Quantum tunneling at zero temperature in the strong friction regime

In the large damping limit we derive a Fokker-Planck equation in configuration space (the so-called Smoluchowski equation) describing a Brownian particle immersed into a thermal environment and subjected to a nonlinear external force. We quantize this stochastic system and survey the problem of escape over a double-well potential barrier. Our finding is that the quantum Kramers rate does not depend on the friction coefficient at low temperatures; i.e., we predict a superfluidity phenomenon in overdamped open systems. Moreover, at zero temperature we show that the quantum escape rate does not vanish in the strong friction regime. This result, therefore, is in contrast with the work by Ankerhold et al. [Phys. Rev. Lett. 87, 086802 (2001)]] in which no quantum tunneling is predicted at zero temperature.     

An internal friction study of the vanadium-deuterium system

The internal friction of deuterium-doped vanadium has been investigated in the temperature range from 65 to 350 K. At low deuterium concentrations (≦0.18 at.−%) a relaxation effect has been observed near 90 K; its activation energy and its limiting relaxation tiem are 0.17 eV and 2×10⁻¹² sec, respectively. A second internal friction peak (precipitation peak) appears in more heavily doped samples; the position of this second peak shifts to higher temperatures as the deuterium content is increased. Possible mechanisms for the two peaks are discussed. The phase diagram of the vanadium-deuterium system at low temperatures has been deduced from the correlation between the deuterium concentration and the position of the precipitation peak.     

Substitutional-interstitial interactions in niobium-titanium alloys: An internal friction study

The influence of additions of interstitial oxygen and nitrogen on the internal friction spetrum of the niobium-1 at-% titanium alloy was studied. The nature of the various observed relaxation processes introduced by the presence of substitutional titanium is discussed. A thermodynamic analysis was carried out for two pronounced interaction peaks attributed to the stress-induced reorientation of single oxygen or nitrogen atoms around single titanium atoms and the respective binding energies were estimated. It was also found that, over the range of oxygen concentration studied, interstitial oxygen is completely removed from random migration by substitutional titanium atoms acting as trapping centres, whilst nitrogen population is always partitioned between mobile and trapped atoms even at very low relative nitrogen concentrationsC _N/C _Ti. A possible reason for the different behavior of oxygen and nitrogen is suggested.     

Positron lifetime study of molybdenum neutron-irradiated at various temperatures

Positron Lifetime measurements reveal that neutron irradiation at 350 and 400°C can produce sub-electron microscopic voids in Mo. The results show that the lifetime in voids reaches a saturation value with increasing void size to a dimension less than 20 Å. It is suggested that the difference among known data for τ₂ ? 450 ps can be attributed to differences in the metal-void interface and gaseous conditions within the void.     

Small-angle neutron scattering studies on the structure of radiation defects in neutron-irradiated synthetic quartz

The supra-atomic structure of single crystals of synthetic quartz with a dislocation density of 54 cm^?2 in their initial state and after irradiation in a VVR-M reactor by fast neutrons with the energy, E _n > 0.1 MeV, at fluences of 2.3 × 10¹⁹ and 4.5 × 10¹⁹ N/cm², has been studied by the method of small-angle thermal neutron scattering. It has been established that fast neutrons create point, linear, and bulk defects throughout the entire material. It has been shown that extended defects have a significant integral length per volume unit equal to ??3 × 10¹¹ cm/cm³, and can form a consolidated network in the sample with a cell size of ??30 nm, through the channels of which the migration of impurity atoms and molecules is possible.     

内耗技术在软物质研究中的一些应用

软物质物理已成为物理学发展迅速的重要前沿学科。描述软物质复杂非平衡态动力学及其运动规律是理解其宏观性质的关键。内耗与力学谱作为材料缺陷与力学性质研究的重要实验技术,它能够很灵敏地探测和研究材料中发生的复杂相变和弛豫动力学过程。文章结合作者近年来的典型工作,介绍了内耗技术在软物质研究中的一些应用,阐述了内耗手段发挥的独特作用。随着内耗测试新方法、新技术和内耗新理论的不断出现和完善,相信未来内耗技术在材料科学和凝聚态物理研究领域将起到越来越重要的作用。     

Positron states and nanoobjects in proton-irradiated quartz single crystals: Positronium atom in quartz

The influence of proton bombardment and metal atom impurities on the structure of quartz single crystals has been studied. The related defects have been studied using positron annihilation spectroscopy (angular correlation of positron-annihilation photons), acoustic absorption, and optical absorption measurements. It is shown that the presence of a narrow component f in the angular distribution of annihilation photons (ADAP), which is related to the formation of parapositronium, determines a high sensitivity of this method with respect to features of the crystal structure of quartz. It is established that the defectness of the structure of irradiated quartz crystals can be characterized by the ratio f/f ₀ of the relative intensities of narrow components in the ADAP curves measured before (f ₀) and after (f) irradiation. Any process leading to a decrease in the probability of positronium formation (e.g., positron loss as a result of the trapping on defects and the interaction with impurity atoms and lattice distortions) decreases the intensity of the narrow component. Based on the ADAP data, estimates of the radii and concentrations of nanodefects in quartz have been obtained and their variation upon annealing at temperatures up to T = 873 K has been studied.     

A study of the effects of deuteride precipitation in tantalum by frequency and internal friction measurements

The effects of deuteride precipitation from Ta-D solid solution was investigated by vibration frequency and internal friction measurements in the temperature range (80–300) K. Initiation of deuteride precipitation is indicated by the inflexion of the frequency curve, whereas the onset of the dislocation-loops formation around the deuteride particles is correlated with the internal friction growth (precipitation peak). The dislocation interstitial drag model which seems to better account for the precipitation peak, is discussed. The phase diagram of the Ta-D system was determined in the range of (0.2–7.5) at.～%.     

Scanning tunneling microscopy of the nonequilibrium interaction of impurity states at semiconductor surfaces

Interaction of two localized impurity states of Si atoms at a GaAs surface was studied by scanning tunneling microscopy and spectroscopy. The effects of a twofold “switching” on and off of the states of each of the interacting atoms, the tunneling-interaction-induced mutual level pulling of these states, and the level stabilization near E _F were observed. These effects are explained in terms of the extended Anderson model.     

Detecting electronic states at stacking faults in magnetic thin films by tunneling spectroscopy

Co islands grown on Cu(111) with a stacking fault at the interface present a conductance in the empty electronic states larger than the Co islands that follow the stacking sequence of the Cu substrate. Electrons can be more easily injected into these faulted interfaces, providing a way to enhance transmission in future spintronic devices. The electronic states associated with the stacking fault are visualized by tunneling spectroscopy, and its origin is identified by band structure calculations.     

Cyclic stress-strain curves and internal friction of steel at ultrasonic frequencies

A new interpretation of some characteristics of material push-pull loaded at a frequency of 23 kHz can be evaluated by measuring the internal friction and elasticity modulus defect at different strain amplitudes. It is possible to obtain interesting relations describing the material's cyclic microplasticity response. The paper presents some basic relationships between the ‘plastic’ internal friction, elasticity modulus defect, hysteresis loop area, plastic strain amplitude and the cyclic deformation hardening coefficient, for low carbon steel with different grain sizes.     

Positron-lifetime study of vacancy annealing in neutron-irradiated GaAs

Positron-lifetime measurements have been used to study the annealing of vacancies in neutron-irradiated GaAs. The vacancies which are interpreted as defects in the Ga sublattice disappear in a single annealing stage (at 500°C in GaAs doped with Si or Zn, and at 600°C in Cr-doped GaAs).