Sudha Srinivas

Behavior of positive muons in highT c superconductors La2?x Sr x CuO4

An unambiguous determination of the μ+ location in the highTc LaSrCuO superconductor is made through zero-field spin relaxation in the paramagnetic phase and its crystal axis dependence by using single crystalline samples. The temperature dependence of the relaxation rates has a maximum between 80 and 150 K, indicating the existence of the onset of quantum diffusion at low temperature.

Interaction of paramagnetic electron with highTc supercurrent in LaSrCuO studied by (μ−O) probe

The paramagnetic state of the muonic atom formed by the negative muon (μ−) bound to the oxygen in highTc LaSrCuO which was found in our recentμ− spin rotation experiments was applied to probe an interaction between the paramagnetic electron and the highTc supercurrent. The observed enhanced spin relaxation rates of the (μ−O) system in the superconducting state revealed the presence of such an interaction.

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.

Local hyperfine field vector of positive muon in mono-domain single crystal of antiferromagnetic La2CuO4

By using a monodomain single crystal La2CuO4 grown by the travelling solvent floating zone method, the strength and direction of the local hyperfine field vector at positive muon sites were determined. Combining with the μ+ location determined separately, we found the importance of the local hyperfine field contributions with dipolar symmetry as well as the distant dipolar field from surrounding point dipoles of Cu atomic moments.

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 hyperfine interactions associated with the negative muon in lanthanum copper oxide high Tc system

We have carried out first‐principles Unrestricted Hartree–Fock Cluster investigations to demonstrate that the Auger holes produced by the entry of μ- into the oxygen ions in La_2CuO_4 lead to stable O-μ- centers at apical and planar sites. The hyperfine fields associated with these two sites suggest that the apical one is responsible for the stronger and more anisotropic Knight shift in μSR measurements and the planar for the weaker and less anisotropic shift, the needed susceptibility arising from the localized magnetic moments on the O-μ- centers. This identification of the susceptibility provides support for the model proposed for the observed increase in μSR relaxation rate below the superconducting temperatures.

Probing interaction of paramagnetic electron with conduction electrons in high Tc superconductor LaSrCuO by (μ-O) spin relaxation

The paramagnetic (μ-O) state formed at the oxygen site in high Tc LaSrCuO was used to probe an interaction between the localized moment of the paramagnetic electron and the conduction electrons via zero field μ- spin relaxation. The enhanced relaxation rates consistently observed in the superconducting state of various Sr concentrations are explained as an effect of spin‐pairing in the high Tc supercurrent.

Theory of Nuclear Quadrupole Interactions in Solid Fluoromethanes with Implanted 19F Nuclei. Coupling of HF* and Host Molecule*

Abstract The Quadrupole Coupling Constant e2qQ and Asymmetry Parameter η of fluorine for fluorine-substitute methane compounds are calculated using the Hartree-Fock Roothaan procedure. Our results are compared with experimental data from measurements by the Time Differential Perturbed Angular Distribution (TDPAD) technique using excited 19F* (spin 5/2) nuclei. The theoretical e2qQ’s for the 19F* nuclei in the fluoromethanes are in good agreement with experimental results. For CH2F2 and CHClF2 molecules, where finite η are expected from symmetry considerations, our results for η are small, 0.12 and 0.05 respectively, in agreement with experimental observation. Besides the 19F* coupling constants associated with the C-F bonds in fluoromethanes, additional interesting NQI parameters, close to those in solid hydrogen fluoride, are observed in the TDPAD measurements. It is demonstrated through investigations of the total energies and electric field gradients that these additional NQI parameters for the fluoromethanes can be explained by a HF* molecule hydrogen-bonded through the hydrogen to a fluorine atom in the host molecular systems. This complexing of an ionic molecule to the host molecules in organic solids containing strongly electronegative atoms is expected to be a general feature in both implantation and conventional techniques.

First principles investigation of19F* nuclear quadrupole interaction in fourth group tetrafluorides

Using the Hartree-Fock-Roothaan procedure the nuclear quadrupole interactions of the19F (spin 5/2) nucleus in CF4, SiF4 and GeF4 are studied. The theoretical results explain within 10% the observed experimental measurements by the time-differential perturbed angular distribution technique, including the important feature of sharp decrease of the19F* nuclear quadrupole coupling constant in going from CF4 to SiF4, followed by an increase in going to GeF4, while a simple consideration of the ionic characters of the C-F, Si-F and Ge-F bonds together with Townes and Dailey theory would lead to a continuous decrease from CF4 to GeF4. The dependence of the results on the choice of basis sets and the role of many-body effects is discussed.