Crystal Growth and Annealing Study of Fragile, Non-Bulk Superconductivity in YFe₂Ge₂

We investigated the occurrence and nature of superconductivity in single crystals of YFe2Ge2 grown out of Sn flux by employing X-ray diffraction, electrical resistivity and specific heat measurements. We found that the residual resistivity ratio (RRR) of single crystals can be greatly improved, reaching as high as ~60, by decanting the crystals from the molten Sn at ~350°C and/or by annealing at temperatures between 550 and 600°C. We found that the samples with RRR ≳ 34 showed resistive signatures of superconductivity with the onset of the superconducting transition Tc ≈ 1.4K. RRR values vary between 35 and 65 with, on average, no systematic change in Tc value, indicating that the systematic changes in RRR do not lead to comparable changes in Tc. Specific heat measurements on samples that showed the clear resistive signatures of a superconducting transition did not show any signature of a superconducting phase transition, which suggests that the superconductivity observed in this compound is either some sort of filamentary, strain-stabilized superconductivity associated with small amounts of stressed YFe2Ge2 (perhaps at twin boundaries or dislocations) or is a second crystallographic phase that is present at level below detection capability of conventional powder X-ray techniques