Studies of the Andreev Conversion Process and Single-Particle Tunneling in Novel and Unconventional Superconductors

非技术摘要超导电性是1911年在极低的温度下在金属中发现的一种以零电阻(完美导电性)和磁场(完美抗磁性)排出为特征的状态。1957年，超导的微观机制被发现；超导体中的单个电子形成对，材料中原子的基本晶格提供了电子配对的基础，即配对相互作用。接近绝对零度的温度被认为是金属成为超导所必需的。1986年，“高温超导体”被发现，这种超导体在更容易获得的温度下超导。不仅那时的应用变得更有前途，而且这些材料代表了一种尚未被理解的新的物质状态。与1986年之前已知的常规超导体不同，在高温超导体中，配对相互作用取决于电子在晶格中移动的方向，因此这些材料被归类为“非常规”材料。在高温超导体之前发现的重费米子超导体也是非常规的，重费米子材料中的电子表现得就像它们非常重一样。非常规超导电性的机制仍然是个谜，所以这个项目的主要任务是从微观上确定电子为什么会配对。为了帮助回答这个问题，我们将研究电子如何通过非常规超导体与规则金属的界面传输。这些研究可能导致开发具有更实用的物理性能和更高的转变温度的超导体，并有可能将超导体和普通金属一起用于微电子电力传输应用。该项目广泛使用材料微观分析技术，并进行了广泛的合作，因此参与的学生将为成为工业、国家实验室和大学中成功的材料物理学家做好准备。这个个人研究人员奖支持对新型和非传统超导体的电子结构和界面电荷传输的实验研究。点接触安德列夫反射光谱(PCARS)和最近发展起来的隧道光谱技术将提供详细的、高分辨率的光谱测量，不仅可以直接探测超导序参数对称性，而且对于阐明超导配对机制也是至关重要的。该项目的重点是重费米子超导体，特别是纯的和掺杂的CeMIn5(M=Co，Ir，Rh)系列超导体，以及其他表现出非磁性和磁性基态的新型超导体。该项目旨在更深入地理解重费米子超导体和正常金属之间安德列夫反射过程的基本原理；这个过程没有被现有的理论解释。为了解决这个问题，将在一系列重费米子正常金属上进行使用超导尖端的pCARS的系统研究。学生将在材料微量分析和电荷传输方面获得广泛的培训。对非传统超导体的基本机制和界面电荷输运的深入了解将有助于创造具有更实用的物理性能和更高的转变温度的超导体，并使超导体和正常金属一起用于微电子电力传输应用。

******NON-TECHNICAL ABSTRACT****Superconductivity, a state characterized by zero resistance (perfect conductivity) and the expulsion of magnetic fields (perfect diamagnetism) was discovered in metals at very low temperatures in 1911. In 1957, the microscopic mechanism for superconductivity was discovered; that the individual electrons in the superconductor form pairs and the underlying lattice of the atoms in the material provided the basis for the electrons to pair, the pairing interaction. Temperatures close to absolute zero were thought to be necessary for metals to become superconducting. In 1986, the "high-temperature superconductors" were discovered, which superconduct at much more easily obtainable temperatures. Not only did applications then become more promising, but these materials represented a new state of matter, yet to be understood. Unlike the conventional superconductors known before 1986, in high-temperature superconductors the pairing interaction depends on the direction that the electron travels in the lattice, so these materials are classified as "unconventional." Heavy-fermion superconductors, discovered just before high-temperature superconductors, are also unconventional, and the electrons within the heavy fermion materials act as if they were very heavy. The mechanism of unconventional superconductivity remains a mystery, so the primary task of this project is to determine, microscopically, why the electrons pair. To help answer this question, investigations of how electrons transport across the interface of an unconventional superconductor with a regular metal will be performed. These studies may lead to the development of superconductors with more practical physical properties and higher transition temperatures and to the potential use superconductors and normal metals together in microelectronics power transmission applications. This project makes extensive use of materials microanalysis techniques and enjoys broad collaborations, so students involved will be well prepared to be successful materials physicists in industry, national laboratories, and universities.******TECHNICAL ABSTRACT****This individual investigator award supports experimental studies of the electronic structure of novel and unconventional superconductors and charge transport across their interfaces. Point-contact Andreev reflection spectroscopy (PCARS) and recently developed tunneling spectroscopic techniques will provide detailed, high-resolution spectroscopic measurements that not only directly probe the superconducting order parameter symmetry, but are also crucial in elucidating the pairing mechanism. The focus of the project is on the heavy-fermion superconductors, particularly the pure and doped CeMIn5 (M=Co,Ir,Rh) series, along with other novel superconductors that exhibit non-magnetic and magnetic ground states. This project is further directed towards a deeper understanding of the fundamentals of the Andreev reflection process between a heavy-fermion superconductor and a normal metal; a process not explained by existing theories. To address this issue, a systematic study of PCARS using superconducting tips will be performed on a range of heavy-fermion normal metals. Students will gain extensive training in materials microanalysis and charge transport. Increased understanding of the basic mechanism and charge transport across the interface of unconventional superconductors will lead to the ability to create superconductors with more practical physical properties and higher transition temperatures, and to potential applications using superconductors and normal metals together in microelectronics power transmission applications.