Extraordinary two-dimensionality in the S=1/2 spatially anisotropic triangular quantum magnet Cu(1,3-diaminopropane)Cl2 with modulated structure

Abstract

The growth of sufficiently large single crystals of Cu(tn)Cl2 (tn=1,3-diaminopropane) enabled specific heat and susceptibility studies in various field orientations. The nearly invisible broad hump in zero-field specific heat at 0.6 K coincides with the change in the characteristic parameters of zero-field muon relaxation spectra. The lack of oscillations in the time spectra and their exponential character preserved down to 40 mK suggest the coexistence of static and fluctuating local fields associated with the prevalence of low-dimensional correlations. The extreme two-dimensionality is also manifested by the nonmonotonous character of the magnetic phase diagram. First-principle calculations of exchange couplings introduced a concept of a quasi-two-dimensional magnetic lattice with many couplings within the magnetic layers. The strongest ones lead to the model of a rectangular lattice with the intrachain coupling J/kB = 4.3 K and the interchain coupling J'/J = 0.46, which provides excellent agreement with the zero-field specific heat. However, the susceptibility data suggest the importance of other weaker interactions in accord with first-principle studies. Significant broadening of fieldinduced specific heat anomalies may be ascribed to potential intrinsic partial magnetic disorder associated with the gradual modulation of tn positions in the crystal structure.