High-pressure Study On Several Pnictide Topological Electronic Materials

In recent years,people have developed the topological band theory by introducing the topological invariants of electronic energy band structure with the help of the topological classification method of closed surface in mathematics.In the topological band theory,topological electronic materials refer to the materials with nontrivial topological electronic band.Successful prediction of topological insulator materials is perhaps the first spectacular triumph of the modern topological band theory.Similar to ordinary insulators,there is a bulk band gap between valence band and conduction band,and electrons can not conduct in the bulk of the material.However,surfaces or edges of topological insulators support metallic or conducting electronic states protected by time-reversal symmetry.By examining how band structures evolve under spin-orbit interaction,many topologically interesting materials have been predicted,including topological semimetals,topological superconductors and so on.The conduction and the valence bands of topological semimetals cross linearly in the momentum space,resulting in a pointlike Fermi surface with a gap of 0,which leads to many peculiar physical properties,such as the ultra-high mobility,light effective mass,extremely large magnetoresistance and negative magnetoresistance phenomenon under parallel magnetic field induced by chiral abnormal et al.Topological superconductors are an analogy of topological insulators in superconductors,which has Majorana bound states,a fully superconducting energy gap and a topologically protected gapless surface state.Due to these novel physical properties,topological electronic materials have potential applications in low energy consumption electronic devices,topological quantum computing and other fields.Pressure has been considered as a clean and effective way to tune crystal lattice,as well as electronic,especially in quantum states.In light of many unique physical properties found in topological electronic materials recently,based on Diamond anvil cell,we performed high-pressure study on several pnictide topological electronic materials through high-pressure X-ray diffraction(XRD),high-pressure electrical transport experiments as well as first-principles calculations.The main research contents and results are included as follows:(i)Structural and transport properties of the topological semimetal TaSb2 at high pressures.No evident trace of structural phase transitions is observed from our XRD up to 63.0 GPa.With increasing pressure,the R-T curves maintain a metallic behavior till 60.0 GPa.From ambient pressure to 27.8 GPa,the low-temperature conduction behaviors of TaSb2 stay almost unchanged,exhibiting a power law with the exponent around 3;meanwhile,the magnetoresistance at 5 K follows a same relationship of MR?Bm with m= 1.45 ± 0.10.These results imply that the topological semimetal state of TaSb2 may be robust subjected to pressure,at least to 27.8 GPa.Besides,with increasing pressure,the suppression of MR ratio correlates directly with the decrease of the residual resistance ratio(RRR).(ii)Field-induced metamagnetic transition and pressure-induced ferromagnetic(FM)Weyl phase in antiferromagnetic(AFM)Dirac semimetal NdSb.At ambient prssure,NdSb displays extremely large magnetoresistance,reaching values of 1.77×106%at 1.3 K and 60 T without any sign of saturation.The field-dependent magnetization at 4.2 K reveals a field-induced metamagnetic transition from the AFM to the ferromagnetic state around 9 T.During the metamagnetic transition,a steplike reduction of resistance occurs,resembling the giant magnetoresistance behavior in metallic multilayers.The investigation of the Shubnikov-de Haas effect in NdSb suggests that the metamagnetic transition triggers the modification of the Fermi surface.In addition,a quasi-2D topologically trivial electron Fermi surfuce pocket is identified in the FM state.On the other hand,under compression to Pc-17 GPa,the XRD data evidence a structural phase transition from an ambient Fm-3m phase to a high-pressure tetragonal P4/mmm phase.Simultaneously,accompanied by structural phase,a rarely observed magnetic topological phase transition occurs from AFM Dirac phase to FM Weyl phase around the critical pressure point.In addition,using high field-high pressure-low temperature technology,we obtain an intact magnetic phase diagram of NdSb.(iii)Pressure-induced irreversible evolution of superconductivity in PdBi2.We found that the superconductivity is initially favored by pressure and then becomes almost pressure independent until a critical pressure of Pc?8 GPa,where a structural transition from monoclinic a phase to tetragonal ? phase is detected.Above Pc,the superconductivity of ?-PdBi2 is robust and the Tc is maintained at about 2.5 K up to the maximum pressure 42.9 GPa investigated in previous reports.Upon decompression,the ? phase is reserved to ambient condition but the T,displays unusual irreversible pressure dependence.(iv)Pressure effects on superconductivity in Mo8Ga41 single crystal.Our high-pressure resistance measurements show that the T,continues to reduces from 0 GPa to 40 GPa before disappearing,except a kink anomaly observed at around 19 GPa.It was shown that the lattice constants monotonously shrink before?19 GPa,where a structural disorder or the initiation of amorphization was detected by XRD.These findings may be favorable to the understanding of the superconductivity in Mo8Ga4i with strong electron-phonon coupling.