Condensed-Matter Physics & Materials Science Seminar

New Condensates of Matter and Light:Macroscopic phase
coherence is one of the most remarkable manifestations of quantum mechanics, yet it seems to be the inevitable ground state of interacting many-body systems. In the last two decades, the familiar examples of superfluid He & conventional superconductors have been joined by exotic & high temperature superconductors, ultra-cold atomic gases, both bosonic & fermionic, & recently systems of excitons, magnons, & exciton-photon superpositions called polaritons, the subject of this talk.
An exciton is the solid-state analogue of positronium, made up of an electron & a hole in a semiconductor, bound together by the Coulomb interaction. The idea that a dense system of electrons & holes would be unstable toward an excitonic (electrical) insulator is one of the key ideas underlying metal-insulator transition physics. The further possibility that an exciton fluid would be a Bose-Einstein condensate was raised over 40 years ago, & has been the subject of an extensive experimental search in a variety of condensed matter systems. Such a condensate would naturally exhibit phase coherence.
Lately, some novel experiments with planar optical microcavities make use of the mixing of excitons with photons to create a composite boson called a polaritons that has a very light mass, and is thus a good candidate for a high-temperature Bose condensate. Good evidence for spontaneous coherence has now been obtained1, though there are special issues to resolve(2) considering the effects of low dimensionality, disorder, strong interactions, & especially strong decoherence associated with decay of the condensate into environmental photons(3), since the condensate is a special kind of laser.
1. J. Kasprzak, et al.Nature, 443, 409-415 (2006).
2. J. Keeling, F. M. Marchetti, M. H. Szymanska, P. B. Littlewood, Semiconductor Sci. & Technol. 22, R1-26 (2007).
3. M. H. Szymanska, J. Keeling, P. B. Littlewood, Phys.Rev.
B 75, 195331 (2007)