ECE Calendar

Abstract:

Harnessing collective motion of electrons is a promising approach to creating ultra-low power logic devices. In a switch that is based on a set of independently-moving electrons, switching requires a change in gate voltage large enough to shift each electron's energy by more than room temperature thermal energy. With collective motion of electrons, one may hope to achieve switching with smaller changes in gate voltage: the energy of an entire ensemble of electrons can be shifted by more than room temperature thermal energy, turning on transport, even though each electron's energy is shifted by only a small fraction of that amount. One of the most spectacular demonstrations of collective behavior in the last 10 years is the giant enhancement in tunnel current seen in semiconductor bilayer devices in the quantum Hall regime [1]. The enhanced tunnel current between the two layers occurs when electrons in the top layer bind with vacancies in the bottom layer to form "indirect excitons," which in turn organize into a Bose-Einstein condensate. Thisi type of behavior may be viewed as spontaneous coherence, or "pseudospin ferromagnetism": selection of a particular superposition of states in the two layers (a particular direction of "pseudospin," where the layer degree of freedom is treated as if it were a spin) for the entire system. In order for this behavior to be useful in the context of a device, pseudospin ferromagnetism must survive at room temperature, rather than being restricted to cryogenic temperatures as in the GaAs heterostructures in which this effect was discovered. Room temperature operation is possible if the carrier densities in the two layers are equal but with opposite polarity: electrons in one layer, holes in the other. This condition is very difficult to engineer in a semiconductor system, but could be feasible in graphene due to its unique linear bandstructure [2,3].

In this talk, I will discuss some of the recent theoretical work on the transport properties of indirect excitons at room temperature in electron-hole bilayer graphene [4]. In particular, I will emphasize the possibility of using this strongly correlated state to make very efficient ultra-low power logic devices.

[1] I.B. Spielman et al., Physical Review Letters 84, 5808 (2000).
[2] Hongki Min et al., arXiv:0802.3462 (2008).
[3] M.J. Gilbert et al., Journal of Computational Electronics (2009).
[4] M.J. Gilbert, Physical Review Letters, in preparation.