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Fractionalization in a strongly correlated many-body system;
Emergent gauge theory in exciton bose condensate

Sung-Sik Lee **MIT**
__Abstract:__
Fractionalization is a phenomenon where a particle is effectively
splintered into multiple particles without actually being broken
apart. In this talk, I will explain how this seemingly paradoxical
phenomenon can happen in a strongly interacting many-body system,
that is, an exciton (electron-hole composite in solid) system.
Once exciton is fractionalized, emergent particles can have completely
different physical properties such as statistics and charge from
the original exciton. At the price of being `liberated' from
exciton, the fractionalized particles are inevitably coupled
to a new force mediator. Interestingly, the dynamics of the emergent
force mediator is governed by gauge theory which has been the
guiding principle to understand interactions of elementary particles.

Fractionalization is a fascinating phenomenon because it not only
constitutes a new state of matter of many-body systems but also sheds
fresh insights on fundamental questions in physics, such as, what
determines statistics of particle? and what is the origin of gauge
principle? I will first review our understanding on statistics
of identical particles and gauge principle based on first quantization
path integral approach. Then, I will introduce an exciton system and
its experimental realization. After generalizing the exciton system
into a multi-band system, I will show how the mundane collection of
non-relativistic neutral particles can end up being described by a
relativistic gauge theory where either fractionalized boson or fermion
arises as a long distance degree of freedom along with an emergent
gauge field.