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Cu_{2}(1,4-diazacycloheptane)_{2}Cl_{4} contains double chains of spin 1/2Cu^{2+} ions. We report ac susceptibility, specific heat, and inelastic neutronscattering measurements on this material. The magnetic susceptibility,$\chi(T)$, shows a rounded maximum at T = 8 K indicative of a low dimensionalantiferromagnet with no zero field magnetic phase transition. We compare the$\chi(T)$ data to exact diagonalization results for various one dimensionalspin Hamiltonians and find excellent agreement for a spin ladder withintra-rung coupling $J_1 = 1.143(3)$ meV and two mutually frustratinginter-rung interactions: $J_2 = 0.21(3)$ meV and $J_3 = 0.09(5)$ meV. Thespecific heat in zero field is exponentially activated with an activationenergy $\Delta = 0.89(1)$ meV. A spin gap is also found through inelasticneutron scattering on powder samples which identify a band of magneticexcitations for $0.8 < \hbar\omega < 1.5$ meV. Using sum-rules we derive anexpression for the dynamic spin correlation function associated withnon-interacting propagating triplets in a spin ladder. The van-Hovesingularities of such a model are not observed in our scattering dataindicating that magnetic excitations in Cu_{2}(1,4-diazacycloheptane)_{2}Cl_{4}are more complicated. For magnetic fields above $H_{c1} \simeq 7.2$ T specificheat data versus temperature show anomalies indicating a phase transition to anordered state below T = 1 K.Comment: 9 pages, 8 postscript figures, LaTeX, Submitted to PRB 8/4/97, e-mail  Comments to dhr@pha.jhu.ed

physical review. b, condensed matter

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Spin gap in a quasi-one-dimensionalS=12antiferromagnet:Cu