Neutron Scattering Investigations On A Kitaev Quantum Spin Liquid Candidate ?-RuCl₃

Quantum spin liquid(QSL)is a novel state of matter in which strong quantumfluctuation prevent long-range magnetic order from being established even at absolutezero temperature.Such a phase is not involved in any spontaneous symmetry breakingand local order parameter,and to understand it is beyond Landau phase transition the-ory.The rich physics and exotic properties of QSL,such as long-range entanglementand non-Abelian anyon excitations,are believed to hold great potentials in quantumcomputation.In addition,it is proposed that high-temperature superconductivity canbe obtained by doping QSLs.Therefore,the study of QSL is helpful to a further under-standing of the high-temperature superconductivity.Among the many classificationsof QSLs,the Kitaev QSL,as one of the main theoretical models,has been extensivelystudied by condensed matter physicist for its novel physical properties.Kitaev model isa spin 1/2 model on a two-dimensional honeycomb lattice.The model has topologicalorder and can be exactly solved.More interestingly,there are fractional excitations inthe system:Majorana fermions and Z2 gauge field.Therefore,searching for real Ki-taev materials it is not only of great significance in the field of basic science research,but also has a high application prospect in quantum communication and quantum in-formation.It has been found that the combination effect of crystal-field effect,spin-orbitcoupling and electronic correlation in Mott insulators can realize the anisotropic Ki-taev interaction.Therefore,the Mott insulator ?-RuCl3 with 4d orbital electrons hasbecome the main candidate for Kitaev quantum spin liquid.Both experimental and the-oretical studies on the Kitaev physics and the QSL excitations in this material appearsin endlessly in recent years.The key feature for defining a QSL is the fractional spin excitations.Among the techniques for magnetic excitation studies,inelastic neutron scattering(INS)is the most powerful one.In this paper,the novel magnetic excitations in Kitaev QSLs candidate ?-RuCl3 are studied by INS experiment.The main researchcontents and conclusions are as follows:1.The study of spin wave excitation in a-RuCl3 by INS experiment.High quality a-RuCl3 single crystals were grown by chemical-vapor-transport method and charac-terized by structural and physical properties.The magnetic ordered ground state of the material is confirmed to be Zigzag ordered state by elastic neutron scattering experi-ment.INS experiment on ?-RuCl3 single crystal is carried out for the first time,and the spin wave spectrum near the high symmetry point M(0.5,0,0)is obtained.By using results obtained from first-principles calculations based on energy bands and fitting the experimental data with a minimal effective-spin model,we determine the two leading interactions,including a anisotropic Kitaev interaction K term of-6.8 meV and the isotropic off-diagonal exchange interaction ? term of 9.5meV,and the K term and the? term are much larger than the Heisenberg exchange coupling J term.This study demonstrate explicitly that the Kitaev interaction can be realized in real materials.2.Polarized neutron scattering studies on the QSL excitation in ?-RuCl3.Some theoretical models and experiments suggest that the continuum excitation located at the Brillouin center ? point is magnetic fractionalized.In ordinary inelastic neutron scattering experiments,there will be some large background caused by nuclear scat-tering and the incoherent scattering when the center of Brillouin region is measured.Polarized neutron scattering can separate magnetic excitations from nuclear scattering processes by measuring the flipping spin in different channels.Here,we use polarized neutron scattering experiments to detect the quantum spin liquid excitation near the ?point and determine that there are temperature-independent magnetic excitations near? point,which is an important evidence for fractional excitation.3.Researches on the novel physics of ?-RuCl3 under applied magnetic field.There is no long range magnetic order in pure QSLs.Although the Kitaev interaction is dominant in this system,the ground state of the material is zigzag magnetic ordering state instead of Kitaev QSL state.Our susptibility and heat capacity measurements results show that the magnetic order of the system will be suppressed by an external magnetic field parallel to the ab plane.At Bc?7.5 T,the magnetic order is completely suppressed,followed by a new disordered phase.The phase diagram of ?-RuCl3 under magnetic field has been systematically studied by NMR,ultralow-temperature thermal conductivity and INS experiments:The results of NMR and ultralow-temperature ther-mal conductivity show that the disordered phase is quantum spin liquid phase.The phase diagram of energy gap evolution with magnetic field is obtained by INS exper-iment.It is found that the magnetic excitation gap in a certain region near the Bc is closed,and then reopened when enter the new partially polarized phase.