Spinon confinement and a sharp longitudinal mode in Yb2Pt2Pb in magnetic fields

W. J. Gannon,M. S. Kim,I. A. Zaliznyak,Y. Qiu,A. E. Feiguin,J. R. D. Copley,F. Demmel,M. C. Aronson,L. S. Wu,A. M. Tsvelik

doi:10.1038/s41467-019-08715-y

Abstract

The fundamental excitations in an antiferromagnetic chain of spins-1/2 are spinons, de-confined fractional quasiparticles that when combined in pairs, form a triplet excitation continuum. In an Ising-like spin chain the continuum is gapped and the ground state is Néel ordered. Here, we report high resolution neutron scattering experiments, which reveal how a magnetic field closes this gap and drives the spin chains in Yb2Pt2Pb to a critical, disordered Luttinger-liquid state. In Yb2Pt2Pb the effective spins-1/2 describe the dynamics of large, Ising-like Yb magnetic moments, ensuring that the measured excitations are exclusively longitudinal, which we find to be well described by time-dependent density matrix renormalization group calculations. The inter-chain coupling leads to the confinement of spinons, a condensed matter analog of quark confinement in quantum chromodynamics. Insensitive to transverse fluctuations, our measurements show how a gapless, dispersive longitudinal mode arises from confinement and evolves with magnetic order.

Highlights

The fundamental excitations in an antiferromagnetic chain of spins-1/2 are spinons, de-confined fractional quasiparticles that when combined in pairs, form a triplet excitation continuum
The intensity measured at 4 T, in the FM state precisely follows the projection of the scattering wave vector on the (110) direction, revealing fluctuations polarized along the inplane Ising moments, which are insensitive to magnetic fields (Fig. 3f)
It is difficult to visualize the nature of the interchain mode itself, as it is purely quantum mechanical in its origin, with no simple classical analog

Summary

Introduction

The fundamental excitations in an antiferromagnetic chain of spins-1/2 are spinons, de-confined fractional quasiparticles that when combined in pairs, form a triplet excitation continuum. The low energy magnetic excitations are spinons, having a quantum continuum for momentum along the chain direction qL that can be measured with inelastic neutron scattering[27], with an excitation bandwidth that is considerably larger than the excitation gap (Fig. 1b).

Results

Conclusion