Superconductivity In Pressurized CeRhGe₃ And Related Noncentrosymmetric Compouds

The superconducting mechanism of unconventional superconductors has been one of the core issues in condensed matter physics for decades.In particular,it is expected that the study of the interplay between magnetism and superconductivity as well as quantum criticality will help to understand the superconducting machnism of unconventional superconductivity.Pressure is an important control parameter to change the crystal and electron structure and the corresponding physical properties without introducing chemical complexity.In this Ph.D thesis,high-pressure physical properties of non-centrosymmetric heavy fermion compound CeRhGe₃ have been studied by a series of high-pressure measurements of resistance,ac susceptibility,specific heat,and synchrotron radiation X-ray diffraction.The obtained experimental results provide some fresh and important information for understanding the superconducting mechanism of Ce-based heavy fermion superconductors.The main contents of this thesis include the following four aspects:In Chapter I,the main discovery and research progress of heavy fermion superconductors are briefly reviewed.Compared with conventional superconductors,the basic features and the typical phase diagram of heavy fermion superconductors are summarized.The issue related to quantum criticality,which is an important issure in heavy fermion superconductors,iron-based superconductors and copper oxide superconductors,is highlighted.For heavy fermion superconductors,its quantum criticality is more complicated due to the localized and itinerant duality of f-electrons.Especially in the case of CeAu₂Si₂,the anomalous phenomenon that the superconducting transition temperature and the antiferromagnetic transition temperature increase with the increase of the pressure was discovered,indicating a more complicated relationship between the magnetism and superconductivity.In addition to the magnetic fluctuations mediated superconductivity,the theory of valence fluctuations mediated superconductivity in heavy fermion compounds and the method of low-temperature resistance scaling analysis are briefly introduced.Forthermore,the special roles of high pressure for the studies on heavy fermion superconductors is given.Finally,the discovery and research progress of non-centrosymmetric heavy fermion superconductors,which is the focus of this thesis,are reviewed.In Chapter II,the typical high-pressure experimental techniques and the experimental equipment used in this study are mainly introduced,including the high pressure measurements of resistance,ac susceptibility and synchrotron radiation X-ray diffraction and absorption in a diamond anvil cell.Moreover,the method of high pressure heat capacity in a Toroid-type pressure cell is briefly introduced.Finally,high-pressure-ultra-low-temperature system set up mainly for the study of heavy fermion compounds is presented.In Chapter III,our high-pressure research results from the studies on a typical CeTX₃-type non-centrosymmetric heavy fermion compound CeRhGe₃,the last non-superconducting member in CeTX₃（T=Co,Rh,Ir,and X=Si,Ge）are presented.The results of high pressure specific heat measurements show that the two antiferromagnetic transition temperatures of CeRhGe₃ increase with increasing pressure below 5 GPa.At higher pressures,resistance measurements were carried out,which show that the two antiferromagnetic transition temperatures merge together at about 13.7 GPa and then decreases with increasing pressure.At around 20 GPa,pressure-induced superconductivity is observed.The results of high pressure XRD show that the pressure-induced superconductivity in CeRhGe₃ does not originate from the structural phase transition.It is consistent with the property of the upper critical field at 21.5 GPa,which is higher than the Pauli paramagnetic limit.In Chapter IV,the high pressure behaviors of the six typical non-centrosymmetric heavy fermion compounds CeTX₃（T=Co,Rh,Ir,asnd X=Si,Ge）are compared.By establishing the relationship among the cell volume(（1₀）at ambient pressure,the critical pressure(_（84）)at which the superconducting transition temperature reaches the maximum(⁸（6）)and the corresponding critical cell volume((1_（84）)at the critical pressure(_（84）),the conclusions that the optimal Kondo hybridization is necessary for the development of superconductivity and the strength of spin-orbit coupling is favor to obtain the highest superconducting transition temperature are achieved.Further,the possible role of valence fluctuations in developing the superconductivity of CeRhGe₃is discussed by a scaling analysis of the low-temperature resistance.Note that there is a critical end point of the first-order valence transition at（²2.0 GPa,^-20.0 K）,but the critical negative temperature means that only a valence crossover is realised instead of a real first-order valence transition.We propose that the valence fluctuations associated with the first-order valence transition still play an important role in the low-temperature physical properties,such as the superconductivity and the non-Fermi liquid behavior at the critical pressure.