Cu2(1,4-diazacycloheptane)2Cl4: A quasi-one-dimensional S=1/2 spin liquid system

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Abstract
The structure of Cu2(1,4‐diazacycloheptane)2Cl4 consists of well‐separated zig‐zag chains of Cu atoms, suggesting the possibility of significant antiferromagnetic near‐neighbor and next‐nearest‐neighbor interactions. We report on measurements of magnetic susceptibility χ(H,T) and heat capacity Cp(T) that show that this material does not behave as a simple linear antiferromagnetic chain with spin S=1/2. χ(H=0,T) shows a broad peak at 8 K indicative of one‐dimensional antiferromagnetic correlations, but below the peak, χ drops dramatically toward zero instead of remaining finite as T→0. For T≪Tpeak, χ(H) is independent of field below a critical field Hc≊6.6 T, and rises sharply above Hc to a plateau at 8 T. Below Tpeak, Cp(T)∝exp(−Δ/T) with an activation energy Δ=6.8 K. These data indicate the presence of a gap between the ground state and the lowest‐lying excited states. This material is thus a candidate for a model quasi‐one‐dimensional S=1/2 system with a disordered ground state that is significantly affected by competing interactions.