Magnons and continua in a magnetized and dimerized spin-1/2 chain

TL;DR

This study investigates how magnetic excitations in a dimerized spin-1/2 chain change with magnetic field, revealing different ground states and the emergence of a continuum indicating a strongly correlated quantum phase.

Contribution

It provides detailed experimental insights into the field-dependent excitation spectrum of copper nitrate, highlighting the transition from magnon modes to a continuum in a quantum critical regime.

Findings

Multiple magnon modes at low and high fields

Observation of a continuum in intermediate fields

Identification of distinct ground states based on magnetic field

Abstract

We examine the magnetic field dependent excitations of the dimerized spin-1/2 chain, copper nitrate, with antiferromagnetic intra-dimer exchange $J_1=0.44$ meV and exchange alternation $\alpha=J_2/J_1=0.24$. Magnetic excitations in three distinct regimes of magnetization are probed through inelastic neutron scattering at low temperatures. At low and high fields there are three and two long-lived magnon-like modes, respectively. The number of modes and the anti-phase relationship between the wave-vector dependent energy and intensity of magnon scattering reflect the distinct ground states: A singlet ground state at low fields $\mu_0H < \mu_0H_{c1} = 2.8$~T and an $S_z=1/2$ product state at high fields $\mu_0H > \mu_0H_{c2} = 4.2$~T. In the intermediate field regime, a continuum of scattering for $\hbar\omega\approx J_1$ is indicative of a strongly correlated gapless quantum state without coherent magnons.