【作者】 何国良；

【导师】 刘楣；

【作者基本信息】 东南大学 ， 凝聚态物理， 2006， 硕士

【摘要】 近年来，在外界驱动电流作用下，Ⅱ类超导体的磁通运动的动力学性质一直引起人们很大的关注。由于磁通系统本身存在的周期性结构和无序因素产生的相互竞争，系统出现了许多复杂的动力学性质。考虑陶瓷超导体的自身的晶体结构和相分离，其超导单元间存在无序的弱耦合，而二维的约瑟夫森结列模型（JJAS）存在分离的晶格结构，JJAS可以控制系统的无序强度和约瑟夫森耦合强度，因此我们可以用无序的JJAS来很好的模拟层状超导体的性质。本文基于RSJ模型,采用Runge-Kutta数值模拟方法,来研究约瑟夫森结列中磁通运动的动力学本质及其噪声频谱的变化情况。主要的结论如下:（1）我们研究了外加磁场条件下，JJAS中磁通系统在不同强度的驱动电流下的动力学特性以及电压关联特性。在弱无序条件下，磁通的脱钉是弹性过程，塑性形变不占据统治地位，脱钉后容易形成弹性流。在强无序条件下，脱钉是一个塑性过程，系统中涡旋反涡旋对的出现先于磁通晶格的脱钉运动，退钉扎后形成较大区域的塑性流动。通过以上的讨论，我们认为磁通晶格的塑性流动相到弹性流动相的转变和涡旋反涡旋对的成核现象都依赖于约瑟夫森结阵列无序度的强弱。（2）我们研究了强无序系统中的磁通晶格在电流驱动下的相变过程。通过计算磁通系统的电压，被激发的涡旋和反涡旋对数，以及磁通运动产生的电压频谱，我们研究了在电流驱动下的磁通运动的动力学相变（DMT）和在各个区域的运动特性，提出了一些有创新的观点。我们首次提出了电流驱动下的二次动力学相变观点：I_p是区分磁通运动塑性流区域和弹性流区域的相变点；I_g是区分弹性流和MBG相的相变点。Koshelev和Vinokur已提出了DMT点I_p已被先前的工作证实。我们提出的二次DMT点I_g最近在实验上也被Kokubo等人的工作所验证。Kokubo等人发现在NbSe₂在磁化峰值以上区域，磁通运动发生从弹性流到MBG相的DMT。第二，我们研究磁通运动产生的电压频谱，得到磁通在三个不同的运动状态下分别产生低频BBN,洛仑兹NBN，和搓衣板噪声NBN，我们得到的图像与最新实验结果相吻合。通过分析磁通运动产生的频谱变化，我们认为电压噪声谱是由磁通间的互作用和系统无序之间的竞争而产生的。当驱动电流较小时，系统的无序钉扎在磁通运动中起主导作用，其频谱中出现明显的BBN；随着外加电流更多还原

【Abstract】 In recent years much theoretical and experimental effort has been made to study the dynamic phenomena of vortex motion driven by an external current in type-II superconductors. Many complex dynamic phenomena in the vortex system arise from a competition between spontaneously periodic structures of interacting vortices and disordered factors. Since the ceramic superconductors, especially in their polycrystalline form, behave in many respects like a random arrangement of weak links between superconducting grains. Thus a discrete lattice structure of two-dimensional (2D) Josephson junction arrays (JJAS), with controlled disorder and controlled Josephson coupling, may shed light on such layered superconductors or film materials. In this dissertation, based on RSJ model and by means of Runge-Kutta numerical simulation methods, dynamical phenomena of moving vortices and voltage noise spectra are studied in disordered Josephson junction arrays. The main results are listed as follows:(1) Using a numerical simulation, we have investigated the threshold currents, voltage–voltage correlation and current-driven dynamics of vortex systems in the disordered JJAS under a magnetic field. For the weak disorder, the depinning of the vortex lattice is driven by elastic deformation, and plastic deformation does not occur dominantly. For the strong disorder, vortex–antivortex pairs are excited prior to the depinning of vortex lattices. Depending on the strength of the disorder, the plastic flow (deformation) of vortex lattices or the nucleation of vortex–antivortex pairs occurs at a certain critical current.(2) For the strong disorder, we study the dynamic melting transition (DMT) driven by current in the vortex lattices. By calculating the voltage drop across the array, the vorticity due to the excitation of vortex and antivortex pairs (VAPS), and the voltage noise spectra of the moving vortices, we study the dynamical phenomena and DMT of moving vortices in disordered JJAS, and propose some innovative viewpoints. We first propose two dynamic melting transitions driven by current: one is at Ipto更多还原