Characterization of a quasi-one-dimensional spin-1/2 magnet which is gapless and paramagnetic for (formula presented) and (formula presented)

High-field magnetization, field-dependent specific heat measurements, and zero-field inelastic magnetic neutron scattering have been used to explore the magnetic properties of copper pyrazine dinitrate (Formula presented) The material is an ideal one-dimensional spin-1/2 Heisenberg antiferromagnet with nearest-neighbor exchange constant (Formula presented) and chains extending along the orthorhombic a direction. As opposed to previously studied molecular-based spin-1/2 magnetic systems, copper pyrazine dinitrate remains gapless and paramagnetic for (Formula presented) at least up to 1.4 and for (Formula presented) at least down to 0.03. This makes the material an excellent model system for exploring the (Formula presented) critical line that is expected in the (Formula presented) phase diagram of the one-dimensional spin-1/2 Heisenberg antiferromagnet. We present accurate measurements of the Sommerfeld constant of the spinon gas versus (Formula presented) that reveal a decrease of the average spinon velocity by 32% in that field range. The results are in excellent agreement with numerical calculations based on the Bethe ansatz with no adjustable parameters.