to test the accuracy of the potentials. Pressure calculations in the liquid, gas, and supercritical states are carried out and are used to explain potential-related effects and systematic discrepancies. The best ab initio potential yields results in good agreement with experiment.
Simplified and improved string method for computing the minimum energy paths in barrier-crossing events
Authors: E Weinan , Ren Weiqing , Vanden-Eijnden Eric
Published: April 2007

We present a simplified and improved version of the string method, originally proposed by E et al. [Phys. Rev. B 66, 052301 (2002)] for identifying the minimum energy paths in barrier-crossing events. In this new version, the step of projecting the potential force to the direction normal to the string is eliminated and the full potential force is used in the evolution of the string. This not only simplifies the numerical procedure, but also makes the method more stable and accurate. We discuss the algorithmic details of the improved string method, analyze its stability, accuracy and efficiency, and illustrate it via numerical examples. We also show how the string method can be combined with the climbing image technique for the accurate calculation of saddle points and we present another algorithm for the accurate calculation of the unstable directions at the saddle points.
Transition state theory for laser-driven reactions
Authors: Kawai Shinnosuke , Bandrauk André D. , Jaffé Charles , Bartsch Thomas , Palacián Jesús , Uzer T.
Published: April 2007

Recent developments in transition state theory brought about by dynamical systems theory are extended to time-dependent systems such as laser-driven reactions. Using time-dependent normal form theory, the authors construct a reaction coordinate with regular dynamics inside the transition region. The conservation of the associated action enables one to extract time-dependent invariant manifolds that act as separatrices between reactive and nonreactive trajectories and thus make it possible to predict the ultimate fate of a trajectory. They illustrate the power of our approach on a driven Hénon-Heiles system, which serves as a simple example of a reactive system with several open channels. The present generalization of transition state theory to driven systems will allow one to study processes such as the control of chemical reactions through laser pulses.
Spectroscopy of cyanodiacetylene in solid argon and the photochemical generation of isocyanodiacetylene
Authors: Coupeaud Anne , Turowski Michał , Gronowski Marcin , Piétri Nathalie , Couturier-Tamburelli Isabelle , Kołos Robert , Aycard Jean-Pierre
Published: April 2007

Following the measurements of UV and mid-IR spectra of cyanodiacetylene, H–(CC)2–CN, isolated in low temperature Ar matrices, the first photochemical study on this compound and on its H2 isotopomer was carried out with the laser light tuned to 267nm and with far-UV discharge lamps. Evidence for the formation of isocyanodiacetylene, H–(CC)2–NC, was found in infrared absorption spectra interpreted with the aid of available theoretical predictions.
Rotationally resolved vacuum ultraviolet pulsed field ionization-photoelectron vibrational bands for HD+(XΣg+2,v+=–20)
Authors: Stimson S. , Evans M. , Hsu C.-W. , Ng C. Y.
Published: April 2007

The authors have obtained rotationally resolved vacuum ultraviolet pulsed field ionization-photoelectron (vuv-PFI-PE) spectrum of HD in the photon energy range of 15.29–18.11eV, covering the ionization transitions HD+(XΣg+2,v+=0–21,N+)←HD(XΣg+1,v″=0,J″). The assignment of rotational transitions resolved in the vuv-PFI-PE vibrational bands for HD+(XΣg+2,v+=0–20) and their simulation using the Buckingham-Orr-Sichel (BOS) model are presented. Rotational branches corresponding to the ΔN=N+-J″=0, ±1, ±2, ±3, and ±4 transitions are observed in the vuv-PFI-PE spectrum of HD. The BOS simulation shows that the perturbation of vuv-PFI-PE rotational line intensities due to near resonance autoionization is very minor at v+⩾5 and decreases as v+ is increased. Thus, the rotationally resolved PFI-PE bands for HD+(v+⩾5) presented here provide reliable estimates of state-to-state cross sections for direct photoionization of HD, while the rotationally resolved PFI-PE bands for HD+(v+<5) are useful data for fundamental understanding of the near resonance autoionizing mechanism. On the basis of the rovibrational assignment of the vuv-PFI-PE bands, the ionization energies for the formation of HD+(XΣg+2,v+=0–20,N+) from HD(XΣg+1,v″=0,J″) and the vibrational constants (ωe, ωeχe, ωeye, and ωeze), the rotational constants (Be and αe), the vibrational energy spacings, and the dissociation energy for HD+(XΣg+2) are determined. As expected, these values are found to be in excellent agreement with high level theoretical predictions.
Insight into the Rydberg states of CH
Authors: Vázquez G. J. , Amero J. M. , Liebermann H. P. , Buenker R. J. , Lefebvre-Brion H.
Published: April 2007

Ab initio electronic structure calculations of a relatively large number of Rydberg states of the CH radical were carried out employing the multireference single and double excitation configuration interaction (MRD-CI) method. A Gaussian basis set of cc-pV5Z quality augmented with 12 diffuse functions was used together with an extensive treatment of electron correlation. The main focus of this contribution is to investigate the 3d Rydberg complex assigned by Watson [Astrophys. J. 555, 472 (2001)] to three unidentified interstellar bands. The authors’ calculations reproduce quite well the absolute excitation energies of the three components of the 3d complex, i.e., Σ+2(3dσ), Π2(3dπ), and Δ2(3dδ), but not the energy ordering inferred from a rotational assignment of the 3d←XΠ2 laboratory spectrum. The computation of the 4d complex is reported for the first time along with a number of other higher lying Rydberg species with an XΣ+1 core. The lowest Rydberg states belonging to series converging to the aΠ3 and AΠ1 excited states of CH+ are also calculated.
Correlation and nuclear distortion effects of Cr-substituted ZnSe
Authors: Tablero C.
Published: April 2007

There is a great deal of interest in the effect of the correlation and effect of the atomic distortion in materials with a metallic intermediate band. This band, situated within the semiconductor band gaps, would be split, thus creating two bands, a full one below the Fermi energy and an empty one above it, i.e., a metal-insulator transition. This basic electronic band structure corresponds to intermediate band materials and is characteristic of transparent-conducting oxides, up and down converters, and intermediate band solar cells. A sufficiently high density of Cr in ZnSe substituting the Zn atoms leads to a microscopic intermediate band, in which these effects will be analyzed. A Hubbard term has been included to improve the description of the many-body effect. This term modifies the bandwidth of the intermediate band, the Fermi energy, and breaks the orbital-occupation degeneracy. From the results, the intermediate band is not split within the range of Hubbard term values analyzed and for Cr substituting Zn from 0.463% to 3.125% of Cr atomic concentration.
Anharmonic force field of cis- and trans-formic acid from high-level ab initio calculations, and analysis of resonance polyads
Authors: Demaison J. , Herman M. , Liévin J.
Published: April 2007

The quadratic, cubic, and semidiagonal quartic force fields of cis- and trans-formic acid have been calculated using three different levels of theory. They all give satisfactory results, including the one at the lowest level of theory which is the MP2 method employing a basis set of triple-ζ quality. The results are used to theoretically analyze resonance polyads, including the one involving the 41, 51, 61, 81, 7191, and 92 vibrational states. A semiexperimental equilibrium structure is derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. The ab initio structure calculated at the CCSD(T) level of theory using a basis set of quintuple-ζ quality is in excellent agreement with the semiexperimental structure.
Noncontact simultaneous dual wavelength photoplethysmography: A further step toward noncontact pulse oximetry
Authors: Humphreys Kenneth , Ward Tomas , Markham Charles
Published: April 2007

We present a camera-based device capable of capturing two photoplethysmographic (PPG) signals at two different wavelengths simultaneously, in a remote noncontact manner. The system comprises a complementary metal-oxide semiconductor camera and dual wavelength array of light emitting diodes (760 and 880nm). By alternately illuminating a region of tissue with each wavelength of light, and detecting the backscattered photons with the camera at a rate of 16frames∕wavelengths, two multiplexed PPG wave forms are simultaneously captured. This process is the basis of pulse oximetry, and we describe how, with the inclusion of a calibration procedure, this system could be used as a noncontact pulse oximeter to measure arterial oxygen saturation (SpO2) remotely. Results from an experiment on ten subjects, exhibiting normal SpO2 readings, that demonstrate the instrument’s ability to capture signals from a range of subjects under realistic lighting and environmental conditions are presented. We compare the signals captured by the noncontact system to a conventional PPG signal captured concurrently from a finger, and show by means of a J. Bland and D. Altman [Lancet 327, 307 (1986); Statistician 32, 307 (1983)] test, the noncontact device to be comparable to a contact device as a monitor of heart rate. We highlight some considerations that should be made when using camera-based “integrative” sampling methods and demonstrate through simulation, the suitability of the captured PPG signals for application of existing pulse oximetry calibration procedures.
Influence of the chain stiffness on the thermodynamics of a Gō-type model for protein folding
Authors: Prieto Lidia , Rey Antonio
Published: April 2007

The relative importance of local and long range interactions in the characteristics of the protein folding process has long been a matter of controversy. Computer simulations based on Gō-type models have been widely used to study this topic, but without much agreement on which type of interactions is more relevant for the foldability of a protein. In this work, the authors also employ a topology-based potential and simulation model to analyze the influence of local and long range interactions on the thermodynamics of the folding transition. The former are mainly used to control the degree of flexibility (or stiffness) of the chain, mostly appreciable in the unfolded (noncompact) state. Our results show the different effects that local and nonlocal interactions have on the entropy and the energy of the system. This implies that a balance between both types of interactions is required, so that a free energy barrier exists between the native and the denatured states. The variations in the contribution of both types of interactions have also a direct effect on the stability of the chain conformations, including the possible appearance of thermodynamic folding intermediates.
Scanning probe imaging of coexistent ferromagnetism and ferroelectricity at room temperature
Authors: Palkar V. R. , Purandare S. C. , Gohil S. , John J. , Bhattacharya S.
Published: April 2007

Room temperature coexistence of ferromagnetism and ferroelectricity in a thin film of nominal composition PbTi0.5Fe0.5O3−δ is probed by standard ferroelectric and ferromagnetic hysteresis loop measurements and by scanning probe microscopy. Both magnetic domains and ferroelectric domains are observed in the same spatial region of the material, implying phase coexistence in this system. Sample morphology strongly affects the roughness of the domain walls for both order parameters.
Effects of channel-length scaling on In2O3 nanowire field effect transistors studied by conducting atomic force microscopy
Authors: Jo Gunho , Maeng Jongsun , Kim Tae-Wook , Hong Woong-Ki , Jo Minseok , Hwang Hyunsang , Lee Takhee
Published: April 2007

Scaling effects of In2O3 nanowire field effect transistors (FETs) were examined as a function of channel length. The channel length was varied from 1μmto20nm by placing a conducting atomic force microscope tip on the In2O3 nanowire as a movable contact. The In2O3 nanowire FET exhibited a variety of channel-length dependent transfer characteristics in terms of the source-drain current, transconductance, threshold voltage, and mobility. Furthermore, the authors were able to extract the contact resistance and distinguish between apparent mobility and intrinsic mobility. The latter was corrected, taking into account the non-negligible contact resistance for short channel devices.