| Frustrated quantum magnetism has emerged as one of the frontier research fields in strongly correlated electron systems.The triangular and kagomélattice Heisenberg antifer-romagnets with strong geometrical frustration are typical two-dimensional frustration sys-tems,which provide an ideal platform for the study of frustrated magnetism.Strong geo-metrical frustration and quantum fluctuation will give rise to the competition of different ordered phases,and the density of states around the ground state is quite high,which leads to quantum spin states beyond the classical Landau theory,e.g.quantum spin liquid.Quan-tum spin liquid is generally considered to emerge in small spin-frustrated system with strong spin quantum fluctuations(e.g.spin-1/2),and system with larger spin(e.g.spin-5/2)will ap-proach the classical limit,so can a frustrated system with a medium-sized spin(e.g.spin-3/2)realize a quantum spin liquid?On the other hand,the spin liquid nature of the ground state of the spin-1/2 kagomélattice Heisenberg antiferromagnet is still under hot debate.This thesis uses the susceptibility,specific heat,electron spin resonance,and neutron scattering to study the quantum magnetism of two typical two-dimensional geometrically frustrated Heisenberg antiferromagnets.The main research contents of this thesis are summarized as follows:1.Based on high quality sample,the frustrated magnetism of the delafossite oxideα-CrOOH has been comprehensively investigated,which experimentally realizes the S=3/2 nearly-Heisenberg antiferromagnetic model on a triangular lattice with weak single-ion anisotropy(D).The electron spin resonance,neutron diffraction,and specific heat measure-ments onα-CrOOH indicate that the long-range 120°Néel order is significantly suppressed and the system is in the vicinity of a spin-liquid phase.By fitting the high-temperature mag-netic susceptibility and the electron spin resonance linewidth,it is found that|D|/J1～5%.Additionally,the perturbation term in this system is proved to be an easy-axis single-ion anisotropy(D<0)by the low-energy spin excitation of the inelastic neutron spectrum.This easy-axis anisotropy competes with the 120°Néel order and thus enhances the quantum spin fluctuations at low temperatures.In addition,the next-nearest neighbor interaction may also give rise to the quantum fluctuation of the system.This work should shed new light on the search for quantum spin liquids in the frustrated systems with S=3/2.2.Based on high-quality single-crystal without evident orphan spins((?)0.8%),the quantum magnetism of the spin-1/2 kagoméHeisenberg antiferromagnet YCu3(OH)6.5Br2.5has been comprehensively investigated.The results of first-principles calculations demon-strate that the site mixing between polar OH~-and nonpolar Br~-gives rise to 70(2)%of randomly distributed hexagons of alternate bonds(J1-△J and J1+△J)and the rest of the almost-uniform hexagons(J1)on the kagomélattice.Simulations of the random exchange model with△J/J1=0.7 show good agreement with experimental observations,including The weak upturn in the low-T susceptibility and the slight polarization in magnetization.No conventional freezing is observed down to 0.36 K,and the raw specific heat exhibits a nearly quadratic temperature dependence below 1 K,which is phenomenologically con-sistent with a gapless Dirac quantum spin liquid(spin gap≤0.025J1).Experimental result sheds light on the theoretical understanding of the randomness-relevant gapless quantum spin liquid behavior in YCu3(OH)6.5Br2.5,as well as in other relevant materials. |
