The Structure And Physical Properties Of Topological Material HgTe And Lead-free Ferroelectric Material NBT-6.5BT Under High Pressure

With the development of in-situ high-pressure measurement technology based on diamond anvil cell（DAC）,studying the physical properties of materials under high pressure is becoming an effective and feasible method in the research of condensed matter physics.Numerous novel physical phenomena caused by high pressure are of great significance for enriching and developing the condensed matter physical theory.In this paper,the crystal structure and electrical properties of two functional materials——topological insulator of HgTe and lead-free ferroelectric material of 0.935(Na_0.5Bi_0.5)TiO₃-0.065BaTiO₃（NBT-6.5BT）under high pressure are systematically studied by in-situ high pressure technology,and some novel research results is obtained.HgTe is a traditional and promising material for quantum Hall effect and device research.Two-dimensional HgTe quantum wells are also the first discovered topological insulators.The unique properties of HgTe,its electronic structure such as energy band inversion,carrier type and its response to chemical doping,magnetic field and compression need to be further studied.In this paper,the relationship between the crystal structure and electrical transport properties of HgTe single crystal film with pressure is systematically studied.In-situ high pressure Raman spectroscopy studies indicate that HgTe single crystal film undergo structural translations at about1.6 GPa,8.7 GPa,12.7 GPa,and 27.1 GPa,respectively.In-situ high pressure electrical transport studies have shown that as the pressure increases,the typical semiconductor behavior of HgTe single-crystal film is gradually suppressed.With the transition of semiconductors to metals,its superconductivity is first discovered at 3.9GPa and the superconductivity transition temperature is about 6.5 K.The superconducting transition still emerges up to 54 GPa in the high-pressure phases of the HgTe single crystal film.The results of in-situ high pressure Hall effect research show that within the pressure range of 5.6-8.6 GPa,the HgTe single crystal film has a change of carrier type that is regulated by pressure and temperature.Finally,the experimental data is comprehensively analyzed from multiple perspectives,and the phase diagram of high pressure structure and electrical performance evolution for HgTe single crystal film is obtained.For the first time,it was discovered that the characteristics of topological insulators of HgTe in phase I,which surface metal state dominates transport at low temperature.It suggests that possibly the surface topologic state co-exists with superconducting state in the high pressure phases of HgTe by analyzing the topological properties of HgTe.2.Perovskite-type relaxed ferroelectric single crystal 0.935(Na_0.5Bi_0.5)TiO₃-0.065BaTiO₃（NBT-6.5BT）is known as the best environment-friendly material that may replace lead-based ferroelectric materials.In this paper,in-situ high pressure Raman spectroscopy was used to systematically study the high pressure crystal structure evolution of NBT-6.5BT single crystal.At ambient pressure,Raman vibration mode show that the structure phase are the coexistence of rhombohedral R3c phase and tetragonal P4bm phase.At 4.6 GPa,new phonon modes appear in the mid-frequency band(300⁴00 cm^-1),indicating a phase transition of the structure from rhombohedral to tetragonal phase.The Raman spectrum information shows that the structure of NBT-6.5BT is still tetragonal phase while being compressed up to 13.1 GPa,but the peak intensity and full width at half maxima（FWHMs）have obvious changes,indicating that NBT-6.5BT may undergo structural distortion under this pressure.In addition,by analyzing the peak intensities of the low-frequency band(100³50 cm^-1)phonon modes related to the displacement of the cation at the A-and B-site,and the high-wavenumber(500⁶50cm^-1)phonon modes related to the octahedron structure,the high pressure suppresses the displacement of the cation at the A-and B-site,and promotes the tilt and vibration of the oxygen octahedron.