Characterization of a quasi-one-dimensional spin-1/2 magnet which is gapless and paramagnetic for gμBH≲J and kBT<<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(l) 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 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 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 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 calculations based on the Bethe ansatz with no adjustable parameters. [S0163-1829(99)11201-3].