| Recently,the manufacture of novel 2D superconducting films with highly crystalline structure and atomic thickness becomes avalaible due to the advances in materials fabrication.However,many of these films,fabricated in ultrahigh vacuum chambers,are highly vulnerable to air exposure,which makes it difficult to measure intrinsic superconducting properties such as zero resistance and perfect diamagnetism with ex situ experimental techniques.One solution to this problem is to grow capping layers on the sample surface,which,however,may introduce changes to the intrinsic properties of the sample surface.A better solution is to perform in situ measurements.In this thesis,we have developed a two-coil mutual inductance setup based on a multifunctional scanning tunneling microscope(STM)which is capable of not only the usual STM functionality but also in situ four-point probe electrical transport measurements owing to a self-designed 4-electrode piezo-scanner tube.With this additional development,the systematic study of a superconducting film can be performed by measuring electrical and diamagnetic properties at varied temperatures(? 0.4 K)and in high magnetic fields(?11 T)all within a single UHV chamber.The performance of the two-coil mutual inductance measurement is validated by experiments on a 10-nm-thick NbN film grown on a Nb-doped SrTiO3 substrate,which exhibits a distinct superconducting transition at 11 K.This system has been recently utilized to study ultrathin superconducting films including the(?)-In/Si(111)superstructure and K doped 2-layer FeSe films grown on a SrTiO3(001)substrate.These reaserches demonstrate convincingly that this noval instrument is a reliable tool for the systematic investigation of intrinsic superconductivity in two-dimensional materials.The monolayer FeSe/SrTiO3 film has attracted tremendous reaserch interests because of its enhanced superconductivity revealed several years ago.However,because of its fragility in air,it is difficult to make direct measurements of its intrinsic superconducting properties including diamagnetism.By using the self-developed two coil mutual inductance setup,we successfully conducted in situ measurements of diamagnetism on the monolayer FeSe/STO film sample grown by MBE method.Meanwhile,the zero-resistence property in the same sample was observed by in situ four-electrode electrical transport measurement,which gives a critical temperature consistent with that found in measurement of diamagnetic response.Moreover,a statistical analysis on numerous samples reveals the superconducting critical temperature of the monolayer FeSe/STO film is not a fixed value.The highest Tc observed in these samples is 28 K.Some of the samples shows no superconducting transition above the lowest measurement temperature(4.4 K)in spite that their growth conditions were the same.An empirical optimal growth conditions for the fabrication of superconducting monolayer FeSe/STO film is proposed in this thesis.Nonetheless,it is still difficult to precisely control the Tc.Futhermore,the reason for the diverse Tc of monolayer FeSe/STO film samples has been revealed.Firstly,comparing the data of tunneling spectrum and diamagnetic response,we found that the opening temperature of energy gap is much higher than Tc,and the value of Tc is uncorrelated to the size of energy gap.In strong contrast,there is a remarkable positive correlation between phase stiffness and Tc.When Tc is lower than about 14 K,the curve of Tc vs.phase stiffness can be well fitted by square root function.Above 14 K,Tc is proportional to the phase stiffness.Due to its low superfluid density,the superconductivity in a monolayer FeSe/STO film is strongly influenced by phase fluctuation.With temperature decrease,the formation of cooper pairs in monolayer FeSe/STO film is prior to the establishment of long rang phase coherence.Therefore,the Tc of monolayer FeSe/STO film is dominated by the stiffness of phase coherence rather than the strength of pair potential. |
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