Study On The Structural And Electrical Transport Properties Of Eu-based Magnetic Topological Materials Under High Pressure

Topological materials have been an important research topic in the field of condensed matter physics due to their novel band structure and potential quantum properties.Magnetic topological materials have attracted much attention because the interaction between magnetic order and topological surface state can break the time inversion symmetry and produce many exotic topological quantum effects,such as the quantum anomalous Hall effect,Majorana bound states and axion insulator states.At present,scientists have confirmed a variety of magnetic topological materials by first-principles calculations and ARPES spectroscopy experiments,including intrinsic magnetic topological insulators and magnetic topological semimetals,etc.which provides new opportunities to explore and study more abundant topological quantum properties.Pressure plays an important role in the research of magnetic topological materials.The application of pressure can not only change the crystal structure and electronic structure of such materials,but also effectively adjust the magnetic order state and the coupling between magnetic order and electron,thus inducing novel structural phase transitions,quantum phase transitions and state transitions.Therefore,the high pressure research is of great significance for the in-depth understanding of the physical properties of magnetic topological materials and the discovery of new quantum states.In this paper,we present a systematic study on the structure and electrical transport properties of three Eu-based magnetic topological materials EuSn₂As₂,EuSn₂P₂,EuMn Sb₂ and EuCd₂As₂under high pressure,by using high-pressure in-situ measurement methods of synchrotron radiation X-ray diffraction,low-temperature resistance and Raman spectroscopy,as well as first-principles calculations.Our important research results are as follows.1.By using first principles calculation,we found thatα-EuSn₂As₂ with layered rhombohedral（R-3m）structure at ambient pressure transformed intoβ-EuSn₂As₂ with three-dimensional monoclinic（C2/m）structure at 14.3 GPa.The transformation path is realized by a"two-stage reconstruction mechanism"with the connecting of Sn-Sn and As-As atoms successively between the buckled Sn As layers.Enthalpy calculation show that theβ-EuSn₂As₂ is the most stable structure above 14.3 GPa.Its dynamic and thermodynamic stability has been also verified by the phonon mode and Gibbs energy analysis.The high-pressure X-ray diffraction measurement results further confirm that EuSn₂As₂ underwent a structural transition from rhombohedral phase（α-EuSn₂As₂）to monoclinic phase（β-EuSn₂As₂）at 12.6 GPa.The lattice parameters and atomic positions under different pressures are obtained by Rietveld refinement and the perfect matching of theoretical and experimental data confirms the structural phase transition.The high-pressure Raman spectrum measurement results are consistent with the XRD pattern and the abnormal changes of Raman mode at 3 and12.45 GPa provide an evidence of the structural phase transition fromα-EuSn₂As₂ toβ-EuSn₂As₂via a two-stage reconstruction mechanism.Electrical resistance measurements reveal an insulator-metal-superconductor transition at low temperature around 5 and 15 GPa,respectively,correspond to the two-stage structural conversion under high pressure.The superconductivity with a T_Cvalue of～4 K is observed up to 30.8 GPa.The antiferromagnetic transition temperature increases monotonously with pressure and disappears at 15 GPa.The magnetic structure calculation shows that the structural phase transition of EuSn₂As₂is accompanied by a magnetic transition from A-to G-type antiferromagnetic ground state.The competition between the two magnetic ground states inhibits the formation of magnetic order.Based on the results of high-pressure in-situ resistance measurement,X-ray diffraction measurement and first-principles calculation,we have created a temperature-pressure phase diagram of EuSn₂As₂ and revealed the relationship between resistance,magnetic order and structural phase transition.2.Combining first-principles calculations and high-pressure in situ X-ray diffraction,low temperature resistance and Raman scattering measurements,we study the structure,electrical transport and vibrational properties of EuSn₂P₂ under pressure.The high-pressure X-ray diffraction measurement results show that a structural phase transition occurred at 10 GPa and completed around 24 GPa.According to the evolution of XRD patterns and the results of Rietveld refinement,we determine that EuSn₂P₂ and EuSn₂As₂ have the same high-pressure structure,that is,a monoclinic C2/m structure.The high-pressure Raman spectroscopy measurement results further confirmed that EuSn₂P₂ undergoes a two-stage structural transition from R-3m to C2/m phase at 3 and 11.2 GPa.These results indicate that our proposed two-stage phase transition mechanism can be used as a reference for other 122 system materials with similar layered structures under high pressure.The high-pressure electrical resistance measurements show that the semiconductor behavior of EuSn₂P₂ below antiferromagnetic temperature changes to a metal behavior at 3 GPa.The resistance value increases abnormally with increasing pressure at 11-22 GPa.And finally,EuSn₂P₂ exhibits a metal behavior at temperatures of 2-300 K above 26 GPa.The energy band calculation shows that the metallicity of EuSn₂P₂ increases obviously with increasing pressure.According to the structural research results,it is found that the abnormal increase of resistance in the pressure range of 11-22 GPa corresponds to the pressure at which the structural phase transformation begins and completes.It is indicated that the competition between the high-pressure and low-pressure phases causes the abnormal change of resistance.3.We investigate the electrical transport and structural properties of EuMn Sb₂under pressure by using high-pressure low-temperature resistance measurements and high-pressure Raman scattering measurements.The high-pressure electrical resistance measurements reveal a semiconductor-semimetal-metaltransition around 5 and 27GPa.The resistance changes discontinuously at about 5 and 10 GPa,and meanwhile,several Raman modes appear obvious splitting,merging or disappearing at 5 and 11GPa,which indicates the structural phase transition occurred at this pressure point and also reveals the correlation between the structural phase transition and the electrical transport properties.4.The structural property of EuCd₂As₂ under high pressure was studied by in situ X-ray diffraction and Raman spectroscopy measurements.The high pressure X-ray diffraction measurement indicates that the P-3m1 structure of EuCd₂As₂ exists stably in the pressure range of 0-30 GPa.The splitting and discontinuous changes of Raman mode indicate EuCd₂As₂ may undergo an electronic phase transition at 12.64GPa.This observed phase transition behavior corresponds to the recently reported insulation-metal transition above T_N by resistance measurements,indicating that the abnormal change in electrical transport properties of EuCd₂As₂ at high pressure is related to the electronic phase transition.