Theoretical Study of Neutron Scattering from Mott Insulators
Abstract
Even though many theoretical works have been done in rm La_2CuO_4 and rm YBa_2Cu_3O_6, the insulating parents of the highT_{c } superconductors, there is no satisfactory theory to describe the lowtemperature neutron scattering experiments in these materials. This dissertation addresses our theoretical study of the neutron scattering of the copper compounds, rm La_2CuO_4, YBa_2Cu_3O_6 and rm Sr_2CUO_2Cl_2, and the related nickel compounds, rm La_2NiO _4, KNiF_3, and NiO. We develop a theory of the spin density which incorporates quantum spin fluctuations (QSF) and covalence effects (COV) simultaneously. We found the spin density, to a good approximations, to be a product of a site spin expectation value which incorporates QSF and a form factor which takes COV into account. The ordered moment is also defined from the spin density, and is found to be affected by both the QSF and COV. We have calculated the magnetic form factor with various theoretical tools and the calculated form factors are compared with experiments. As a simple procedure to obtain the form factor for rm La_2NiO _4, we follow the model which was developed by Hubbard and Marshall. The Hubbard and Marshall procedure failed to explain the observed large covalence in rm La_2NiO_4. We have carried out ab initio cluster calculations of the form factor in a solid using a cluster approximation based on quantum chemistry technique. We have used restricted HartreeFock (RHF) and unrestricted HartreeFock (UHF) procedures with correlation corrections via a restricted multiconfiguration selfconsistentfield (MCSCF) approach to obtain the ground state cluster wave functions needed in the neutron form factor calculations. We find that the ab initio cluster calculations describe the experimental form factor in KNiF_3 and NiO extremely well, but fail badly in rm La_2NiO_4. We applied the same method to the cuprate materials, rm La_2CuO_4, YBa_2Cu_3O _6 and rm Sr_2CuO_2Cl _2. The calculated form factors for these cuprates turn out to agree reasonably well with experiments, although there are other indications that the degree of covalence is underestimated in our calculations.
 Publication:

Ph.D. Thesis
 Pub Date:
 1995
 Bibcode:
 1995PhDT.......226C
 Keywords:

 SUPERCONDUCTORS;
 Physics: Condensed Matter