First Principles Investigation of Electronic Structures and Hyperfine Properties of Semiconductors and High Critical Transition Temperature Superconductors

We have applied the first principles Unrestricted Hartree-Fock Cluster procedure to investigate the electronic structures and associated hyperfine properties of several categories for solid systems. The first category is concerned with the location and nuclear quadrupole interactions (NQI) of fluorine impurity centers in crystalline silicon (c -Si). The Time Differential Perturbed Angular Distribution experiments show that when excited nuclear static fluorine (^{19}F*) is implanted into c-Si, two ^{19}F* centers are formed characterized by two unique axially symmetric electric field gradients (efg). We have examined three models to determine the stable ^{19 }F* sites in the bulk c-Si namely, Intra-Bond (IB), Anti-Bond (AB) and Substitutional models. Our investigation shows that the IB and AB models can explain very well the origin of the two observed NQI data of ^ {19}F* in c-Si. The two models, IB and AB, are also able to explain the experimental ^{19}F* NQI data in crystalline germanium where two centers with axially symmetric efg are observed. The experimental trends of ^ {19}F* NQI in going from c-Si to c-Ge are well reproduced by our investigation using the IB and AB models. The second category of the systems we have investigated deals with the NQI and magnetic hyperfine interaction of ^{63}Cu in rm La_2CuO_4 and rm YBa _2Cu_3O_6 as well as the NQI of ^{139}La and ^{135}Ba in the former and the latter compounds respectively. Satisfactory agreements are found between our calculated ^{63 }Cu nuclear quadrupole coupling constants (e ^2qQ) and the observed ones except for the planar copper in rm YBa_2Cu _3O_6. Our calculated hyperfine fields at ^{63}Cu in both systems are also in satisfactory agreement with experiment. On the other hand, very good agreements are found between UHF theory and experiment for the NQI at ^ {139}La and ^{135 }Ba. We consider these to be a signature of significant electron correlation effects associated with copper and its neighboring oxygen ions. In the third category, we investigate the possible stable sites of the Muon Spin Rotation (mu SR) probe atom, positive muon (mu ^+) in rm La_2CuO_4 , and the hyperfine field (H_{ rm hyp}) at mu^+ site in the antiferromagnetic phase of the system. From the total energy analysis for mu^+ at several positions, we found that the most stable mu^+ site is at (0.121a, 0.0, 0.110c) of the tetragonal rm La_2CuO _4 unit cell. The value of H_ {rm hyp} at this site is in reasonable order of magnitude with the observed one, and the direction of the field is found to agree qualitatively with preliminary results of a muSR measurement. Possible reasons for the differences between theory and experiment as well as directions for future investigations are proposed.