Effects Of Pressure And Strain On La-based214Cuprate Superconductors

Superconductivity is an advanced subject of intense research in contemporaryscience due to its wide application and great potential in various fields, such as energy,information, traffic, scientific instruments, national defense, mega-projects of scienceresearch and so on. The high-Tccuprate superconductor is one of the most importantmaterials in superconducting researches, because of its high superconductingtransition temperature and interesting properties. The crystal structure andsuperconductivity of high-Tccuprate superconductor could be modulated by stress.Researches on the relation between the crystal structure and superconductivity of thecuprate superconductor under stress have been shown to be effective in elucidatingthe mechanism of superconductivity and in providing clues that are useful in thesearch for new materials with higher critical temperatures by using stress.We investigate effects of pressure and strain on La-based214cupratesuperconductors.The compound of La_1.96Ba_0.04Cu_O4is non-superconducting and in a spin glassstate. We have investigated the structure properties with the powder synchrotronX-ray diffraction in the pressure range from ambient pressure to27.8GPa at roomtemperature. A phase transition could be found from an orthorhombic structure （spacegroup，Bmab） to a tetragonal phase （space group, I4/mmm） at about1.9GPa,accompanied by the expansion of and the crystal parameter c and the shrinkages of aand b. It is well known that there is a strong relation between crystal structure andsuperconductivity. The resistance of this compound was measured at various pressuresup to25GPa. Through resistance measurements, La_1.96Ba_0.04Cu_O4is insulating-like atlow pressure. Signatures of pressure-induced superconductivity could not be found.But a precursor superconducting state was found at low temperature under3.8GPa,and it disappeared above6.0GPa. It is known that pressure changes the carrierconcentration. But there are other factors which influence the superconductivity.High-pressure studies are important to explore the superconducting mechanism. We have measured the resistance of La_1.89Sr_0.11CuO₄single crystal at pressures.The superconducting transition temperature Tcincreases at pressures. Upon7.9GPa,La_1.89Sr_0.11CuO₄is insulate-liking. However, the insulating state of La_1.89Sr_0.11CuO₄under pressure could not be well interpreted within Anderson localization and Kondoeffect. Thus, the mechanism of pressure-induced superconductor-insulator transitionof La_1.89Sr_0.11CuO₄is still not clear.We obtained high quality of La_2-xSr_xCuO₄thin epitaxial films by using molecularbeam epitaxy （MBE）. The structure and electrical transport properties ofLa_1.9Sr_0.1CuO₄films have been investigated. The crystal parameter c of theLa_1.9Sr_0.1CuO₄film is smaller than that of La_1.9Sr_0.1CuO₄bulk. The films ofLa_1.9Sr_0.1CuO₄are insulate-liking and non-superconducting. We find asuperconductor-insulator transition in the superconducting state of La_1.9Sr_0.1CuO₄films. Variable range hopping （VRH）, Anderson localization and Kondo effect couldnot fully account for the insulating state of La_1.9Sr_0.1CuO₄films. The reason may beabsence of the oxygen and the effect of epitaxial strain.