  High field mangetization, field-dependent specific heat measurements, and
zero field inelastic magnetic neutron scattering have been used to explore the
magnetic properties of copper pyrazine dinitrate (Cu(C4H4N2)(NO3)2). The
material is an ideal one-dimensional spin-1/2 Heisenberg antiferromagnet with
nearest neighbor exchange constant J=0.90(1) meV and chains extending along the
orthorhombic a-direction. As opposed to previosly studied molecular-based
spin-1/2 magnetic systems, coppyer pyrazine dinitrate remains gapless and
paramagnetic for g mu_B H/J at least up to 1.4 and for k_B T/J at least down to
0.03 this makes the material an excellent model system for exploring the T=0
critical line which is expected in the H - T phase diagram on the
one-dimensional spin-1/2 Heisenberg antiferromagnet. As a first example of such
a study we present accurate measurements of the Sommerfeld constant of the
spinon gas versus g mu_B H/J < 1.4 which reveal a decrease of the averages
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.
