Heuristic bounds on superconductivity and how to exceed them

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