Today and tomorrow I’m visiting National Cheng Kung University (成功大學) in Tainan. I was graciously invited to give a seminar by Prof. Ching-Hao Chang (thanks!). If anyone reading this wants to chat, I’ll be visiting until Tuesday afternoon. I’m staying in the fourth-floor visitor’s office.

**Title**: Accessing quantum criticality with magnetic field effects: metamagnetism and deconfined quantum criticality

**Abstract:** Simple models of interacting quantum spins (like the Heisenberg model) are remarkable tools for understanding strong quantum fluctuations, but relatively few studies have considered the effects of external magnetic fields on these systems. I investigate the influence of magnetic fields in the J-Q model, an antiferromagnetic Heisenberg model with an added 4-spin interaction (Q). This model is known to harbor a direct, continuous phase transition between the Nel state and a valence-bond solid. This transition is believed to be an example of deconfined quantum criticality, where the excitations are exotic fractionalized particles known as spinons (S=1/2 bosons). We study the thermodynamics of the excitations and find direct evidence that they are indeed fractional. Separately, we also find that the four-spin term changes the nature of the saturation transition from “zero-scale-factor” universality to metamagnetism (magnetization jumps).

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