Magnetism without Magnetic Ions

Lian Li, University of Wisconsin-Milwaukee, DMR 0706359

We conduct a systematic study of the doping of wide bandgap semiconductors (e.g. MnGeN2 and GaN) with non-magnetic impurities such as Cu by molecular beam epitaxy. First-principles calculations show that the incorporation of Cu in GaN leads to a net moment of 1 μB/Cu, and p-like doping. However, the Cu is strongly attracted to N split-interstitial defects, which suppress the moment. These results indicate that the interaction between intrinsic defects and dopants can significantly alter the electronic and magnetic properties of magnetic semiconductors.

Calculated isosurfaces of the electron density at the Fermi level for CuGa dopant in GaN (left), and for a CuGa next to a N split-interstitial (right, yellow atoms are the N atoms of the split interstitial), where the Cu is effectively 5-fold coordinated. For the simple substitution, this density is spin-polarized, while for the split-interstitial, the system is non-magnetic. (Cu: green, Ga: blue, N: red.)

RET: Color Mapping of Atomic Layers

Lian Li, University of Wisconsin-Milwaukee, DMR 0706359

Since 2005, the PI has established a close relationship with Hamilton High School in Sussex, WI. The school’s Physics teacher, Tim Moeller (shown in the picture holding a giant soap bubble) spent eight weeks in the PI’s lab, learning interference effects for visible light in thin films such as soap bubbles and oil slicks.

Research Experience for Teachers (RET)

The PI (Li) examines a bubble made by Tim.

He will then bring that to life in his high school classroom by presenting “Color Mapping of Atomic Layers”. The students will first make the thinnest sample of graphene possible by the Mechanical Exfoliation Method (simple, repetitive, mechanical “peeling” off layers of Graphite). They will then use a reflection / interference “Color Map/Chart” to identify, under light microscope (by reflected color), the precise thickness of the graphene sample they produced. A single layer of graphene will be violet in color when placed on a 300 nm SiO2/Si wafer.