Effect of hydrostatic pressure and uniaxial stress on the competing phases in iron-based superconductors*

Understanding the phase interplay in iron-based superconductors is considered to be crucial for unravelling the mechanism behind their superconductivity. In this talk, we focus on new insights into the phase relation under pressure [1] of two members, FeSe and BaFe₂As₂, from specific heat [2] and elastoresistance measurements [3] made under hydrostatic pressures up to ~ 2.5 GPa.
First, we address ongoing debates about the phase diagram of FeSe under hydrostatic pressure. We demonstrate [4] that for high pressures superconductivity is bulk and competes with magnetism. Further, our data suggests that, whenever magnetism is present, magnetic and superconducting fluctuations exist across wide ranges of temperatures above their respective, bulk transition temperatures. We compare this finding to the phase diagrams of other high-temperature superconductors.
Second, we present the first realization of elastoresistance measurements under hydrostatic pressures [3] and show our proof-of-principle data on BaFe₂As₂. This novel approach will now allow for the study of the interaction between nematicity and superconductivity by fine-tuning iron-based superconductors without introducing additional disorder.

[1] Gati et al., Ann. Phys. 532, 2000248 (2020); [2] Gati et al., Rev. Sci. Instrum. 90, 023911 (2019); [3] Gati et al., Rev. Sci. Instrum. 91, 023904 (2020); [4] Gati et al., PRL 123, 167002 (2019).