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Abstract:
We report an experimental and theoretical study of the low-temperature specific heat C and magnetic susceptibility chi of the layered anisotropic triangular-lattice spin-1/2 Heisenberg antiferromagnets Cs2CuCl4-xBrx with x= 0, 1, 2, and 4. We find that the ratio J'/J of the exchange couplings ranges from 0.32 approximate to 0.78, implying a change (crossover or quantum phase transition) in the materials' magnetic properties from one-dimensional (1D) behavior for J'/J < 0.6 to two-dimensional (2D) behavior for J'/J approximate to 0.78. For J'/J < 0.6, realized for x= 0, 1, and 4, we find a magnetic contribution to the low-temperature specific heat, C-m proportional to T, consistent with spinon excitations in 1D spin-1/2 Heisenberg antiferromagnets. Remarkably, for x = 2, where J'/J approximate to 0.78 implies a 2D magnetic character, we also observe C-m( )proportional to T. This finding, which contrasts the prediction of C-m proportional to T-2 made by standard spin-wave theories, shows that Fermi-like statistics also plays a significant role for the magnetic excitations in spin-1/2 frustrated 2D antiferromagnets.
