Pressure-induced Superconductivity And Structural Transitions In Topological Insulators AB₂Te₄(A=Sn;B=Bi,Sb)

The study of superconductivity is always the forefront of condensed matter physics.The natural Van der Waals compounds AB₂X₄(A=Sn;B=Bi,Sb;X=Te)are new three-dimensional topological insulators,which are typical layered compounds.These compounds have provided a good platform for us to search for topological superconductors and study the relationship between superconductivity and other physical phenomena.Herein,we systematically studied the crystal structure and electrical transport properties on topological insulators SnBi₂Te₄and SnSb₂Te₄ with diamond anvil cell technique,in-situ high pressure synchrotron X-ray diffraction technique and in-situ high pressure electrical measurements technique.The results are as follows:1.In-situ high pressure low temperature electrical measurement results show that the electronic band structure of SnBi₂Te₄ changes.The pressure-induced superconductivity occurs at 16.1 GPa and the critical temperature of superconductivity is 6.5 K.As the pressure increases,three distinct pressure-induced superconducting states have been observed,one in C2/m(phase ?)and two in bcc(phase ?).In each of the pressure-induced superconducting states,the critical temperature T_cof superconductivity decreases with pressure.The abnormal change of T_c with pressure indicates that the superconductivity of SnBi₂Te₄ at high pressure is an unconventional superconducting mechanism.At 22.3 GPa,T_c reaches a maximum of8.9 K,which is obtained in the third superconducting state.The study of high pressure crystal structure shows that,with the increase of the pressure,SnBi₂Te₄ transforms from the initial layered quasi-two-dimensional R-3m structure(phase I)to the monoclinic C2/m structure(phase ?),and eventually develops into a site-disordered body centered cubic bcc structure(phase ?),where superconductivity is caused by the phase transition of the C2/m structure.The change of resistance with temperature was measured at different external magnetic fields intensity.The critical temperature T_c of superconductivity decreases with increasing magnetic field intensity,which further confirms that the observed zero resistance state is indeed the intrinsic superconducting property of the sample.The upper critical field is estimated to be 3 T.2.High pressure synchrotron radiation studies show that SnSb₂Te₄ underwent three structural phase transitions,and the phase transition sequence is R-3m?C2/m?phase ??disordered Im-3m.These structural changes are accompanied by changes in the electronic structure.The occurrence of zero resistance in high pressure and low temperature electrical measurements indicates that topological insulator SnSb₂Te₄ has superconductivity.The superconductivity transition begins at 12.3 GPa,and the zero resistance state is achieved at 13.8 GPa.The superconductivity may be caused by C2/m structural phase transition.The critical temperature T_c of superconductivity increases with pressure and reaches a maximum of 8.2 K at 17.1GPa.It is worth noting that after 17.1 GPa,the critical temperature T_c of superconductivity shows a downward trend as the pressure continues to increase,indicating the emergence of a new superconducting phase.This pressure point is also the transition point of the two high pressure phases(phase ? and phase IV),so the new superconducting phase may be caused by the transition of the structure phase ? to phase IV.The obtained phase diagram reveals a relationship between the structural transition and the appearance of superconductivity.