田中 秋広 氏（物質・材料研究機構 計算材料科学研究センター）
"Spin Berry phases in the vicinity of the pi-flux phase"
Senthil and company have recently stirred up a new wave in the physics of 2d antiferromagnets (AF), by coming up with the possibility of a new class of quantum critical phenomena which they coined the deconfined quantum critical point (DQCP).
Berry phases associated with monopole events play a central role in the DQCP theory. With an extension towards the doped AF as realized in the cuprates being an obvious prospect, a fermionic framework for this and related problems may be of some interest.
For some time we have been trying to formulate a 2+1D theory for extremely underdoped AFs by extension of a previous 1+1D work. It has occurred to us that at half-filling, this method can be applied quite naturally to deal specifically with the DQCP issue.
Here we will discuss the effect of the competition between Neel ordering and valence -bond -solid -like ordering on the Berry phases of the Affleck-Marston pi-flux phase. We find that the effective field theory describing the DQCP is a variant of the 2+1D SO(5) nonlinear sigma model with a Wess-Zumino term (i.e. Berry phase term).