Creation of classical and quantum fluxons by a current dipole in a long Josephson junction
Creation of classical and quantum fluxons by a current dipole in a long Josephson junction
复制标题
通过长约瑟夫森结中的电流偶极子创建经典和量子通量子
DOI：
10.1103/physrevb.69.064502
复制
发表时间：
2003
影响因子：
3.7
通讯作者：
A. Ustinov
中科院分区：
文献类型：
--
作者：
B. Malomed;A. Ustinov
本文研究了长环形Josephson结（JJ）中在一点注入电流并在另一点收集电流的通量子的静态和动态特性。均匀分布的直流偏置电流密度为$\ensuremath{\gamma}$。我们证明，在电流偶极子的无限小尺寸的极限下，对于长度远小于约瑟夫森穿透长度的环形JJ，最近发现的（以弗劳恩霍夫解析形式）的临界值$\ensuremath{\gamma},$不存在静态相位分布，与结的长度无关，包括无限长的JJ。在长环形JJ中，如果$\ensuremath{\gamma}$超过临界值，偶极子产生自由通量(s)。对于长JJ，我们还发现了另一个临界值（也是解析形式），它总是略小于弗劳恩霍夫值，除了偶极子强度为$2\ensuremath{\pi}N$的整数N点，两个值都消失了。产生新临界值的静态相位配置是基于不稳定的通量-反通量束缚态，因此它可能不会在通常的（经典）状态中表现出来。然而，它为自由通量子的隧道诞生提供了一个主导的瞬子配置，因此有望确定超低温下通量子的量子诞生阈值。我们还考虑了自由通量子与由电流偶极子和由其固定的反通量子组成的络合物的相互作用。用解析的形式得到了一个抑制净相互作用力的条件，使得长JJ对自由通量几乎是均匀的。分析结果与数值模拟结果进行了比较。本文的分析与最近提出的将通量子插入环形约瑟夫森结的新实验技术有关。
We study static and dynamical properties of fluxons in a long annular Josephson junction (JJ) with a current injected at one point and collected back at a close point. Uniformly distributed dc bias current with density $\ensuremath{\gamma}$ is applied too. We demonstrate that, in the limit of the infinitely small size of the current dipole, the critical value of $\ensuremath{\gamma},$ above which static phase distributions do not exist, that was recently found (in the Fraunhofer's analytical form) for the annular JJ with the length much smaller than the Josephson penetration length is valid irrespective of the junction's length, including infinitely long JJ's. In a long annular JJ, the dipole generates free fluxon(s) if $\ensuremath{\gamma}$ exceeds the critical value. For long JJ's, we also find another critical value (in an analytical form too), which is always slightly smaller than the Fraunhofer value, except for points where the dipole strength is $2\ensuremath{\pi}N$ with integer N, and both values vanish. The static phase configuration which yields the new critical value is based on an unstable fluxon-antifluxon bound state, therefore it will probably not manifest itself in the usual (classical) regime. However, it provides for a dominating instanton configuration for tunnel birth of a free fluxon, hence it is expected to determine a quantum-birth threshold for fluxons at ultralow temperatures. We also consider the interaction of a free fluxon with the complex consisting of the current dipole and antifluxon pinned by it. A condition for suppression of the net interaction force, which makes the long JJ nearly homogeneous for the free fluxon, is obtained in an analytical form. The analytical results are compared with numerical simulations. The analysis presented in the paper is relevant to the recently proposed new experimental technique of inserting fluxons into annular Josephson junctions.