Observation of "Stick" and "Handle" intermediates along the fullerene road

The hypothesis that fullerenes grow in a carbon plasma by the addition of C2 units (the "fullerene road") has been widely acclaimed as the most plausible mechanism for formation of larger fullerenes including C60 and C70. Calculations suggest that the association of C2 with fullerenes proceeds through two classes of intermediates, "sticks" and "handles." Here we report the observation of these species using high-resolution ion-mobility measurements for C(n) cations generated by laser vaporization of graphite and laser desorption of C60. Sticks with up to eight-atom chains have also been found.     

最具潜力的新型"奇迹材料"——石墨烯

2009年4月29日据物理世界网报道,意大利的科研人员成功地用石墨烯制造了首枚包含两个晶体管的集成电路(见封面主图),它拥有简单的计算能力,标制着碳基电子学时代的到来.这枚只有两个晶体管的集成电路虽然很小,却是向制造碳基高性能电子器件迈出的重要一步.     

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.     

Comment on "Pushing the hyperpolarizability to the limit"

It is shown that energy/length scaling complicates maximizing the first hyperpolarizability of a single electron as a function of the potential. A more transparent formula for this hyperpolarizability is given. Examining this formula demonstrates that Zhou et al.(1) have not proved that modulated conjugation results in large hyperpolarizability.     

创造条件 做好“传感器”的实验

新课程把传感器编入教材,给中学物理教学带来了全新的时代气息.物理教学由概念辨析到实际应用,由抽象模型到元器件,由繁难运算到设计操作,学生第一次感到物理有用,物理神奇,物理好学.在"传感器"一章的教学中要达到以上教学效果就要做好实验.     

Viewing the proton through "color" filters

While the form factors and parton distributions provide separately the shape of the proton in coordinate and momentum spaces, a more powerful imaging of the proton structure can be obtained through phase-space distributions. Here we introduce the Wigner-type quark and gluon distributions which depict a full-3D proton at every fixed light-cone momentum, like what is seen through momentum ("color") filters. After appropriate phase-space reductions, the Wigner distributions are related to the generalized parton distributions (GPDs) and transverse-momentum dependent parton distributions, which are measurable in high-energy experiments. The new interpretation of GPDs provides a classical way to visualize the orbital motion of the quarks, which is known to be the key to the spin and magnetic moment of the proton.     

On the concept of "tractor beams"

We demonstrate the existence of a class of optical beams where the nonconservative forces can be locally oriented in a direction opposite to the propagation wave vector. Objects placed in the vicinity of these locations will move toward the source of light. The behavior of these negative forces is discussed for the particular case of nondiffracting rotating scale-invariant vector electromagnetic waves.     

Unraveling the "pressure effect" in nucleation

The influence of the pressure of a chemically inert carrier gas on the nucleation rate is one of the biggest puzzles in the research of gas-liquid nucleation. Experiments can show a positive effect, a negative effect, or no effect at all. The same experiment may show both trends for the same substance depending on temperature, or for different substances at the same temperature. We show how this ambiguous effect naturally arises from the competition of two contributions: nonisothermal effects and pressure-volume work. Our model clarifies seemingly contradictory experimental results and quantifies the variation of the nucleation ability of a substance in the presence of an ambient gas. Our findings are corroborated by molecular dynamics simulations and might have important implications since nucleation in experiments, technical applications, and nature practically always occurs in the presence of an ambient gas.     

对"胡不归问题"的思考

从前，有一个小伙子在外地当学徒，当他获悉在家乡的年老父亲病危的消息后，便立即启程赶路．由于思念心切，他选择了全是砂砾地带的直线路径．当他气喘吁吁地赶到父亲眼前时，老人刚刚咽了气．小伙子不觉失声痛哭，邻舍劝慰小伙子说，老人在弥留之际还不断喃喃地叨念“胡不归？胡不归？”．对这个古老的传说，引起了人们很多的思索，小伙子要能稍早些到家，就不会出现这种遗憾了．     

球形开口空腔“实际”发射率的计算

球形开口空腔在辐射计量、辐射定标和激光能量测量等方面得到了广泛应用。当考虑辐射热交换时，实际的发射率是一个很重要的参数。由于辐射能在壁面进行了多次反射吸收，所以开口空腔系统吸收的能量要比相同尺寸和相同发射率的平板吸收的能量大。这种由于辐射在空腔内多次反射和吸收使表面吸收率和表面发射率都增大的现象称之为“空腔效应”。针对空腔效应，引入有效辐射和视在发射率概念，通过球形开口空腔的视在发射率简单计算了开口空腔的辐射热损失。     

"光谱"教学中的几个问题

中学物理教学中"光谱及光谱分析",虽然篇幅不多,但内容丰富.由于这部分内容有些方面所需的知识,超过中学的要求,所以学生在学习中常会提出许多问题.这里选择若干典型问题,作一定性解释,以帮助学生进一步理解这些内容.     

"荧光-1"实验装置物理设计

本文主要介绍"荧光-1"实验装置物理参数设计, 并依据半经验公式预估在实验装置上可能达到的磁化等离子体状态参数. 理论设计结果表明: "荧光-1"实验装置最大放电电流1.5 MA, 四分之一周期3 μs, 最大反向磁场4 T; 以此为实验平台, 当θ箍缩线圈内充气压力50 mTorr(D₂气体)时, 形成的等离子体靶直径约为2 cm, 长度17 cm; 等离子体靶密度6.6×10¹⁶ cm^-3, 温度 (T_i+T_e) 约300 eV; 等离子体平均β值为0.95. 该状态参数接近磁化靶聚变所要求的等离子体靶初始状态参数. 关键词： 脉冲功率技术 反场构形 磁化等离子体 磁化靶聚变