title: Quantum criticality in an organic magnet.
abstract: 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.
authors.affiliation: Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennennesse 37831, USA.
authors.forename: M B
authors.initials: MB
authors.lastname: Stone
authors.forename: C
authors.initials: C
authors.lastname: Broholm
authors.forename: D H
authors.initials: DH
authors.lastname: Reich
authors.forename: O
authors.initials: O
authors.lastname: Tchernyshyov
authors.forename: P
authors.initials: P
authors.lastname: Vorderwisch
authors.forename: N
authors.initials: N
authors.lastname: Harrison