Periodic Approach to the Electronic Structure and Magnetic Coupling in KCuF_3,K_2CuF_4,and Sr_2Cu0_2Cl_2 Low-Dimensional Magnetic Systems

摘要

The ab initio periodic unrestricted Hartree-Fock method and several generalized gradient algorithm (GGA) and hybrid density functional theory (DFT) approaches have been used to investigate the ground-state electronic and magnetic properties of three low-dimensional magnetic copper insulators,namely,KCuF_3,K_2CuF_4,and Sr_2CuO_2Cl_2.A Gaussian atomic basis set optimized for the crystal environment has been used to construct the crystalline orbitals in the LCAO approximation as implemented in the CRYSTAL code.The interaction between magnetic moments on Cu~92+) ions (d~9 local configuration) strongly depends on the crystal structure and hence on the resulting electronic structure:Sr_2CuO_2Cl_2 is representative of 2-D antiferromagnetic systems,closely related to the high-T_c parent cuprate superconductorsK_2CuF_4 is a 2-D ferromagnetic system and KCuF_3 behaves as a quasi-1-D antiferromagnetic system.The most stable electronic state,the relevant magnetic coupling constants,and the magnetic form factors are calculated by means of the various Hamiltonians.The systems turn out to be large band-gap insulators in the UHF and hybrid DFT approximations,whereas they become metallic or narrow band-gap semiconductors in the local density approximation or GGA approximations.UHF gives a qualitatively correct description of this kind of magnetic insulators and,more important,good relative values for the relevant magnetic coupling constants.Hybrid functionals (like B3LYP) largely improve,with respect to the other Hamiltonians,the magnetude and nature of the band-gap,the spin densities on the Cu~(2+) ions,and the most important magnetic coupling constants,providing a reasonable,semiquantitative description of this kind of strongly correlated materials.In addition,those functionals correctly account for the distoritions due to the Jahn-Teller effect in KCuF_3,in contrast to LDA and PW-GGA,where no stabilization appears when the distortion is included.