| As one of the key issues in condense matter physics,the charm of high temperature superconductors is mainly manifested by two points.One is its high superconducting transition temperature Tc,which enormously expands the application of superconductors.The other is the strong correlation between its electrons,which closely relates the high-Tc mechanism with the core of condensed matter theory nowadays,i.e.strong-correlated physics.As for the research of high-Tc mechanism,it now mainly faces two challenges.One is that the strong-correlated system haven't be well described by theory yet.The other is due to the complicated properties of high temperature superconductors,in which abundant orders such as antiferromagnetism,charge density wave and nematic phase have been observed.As a consequence,it is hard for us to regulate one of them without influence the others in experiments.So that,the relations between them and superconductivity can hardly be well determined.Therefore,during the research of high-Tc mechanism,on one hand,we can pursue theory to well describe the strong-correlated system or develop experimental techniques to regulate the properties of the high temperature superconductors rigidly.On the other hand,we also can search clues for high-Tc mechanism in other superconducting systems.As we know,BCS theory built by Bardeen,Cooper and Schrieffer in 1957 successfully described the superconductivity of conventional superconductors,such as metal,alloy and simple compound superconductors.Although the BCS theory cannot completely explain the high-Tc mechanism,its core concept that electrons are combined to form cooper pairs can still be used,only the pairing mechanism of high temperature superconductor has not totally been clarified yet.The main content of my dissertation is to explore key information on pairing symmetry.At the first stage,we have studied the paring symmetry of electron-doped cuprate superconductors.At the second stage,we mainly concentrate our attention on the superconductivity of spinel oxide superconductors,which are complementary to cuprate superconductors.The main results are summarized below:?.We have studied the Hall sign reversals of optimally electron-doped La2-xCexCuO4?x=0.105?in the vortex liquid region.Both quantitative and qualitative analyses point to that the superconducting coupling between the electron and hole band is weak.Moreover,in the absence of coupling,the hole band has a higher Tc at zero field,while the electron band possesses a larger upper critical field Hc2 at zero temperature.Such result supports the two-gap model of optimally electron-doped cuprate superconductors:the pairing symmetries of hole and electron band are d wave and s wave,respectively.Therefore,it also points out that in optimally electron-doped cuprate superconductors,analyses on temperature dependent superfluid densities should take this two-gap model in account.?.According to the results of the first stage,we have found that the superconducting properties of spinel oxide superconductor Li Ti2O4-?are much similar to those of the electron band in electron-doped cuprate superconductor,indicating the unconventional pairing mechanism of Li Ti2O4-?.Furthermore,by tuning the Tc of Li Ti2O4-?from?11 K to?14 K via electrochemical gating of ionic liquid,it is found that the Tc of Li Ti2O4-?is closely related to its linear resistivity behavior.?.Through pulsed laser deposition technique,we have successfully suppressed the orbital ordering in a spinel oxide MgTi2O4 with Mottness and obtained superconductivity in it.This superconductor exhibits the following features:Hc2 at zero temperature surpasses the Pauli limit;Tc is positively correlated with the length of c-axis parameter.Those further indicate that the pairing mechanism of spinel oxide superconductors is unconventional.Compared to cuprate superconductors,the normal states of spinel oxide superconductors are not so complicated and the growth is relatively simple.Therefore,it is suitable for more groups to carry out extensive study. |
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