Classification of materials with divergent magnetic Grüneisen parameter

At any quantum critical point (QCP) with a critical magnetic field , the magnetic Grüneisen parameter , which equals the adiabatic magnetocaloric effect, is predicted to show characteristic signatures such as a divergence, sign change and scaling. We categorise 13 materials, ranging from heavy fermion metals to frustrated magnets, where such experimental signatures have been found. Remarkably, seven stoichiometric materials at ambient pressure show . 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 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 divergence is insufficient and also the Grüneisen ratio of thermal expansion to specific heat must diverge.

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