Experimental Study On The Collective Excitation Mode Of Pb Thin Films

In recent years,the research of low dimensional materials has been an important field in condensed matter physics.With the decrease of material's dimension,the quantum size effect becomes more significant in the film,and the influence of the substrate on the film becomes more important.Therefore,the properties of thin-film materials tend to be richer and more complex than the bulk structure.Due to the break of symmetry,the properties of the surface are completely different from that of the bulk.Although Pb is a simple metal,it is a superconductor with a relative high superconducting transition temperature of 7.23 K.Therefore,electron-phonon coupling in Pb films has always been the focus of attention.Meanwhile,there is a strong quantum size effect in Pb films,and the quantum well states can be observed in rather thick Pb films.According to the previous research on alkali metal thin films,the quantum well state can significantly influence the surface plasmon of the film.In this context,the current thesis employs the technique of high-resolution electron energy loss spectroscopy to study the surface plasmon and phonons of Pb films with different thickness.The first part of this thesis is to study the variation of the surface plasmon of Pb films with the change of the film thickness.First,in the thick film of Pb(40 monolayer,about 11 nm)we observed three energy loss modes.Through comparing with previous theoretical and experimental results,we identified the origins of the modes.The energy loss peak of 1.75 eV originates from the spin-orbit coupling effect of Pb bands,the 7-eV mode is from the interband transition,and the 10 eV mode is assigned to the thin-film surface plasmon.In thick Pb films,surface plasmon starts damping at a very small momentum.This phenomenon indicates that the quantum well state in Pb films enhances the coupling between the surface and the substrate.With decreasing the thickness of the film,the critical momentum where surface plasmon starts damping has increased gradually,but far less than the theoretical calculation.These results indicate that the surface plasmon of Pb films is formed from mutual coupling between the thin-film surface and the substrate interface,consolidating that the quantum well states in Pb films can enhance the coupling effect.In the second part,the change of surface phonons in Pb films with the thickness from zero to 40 monolayer on Si(111)substrates is studied.When the coverage of Pb is 1/3 monolayer,the detected vibration modes are significantly different from those of the Si substrates,indicating that a small amount of Pb atoms would significantly change the lattice dynamics of Si surfaces.Besides,the detailed analysis of the vibration modes shows that when the coverage of Pb is less than 4/3 monolayer,the Pb thin film cannot form the vibration modes between Pb-Pb atoms.When the Pb coverage is increased further,the acoustic and optical phonon dispersions are observed,indicating the vibrations formed by the Pb-Pb atoms.These results show that the thickness of Pb films has a decisive influence on its lattice dynamics.The appendix part of this thesis introduces another work related to a study of the anomalous lattice shrinking in a heterostructure of two transition metal oxides.The transition metal oxides with the ABO₃ perovskite structure has a special configuration along the(111)direction,which strongly affect the heteroepitaxy material properties of the interface.We had successfully grown LaTiO₃ thin films on Sr Ti O₃(111)substrate by molecular beam epitaxy.The scanning transmission electron microscopy reveal that the films with the thickness up to 60 monolayers bear the in-plane compressive strain,while the x-ray diffractions indicate the contracted out-of-plane interspacing.Such an anomalous lattice shrinking may be related to the formation of oxidized LaTiO_3+?,and more importantly,the three-dimensional electron transfer from the film to the substrate.