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Quantum Criticality in an Organic Magnet

M. B. Stone·C. Broholm·D. H. Reich·O. Tchernyshyov·P. Vorderwisch·N. Harrison
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Abstract

Exchange interactions between $S=\frac{1}{2}$ 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: $H_{c1}=7.5$ T separates a quantum paramagnet phase from a three dimensional, antiferromagnetically-ordered state while $H_{c2}=37$ T 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 $H_{c1}$.

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M. B. Stone, C. Broholm, D. H. Reich, O. Tchernyshyov, P. Vorderwisch, N. Harrison. 2006-02-15. Quantum Criticality in an Organic Magnet. https://doi.org/10.1103/physrevlett.96.257203

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