层状超导体中的磁通静态结构和动力学行为及介观超导体的磁通分布的研究

发布时间：2018-02-27 02:34

本文关键词： 涡旋饼磁通格子磁通钉扎高温超导体介观超导体　出处：《东南大学》2016年博士论文　论文类型：学位论文

【摘要】：层状超导体中磁通的输运性质以及相关机理的研究一直是凝聚态物理研究的热点。本文首先在x和y方向使用周期性边界条件,z方向使用开放性边界条件,在全部钉扎强度区域研究了静态和动态情形下3D系统中涡旋饼的结构分布,给出了涡旋饼在不同钉扎强度和不同密度下的静态结构相图;总结出了涡旋饼在外推力作用下的动力学规律及奇异特征。考虑到最接近实际的模型应该采用长周期性边界条件,我们通过降维(二维或准一维)和增加层间距的方法来减少计算量,并利用这一模型继续研究层状超导体中涡旋饼的性质,寻找第二峰效应等实验现象的新解释。近年来介观超导体中的磁通分布也引起了人们的广泛关注,不同于大块超导体,介观超导体样品尺寸很小,磁通的分布不仅由磁通间的相互作用决定,还由边界的限制决定。故不同形状的介观超导体对应的磁通分布亦不同,我们主要研究的是等腰直角三角形介观超导体。本文主要内容分为如下几部分:1、我们用数值模拟的方法研究了层状超导体中涡旋饼的静态结构分布,分析了随机钉扎引起的无序和层间(以及层内)的有序性的竞争关系。通常,对于层间有序的形成,来源于强层间耦合弱钉扎条件下涡旋饼或磁通线片段的耦合,即形成3D状态。相反,2D状态则是由弱层间耦合强钉扎条件下退耦合的独立涡旋饼形成。对层间有序性,随着钉扎力的增加,层内的结构涉及到晶体(Crystal),布拉格玻璃(Bragg Glasses,BG),磁通玻璃(Vortex Glasses,VG)和液体状(Liquid-like,LQ)结构。改变磁通密度,我们发现在同一层内部和层与层之间都存在一个先快速无序然后缓慢有序的过程,这可能与第二峰效应有关联,我们对这种非单调行为的原因也做了讨论。研究结果总结为"层间耦合强度Sm和钉扎强度fp "平面内的相图。2、使用分子动力学模拟的方法研究了外力驱动下的层状超导体中的涡旋饼。我们发现,随着外加驱动力的增加,对于强的层间耦合,预先存在的磁通线要么直接脱钉,要么在脱钉之前先转变为二维(2D)钉扎态,是哪种情况要取决于钉扎的强弱;在一个很窄的钉扎范围内,我们还发现了有趣的再次钉扎过程,它导致了负微分电阻现象的出现。对于弱的层间耦合,随外加驱动的增加,被独立钉扎的涡旋饼先是形成无序的2D流动态,然后转变为有序的3D流动态;然而,对于极强的钉扎情形,随机钉扎导致一个类似热涨落力的作用效果出现,这一作用可以融化3D磁通线,导致一个持续的2D流动态出现在快速流动区,而且,在它的中部区域关联函数有一个尖峰现象出现:随外加驱动力增加,运动的涡旋饼先晶体化形成运动的3D磁通线,然后这些3D磁通线融化,导致重新形成2D流动态。我们的结果总结成了一个动力学相图。3、我们采用了最接近真实系统的长周期性边界条件的模型,但由于计算量大,用了两种方法修改模型:降维的方法(保留z方向自由度,将x-y平面的维度缩减为一维)和增加层间距(同时微调涡旋饼相互作用)的方法。对于降维后的系统,不再是三维系统(或传统二维x-y平面)的三角格子的分布,这是因为去掉了y方向的维度,涡旋饼的有序排列为平行于z轴方向的等间距直线。我们计算了不同钉扎强度、不同层数时,稳定态涡旋饼分布的关联函数随磁通密度的变化,并观察磁通分布图,发现随磁通密度的增大,系统会由有序的1D状态退耦合为无序的2D状态,分析原因是在高磁通密度下,晶格常数变小,相互作用势的竞争将导致退耦合态与耦合态的能量差降低,导致了退耦合相变,这个退耦合与实验上观察到的第二峰效应相符合。4、我们研究了等腰直角三角形介观超导体中的磁通分布。我们使用格林函数方法寻找伦敦方程的解析解,得到了任意磁通分布的吉布斯自由能表达式,并使用镜像方法把磁通-磁通和磁通-边界之间的相互作用转化为磁通-反磁通之间的作用,然后我们结合分子动力学方法,模拟退火获得磁通涡旋的稳定态分布。研究发现随外磁场增加,磁通涡旋的填充规则如下:其分布按一层层"壳"状排列,在小涡旋度时,磁通呈轴对称分布;随涡旋度的增大,磁通开始出现不对称的分布;涡旋度越大,磁通的分布模式越复杂。另外我们还研究了系统的尺寸效应,改变系统尺寸,磁通的分布模式也会有变化,我们的结果总结为一张表。
[Abstract]:Study on layered superconductors in magnetic flux transport properties and related mechanism has been a hot topic in condensed matter physics research. Firstly, using periodic boundary conditions in X and Y direction, the open boundary conditions using Z in all directions, nail structure distribution of tie strength area of static and dynamic 3D system under the situation of vortex cake the cake is given in different vortex pinning strength and static structure under different density of phase diagram; summed up the dynamics of vortex cake in thrust and singular feature. Considering the most close to the actual model should be adopted in long periodic boundary conditions, we through dimensionality reduction (2D or 1D) and method of increasing the layer spacing to reduce the amount of calculation, and use this model to study the properties of layered ultra vortex pancake in the conductor, a new explanation for second peak effect experimental phenomena. In recent years in the mesoscopic superconductor The flux distribution has also aroused people's attention, is different from the bulk superconductor mesoscopic superconductor, the sample size is very small, not only the flux distribution is determined by the interaction between the magnetic flux, is determined by the boundary constraints. The flux distribution is also different so different shapes corresponding to the mesoscopic superconductor, we study the isosceles right the triangle mesoscopic superconductor. Main contents of this dissertation are as follows: 1, we use the method of numerical simulation of the distribution of static structure of layered ultra vortex pancake in the conductor, analyzed the random pinning caused by disordered and inter layer (and layer) competition between order. Usually, to form an orderly between the layers, from the strong coupling between layers under the condition of weak coupling with nail cake or vortex flux line segments, namely the formation of 3D. On the contrary, the 2D state is weak interlayer coupling strong pinning conditions under coupling of independent vortex pie . the interlayer order, with the increase of the pinning force, structure layer related to the crystal glass (Crystal), Prague (Bragg Glasses, BG), (Vortex Glasses, VG glass flux) and liquid (Liquid-like, LQ). The structure change of flux density, we found that in the same layer and the layers have a first fast and then slow disorderly orderly process, which may be associated with a second peak effect, also discussed the reasons for our non monotonic behavior. The research results are summarized as the coupling strength between the Sm layer and the pinning strength phase diagram of.2 FP plane, is studied the external driving force, the vortex cake layered superconductors using molecular dynamics simulation method. We found that, with increasing the driving force for the strong coupling between layers, pre-existing flux lines either directly or in off the nail, nail off before you into a two-dimensional (2D) pinning state is. What kind of depends on the pinning strength; in a very narrow range of pinning, we also found interesting again pinning process, it leads to a negative differential resistance phenomenon. For weak coupling between layers, with the increase of external driving, was first formed out of order 2D flow dynamic nail pierced scroll cake independently, then turned into an orderly flow of dynamic 3D; however, for strong pinning, random pinning effect leads to a similar thermal fluctuating force, this effect can melt 3D flux line, resulting in a continuous flow 2D dynamic in the rapid flow area moreover, in the central region, the correlation function which has appeared a phenomenon: with the driving force increasing, the vortex movement to form crystal cake first 3D flux line movement, and then the 3D flux line leads to formation of 2D melt flow dynamics. We summarize the results of a move We use the phase diagram of.3 mechanics, boundary condition of long period the most close to the real model of the system, but due to the large amount of calculation, using the two methods to modify the model: dimension reduction method (retention direction Z degrees of freedom, X-Y will reduce the plane dimension to one dimension) and increase the layer spacing (with a fine scroll cake interaction) method. For the system after dimensionality reduction, is no longer a three-dimensional system (or traditional 2D X-Y plane) distribution of the triangular lattice, this is because removing the Y direction dimension ordered vortex cake is parallel to the Z axis space with a linear. We calculated the different pinning strength. Different layers, stable state correlation function of cake distribution changes with vortex flux density, and to observe the flux distribution, found with increasing magnetic flux density, the system will by ordered 1D state decoupling 2D disordered state, the reason is analyzed in high flux density, crystal The lattice constant is smaller, the interaction potential competition will lead to decoupling state and coupling state energy difference decreases, leads to the decoupling transition, the second peak effect decoupling with the experimentally observed phase with.4, we studied the isosceles triangle mesoscopic flux distribution in superconductors. We use Green function method the analytical solution for the London equation, Gibbs free flux distribution can be arbitrary expressions, and use the image method to the interaction between magnetic flux and magnetic flux - boundary into flux between reverse flux, then we combine molecular dynamics method, simulated annealing to obtain stable distribution of vortex. The study found that increased with the external magnetic field, filling rules are as follows: the vortex distribution by layers "shell" like arrangement, in small vorticity flux, are symmetric; with the increase of vorticity flux. The asymmetric distribution began to appear. The larger the swirl, the more complex the distribution pattern of the magnetic flux. Besides, we also studied the size effect of the system, changing the size of the system, and the distribution pattern of the flux will also change. Our results are summarized as a table.

【学位授予单位】：东南大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O511.3

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