Study On The Kondo Effect And Oxidation Of FeTe Ultra-thin Films

The rapid development of science and technology stimulates the research on material science gradually moves from macro to nano scale.In particular,twodimensional materials have attracted extensive interests due to their novel properties,such as tunable band gap,superconductivity,ferromagnetism and topological properties.Layered Fe1+yTex materials have abundant quantum states.Superconductivity,antiferromagnetic order,charge density wave and two-dimensional ferromagnetism have been found in such single material system,which provide ideal platforms for studying the relationship among different quantum states.In this thesis,we firstly prepared high quality FeTe films with molecular beam epitaxy(MBE)technology,and transferred them to various in-situ characterization devices through ultra-high vacuum interconnecting pipes,which ensures the intrinsic properties during the transfer process.Combined scanning tunneling microscope(STM),angle-resolved photoelectron spectroscopy(ARPES),and X-ray photoelectron spectroscopy(XPS),the morphology,electronic states and band structure of the materials were obtained.We observed the Kondo effect in 1 UC FeTe films.The main results of this thesis are listed as follows:(1)We successfully prepared high-quality single unit cell(1 UC)and 10 UC FeTe films on the SrTiO3(001)substrates.STM images show the films exhibit tetragonal structures with atomic flatness.The scanning tunneling spectroscopies(STS)show that the films have uniform electronic states.A representative "V" shape electronic state near the Fermi surface is observed,indicating the metallic property in FeTe film.We also found the STO substrate induced lattice expansion and electron doping on the FeTe film.(2)The Kondo effect is found in 1 UC FeTe system.The transition temperature can be tuned by electron doping after the Sr treatment of the sample.Both pristine FeTe and electron doped FeTe were characterized.Kondo resonance were observed near EF investigated by STM and ARPES measurements.The Kondo temperature decreases upon electronic doping.The origin of the Kondo effect in FeTe may due to the hybridization between localized dxy orbital of Fe and pz orbit of Te.(3)Metallic TeO2 islands with rutile structure were observed on the surface of 10 UC FeTe film after oxygen treatment.The STM images show that the TeO2 islands have two different phases:the long large islands and the surrounding small islands.There are two kinds of bright strips on the surface of big islands,one is "wrinkle",with lattice distortion and "V" shape electronic state;The other is "ripple" with "U" shape electronic state.The small islands have various thickness.The electronic state is not sensitive to the thickness of the small islands,which differs from that of the big islands.After preparing high-quality FeTe film with controllable layer thickness,the effects of Sr treatment on STO substrate and O treatment on FeTe film were studied by in-situ ARPES and STM.The Kondo effect has been found in 1 UC FeTe films.Our study provides excellent platforms to study the Kondo effect in 3d electronic systems.