High Pressure Studies On Iron Base-112 Type And 1111 Type Superconductor And Rare Earth Borides

This Ph.D thesis presents the high pressure studies on two kinds of iron-based superconductors and rare earth hexaborides.In the high pressure experiments,two kinds of high pressure cells have been adopted for in-situ high pressure measurements of resistance,Hall coefficient,magnetic susceptibility and heat capacity.In combination with synchrotron radiation facilities,high-pressure X-ray diffraction(XRD)and X-ray absorption experiments were also carried out.On basis of interesting phenomena observed,the underlying physics have been discussed.The findings achieved from the studies on iron-based superconductors are expected to inspire understandings on the superconducting mechanism,and the phenomena revealed from the strongly correlated systems with f electron are also expected to be helpful for solving the puzzling problems in these materials.This thesis mainly includes the following four chapters:In the first chapter,the history and current situation of the research on unconventional superconductors are reviewed.Then comparison between the cuprates and iron-based superconductors is made.Finally,the development of high pressure technology and its unique role in tuning superconductivity is described.In the second chapter,the methods adopted in this thesis work,including in-situ high-pressure resistance,Hall coefficient and ac susceptibility measurements,as well as synchrotron X-ray diffraction and absorption measurements in a diamond anvil are introduced.Moreover,the means of material synthesis under high pressure and high temperature are addressed.In the third chapter,the high pressure studies on Ca0.73La0.27FeAs2 and GdFeAsO0.85 have been presented.By applying complementary measurements of high-pressure resistance,ac susceptibility heat capacity,a bi-critical point between antiferromagnetic and superconducting phases has been discovered in Ca0.73La0.27FeAs2.This phenomenon is found for the first time in high-temperature superconductors.The results obtained from the high-pressure heat capacity measurements on GdFeAsO0.85 were reported.It was found that the?C/TC ? T 2 of the GdFe AsO0.85,a "1111" type of iron-based superconductor,also follows the BNC scale rate,although its value is smaller than that of "122" type.In the fourth chapter,the high pressure behaviors of the hexaborides,SmB6 and YbB6 have been described.By performing high-pressure resistance,Hall,X-ray diffraction and X-ray absorption experiments in a diamond anvil cell,we associate the gaps of SmB6 with the bulk Kondo hybridization and demonstrate an insulator-to-metal transition at ~4 GPa.At the transition pressure,a large change in the Hall number and a divergence tendency of the electron-electron scattering coefficient provide evidence for a destruction of the Kondo entanglement in the ground state.Our results raise the new prospect for studying topological electronic states in quantum critical materials settings.In the high pressure studies on YbB6,the observation of two pressure-induced quantum phase transitions,i.e.,from a topologically trivial semiconductor to an intermediate semimetal and then from the semimetal to a possible topologically nontrivial high-pressure gapped,was found phase through in-situ high-pressure transport and synchrotron x-ray diffraction measurements.Our high-pressure absorption results reveal that the mixed-valence state in pressurized YbB6 above 15 GPa plays an important role in the formation of the high-pressure state with a reopened gap.These high-pressure results may be helpful for shedding light on the intriguing relation between the topology and the 4f electrons in the rare-earth hexaborides.