Unusually large shift in low-temperature susceptibility peak of Fe2.8Ti0.2O4 with frequency

The temperature dependence of magnetic susceptibility,x, of titanomagnetites (Fe_3−x Ti _x O₄, 0⩽x⩽0.2) at different frequencies in the range of 13 Hz to 117 kHz is reported. Whereas the position of thex-peak of Fe₃O₄ at 120 K shows no significant shift over this frequency range, that of Fe_2.8Ti_0.2O₄ shifts from 70 K at 13 Hz to 115 K at 117 kHz. This seems to be the first observation of such large shift in the position of ax-peak observed at low-temperatures is attributed to the occurrence of an isotropic point (temperature at which first order anisotropy constantK ₁=0) associated with change in crystal structure. In such a case a shift inx-peak with frequency is not expected normally. Thus, the observed large shift in the position ofx-peak with frequency is an unusual phenomenon and some of its repercussions are discussed.     

Investigation of three-terminal voltage-controlled switching devices prepared by molecular beam epitaxy

Three-terminal GaAs switching devices prepared by molecular beam epitaxy using p ⁺-n ^–-(p ⁺)-n ^–-n ⁺ structures are fabricated. The effects of the third-electrode position and the possible voltage-controlled operation on the device performance are discussed. Concepts are proposed to obtain new and improved voltage-controlled properties. The internal barrier of one proposed structure can be modulated directly and is found to be effective for the studied structures. The position of the third-electrode is found to affect the electrical properties profoundly due to different dominant mechanisms. Comparisions are made by defining a control efficiency. Due to the idea of varying the gate position, a conceptual understanding of such a set of results would enhance our understanding of the physics of bulk barrier devices in general.     

The analysis of rovibrational motion equations of a diatomic molecule in the linear regime

The motion equations of diatomic molecules are here extended from the absolute vibrational case to a more general and real rotational and vibrational (rovibrational) case. The rovibrational Hamiltonian is heuristically formed by substituting the respective number and angular momentum operators for the vibrational and rotational quantum numbers in the energy eigenvalues of a diatomic molecule which was first introduced by Dunham. The motion equations of observable quantities such as the position and linear momentum are then determined by implementing the well-known Heisenberg relation in quantum mechanics. We face with the second-order imaginary differential equations for describing the temporal variations of the relative position and the linear momentum of two oscillating atoms, which are coupled in the x–y horizontal plane. The possible rovibrational oscillations are distinguished by the three quantum numbers n, l and m associated with the energy and angular momentum quantities. It is finally demonstrated that the simultaneous solutions of rovibrational equations satisfy the energy conservation during all quantised oscillations of a diatomic molecule in space.     

Exponential Mixing and Time Scales¶in Quantized Hyperbolic Maps on the Torus

We study the behaviour, in the simultaneous limits , of the Husimi and Wigner distributions of initial coherent states and position eigenstates, evolved under the quantized hyperbolic toral automorphisms and the quantized baker map. We show how the exponential mixing of the underlying dynamics manifests itself in those quantities on time scales logarithmic in . The phase space distributions of the coherent states, evolved under either of those dynamics, are shown to equidistribute on the torus in the limit , for times t between and , where γ is the Lyapounov exponent of the classical system. For times shorter than , they remain concentrated on the classical trajectory of the center of the coherent state. The behaviour of the phase space distributions of evolved position eigenstates, on the other hand, is not the same for the quantized automorphisms as for the baker map. In the first case, they equidistribute provided t→∞ as , and as long as t is shorter than . In the second case, they remain localized on the evolved initial position at all such times. Received: 1 October 1999/Accepted: 4 January 2000     

Variance and covariance in quantum mechanics and the spreading of position probability

A rigorous discussion of the concept of expectation value of an unbounded observable is given, and of its variance. It is shown that ifA andB are observables for which the expectations <A ²> and <B ²> exist, and such that A+B is also an observable for some real numbers and , neither of which vanishes, then a quantum mechanical analog of covariance and correlation coefficient can be defined. The quadratic variation with time of the variance of position of a particle moving freely in one dimension is deduced rigorously, assuming only that there is a time at which the variances of position and momentum exist.     

Nonlocal potential and optical properties of solids

Summary The theory of the RPA optical response of a solid has been generalized in order to take into account also the possible presence of spatially nonlocal potentials in the Hamiltonian. Explicit expressions for first- and second-order susceptibilities are given in the new framework. The expressions obtained depend on the matrix elements of operators of the form of a commutator of a component of the position operatorr and an operator that commutes with the lattice translations. The problem of the evaluation of these matrix elements is solved in a simple manner by introducing an auxiliary, periodic position operator,XXXr. In such a way a general formulation is obtained that preserves the gauge invariance. As an application of the new theory, the second harmonic generation (SHG) from a semiconductor in a simple two-band model has been studied. The differences between our correct gauge-invariant results and those obtained in the usual local approximation is an indication of a slow convergence of the expressions obtained in the local approximation.     

Paramagnetic Centres in Be-Doped MgO Single Crystals

Using a variation of arc-fusion technique, Be-doped MgO single crystals were grown, in which about 0.01% of the cation sites are occupied by Be^2?. This gives rise to a variety of Be-containing paramagnetic centres, easily detectable by EPR. The models of the centres are proposed and the values of their spin-Hamiltonian parameters are determined and discussed. Two of them—V_OH-Be and H-Be centres—stem from well-known paramagnetic centres such as V_OH, and interstitial H atom. In addition, because of the non-central position of the ion, an isolated Be^2? can trap a hole forming a Be^2?O^? centre. The symmetry of the Be^2?O^? centre at T<30K is rhombic, at a higher temperature a motional averaging of the spectrum takes place. It is shown that unusually for V centres physical properties of the V_OH-Be centre (a relatively small g-factor anisotropy and high thermal stability, optical absorption energy and spin-lattice relaxation time) arc caused by the non-central position of the Be^2? ion.     

采用密集波分复用技术的光纤水听器阵列研究

利用密集波分复用和时分多路复用技术相结合的大规模阵列结构,以Mach-Zehnder干涉型光纤水听器为例,分析了采用相位产生载波技术的频分多路复用,提出了密集波分复用技术在干涉型光纤水听器阵列应用的新方法,给出了复用体系结构,并分析了其在工程上可行性.     

Influence of the spin-orbit couplingon nuclear superfluidity along the N = Z line

We show that the spin-orbit potential of the nuclear mean field destroys isoscalar superfluid correlations in self-conjugate nuclei. Using group theory and boson mapping techniques on a Hamiltonian including single particle splittings and a SO _ST(8) pairing interaction, we give analytical expressions for the spin-orbit dependence of some N = Z properties such as the relative position of T = 0 and T = 1 states in odd-odd systems or double binding-energy differences of even-even nuclei. Received: 12 April 2000 / Accepted: 25 May 2000     

A Hamiltonian yielding damped motion in an homogeneous magnetic field: quantum treatment

In earlier work, a Hamiltonian describing the classical motion of a particle moving in two dimensions under the combined influence of a perpendicular magnetic field and of a damping force proportional to the particle velocity, was indicated. Here we derive the quantum propagator for the Hamiltonian in different representations, one corresponding to momentum space, the other to position, and the third to a natural choice of “velocity” variables. We call attention to the following noteworthy fact: the Hamiltonian contains three parameters which do not in any way influence the motion of the position of the particle. However, at the quantum level, the propagator, even in the position representation, depends in an intricate way on these classically irrelevant parameters. This creates considerable doubt as to the validity of such a quantization procedure, as the physical results predicted differ for various Hamiltonians, all of which describe the dissipative dynamics equally well.     

Stopping cross sections of 50- to 230-keV nitrogen ions in silicon measured by ion backscattering spectroscopy

The energy dependence of the total stopping cross section of 50- to 230-keV nitrogen ions in silicon (σ _S (E)) is measured in order to develop the diagnostics of heavy impurities in films of a nanometer thickness by heavy ion backscattering (HIBS) spectroscopy. At ion energies lower than 150 keV, this σ _S (E) dependence occupies an intermediate position between the dependences given in the SRIM and MSTAR data-bases; at higher energies, our dependence is closer to the former dependence. The estimation of the effect of inelastic processes on the stopping cross section demonstrates that the effect of these processes for nitrogen ions can be neglected when heavy impurities in such films are studied by HIBS.     

On symmetries and averaging of the G-equation for premixed combustion

It is demonstrated that the G-equation for premixed combustion admits a diversity of symmetries properties, i.e. invariance characteristics under certain transformations. Included are those of classical mechanics such as Galilean invariance, rotation invariance and others. Also a new generalized scaling symmetry has been established. It is shown that the generalized scaling symmetry defines the physical property of the G-equation precisely. That is to say the value of G at a given flame front is arbitrary. It is proven that beside the symmetries of classical mechanics, particularly the generalized scaling symmetry uniquely defines the basic structure of the G-equation. It is also proven that the generalized scaling symmetry precludes the application of classical Reynolds ensemble averaging usually employed in statistical turbulence theory in order to avoid non-unique statistical quantities such as for the mean flame position. Finally, a new averaging scheme of the G-field is presented which is fully consistent with all symmetries of the G-equation. Equations for the mean G-field and flame brush thickness are derived and a route to consistent invariant modelling of other quantities derived from the G-field is illustrated. Examples of statistical quantities derived from the G-field both in the context of Reynolds-averaged models as well as subgrid-scale models for large-eddy simulations taken from the literature are investigated as to whether they are compatible with the important generalized scaling symmetry.     

Information-theoretical complexity for the hydrogenic abstraction reaction

In this work, we have investigated the complexity of the hydrogenic abstraction reaction by means of information functionals such as disequilibrium (D), exponential entropy (L), Fisher information (I), power entropy (J) and joint information-theoretic measures, i.e. the I–D, D–L and I–J planes and the Fisher–Shannon and López–Mancini–Calbet (LMC) shape complexities. The analysis of the information-theoretical functionals of the one-particle density was computed in position (r) and momentum (p) space. The analysis revealed that all of the chemically significant regions can be identified from the information functionals and most of the information-theoretical planes, i.e. the reactant/product regions (R/P), the transition state (TS), including those that are not present in the energy profile such as the bond cleavage energy region (BCER), and the bond breaking/forming regions (B–B/F). The analysis of the complexities shows that, in position as well as in the joint space, the energy profile of the abstraction reaction bears the same information-theoretical features as the LMC and FS measures. We discuss why most of the chemical features of interest, namely the BCER and B–B/F, are lost in the energy profile and that they are only revealed when particular information-theoretical aspects of localizability (L or J), uniformity (D) and disorder (I) are considered.     

Investigation of the effect of plasma waves excitation on target normal sheath ion acceleration using fs laser‐irradiating hydrogenated structures

Measurements of ion acceleration in polymethylmethacrylate foils covered by a thin copper film irradiated by fs laser in target normal sheath acceleration regime are presented. The ion acceleration depends on the laser parameters, such as the pulse energy; depends on the irradiation conditions, such as the focal point position of the laser with respect to the target surface; and depends on the target properties, such as the metallic film thickness. The proton acceleration increases in the presence of the metallic film enhancing the plasma electron density, reaching about 1.6 MeV energy for a focal position on the target surface. The plasma diagnostics uses SiC detectors, absorber foils, Faraday cups, and gafchromic films. Employing p‐polarized laser light and a suitable oblique incidence, it is possible to increase the proton acceleration up to about 2.0 MeV thanks to the effects of laser absorption resonance due to plasma waves excitation.     

On the validity of the amphoteric-defect model in gallium arsenide and a criterion for Fermi-level pinning by defects

Using the theoretically calculated point-defect total-energy values of Baraff and Schlüter in GaAs, anamphoteric-defect model has been proposed by Walukiewicz to explain a large number of experimental results. The suggested amphoteric-defect system consists of two point-defect species capable of transforming into each other: the doubly negatively charged Ga vacancyV _Ga ^2– and the triply positively charged defect complex (AS_Ga+V _As)³⁺, with As_Ga being the antisite defect of an As atom occupying a Ga site andV _As being an As vacancy. When present in sufficiently high concentrations, the amphoteric defect systemV _Ga ^2– /(As_Ga+V _As)³⁺ is supposed to be able to pin the GaAs Fermi level at approximately theE _v +0.6 eV level position, which requires that the net free energy of theV _Ga/(As_Ga+V _As) defect system to be minimum at the same Fermi-level position. We have carried out a quantitative study of the net energy of this defect system in accordance with the individual point-defect total-energy results of Baraff and Schlüter, and found that the minimum net defect-system-energy position is located at about theE _v +1.2 eV level position instead of the neededE _v +0.6 eV position. Therefore, the validity of the amphoteric-defect model is in doubt. We have proposed a simple criterion for determining the Fermi-level pinning position in the deeper part of the GaAs band gap due to two oppositely charged point-defect species, which should be useful in the future.     

<Emphasis Type="Italic">X-Y</Emphasis> photoelectric sensors based on homogeneous conducting semiconductor films

The distributions of the dark voltage and photovoltage over the surface of a conducting semiconductor disk (film) with current is calculated analytically. For a photodetector with five ohmic contacts arranged in an appropriate pattern at the circumference of the disk, a relationship between the output signal, current, and local illumination position is found. Experimental data indicate the feasibility of using such photodetectors as X-Y optical sensors.     

光电探测器阵列光敏元宽度对谱线峰值位置影响的研究

本文针对采用光电阵列探测器进行信号采集的光谱分析系统,分析了探测器阵列光敏元宽度对光谱峰值定位的影响.指出由于光敏元具有一定宽度,导致非对称型光谱峰值的亚象元定位误差,并提出用反卷积消除这种误差.     

Density of stationary points in a high dimensional random energy landscape and the onset of glassy behavior

The density of stationary points and minima of a N ≫ 1 dimensional Gaussian energy landscape has been calculated. It is used to show that the point of zero-temperature replica symmetry breaking in the equilibrium statistical mechanics of a particle placed in such a landscape in a spherical box of size L = R √N corresponds to the onset of exponential in N growth of the cumulative number of stationary points, but not necessarily the minima. For finite temperatures, a simple variational upper bound on the true free energy of the R = ∞ version of the problem has been constructed and it has been shown that this approximation can recover the position of the whole de-Almeida-Thouless line. The text was submitted by the authors in English.     

Carbon K‐edge spectra of carbonate minerals

Carbon K‐edge X‐ray spectroscopy has been applied to the study of a wide range of organic samples, from polymers and coals to interstellar dust particles. Identification of carbonaceous materials within these samples is accomplished by the pattern of resonances in the 280–320 eV energy region. Carbonate minerals are often encountered in the study of natural samples, and have been identified by a distinctive resonance at 290.3 eV. Here C K‐edge and Ca L‐edge spectra from a range of carbonate minerals are presented. Although all carbonates exhibit a sharp 290 eV resonance, both the precise position of this resonance and the positions of other resonances vary among minerals. The relative strengths of the different carbonate resonances also vary with crystal orientation to the linearly polarized X‐ray beam. Intriguingly, several carbonate minerals also exhibit a strong 288.6 eV resonance, consistent with the position of a carbonyl resonance rather than carbonate. Calcite and aragonite, although indistinguishable spectrally at the C K‐edge, exhibited significantly different spectra at the Ca L‐edge. The distinctive spectral fingerprints of carbonates provide an identification tool, allowing for the examination of such processes as carbon sequestration in minerals, Mn substitution in marine calcium carbonates (dolomitization) and serpentinization of basalts.     

On the determination of the long-range form of the interatomic potential from quasi-bound levels

Calculations have been made on the quasi-bound (QB) vibrational levels of BeH and HgH to test the hypothesis that the position of such levels provides an accurate method of determining the form of the long range part of the interatomic potential and the dissociation energy. It is found that if the potential is required to join the spectroscopic RKR potential at some point r _sp then it is only for low J values that the positions of the QB levels are sensitive to the long range part of the potential. It is concluded that the available data on BeH and HgH does not allow an accurate estimate of the long range potential.