Ordering and Frustration of Flux Quantum Arrangement in Superconducting Networks

本课题的目的是通过研究超导回路中磁通量量子的排列，阐明在磁场作用下超导细导线网（超导网络）中令人沮丧的超导状态。本文利用由细铝丝制成的二维超导网络样品，通过测量超导转变温度的微小变化，研究了磁场作用下超导状态的稳定性。首先，我们建立了利用电子束光刻技术制作网络样品的制备工艺。然后，我们构建了两种类型的实验系统来检测超导转变温度的微小变化。第一个是系统在恒定温度下可以检测到微小的电阻变化，但灵敏度不够。二是精确地将样品温度保持在超导转变的中点，并直接测量作为磁场函数的微小温度变化的系统。在这种情况下，样品被用作非常灵敏的温度计，因此我们可以获得足够的灵敏度。利用该系统，我们研究了“三角形”和“菱形”晶格网络，并成功地观察了叠加在通量量化振荡上的精细结构。这些特征反映了受挫通量量子排列的稳定性。通过与理论计算的比较，我们估计相干长度为亚微米级。我们也制作了带有小霍尔探针阵列的网络样品来直接检测磁通排列，但尚未完成系统的实验。

The purpose of the present project is to clarify the frustrating superconducting states in superconducting thin wire nets (superconducting networks) under magnetic fields by studying the arrangement of magnetic flux quanta in superconducting loops of the system. In this work, using the two-dimensional superconducting network samples made of thin wire Al, we investigated the stability of the superconducting state under magnetic fields by measuring the very small change of the superconducting transition temperature. First, we established the fabrication process to make the network samples using the electron beam lithography technipue. Then, we constructed two types of experimental sytems to detect the small change of the superconducting transition temperature. The first one was the system to detect small resistance change at a constant temperature, but its sensitivity was not enough. The second one was the system which exactly maintains the sample temperature at the middle point of the superconducting transition and directly measures the small temperature change as a function of magnetic fields. In this case, the sample was used as a very sensitive thermometer, so that we could obtain the sufficient sensitivity. Using this system, we studied the "triangular" and "rhombic" lattice networks, and successfully observed the fine structures superposing on the flux quantization oscillations. These features reflect the stability of frustrating flux quantum arrangement. We estimated the coherence length as sub-microns from the comparison with the theoretical calculation. We also fabricated the network samples with small Hall probe array to detect the flux arrangement directly, but the systematic experiments have not completed yet.