Semifluxons in ferromagnetic Josephson junctions

在O-π约瑟夫森结中，只携带一半磁通量量子(半通量)的涡旋可能自发出现。鉴于其新颖的基础物理和在超导电子学中的潜在应用，这类涡旋的物理引起了人们的极大兴趣。利用superconductor-insulator-ferromagnet-superconductor(SiFS)技术，可以制作出低损耗的O-π约瑟夫森结(当使用约瑟夫森结作为有源元件时所需要的)和几乎任意形状的结和O-π不连续。然而，现有技术存在缺陷：结的π部分的最大超流密度JCπ很低(40A/cm2或更低)，导致涡旋尺寸()经常超过60微米。因此，包含多个半流子排列的结构几乎不可能实现。在本项目中，我们专注于(A)铁磁性O-π约瑟夫森结技术的进一步发展和(B)此类结中的半流子的实验研究。技术目标是大幅增加π处于π状态(理想情况下至少增加一个数量级)，同时保持结的正常状态下的高电阻Rn和低损耗。这将使分数涡旋(和器件)变得小得多，适合于在经典或量子体制下运行的电路。这一目标将通过以下两条路线实现。首先，将对现有的(S|我|F|S)约瑟夫森结技术进行修改，以包括一个清洁的低阻尼铁磁体。第二，现有的超导体-铁磁绝缘体-超导体(S|FIRS)约瑟夫森结技术(例如，FI=FeSi)将在化学计量比和F1层厚度方面进行优化。利用这些结构，在300MK到6K的温度范围内进行了实验，分数阶涡旋可以看作是经典的非线性物体。减小的涡旋尺寸将使尚未探索的多涡旋系统的研究成为可能，例如两个或三个涡旋分子，甚至ID涡旋晶体。该项目将研究分数阶涡旋之间的相互作用，例如本征频率的相互翻转或分裂，以及与整数通量子的相互作用，从而推动铁磁0-π约瑟夫森结技术的发展，该技术也适用于许多其他应用，如自偏置经典电路和量子电路。该项目将提高对分数涡物理的理解，并可能提出新的应用。

In a O-π Josephson junction, vortices carrying only a half of the magnetic flux quantum (semifluxons) may appear spontaneously. The physics of such vortices has attracted a lot of interest both in view of their novel fundamental physics and in view of potential applications in superconducting electronics. Using superconductor-insulator-ferromagnet-superconductor (SIFS) technology O- π Josephson junctions with low damping (required when using Josephson junctions as active elements) and of almost arbitrary shape of both the junction and the O- π discontinuity can be fabricated. However, the existing technology has drawbacks: the maximum supercurrent density jc π in the π parts of the junctions is low (40 A/cm² or less), leading to a vortex size ( ) often exceeding 60 µm. Consequently, structures containing arrangements of several semifluxons are almost impossible to realize.Within this project we focus on (a) the further development of the ferromagnetic O- π Josephson junction technology and (b) the experimental investigation of semifluxons in such junctions.The technological aim is to substantially increase jc π in the π state (ideally by at least one order of magnitude), still keeping the junction normal state resistance Rn high and thus damping low. This will make fractional vortices (and devices) much smaller and suitable for circuits operating in either the classical or quantum regime. This aim will be persued by the following two routes. First, the existing (S|I|F|S) Josephson junction technology will be modified to include a clean ferromagnet with low damping. Second, the existing Superconductor-Ferromagnetic lnsulator-Superconductor (S|FIlS) Josephson junction technology (e.g., FI=FeSi) will be optimized in terms of stoichiometry and the thickness of the Fl-layer. The crossover from S|FI|S to S|F|S limit at high concentration of the ferromagnet will be investigated.Using these structures experiments will be performed in the temperature range between 300 mK and 6 K where the fractional vortices can be considered as classical nonlinear objects. The reduced vortex size will enable investigation of not yet explored multi-vortex systems such as two- or three-vortex molecules and even ID vortex crystals. The interaction of fractional vortices with each other, e.g., mutual flipping or splitting of eigenfrequencies, as well as interaction with integer fluxons will be studied.As a result, this project will push forward the ferromagnetic 0- π Josephson junction technology, which is also useful for many other applications such as self-biased classical and quantum circuits. The project will improve the understanding of the fractional vortex physics and may suggest new applications.