Superconductivity Of Pressurized Novel Transition-metal Compounds And Weyl Semimetals

The continuous discovery of new superconductors has expanded the understanding of superconductivity and promoted the perfection of related theories of superconductivity.In recent years,the discovery of numerous new materials,such as transition-metal compounds and topological materials,provides important opportunities for exploring unconventional superconductors and the evolution between superconductivity and other novel states.Pressure is an effective way of tuning electronic states and inducing superconductivity in a variety of materials,which plays an important role for exploring unconventional superconducting mechanism and discovering novel superconductors.With the help of high-pressure experimental technologies,especially the diamond anvil cell（DAC）technology and related high-pressure physical property measurement methods,much important progress has been made in the research field for superconductivity.In this dissertation,we investigated the superconductivity under pressures for several novel materials that have recently attracted attention of the field,including a quasi-one-dimensional Mn-based compound,a layered oxide selenide and a Weyl semimetal,mainly focusing to understand the connections of superconductivity with magnetic order,extraordinary crystal structure and topological states,which is at the forefront of the research field for superconductivity.One of the universal features of unconventional superconductors is that the superconducting（SC）state is developed in the proximity of an antiferromagnetic（AFM）state.Unified understanding the interplay between these two states in different superconducting systems is one of the key issues to uncover the underlying physics of unconventional SC mechanism.Through the high-pressure transport measurements,we report the pressure-induced superconductivity in a new quasi-one-dimensional Mn-based compound CsMn₆Bi₅ that bears an antiferromagnetic state at ambient pressure.The superconducting state appears on the border of pressure-suppressed antiferromagnetic order at the critical pressure（P_c）of～12 GPa and stabilizes up to～26GPa.The high-pressure X-ray diffraction measurements on CsMn₆Bi₅ indicate that no structural phase transition occurs at the P_c,indicating that the AFM-SC transition is electronic origin.By comparing the previous results of AMn₆Bi₅（A=K,Rb）,it is the first time to identify that all members of the family possess the genetic flipping behavior of AFM-SC states at almost the same P_c,though their ambient-pressure unit-cell volumes vary quite differently.The theoretical calculations suggest that the pressure-induced changes of partial density of state contributed by the(9 and(9/(9 orbital electrons near Fermi energy may be associated with the origin of the AFM-SC flipping.These results provide a diverse picture of the interplay between the AFM and SC states in the 3d-transition-metal compounds and a novel platform for understanding the mechanism of unconventional superconductivity.Crystal structure often has a significant impact on superconducting properties.The discovery of two families of high-T_c superconductors,cuprates and iron-based superconductors,has inspired the search for new superconductors in compound systems with similar structural characteristics.We have carried out high-pressure experimental research on a new layered oxide selenide compound Ba₂CuO₂Cu₂Se₂.The compound contains the lattice of CuO₂ planes same as in the cuprates and the antifluorite Cu Se layers similar to the FeAs/Se layers in the iron-based superconductors.A transition from insulator to metal is found in Ba₂CuO₂Cu₂Se₂ under pressure,and a possible superconducting transition is observed above～130 GPa.The unique transport behavior under pressure indicates that the compound may be a candidate parent compound of a new unconventional superconductor.The high-pressure properties of Ba₂CuO₂Cu₂Se₂need further experiments to clarify.The non-trivial band structure of topological materials gives rise to fascinating phenomena in transport properties,and finding unconventional superconductivity such as topological superconductivity in topological materials is one of the frontier directions in related fields.We report the observation of pressure-induced superconductivity in type-II Weyl semimetal（WSM）candidate NbIrTe₄ and the coevolution of its Hall coefficient R_H,magnetoresistance MR and lattice.Combined with the results of high-pressure transport measurement and XRD measurement,it is found that a structural phase transition occurs and a pressure-induced superconducting（SC）state emerges synchronously at about 27 GPa,and then the WSM state and the SC state coexist until the orthorhombic phase that protects the WSM state converts to the high-pressure phase completely at 40 GPa.Compared with the high-pressure experimental results of the sister compound TaIrTe₄,it is found that the pressure-tuned evolution from the WSM to the SC state in these two compounds exhibit the same trend,and especially,above40 GPa,an identical high-pressure behavior characterized by almost the same value of R_H and MR in its normal state and the same value of T_c in its SC state appears in both compounds.These results reveal the universal high-pressure behavior of these ternary WSMs and provide a significant information to achieve a better understanding on the connection between the WSM state and the SC state.