Strain-Driven Approach to Quantum Criticality inAFe2As2withA=K, Rb, and Cs

The iron-based superconductors $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ with $A=\mathrm{K}$, Rb, Cs exhibit large Sommerfeld coefficients approaching those of heavy-fermion systems. We have investigated the magnetostriction and thermal expansion of this series to shed light on this unusual behavior. Quantum oscillations of the magnetostriction allow identifying the band-specific quasiparticle masses which by far exceed the band-structure derived masses. The divergence of the Grüneisen ratio derived from thermal expansion indicates that with increasing volume along the series a quantum critical point is approached. The critical fluctuations responsible for the enhancement of the quasiparticle masses appear to weaken the superconducting state.