Spatially Resolved Quasiparticle Tunneling Spectroscopic Studies of Cuprate and Iron-Based High-Temperature Superconductors

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Technical Abstract:This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). The pairing mechanism for cuprate superconductivity remains elusive to date despite much progress since 1986. Cuprate superconductors are doped anti-ferromagnetic Mott insulators with strong electronic correlation. Depending on the doping level, the ground state may consist of various competing orders besides superconductivity. The existence of competing orders naturally leads to quantum criticality and quantum fluctuations, which significantly affects the low-energy excitations and results in numerous unconventional phenomena. Despite apparent effects associated with competing orders, whether competing orders are relevant to superconductivity remains unclear. One possible approach to addressing this issue is to compare the cuprates with a new class of superconductors, the iron pnictides (with a maximum transition temperature ~ 52 K). These iron pnictides exhibit interesting similarities and contrasts to the cuprates. Therefore, parallel studies of both systems may provide useful insights into the fundamental issue of pair formation in superconductors. The technical approach will focus on investigating the atomically resolved quasiparticle tunneling spectra of both cuprate- and iron-based superconductors as functions of magnetic fields using a homemade high-field cryogenic scanning tunneling microscope and the numerical tools established in this group for data analysis and modeling. The objective is to develop a comprehensive phenomenology for the low-energy excitations in both systems, thereby finding the guiding principles for Cooper pairing in unconventional superconductors. Additionally, the trainings required for conducting the research involve advanced technologies and theoretical knowledge, and are therefore effective for the education of young scientists.Non-Technical Abstract:This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). The discovery of high-temperature superconductivity in a class of oxide materials known as cuprates in 1986 had simulated intense worldwide research efforts because of high expectations for potentially a wide range of applications. Although much progress has been made both scientifically and technologically, the physical cause for the occurrence of high-temperature superconductivity remains unknown. Moreover, the complexity of these materials makes applications of them far more challenging than originally anticipated. In 2008 a class of new superconductors, known as the iron pnictides (with a maximum transition temperature ~ 52 K), was discovered. These new iron-based superconductors reveal interesting similarities and contrasts to the cuprates. The primary thrust of this research project is to conduct parallel studies and comparison of the physical properties of both cuprate and iron-pnictide superconducting systems. The objective of these studies is to provide insights into means of making superconductors with higher transition temperatures and better physical properties, thereby advancing better applications based on superconducting materials. Additionally, the training required for conducting this research project involves a wide variety of advanced technologies and theoretical knowledge, and are therefore effective for the education of young researchers.