My primary research interest is materials physics, in particular, the physics of ceramic materials. Ceramics belong to a huge class of materials including a multitude of compounds, many of which have technologically important applications such as thermal barrier coating, oxygen sensors, ferroelectric memory chips, and high transition temperature superconductor devices.

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Technique

I use time-differential perturbed angular correlation spectroscopy (PAC) to study structure and defects in ceramic materials on a microscopic level. PAC is a hyperfine interaction method and uses a nuclear probe to measures its interaction with electric and magnetic fields due to surrounding charges and spins. This technique allows the characterization of crystalline order, the study of local defect structures, as well as the investigation of structural and dynamic processes, such as phase transitions, the nature of defects, vacancy migration, etc. Details on PAC can be found here.

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Ongoing Work

Presently, ongoing work includes the following:

The electric field gradient in crystalline zircon (ZrSiO₄)

Radiation damage in naturally occurring zircons

Rare earth titanate and zirconate pyrochlores

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PAC Spectroscopy

Time-Differential Perturbed Angular Correlation Spectroscopy (TDPAC, or short PAC) is a nuclear probe technique that is sensititve to the interaction of radioactive probe nuclei with electric and magnetic fields due to surrounding charges and spins. The probe nucleus decays from an excited state to the ground state via subsequent emission of two $\gamma$-photons, $\gamma$₁ and $\gamma$₂. Since $\gamma$-photons are emitted preferentially along the nuclear spin direction, detection of $\gamma$₁ by a fixed detector selects an ensemble of nuclei with spins oriented along the detector axis. Subsequent emission of $\gamma$₂ from this set of oriented nuclei is now anisotropic and is referred to as the (unperturbed) angular correlation of $\gamma$₂ with respect to the direction of $\gamma$₁. This angular correlation depends on the nuclear spins of the involved states of the probe nucleus and the multipolarity of the emitted $\gamma$-photons, as well as the angle between the direction of emission of $\gamma$₁ and $\gamma$₂.

If a hyperfine field is present, the degeneracy of the nuclear sub-states (m-states) is lifted, and the initial population of the intermediate sub-states changes between the emission of $\gamma$₁ and $\gamma$₂. Semi-classically this interaction can be visualized as precession of the nuclear spin in the extra-nuclear hyperfine field. This spin precession perturbs the angular correlation and results in an oscillation of the delayed coincidence rate of $\gamma$₂ with respect to $\gamma$₁. The precession frequency is characteristic for the type of interaction and its strength.

[to be continued]

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PAC poster of the Konstanz group (click image for larger view)

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• Universität Konstanz, Germany (in German)
• Pac Poster, Universität Konstanz, Germany
• Universität des Saarlandes, Germany
• Universität Bonn, Germany (in German)
• Universität Leipzig, Germany
• Katholieke Universiteit Leuven, Belgium
• Royal Vet. and Ag. University (KVL)  (pdf-file)
• Washington State University
• ISAC facility at TRIUMF (Accelerator-produced PAC isotopes)

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