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

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******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.