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An electronic solid with itinerant carriers and localized magnetic moments represents a paradigmatic strongly correlated system. The electrical transport properties associated with the itinerant carriers, as they scatter off these local moments, have been scrutinized across a number of materials. Here, we analyze the transport characteristics associated with ultraclean PdCrO ${}_2$—a quasi-two-dimensional material consisting of alternating layers of itinerant Pd-electrons and Mott-insulating CrO ${}_2$layers—which shows a pronounced regime ofT-linear resistivity over a wide range of intermediate temperatures. By contrasting these observations to the transport properties in a closely related material PdCoO ${}_2$, where the CoO ${}_2$layers are band-insulators, we can rule out the traditional electron–phonon interactions as being responsible for this interesting regime. We propose a previously ignored electron-magneto-elastic interaction between the Pd-electrons, the Cr local moments and an out-of-plane phonon as the main scattering mechanism that leads to the significant enhancement of resistivity and aT-linear regime in PdCrO ${}_2$at temperatures far in excess of the magnetic ordering temperature. We suggest a number of future experiments to confirm this picture in PdCrO ${}_2$as well as other layered metallic/Mott-insulating materials. 

Abstract What limits the value of the superconducting transition temperature ( T c ) is a question of great fundamental and practical importance. Various heuristic upper bounds on T c have been proposed, expressed as fractions of the Fermi temperature, T F , the zero-temperature superfluid stiffness, ρ s (0), or a characteristic Debye frequency, ω 0 . We show that while these bounds are physically motivated and are certainly useful in many relevant situations, none of them serve as a fundamental bound on T c . To demonstrate this, we provide explicit models where T c / T F (with an appropriately defined T F ), T c / ρ s (0), and T c / ω 0 are unbounded.