Search for New Quantum Effects in Superconducting Vortices

本研究利用表面阻抗测量、比热测量和扫描隧道显微镜（STM）技术，研究了高T_c铜氧化物、NbSe_2和YNi_2B_2C等清洁超导体的涡旋态电子态，并成功地建立了在磁场作用下工作在0.5K以下的表面阻抗测量装置。用该装置测量了YBa_2Cu_3O_y和强各向异性s波超导体YNi_2B_2C的磁通流电阻率。通过分析发现，YBa_2Cu_3O_y的涡旋核电子态处于所谓的“适度干净”区，涡旋核与经典的正常态核有些不同。在YNi_2B_2C中，磁通流电阻率几乎是传统的Barnes-Stephen模型所预期的两倍。我们还测量了不同NbSe_2晶体的电子比热系数随磁场H的变化，发现随着样品纯度的增加，准粒子态密度（QPDOS）偏离了传统的H线性行为。通过对重离子辐照引入的柱状缺陷样品的测量，发现QPDOS异常是由涡核外的准粒子引起的。从电阻率测量和STM观测的比较中，我们得出结论，QPDOS异常与超导共存的电荷密度波的演化密切相关。

In this study, we examined the electronic states of vortex states of various clean superconductors such as high-T_c cuprates, NbSe_2 and YNi_2B_2C by utilizing the surface impedance measurements, specific heat measurements and scanning tunneling microscopy (STM).We succeeded in building a surface impedance measurement apparatus which works at very low temperatures down to 0.5K under magnetic fields. By using this apparatus, we measured flux flow resistivity of YBa_2Cu_3O_y and strongly anisotropic s-wave superconductor YNi_2B_2C. From the analysis, we found that the electronic state of vortex core of YBa_2Cu_3O_y is in the so-called "moderately clean" regime and vortex core is somewhat different from classical normal state core. In YNi_2B_2C, flux flow resistivity was almost twice as large as that expected from the conventional Bardeen Stephen model. This result indicates that the energy dissipation in the vortex core is strongly affected by the anisotropy of the superconducting gap.We also measured the magnetic field H dependence of electronic specific heat coefficients of various NbSe_2 crystals and found that the quasi-particle density of states (QPDOS) deviates from conventional H-linear behavior with increasing sample purity. From measurements on the sample containing columnar defects introduced by heavy ion irradiation, we found that the QPDOS anomaly is brought by quasi particles outside of vortex cores. From the comparative resistivity measurements and STM observations, we conclude that the QPDOS anomaly is strongly correlated with the evolution of charge-density wave which coexists with superconductivity.