Kagome Quantum Spin Liquid Materials Exploration And The Studies Of Their Physical Properties

When a magnetic system suffers strong quantum frustration,it may form quantum spin liquid?QSL?ground state at 0 K which is a quantum entangled state.Initially,QSL was considered to be the ground state of the triangular antiferromagnetic system,and then it experienced a revival in interests simulated by the high-temperature superconductivity.Now,QSL attracted a lot of attention since its relationship between quantum entanglement and topological order.Therefore,the study of QSL is valuable and meaningful.Theoretically,QSL breaks the Landau-Ginzburg symmetry-breaking theory paradigm and promote the development of quantum theory.Experimentally,we may find spinon excitations in QSL material and QSL is believed to be used to realize quantum calculation.Currently,kagome Heisenberg model is promising to realize QSL.In this thesis,we will talk about our works about kagome QSL materials and their kagome QSL properties.Firstly,we synthesized new QSL candidate Cu₃Zn?OH?₆FBr using hydrothermal method and Cu²⁺ions consist of two-dimensional kagome lattice.Strong antiferromagnetic interaction?²00 K?exists between Cu²⁺ions whereas no visible magnetic order is observed down to 50mK.Furtherly,pure spin magnetization from kagome plane is obtained by performing NMR measurement on F element.Our data reveals clearly spin gap opening at low temperature.We identified the quasiparticles are S=1/2 spinon excitations in kagome QSL candidate for the first time.Secondly,we systematically analyzed the influence of Zn content on kagome magnetism in Cu_4-xZn_x?OH?₆FBr system and tried to find the quantum phase transition from 3-dimensional antiferromagnetic order to QSL ground state.At room temperature,Cu₄?OH?₆FBr shows the same crystal structure with Cu₃Zn?OH?₆FBr.In this system,we find that three-dimensional antiferromagnetic order would be suppressed with increasing x,while another second local ferromagnetic order may exist until x equals to 0.8.Most interestingly,the kagome properties related to QSL appear when x is larger than 0.4 and we believe this system may host the long-thought quantum-phase transition between long-range order and topological QSL ground state.At last,we introduce our exploration of new QSL candidate and topological materials briefly.On the one hand,we synthesized another new QSL candidate Cu₃Zn?OH?₆FCl which is very similar to Cu₃Zn?OH?₆FBr.This material provides new research matter for the study of QSL properties.On the other hand,we synthesized high-quality MoP single crystal for the first time.It is a new kind of topological semimetal beyond the conventional Weyl-Dirac-Majorana classification,and we are the first to observethe three-degenerate band-crossing point by ARPES measurement in MoP energy band.This observation may promote the development of topological physics and provide the motivation of searching for“new fermion”in condensed matter.Those are my main thesis works.QSL and topological materials as two topic issues in condensed matter physics,they come from the discovery of fractional quantum Hall effect?FQHE?and high T_c superconductor last century.As“the two dark clouds”developed into the foundation of physic—quantum mechanism and relativity—at 20century,we believe the studies on topological materials arising from FQHE and QSL,which is related to high T_c superconductivity would renew our knowledge about condensed matter physics.