Crystal growth and annealing study of fragile, non-bulk superconductivity in YFe2Ge2

We investigated the occurrence and nature of superconductivity in single\ncrystals of YFe$_2$Ge$_2$ grown out of Sn flux by employing x-ray diffraction,\nelectrical resistivity, and specific heat measurements. We found that the\nresidual resistivity ratio (RRR) of single crystals can be greatly improved,\nreaching as high as $\\sim$60, by decanting the crystals from the molten Sn at\n$\\sim$350$^\\circ$C and/or by annealing at temperatures between 550$^\\circ$C and\n600$^\\circ$C. We found that samples with RRR $\\gtrsim$ 34 showed resistive\nsignatures of superconductivity with the onset of the superconducting\ntransition $T_c\\approx1.4$ K. RRR values vary between 35 and 65 with, on\naverage, no systematic change in $T_c$ value, indicating that systematic\nchanges in RRR do not lead to comparable changes in $T_c$. Specific heat\nmeasurements on samples that showed clear resistive signatures of a\nsuperconducting transition did not show any signature of a superconducting\nphase transition, which suggests that the superconductivity observed in this\ncompound is either some sort of filamentary, strain stabilized\nsuperconductivity associated with small amounts of stressed YFe$_2$Ge$_2$\n(perhaps at twin boundaries or dislocations) or is a second crystallographic\nphase present at levels below detection capability of conventional powder x-ray\ntechniques.\n