Crystal growth and annealing study of fragile, non-bulk superconductivity in YFe$_2$Ge$_2$

TL;DR

This study explores the fragile, filamentary superconductivity in YFe$_2$Ge$_2$ crystals, showing that growth and annealing improve crystal quality but do not produce bulk superconductivity, suggesting strain or secondary phases as possible causes.

Contribution

It demonstrates that high-quality YFe$_2$Ge$_2$ crystals exhibit superconductivity signatures without bulk phase evidence, highlighting the role of strain or minor phases in superconductivity.

Findings

RRR can be increased to ~60 through specific growth and annealing.

Superconductivity signatures appear in resistivity but not in specific heat.

Superconductivity may be filamentary or due to secondary phases.

Abstract

We investigated the occurrence and nature of superconductivity in single crystals of YFe$_2$Ge$_2$ 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 $\sim$60, by decanting the crystals from the molten Sn at $\sim$350$^\circ$C and/or by annealing at temperatures between 550$^\circ$C and 600$^\circ$C. We found that samples with RRR $\gtrsim$ 34 showed resistive signatures of superconductivity with the onset of the superconducting transition $T_c\approx1.4$ K. RRR values vary between 35 and 65 with, on average, no systematic change in $T_c$ value, indicating that systematic changes in RRR do not lead to comparable changes in $T_c$. Specific heat measurements on samples that showed 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 YFe$_2$Ge$_2$ (perhaps at twin boundaries or dislocations) or is a second crystallographic phase present at levels below detection capability of conventional powder x-ray techniques.