Field-driven phase transitions in a quasi-two-dimensional quantum antiferromagnet

M. B. Stone, C. Broholm, D. H. Reich, P. Schiffer, O. Tchernyshyov, P. Vorderwisch, N. Harrison

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33 Scopus citations


We report magnetic susceptibility, specific heat, and neutron scattering measurements as a function of applied magnetic field and temperature to characterize the S = 1/2 quasi-two-dimensional (2D) frustrated magnet piperazinium hexachlorodicuprate (PHCC). The experiments reveal four distinct phases. At low temperatures and fields the material forms a quantum paramagnet with a 1meV singlet triplet gap and a magnon bandwidth of 1.7meV. The singlet state involves multiple spin pairs some of which have negative ground state bond energies. Increasing the field at low temperatures induces 3D long-range antiferromagnetic order at 7.5 Tesla through a continuous phase transition that can be described as magnon Bose-Einstein condensation. The phase transition to a fully polarized ferromagnetic state occurs at 37 Tesla. The ordered antiferromagnetic phase is surrounded by a renormalized classical region. The crossover to this phase from the quantum paramagnet is marked by a distinct anomaly in the magnetic susceptibility which coincides with closure of the finite temperature singlet-triplet pseudo gap. The phase boundary between the quantum paramagnet and the Bose-Einstein condensate features a finite temperature minimum at T = 0.2 K, which may be associated with coupling to nuclear spin or lattice degrees of freedom close to quantum criticality.

Original languageEnglish (US)
Article number31
JournalNew Journal of Physics
StatePublished - Feb 16 2007
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • General Physics and Astronomy


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