Elastocaloric determination of the phase diagram of Sr2RuO4

One ofthe main developments in unconventional superconductivity in the past two decades has been the discovery that most unconventional superconductors form phase diagramsthat also contain other strongly correlated states. Many systems of interest are therefore close to more than one instability, and tuning between the resultant ordered phases is the subject of intense research(1). In recent years, uniaxial pressure applied using piezoelectric-based devices has been shown to be a particularly versatile new method oftuning(2,3), leading to experimentsthat have advanced our understanding ofthe fascinating unconventional superconductor Sr2RuO4 (refs.(4-9)). Here we map out its phase diagram using high-precision measurements ofthe elastocaloric effect in what we believe to be the first such study including both the normal and the superconducting states. We observe a strong entropy quench on entering the superconducting state, in excellent agreement with a model calculation for pairing at the Van Hove point, and obtain a quantitative estimate ofthe entropy change associated with entry to a magnetic state that is observed in proximity to the superconductivity. The phase diagram is intriguing both for its similarity to those seen in other families of unconventional superconductors and for extra features unique, so far, to Sr2RuO4.