Theoretical Study Of Localized Magnetism And Phase Transition In Large-spin Systems

In recent years,the magnetic properties of large-spin quantum multi-body systems have attracted considerable attention.The large-spin in solid materials is usually caused by the Hund's rule coupling of unpaired electrons in atoms.Another system that can achieve and simulate the large-spin is ultracold atoms in an optical lattice.Some strong correlation effects of itinerant ferromagnetism,Mott transition and others in solid materials have received renewed interest and have been intensely studied in large-spin ultracold atomic systems.Motivated by these considerations,in this thesis,Anderson magnetic impurity and Heisenberg local magnetism are re-examined and explored in large-spin systems.Frist,we extend the Anderson impurity model to a large-spin Fermi system with spin-3/2.The condition required for the spontaneous formation of local magnetic moments is examined and the ground-state mean-field magnetic phase diagram is explored carefully.We find that phases of the spin-3/2 Anderson impurity system are considerably richer than those of the spin-1/2 system.There are three magnetic phases ?,?,and ? that correspond to one,two,and three particle/hole occupation,respectively.The regions ?,?,and ? for holes are interpreted as the regions ?,?,and ? for particles,signaling the symmetry of the Anderson impurity model.Additionally,we observe that all the three magnetic phases have fourfold-degenerate ground states,and that the phase transition between the three phases seems to be of the first order.Second,ground-state magnetism and thermodynamics of three-dimensional isotropic frustrated antiferromagnets with a large-spin are studied based on the Heisenberg model.The dependence of magnetization,phase transition temperature,internal energy,and free energy on spin quantum number and frustration strength are calculated and discussed carefully.The results show that quantum spin fluctuation is depressed by spin quantum number,but is strengthened by frustration strength.At a finite temperature,both thermodynamic fluctuation and quantum spin fluctuation coexist.The former is believed to be isotropic for different frustration,while the latter seems to be anisotropic,and thermodynamic fluctuation dominates for a large-spin quantum number.Finally,the effects of spin anisotropy and single-ion anisotropy on the magnetic properties of three-dimensional frustrated antiferromagnets with spin-1 are calculated and discussed.The results indicate that both spin anisotropy and single-ion anisotropy suppress quantum spin fluctuations,but their effects on ground-state energy are completely opposite.When these two types of anisotropy coexist,the effect of spin anisotropy on magnetization and internal energy is larger than that of single-ion anisotropy.