Franziska Weickert, Robert Kuechler, Alexander Steppke, Luis Pedrero, Michael Nicklas, Manuel Brando, Frank Steglich, Marcelo Jaime, Vivien S. Zapf, Armando Paduan-Filho, Khaled A. Al-Hassanieh, Cristian D. Batista, Pinaki Sengupta
We present a comprehensive experimental and theoretical investigation of the thermodynamic properties: specific heat, magnetization and thermal expansion in the vicinity of the field-induced quantum critical point (QCP) around the lower critical field $H_{c1} \approx 2$\,T in DTN . A $T^{3/2}$ behavior in the specific heat and magnetization is observed at very low temperatures at $H=H_{c1}$ that is consistent with the universality class of Bose-Einstein condensation of magnons. The temperature dependence of the thermal expansion coefficient at $H_{c1}$ shows minor deviations from the expected $T^{1/2}$ behavior. Our experimental study is complemented by analytical calculations and Quantum Monte Carlo simulations, which reproduce nicely the measured quantities. We analyze the thermal and the magnetic Gr\"{u}neisen parameters that are ideal quantities to identify QCPs. Both parameters diverge at $H_{c1}$ with the expected $T^{-1}$ power law. By using the Ehrenfest relations at the second order phase transition, we are able to estimate the pressure dependencies of the characteristic temperature and field scales.
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http://arxiv.org/abs/1204.3064
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