退火对a-SixC1-x:H薄膜带尾态的影响

本文研究了退火后的a-Si_xC_1-x:H薄膜的光致发光光谱,采用发光光谱的峰值能量与测量温度之间的关系判别带尾宽度的大小。实验表明,发光峰值能量随退火温度的升高而移向低能。用指数带尾模型计算了带尾宽度。结果表明,随着退火温度的上升带尾变小。文中对这些结果进行了讨论。 关键词：     

中国近50年气候破纪录温度事件发生概率分析

随着全球变暖趋势的不断增强,破纪录温度事件（record breaking temperature events, RBTE）发生的频次呈现不断增加的特点.基于已知当前年份的温度记录,未来五年内这个记录被打破的概率是多少？即未来五年内发生新的RBTE的概率是多少？这是目前国内外学术界关注的前沿科学问题之一.文中基于概率统计模型研究在不同情景下RBTE发生概率,并与实际观测资料检测结果作对比分析,研究近50年中国RBTE发生概率的变化与全球增暖的关系.结果显示,RBTE发生概率的大小依赖于温度的增长趋势和相 关键词： 破纪录温度事件 全球增暖 极端事件     

Analysis of the absorption coefficient in carbon implanted Nd:YVO<Subscript>4</Subscript> by an annealing process

This work reports the behaviour of the absorption coefficient in a C²⁺-implanted Nd:YVO₄ crystal by successive annealing steps. The absorption coefficient was monitored with respect to time and temperature after an annealing process. Firstly, the annealing duration increased in a constant form keeping a constant temperature and secondly, the annealing temperature was increased with equal time intervals. The results indicate that the variation of the absorption coefficient has an exponential behaviour.     

Anomalous magnetic relaxation in thin Pd<Subscript>0.99</Subscript>Fe<Subscript>0.01</Subscript> films

The magnetic relaxation in Pd_0.99Fe_0.01 films, which have the thicknesses that are practically important for cryoelectronics (25 and 40 nm), is detected and experimentally studied. The relaxation is shown to be substantial only in thin films. The magnetization relaxation is found to be well described by the sum of two exponential functions with characteristic times that differ by an order of magnitude from each other. The characteristic relaxation time and the ratio of the contributions of two relaxations depend on temperature. The activation energies of the relaxation processes are determined. The activation volume is shown to correspond to a 20-nm ferromagnetic cluster. The results obtained agree with the model of two-component magnetization in thin PdFe films [6].     

The temperature dependence of the symmetric exchange interaction by optical phonon modulation of the exchange integral

By assuming an anharmonic intermolecular potential for lattice displacement and an exponential form for the exchange energy, the exchange interaction is shown to be temperature dependent in the paramagnetic region. Bond strengths are taken from known tabulated results and overlap integrals are calculated with Slater-type orbitals so there are no adjustable parameters. Agreement with experimental results on K₂CuCl₄ · 2H₂O and other layered compounds is shown.     

Magnetic anisotropy-induced spin-reorientation in spinel FeCr2S4

In this paper, the low-field magnetic behavior of polycrystalline FeCr₂S₄ was investigated by thermal circling the sample from 5 to 200 K at a constant magnetic field. We show that, upon cooling the sample at 50 Oe to a temperature below the Curie temperature then warming back, the magnetization displays irreversibility between cooling and warming sequence. The irreversible behavior was suppressed gradually with increasing magnetic field. By considering spin-reorientation due to the increase of magnetic anisotropy upon cooling, the irreversible behavior has been explained qualitatively.     

Untersuchung der Lumineszenzabklingung von Wolframatleuchtstoffen nach Anregung mit UV-Licht und Elektronen

The decay of the luminescence of CaWO₄, CdWO₄, MgWO₄, and ZnWO₄ after excitation with short pulses of light and electron beams has been investigated at different temperatures. Excitation with UV-light yields an exponential decay, and from the temperature dependence of the decay time it can be concluded that at high temperatures non-radiative transitions are prevalent. After excitation with electrons, there are additional effects by the heating up of the “excitation channel”. In this case the decay is not purely exponential and thermoluminescence is observed. This indicates the existence of traps, which are assumed to be located in the centers of the luminescence.     

气候变化中高温破纪录事件的蒙特卡罗模拟研究

采用蒙特卡罗方法,分析了南京市1961年—2000年40?a日观测温度资料中的高温破纪录事件的统计规律,并比较了20世纪全球变暖和南京市近40年来区域变暖背景对其统计规律的影响. 理论分析和蒙特卡罗模拟结果均表明：近40年来南京市第ｋ个高温破纪录事件的最概然发生强度与k（k=1,2,3,\:）呈线性增长,而年发生高温破纪录事件的频率随时间t呈1/（t+1）的衰减趋势,且平均温度高的年份,发生高温破纪录事件的概率较大,反之,概率较小. 结果还表明：20世纪的全球变暖速率（ｖ=0.006?℃/a）和南京市区域的变暖速率（ｖ=0.017?℃/a）在短期内还不至于引起高温破纪录事件的发生强度和发生频率有明显变化,但持续变暖最终将会使年发生高温破纪录事件的频率渐渐地收敛于一个常数,近似等于变暖速率的值. 此外,还研究了日温度之间的自相关和方差变化对高温破纪录事件的影响,研究发现异方差和弱的自相关对高温破纪录事件的发生强度和概率的影响基本可以忽略. 关键词： 高温破纪录事件 蒙特卡罗模拟 全球变暖     

Two-phonon scattering with zero-point vibrations of atoms

We discuss a method of calculating two-phonon thermal diffusion scattering (I₂) in monatomic cubic crystals for temperatures below the Debye temperature. The quantity I₂ was calculated and measured for an Mo single crystal over a wide region of the reciprocal space at different temperatures. It is shown that good agreement between the calculated and experimental results can be obtained only when the quantum correction to the phonon energy is included.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 82–86, February, 1989.     

Temperature-dependent modifications in the surface composition of Finemet: A ToF-SIMS study

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was employed as an in situ tool to study the temperature-induced alteration of the surface composition of amorphous Finemet, Fe₇₃Si_15.8B_7.2Cu₁Nb₃. Temperature was changed reversibly by cooling from room temperature to 118 K and warming back to room temperature. As a general result, the ion intensities and, consequently, the surface concentrations of the alloy constituents were found to vary non-monotonously. Therefore segregation processes were in operation the extent of which was element-specific. Most importantly, while cycling the temperature hysteresis behaviour was observed with concentration of Fe developing just opposite to that of the alloying elements. Accordingly, on cooling the alloy, the surface enrichment with B, Si, Nb, Cu attained first a maximum in the range of 248-193 K before the segregation changed the trend to establish appreciable depletion of these elements at 118 K (as compared to room temperature). By contrast, the surface iron content developed inversely and decreased first to a minimum at ∼223 K before reaching enrichment at 118 K. During warming, a maximum segregation of boron and silicon was observed at about 223 K - similar as on cooling - so that this temperature can be considered characteristic of the segregation process. Dissociative adsorption of water from the residual atmosphere occurring at low temperatures was responsible for the formation of surface hydroxides of iron, silicon and niobium; an enhanced adsorption of molecular water was observed at temperatures below 153 K. The temperature-dependent segregation and adsorption-desorption processes were found to be largely reversible, so that the surface composition of Finemet was practically restored after finishing the cooling-warming cycle. The processes and factors governing the non-monotonous temperature dependence of the surface segregation in the amorphous alloy are discussed within the frame of segregation theory and the influence of temperature-induced tensile stress on segregation.     

Strong field effects in MOS systems containing compensated donors and traps distributed in energy

The current-field temperature relationships of the Ta-Ta₂O₅-MnO₂ system are studied when the tantalum has positive polarity. It is demonstrated that the conductivity of such systems is determined by the Poole-Frenkel effect (PFE) for partially compensated donors. We compare our results with previous results [9]. A theory of the PFE is developed for a model which includes traps with an exponential energy distribution; acceptors, and deep discrete donors.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 3–6, October, 1981.     

Electric-field-controlled photoluminescence of CdSe nanocrystal-doped SiO2 on Si

Dense-packed CdSe nanoclusters synthesized by sequential ion implantation of Cd⁺ and Se⁺ in thermally grown SiO₂ are subjected to high electric field strengths in a metal oxide semiconductor (MOS) structure. The nanocrystal-containing device shows efficient CdSe band-edge photoluminescence (PL) when excited by a cw-HeCd laser operating at a wavelength of 442 nm at room temperature. An effective PL quenching and enhancement has been observed. Depth-resolved μ-PL measurements reveal an exponential decrease, which is depth-correlated with a layer of nanoparticles near the surface, whereas the optical non-linearity of the PL increases in parallel. The PL spectra and particle size distribution suggest an energy transfer from the nanoscopic to adjacent large particles. It can be concluded from these results that charge injection into the near-surface region of the nanocluster/SiO₂ system might be the reason for the asymmetric and hysteretic electro-optic response.     

Electron paramagnetic resonance of Yb<Superscript>3+</Superscript> ions in a concentrated YbRh<Subscript>2</Subscript>Si<Subscript>2</Subscript> compound with heavy fermions

The EPR signal from localized ytterbium ions was observed in an undoped YbRh₂Si₂ compound with heavy fermions in the temperature range from 1.5 to 25 K. The exponential contribution dominating the temperature dependence of EPR line width at temperatures above 15 K was shown to be caused by the random transitions from the ground to the first excited Stark sublevel of the Yb³⁺(4f¹³) ion with the activation energy Δ=115 K.     

Influence of molecular motions on NQR echoes

The influence of thermal molecular motions on spin echo decay in pure nuclear quadrupole resonance (NQR) is considered. Our calculations show that the Hahn echo decay is caused by dipole-dipole interaction of the nuclear spins and is strongly affected by molecular mobility that can lead to the shortening of the echo decay with increased temperature. Slow molecular motion yields an exponential τ₃ time dependence, while fast motion yields an exponential decay. The outlined theory allows us to explain an unusual shortening of the³⁵Cl NQR echo decay on heating in thiourea-C₂Cl₆ inclusion compound.     

Bias voltage dependence of tunneling magnetoresistance in granular C60–Co films with current-perpendicular-to-plane geometry

Voltage-dependence of the tunneling magnetoresistance effect in the granular C₆₀–Co films has been investigated for the samples with the current-perpendicular-to-plane geometry. The transport measurements under this geometry demonstrate that the granular C₆₀–Co films show an unusual exponential bias voltage dependence of the magnetoresistance ratio down to zero voltage. Small characteristic energies of less than 10's meV are derived from the temperature dependences of the characteristic voltage in the exponential relationship. Considering the magnitudes of the voltage drop between Co nanoparticles and also the effect of cotunneling on the energy values, the characteristic energies for the voltage-induced degradation of the spin polarization are found to show a satisfactory agreement with that for the thermally-induced one. It can be reasonably expected that the onset of magnetic disorder to the localized d-electron spins at the interface region of the C₆₀-based matrix (C₆₀–Co compound) with Co nanoparticles leading to the unusual voltage and temperature dependence of the magnetoresistance ratio and the spin polarization at low temperatures.     

Energie- und Winkelverteilung sekundärer Photonen bei der Wechselwirkung eines 6,2 GeV-Gammastrahls mit komplexen Kernen

The energy spectra of secondary photons from the interaction of a 6.2 GeV bremsstrahlung beam with targets of H₂, Be, C, Al and Cu are measured for laboratory angles from 35° to 120°. All spectra exhibit an exponential decrease of the photon yield with increasing momentum. Difference measurements with energies of the bremsstrahlung beam from 3.5 to 6.2 GeV for a center of mass angle of 90° are compared with the thermodynamical model of Fermi and the reaction temperature is determined to be 83MeV±8%. The results agree approximately with the measurements of p-Be-collisions at small laboratory angles.     

SO2 cooling and detection of the greenhouse effect

Summary Greenhouse effect calculations normally take into account the greenhouse gases’s (CO₂ mainly) warming effect. Only recently the inclusion of the cooling effect due to different gases (SO₂ mainly) has been suggested. Zecca and Brusa have proposed that the historical evolution of SO₂ production might have caused the apparent global cooling in the period 1940–1970 and might still now hinder the detection of greenhouse warming. In this paper it is shown that the SO₂ hypothesis could lead to a definite detection of the greenhouse warming provided that the globally averaged minimum daily and maximum daily temperatures were studied.     

Magnetization modification by superconductivity in Nb/Ni80Fe20/Nb trilayers

We present experimental evidences for magnetization modification by superconductivity in a series of Nb/Ni₈₀Fe₂₀/Nb trilayers. By monitoring the magnetization in a zero field as a function of temperature, we observed an irreversibility in magnetization between the cooling and warming branches just above the superconducting transition temperature T_c. These results suggest that the magnetization of the ferromagnetic Ni₈₀Fe₂₀ layer is reduced by the mutual interactions between the ferromagnet and superconductor. Moreover, this effect diminishes with increasing thickness of the Ni₈₀Fe₂₀ layer, which indicates that the interaction between the superconducting and magnetic layers occurs mainly at the vicinity of the interfaces.     

Theoretical and experimental study on the photoluminescence in BaTiO3 amorphous thin films prepared by the chemical route

A polymeric precursor method was used to synthesize BaTiO₃ amorphous thin film processed at low temperature. The luminescence spectra of BaTiO₃ amorphous thin films at room temperature revealed an intense single-emission band in the visible region. The visible emission band was found to be dependent of the thermal treatment history. Photoluminescence (PL) properties for different annealing temperatures were investigated. It was concluded that the intensity of PL is strongly dependent on both the heat treatment of the films and the presence of an inorganic disordered phase. Experimental optical absorption measurements showed the presence of a tail. These results are interpreted by the nature of these exponential optical edges and tails, associated with defects promoted by the disordered structure of the amorphous material. We discuss the nature of visible PL at room temperature in amorphous barium titanate in the light of the results of recent experimental and quantum mechanical theoretical studies. Our investigation of the electronic structure involved the use of first-principle molecular calculations to simulate the variation of the electronic structure in the barium titanate crystalline phase, which is known to have a direct band gap, and we also made an in-depth examination of amorphous barium titanate.     

Enhancement of thermopower in the highT c superconductor YBa2Cu3O7 and related compounds

The absolute thermopower of single phase YBa₂Cu₃O₇ and Y_0.8Er_0.2Ba₂Cu₃O₇ has been measured in the range 250 K to the superconducting transition temperature. It is found that these compounds show a large enhancement of thermopower in the range 150 K down toT _c. This enhancement shows a steep exponential drop as the temperature increases from the transition temperature. The temperature variation of the enhancement is too steep to be accounted for by electron-phonon or electron-local structural excitation mechanisms.