Characterization of a quasi-one-dimensional spin-1/2 magnet which is gapless and paramagnetic for gμ_BH≲J and k_BT≪J

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 [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 previously studied molecular-based spin-1/2 magnetic systems, copper pyrazine dinitrate remains gapless and paramagnetic for gμ_BH/J at least up to 1.4 and for k_BT/J at least down to 0.03. This makes the material an excellent model system for exploring 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 accurate measurements of the Sommerfeld constant of the spinon gas versus gμ_BH/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 calculations based on the Bethe ansatz with no adjustable parameters.