一种“类耗散系统”中的“类Ⅴ型阵发”

一类不连续不可逆保面积映象可以展示类似耗散的行为,因此可称其为“类耗散系统”.在一种类耗散系统中观察到了椭圆周期轨道及其周围的椭圆岛与映象不连续边界碰撞而消失的现象.周期轨道消失后,经过一系列过渡椭圆周期轨道之后,系统的行为由一个混沌类吸引子主导.在混沌类吸引子刚刚出现时,混沌时间序列呈现层流相与湍流相的无规交替.这一切都与不连续耗散系统中发生的Ⅴ型阵发的相应性质十分相似,因此可称为“类Ⅴ型阵发”.然而,当混沌类吸引子刚刚出现时,仅可以找到最后一个过渡椭圆岛的“遗迹”,并不存在它的“鬼魂”,因此类Ⅴ型阵发不遵从Ⅴ型阵发的特征标度规律.反之,混沌类吸引子的鬼魂却存在于最后一个过渡椭圆周期轨道的类瞬态过程中,因此在类Ⅴ型阵发导致混沌运动的临界点之前,由此“类瞬态混沌奇异集”中逃逸的规律就成为标志这一种临界现象的标度律.这与Ⅴ型阵发又根本不同. 关键词： 类耗散性 类混沌吸引子 类Ⅴ型阵发     

Pressure-driven "molecular metal" to "atomic metal" transition in crystalline Ga

We present the ultrasonic study of gallium (Ga I) under high pressure up to 1.7 GPa, including the measurements of the density and elastic properties during phase transitions to Ga II and to a liquid state. The observed large drop of both bulk and shear moduli (by 30% and 55%, correspondingly) during the phase transition to Ga II, as well as the increase of the Poisson's ratio from typically "covalent" ( approximately 0.22) to "metallic" ( approximately 0.32) values, experimentally testifies to the coexistence of a molecular and metallic behavior in Ga I and to the disappearance of the "covalency" during the transition to Ga II. A high value of the pressure derivative of the bulk modulus for Ga I and the increase in the Poisson's ratio can be associated with the weakening of the covalency in compressed Ga I and considered as a precursor of the transition to normal metal.     

"Bottleneck effect" in two-dimensional microfluidics

An anomalously long transient is needed to achieve a steady pressurization of a fluid when forced to flow through micronarrowed channels under constant mechanical driving. This phenomenon, known as the "bottleneck effect" is here revisited from a different perspective, by using confined displacements of interfacial fluids. Compared to standard microfluidics, such effect admits in this case a neat quantitative characterization, which reveals intrinsic material characteristics of flowing monolayers and permits to envisage strategies for their controlled micromanipulation.     

Resonant "forbidden" reflections in magnetite

Resonant x-ray scattering was used to investigate electronic fluctuations of the octahedral iron atoms in magnetite. We measured the (002) and (006) "forbidden" x-ray diffraction reflections permitted by the anisotropy of the iron anomalous scattering factor. The energy and azimuthal angle dependencies of these reflections, and the polarization analysis, are shown and discussed. The results clearly show p and d iron empty states ordering in magnetite at room temperature. Moreover, the octahedral iron atoms are electronically equivalent in a time scale lower than 10(-16) sec. Therefore, magnetite should be considered as an itinerant magnet and not as a fluctuating mixed valence material.     

磁性固体中的"冰原则"

按照热力学第三定律,所有物质在Ｔ=0Ｋ的温度下它的熵应等于零.也就是说,在零温平衡状态下,任何一种物质的分子只具有唯一的排列方式.然而,在实验上,人们仍然能够观察到一些零温下的“剩熵”现象———某些物质在Ｔ=0Ｋ具有“零点熵”.一般认为,实验上测出的零点熵起因于?..     

“黑盒子”实验中的电容器

以物理竞赛实验试题为例,讲述了黑盒子实验中电容器元件的判断方法。     

Ccontrolling the Transition Between "Higher" and "Lower" State of OB in Coherent Atomic Medium

We present a new experimental demonstration that the threshold points and shape of OB hysteresis curve in a four-level Rb atomic system inside an optical cavity were controllably changed by optical signals.Tuning the triggering optical signal to two different transitions of the atomic system we found that the OB threshold points shift toward different direction and at the same time the shape of the OB curve is also changed.For a given intensity of probe laser the OB system can be reliably inversed from its lower stable state to upper state under the triggering of a suitably tuned optical pulse signal (we named it up-controlling signal).More interesting, the inversed OB state does not drop to its initial lower stable state after the triggering pulse signal has passed, but moves to the upper branch of the initial OB curve and stably stays there until another optical signal tuned on other atomic transition (named down-controlling signal) coming.The result shows that there is the ability of information storage in the OB system.The experiments are reasonably explained with the change of cavity resonant condition resulting from the enhanced third-order nonlinearity in multilevel atomic system owing to atomic coherence.     

"整体法"理念在力学解题中的运用

物理解题方法中有隔离法和整体法,而整体法思维方式在物理解题中若能正确运用,则解题会得心应手．整体法是在一个物理过程中把两个或者多个物体看成一个整体;这样整体内部的物体之间的所有作用力都是内力,整体以外的物体对它的作用力都叫外力．应用整体法时应注意“系统”的选取、“内力”和“外力”的确定和分析．     

One-dimensional "turbulence" in a discrete lattice

We study a one-dimensional discrete analog of the von Karman flow, widely investigated in turbulence. A lattice of anharmonic oscillators is excited by both ends in order to create a large scale structure in a highly nonlinear medium, in the presence of a dissipative term proportional to the second order finite difference of the velocities, similar to the viscous term in a fluid. In a first part, the energy density is investigated in real and Fourier space in order to characterize the behavior of the system on a local scale. At low amplitude of excitation the large scale structure persists in the system but all modes are however excited and exchange energy, leading to a power law spectrum for the energy density, which is remarkably stable against changes in the model parameters, amplitude of excitation, or damping. In the spirit of shell models, this regime can be described in terms of interacting scales. At higher amplitude of excitation, the large scale structure is destroyed and the dynamics of the system can be viewed as resulting from the creation, interaction, and decay of localized excitations, the discrete breathers, the one-dimensional equivalents of vortices in a fluid. The spectrum of the energy density is well described by the spectrum of the breathers, and shows an exponential decay with the wave vector. Due to this exponential behavior, the spectrum is dominated by the most intense breathers. In this regime, the probability distribution of the increments of velocity between neighboring points is remarkably similar to the experimental results of turbulence and can be described by distributions deduced from nonextensive thermodynamics as in fluids. In a second part the power dissipated in the whole lattice is studied to characterize the global behavior of the system. Its probability distribution function shows non-Gaussian fluctuations similar to the one exhibited recently in a large class of "inertial systems," i.e., systems that cannot be divided into mesoscopic regions which are independent. The properties of the nonlinear excitations of the lattice provide a partial understanding of this behavior.