Breakdown of Hooke's law of elasticity at the Mott critical endpoint in an organic conductor
The coupling of the critical electronic system to a compressible lattice is found to drastically alter the Mott criticality. The Mott metal insulator transition, a paradigm of strong electron electron correlations, has been considered as a source of intriguing phenomena. Despite its importance for a wide range of materials, fundamental aspects of the transition, such as its universal properties, are still under debate. We report detailed measurements of relative length changes DL/L as a function of continuously controlled helium gas pressure P for the organic conductor k (BEDT TTF)2Cu[N(CN)2]Cl across the pressure induced Mott transition. We observe strongly nonlinear variations of DL/L with pressure around the Mott critical endpoint, highlighting a breakdown of Hooke's law of elasticity. We assign these nonlinear strain stress relations to an intimate, nonperturbative coupling of the critical electronic system to the lattice degrees of freedom. Our results are fully consistent with mean field criticality, predicted for electrons in a compressible lattice with finite shear moduli. We argue that the Mott transition for all systems that are amenable to pressure tuning shows the universal properties of an isostructural solid solid transition.
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