D. C. Dender

Direct observation of field-induced incommensurate fluctuations in a one-dimensional S=1/2 antiferromagnet

Neutron scattering from copper benzoate, Cu(C6D5COO)(2) . 3D(2)O, provides the first direct experimental evidence for field-dependent incommensurate low energy modes in a one-dimensional spin S = 1/2 antiferromagnet. Soft modes occur for wave vectors (q) over tilde = pi +/- delta (q) over tilde(H), where delta (q) over tilde(H) approximate to 2 pi M(H)/g mu(B) as predicted by Bethe ansatz and spinon descriptions of the S = 1/2 chain. Unexpected was a field-induced energy gap Delta(H) alpha H-alpha, where alpha = 0.65(3) as determined from specific heat measurements. At H = 7 T (g mu(B)H/J = 0.52), the magnitude of the gap varies from 0.06J to 0.3J depending on the orientation of the applied field.

Magnetic studies of the two-dimensional, S=1/2 Heisenberg antiferromagnets (5CAP)2CuCl4 and (5MAP)2CuCl4

The magnetic materials (5CAP)2CuCl4 and (5MAP)2CuCl4 form effective square planar lattices of CuCl42− anions. The Cu2+ ions have weak antiferromagnetic interactions in the plane through Cl–Cl contacts. These planes are well separated by the 5MAP and 5CAP organic groups. High temperature susceptibility measurements give J=0.57 K for (5CAP)2CuCl4 and J=0.38 K for (5MAP)2CuCl4. χ(T,H=0) for these materials has a rounded maximum at Tpeak=1.1 K for (5CAP)2CuCl4 and Tpeak=0.62 K for (5MAP)2CuCl4. A kink in χ(T,H=0) for the (5MAP)2CuCl4 suggests a phase transition to three-dimensional long range order at TC=0.44 K. Measurements of χ(T,H) show that the saturation field HC for the transition to a fieldinduced saturated state is 3.78 T for (5CAP)2CuCl4 and 2.17 T for (5MAP)2CuCl4. Thus these materials are excellent model systems in which to explore the full H–T phase diagram of the two-dimensional S=1/2 Heisenberg antiferromagnet.

Magnetized States of Quantum Spin Chains

Quantum spin chains display complex cooperative phenomena that can be explored in considerable detail through theory, numerical simulations, and experiments. Here we review neutron scattering experiments that probe quantum spin chains in high magnetic fields. Experiments on copper-containing organometallic systems show that the uniform antiferromagnetic spin-1/2 chain has a gapless continuum of magnetic excitations and is critical in zero field. Application of a magnetic field creates incommensurate soft modes with a characteristic wave-vector that grows in proportion to the magnetization. These experimental results are evidence that the spins-1/2 chain maps to a one dimensional Luttinger liquid. Experiments on antiferromagnetic spin chains built from spin-1 nickel atoms show a Haldane gap to bound triplet excitations and a finite critical field that must be exceeded to induce magnetization at low temperatures. These results indicate that the integer spin chain has an isolated singlet ground state with hidden topological order. For both spin-1/2 and spin-1 systems, site alternation leads to a field induced gap in the excitation spectrum.

Neutron Scattering Studies of Non-Metallic Low-Dimensional Quantum Antiferromagnets

Low energy properties of non-metallic magnets can be described by a spin Hamiltonian of the form

Magnetic properties of a quasi-one-dimensional S=1/2 antiferromagnet: Copper benzoate.

\mathcal{H} = \sum\limits_{{ }} {{{J}_{{\vec{R}\vec{R}\prime }}}{{{\vec{S}}}_{{\vec{R}}}} \cdot {{{\vec{S}}}_{{\vec{R}\prime }}} + \sum\limits_{{\vec{R}}} {[G({{{\vec{S}}}_{{\vec{R}}}}) - g{{\mu }_{B}}{{{\vec{S}}}_{{\vec{R}}}} \cdot \vec{H}],} }

Dynamical Properties of Unconventional Magnetic Systems

(1) where the double summation visits each spin pair once.

Heat Capacity Study of the Field-Induced Gap in the Linear, S=1/2, Antiferromagnetic Heisenberg Spin Chain Copper Benzoate

Quasi‐one‐dimensional magnetic materials provide important test beds for rigorous models of interacting many‐body systems. Nevertheless, there have been few studies of finite temperature effects on spin correlations in low‐spin systems, though these are most likely to exhibit quantum effects. Copper benzoate, Cu(C6D5COO)2⋅3D2O, is a good realization of the S=1/2 Heisenberg antiferromagnetic chain (HAFC). We have performed ac susceptibility and inelastic neutron scattering experiments on this material which establish the Hamiltonian and an excitation spectrum which is consistent with the spinon continuum ansatz of Muller et al. We present inelastic neutron scattering measurements of the temperature evolution of the dynamic spin–spin correlation function S zz (π,ω). Comparison will be made to the classical prediction and to a finite temperature field theory of Schulz as applied to this S=1/2 HAFC.