| Topological states have become one of the core research subjects in condensed matter physics through decades of development.At present,the topological classification of non-correlated systems has been basically established.Substances with nontrivial topology can be classified into different topological insulators or topological semimetals.Further on this research,it is important to extend the research of topological states into strongly correlated systems.Strongly correlated systems have been always the core field in condensed matter physics,which provide a frontier for the research of manybody interaction.In previous studies,the strongly correlated systems exhibit rich physical phenomena,demonstrating the principle of "more is different" in condensed matter physics.The research of strongly correlated systems focus on the amplitudes of electronic wave function.In contrast,topological states concern the phases of electronic wave function.The combination of both research fields offers a comprehensive way to improve our understanding on physical phenomena.Nowadays,the conjunction of topological states and strong electronic correlation effect presents a common challenge in both fields.The quasiparticles in most topological materials are massless or massive Dirac/Weyl fermions,which brings about characteristic linear dispersion.The large bandwidth and electron kinetic energy originating from the linear dispersion is harmful to achieve strongly correlated topological electronic states.Finding the correlation effect in topological states has become one of the current research hotspots.This dissertation focuses on searching electronic correlation effect in topological materials.The research approaches contain strengthening the electron correlation or finding special topological band structure with small bandwidth.The pass researches on strongly correlated systems have discovered the important role of the magnetic field on enhance the electron correlation strength.The key is the quench of electronic kinetic energy by Landau quantization.Therefore,the magnetic field can be applied to enhance the electron correlation strength in topological materials.Besides,some special topological materials own topological flat bands with small bandwidth which is naturally suitable for forming topological correlated states.,Following above two research approaches,this dissertation summarizes three works on node-line semimetal and Kagome lattice materials,respectively.The nuclear magnetic resonance(NMR)technology was used to detect the possible electron correlation effect.NMR is a lowenergy spectroscopy method which is very sensitive to the low-energy electronic excitation.It is able to detect weak electron correlation effect and appropriate for the aim of this dissertation.Besides,as important tunning tools in these works,magnetic field and hydrostatic pressure can easily combine with NMR.The detail of three works are as follows.In chapter 3,detailed nuclear magnetic resonance measurements were carried out on compressed black phosphorus.Black phosphorus undergoes a topological transition from semiconductor to nodal-line semimetal under pressuring.Its low quantum limit near the topological phase transition is benefit to realizing Landau quantization.Through the measurements,an anomalous 1/T1T upturn is revealed at low temperature with the magnetic field parallel to c axis.This phenomenon disappears when the magnetic field is parallel to a axis.1/T1T is proportional to imagine part of dynamical magnetic susceptibility.After excluding the interferences of impurities or chemical potential,this phenomenon is ascribed to intrinsic spin fluctuations.According to theoretical analysis,the lowest Landau level is Weyl-like band structure with the magnetic field parallel to c axis but is a trivial semimetal with the magnetic field parallel to a axis.Further experiments and analysis on the pressure-and field-dependent 1/T1T establish the partner relationship between anomalous spin fluctuations and Weyl-like band structure.Further theoretical analysis excludes the Fermi surface nesting scenario under weak electronic correlation but points to an extra electronic scattering between Weyl points,which demonstrates the existence of significant electronic interactions.In chapter 4,the evolution of different phases of CsV3Sb5 under pressures was systematically studied by utilizing nuclear magnetic resonance technology.The band structure of CsV3Sb5 owns Dirac points and Van Hove singularities simultaneously.CsV3Sb5 exhibits multiple ordered states like changer density wave,electronic nematic phase and superconductivity under ambient pressure,.Meanwhile,the anomalous Hall effect was also observed in CsV3Sb5.First,a emergent charge density wave,which is possible with stripe-like modulation,was uncovered under pressure through analyzing the temperature-and pressure-dependent nuclear magnetic resonance spectrum.Second,the two-dome-like superconductivity phase diagram,as well as the significant superconducting transition broadening between two domes,was confirmed by our measurements.We also found three superconducting phases under pressures and third phase is possible unconventional superconductivity.The one-to-one relationship between different charge density waves and superconducting phases was revealed by comparing their pressure phase diagrams.The broaden superconductivity was just below the possible stripe-like charge density wave.Finally,a pressured-insensitive Curie-Weiss type 1/T1T was observed above the charge density wave transition under all pressures in this work.We speculated that it should attribute to certain fluctuation of electronic degree of freedom.This work unveils the complicated interplay between charge density wave and superconductivity and the important role of electronic correlation inside these phenomena.In chapter 5,the properties of kagome lattice compound CoSn was studied by applying nuclear magnetic resonance measurement.The topological flat band near Fermi surface in CoSn is beneficial for the electron correlation effect.Combing the Knight shift of Co atoms,Knight shift of both sites Sn atoms and the ab initio calculation,it was revealed that the different 3d electrons from Co couple to Sn atoms on different sites.The nuclear magnetic resonance measurement become orbital-selective by measuring Sn atoms on different sites.The hyperfine coupling tensor of in-plane 3d electrons is unusually small in stark contrast to other 3d electrons.We suspected that it results from cancellation of spin susceptibility.Besides,the in-plane ferromagnetic fluctuation and out-of-plane antiferromagnetic fluctuation were uncovered through analyzing Korringa relation belong to different 3d electrons.It is strong evidence that the exchange interaction is quite strong and CoSn is closed to a magnetic phase transition.The flat band should be responsible for these phenomena.To achieve the electron correlation effect in topological materials,two types of topological materials are studied following different research approaches.Utilizing applied magnetic field to enhance the electron interaction strength,anomalous spin fluctuations are observed as an evidence of electron correlation effect in node-line semimetal state of black phosphorous.Kagome lattice materials are natural platform to explore strong correlated topological states due to the coexistence of topological band and Von Hove singularities or flat bands.Different electron correlation effects on both kagome lattice materials CsV3Sb5 and CoSn are confirmed through our studies.In a summary,the concrete evidences of electron correlation effect have been found in different topological materials through NMR technology combined with strong magnetic field and hydrostatic pressure this dissertation. |
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