Extended quantum critical phase in a magnetized spin-1/2 antiferromagnetic chain

TL;DR

This study investigates the magnetic field effects on excitations in a spin-1/2 antiferromagnetic chain, revealing an extended quantum critical phase with multiple continua and confirming theoretical predictions through precise measurements.

Contribution

It provides the first detailed experimental measurement of the excitation spectrum in a magnetized spin-1/2 chain, confirming theoretical models and revealing an extended quantum critical phase.

Findings

Quantitative agreement with two-spinon continuum at zero field

Observation of multiple overlapping continua at high field

Confirmation of theoretical predictions of incommensurate soft modes

Abstract

Measurements are reported of the magnetic field dependence of excitations in the quantum critical state of the spin S=1/2 linear chain Heisenberg antiferromagnet copper pyrazine dinitrate (CuPzN). The complete spectrum was measured at k_B T/J <= 0.025 for H=0 and H=8.7 Tesla where the system is ~30% magnetized. At H=0, the results are in quantitative agreement with exact calculations of the dynamic spin correlation function for a two-spinon continuum. At high magnetic field, there are multiple overlapping continua with incommensurate soft modes. The boundaries of these continua confirm long-standing predictions, and the intensities are consistent with exact diagonalization and Bethe Ansatz calculations.