Collective excitations of dynamic Fermi surface deformations in BaFe2(As0.5P0.5)2

We use electronic Raman scattering to study the low-energy excitations in BaFe₂(As_0.5P_0.5)₂ (Tc≈16 K) samples. In addition to a superconducting pair breaking peak (2Δ=6.7 meV) in the A_1g channel with a linear tail towards zero energy, suggesting a nodal gap structure, we detect spectral features associated to Pomeranchuk oscillations in the A_1g, B_1g and B_2g channels. We argue that the small Fermi energy of the system is an essential condition for these Pomeranchuk oscillations to be underdamped. The Pomeranchuk oscillations have the same frequencies in the B_1g and B_2g channels, which we explain by the mixing of these symmetries resulting from the removal of the σ_v and σ_d symmetry planes due to a large As/P disorder. Interestingly, we show that the temperature at which the peaks corresponding to the Pomeranchuk oscillations get underdamped is consistent with the non-Fermi liquid to Fermi liquid crossover determined by transport, suggesting that the Pomeranchuk instability plays an important role in the low-energy physics of the Fe-based superconductors.