Scanning Tunneling Microscopy Studies On Iron-based Superconductors

As the second family of high temperature superconductors after cuprates,the dis*covery of iron-based superconductors triggers the renewed research on the origin of unconventional superconductivity.As in the case of cuprates,iron-based superconduc-tors(IBS)are hard to induce strong electron-phonon coupling,thus fails to describe the origin of the superconductivity in IBS.Meanwhile,from the point of phase dia-gram,the optimal superconductivity in IBS always emerges at the border of the anti-ferromagnetic instability,which suggests the possibility of the spin fluctuations as the bosonic mode for the superconducting pairing in these materials.Neutron scattering experiments have observed that the spin resonance peaks exist in almost all IBS.Since the interaction potential based on spin fluctuations is repulsive,the signs of the super-conducting gaps in hole and electron Fermi surface connected by the wave vector of antiferromagnetic order should be reverse,which is called s± pairing mechanism and widely accepted in FeAs-based systems.However,as a multi-band system,the Fermi surface topography in IBS could change strongly with the doping of the carriers,which makes the pairing symmetry unclear in these systems without hole pockets.How to u-nify the pairing symmetry in IBS with or without the hole Fermi surface will help to understand and solve the microscopic mechanism of high-temperature superconductiv-ity.In this thesis,focusing on the pairing symmetry,we report detailed scanning tun-neling microscopy/spectroscopy(STM/STS)researches on(Li1-xFex)OHFeSe,FeS and CsFe2As2.The result consists of four parts listed as follows.(1)Scrutinizing the double superconducting gaps and strong coupling pairing in(Li1-xFex)OHFeSe.Angle resolved photoemission spectroscopy(ARPES)experi-ments show that(Li1-xFex)OHFeSe is in the extremely electron-doping level and with only electron Fermi surface.However we find two superconducting gaps at 14.3 and 8.6 meV from the tunnelling spectrum.Taking the gap of 14.3 meV into calculation 2△max/kBTc,we get the result 8.7,which is much larger than the electron-phonon coupling based BCS theory prediction,indicating the strong coupling mechanism in this superconducting material.Meanwhile,in order to locate the superconducting gaps in Fermi surface,we use quasiparticle interference(QPI)to do further research.Based on the analysis on QPI,we successfully assign the smaller(larger)gap to the inner(outer)Fermi pockets(after folding)associating with the dyz(xz)(dxy)orbitals.(2)Sign reversal of the order parameter in(Li1-xFex)OHFe1-yZnySe.In last work,we have proved there are inner and outer electron fermi pockets in(Li1-xFex)OHFeSe.Since the absence of the hole pocket in this material,the pairing symmetry becomes puzzling,which motivates our further researches.In(Li1-xFex)OHFe1-yZnySe,we have found the in-gap bound state possibly induced by the Zn impurity.The applied magnetic field does not shift the energy of the bound state,indicating the non-magnetic nature of the impurity.Non-magnetic impurity induced in-gap bound states suggest there is a sign-reversal gap.Further analysis based on the phase-sensitive QPI tech-nique find that the superconducting gaps in(Li1-xFex)OHFe1-yZnySe does change sign,generalizing the pairing symmetry in IBS with and without hole pockets.(3)Strong-coupling superconductivity revealed by scanning tunneling microscope in tetragonal FeS.FeSe has not only the simplest atomic structure among all IBS,but also rich physics.As an isostructure of the FeSe superconductor,superconductivity has been reported with Tc = 4.5 K in tetragonal FeS.Previous band structure calculations have shown that the electronic structure of FeS is quite similar to FeSe.Therefore,it is intriguing to know whether the gap structure and the pairing mechanism of FeS are similar to that of FeSe systems.It is found that the superconducting tunneling spectrum can be fitted well with the anisotropic s wave or s + d waves by using Dynes model.The corresponding gap ratio of the 2△max/kBTC is larger than the predicted one by the BCS theory in the weak coupling limit,suggesting strong coupling in this compound.We also observe two kinds of defects on the surface.The defect located at Fe site gives negligible influences on the STS spectra and the superconducting gap,while the defect located at S site induces clear in-gap states with the peak locating at zero energy.(4)Investigation of vortex lattice and vortex bound states on CsFe2As2.The vor-tex state is a typical character of type-II superconductors.The repulsive interaction be-tween vortices will make the vortices form an ordered lattice which is called Abrikosov lattice(usually triangular)in the sample with weak pinning.In CsFe2As2,we observe a possible structural transition of vortex lattice,i.e.,the vortex lattice changes from a distorted hexagonal lattice to a distorted tetragonal one.Some qualitative supports are obtained from a simple calculation of the vortex-interaction energy.The vortex bound state is also observed in the vortex center.The tunneling spectra crossing a vortex show that,the bound-state peak position holds near zero bias with STM tip moving away from the vortex core center.Based on the analysis of vortex bound state,we infer that the Fermi energy of some band(s)of CsFe2As2 could be very small.