Full recovery of electron damage in glass at ambient temperatures

An unusually complete recovery of extensive electron-beam-induced damage in a thin film of a CaO-Al2O3-SiO2 glass was discovered. Nanoscale measurements show that the Ca ions migrate about 10 nm away during irradiation and return during recovery. Oxygen atoms are trapped largely as molecular oxygen and do not migrate. Electron energy loss measurements demonstrate that the glass returns completely to the original compositional and structural state thus indicating that the glass is in a deep thermodynamic energy minimum.     

Out-of-equilibrium kondo effect in double quantum dots

We present results of high resolution experiments on kinetic roughening of slow combustion fronts in paper, focusing on short length and time scales. Using three different grades of paper, we find that the combustion fronts show apparent spatial and temporal multiscaling at short scales. The scaling exponents decrease as a function of the order of the corresponding correlation functions. The noise affecting the fronts reveals short range temporal and spatial correlations, and non-Gaussian noise amplitudes. Our results imply that the overall behavior of slow combustion fronts cannot be explained by standard theories of kinetic roughening.     

Ultrasonic characterization of ceramic fibres at ambient and elevated temperatures

A non-destructive laser-generated ultrasonic inspection system has been developed to evaluate the elastic properties of ceramic fibres. The approach uses a pulsed Nd:YAG laser to excite ultrasonic signals in fibres. The signal is detected by a piezoelectric acoustic emission transducer to obtain the appropriate frequency response suitable for an elastically one-dimensional sample. By using a differential time-of-flight system, a very accurate measure of the velocity can be obtained in the fibre, with a total scatter of less than 0.5%. This approach has been used to investigate the Young's modulus of polycrystalline carbon and boron fibres as a function of stress. Both types of fibres were found to have a Young's modulus increase as greater applied loads were imposed. The carbon and boron fibres, along with silicon carbide fibres, were evaluated at elevated temperatures up to 700 °C. The carbon fibres were found to have an immediate decrease in the Young's modulus as the temperature was increased, due to oxidation of the carbon. The Young's modulus of the boron fibres decreased only at temperatures higher than 200 °C, probably the result of a microstructural transformation or relaxation. The silicon carbide fibres were found to have no significant change in the elastic properties up to 700 °C. The ultrasonic technique was also applied to polycrystalline alumina fibres and fibre tows between ambient temperature and 1200 °C in a specially designed furnace. Using this technique, it was possible to distinguish the changes in the elasticity of the alumina fibres as they were processed into -alumina. The change in the Young's modulus was readily apparent during phase transformations to -alumina. In addition, the ultrasonic velocity can be used to infer information concerning any coatings that were applied to the alumina fibres. This can be used to aid in the quantification of the coating thickness and uniformity. The application of the ultrasonic inspection system has demonstrated the ability to determine rapidly and non-destructively the elastic properties in ceramic fibres. The information gained from the measurements can be used as a quality assurance technique, or can be modified to be a real-time process control/process monitoring system.     

Sensing characteristics of a solid-state ammonia sensor at ambient temperatures

The sensing characteristics of a solid-state gas sensor using a proton conductor (antimonic acid) to detect small amounts of ammonia in air at room temperature have been investigated. The sensor consists of four-probes with two outer platinum electrodes and two inner silver probes. Under the short-circuit conditions, the potential difference between the silver probes exhibits a logarithmic variation with ammonia concentration 50 ppm to 5030 ppm) in air. The most-likely sensing mechanism is also proposed.     

Characteristics of DC discharge in a wire-cylinder configuration at high ambient temperatures

In the present work, the characteristics of direct-current (DC) discharge in a wire-cylinder configuration at an ambient temperature range of 350–850 °C were studied by analyzing photographs of the discharging process and the corresponding V–I characteristics, with the aim of facilitating the application of plasma technology in the fields of energy and the environment. The influences of the ambient temperature, the inter-electrode gap, the gas medium and the cathode material on the DC discharge were investigated. The corona-onset threshold voltage (COTV) and the spark-breakdown threshold voltage (SBTV) decrease as the ambient temperature increases, and the SBTV decreases more rapidly. Increasing the inter-electrode gap enlarges the difference between the SBTV and the COTV. After spark breakdown, in an air atmosphere, glow discharge is more likely to take place under conditions of high ambient temperatures or small inter-electrode gaps. The values of the SBTV in different atmospheres have the following relation: air ≈ O₂ > CO₂. At an ambient temperature range of 350–850 °C and in an atmosphere of N₂, glow discharge and arc discharge occur successively as the output voltage of the power supply is increased, while in an atmosphere of O₂ and CO₂, only corona and arc discharge are successively observed. In an air atmosphere, when the inter-electrode gap is 29 mm, corona, glow and arc discharge occur successively with increasing output voltage when the ambient temperature is 850 °C, while only corona and arc discharge appear when the temperature is 350–750 °C. When the inter-electrode gap is 5 mm in an air atmosphere, corona, glow and arc discharge occur successively in an ambient temperature range of 350–850 °C. The cathode material has a minor influence on the COTV and a more significant influence on the SBTV. In a device using a cathode with a low work function, the SBTV is low, and the power to maintain arc discharge is small.     

Pair-breaking in kondo superconductors

An earlier theory for superconductors with magnetic impurities is extended by taking into account explicitly the energy dependence of pair breaking. Results on the transition temperature and the specific heat jumps are in very good agreement with experimental data on (La_1-cCe_c)Al₂.     

A method for measuring infrared emissivities of near-black surfaces at ambient temperatures

The emissivity ε of a surface is an essential quantity in i.r. thermography and thermometry. Since very few data are available, in particular for the 5 μm band, a technique has been worked out for the assessment of i.r. emissivities of surfaces at approximately room temperature. The method provides the reflectivity ρ from a differential temperature measurement of the same object in two different environments. This is equivalent to a determination ofϵ through the relationship ϵ + ρ = 1. Measurements have been made in the 5μm band with an AGA 680 camera and in the 10μm band with a Heimann KT4 thermometer. Surfaces studied include leaves of trees and green plants, wood, plastics, paper, human skin, and some other materials. The accuracy of the results is shown to be 0.005 or better for ϵ values of 0.92 or higher. Natural surfaces have emissivities in this range, with ϵ usually slightly higher in the 10μm band. Other materials may have a significantly lower ϵ.     

Influence of nonmagnetic impurities on the kondo effect

The influence of scattering by nonmagnetic impurities is studied in perturbation theory. While the finite lifetime of the electrons in intermediate states due to scattering by nonmagnetic impurities does not lead to a change in the logT-behaviour of the third-order self-energy, certain vertex-corrections give rise to an additional term which varies like 1/√T at low temperatures. Similar correction terms are found to occur in the higher order self-energy contributions. Although these terms diverge more strongly atT=0 than the logarithmic contributions they are quite small at finite temperatures since they depend on the lifetime τ of the electrons through a factor of (? _F τ)^?5/2 (? _F Fermi energy). The possibility of observing these interference effects experimentally is discussed.     

吸附位置对分子近藤效应的影响

文章报道了利用低温扫描隧道显微镜观察到磁性分子酞菁铁（iron phthalocyanine, FePc）在金属表面Au(111)上的近藤效应（Kondo effect）.从实验上观察到两种由近藤效应引起的不同的Fano共振现象.通过理论模拟和分析,文章作者发现这两种Fano共振分别对应于不同的分子吸附位置.吸附位置不仅影响Fano共振的线型,而且还影响自旋电子的耦合强度,是调控金属表面磁性杂质自旋态的重要途径.     

吸附位置对分子近藤效应的影响

文章报道了利用低温扫描隧道显微镜观察到磁性分子酞菁铁(iron phthalocyanine,FePc)在金属表面Au(111)上的近藤效应(Kondo effect).从实验上观察到两种由近藤效应引起的不同的Fano共振现象.通过理论模拟和分析,文章作者发现这两种Fano共振分别对应于不同的分子吸附位置.吸附位置不仅影响Fano共振的线型,而且还影响自旋电子的耦合强度,是调控金属表面磁性杂质自旋态的重要途径.     

Current analysis of DC negative corona discharge in a wire-cylinder configuration at high ambient temperatures

To study the characteristics of DC negative corona discharge in a wire-cylinder configuration at an ambient temperature range of 350–850 °C, the I–V characteristics and the current composition are analyzed under different conditions. A simple method is proposed to determine the DC corona onset threshold voltage. At high ambient temperatures, in the DC negative corona discharge gap, some electrons are not attached to the electronegative gas molecules and move to the anode tube. Thus, these electrons form an electron current, which may account for most of the total discharging current. The ratio of the electron current to the total discharging current increases with increasing temperature. In a mixture of O₂ and N₂ and a mixture of CO₂ and N₂, the ratio of electron current increases with increasing N₂ content in the mixtures. The cathode material has little influence on the corona discharge characteristics at high ambient temperatures.     

Self-association of the molecules of alcohols in a condensed phase at different temperatures

Using a probability approach, we investigate the process of self-association of the molecules of alcohols in a condensed phase. We obtained data on the nature of association, in particular, on the association degree distribution of molecules and concerning the dependence of the form of this distribution on temperature. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 4, pp. 497–503, July–August, 1998.     

Two-photon laser-induced fluorescence of H O and D O molecules at ambient pressure

We have carried out experiments of two-photon excitation of vapor phase H₂O and D₂O molecules at atmospheric pressure. A narrow-band tunable UV OPO laser is used in the experiments. Transient B ₁ → B ₁ emission from the excited predissociating state is seen in both cases. The complete B ₁ ← A ₁ fluorescence excitation spectrum in the spectral range of 245-250 nm is measured and compared with theory. It is shown that the predissociation rate increases with the rotational quantum number K' _a > 2 more strongly than with K ^′2 _a . No perturbation effects on the measured LIF spectra are observed at a laser power density below 2 GW/cm². Experimental results indicate a negligible contribution from both molecular association and collisions with atmospheric gases. Only an extremely weak vibrational progression belonging to the second positive system of N₂ has been observed, which appears to be due to energy and charge transfer in N ⁺ ₂ ^* + H ₂ O collisions. Received 25 January 2002 / Received in final form 27 May 2002 Published online 4 March 2003     

Thermomagnetic effect in a Knudsen gas at low temperatures

The thermomagnetic effect (TME) in nitrogen interacting with a gold surface is investigated. It has been found that at temperatures T<100K the sign of the effect and the dependence of the heat flux on the magnitude and direction of the field do change.     

不同温度条件下单层石墨烯纳米带弛豫性能的分子动力学研究

分别采用Tersoff-Brenner势和AIREBO势,对三种长宽比的单层石墨烯纳米带在不同热力学温度(0.01—4000 K)下的弛豫性能进行了分子动力学模拟.对基于两种势函数模拟的石墨烯纳米带弛豫的能量曲线和表面形貌进行了分析对比,研究了石墨烯纳米带在弛豫过程中的动态平衡过程.模拟结果表明:单层石墨烯纳米带并非完美的平面结构,边缘处和内部都会呈现一定程度的起伏和皱褶,这与已有的实验结果相符合;石墨烯纳米带的表面起伏程度随长宽比的减小而减小,并且在不同温度条件下,系统动能对石墨烯纳米带的弛豫变形的影响很大,即系统温度越高,石墨烯纳米带的弛豫变形幅度愈大;高长宽比纳米带在一定温度条件下甚至会出现卷曲现象.最后,对采用Tersoff-Brenner势和AIREBO势进行石墨烯的分子动力学模拟进行了深入分析.     

Efficient production of ground-state potassium molecules at sub-mK temperatures by two-step photoassociation

We have developed a two-step " R-transfer" method that efficiently produces translationally ultracold potassium molecules in the X (1)Sigma(+)(g) electronic ground state. Laser-cooled atoms are initially photoassociated at large internuclear separation R to form molecules in high vibrational levels of the 1 (1)Pi(g) state, which are in turn excited by an additional laser to shorter-range Rydberg states such as 5 (1)Pi(u) and 6 (1)Pi(u). Subsequent radiative decay produces ground-state molecules at rates up to 10(5) molecules/second per vibrational level.     

The orientational relaxation of methane molecules in the solid phase II at low temperatures

A model for the dynamics of the coupling between the orientations of the ordered CH₄ molecules in phase II of solid methane at low temperatures is proposed. The model is equivalent to the dynamics of disordered solid hydrogen. The effective interaction strength is determined by the overlap of the librational ground states in the molecular field potential and vanishes in the classical limit. An approximate expression for the effective interaction strength is derived, showing an exponential dependence on the uncertainty of orientation in the librational ground states. This parameter is estimated from the experimental values of the tunnel energies. The second moments of the spectral densities of several anisotropic operators are evaluated in the infinite temperature limit. The resulting gaussian approximations for the spectra are applied in a derivation of the spin lattice relaxation time. The calculated values of the spin lattice relaxation time are compared to experiment.     

Kondo effect in a magnetic field and the magnetoresistivity of kondo alloys

The effect of a magnetic field on the spectral density of a S = 1/2 Kondo impurity is investigated at zero and finite temperatures by using Wilson's numerical renormalization group method. A splitting of the total spectral density is found for fields larger than a critical value H(c)(T = 0) approximately 0.5T(K), where T(K) is the Kondo scale. The splitting correlates with a peak in the magnetoresistivity of dilute magnetic alloys which we calculate and compare with the experiments on CexLa1-xAl2,x = 0.0063. The linear magnetoconductance of quantum dots exhibiting the Kondo effect is also calculated.     

Theory of pressure-induced vibrational and rotational absorption of diatomic molecules at high temperatures

A derivation is given for the integrated absorption coefficient of pressure-induced pure rotational and vibrational transitions in binary collisions of homonuclear diatomic molecules of the same chemical species. The previously neglected effects of excited vibrational states, mechanical anharmonicity, and vibration-rotation interaction are taken into account to obtain more accurate absorption coefficients at high temperatures. In the region of the fundamental wave number the excited vibrational states make more of a contribution to the absorption than their relative population would lead one to expect.