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.
@article{arxiv.cond-mat/0303364,
title = {Extended quantum critical phase in a magnetized spin-1/2 antiferromagnetic chain},
author = {M. B. Stone and D. H. Reich and C. Broholm and K. Lefmann and C. Rischel and C. P. Landee and M. M. Turnbull},
journal= {arXiv preprint arXiv:cond-mat/0303364},
year = {2009}
}