The Preparation Of Cerium-lanthanum Alloy Thin Film By Molecular Beam Epitaxy And Its Phase Transition And Electrical Transport Research

Lanthanides and actinides with f-electrons are the most complex elements in the periodic table,and tend to exhibit peculiar macroscopic physical properties,among which the y-a phase transition in cerium is one of the most typical representatives.This iso-structual phase transition involves both a volume collapse of more than 15%and the magnetic properties transition from Curie-Weiss like to Pauli-like paramagnetism.Over the years,a variety of theoretical and experimental researches have been dedicated to understand the microscopic mechanism of the y-a phase transition in cerium,and have proved it to be related to the dual nature,i.e.,localization and itinerancy,of the 4f electrons and their hybridization with conduction electrons.The electronic configuration of cerium is 4f¹5d¹6s²,in which the 4f electrons are localized at room temperature but their energy level is close to that of the 5d and 6s electronic energy levels,resulting in its dual characteristics of localization and itinerancy of the 4f electrons and complex physical properties.Lanthanum,with an electronic configuration of 5d¹6s²,has similiar electronic structure with Ce except for the absence of 4f-electrons.What is more,La can form alloys with Ce with arbitary composition,making it one of the best elements to be alloyed with Ce and manipulate the behaviour of 4f electrons and y-a phase transition in Ce.Due to the large volume collapse of the y-a transition in Ce,the latent heat is massive,resulting in significant hysteresis phenomena.The onset temperature of γ→α phase transition（M_s=～100 K）is much lower than that of the α→γ phase transition（A_s=～170 K）.Besides,the electrical resistivity of a-Ce is much samllar than that of γ-Ce.Consequently,there is a significant hysteresis loop in the electrical resistivity versus temperature（ρ-T）curve of Ce between 2 to 300 K,which clearly reflects the feature of γ-α transition.By synthesizing a series of Ce_xLa_1-x alloys with different La concentration and systematically studying their hysteresis loops of p-T curves,we could have a better insight into the 4f-electrons property in Ce and its relationship to the y-a transition.In this thesis,on the one hand,a series of Ce_xLa_1-x/Al₂O₃（11-20）polycrystalline thin films were synthesized by Molecular Beam Epitaxy（MBE）.Their crystal structure was characterized by XRD and electrical transport measurements between 2 to 300 K were carried out by Physical Property Measurement System（PPMS）.On the other hand,we have grown Graphene on 6H-SiC（0001）single crystal substrates with low and high resistivity（LR and HR）,respectively.High quality single crystalline Ce thin films were successfully synthesized on Graphene/6H-SiC（0001）（LR）and Graphene/6H-SiC（0001）（HR）substrates by MBE.The crystal structure of the single crystalline thin films were characterized by in-situ Reflected High Energy Electron Diffraction（RHEED）and ex-situ room temperature XRD and varied temperature XRD（VT-XRD）.Electrical transport measurements were also carried out between 2 and 200 K by PPMS.The main results of our researches are summarized as follows:（1）For the first time,we have successfully synthesized a series of high quality,highly orientated,FCC structured polycrystalline Ce_xLa_1-x alloy thin films on Al₂O₃（11-20）substrates by MBE method.（2）We have confirmed that the cooling and heating cycles between 2 and 300 K would result in the accumulation of β-phase Ce in both Ce and Ce_xLa_1-x alloy films.The concentration of β-phase does not affect y-a phase transition temperatures;（3）The y-a phase transition was observed in the Ce_0.967La_0.033 alloy film,and the formation and stable existence of a-Ce phase in the alloy were suppressed.We think that the reason was the increase of the localization of the 4f electrons in the the alloyed thin film.（4）It was found that the existence of α-phase Ce will be completely suppressed with a La concentration over 9.2%.And β-phase Ce will dominate at low temperature,indicating the 4f electrons for La-alloyed thin films were more localizded.（5）We have grown Graphene on low-resistivity 6H-SiC（0001）（LR）single crystal substrates and high-resistivity 6H-SiC（0001）（HR）single crystal substrates.High quality y-Ce single crystal films were synthesized on those substrates.（6）It was confirmed that the semi-insulating Graphene/6H-SiC（0001）（HR）substrates served as perfect substrate for the electronic transport measurements of Ce single crystalline thin films.And the y-a phase transition temperature of a 200 nm Ce single crystal film was obtained.