Stone, M.B.; Broholm, C.; Reich, D.H.; Tchernyshyov, O.; Vorderwisch, P.; Harrison, N.: Quantum criticality in an organic magnet. Physical Review Letters 96 (2006), p. 257203/1-4
10.1103/PhysRevLett.96.257203
Abstract:
Exchange interactions between spin-1/2 sites in piperazinium hexachlorodicuprate produce a frustrated bilayer antiferromagnet with a bond ordered singlet ground state. We report the field-temperature phase diagram of this system as determined via high field susceptibility and neutron scattering experiments. There are two quantum critical points: Hc1 = 7.5 T separates the singlet phase from a three dimensional, antiferromagnetically-ordered state while Hc2 = 37 T marks the onset of a fully spin-polarized state. The long range ordered phase is embedded in a gapless quasi-two-dimensional paramagnetic regime with short range transverse spin correlations. Close to the low field quantum critical point, a reentrant phase transition between long range order and the singlet phase indicates that weak interactions with lattice or nuclear spin degrees of freedom become important.