Research for Superconducting Pairing Symmetry in Unconventional Superconductors by Geometry Effects

实验结果表明，层状钌酸盐Sr_2RuO_4是具有简并手性d向量的自旋三重态超导体。在具有手性的自旋三重态超导体中，在具有相反手性的两个超导畴上的d矢量动力学有望被实验观察到。为了研究Sr_2RuO_4单晶样品的手性超导特性和涡旋夹销特性，在平行于[001]轴的场方向上进行了Little-Parks振荡和静态磁化测量。为了进行Little-Parks振荡测量，用FIB法制备了带有小孔的Sr_2RuO_4微小单晶样品。我们成功地观察到了样品的电阻率随场周期性振荡的现象，并验证了这种振荡是否真的是超导性所固有的。在超导混合态磁化曲线的弱场处发现了反常的钉钉特征：在温度至少达到0.2 K时，在零场附近连续出现微小的磁通跳变。在较高温度下，在0.1 kOe以下出现一个小的磁滞磁化峰，即所谓的第二磁化峰（SMP）。磁滞磁化的大小强烈地依赖于（独立于）在SMP出现的HSMP场下方（上方）的场梯度。这些现象可以通过假设超导畴在场梯度或超电流的作用下容易移动，导致畴边界处的有效钉合力减小来解释。HSMP周围异常磁化特征的另一个可能来源是涡旋态从布拉格玻璃到涡旋玻璃的拓扑转变，偶尔在“超清洁”II型超导体中观察到。

From several experimental results, the layered ruthenate Sr_2RuO_4 is evidenced to be a spin-triplet superconductor with the degenerate chiral d-vector. In the spin-triplet superconductors with chiral d-vectot dynamics on the two superconducting domains with opposite chiralities is expected to be experimentally observed. In order to investigate the chiral superconducting characteristics as well as the vortex-pinning properties, the measurements of Little-Parks oscillation and static magnetization on the single crystalline samples Sr_2RuO_4 have been performed in the field direction parallel to the [001) axis.For the Little-Parks oscillation measurement, tiny single crystalline samples of Sr_2RuO_4 with small holes were prepared by using FIB. We succeed in observing the phenomena that the resistivity of the samples periodically oscillates with field, and we are checking whether this oscillation is really intrinsic to the superconductivity or not.Anomalous pinning features are found at weak fields in the magnetization curves in the superconducting mixed state: Tiny flux-jumps successively appear around zero-field at temperature up to at least 0.2 K. At higher temperatures, a small peak of the hysteretic magnetization is observed below 0.1 kOe, which is so-called second magnetization peak (SMP). The magnitude of the hysteretic magnetization is strongly dependent on (independent of) the field-gradient in the field region below (above) the field HSMP where the SMP appears. These phenomena can be explained by assuming that the superconducting &domains easily move under the field-gradient or super current flow, leading to the reduction of the effective pinning-force at the domain boundaries. Another possible origin of the anomalous magnetization features around HSMP is the topological transition in the vortex-state from Bragg-glass to vortex-glass, occasionally observed in "super-clean" type II superconductors.