非磁性杂质对碳纳米管态密度的影响研究

针对非磁性杂质对碳纳米管的影响问题作了系统理论的分析,为设计和实现具有优良性能的基于碳纳米管的量子器件提供理论依据.应用格林函数方法计算态密度的结果表明,单个点缺陷在碳纳米管中引起准束缚态;给出准束缚态能级和峰宽的解析公式,分析它们与碳纳米管手型和直径之间的关系;逐点计算局域态密度发现,在实空间准束缚态是一个非常局域化...     

非磁性杂质对碳纳米管态密度的影响研究

针对非磁性杂质对碳纳米管的影响问题作了系统理论的分析,为设计和实现具有优良性能的基于碳纳米管的量子器件提供理论依据.应用格林函数方法计算态密度的结果表明,单个点缺陷在碳纳米管中引起准束缚态;给出准束缚态能级和峰宽的解析公式,分析它们与碳纳米管手型和直径之间的关系;逐点计算局域态密度发现,在实空间准束缚态是一个非常局域化的效应,离开缺陷即迅速衰减.同时,缺陷在碳纳米管中会引起Friedel振荡,其方向与碳纳米管手型有关.     

Superconductivity in lithium below 0.4 millikelvin at ambient pressure

Elements in the alkali metal series are regarded as unlikely superconductors because of their monovalent character. A superconducting transition temperature as high as 20 K, recently found in compressed lithium (the lightest alkali element), probably arises from pressure-induced changes in the conduction-electron band structure. Superconductivity at ambient pressure in lithium has hitherto remained unresolved, both theoretically and experimentally. Here we demonstrate that lithium is a superconductor at ambient pressure with a transition temperature of 0.4 mK. As lithium has a particularly simple conduction electron system, it represents an important case for any attempts to classify superconductors and transition temperatures, especially to determine if any non-magnetic configuration can exclude superconductivity down to zero temperature. Furthermore, the combination of extremely weak superconductivity and relatively strong nuclear magnetism in lithium would clearly lead to mutual competition between these two ordering phenomena under suitably prepared conditions.     

负压状态下无衬砌土拱的稳定性分析

无衬砌土质输水通道或洞穴中常会出现负压力 ,它对周壁的土体将产生气蚀切割和反向渗透潜蚀破坏 ,从而导致拱顶土体的垮落 ,以达到新的平衡。随着负压力的增大 ,拱顶垮落高度也不断增大 ,但增长幅度却逐渐减小 ,直至趋于垮落极限值 1 333b1 /fk。这表明对有负压存在的土质通道或洞穴 ,其上覆土层的最小保护层厚度应大于该值 ,否则将会产生地面塌陷等灾害现象。     

超高压力下通电烧结制备钨铁功能梯度材料

采用超高压力下通电烧结技术制备了钨体积分数为0%、25%、50%、75%和100%的钨铁功能梯度材料.研究了材料组分和工艺参数对W/Fe功能梯度材料的显微形貌及力学性能的影响.结果表明:当施加压力为9 GPa,通电功率为11 kW,通电时间60 s时可以获得相对密度大于98%的钨铁功能梯度材料,其组分分布与设计成分保持一致.     

Superconductivity in single crystals of the fullerene C70.

The observation of superconductivity in doped C60 has attracted much attention, as these materials represent an entirely new class of superconductors. A maximum transition temperature (Tc) of 40 K has been reported for electron-doped C60 crystals, while a Tc of 52 K has been seen in hole-doped crystals; only the copper oxide superconductors have higher transition temperatures. The results for C60 raise the intriguing questions of whether conventional electron-phonon coupling alone can produce such high transition temperatures, and whether even higher transition temperatures might be observed in other fullerenes. There have, however, been no confirmed reports of superconductivity in other fullerenes, though it has recently been observed in carbon nanotubes. Here we report the observation of superconductivity in single crystals of electric-field-doped C70. The maximum transition temperature of about 7 K is achieved when the sample is doped to approximately four electrons per C70 molecule, which corresponds to a half-filled conduction band. We anticipate superconductivity in smaller fullerenes at temperatures even higher than in C60 if the right charge density can be induced.     

基于IP欺骗攻击的状态分析法研究

提出了一种能够在网络中检测并防范IP欺骗攻击的方法．通过获取网络结点的工作状态,分析网络中出现的异常活动,给出了因攻击而引起的网络状态的迁移过程．通过预测分析,及时检测出已经存在的IP欺骗攻击;通过回溯分析,试图找到发起攻击的攻击者．将状态分析法运用于入侵检测,可增强网络抵御IP欺骗攻击的能力．     

超导电流的动量守恒与超导电子对隧道效应

讨论了稳恒电流的动量守恒现象,由此分析了直流约瑟夫森隧道效应。指出,描述约瑟夫森隧道效应的2个基本方程遵循稳恒电流动量守恒定律;约瑟夫森隧道结上的超导电子对质心定向运动速度比隧道结2侧超导体内的超导电子对质心定向运动速度大3个数量级：vS2～1 03 vS1;约瑟夫森隧道结上超导电子对的质心定向运动速度和动量很大的原因在于其运动遵循稳恒电流动量守恒定律;约瑟夫森隧道结上超导电子对质心定向运动速度可以达到费米速度的数量级、其定向运动动能可以达到费米能的数量级,从而使隧道结由超导态转变成正常态。     

压力校直下压量计算方法的比较分析

校直下压量是压力校直的主要工艺参数。校直下压量有三种计算方法：基于弹塑性理论基础的解析计算方法,有限元方法和经验公式的计算方法。通过实例计算结果与实验数据比较,证实了计算方法的实用性,对开展校直工艺理论研究和开发设计自动校直机具有一定的指导意义。     

Superconductivity in the non-oxide perovskite MgCNi3

The interplay of magnetic interactions, the dimensionality of the crystal structure and electronic correlations in producing superconductivity is one of the dominant themes in the study of the electronic properties of complex materials. Although magnetic interactions and two-dimensional structures were long thought to be detrimental to the formation of a superconducting state, they are actually common features of both the high transition-temperature (Tc) copper oxides and low-Tc material Sr2RuO4, where they appear to be essential contributors to the exotic electronic states of these materials. Here we report that the perovskite-structured compound MgCNi3 is superconducting with a critical temperature of 8 K. This material is the three-dimensional analogue of the LnNi2B2C family of superconductors, which have critical temperatures up to 16 K (ref. 2). The itinerant electrons in both families of materials arise from the partial filling of the nickel d-states, which generally leads to ferromagnetism as is the case in metallic Ni. The high relative proportion of Ni in MgCNi3 suggests that magnetic interactions are important, and the lower Tc of this three-dimensional compound-when compared to the LnNi2B2C family-contrasts with conventional ideas regarding the origins of superconductivity.     

难熔金属钽、钼、钨的高温高压状态方程及声速的第一性原理研究

难熔金属的高压熔化曲线在动-静高压实验之间存在巨大争议,而在发生冲击熔化之前是否存在固-固相变是目前的研究热点问题.本文以3种典型难熔金属钽、钼、钨为例,通过第一性原理晶格动力学方法,计算了钽、钼、钨的声子色散曲线.采用准谐近似的方法,获得了Hugoniot状态方程以及Hugoniot声速.对于钽和钨的声速计算表明,其基态体心立方结构在高压下一直保持其稳定性;而钼的晶格动力学计算表明其基态结构的稳定性在高压下消失,而钼的Hugoniot声速在175–275GPa区域内发生了拐折,这一结果证实了冲击波实验中对于钼的声速测量的结果:在210GPa压力附近声速发生间断.     

Superconductivity in CaCuO2 as a result of field-effect doping.

Understanding the doping mechanisms in the simplest superconducting copper oxide-the infinite-layer compound ACuO2 (where A is an alkaline earth metal)-is an excellent way of investigating the pairing mechanism in high-transition-temperature (high-Tc) superconductors more generally. Gate-induced modulation of the carrier concentration to obtain superconductivity is a powerful means of achieving such understanding: it minimizes the effects of potential scattering by impurities, and of structural modifications arising from chemical dopants. Here we report the transport properties of thin films of the infinite-layer compound CaCuO2 using field-effect doping. At high hole- and electron-doping levels, superconductivity is induced in the nominally insulating material. Maximum values of Tc of 89 K and 34 K are observed respectively for hole- and electron-type doping of around 0.15 charge carriers per CuO2. We can explore the whole doping diagram of the CuO2 plane while changing only a single electric parameter, the gate voltage.     

钡钇铜氧化物陶瓷超导电性的研究

赵忠贤等报导了对Ba_(0.5)Y_(4.5)Cu_5O_(5(3-y)陶瓷超导电性的早期研究结果,经1000℃烧结6小时后炉冷试样的零电阻超导温度为78.5K,转变宽度为4K.测量电阻-温度曲线所用的电流密度为0.04A/cm~2,未给出临界电流密度Jc的值. 本文所用试样的成份为Ba_2Y_2Cu_3O_(3(3-y′).混合均匀的金属氧化物粉末由草酸盐分解法制得.氧化物粉末经压制成型后于空气中加热至920℃至1050℃烧结6小时炉冷.X-衍射分析结果表明,试样基本上为单相的准正交相,其晶格常数为a=3.840A.b=3.906A,c=11.718A.断口扫描测试表明,该试样的平均晶粒尺寸小于5μ,晶粒大小均匀.断口相呈部份穿晶断裂,有不太明显的断裂台阶,韧性及强度均较好.能谱