Extended Quantum Critical Phase in a Magnetized Spin-[Formula presented] Antiferromagnetic Chain

Measurements are reported of the magnetic field dependence of excitations in the quantum critical state of the spin [Formula presented] linear chain Heisenberg antiferromagnet copper pyrazine dinitrate (CuPzN). The complete spectrum was measured at [Formula presented] for [Formula presented] and [Formula presented], where the system is [Formula presented] magnetized. At [Formula presented], the results are in agreement with exact calculations of the dynamic spin correlation function for a two-spinon continuum. At [Formula presented], 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.