本文關(guān)鍵詞： 渦旋餅 磁通格子 磁通釘扎 高溫超導(dǎo)體 介觀超導(dǎo)體　出處：《東南大學(xué)》2016年博士論文　論文類型：學(xué)位論文

【摘要】：層狀超導(dǎo)體中磁通的輸運(yùn)性質(zhì)以及相關(guān)機(jī)理的研究一直是凝聚態(tài)物理研究的熱點(diǎn)。本文首先在x和y方向使用周期性邊界條件,z方向使用開放性邊界條件,在全部釘扎強(qiáng)度區(qū)域研究了靜態(tài)和動(dòng)態(tài)情形下3D系統(tǒng)中渦旋餅的結(jié)構(gòu)分布,給出了渦旋餅在不同釘扎強(qiáng)度和不同密度下的靜態(tài)結(jié)構(gòu)相圖;總結(jié)出了渦旋餅在外推力作用下的動(dòng)力學(xué)規(guī)律及奇異特征。考慮到最接近實(shí)際的模型應(yīng)該采用長(zhǎng)周期性邊界條件,我們通過降維(二維或準(zhǔn)一維)和增加層間距的方法來減少計(jì)算量,并利用這一模型繼續(xù)研究層狀超導(dǎo)體中渦旋餅的性質(zhì),尋找第二峰效應(yīng)等實(shí)驗(yàn)現(xiàn)象的新解釋。近年來介觀超導(dǎo)體中的磁通分布也引起了人們的廣泛關(guān)注,不同于大塊超導(dǎo)體,介觀超導(dǎo)體樣品尺寸很小,磁通的分布不僅由磁通間的相互作用決定,還由邊界的限制決定。故不同形狀的介觀超導(dǎo)體對(duì)應(yīng)的磁通分布亦不同,我們主要研究的是等腰直角三角形介觀超導(dǎo)體。本文主要內(nèi)容分為如下幾部分:1、我們用數(shù)值模擬的方法研究了層狀超導(dǎo)體中渦旋餅的靜態(tài)結(jié)構(gòu)分布,分析了隨機(jī)釘扎引起的無序和層間(以及層內(nèi))的有序性的競(jìng)爭(zhēng)關(guān)系。通常,對(duì)于層間有序的形成,來源于強(qiáng)層間耦合弱釘扎條件下渦旋餅或磁通線片段的耦合,即形成3D狀態(tài)。相反,2D狀態(tài)則是由弱層間耦合強(qiáng)釘扎條件下退耦合的獨(dú)立渦旋餅形成。對(duì)層間有序性,隨著釘扎力的增加,層內(nèi)的結(jié)構(gòu)涉及到晶體(Crystal),布拉格玻璃(Bragg Glasses,BG),磁通玻璃(Vortex Glasses,VG)和液體狀(Liquid-like,LQ)結(jié)構(gòu)。改變磁通密度,我們發(fā)現(xiàn)在同一層內(nèi)部和層與層之間都存在一個(gè)先快速無序然后緩慢有序的過程,這可能與第二峰效應(yīng)有關(guān)聯(lián),我們對(duì)這種非單調(diào)行為的原因也做了討論。研究結(jié)果總結(jié)為"層間耦合強(qiáng)度Sm和釘扎強(qiáng)度fp "平面內(nèi)的相圖。2、使用分子動(dòng)力學(xué)模擬的方法研究了外力驅(qū)動(dòng)下的層狀超導(dǎo)體中的渦旋餅。我們發(fā)現(xiàn),隨著外加驅(qū)動(dòng)力的增加,對(duì)于強(qiáng)的層間耦合,預(yù)先存在的磁通線要么直接脫釘,要么在脫釘之前先轉(zhuǎn)變?yōu)槎S(2D)釘扎態(tài),是哪種情況要取決于釘扎的強(qiáng)弱;在一個(gè)很窄的釘扎范圍內(nèi),我們還發(fā)現(xiàn)了有趣的再次釘扎過程,它導(dǎo)致了負(fù)微分電阻現(xiàn)象的出現(xiàn)。對(duì)于弱的層間耦合,隨外加驅(qū)動(dòng)的增加,被獨(dú)立釘扎的渦旋餅先是形成無序的2D流動(dòng)態(tài),然后轉(zhuǎn)變?yōu)橛行虻?D流動(dòng)態(tài);然而,對(duì)于極強(qiáng)的釘扎情形,隨機(jī)釘扎導(dǎo)致一個(gè)類似熱漲落力的作用效果出現(xiàn),這一作用可以融化3D磁通線,導(dǎo)致一個(gè)持續(xù)的2D流動(dòng)態(tài)出現(xiàn)在快速流動(dòng)區(qū),而且,在它的中部區(qū)域關(guān)聯(lián)函數(shù)有一個(gè)尖峰現(xiàn)象出現(xiàn):隨外加驅(qū)動(dòng)力增加,運(yùn)動(dòng)的渦旋餅先晶體化形成運(yùn)動(dòng)的3D磁通線,然后這些3D磁通線融化,導(dǎo)致重新形成2D流動(dòng)態(tài)。我們的結(jié)果總結(jié)成了一個(gè)動(dòng)力學(xué)相圖。3、我們采用了最接近真實(shí)系統(tǒng)的長(zhǎng)周期性邊界條件的模型,但由于計(jì)算量大,用了兩種方法修改模型:降維的方法(保留z方向自由度,將x-y平面的維度縮減為一維)和增加層間距(同時(shí)微調(diào)渦旋餅相互作用)的方法。對(duì)于降維后的系統(tǒng),不再是三維系統(tǒng)(或傳統(tǒng)二維x-y平面)的三角格子的分布,這是因?yàn)槿サ袅藋方向的維度,渦旋餅的有序排列為平行于z軸方向的等間距直線。我們計(jì)算了不同釘扎強(qiáng)度、不同層數(shù)時(shí),穩(wěn)定態(tài)渦旋餅分布的關(guān)聯(lián)函數(shù)隨磁通密度的變化,并觀察磁通分布圖,發(fā)現(xiàn)隨磁通密度的增大,系統(tǒng)會(huì)由有序的1D狀態(tài)退耦合為無序的2D狀態(tài),分析原因是在高磁通密度下,晶格常數(shù)變小,相互作用勢(shì)的競(jìng)爭(zhēng)將導(dǎo)致退耦合態(tài)與耦合態(tài)的能量差降低,導(dǎo)致了退耦合相變,這個(gè)退耦合與實(shí)驗(yàn)上觀察到的第二峰效應(yīng)相符合。4、我們研究了等腰直角三角形介觀超導(dǎo)體中的磁通分布。我們使用格林函數(shù)方法尋找倫敦方程的解析解,得到了任意磁通分布的吉布斯自由能表達(dá)式,并使用鏡像方法把磁通-磁通和磁通-邊界之間的相互作用轉(zhuǎn)化為磁通-反磁通之間的作用,然后我們結(jié)合分子動(dòng)力學(xué)方法,模擬退火獲得磁通渦旋的穩(wěn)定態(tài)分布。研究發(fā)現(xiàn)隨外磁場(chǎng)增加,磁通渦旋的填充規(guī)則如下:其分布按一層層"殼"狀排列,在小渦旋度時(shí),磁通呈軸對(duì)稱分布;隨渦旋度的增大,磁通開始出現(xiàn)不對(duì)稱的分布;渦旋度越大,磁通的分布模式越復(fù)雜。另外我們還研究了系統(tǒng)的尺寸效應(yīng),改變系統(tǒng)尺寸,磁通的分布模式也會(huì)有變化,我們的結(jié)果總結(jié)為一張表。
[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.

【學(xué)位授予單位】：東南大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2016
【分類號(hào)】：O511.3

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本文編號(hào)：1540777