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

The structure of Cu-2(1,4-diazacycloheptane)(2)Cl-4 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 (chi)(H,T) and heat capacity C-p(T) that show that this material does not behave as a simple linear antiferromagnetic chain with spin S=1/2. (chi)(H=0,T) shows a broad peak at 8 K indicative of one-dimensional antiferromagnetic correlations, but below the peak, chi drops dramatically toward zero instead of remaining finite as T-->0. For T much less than T (peak), (chi)(H) is independent of field below a critical field H-c approximate to 6.6 T, and rises sharply above H-c to a plateau at 8 T. Below T-peak, C-p(T)proportional to exp(-Delta/T) with an activation energy Delta=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. (C) 1996 American Institute of Physics.