Fermi Surface and Mass Renormalization in the Iron-Based Superconductor YFe_{2}Ge_{2}.

Interaction-enhanced carrier masses are central to the phenomenology of iron-based superconductors. Quantum oscillation measurements in the new unconventional superconductor YFe_{2}Ge_{2} resolve all four Fermi surface pockets expected from band structure calculations, which predict an electron pocket in the Brillouin zone corner and three hole pockets enveloping the centers of the top and bottom of the Brillouin zone. Carrier masses reach up to 20 times the bare electron mass and are among the highest ever observed in any iron-based material, accounting for the enhanced heat capacity Sommerfeld coefficient ≃100  mJ/mol K^{2}.

Unconventional Bulk Superconductivity in YFe_{2}Ge_{2} Single Crystals.

Sharp superconducting transition anomalies observed in a new generation of single crystals establish that bulk superconductivity is intrinsic to high purity YFe_{2}Ge_{2}. Low temperature heat capacity measurements suggest a disorder and field dependent residual Sommerfeld coefficient, consistent with disorder-induced in-gap states as expected for a sign-changing order parameter. The sevenfold reduction in disorder scattering in these new crystals to residual resistivities ≃0.