Bose-Einstein condensation of triplons in the 5 = 1 tetramer antiferromagnet K_2Ni_2(MoO_4)_3: A compound close to a quantum critical point

摘要

The structure of K_2Ni_2(MoO_4)_3 consists of S = 1 tetramers formed by Ni~(2+) ions. The magnetic susceptibility X(T) and specific heat C_P(T) data on a single crystal show a broad maximum due to the low dimensionality of the system with short-range spin correlations. A sharp peak is seen in x(T) and C_P(T) at about 1.13 K, well below the broad maximum. This is an indication of magnetic long-range order, i.e., the absence of spin gap in the ground state. Interestingly, the application of a small magnetic field (H > 0.1 T) induces magnetic behavior akin to the Bose-Einstein condensation (BEC) of triplon excitations observed in some spin-gap materials. Our results demonstrate that the temperature-field (T-H) phase boundary follows a power law (T - T_N) ∝ H~(1/a) with the exponent 1/a close to 2/3, as predicted for the BEC scenario. The observation of BEC of triplon excitations in small H infers that K_2Ni_2(MoO_4)_3 is located in the proximity of a quantum critical point, which separates the magnetically ordered and spin-gap regions of the phase diagram.