Research On Superconducting Topological Materials And Non-Centrosymmetric Superconductors Ta_xRe_1-x（0.1≤x≤0.25）

Unlike the common Dirac fermions,Majorana fermions are exotic particles which represent their own antiparticle.Majorana fermions satisfy non-Abelian statistics and can form topological qubits for fault-tolerant topological quantum computing,which can be used to make quantum computers and have great application potential.In recent years,theoretical research and experimental exploration of Majorana fermions have occupied a large area of scientific research in condensed matter physics.The topological superconductor is composed of a bulk state with a paired energy gap and a gapless surface state composed of Majorana fermions.Therefore,topological superconductors have been an ideal platform for studying Majorana fermions,and they are also being an important direction for researchers’ pursuit.Superconducting topological materials and non-centrosymmetric superconducting materials are two major systems that can realize topological superconductivity.This paper mainly studies the superconducting topological materials TaOsSi、PtPb₄ and non-centrosymmetric superconducting materials Ta_xRe_1-x（0.1≤x≤0.25）series compounds,focusing the low-temperature superconducting properties and potential topological properties.The main research content and conclusions of this article as follows:1.We synthesized TaOsSi poly crystalline sample and measured specific heat,magnetic susceptibility,and upper critical magnetic field.We found that TaOsSi is a superconductor with critical transition temperature 5.8 K,and the superconducting energy gap exhibits a two-band behavior.On the other hand,the time-reversal symmetry breaking in the TaOsSi superconducting state has been investigated by the help of μSR measurement,implying possible the unconventional superconducting state with spin triplet pairing.The first-principal calculations results demonstrated there are Dirac cones near the Fermi level,one of which is gapped due to the spin-orbital coupling and forms Surface state.Topological index indicated that TaOsSi is a weak topological material.Additionally,the evolution of the electrical resistivity with pressure（up to 50 GPa）was also investigated,and a "V-shaped" diagram of Tc vs P was found.2.It had been experimentally and theoretically confirmed that Pt/PdSn4 are nodal-line semimetal with orthorhombic structure,and PtPb₄ had also been crystallized in the same structure as Pt/PdSn4 rather than tetragonal that was claimed in previous study.The Angle Resolved Photoemission Spectroscopy experiment found that the Rashba effect exists in the material.In addition,PtPb₄ is an intrinsic superconductor with a superconducting transition temperature of 2.7 K,previous study on superconductivity is only limited to the superconducting transition temperature,we hence here studied its low-temperature properties in detail.In this section,we synthesized high-quality PtPb₄ single crystals and found topological nontrivial Berry phases by means of quantum oscillatory measurements of magnetic susceptibility（dHvA）.First-principles calculations found that the material is a weak topological material.For weak topological materials,topological surface states exist only on specific surfaces.What’s more,we show that in PtPb₄,the specific heat jump at Tc reaches ΔC/γTc～1.70±0.04,larger than the 1.43 expected for weak-coupling BCS superconductors.3.Non-centrosymmetric superconductors have anti-symmetric spin-orbital coupling because their crystal structures do not have symmetry center.The antisymmetric spin-orbital coupling can give rise to the mixture superconducting state of spin-singlet and spin-triplet,so it has always been considered as a candidate system for realizing topological superconductivity.On the other hand,recent studies have found that the time reversal symmetry is always broken in Re-based superconductors(e.g.,Re₂₄Ti₅、Re_0.82Ti_0.18、Re₆Ti、Re₆Zr、Re₆Hf),which is related to the electronic structure of Re.In this chapter,we synthesized Ta_xRe_1-x（0.1≤x≤0.25）series samples and found that the superconducting transition temperature of the optimal doping level Ta_0.15Re_0.85 reached as high as 8K.The superconducting gap of Ta_0.15Re_0.85 can be fitted with two-gap s-wave or anisotropic s-wave.In contrast with other Re-based α-Mn superconductors,in Ta_0.15Re_0.85,the absence of timereversal symmetry break is observed.For TaOsSi and PtPb₄,while both our experiments and calculations indicate potential topological superconductivity,the topological superconducting state has not been directly observed due to our limited measurement instrument.Therefore,it still needs more abundant and direct experimental means（such as point contact spectroscopy,STM,nuclear magnetic resonance）to verify.