Iron-based High-temperature Superconductors, Specific Heat And Very Low Thermal Transport Properties

Since the discovery of superconductivity at the beginning of last century, scientists have been searching for superconducting materials with higher critical temperature (Tc),and to find room temperature superconductor is one of the biggest dreams to the scientists. After the breakthrough of cuprates, the research on high Tc superconductivity is getting slow in the 21st century. At present, the physical mechanism of high Tc superconductivity is still a controversial issue, thus researchers are hoping to find new high Tc superconductors other than cuprates.Just in the early of 2008, a Japanese group announced the discovery of a new high Tc superconductor-LaO1-xFxFeAs with Tc=26 K. Since then, new series of so-called iron-based superconductors are found successively, and a new round of research on high Tc superconductor has begun.In this dissertation, we will make a particular research and discussion on the specific heat and ultra-low temperature thermal transport experimental results about several kinds of iron-based superconductor systems.In the first three chapters, we will give a detailed introduction on the research motivation and relevant theory backgrounds, including research progress on iron-based high Tc superconductors, theories on transport measurements and corresponding experimental facilities. The rest part of this dissertation will report our specific heat and ultra-low temperature thermal transport on three different kinds of iron-based high Tc superconductor materials:1111,122 and 11 systems.The content structure of this dissertation is as follows:Chapter One:A brief historical retrospect on superconduction and high Tc superconductor, the discovery and experimental and theoretical progress on iron-based high Tc superconductors in time series.Chapter Two:Fundamental theories of specific heat and thermal conduction, and their applications on superconduction research and superconducting gap behavior in ultra-low temperature thermal conduction measurements.Chapter Three:An introduction to experimental facilities we are using, including PPMS system with specific heat option and ultra-low temperature thermal transport measurement system.Chapter Four:Report of specific heat research on iron-based superconductor of 1111 system-SmO1-xFxFeAs. Chapter Five:Report of specific heat research on iron-based superconductors of 122 system-Ba1-xKxFe2As2 and Ca1-xNaxFe2As2.Chapter Six:Report of ultra-low temperature thermal transport on electron-doped iron-based superconductor of 122 system-BaFe1.9Ni0.1As2.Chapter Seven:Report of ultra-low temperature thermal transport on iron-based superconductor of 11 system-FeSex.Chapter Eight:A conclusion of all our research works.