Ammonothermal Synthesis Of High-t_c Superconductor Li_x(C₃H₁₀N₂)_yFe₂Se₂ And DC-current Control Of The Mott Transition In Ca₃(Ru_1-xTi_x)

Strongly correlated electronic systems are important frontier topics in condensed mat-ter physics.The strong correlative interactions among various quantum orders make the system exhibit novel physical properties,such as high-temperature superconductivity,metal-insulator transition,the non-equilibrium steady states?NESS?of strongly corre-lated electronic materials under direct current?DC?,topological insulators,giant mag-netoresistance?MR?effect,and so on.In this thesis,we mainly study on two types of strongly correlated electronic systems:layered FeSe-based high temperature supercon-ductor Li_x(C₃H₁₀N₂)yFe₂Se₂ and bilayered ruthenate Mott insulator Ca₃yRu1-xTix)2O7.We have studied the physical properties of these two systems mainly by means of mag-netic and transport experiments.By applying different current density?J?to single crystals of Ca₃(Ru_1-xCTi_x)₂O₇,diamagnetism can be induced in this system.According to the transition temperatures?T?,we also mapped out the T vs J and T vs P/v phase diagrams.This dissertation consists of four parts as follows:1.Brief overview of FeSe-based superconductors and layered ruthenatesIn this chapter,the history of iron-based superconductors and the establishment of BCS theory are briefly reviewed.Then,we introduced the main crystal structure,mag-netic structure and several synthesis methods of iron-based superconductors.The possi-ble superconducting mechanism and phase diagrams of typical systems of iron-based su-perconductors are described.Finally,the magnetic and electronic properties of ruthen-ates in its NESS under DC are introduced.The study of NESS in strongly correlated materials is at a rather infant stage,but it has attracted widespread attention.2.Synthesis and superconducting properties of Li_x(C₃H₁₀N₂)yFe₂Se₂This chapter introduced a newly FeSe-based superconductor Li_x(C₃H₁₀N₂)yFe₂Se₂ synthesized by the hydrothermal method.Through a combination of powder X-ray diffraction?XRD?,element analysis and inductively coupled plasma-atomic emission spectrometry?ICP-AES?results,the crystal structure of this new superconductor was determined,with alternate stacking of FeSe layers and clathrate layers formed of alkali-metal Li and 1,2-Diaminopropane along c-axis.Our work demonstrates that the hy-drothermal method has unique advantages in synthesizing FeSe-based superconduc-tors,and the Tc has a strong dependence on carrier concentration.The effect of post-annealing on the crystal structure and Tc of the superconductor?x=2?was also care-fully analyzed.After post-annealing,crystal structure remains unchanged,but the Tc changes from 36 K to 45 K which is confirmed by the magnetic susceptibility and resis-tance measurements.3.Two-step method for synthesizing Li_x(C₃H₁₀N₂)yFe₂Se₂ superconductorOrganic and metal co-intercalated FeSe-based superconductors prepared by sono-chemical method have the advantages of pure phase,higher crystallinity and larger su-perconducting volume fractions.In this chapter,we make good use of the hydrothermal method to re-synthesize the Li_x(C₃H₁₀N₂)yFe₂Se₂ superconductor based on sonochem-ical method,which is called two-step method.The Tc of this newly synthesized su-perconductor is 37 K,and diamagnetic shielding fractions exceeds 90%at 2 K which fully indicates the characteristics of bulk superconductivity.According to the result of powder XRD,the Li_x(C₃H₁₀N₂)yFe₂Se₂ superconductor synthesized by the hydrother-mal method or synthesized by the two-step method belongs to the tetragonal system.In order to detect the air sensitivity of this superconductor,we measured the R of this sample exposed to air for different lengths of time?t?.For t?2h,the Tc indicated by the resistance curve is almost unchanged,but the resistivity of the sample increase as a whole.The sample almost lost its superconductivity when the time was t=5 h,indicating that the sample synthesized by this method had a certain stability in the air atmosphere.4.Universality of current-induced diamagnetism in layered ruthenatesNon-equilibrium steady states?NESS?induced by the flow of electric current are an emerging research field in condensed matter physics,holding great potential to un-cover novel material properties.Recently,a large current-induced diamagnetism has been reported in two layered ruthenates with similar magnetic ground states,but the physical understanding of what triggers this effect remained elusive.In this work,we induce diamagnetism in Ca₃Ru_1-xTi_x)₂O₇?x=10%?,a bilayer ruthenate Mott insu-lator with another magnetic structure,by using a current either along the RuO2 planes or in the out-of-plane direction.Through the analysis of the dependence of the dia-magnetic transition temperature on several physical quantities,we identify the power density as a universal,direction-independent control parameter for current-induced dia-magnetism.We show that current-induced diamagnetism is a universal phenomenon in layered ruthenates with a Mott insulating ground state,irrespective of the underlying magnetic structure and current direction.The use of NESS as an interesting playground to induce novel properties.5.Conclusion and outlook...