High-pressure Study Of Novel Topological Materials MnBi₂Te₄ And CsV₃Sb₅

Topological materials are insensitive to perturbations such as impurities and defects due to their symmetry-protected topological electronic states.Therefore,they have potential applications in the fields of information storage and transmission,and have attracted much attention in recent years.In topological materials,if there are electronic states such as magnetic order,superconductivity,and charge density waves at the same time,they can often interact with topological states and form unique properties.High pressure can effectively tune the lattice constant,thus to fine-tune various electronic order states in topological materials,which can not only reveal its microscopic mechanism but also explore new states of matter.In this paper,two important topological material systems,the antiferromagnetic topological insulator MnBi₂Te₄ and the kagome metal CsV₃Sb₅,their doped systems MnBi_2-xSb_xTe₄（x=0.63,1 and 2）and CsV_3-xTi_xSb₅（x=0.04 and 0.15）and a possible topological material Ca Mn₂Bi₂ are investigated.We carried out several physical property measurement methods under high pressure,low temperature,and other comprehensive extreme conditions and revealed the evolution of the magnetic order,charge order and superconductivity under hydrostatic pressure in detail.Innovative research results were obtained as follows.1.MnBi₂Te₄ is the first intrinsic antiferromagnetic topological insulator.We first carried out a detailed high-pressure study on the single crystal of MnBi₂Te₄ and established its temperature-pressure phase diagram through high pressure experiments such as resistivity,ac susceptibility,and X-ray diffraction（XRD）.The experimental results show that the antiferromagnetic order is initially enhanced by pressure,but gradually suppressed after 2 GPa until it completely disappears at about 7 GPa.Its resistivity increases abnormally with the increase of pressure and changes from metallic behavior to semiconductor-like behavior at P>7 GPa.High-pressure XRD shows that the sample has no crystal structure transformation within 12.8 GPa.Based on the results of first-principles calculations,we explain the experimental phenomena in terms of the competition of magnetic interactions and electron transitions from local to itinerant states.On this basis,we then carried out systematic high-pressure measurements on the Sb-doped MnBi₂Te₄ system,namely MnBi_2-xSb_xTe₄（x=0.63,1 and 2）,and established the temperature-pressure phase diagrams with different doping concentrations.For the x=0.63 sample,the anomalous behavior of resistivity near the magnetic order temperature T_N changes significantly under pressure.Based on the detailed analysis of the magnetoresistance and Hall effect data,we speculate that the change in the anomalous behavior of resistance near T_N may originate from pressure-induced changes in magnetic order.For the x=1 sample,the magnetic order is quickly suppressed by pressure.While the T_N for the x=2 sample increases with increasing pressure.These results indicate that in these samples chemical pressure and physical pressure have different effects on magnetism.2.CsV₃Sb₅ is a Z₂ topological metal with kagome structure.Through detailed high-pressure measurements on single crystals CsV₃Sb₅,we established a complete phase diagram of charge density waves and superconductivity,revealed the competitive relationship between them,and discovered a double-dome superconducting phase diagram.According to the change of the charge density wave phase transition under pressure,we pointed out the close correlation between the double-dome superconducting phase and the charge density wave and revealed the strong correlation nature in its rich physical properties.In addition,we found that the superconducting transition temperature of CsV₃Sb₅ is increased to 8 K at around 2 GPa,which is three times higher than that under ambient pressure,indicating that the superconducting transition temperature of vanadium-based kagome superconductors can be further improved.To reveal the effect of chemical doping and pressure on the charge density wave and double-dome superconducting phase,we further carried out detailed high-pressure studies on Ti-doped CsV_3-xTi_xSb₅（x=0.04 and 0.15）single crystals.We compared the results with the parent material CsV₃Sb₅.At ambient pressure,the charge density wave of the x=0.04 sample is partially suppressed,while the charge density wave of the x=0.15 sample has been completely suppressed.The high-pressure results found that chemical doping gradually suppressed the low-pressure dome while suppressing the charge density wave,further proving that the feature of the double-dome superconducting phase in CsV₃Sb₅ is closely related to charge density wave.Although the x=0.15 sample has no long-range charge density waves,superconducting domes and quantum critical point-like behavior are still observed,probably caused by the existence of short-range charge density waves competing with superconductivity.3.In addition,we also carried out high-pressure studies on Ca Mn₂Bi₂ single crystals.Combined with the high-pressure structural research of our collaborators,we found a semiconductor to metal transition along with a structure transition from wrinkled Mn-Mn honeycomb layered 2D structured to quasi-1D Mn-Mn chain-like structure in Ca Mn₂Bi₂ at P_c≈2.35 GPa.In addition,the high-pressure resistivity shows that the pressure enhances antiferromagnetic order in the low-pressure phase and its phase transition temperature increases.The high-pressure phase has two phase transitions with opposite pressure effects,one of which is thought to be the antiferromagnetic transition and the other possibility is the charge density wave phase transition.