Mar 07, 2018
Research: Quantum criticality in the one-dimensional quantum magnet copper pyrazine dinitrate
Phase diagram of copper pyrazine dinitrate as a function of magnetic field and temperature with a quantum phase transition at a field of approximately 14 Tesla. The material consists of chains of copper atoms whose magnetic moments are represented by blue arrows. The colour shading reflects the measured specific heat.
Low-dimensional quantum magnets are a playground for fascinating quantum phenomena. The confinement of spin fluctuations to two-dimensional planes or one-dimensional chains promote strong correlations that can give rise to exotic spin-liquid ground states without long-range magnetic order. Such spin-liquids are unstable however in the presence of a sufficiently strong magnetic field giving rise to a quantum phase transition into a field-polarized state. A paradigm of such a transition is found in the Heisenberg spin-½ chain which is exactly solvable by Bethe-Ansatz methods so that its quantum critical thermodynamics is theoretically well understood.
Such Heisenberg chains are approximately realized in copper pyrazine dinitrate. The magnetic moments of different chains interact very weakly in this material so that long-range magnetic order only materializes below temperatures of 0.1 Kelvin. Above this temperature, there is a large regime of temperatures and magnetic fields where thermodynamics is controlled by the magnetic Heisenberg chains providing a unique opportunity for a quantitative comparison between theory and experiment. As reported in Science Advances [1], such a comparison of a comprehensive set of thermodynamic data was carried out by a collaboration between experimental and theoretical groups of the University of Cologne, the University of Wuppertal and the TU Dresden, and excellent agreement between theory and experiment were found. This study establishes copper pyrazine dinitrate as an instructive reference material for the emergence of quantum criticality in a quantum magnet illustrating fundamental principles of quantum critical thermodynamics.
[1] O. Breunig, M. Garst, A. Klümper, J. Rohrkamp, M. M. Turnbull, T. Lorenz,
Quantum Criticality in the spin-1/2 Heisenberg chain system copper pyrazine dinitrate,
Science Advances 3 eaao3773 (2017)