title: Extended quantum critical phase in a magnetized spin-1/2 antiferromagnetic chain.
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< or =0.025 for H=0 and H=8.7 T, where the system is approximately 30% magnetized. At H=0, the results are in agreement with exact calculations of the dynamic spin correlation function for a two-spinon continuum. At H=8.7 T, 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.
authors.affiliation: Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA.
authors.forename: M B
authors.initials: MB
authors.lastname: Stone
authors.forename: D H
authors.initials: DH
authors.lastname: Reich
authors.forename: C
authors.initials: C
authors.lastname: Broholm
authors.forename: K
authors.initials: K
authors.lastname: Lefmann
authors.forename: C
authors.initials: C
authors.lastname: Rischel
authors.forename: C P
authors.initials: CP
authors.lastname: Landee
authors.forename: M M
authors.initials: MM
authors.lastname: Turnbull