Characterizations And Crystal Field Excitations In Novel Kitaev Material ErOCl

Quantum spin liquids are a type of Mott insulator that do not exhibit symmetry breaking even at absolute zero temperature.They have attracted widespread atten-tion in recent years due to their close relationship with the parent compound of high-temperature cuprate superconductors.Conventional geometric frustration leads to a large ground state degeneracy in the system,where magnetic order is suppressed as the system cannot simultaneously satisfy the minimum energy requirements.Common geometric frustration models include edge-sharing triangular lattice and corner-sharing Kagome lattice.However,in actual materials,quantum spin liquid candidates with ge-ometric frustration are often very sensitive to lattice distortions and defects.In 2006,Alexei Kitaev proposed a quantum frustration model based on the Heisenberg uncer-tainty principle.In this model,spins with=1/2 are arranged on a two-dimensional honeycomb lattice without geometric frustration,forming bond-dependent anisotropic Ising-type exchange interactions.Due to the non-commutation of spin operators,the spin on the central lattice point experiences frustration,leading to the quantum frustra-tion effect.In 2009,Jackeli and Khaliullin suggested that strong spin-orbital coupling can provide the anisotropy required for Kitaev interactions.As a result,we explored the family of two-dimensional honeycomb lattice rare-earth chalcohalides compounds RECh X（RE=rare earth;Ch=O,S,Se,Te;X=F,Cl,Br,I）.Through the self-flux and solid-state reaction method,we successfully grew light pink transparent ErOCl single crystal and pink polycrystalline samples.The crystal cell parameters and atomic positions were refined from XRD data,SEM-EDS tests show that the atomic ratio is in good agreement with chemical formula Er:O:Cl=1:1:1.The material’s space group is R（?）m.The interlayer distance of Er³⁺ions（d₁=3.584（?））is smaller than the intralayer distance（d₂=3.768（?））.Er³⁺ions form a triangular lat-tice within the layers,and they combine interlayer to form a free distortion honeycomb lattice.The strong spin-orbit coupling of Er³⁺ions inherently provides the anisotropic exchange interactions required by the Kitaev model.Magnetic measurements of ErOCl single crystal down to 1.8 K show no magnetic phase transition occurs,with the sys-tem dominated by antiferromagnetic interactions and no hysteresis phenomena.The material exhibits good paramagnetism in the temperature range of 25 K-300 K.Zero-magnetic-field specific heat measurements on single-crystal ErOCl above 1.8 K reveal the presence of two smooth peaks at low temperatures.After numerical calculations,we believe that the magnetic specific heat peak around 13 K comes from the contribution of crystal field excitation energy leavels.At zero magnetic field,another magnetic heat capacity peak is observed below 1.8 K,which we believe is related to the low-energy effective spin-1/2 exchange interaction in the doubly degenerate ground state configura-tion.When an external magnetic field is applied,the magnetic heat capacity peak shifts towards higher temperatures.Upon increasing the external magnetic field to 3 T,the magnetic heat capacity peak moves to 2.9 K,indicating the occurrence of zeeman split-ting in the crystal field ground state energy levels.The two-level system of the ground state contributes to the Schottky anomaly observed in the lower termperature magnetic heat capacity peak.In order to study the ground state properties of the material,it is necessary to determine the magnitude of the first crystal field excitation energy level.The ground state spectral term of Er³⁺under spin-orbital coupling is ⁴I_15/2.In ErOCl crystal,the point group symmetry of the crystal field environment of Er³⁺is C_3v,and its ground state spectral term will split into eight doubly degenerate energy levels under the ac-tion of this crystal field environment and time-reversal symmetry.Starting from the effective magnetic hamiltonian of the ErOCl system and considering the mean field approximation of the exchange interaction between magnetic ions,we numerically fit and calculate the magnetic and heat capacity data at finite temperatures,obtaining six crystal field B_mⁿ parameters,eight doubly degenerate crystal field energy levels,and crystal field wave functions.At the same time,we also obtain the exchange parameters of the angular momentum operators after considering the spin-orbital coupling in the mean field approximation.In ErOCl material,the eight doubly degenerate crystal field energy levels of Er³⁺are divided into two parts.The first crystal field excitation en-ergy level is 31.2 K,the first three crystal field excitation energy levels are below 50 K,and the last four crystal field excitation energy levels are above room temperature.The exchange parameters of Er³⁺ions are J_±=0.007 K and J_zz=0.029 K.By performing temperature-dependent Raman measurements on ErOCl single crystals under zero mag-netic field,we identify all six phonon peaks and observe seven crystal field excitations from the ground state to the excited state due to the crystal field.After fitting the peak positions,it shows that they are in good agreement with the crystal field energy levels obtained from numerical calculations.