High Pressure Studies On The Superconductivity From Quantum Materials With Novel States

Topological materials show exotic properties due to the novel topological band structure and have drawn great interest,therefore searching for new topological materials and studying the topological properties have been hot topics in the condensed matter physics in the recent years.The potential application of topological materials may provide great chance for the development of science and technology.This Ph.D.thesis mainly focuses on the study of the newly found tetradymite topological insulators(Bi₂Te₂Se and Bi_1.1Sb_0.9Te₂S)and the type-II Weyl semimetals?TaIrTe₄?.We performed systematic studies on these materials through complementary measurements of in-situ high pressure resistance,Hall coefficient and ac susceptibility.The synchrotron high pressure X-ray diffraction measurements are also adopted for observing the variation of the crystal structure under high pressure.In our experimental results,the properties of topological surface states under high pressure condition are shown.Our observation of superconductivity also gives experimental evidence of the correlation between superconductivity and crystal structure.The main contents are as follows:In chapter I,the history of the development of superconductors and topological materials are reviewed.Basic theoretical concepts and experimental evidences are also shown.Finally,a brief introduction to the advantages of high-pressure techniques adopted in the studies of these novel quantum materials is also given.In chapter II,the experimental apparatus is introduced.The specific high-pressure techniques and methods including in-situ resistance measurements,Hall coefficient measurements and ac susceptibility measurements are reviewed.In chapter III,the high pressure studies of the newly found tetradymite topological insulators(Bi₂Te₂Se and Bi_1.1Sb_0.9Te₂S)are presented.Topological insulators possess insulating bulk but metallic surface states due to band inversion.This chapter mainly focuses on the effects of pressure and temperature on the topological surface state and insulating bulk state.The experimental results show that the conduction of bulk state increases orders of magnitude while applying pressure,however,surface state conductance remains constant.The two-band fit shows excellent consistence with the experimental results.Our finding shows a straight forward way in confirming the independent behavior of the bulk state and the surface state in the topological insulators.In chapter IV,the pressure-induced superconductivity is observed in the newly found topological insulators(Bi₂Te₂Se and Bi_1.1Sb_0.9Te₂S.Here,they are defined as BTS and BSTS).BTS and BSTS become superconducting at⁸ GPa and¹2 GPa,respectively.Through temperature dependence of resistance measurements,two distinguished superconducting phases are observed under high pressure,which is associated with the phase transition as revealed by our high pressure synchrotron XRD measurements.By comparing the results with other topological insulators in the tetradymite family,the universal superconducting phase diagram is obtained,which illustrates the correlation between superconductivity and the crystal structure.In chapter V,the effects of pressure on the type-II Weyl semimetal candidate TaIrTe₄ are investigated.Superconductivity is observed and confirmed at²3.8 GPa.The high pressure synchrotron XRD measurements show that lattice distortion occurs at that critical pressure where superconductivity emerges.Fermi surface reconstruction due to the lattice distortion may lead to the emergence of the superconductivity in this material.Our results establish the correlation among superconductivity,topology and lattice distortion.