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(C 4 H 4 N 2 )(NO 3 ) 2 ͔. The material is an ideal one-dimensional spin-1/2 Heisenberg antiferromagnet with nearest-neighbor exchange constant Jϭ0.90(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 g 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 Tϭ0 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 B H/JϽ1.4 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. ͓S0163-1829͑99͒11201-3͔

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Measurements are reported of the magnetic field dependence of excitations in the quantum critical state of the spin S 1=2 linear chain Heisenberg antiferromagnet copper pyrazine dinitrate (CuPzN). The complete spectrum was measured at k B T=J 0:025 for H 0 and H 8:7 T, where the system is 30% magnetized. At H 0, the results are in agreement with exact calculations of the dynamic spin correlation function for a two-spinon continuum. At H 8:7 T, there are multiple overlapping continua with incommensurate soft modes. The boundaries of these continua confirm long-standing predictions, and the intensities are consistent with exact diagonalization and Bethe ansatz calculations.

The complex (2,3-dmpyH)2CuBr4 has been synthesized and its crystal packing determined by single-crystal X-ray diffraction (2,3-dmpyH = 2,3-dimethylpyridinium). The compound crystallizes in the triclinic space group P1. The crystal packing is characterized by the formation of a ladder structure for the CuBr4 anions showing short Br...Br contacts. The rungs of the ladder are formed via a crystallographic inversion center, while the rails are formed via unit cell translations. Variable temperature magnetic susceptibility measurements agree very well with the ladder model [Jrung = -3.10 cm-1 (-4.34 K) and Jrail = -6.02 cm-1 (-8.42 K)]. The assignment as a magnetic ladder is confirmed by first principles bottom-up theoretical calculations which conclude that Jrung = -3.49 cm-1 (-4.89 K) and Jrail = -7.79 cm-1 (-10.9 K), in very good agreement with the experimental values. They also support the absence of additional significant magnetic exchange within the crystals. Thus, (2,3-dmpyH)2CuBr4 represents the second reported example of a weak-exchange limit magnetic ladder (that is, one in which the exchange along the rail is stronger than that across the rung).

We present an experimental study of the two-dimensional S=1/2 square-lattice antiferromagnet Cu(pz)2(ClO4)2 (pz denotes pyrazine -C4H4N2) using specific heat measurements, neutron diffraction and cold-neutron spectroscopy. The magnetic field dependence of the magnetic ordering temperature was determined from specific heat measurements for fields perpendicular and parallel to the square-lattice planes, showing identical field-temperature phase diagrams. This suggest that spin anisotropies in Cu(pz)2(ClO4)2 are small. The ordered antiferromagnetic structure is a collinear arrangement with the magnetic moments along either the crystallographic b-or c-axis. The estimated ordered magnetic moment at zero field is m0 = 0.47(5) µB and thus much smaller than the available single-ion magnetic moment. This is evidence for strong quantum fluctuations in the ordered magnetic phase of Cu(pz)2(ClO4)2. Magnetic fields applied perpendicular to the squarelattice planes lead to an increase of the antiferromagnetically ordered moment to m0 = 0.93(5) µB at µ0H = 13.5 T -evidence that magnetic fields quench quantum fluctuations. Neutron spectroscopy reveals the presence of a gapped spin excitations at the antiferromagnetic zone center, and it can be explained with a slightly anisotropic nearest neighbor exchange coupling described by J xy 1 = 1.563(13) meV and J z 1 = 0.9979(2)J xy 1 .