Epitaxial Growth And Physical Properties Study Of Emerging Superconducting Nickelates Thin Films

Condensed matter physics,as the extension of solid physics,has become one of the most important branches of physics today.While the traditional condensed matter theory,which is based on the Landau Fermi liquid theory and the Landau–Ginzburg–Wilson phase transition theory,including symmetry-breaking theory and renormalization group theory,has become an important paradigm for exploring condensed matter.The completeness of this traditional theory was broken by the subsequent discovery of the fraction quantum Hall effect and the high-temperature superconducting cuprate.The modern condensed matter paradigm is excited by the limitations of traditional condensed matter theory.However,there is no consensus on the mechanism of superconductivity in cuprates till now.Researchers have been looking for superconductivity in other transition metal compounds,to reveal the mechanisms of high-temperature superconductivity in the past 30 years.Superconductivity is finally achieved in the recently discovered infinite-layer nickelates R_1–xA_xNiO₂（R=La,Pr,Nd;A=Ca,Sr）,which quickly became the theme of many experimental and theoretical research.Even though,research on nickelates superconductor progresses at a slow pace due to the difficulty in sample preparation.In this dissertation,we study the environmental stability of emerging superconducting Nd_0.8Sr_0.2NiO₂and parent Nd NiO₂ thin film.The dependence between hybridization of Nd–Ni orbitals and the intensity of charge density waves are also revealed in parent compounds.In addition,we provide the first experimental evidence of hygendron in nicklates superconductor,as well as the hygedron doping dependent superconducting phase diagram.From the perspective of synthesis,crystal structure and low-energy excitation,the properties of nickelates are systematically studied.The research content includes the following three parts:（1）We give the synthesis procedure for infinite-layer nickelates films and study their physical properties.The growth windows of perovskite precursor,i.e.,Nd NiO₃and Nd_0.8Sr_0.2NiO₃ thin films and their topotactic reduction conditions to Nd NiO₂and Nd_0.8Sr_0.2NiO₂,are provided.The Nd_0.8Sr_0.2NiO₂ thin films show a superconducting transition temperature up to 11.1K.High-resolution X-ray diffraction indicates all of the infinite-layer nickelates films are fully strained.Furthermore,superconducting Nd_0.8Sr_0.2NiO₂ films were put in the air,water and ionic liquid to study their crystal structure and superconducting stability under different conditions.These results are important for the storage and application of the emerging nickelates superconductor.（2）The charge order is found in parent Nd NiO₂ films.The Ni L₃ edge excited resonance inelastic X-ray scattered spectrum shows the existance of charge density waves in Nd NiO₂ samples,indicating this property is universal in parent compounds.Charge density waves form in Nd 5d and Ni 3d orbitals at the same commensurate wavevector（0.333,?0）reciprocal lattice units.Further analysis reveales a strong connection between charge density waves and Nd 5d–Ni 3d orbital hybridization.Charge density waves are fully suppressed by 20%Sr doping.While previous reports focus on their spin order,this work highlights the charge order in nickelates superconductors.（3）For the first time,the existence of the H element in superconducting Nd_0.8Sr_0.2NiO₂ thin film is experimentally proved.Ca H₂-assisted topotactic reduction may introduce hydrogen that affects the physical properties of nickelates,which has caused widespread controversy.Through the high-resolution time of flight secondary ion mass spectroscopy,we find that hydrogen was introduced into infinite-layer nickelates during the topotactic reduction process to form Nd_0.8Sr_0.2NiO₂H_x.x-dependent resistivity revealed a brand-new“weakly insulating–superconducting–weakly insulating”phase diagram.Combined with density functional theory calculation and resonance X-ray scattering spectrum,we believe that the H ions in the lattice occupy the position of apical oxygens,forming monovalent H^–.