Unconventional Superconductivity in the Layered Iron Germanide YFe2Ge2

Abstract

Since the discovery of superconductivity in LaFePO, numerous iron-based superconductors have been identified within diverse structure families. Superconductivity in the layered iron germanide YFe$_2$Ge$_2$ was first reported in 2014. It stands out from the commonly known iron- based superconductor families for not containing either Group-V or Group-VI elements and has since been predicted to be an unconventional superconductor. The intermetallic $d$-electron system YFe$_2$Ge$_2$ exhibits an unusually high Sommerfeld coefficient of $\approx \SI{100}{\milli\joule/\mole\kelvin^2}$, signalling strong electronic correlations. Its low-temperature normal-state resistivity displays a $T^{1.5}$ power-law temperature dependence, which is an indication of non-Fermi-liquid behaviour. While superconductivity in YFe$_2$Ge$_2$ has been widely observed below $T_c \approx \SI{1.9}{\kelvin}$ in electric transport measurements, evidence of a bulk superconducting transition has proved elusive. This has prompted significant efforts into improving the crystal quality. In this thesis, I present the crystal growth methods which have successfully produced high-quality poly- and single-crystal YFe$_2$Ge$_2$ samples. Measurements on these samples have led to conclusive evidence that superconductivity is an intrinsic property of this compound. Disorder effects on both the poly- and single-crystals have been studied through structural investigations, in which anti-site disorder of germanium substitution on the iron site was found to be the dominant factor. The fast suppression of the superconducting transition temperature, $T_c$, of YFe$2$Ge$2$ by disorder suggests an unconventional pairing mechanism. Using a liquid transport flux method, single crystals with residual resistivity ratios ($\mathrm{RRR} = \mathrm{\rho}{\SI{300}{\kelvin}}/\mathrm{\rho}{\SI{2}{\kelvin}}$) reaching 470 have been synthesised. These crystals exhibit clear bulk superconducting transitions. Low-temperature specific heat and $\mu$SR measurements performed on these crystals provided evidence for multi-gap superconductivity, most likely of the $s^\pm$-wave nature, which is compatible with theoretical predictions. Moreover, quantum oscillations have been detected for the first time in dHvA susceptibility and tunnel-diode oscillation measurements of high-quality YFe$_2$Ge$_2$ single crystals. Although unable to account fully for the high Sommerfeld coefficient, the current results have confirmed significant mass enhancements in the detected Fermi surface sheets.