Interplay of frustration and magnetic field in the two-dimensional quantum antiferromagnet Cu (tn) Cl2

Specific heat and ac magnetic susceptibility measurements, spanning low temperatures (T ≥ 40 mK) and high-magnetic fields (B ≤ 14 T), have been performed on a two-dimensional (2D) antiferromagnet Cu (tn) Cl2 (tn=1,3-diaminopropane= C3 H10 N2). The compound represents a S=1/2 spatially anisotropic triangular antiferromagnet realized by a square lattice with nearest-neighbor (J/kB =3K), frustrating next-nearest-neighbor (0< J′ /J<0.6), and interlayer (| J″ /J'' | ≈ 10-3) interactions. The absence of long-range magnetic order down to T=60 mK in B=0 and the T2 behavior of the specific heat for T ≤ 0.4K and B ≥ 0 are considered evidence of a high degree of 2D magnetic order. In fields lower than the saturation field, Bsat =6.6 T, a specific heat anomaly, appearing near 0.8 K, is ascribed to bound vortex-antivortex pairs stabilized by the applied magnetic field. The resulting magnetic phase diagram is remarkably consistent with the one predicted for a square lattice without a frustrating interaction, expect that Bsat is shifted to values lower than expected. Potential explanations for this observation, as well as the possibility of a Berezinski-Kosterlitz-Thouless (BKT) phase transition in a spatially anisotropic triangular magnet with the collinear Néel ground state, are discussed.