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TT: Fachverband Tiefe Temperaturen

TT 45: Correlated Electrons: Poster Session

TT 45.65: Poster

Donnerstag, 29. März 2012, 15:00–19:00, Poster B

Phase diagram of the Ising chain compound Nb₂O₆ in transverse magnetic field — •Simon Scharffe, Oliver Breunig, Johanna Frielingsdorf, Martin Valldor, Michael Seher, and Thomas Lorenz — II. Physikalisches Insitut, Universität zu Köln

CoNb₂O₆ is a model system to investigate a quantum phase transition in magnetic fields. The structure consists of layers of CoO₆ octahedrons separated by non-magnetic NbO₆ layers. Within each layer the Co²⁺spins are linked through superexchange and form 1D ferromagnetic zigzag chains along the c axis. Due to crystal field effects (compression of the CoO₆ octahedrons) an easy-axis anisotropy of the Co²⁺moments is present, which leads to an effective spin-1/2 system described by the Ising model. The easy axis is in the a c plane, and alternates by ± 31^∘ with respect to the c axis for neighboring chains. Due to small inter-chain couplings J_|| ≈ 0.01 · J_⊥ the system shows long-range antiferromagnetic order, which is insommensurate below T_N1=2.9K and becomes commensurate at T_N2=1.9K. A magnetic field parallel to the b axis is normal to the easy axis and allows to study the quantum phase transition of an Ising-spin chain in tranverse field. The system is driven to its quantum critical point between a quasi 1D ferromagnet to a quantum paramagnet. Only few studies of the transverse field case are available so far. We present measurements of specific heat and magnetization in a temperature range from about 0.3 to up to 10 K and discuss the phase diagram.
This work was supported by the DFG through SFB 608.