Development Of STM-based Micro-four-point Probes And In-situ Measurements Of Interface Superconductivity

Existence of crystal surfaces and interfaces reduces spatial degrees of freedom,resulting in novel quantum phenomena related to the reduction of dimensionality.Measuring electrical transport properties of crystal surfaces or thin films thereon is a promising way for both fundamental researches and potential low-dimensional devices.Electrical transport measurement is a dominant method to characterize electrical conductance of a material.In the process of traditional approach to conduct electrical transport measurements,however,samples may inevitably be exposed to atmosphere.For some kinds of samples,it is easy to bring damage to surfaces,and may alter the original physical properties of samples.In comparison,it is more reliable to obtain transport properties in situ.In-situ measurements incorporate sample preparation and surface measurements in a same ultra-high vacuum(UHV).The results of in-situ measurements reveal exact properties of samples by avoiding the influences of the air.On the other hand,the size of electronic devices shrinks gradually as the technology evolves,and reaches micrometers or even nanometers now.Measurements in microscopic scale are required in deriving transport properties of nano-scale samples.Micro-four-point probes(M4PPs),integrating together four probes with micrometer spacing for micro-scale electrical transport measurements,are very suitable for in-situ measurements.M4 PPs,after being developed,are employed to in situ measure transport properties of surfaces and ultrathin superconductors.In this thesis,based on a commercial scanning tunnelling microscope(STM),we modified the STM unit to be compatible with four-point probes(4PP),while the basic functions of STM remain as well.An STM is a powerful tool to display the arrangement of atoms and to measure the electronic states on surfaces.On the other hand,crystal structures and electronic states on surfaces of the sample make substantial influences on the transport properties.Combining insitu transport measurements by M4 PP with an STM,one can obtain surface morphology,surface electronic states and transport properties in situ,and understand physics of surfaces correctly.Moreover,affiliated environments of a commercial STM,e.g.low temperature,high magnetic field and UHV,are often required in transport measurements.In addition,we developed measurement circuits and software programmes for automatic measuring by 4PP.The measurement system has a resistance resolution down to 100 n?.Using the electrical measurement system,we have successfully detected the zero-resistance transition of high-temperature cuprate superconductors,with a consistent critical temperature as expected.In general,we developed the first experimental system that combines together in situ sample preparation by molecular beam epitaxy(MBE),in situ surface morphology and local density of states characterization by STM and in situ electrical transport measurements by 4PP at a temperature down to 0.4 K and magnetic field up to 11 T.This experimental system opens up a new way for systematic investigation of novel low-dimensional materials and exhibits significant capability in scientific research of surface physics.Interface effects were found to induce superconductivity between insulating oxides,as well to enhance critical temperature of superconducting films,which matches the technology of device manufacturing perfectly.Recent studies show a superconducting energy gap of 20 meV in single-layer FeSe films grown on strontium titanate(STO)substrates,by which a critical temperature over 80 K can be estimated.Using our new STM-4PP experimental system,we prepared single-layer FeSe films on STO substrates by MBE,and characterized in situ interface superconductivity in these samples.The main results are as follows:(1)We succeeded in epitaxial growth of high quality single-layer FeSe films on Nb-doped STO substrates.We employed Se molecular etching in annealing Nb-doped STO substrates to obtain flat surface terraces.By real-time monitoring the preparation of single-layer FeSe films,we identified the growth mode to be two-dimensional layer-by-layer mode,which is in agreement with STM topography of various coverage.The atomic resolved STM images of single-layer FeSe films show a square lattice with fourfold symmetry,with lattice constant of3.8 ?and 5.3 ?.On the other hand,we prepared insulating STO substrates by acid etching,and tried to grow single-layer FeSe films on insulating STO substrates,but FeSe forms hexagonal lattice on Sr-terminated layers,rather than square lattice on Ti-terminated surface.FeSe grows in three-dimensional mode on Sr-terminated surfaces,which hinders successive growth of FeSe on the substrate.Systematic investigations on the structure of single-layer FeSe films grown on STO substrates are important to understand the electronic structure,origin of the superconductivity and electrical transport properties.(2)By utilizing the self-developed experimental system,we detected zero-resistance states in situ in single-layer FeSe films on Nb-doped STO substrates up to 109 K.Temperature dependence of the critical current coincides with the Ginzburg-Landau(G-L)theoretical model,and results in a consistent critical temperature.To verify that superconductivity can be suppressed by external magnetic field,we conducted transport measurements under various magnetic field.Superconducting critical current and temperature decreases with increasing magnetic field,and critical field emerges when changing the field at fixed temperatures.The upper critical field is estimated to be 116 T,which corresponds to a G-L coherence length of 1.7 nm.In addition,we measured the superconducting energy gap in single-layer FeSe films on Nb-doped STO substrates by scanning tunneling spectroscopy.Based on both our results and related follow-up research,we attribute the origination of superconductivity in single-layer FeSe/STO system to interface-enhanced electron-phonon coupling effects.Our findings not only provide an ideal platform with the simplest structure to reveal the mechanism of high-temperature superconductivity,but also prove interface enhancement effects to be an effective way in searching for superconductors with a higher critical temperature.