Study On The Superconductivity Of Mo₃Sb₇ And FeSe Under Multiple-extrene Conditions

One of the important research fields in the condensed matter physics is to study the Quantum Criticality phenomena,especially in the superconductivity research,in which the superconducting transition temperature Tc can usually be enhanced by suppressing a competing electronic order in the normal state via chemical doping or the application of high pressure.To unravel the competitive nature of superconductivity with other electronic orders can not only clarify the key factors governing Tc,but also deepen our understanding on the pairing mechanism for the observed superconductivity.Studies on these phenomena can improve our understanding on the complicated interactions involved in the unconventional superconductors.Therefore,in this dissertation we studied in detail on several strongly correlated electronic systems with unusual properties and revealed their origins.Firstly,We have monitored the resistivity of Mo3Sb7 single crystals under various hydrostatic pressures up to 15 GPa.The application of external pressure enhances Tc but suppresses Ts until Pc ? 10 GPa.The cubic phase above 12 GPa is also found to be superconducting with a higher Tc ? 6 K that decreases slightly with further increasing pressure.The variations with pressure of Tc and Ts satisfy the Bilbro-McMillan equation,i.e.Tcn*Ts1-n = constant,thus suggesting the competition of superconductivity with the structural transition that has been proposed to be accompanied with a spin-gap formation at Ts.The first-principles calculations suggest the importance of magnetism that competes with the superconductivity in Mo3Sb7.Secondly,we studied high-pressure magnetotransport measurements in FeSe up to?15 GPa,which uncover the dome shape of magnetic phase superseding the nematic order.Unlike other iron-based superconductors,FeSe exhibits nematic ordering without magnetism whose relationship with its superconductivity remains unclear.Moreover,a pressure-induced fourfold increase of Tc has been reported,which poses a profound mystery.Above-6 GPa the sudden enhancement of superconductivity(Tc?38.3 K)accompanies a suppression of magnetic order,demonstrating their competing nature with very similar energy scales.Above the magnetic dome,we find anomalous transport properties suggesting a possible pseudogap formation,whereas linear-in-temperature resistivity is observed in the normal states of the high-Tc phase above 6 GPa.The obtained phase diagram highlights unique features of FeSe aMong iron-based superconductors,but bears some resemblance to that of high-Tc cuprates.