Quantum Critical Spinon Deconfinement

TL;DR

This paper investigates the nature of deconfined quantum critical points in 2+1 D antiferromagnets, showing that instanton events vanish at criticality, leading to free spinons with unique experimental signatures.

Contribution

It demonstrates that instanton tunneling events disappear at the quantum critical point, resulting in deconfined, free spinons with an anomalous susceptibility exponent of one.

Findings

Instanton events vanish at the critical point.

Deconfined spinons are strictly free particles.

Susceptibility exponent eta equals one.

Abstract

The Neel magnetization of 2+1 D antiferromagnets is composed of quark-like spin 1/2 constituents, the spinons, as follows from the CP^1 mapping. These quark spinons are confined in both the Neel ordered phase and quantum paramagnetic phases. The confinement in the quantum paramagnetic phase is understood as arising from quantum tunneling events, instantons or hedgehog monopole events. In the present article, we study the approach to the quantum critical point, where the quantum paramagnetic phase ceases to exist. We find that irrespective of the intrinsic spin of the antiferromagnet, instanton events disappear at the deconfined critical point because instanton tunelling becomes infinitely costly and have zero probability at the quantum critical point. Berry phase terms relevant to the paramagnetic phase vanish at the quantum critical point, but make the confinement length scale diverge more strongly for half-integer spins, next strongest for odd integer spins, and weakest for even integer spins. There is an emergent photon at the deconfined critical point, but the "semimetallic'' nature of critical spinons screens such photon making it irrelevant to long distance physics and the deconfined spinons are strictly free particles. A unique prediction of having critical free spinons is an anomalous exponent eta for the susceptibility exactly equal to one. Experimentally measurable response functions are calculated from the deconfined spinon criticality.