Infrared Hall effect in conventional and unconventional materials John Cerne, SUNY at Buffalo, DMR 1006078.
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In metals, magnetic fields (H) deflect moving charges to produce an electric field (EH) perpendicular to the flow of the charges. This phenomenon is known as the Hall effect and is critical to characterizing materials as well as fundamental science. Although this effect was discovered in 1879, two Nobel prizes in the last few decades have been awarded for the integer and fractional quantum Hall effect (QHE) in semiconductors. Unlike the dc Hall effect where charges move only in one direction at a constant speed, in our labs we look at how oscillating charges are deflected by magnetic fields. The frequency dependence of the Hall effect provides new insights into conventional materials, such as semiconductors (Fig. b) as well as unconventional materials such as graphene (Fig. a) and topological insulators.
In semiconductors, the dc (zero frequency) Hall effect produces quantized steps in EH as a function of carrier density. Although theoretical predictions suggest that the QHE may persist at higher frequencies, it is surprising that one can still observe this behavior when charges oscillate 1012 times per second (1 THz). By looking at the polarization of transmitted light (Faraday rotation, see Fig. b), which is sensitive to EH we have clearly observed QHE steps at up to 2.5 THz, which is the highest frequency at which the QHE has been observed thus far. We also have studied topological insulators using these techniques and have observed dramatic polarization changes. In graphene, we see that magnetic fields can produce rich structure in the polarization of reflected light at even higher frequencies (29 THz) as can be seen in Fig. a. To the best of our knowledge, we are the only group that has made polarization sensitive measurements on graphene in this frequency range. The data show that the sample consists of single and multi-layer graphene, which can be accurately modeled by theory. Figures a) and b) are from two recent presentations by our groups at the Electronic Properties of Two-Dimensional Systems in July 2011.
On July 12 and 13, 2011, seven students from a Buffalo public middle school who are participating in UB’s Center for Excellence in Education through Technology and Arts spent their afternoons with the PI and co-PI, Prof. Andrea Markelz. They were led through interactive displays and demos in the Department, and participated in several demonstrations on liquid nitrogen and high temperature superconductors.
The final set of demonstrations used plans and resources from the PI’s NSF-funded work on starting a radio controlled flying club in a Buffalo public high school. The students built and tested their own gliders, constructed from 9” foam plates (www.modelaircraft.org/education/fpg-9.aspx). The students were very excited about building and flying their gliders. One student was jubilant when her glider set the flight distance record for the group as it gently banked into a 20 m long flight from the back of the sloped lecture hall to the chalkboard at the front.
After a radio control flight demonstration, each student flew one of the club’s radio controlled airplanes in the lecture hall. The flying not only gave us the opportunity to discuss basic physics concepts such as forces and acceleration, but also highlighted the great advances in technology that have revolutionized radio controlled flight over the past few years.
The students’ enthusiasm resulted in the tours running well past the allotted time on both days and many of the students were very disappointed that the tours would not continue for more days.
PI showing students how the transmitter controls the model airplane (circled in red). The airplane’s pitch, yaw and thrust (3 channels) are controlled by the pilot. The balloons and dewar behind the students were used in liquid nitrogen demonstrations.
A student flying a 3-channel, 14 g airplane (Parkzone Night Vapor) inside the lecture hall. The student is using a second transmitter so that control of the airplane can pass between the instructor and student with a flip of a switch on the instructor’s transmitter. This allows the instructor to take control when necessary.