Tools for magnetostructural correlations for the 3d8(3A2 state) ions at orthorhombic sites: Comparative study with applications to Ni2+ ions in Y2BaNiO5 and Nd2BaNiO5

Quantum phase transitions driven by rhombic-type single-ion anisotropy in the S=1 Haldane chain

“…It was stimulated by the Haldane conjecture [1] that the lowest excitation in the antiferromagnetic Heisenberg model are gapped if and only if the spin S is an integer. Experimental evidences for the Haldane gap were discovered in several S=1 quasi-one dimensional (Q1D) materials, e.g., CsNiCl 3 [2,3], Y 2 BaNiO 5 [4][5][6], Ni(C 2 H 8 N 2 ) 2 NO 2 (ClO 4 ) (NENP) [7,8], and [Ni(C 2 H 8 N 2 ) 2 NO 2 ]BF 4 (NENB) [9]. Due to the crystal field and the spin-orbit coupling, the microscopic effective Hamiltonian for the Q1D spin chains involves the single-ion anisotropies,…”