Research Interests

Correlated Electron Systems: crystal growth and neutron scattering

In quantum mechanics, analytic solutions are generally only possible for two interacting particles. For three or more particles, one must either solve a problem numerically (using computers) or one must make approximations to simplify the problem. In condensed matter physics, this problem is especially accute, because the number of interacting particles can easily reach 10^23 or more. In the face of such large numbers of particles, we can only make progress by using approximations, and condensed matter physics is largely a search for which approximations we can use for a given system of interest.

One of the early approaches to studying the properties of materials at the level of quantum mechanics is known as band theory, which treats the electrons of a material as occupying states within bands formed by taking the orbitals of isolated atoms and spreading them out in energy due to the interaction between atoms in the solid. Band theory was very successful at describing a wide range of material properties, including the difference between metals and insulators, and it forms much of the basis for our understanding of semiconductors which make up the heart of computers and most of our electronic devices. The most important approximation used by band theory is that interactions between electrons are not strong and so can largely be brushed under the rug. This approximation works well for many materials, but not all. For materials known as highly correlated electron systems, electron-electron interactions are large enough that they cannot be brushed under the rug, and they can lead to interesting and important effects which cannot be explained by band theory, such as collosal magnetoresistance, high-temperature superconductivity, and Mott insulating behavior.

My research focuses mainly on the magnetic properties of highly correlated electron systems. I am currently studying how magnetic order and ferroelectric order can interact with each other in materials which are both magnetic and ferroelectric (so-called multiferroics). While ferroelectric materials and magnetic materials are both common in nature, very few materials have both types of order simultaneously. The presence of both types of order creates the possibility for large magnetoelectric susceptibilities, which could be used to change ferroelectric polarization with an applied magnetic field or magnetic polarization with an applied electric field. This could open the door for many new device applications, but the mechanisms of coupling between ferroelectric and magnetic order which create these magnetoelectric susceptibilities in multiferroics are not well understood.

My primary experimental tool is neutron scattering, which can be used to measure both the structure and the motion of atoms and electron spins in these materials. Neutron scattering is one of the most powerful experimental probes of magnetic interaction, and allows us to extract information about atomic-scale interactions within materials. Most measurements are done on the tripple-axis spectrometer (TRIAX) at the Missouri University Research Reactor (MURR). Neutrons interact weakly with most materials, so to ensure that enough neutrons scatter from our samples, we want to use large samples. In order to extract the maximum amount of information from our scattering experiments, we also want to use single-grain crystal samples so that we know the orientation of each scattered neutron relative to the crystal structure. In order to obtain single-grain crystals of sufficient size, we grow our own crystals in my lab using a technique known as Traveling-Solvent Floating-Zone growth. A specialized furnace uses high-powered lamps and focusing mirrors to melt the material, which is suspended (floated) by surface tension between a rod of feed material and a seed crystal. Because the melt is never in contact with a crucible, it is a very clean technique, and can be used to prepare large (5mm x 5cm) single-grain crystals of a wide variety of oxides and intermetallics.

Selectd Publications

“Quantum Impurities in the twodimensional spin one-half Heisenberg antiferromagnet,” O. P. Vajk, P. K. Mang, M. Greven, P. M. Gehring, J. W. Lynn, Science 295, 1691 (2002).

“Quantum versus geometric disorder in a two-dimensional Heisenberg antiferromagnet,” O. P. Vajk, M. Greven, Phys. Rev. Lett. 89, 7202 (2002).

“Magnetic Order and Spin Dynamics in Ferroelectric HoMnO3,” O. P. Vajk, M. Kenzelmann, J. W. Lynn, S. B. Kim, S.-W. Cheong, Phys. Rev. Lett. 94, 087601 (2005).

“Neutron scattering studies of magnetism in multiferroic HoMnO3,” O. P. Vajk, M. Kenzelmann, J. W. Lynn, S. B. Kim, S.-W. Cheong, J. App. Phys. 99, 08E301 (2006).