Scanning Tunneling Microscopy/Spectroscopy Study Of Surface Enhanced Superconductivity Of KFe₂As₂ And Charge-Density Wave Of ZrTe₃

As an electronic characterization method with high spatial resolution,Scanning Tunneling Microscopy/Spectroscopy(STM/S)has become an important surface characterization method in condensed matter physics.Ultra-low temperature and high magnetic fields are playing an increasingly important role in the studies of material science.Ultra-low temperature can greatly reduce thermal disturbance of electrons and improve the energy resolution of Scanning Tunneling Microscope(STM),and high magnetic field provides an important way for sample regulation,and the vector magnetic field greatly improves the degree of freedom of regulation,which plays a vital role in the study of vortices for superconductors and magnetic topological materials.This paper introduces a homemade ultra-low temperature high vector magnetic field STM,the base temperature is below 30 m K,the magnetic field is up to 18 T,and the angle between the sample surface and the magnetic field direction can be arbitrarily adjusted.Then,this paper introduces the research of unconventional superconductors using another homemade low-temperature vector magnetic field STM,including iron-based superconductors,asymmetric superconductors,etc.In this paper,an over-doped 122 iron-based superconductor KFe₂As₂ has been cleaved at low temperature by cryogenic in-situ cleavage technique,and the in-situ STM/S measurements have been made.After lots of repeated experiments,the As,50%K,and 100%K terminated surfaces of KFe₂As₂ were determined,and the superconducting transition temperature and superconducting energy gap on the 100%K cleavage surfaces are 4 times higher than that in the bulk.Then,we studied the scattering patterns on different cleaving surfaces by using the Quasiparticle Interference(QPI)technique,which does not accord with the prediction of band shift induced by doping.Therefore,the surface-enhanced superconductivity on the 100%K surface is not simply from the band shift induced by the change of doping but is more likely a new surface state.The vortices measurements on the 100%K surface clearly show the star shape and the exotic vortices structure at different sample bias voltage,which provides important evidence for revealing the pairing symmetry in iron-based superconductors.Then,we use STM/S to study PtBi₂.PtBi₂ has recently been found existing superconductivity under ultra-low temperature,which,combined with its topological nontrivial properties,makes it a potential candidate for topological superconductors.Exploring quantum materials with nontrivial lattice geometry is emerging as a frontier in fundamental physics.The kagome lattice,made of conner-sharing triangles,can naturally host both realistic electrons and a flat band,providing nontrivial band topology and strong electron correlation.In this paper,we report our scanning tunneling microscopy observation of an unexpected kagome-like lattice of Bi on the surface of PtBi₂,which does not contain perfect kagome lattice in the bulk crystal structure.We find that the topographic image exhibits an unusual contrast inversion phenomenon as a function of bias voltage,and the kagome-like lattice is uncovered at very low bias voltage within 10meV.We further measure the local density of states to provide insights into the contrast inversion phenomenon.Moreover,in comparison with another Bi-based honeycomb-like lattice,we find the Bi-based kagome-like lattice features an additional pronounced electronic state,which may be a candidate of the kagome flat band and deserves future theoretical confirmation.Our observation of the Bi-based kagome-like lattice provides a platform for studying physics with both geometrical frustration and strong spin-orbit interaction.Finally,the Charge Density Wave(CDW)of quasi-one-dimensional material ZrTe₃ is studied by the STM/S technique.Theory predicts that incommensurate CDW will slip without resistance under a certain electrical field,resulting in a supercurrent.However,due to the presence of impurities,defects,lattice distortions,and other pinning centers existing in the lattice,CDW will be pinned by them.This kind of pinning phenomenon has received a lot of macroscopic studies,but its direct microscopic observation is rarely reported.In this paper,we use atomic resolved STM directly observed the pinning phenomenon of CDW,and the thermal dynamics of the pinning effect are studied.we found the impurity pinning evolution from weak to strong as the temperature increases,revealing the interaction between the impurity pinning and CDW phase rigidity.