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Unconventional Superconductivity in the Layered Iron Germanide YFe2Ge2



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Chen, Jiasheng 


Since the discovery of superconductivity in LaFePO, numerous iron-based superconductors have been identified within diverse structure families. Superconductivity in the layered iron germanide YFe2Ge2 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 YFe2Ge2 exhibits an unusually high Sommerfeld coefficient of ≈\SI100\milli\joule/\mole\kelvin2, signalling strong electronic correlations. Its low-temperature normal-state resistivity displays a T1.5 power-law temperature dependence, which is an indication of non-Fermi-liquid behaviour. While superconductivity in YFe2Ge2 has been widely observed below Tc≈\SI1.9\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 YFe2Ge2 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, Tc, of YFe2Ge2 by disorder suggests an unconventional pairing mechanism. Using a liquid transport flux method, single crystals with residual resistivity ratios (RRR=ρ\SI300\kelvin/ρ\SI2\kelvin) reaching 470 have been synthesised. These crystals exhibit clear bulk superconducting transitions. Low-temperature specific heat and μSR measurements performed on these crystals provided evidence for multi-gap superconductivity, most likely of the s±-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 YFe2Ge2 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.





Grosche, Malte


Iron-based superconductor, Unconventional superconductivity, YFe2Ge2, Iron germanide


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
Trinity College; EPSRC of the UK (Grants No. EP/K012894 and EP/P023290/1)