The halon: a quasiparticle featuring critical charge fractionalization

TL;DR

This paper investigates the halon, a critical impurity state in quantum-critical environments, demonstrating charge fractionalization into a core and halo near a boundary quantum critical point, confirmed through large-scale Monte Carlo simulations.

Contribution

It introduces the concept of halons as quasiparticles with fractional charge in quantum-critical systems and characterizes their universal features near boundary quantum critical points.

Findings

Confirmation of halon existence in O(2) and O(3) systems

Quantification of universal features of halons

Demonstration of charge fractionalization phenomenon

Abstract

The halon is a special critical state of an impurity in a quantum-critical environment. The hallmark of the halon physics is that a well-defined integer charge gets fractionalized into two parts: a microscopic core with half-integer charge and a critically large halo carrying a complementary charge of $\pm 1/2$. The halon phenomenon emerges when the impurity--environment interaction is fine-tuned to the vicinity of a boundary quantum critical point (BQCP), at which the energies of two quasiparticle states with adjacent integer charges approach each other. The universality class of such BQCP is captured by a model of pseudo-spin-$1/2$ impurity coupled to the quantum-critical environment, in such a way that the rotational symmetry in the pseudo-spin $xy$-plane is respected, with a small local "magnetic" field along the pseudo-spin $z$-axis playing the role of control parameter driving the system away from the BQCP. On the approach to BQCP, the half-integer projection of the pseudo-spin on its $z$-axis gets delocalized into a halo of critically divergent radius, capturing the essence of the phenomenon of charge fractionalization. With large-scale Monte Carlo simulations, we confirm the existence of halons---and quantify their universal features---in O(2) and O(3) quantum critical systems.