Search for New Quantum Effects in Superconducting Vortices

点击翻译按钮获取中文摘要

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.