C. P. Landee

Characterization of a quasi-one-dimensional spin-1/2 magnet which is gapless and paramagnetic for g μ B H ≲ J and k B T ≪ J

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.

Two-dimensional S = 1 2 Heisenberg antiferromagnets: Synthesis, structure, and magnetic properties

The magnetic susceptibility and magnetization of two layered S=½ Heisenberg antiferromagnets with moderate exchange are reported. The two isostructural compounds, (2-amino-5-chloropyridinium) 2 CuBr 4 [(5CAP) 2 CuBr 4 ] and (2-amino-5-methylpyridinium) 2 CuBr 4 [(5MAP) 2 CuBr 4 ], contain S=½, Cu(II) ions related by C centering, yielding four equivalent nearest neighbors The crystal structure of the synthesized compound, (5CAP) 2 CuBr 4 , shows the existence of layers of distorted copper(II)-bromide tetrahedra parallel to the ab plane, separated by the organic cations along the c axis. Magnetic pathways are available through the bromide-bromide contacts within the layers and provide for moderate antiferromagnetic exchange. Susceptibility measurements indicate interaction strengths to he 8.5(2) K and 6.5(2) K and ordering temperatures of 5.1(2) K and 3.8(2) K for (5CAP) 2 CuBr 4 and (5MAP) 2 CuBr 4 , respectively. High-field magnetization experiments on both compounds show upward curvature of M(H,T). Magnetization measurements made at T = 1.3 K show saturation occurs in (5MAP) 2 CuBr 4 at 18.8 T and in (5CAP) 2 CuBr 4 at 24.1 T. The magnetization curves are consistent with recent theoretical predictions. Single-crystal magnetization measured at 2.0 K indicates a spin-flop transition at 0.38 T and 0.63 T for (5CAP) 2 CuBr 4 and (5MAP) 2 CuBr 4 , respectively.

Extended quantum critical phase in a magnetized spin-1/2 antiferromagnetic chain.

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< or =0.025 for H=0 and H=8.7 T, where the system is approximately 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.

Field-induced Tomonaga-Luttinger liquid phase of a two-leg spin-1/2 ladder with strong leg interactions

We study the magnetic-field-induced quantum phase transition from a gapped quantum phase that has no magnetic long-range order into a gapless phase in the spin-1/2 ladder compound bis(2,3-dimethylpyridinium) tetrabromocuprate (DIMPY). At temperatures below about 1 K, the specific heat in the gapless phase attains an asymptotic linear temperature dependence, characteristic of a Tomonaga-Luttinger liquid. Inelastic neutron scattering and the specific heat measurements in both phases are in good agreement with theoretical calculations, demonstrating that DIMPY is the first model material for an S=1/2 two-leg spin ladder in the strong-leg regime.

Molecular-based quantum magnets: the isotropic spin ladder Cu(quinoxaline)Br 2

Abstract The structure, susceptibility, and high-field magnetization of μ-quinoxalinecopper(II) dibromide are reported. The compound crystallizes in the space group C2/c as a coordination polymer of neutral Cu2Br4 dimers bridged into chains by quinoxaline molecules forming a ladder structure. The susceptibility data are well described by a spin-ladder model with two antiferromagnetic exchange strengths 2J⊥=35.0 K within the dimers, and 2J=30.3 K through the quinoxaline groups. The ratio of exchange strengths, R J⊥/J=1.16, is the closest to unity yet found for a spin ladder. The magnetization study shows the compound at low temperatures to be in a non-magnetic singlet state for low fields, but that a phase transition to a magnetized state occurs near 14 T, indicating the presence of an energy gap Δ≈20 K.

Quantum effects in a weakly frustrated s=1/2 two-dimensional heisenberg antiferromagnet in an applied magnetic field.

We have studied the two-dimensional S=1/2 square-lattice antiferromagnet Cu(pz)_{2}(ClO4)_{2} (where pz denotes pyrazine), using neutron inelastic scattering and series expansion calculations. We show that the presence of antiferromagnetic next-nearest-neighbor interactions enhances quantum fluctuations associated with resonating valence bonds. Intermediate magnetic fields lead to a selective tuning of resonating valence bonds and a spectacular inversion of the zone-boundary dispersion, providing novel insight into 2D antiferromagnetism in the quantum limit.

Two-dimensional square-lattice S=1/2 antiferromagnet Cu(pz)(2)(ClO4)(2)

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 m(0)=0.47 (5)mu(B) 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 square-lattice planes lead to an increase in the antiferromagnetically ordered moment to m(0)=0.93 (5)mu(B) at mu H-0= 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(1)(xy).

Quantum critical dynamics of an S = 1 2 antiferromagnetic Heisenberg chain studied by C 13 NMR spectroscopy

We present a

Dynamics of the two-dimensional S=1/2 dimer system (C5H6N2F)2CuCl4

^{13}\text{C}

Dynamics of a Heisenberg spin chain in the quantum critical regime: NMR experiment versus effective field theory

NMR study of the magnetic field driven transition to complete polarization of the