Scanning Tunneling Microscopy Studies On The Pairing Symmetries Of High Temperature Superconductors

The pairing mechanism of unconventional high-temperature superconductors is an important scientific issue in condensed matter physics.The key point of this issue is to know how the two electrons can be bounded into a Cooper pair.The pairing sym-metry and superconducting gap function can actually reveal the pairing interaction in a superconductor.Hole doped cuprate superconductors have a d-wave gap,i.e.,the gap amplitude remains the same but the gap phase is reversed when rotating 90 degrees in the momentum space.The angle dependent gap amplitude of a d-wave gap can be distinguished by many experiments,however,very few experiments can demonstrate the sign change of the gap.In Fe-based high-temperature superconductors,the the-ory of s^±pairing was proposed for the case of the FeAs family with nearly nesting character of electron-like and hole-like Fermi surfaces based on the weak coupling s-cenario.The antiferromagnetic spin fluctuations mediated electron pairing will lead to an s-wave gap symmetry,but the gap sign changes between the two kinds of Fermi sur-faces.For some electron doped FeSe based superconductors,there exist only electron pockets with the absence of hole pockets,which seemingly challenges the theoretical basis of the s^±pairing.Thus,we need to carefully examine the gap sign in this kind of materials.The element nickel is just between Feand Cu which are basic elements of known high-temperature superconductors.In nickelate Nd_1-xSr_xNi O₂films,super-conductivity with the critical temperature of 9?15 K was discovered in 2019.The important discovery immediately stimulates lots of interests in this field.However,the research is very rare on physical properties of this material,and experimental results of the paring symmetry is still lacking after the discovery.Thus,it is urgent to conduct spectroscopic measurements to directly determine the gap structure.In this thesis,I will report the experimental results of the scanning tunneling mi-croscope(STM)to investigate the pairing symmetry of some superconducting material-s,including Fe-based superconductor(Li_1-xFe_x)OHFeSe single crystal,the optimally doped cuprate superconductor Bi₂Sr₂Ca Cu₂O_8+?(Bi2212)single crystal and the new-ly found nickelate superconductor Nd_1-xSr_xNi O₂film.The main content is listed as follows:(1)We demonstrate that there exists a sign reversal gap on the two set of electron-like Fermi pockets in Zinc doped(Li_1-xFe_x)OHFeSe.We successfully synthesize Zn doped(Li_1-xFe_x)OHFeSe.By using STM,we find out locations of Zn impurities on the topographic images.A strong in-gap bound state is observed on the spectrum,which is induced by nonmagnetic Zn impurities.This is a proof of the gap sign reverse in this material.We measure quasiparticle interference(QPI)patterns around Zn im-purities at different energies.Base on the phase-referenced QPI of impurity-induced bound states,we successfully demonstrate that a sign reversal gap is established be-tween the two folded or hybridized electron-like Fermi pockets.This work can provide a unified understanding about the s^±pairing theory in Fe-based superconductors with and without hole-like Fermi pockets.(2)We directly visualize the sign change of the d-wave gap in cuprate supercon-ductor Bi2212.We measure QPI images at different energies in the optimally doped Bi2212 single crystal,and obtain the characteristic scattering wave vectors connecting the octet ends of the banana-shaped contour of Fermi surface in Fourier transformed QPI patterns.Subsequently,we develop the multi-impurity phase referenced QPI anal-ysis and clearly visualize the sign change of the d-wave gap by using this method.The related conclusion is supported by theoretical calculations.The new method can be used to detect the gap-sign change of other unconventional superconductors.(3)We find two types of superconducting gaps in the newly found nickelate super-conductor Nd_1-xSr_xNi O₂.Using molecular beam epitaxy technique and soft-chemistry topotactic reduction method,we successfully synthesize the high quality superconduct-ing Nd_1-xSr_xNi O₂thin films.We conduct the single particle tunneling spectrum mea-surements at 0.35 K,and find the features of two types of superconducting gaps on measured spectra.One kind of spectra shows V-shaped feature near zero bias,which suggests the existence of a d-wave gap with the gap maximum of 3.9 me V;another kind shows U-shaped feature near zero bias,corresponding to a full gap with the gap value of about 2.4 me V.The mixed feature of the two kinds of gaps is also observed on some spectra,which further confirms the coexistence of two types of gaps in the system.Our results indicate both similarities and distinctions between the newly found Ni-based superconductors and cuprates,and shine new light on studies of the newly found Ni-based superconductors.