College Park, Maryland      June 6 - 10 , 2004

MP35: Quantum Critical Phase Diagram of a Quantum Magnet

M. B. Stone (Johns Hopkins University; Pennsylvania State University), C. Broholm (Johns Hopkins University; NIST Center for Neutron Research), D. H. Reich (Johns Hopkins University), I. Zaliznyak (Brookhaven National Laboratory), P. Vorderwisch (Hahn-Meitner-Institut Berlin), N. Harrison (National High Magnetic Field Laboratory (LANL))

Piperazinum hexachlorodicuprate (PHCC) has been shown to be a quasi-two-dimensional Heisenberg antiferromagnet with a bilayer structure (M. B. Stone et al PRB, 64, 144405 (2001)). The bilayer structure and frustrated interactions leads to a cooperative singlet ground state at T = 0 and H = 0. We present the field-temperature phase diagram of this unique quantum spin liquid as determined by neutron scattering and high field magnetic susceptiblity measurements. Two field dependent quantum critical points were accessed: HC1 = 7.6 T which is the field that closes the spin-gap and leads to three-dimensional long-range order for sufficiently low T and HC2 = 37 T where the system transitions to a field induced ferromagnetic state. A gapless disordered phase with short-range spin correlations encloses the antiferromagnetically ordered phase in the field-temperature phase diagram. Close to the low field quantum critical point, a minimum in the phase boundary to long-range order indicates that weak interactions with lattice or nuclear spin degrees of freedom become important.

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