Exotic Quantum States In Several Novel Topological Electronic Materials Under High Pressure

Phase transition refers to the transformation of matter from one phase to another under continuous changes in external parameters.The Landau theory of phase transition has long been regarded as the "ultimate theory" for classifying condensed matter based on the concept of symmetry and order parameters.In the 1980s,German scientist von Klitzing discovered the quantum Hall effect in a two-dimensional system,ushering in a new era of research on topological physics in condensed matter physics,which profoundly changed people’s understanding of phase transitions and state classification.Especially in the past decade,research on topological quantum states and topological quantum materials has become one of the most active and important frontiers in the field of condensed matter physics.Based on a series of recent discoveries in the field of topological electronic materials,we have utilized diamond anvil cell technology,combined with high-pressure electrical transport,high-pressure synchrotron X-ray diffraction,and high-pressure Raman spectroscopy experiments,supplemented by firstprinciples calculations,to investigate several novel topological electronic materials,including topological nodal\line semimetal Ta2PdSe6,high-order topological insulator Ta2Ni3Te5,topological nodal-line superconductor PbTaSe2,and topological crystalline insulator NaCd4As3.The main results are as follows:(1)We have made a remarkable discovery of pressure-induced superconductivity in quasi-one-dimensional topological nodal semimetal Ta2PdSe6.High-pressure electrical transport measurements reveal that Ta2PdSe6 undergoes a superconducting transition with Tc～2 K at the critical pressure Pc=18.3 GPa,and the superconducting properties persist at high pressures up to 62.6 GPa.In the vicinity of Pc,we have also observed a series of anomalous evolutions in the transport properties of Ta2PdSe6,including a sudden increase in the overall electrical resistivity,a change in the sign of the Hall coefficient,and a suppression of the magnetoresistance.Room temperature high-pressure synchrotron X-ray diffraction experiments and high-pressure phonon spectrum calculations indicate that the emergence of the superconducting state is not related to a structural phase transition.Combined with theoretical calculations,we suggest that the pressure-induced Lifshitz phase transition may be the origin of the superconductivity phenomenon in Ta2PdSe6.(2)We have made a profound discovery of pressure-induced metallization.topological phase transition,and the emergence of superconductivity within the transition metal chalcogenide Ta2Ni3Te5,which boasts a monolayer with the potential to be a high-order topological insulator.High-pressure electrical transport measurements show that Ta2Ni3Te5 undergoes metallization at 3.3 GPa and then experiences a superconducting transition around 0.5 K at Pc=21.3 GPa.Room temperature synchrotron X-ray diffraction experiments indicate that the crystal structure of Ta2Ni3Te5 can remain stable under pressure.First-principles calculations reveal that Ta2Ni3Te5 transforms into an electron-hole compensated ordinary semimetal at 2.1 GPa and then undergoes a topological phase transition at 4.0 GPa.In addition,the pressure-induced nontrivial topological band structure can be maintained up to at least 41.1 GPa,suggesting that Ta2Ni3Te5 may be a candidate material for topological superconductivity under pressure.(3)We have made a momentous discovery of a sequence of pressure-driven structural phase transitions in the topological nodal-line superconductor PbTaSe2.The findings from high-pressure synchrotron X-ray diffraction provide compelling evidence that the crystal structure of PbTaSe2 undergoes a transition from the P6m2(α-phase)to P63mc(β-phase)between the pressure range of 0.5 GPa.Subsequently,it transforms into P6/mmm(y-phase)at 7.5 GPa and finally changes to Pmmm(δ-phase)at 44.1 GPa.Moreover,our first-principles calculations reveal that the electronic structure of PbTaSe2 experiences significant alterations concurrent with the structural phase transition,giving rise to a plethora of captivating topological band structures,including tilted Weyl nodal lines,tilted Dirac cones,and dual topological nodal surfaces.(4)We have made a remarkable discovery of pressure-induced amorphous phase transition and the emergence of superconductivity in the topological crystalline insulator NaCd4As3.High-pressure synchrotron X-ray diffraction experiments have revealed that NaCd4As3 undergoes an amorphous phase transition at Pc1=18.6 GPa,a process that remains incomplete until reaching Pc2=33.5 GPa.Electrical transport experiments reveal the occurrence of metallization and the remarkable emergence of a superconducting transition at approximately 2.5 K in NaCd4As3.Upon further pressurization,the Tc of NaCd4As3 rapidly increases to 3.2 K and remains largely unchanged until reaching an abrupt enhancement to 4.6 K at Pc3=60.0 GPa,where it continues to persist.The pressure-cycling Raman experiments indicate the existence of an implicit amorphous-to-amorphous phase transition in the vicinity of Pc3.Thus,we attribute the appearance and enhancement of superconductivity in NaCd4As3 to its intricate correlation with the continuous evolution of its amorphous local structure.