Effects Of Pressure On Superconductivity In Alkali Chromium Arsenide

This Ph.D thesis focuses on the high pressure studies for the electrical and structural properties in the alkali chromium arsenides A₂Cr₃As₃?A=K,Rb?,a new type of quasi-one dimensional non-centrosymmetric superconductors.By combining in-situ high pressure ac susceptibility,synchrotron X-ray diffraction and resistance measurements,physical properties of the superconductors and its�brother�non-superconducting compound KCr₃As₃ have been studied.The results from our studies are expected to shed a new light on the underlying mechanism of the superconductivity in these Q1D superconductors and also to provide new perspective in understanding other non-centrosymmetric superconductors.The main contents are as follows:In chapter I,the background of the studied materials is briefly reviewed.First,the history and progress of superconductivity are presented.Then,the related issues on the unconventional superconductors and non-centrosymmetric superconductors are focused.Finally,the important role of high pressure technology in finding new superconductors and revealing corresponding physics are highlighted.In chapter II,the high pressure equipment and methods are briefly introduced.First,the diamond anvil cell,Toroid cell and the combined system of high pressure-low temperature-high magnetic field with comprehensive measurements are presented,including about the measurements of in-situ high pressure resistance,ac susceptibility as well as synchrotron X-ray diffraction.Finally,the methods of pressure determination used in our study are also addressed.In chapter III,the high pressure studies on A₂Cr₃As₃?A=K,Rb?have been reported.We are the first to find that the extent of non-centrosymmetry of A₂Cr₃As₃?A=K,Rb?can be tuned by pressure and it has a positive correlation with the superconducting temperature of the materials.Our in-situ high pressure ac susceptibility and synchrotron x-ray diffraction measurements reveal that the bond angle of As-Cr-As?defined as??in the CrAs chains can be taken as a key factor controlling the superconductivity of the certain superconductor.We also find that the larger value of?-?,which is associated with the extent of the non-centrosymmetry of the lattice structure,is in favor of superconductivity,and a smaller L_CrAs between these two different superconductors benefits a higher T_c.The estimated high pressure electron density distribution?EDD?for K₂Cr₃As₃ shows that the?EDD and the corresponding T_c decrease with increasing pressure,revealing that the extent of inhomogeneous electron state originated from the change of non-centrosymmetric lattice structure is the intrinsic factor for developing/stabilizing the superconductivity in this kind of Q1D superconductors.In chapter IV,the high pressure studies on KCr₃As₃ have been presented.The high pressure resistance measurements reveal that the semiconducting behavior and the formation of spin clusters in KCr₃As₃ can be suppressed by pressure.The activation energy-pressure phase diagram manifests that the gap will be closed at around 31 GPa.