High-pressure Regulation And Exploration Of New Superconducting Materials

Since its discovery in 1911,superconductivity has remained at the forefront of modern physics for more than 110 years.Exploring and discovering new superconducting material systems and revealing their regulation laws and microscopic mechanisms have always been important research contents,which is also the only way to achieve the grand goal of room temperature superconductivity.As another important parameter besides temperature and chemical composition,pressure has a great influence on the structure and physical properties of materials.Pressure has shown unique advantages in the synthesis and regulation of new superconducting materials.Compared with chemical doping,high pressure does not introduce lattice disorder and additional charge carriers,which is thus a clean,fine and controllable tuning method.For example,the obtained relationship between the superconducting critical temperature and pressure of new superconductors can not only guide the optimal design of superconducting materials at ambient pressure,but also help to clarify the superconducting mechanism.To this end,we carried out detailed high-pressure study on the newly discovered Ni-based infinite layer superconducting Pr_0.82Sr_0.18Ni O₂ thin films and the V-based kagome superconductor Rb V₃Sb₅.On this basis,attempts have been made to explore new superconducting materials using high pressure and high temperature method.The innovative experimental results we obtained are as follows:1,The infinite-layer nickelate superconductors have many similarities with Cu-based high-temperature superconductors,but their optimal T_c（～9-15 K）is much lower than that of cuprate superconductors.Whether their T_c can be further improved is an urgent question to be answered.We studied the pressure effects on the high-quality superconducting Pr_0.82Sr_0.18Ni O₂ thin films and successfully obtained its’superconducting T_c（P）phase diagram by means of the excellent hydrostatic pressure environment provided by a cubic anvil cell apparatus.The obtained T_c（P）phase diagram reveals a positive pressure effect on the superconducting transition temperature,which increases monotonically from～17 K at ambient pressure to～31 K at 12.1 GPa without showing signatures of saturation.Based on the effect of pressure on lattice and electronic band structure,we discussed the physical mechanism of the observed positive pressure effect of T_c in Pr_0.82Sr_0.18Ni O₂ thin films.This is the first time that the T_c of an infinite-layer nickelate superconducting thin film has been increased to above30 K,indicating that the T_c of infinite-layer nickelate superconductors can be further raised up.For example,epitaxial stain can be used to further improve its T_c at ambient pressure.2,The recently discovered kagome superconductors AV₃Sb₅（A=K,Rb,and Cs）have attracted numerous attention due to its novel properties such as anomalous charge density waves（CDW）,nematic order and topological electronic states.Uncovering the relationship between superconductivity and other electronic orders is critical for regulating and understanding the superconductivity of this system.Firstly,the superconductivity of Rb V₃Sb₅ single crystal was systematically studied by anisotropic magnetic and electrical transport measurements at ultra-low temperature.The in-plane and out-of-plane critical magnetic fields,penetration depths,and coherence length were obtained,which revealed the anisotropic superconducting properties of Rb V₃Sb₅.We further performed a comprehensive high-pressure study on Rb V₃Sb₅ by using the piston cylinder cell and the cubic anvil cell.It is found that the CDW order of Rb V₃Sb₅experiences a subtle modification at P_c1（?）1.5 GPa before it is completely suppressed around P_c2（?）2.4 GPa.Accordingly,the superconducting transition T_c（P）exhibits a shallow M-shaped double superconducting dome with the extrema of T_c^onset（?）4.4 K around P_c1,leading to a fourfold enhancement of T_c with respect to that at ambient pressure.The competitive relationship between CDW and SC in Rb V₃Sb₅ is clearly revealed by the constructed T-P phase diagram which resembles that of Cs V₃Sb₅,and shares similar features as many other unconventional superconducting systems with intertwined competing electronic orders and quantum criticality.Our results shed more light on the intriguing physics involving intertwined electronic orders in this novel topological kagome metal family.3,Exploring new V-based kagome metal has become a research frontier as motivated by the extensive study of AV₃Sb₅（A=K,Rb,and Cs）family.The polycrystalline samples of V₃Sb₂ with a bilayer kagome lattice of V atoms were synthesized through conventional solid-state-reaction method and its structure,electrical transport,magnetic and thermodynamic properties were characterized via x-ray powder diffraction,resistivity,magnetic susceptibility and specific heat measurements.We observed no long-range magnetic order in V₃Sb₂ above 1.8 K,which is similar with the kagome metal AV₃Sb₅（A=K,Rb,Cs）family.In addition,a density-wave-like anomaly was evidenced around 160 K in V₃Sb₂,making it more similar to the AV₃Sb₅（A=K,Rb,Cs）family.Moreover,we find that the density-wave-like transition can be gradually suppressed by pressure but no sign of superconductivity can be observed down to 1.5 K near the critical pressure where the density wave order disappears.Density functional theory calculations indicate that V₃Sb₂ shows a non-trivial topological crystalline property.Thus,our study makes V₃Sb₂ a new candidate of V-based metallic kagome compound to study the interplay between density-wave-order,nontrivial band topology and possible superconductivity.4,Th₄H₁₅ is the first discovered hydride superconductor,but the polycrystalline samples obtained via hydrogenation of the Th metal under traditional hydrogen-gas pressure are relatively loose,which is not conducive to obtaining accurate physical properties.Here,the dense polycrystalline Th₄H₁₅ samples were synthesis under 5 GPa and 800℃ through high pressure and high temperature technique and characterized its electrical transport,magnetic,thermodynamics properties and pressure effects.It was found that the obtained Th₄H₁₅ sample shows a bulk superconducting transition at T_c（?）6 K,which is about 2 K lower than that reported previously.Various characteristic superconducting parameters have been extracted and unusual lattice dynamics were evidenced from the specific-heat analysis.The high-frequency phonons associated with the vibration of hydrogen cage may be responsible for the observed superconductivity in Th₄H₁₅.A non-monotonic T_c（P）phase diagram of Th₄H₁₅ was obtained through measurements of resistivity under various pressure,which implies that two competing factors should be at play under pressure.