Research: Elastocaloric effect in distrontium ruthenate

Distrontium ruthenate Sr2RuOis a fascinating unconventional superconductor. Despite decades of intensive research, questions about the symmetry of its order parameter and the associated pairing mechanism still remain unsettled. Recently, it was discovered that its critical temperature Tc can be increased with the help of a uniaxial strain that drives the system towards a Van Hove singularity. At the Van Hove singularity Tc reaches a maximum and decreases again for larger strain.

In a collaborative effort within the CRC TRR 288 involving experimental and theoretical groups, the elastocaloric effect of Sr2RuO4 was now investigated as a function of temperature and strain. The elastocaloric effect is also known as the Grüneisen parameter, and it gives important information about the entropy distribution in the phase diagram. It is related to a combination of thermal expansion, specific heat and compressibility. Instead of measuring these three quantities separately, in the present work the elastocaloric effect was directly determined with high precision using a newly developed apparatus that allows to work with a strain oscillating in time.

These measurements found that above Tc  the entropy as a function of strain exhibits a maximum at the Van Hove singularity that abruptly turns into a minimum below the critical temperature. This imposes constraints on the superconducting order. Theoretical calculations performed in collaboration with the Institute of Theoretical Condensed Matter Physics (TKM) suggest that the superconducting order parameter cannot possess a node at the location of the Van Hove singularity in wavevector space. This provides important information on the pairing symmetry of Sr2RuO4.

You-Sheng Li, Markus Garst, Jörg Schmalian, Sayak Ghosh, Naoki Kikugawa, Dmitry A. Sokolov, Clifford W. Hicks, Fabian Jerzembeck, Matthias S. Ikeda, Zhenhai Hu, B. J. Ramshaw, Andreas W. Rost, Michael Nicklas and Andrew P. Mackenzie, Elastocaloric determination of the phase diagram of Sr2RuO4, Nature 607, 276 (2022).

Phase diagram of distrontium ruthenate You-Sheng Li
Phase diagram of distrontium ruthenate in the strain-temperature plane. The coloring represents the elastocaloric effect.