Spin gap in a quasi-one-dimensionalS=12antiferromagnet:Cu2(1,4−diazacycloheptane)2Cl4

Abstract

${\mathrm{Cu}}_{2}$(1,4-diazacycloheptane)${}_{2}{\mathrm{Cl}}_{4}$ contains double chains of spin-$\frac{1}{2}$ ${\mathrm{Cu}}^{2+}$ ions. We report ac susceptibility, specific heat, and inelastic neutron-scattering measurements on this material. The magnetic susceptibility $\ensuremath{\chi}(T)$ shows a rounded maximum at $T=8$ K indicative of a low-dimensional antiferromagnet with no zero-field magnetic phase transition. We compare the $\ensuremath{\chi}(T)$ data with exact diagonalization results for various one-dimensional spin Hamiltonians and find excellent agreement for a spin ladder with intrarung coupling ${J}_{1}=1.143(3)$ meV and two mutually frustrating interrung interactions ${J}_{2}=0.21(3)$ meV and ${J}_{3}=0.09(5)$ meV. The specific heat in zero field is exponentially activated with an activation energy $\ensuremath{\Delta}=0.87(1)$ meV. A spin gap is also found through inelastic neutron scattering on powder samples that identify a band of magnetic excitations for $0.8<\ensuremath{\Elzxh}\ensuremath{\omega}<1.5$ 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 ${\mathrm{Cu}}_{2}$(1,4-diazacycloheptane)${}_{2}{\mathrm{Cl}}_{4}$ are more complicated. For magnetic fields above ${H}_{c1}\ensuremath{\simeq}7.2$ T specific-heat data versus temperature show anomalies indicating a phase transition to an ordered state below $T=1$ K.

Spin gap in a quasi-one-dimensionalS=12antiferromagnet:Cu2(1,4−diazacycloheptane)2Cl4

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Abstract

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