Theoretical study for the superconducting state in nano-superconductors

在这个研究项目中，我们从基础物理和量子器件应用的角度研究了非常规d波和常规S波超导体组成的小的复杂系统中的约瑟夫森效应。当d(x^2-y^2)对称的d波超导体嵌入到S波超导矩阵中时，系统形成一个闭合的0-π约瑟夫森结，在该结中自发产生具有半通量量子的涡旋。我们构造了一个描述这个复杂系统中的相动力学的理论。当系统的大小足够小时，量子涨落主导着系统。我们严格地证明了该系统在量子区域内表现为量子二能级系统。讨论了量子比特的应用。在本研究项目中，我们还研究了在高温超导体Bi2212中观察到的本征约瑟夫森效应。阐明了在平行于各层的磁场作用下，在Bi2212中观察到的微波共振吸收光谱的反常场依赖性的原因。文中还讨论了本征约瑟夫森结的I-V特性和太赫兹辐射。

In this research project we studied the Josephson effects in small complex systems composed of unconventional d-wave and conventional s-wave superconductors from viewpoints of both basic physics and quantum devise applications. When a d-wave superconductor with d(x^2-y^2)-symmetry of square shape is embedded in a s-wave superconducting matrix, the system forms a closed 0-π Josephson junction in which vortices with half-flux quanta appear spontaneously. We construct a theory describing the phase dynamics in this complex system. When the size of the system is small enough, the quantum fluctuations dominate the system. We rigorously prove that this system behaves as a quantum two-level system in the quantum regime. The qubit applications were discussed. In this research project we also investigated the intrinsic Josephson effect observed in Bi-2212 high temperature superconductor. The origin of unusual field dependence of the microwave resonance absorption spectrum observed in Bi-2212 under a magnetic field parallel to the layers was clarified. The I-V characteristics and terahertz emission in a stack of intrinsic Josephson junctions were also discussed.