Aging effects across the metal-insulator transition in two dimensions

Aging effects in the relaxations of conductivity of a two-dimensional electron system in Si have been studied as a function of carrier density. They reveal an abrupt change in the nature of the glassy phase at the metal-insulator transition (MIT): (a) while full aging is observed in the insulating regime, there are significant departures from full aging on the metallic side of the MIT, before the glassy phase disappears completely at a higher density n(g): (b) the amplitude of the relaxations peaks just below the MIT, and it is strongly suppressed in the insulating phase. Other aspects of aging, including large non-Gaussian noise and similarities to spin glasses, also have been discussed.     

Nonexponential relaxations in a two-dimensional electron system in silicon

The relaxations of conductivity have been studied in a strongly disordered two-dimensional (2D) electron system in Si after excitation far from equilibrium by a rapid change of carrier density ns at low temperatures T. The dramatic and precise dependence of the relaxations on ns and T strongly suggests (a) the transition to a glassy phase as T-->0, and (b) the Coulomb interactions between 2D electrons play a dominant role in the observed out-of-equilibrium dynamics.     

Atomic mechanism of the heating-induced phase transitions of the simple monatomic glasses

Atomic mechanism of the heating-induced phase transitions of the monatomic Lennard-Jones (LJ) glass has been studied via molecular dynamics (MD) simulations. Monatomic LJ glass was heated up at two different heating rates, crystallization occurs at the lowest one and further heating leads to the melting of LJ crystal. Thermodynamics of the phase transitions and corresponding evolution of structural properties upon heating have been analyzed in details. Atomic mechanism of a crystallization of the glassy state was monitored via spatio-temporal arrangements of the atoms involved in the 1421 bond-pair of the fcc crystalline structure. The 1421 bond-pair was detected via the Honeycutt-Andersen analysis [J.D. Honeycutt, H.C. Andersen, J. Phys. Chem. 91 (1987) 4950]. We found that crystallization of the monatomic LJ glass occurs via homogeneous local rearrangements of atoms in the glassy matrix and we found an important role of the liquid-like atoms (existed in the glassy state) in crystallization of the system. In addition, spatio-temporal arrangements of the liquid-like atoms in the system upon further heating were shown in order to clarify the atomic mechanism of a melting of the obtained LJ crystal. Liquid-like atoms were defined by the Lindemann melting criterion. Our results provide previously un-reported data and give deeper understanding of the heating-induced phase transitions in the less stable metallic glasses, which have been observed in practice.     

Phase retrieval of speckle fringe pattern with carriers using 2D wavelet transform

A novel two-dimensional (2D) Gabor continuous wavelet transform (CWT) for phase retrieval and fringe filtering of speckle fringe patterns with spatial carriers is proposed. Theoretical analysis of 2D Gabor CWT is presented and results are compared with advanced fan 2D CWT using both the computer simulated and experimental speckle fringe patterns. It is shown that noise reduction by 2D Gabor CWT demonstrates better results than that of the advanced fan 2D CWT. Two-dimensional Gabor CWT is also compared with 2D Fourier transform and results show that 2D Gabor CWT algorithm has better noise immunity.     

Thermal jamming of a colloidal glass

We investigate the effect of temperature on structure and dynamics of a colloidal glass created by tethering polymers to the surface of inorganic nanoparticles. Contrary to the conventional assumption, an increase in temperature slows down glassy dynamics of the material, yet causes no change in its static structure factor. We show that these findings can be explained within the soft glassy rheology framework if the noise temperature X of the glass phase is correlated with thermodynamic temperature.     

Glassy synchronization in a population of coupled oscillators

A phase model for a population of oscillators with random excitatory and inhibitory mean-field coupling and subject to external white noise random forces is proposed and studied. In the thermodynamic limit different stable phases for the oscillator population may be found: (i) an incoherent state where all possible values of an oscillator phase are equally probable, (ii) a synchronized state where the population has a nonzero collective phase; (iii) a glassy phase where the global synchronization is zero but the oscillators are in phase with the random disorder; and (iv) a mixed phase where the oscillators are partially synchronized and partially in phase with the disorder. These predictions are based upon bifurcation analysis of the reduced equation valid at the thermodynamic limit and confirmed by Brownian simulation.     

Simulation of flux lines with columnar pins: bose glass and entangled liquids

Using path integral Monte Carlo we simulate a 3D system of up to 1000 magnetic flux lines by mapping it onto interacting bosons in (2+1)D. With increasing temperatures we find first order melting from an ordered solid to an entangled liquid signaled by a finite entropy jump and sharp discontinuities of the defect density and the structure factor S(G). For a particular density of strong columnar pins the crystal is transformed into a Bose glass phase with patches of crystalline order disrupted by the trapped vortices at the pinning sites but with no overall positional or orientational order. This glassy phase melts into a defected entangled liquid through a continuous transition.     

Resistance noise scaling in a dilute two-dimensional hole system in GaAs

We have measured the resistance noise of a two-dimensional (2D) hole system in a high mobility GaAs quantum well, around the 2D metal-insulator transition (MIT) at zero magnetic field. The normalized noise power S(R)/R(2) increases strongly when the hole density p(s) is decreased, increases slightly with temperature (T) at the largest densities, and decreases strongly with T at low p(s). The noise scales with the resistance, S(R)/R(2) approximately R2.4, as for a second order phase transition such as a percolation transition. The p(s) dependence of the conductivity is consistent with a critical behavior for such a transition, near a density p(*) which is lower than the observed MIT critical density p(c).     

Atomic structure and phonons of a Pb ultrathin film on the Al(100) surface

The effect of high dynamic pressures on the electrical conductivity of the amorphous conducting carbon phase (glassy carbon) has been studied. The electrical conductivity of glassy carbon samples has been measured under the condition of shock compression and subsequent release wave. The history of the shock loading of glassy carbon has been calculated with the developed semiempirical equations of state. It has been shown the electrical conductivity of glassy carbon samples in the compression phase at a pressure of 45(5) GPa decreases abruptly by two orders of magnitude. In the relief phase, partially reversible change in the electrical conductivity of an amorphous carbon sample occurs. The recorded effect has been treated as a result of a partially reversible physicochemical transformation of shock-compressed amorphous carbon.     

On-Off Intermittency in Continuous Systems

On-off intermittency is investigated for the model χ= (a + Г(t))χ-χ³ with Г(t) being a stochastic force. The laminar phase distribution ω(T) is studied in the parameter space of bifurcation parameter a, noise intensity D and noise correlation time τ. It is found that increasing D may stabilize the fixed point χ= 0 and reduce the exponential tail in ω(T) for a>0. An analytical solution of the laminar phase distributions is obtained for white noise and colored noise cases, respectively, which agrees with numerical simulations well.     

Universal conductance fluctuations in three dimensional metallic single crystals of Si

In this paper we report the measurement of conductance fluctuations in 3D crystals of Si made metallic by heavy doping. ( L/L(straight phi) approximately 10(3), where L(straight phi) is the phase coherence length.) Temperature and magnetic field dependence of noise strongly indicate the universal conductance fluctuations as a predominant source of the observed magnitude of noise. Conductance fluctuations within a single phase coherent region of L(3)(straight phi) were found to be saturated at <(deltaG(straight phi))(2)> approximately (e(2)/h)(2). An accurate knowledge of the level of disorder enables us to calculate the change in conductance deltaG1 due to movement of a single scatterer as <(deltaG1)(2)> approximately (e(2)/h)(2), which is approximately 2 orders of magnitude higher than its theoretically expected value in 3D systems.     

Cardiac-induced physiological noise in 3D gradient echo brain imaging: effect of k-space sampling scheme

The physiological noise in 3D image acquisition is shown to depend strongly on the sampling scheme. Five sampling schemes are considered: Linear, Centric, Segmented, Random and Tuned. Tuned acquisition means that data acquisition at k-space positions k and -k are separated with a specific time interval. We model physiological noise as a periodic temporal oscillation with arbitrary spatial amplitude in the physical object and develop a general framework to describe how this is rendered in the reconstructed image. Reconstructed noise can be decomposed in one component that is in phase with the signal (parallel) and one that is 90° out of phase (orthogonal). Only the former has a significant influence on the magnitude of the signal. The study focuses on fMRI using 3D EPI. Each k-space plane is acquired in a single shot in a time much shorter than the period of the physiological noise. The above mentioned sampling schemes are applied in the slow k-space direction and noise propagates almost exclusively in this direction. The problem then, is effectively one-dimensional. Numerical simulations and analytical expressions are presented. 3D noise measurements and 2D measurements with high temporal resolution are conducted. The measurements are performed under breath-hold to isolate the effect of cardiac-induced pulsatile motion. We compare the time-course stability of the sampling schemes and the extent to which noise propagates from a localized source into other parts of the imaging volume. Tuned and Linear acquisitions perform better than Centric, Segmented and Random.     

Consumers don’t play dice,influence of social networks and advertisements

Empirical data of supermarket sales show stylised facts that are similar to stock markets, with a broad (truncated) Lévy distribution of weekly sales differences in the baseline sales [R.D. Groot, Physica A 353 (2005) 501]. To investigate the cause of this, the influence of social interactions and advertisements are studied in an agent-based model of consumers in a social network. The influence of network topology was varied by using a small-world network, a random network and a Barabási–Albert network. The degree to which consumers value the opinion of their peers was also varied.On a small-world and random network we find a phase transition between an open market and a locked-in market that is similar to condensation in liquids. At the critical point, fluctuations become large and buying behaviour is strongly correlated. However, on the small world network the noise distribution at the critical point is Gaussian, and critical slowing down occurs which is not observed in supermarket sales. On a scale-free network, the model shows a transition between a gas-like phase and a glassy state, but at the transition point the noise amplitude is much larger than what is seen in supermarket sales.To explore the role of advertisements, a model is studied where imprints are placed on the minds of consumers that ripen when a decision for a product is made. The correct distribution of weekly sales returns follows naturally from this model, as well as the noise amplitude, the correlation time and cross-correlation of sales fluctuations. For particular parameter values, simulated sales correlation shows power-law decay in time. The model predicts that social interaction helps to prevent aversion, and that products are viewed more positively when their consumption rate is higher.     

High-resolution gadolinium-enhanced 3D MRA of the infrapopliteal arteries. Lessons for improving bolus-chase peripheral MRA

Peripheral magnetic resonance angiography (MRA) is growing in use. However, methods of performing peripheral MRA vary widely and continue to be optimized, especially for improvement in illustration of infrapopliteal arteries. The main purpose of this project was to identify imaging factors that can improve arterial visualization in the lower leg using bolus chase peripheral MRA. Eighteen healthy adults were imaged on a 1.5T MR scanner. The calf was imaged using conventional three-station bolus chase three-dimensional (3D) MRA, two dimensional (2D) time-of-flight (TOF) MRA and single-station Gadolinium (Gd)-enhanced 3D MRA. Observer comparisons of vessel visualization, signal to noise ratios (SNR), contrast to noise ratios (CNR) and spatial resolution comparisons were performed. Arterial SNR and CNR were similar for all three techniques. However, arterial visualization was dramatically improved on dedicated, arterial-phase Gd-enhanced 3D MRA compared with the multi-station bolus chase MRA and 2D TOF MRA. This improvement was related to optimization of Gd-enhanced 3D MRA parameters (fast injection rate of 2 mL/sec, high spatial resolution imaging, the use of dedicated phased array coils, elliptical centric k-space sampling and accurate arterial phase timing for image acquisition). The visualization of the infrapopliteal arteries can be substantially improved in bolus chase peripheral MRA if voxel size, contrast delivery, and central k-space data acquisition for arterial enhancement are optimized. Improvements in peripheral MRA should be directed at these parameters.     

相干X射线衍射成像三维重建的数字模拟研究

利用相干X射线衍射成像“过采样”采集样品的远场相干衍射图样, 结合相位重建迭代算法重建物空间样品信息. 三维重建过程中, 边界约束条件是相位重建算法中最为关键的部分. 本文采用数字模拟的方法, 利用灰度值图像作为物空间的样品, 研究并实现了边界条件的自动寻找, 重建结果显示比以往采用较“松”的边界约束更为精确. 利用噪声模拟方法, 研究了衍射图样中不同噪声类型的滤除对重建结果的影响. 研究发现传统的去噪方法在高噪声情况下不能直接应用于相干X射线衍射成像, 并找到了适用于相干X射线衍射成像噪声滤除的有效方法. 研究表明, 此方法能非常有效地降低噪声对重建结果的影响. 利用模拟三维纳米金颗粒作为样品, 完成了对纳米金颗粒中电子密度分布的三维重建, 在有随机噪声的影响下, 也得到了很好的重建结果, 并找到了成功实现三维重建的噪声限为信噪比不低于27. 关键词： X射线相干衍射成像 过采样 三维相位重建 显微成像     

Mean First-Passage Time of an Asymmetric Kinetic Model Driven by Two Different Kinds of Colored Noises

The mean first-passage time （MFPT） of an asymmetric bistable system between multiplicative non-Gaussian noise and additive Gaussian white noise with nonzero cross-correlation time is investigated. Firstly, the non-Markov process is reduced to the Markov process through a path-integral approach; Secondly, the approximate Fokker Planck equation is obtained by applying the unified colored noise approximation and the.Novikov Theorem. The steady-state probability distribution （SPD） is also obtained. The basal functional analysis and simplification are employed to obtain the approximate expressions of MFPT T^±. The effects of the asymmetry parameter β, the non-Gaussian parameter （measures deviation from Gaussian character） r, the noise correlation times τ and τ2, the coupling coefficient A, the intensities D and a of noise on the MFPT are discussed. It is found that the asymmetry parameter β, the non-Gaussian parameter r and the coupling coefficient A can induce phase transition. Moreover, the main findings are that the effect of self-existent parameters （D, α, and τ） of noise and cross-correlation parameters （A, 7-2） between noises on MFPT T^± is different.     

Enhanced electrochemical performance of LiMnBO<Subscript>3</Subscript> with conductive glassy phase: a prospective cathode material for lithium-ion battery

LiMnBO₃ has been identified as a promising cathode material for next-generation lithium-ion batteries. In this study, LiMnBO₃ along with glassy lithium borate material (LiMnBO₃ (II)) is synthesized by sol-gel method. X-ray diffraction (XRD) analysis depicts the existence of LiBO₂ glassy phase along with m-LiMnBO₃ phase. Transmission electron microscopy (TEM) analysis confirms the presence of LiBO₂ glassy phase. An enhanced electrical conductivity of 3.64 × 10^?7 S/cm is observed for LiMnBO₃ (II). The LiBO₂ glassy phase is found to promote the Li reaction kinetics in LiMnBO₃ (II). The synthesized LiMnBO₃ (II) delivers a first discharge capacity of 310 mAh g^?1 within a potential window of 1.5–4.5 V at C/10 rate. Further, a discharge capacity of 186 mAh g^?1 at the 27th cycle shows a better cycle performance. The enhanced capacity is due to the presence of LiBO₂ glassy phase and more than one Li-ion transfer in the lithium-rich stoichiometry of LiMnBO₃ (II). Density functional theory calculation reveals the exact electronic structure of m-LiMnBO₃ with a band gap of 3.05 eV. A charge transfer mechanism is predicted for delithiation process of m-LiMnBO₃.     

Detection of tones in reproducible narrow-band noise

Hit and false-alarm rates were measured for detection of a 500-Hz tone target in each of ten reproducible samples of 1/3-oct bandwidth noise centered at 500 Hz for both NoS pi and NoSo conditions. The effects on hit rates of the starting phase of the target relative to individual noise samples were investigated with two target phase angles for three subjects. The major results are: (1) performance varies significantly over masker waveforms; (2) for NoS pi conditions, the effect of target-to-marker phase angle on hit rates is not significant for these narrow-band maskers; (3) for NoSo conditions, the target-to-masker phase angle has a large effect; (4) no significant correlation between NoSo performance and NoS pi performance is seen across masker waveforms. These results are generally consistent wuth previously reported results for wideband maskers [R.H. Gilkey, D.E. Robinson, and T.E. Hanna, "Effects of masker waveform and signal-to-masker phase relation on diotic and dichotic masking by reproducible noise," J. Acoust. Soc. Am. 78, 1207-1219 (1985)] with an important exception. Specifically, in the wideband experiment, significant correlation between NoSo and NoS pi performance across noise samples was found. In addition, in the wideband experiment, a small yet statistically significant effect of target-to-masker phase was observed in the NoS pi condition.     

Phase unwrapping algorithm based on singularity compensation for three-dimensional shape measurement

Three-dimensional (3D) imaging continues to attract much research interest for its wide applications. In 3D shape measurement, the phase carries information about the object. However, phase mapping is ambiguous as the extracted phase is returned in a form that suffers from 2π phase jumps. In this case, the phase data must be unwrapped to be fit for use. Furthermore, sometimes the presence of noise in the measured data, in which many singular points (SPs) are found, makes general phase unwrapping algorithms fail to produce accurate unwrapped results. For this reason, we propose a new phase unwrapping method for dynamic 3D shape measurement. The new algorithm is based on compensating the singularity of discontinuity sources. It uses direct compensators for adjoining SP pairs and uses rotational compensators for other SP pairs. The proposed algorithm has been evaluated and compared with past phase unwrapping methods. Results show that the proposed method gives satisfactory unwrapped results with a low computation time.     

基于平面估计的三维测量相位展开新方法

工业自动化生产中微米级的三维测量大多采用相位测量轮廓术,但其中现有的包裹相位展开方法容易受图像噪声和相位突变的干扰;在电路板贴片安装的锡膏三维测量中,利用电路板的平面区域包裹相位信息进行平面估计,然后全局相位展开向拟合平面靠拢,根据统计信息确定参数,得到连续相位;利用展开后的相位再进行电路板平面的二次曲面拟合,提高基平面的拟合精度和相对高度;对比实验证明了该方法的鲁棒性和快速性,处理时间短,不受噪声、相位突变、阴影等干扰。