Classification of materials with divergent magnetic Grüneisen parameter

Abstract

At any quantum critical point (QCP) with a critical magnetic field $H_c$, the magnetic Grüneisen parameter ${\Gamma }_{\mathrm{H}}$, which equals the adiabatic magnetocaloric effect, is predicted to show characteristic signatures such as a divergence, sign change and $T/(H-H_c{)}^{\epsilon }$ scaling. We categorize thirteen materials, ranging from heavy fermion metals to frustrated magnets, where such experimental signatures have been found. Remarkably, seven stoichiometric materials at ambient pressure show $H_c=0$. However, additional thermodynamic and magnetic experiments suggest that most of them do not show a zero-field QCP. While the existence of a pressure insensitive "strange metal" state is one possibility, for some of the materials ${\Gamma }_{\mathrm{H}}$ seems influenced by impurities or a fraction of moments which are not participating in a frozen state. To unambiguously prove zero-field and pressure sensitive quantum criticality, a ${\Gamma }_{\mathrm{H}}$ divergence is insufficient and also the Grüneisen ratio of thermal expansion to specific heat must diverge.

10 pages, 6 figs, manuscript for the proceedings of SCES 2016

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