Quantum criticality in an organic magnet.
Exchange interactions between S=1/2 sites in piperazinium hexachlorodicuprate produce a frustrated bilayer magnet with a singlet ground state. We have determined the field-temperature phase diagram by high field magnetization and neutron scattering experiments. There are two quantum critical points: Hc1=7.5 T separates a quantum paramagnet phase from a three dimensional, antiferromagnetically ordered state while Hc2=37 marks the onset of a fully polarized state. The ordered phase, which we describe as a magnon Bose-Einstein condensate (BEC), is embedded in a quantum critical regime with short range correlations. A low temperature anomaly in the BEC phase boundary indicates that additional low energy features of the material become important near Hc1.
Stone
M B
MB
Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennennesse 37831, USA.
Broholm
C
C
Reich
D H
DH
Tchernyshyov
O
O
Vorderwisch
P
P
Harrison
N
N