Spin gap in a quasi-one-dimensional S=1/2 antiferromagnet: Cu₂(1,4-diazacycloheptane)₂Cl₄

Cu₂(1,4-diazacycloheptane)₂Cl₄ contains double chains of spin-1/2 Cu²⁺ ions. We report ac susceptibility, specific heat, and inelastic neutron-scattering measurements on this material. The magnetic susceptibility χ(T) shows a rounded maximum at T=8 K indicative of a low-dimensional antiferromagnet with no zero-field magnetic phase transition. We compare the χ(T) data with exact diagonalization results for various one-dimensional spin Hamiltonians and find excellent agreement for a spin ladder with intrarung coupling J₁=1.143(3) meV and two mutually frustrating interrung interactions J₂=0.21(3) meV and J₃=0.09(5) meV. The specific heat in zero field is exponentially activated with an activation energy Δ=0.87(1) meV. A spin gap is also found through inelastic neutron scattering on powder samples that identify a band of magnetic excitations for 0.8<ħω<1.5 meV. Using sum rules we derive an expression for the dynamic spin-correlation function associated with noninteracting propagating triplets in a spin ladder. The Van Hove singularities of such a model are not observed in our scattering data, indicating that magnetic excitations in Cu₂(1,4-diazacycloheptane)₂Cl₄ are more complicated. For magnetic fields above H_c1≃7.2 T specific-heat data versus temperature show anomalies indicating a phase transition to an ordered state below T=1 K.