Narayan Sahoo

First principles cluster investigation of electronic structure and hyperfine interaction for nitrogen in diamond

The self-consistent field Unrestricted Hartree-Fock cluster procedure has been used to study the location, electronic structure and hyperfine properties of nitrogen impurity in diamond. From the analysis of the potential energy curve for nitrogen along the <111> axis, it was found that nitrogen is located at a position 0.3A away from the substitutional site towards the plane formed by the three nearest neighbour carbon atoms. The calculated values of the magnetic hyperfine constants and nuclear quadrupole coupling constants for14N agree very well with values obtained from electron paramagnetic resonance and electronnuclear double resonance measurements.

High-Pressure phase transition in67ZnO

Using a high-pressure Mössbauer spectrometer for the 93.3-keV resonance in67Zn and an X-ray diffractometer of the Guinier type67ZnO powder was investigated at pressures up to ≈19% at the phase transition. Surprisingly, the center shift (CS) of the cubic phase is by ≈20μm/s more negative than CS of the wurtzite structure. The Mössbauer data as well as theoretical cluster calculations show that the volume collapse is mainly due to the increase in coordination number of nearest-neighbour (nn) atoms. Covalency of the Zn−O bond, however, is reduced due to a≈7% increase of nn distance.

Hartree-Fock cluster investigation of location and hyperfine properties of normal muonium in silicon-trend with respect to diamond

Using the first-principles Hartree-Fock Cluster procedure employed earlier for normal muonium (Mu) in diamond, the total energy and hyperfine field at the muon site in silicon have been studied as a function of muon position along the <111> direction. The muon was found to be localized in the tetrahedral interstitial region, although the potential was significantly shallower as compared to diamond. The vibrationally averaged hyperfine constant for the muon shows a correct trend compared to diamond but is somewhat larger than experiment, possible reasons for which will be discussed. Results for the superhyperfine constants in silicon will be presented and compared with those for diamond.

Location and associated hyperfine properties of μ+ in La2CuO4

The location of the positive muon used as a probe in highTc systems is investigated using the unrestricted Hartree-Fock cluster procedure. Our calculations indicate that μ+ is located in thea–c plane at a distance of 1.08 Å from the apical oxygen at a μ+-O(a)-Cu angle of 25°. The hyperfine field at this site is also calculated. Our results show the importance of including the local contact and dipolar contributions to the hyperfine field which arise from the unpaired electron spin distribution in the vicinity of the muon.

Theory of location and associated hyperfine properties of the positive muon in La2CuO4

The current understanding regarding the location of the positive muon, which is a valuable tool for probing the magnetic properties of copper oxide superconducting systems, will be reviewed for La2CuO4. The results of our present investigations by the Unrestricted Hartree-Fock Cluster procedure, which leads to a location for the muon of about 1.08 Å away from an apical oxygen and with the O-μ+ direction making an angle of 25° with the O-Cu direction, will be discussed, including the magnitude and direction of the hyperfine field obtained from the calculated wave functions. These latter results, which are in reasonable agreement with earlier muon spin rotation data in powdered samples and recent data in single crystals, show the importance of including the local contact and dipolar contributions to the hyperfine field associated with the unpaired electron spin distribution in the neighborhood of the muon. Possible additional factors besides those included here, that will involve substantial computational efforts but could lead to a bridging of the remaining quantitative differences with experimental hyperfine data, will be discussed.

Theory of electron distributions and63Cu and17O nuclear quadrupole interactions in YBa2Cu3O7 and YBa2Cu3O6

Ab initio unrestricted Hartree-Fock Cluster investigations have been carried out on the electronic structures of the YBa2Cu3O7 and YBa2Cu3O6 systems. The results of these investigations provide satisfactory explanations of available63Cu and17O nuclear quadrupole interaction data. The electron distributions obtained rule out the presence of Cu3+ ions and are supportive of the presence of Cu2+, Cu1+, O1− ions in the O7 system and Cu2+, Cu1+ and O2− in the O6 system with actual charges departing significantly from the formal charges, especially in the O7 system, indicating the importance of covalency effects. Suggestions will be made regarding possible sources that can bridge the remaining gap between theoretical and experimental results for the nuclear quadrupole interaction parameters.

Self-consistent field cluster study of hyperfine properties of normal and anomalous muonium in elemental semiconductors

Using the Unrestricted Hartree Fock (UHF) Cluster Procedure, it is shown that for the normal muonium (Mu) center, the tetrahedral site is the most favorable in the two systems diamond and silicon investigated, while for the anomalous muonium (Mu*) center, a site displaced in the <111> direction with respect to a vacancy in a double-positively charged environment is the appropriate one for all three elemental semiconductors. Using our calculated electronic wave-functions, one is able to explain all features of the observed hyperfine properties of both centers and, in a number of cases, obtain good quantitative agreement with experiment.

Vacancy associated model for anomalous muonium in diamond, silicon and germanium

Through first-principles investigations on a number of models for anomalous muonium in diamond using the Unrestricted Hartree-Fock Cluster procedure, it is demonstrated that a muonium trapped near a double-positively charged vacancy is the most viable model for this center. This model is shown to successfully explain all the observed features of the hyperfine tensors A⇉ in diamond, silicon and germanium, namely, oblateness, opposite signs of A│ and A┼ in diamond and same signs for silicon and germanium, the trend in the strengths of the hyperfine tensors from diamond to germanium and the negative sign for A┼ in diamond.

First principles theory of hyperfine interactions in spinels: ZnAl2O4

The nuclear quadrupole interactions of27Al and67Zn, both at the B-site in the spinel ZnAl2O4 have been studied using the Hartree-Fock cluster procedure including the influence of the ions outside the cluster. The theoretical value −3.85 MHz ofe2qQ for27Al is in very good a agreement with the experimental value of 3.68 MHz (sign undetermined). For67Zn at the Al site the theoretical value is −8.26 MHz in reasonable agreement with the experimental value of −11.34 MHz indicating that lattice distortion effects associated with Zn as an impurity at the Al site are relatively small.

Nuclear quadrupole interactions associated with adsorbed halogen and chalcogen atoms at semiconductor surfaces

As part of a program for studying hyperfine interactions at surfaces of condensed matter systems, we have studied the nuclear quadrupole interactions associated with group VI and VII atoms adsorbed on silicon surface. The electronic structures used for these investigations were determined through the self-consistent Hartree-Fock cluster procedure using sizeable clusters of atoms to simulate the infinite systems. Results will be presented for the nuclei of halogen atoms adsorbed on the silicon (111) surface and for sulfur and selenium atoms on the (110) surface.