Characterization of a quasi-one-dimensional spin-1/2 magnet which is gapless and paramagnetic for g mu H-B less than or similar to J and k(B)T << J

High-field magnetization, field-dependent specific heat measurements, and z ero-field inelastic magnetic neutron scattering have been used to explore t he magnetic properties of copper pyrazine dinitrate [Cu(C4H4N2)(NO3)(2)] Th e material is an ideal one-dimensional spin-1/2 Heisenberg antiferromagnet with nearest-neighbor exchange constant J=0.90(l) meV and chains extending along the orthorhombic a direction. As opposed to previously studied molecu lar-based spin-1/2 magnetic systems, copper pyrazine dinitrate remains gapl ess and paramagnetic for g mu(B)H/J at least up to 1.4 and for k(B)T/J at l east down to 0.03. This makes the material an excellent model system for ex ploring the T=0 critical line that is expected in the H-T phase diagram of the one-dimensional spin-1/2 Heisenberg antiferromagnet. We present accurat e measurements of the Sommerfeld constant of the spinon gas versus g mu(B)H /J<1.4 that reveal a decrease of the average spinon velocity by 32% in that field range. The results are in excellent agreement with numerical calcula tions based on the Bethe ansatz with no adjustable parameters. [S0163-1829( 99)11201-3].