Unconventional Superconductivity in the Layered Iron Germanide YFe2Ge2

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 \approx \SI{100}{\milli\joule/\mole\kelvin²}, 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 YFe₂Ge₂ 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 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, T_c, of YFe₂Ge₂ by disorder suggests an unconventional pairing mechanism. Using a liquid transport flux method, single crystals with residual resistivity ratios (RRR = \rho_{\SI{300}{\kelvin}}/\rho_{\SI{2}{\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^\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 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.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2020

Author dc:creator

• Chen, Jiasheng

Advisor dc:contributor.advisor

• Grosche, Malte

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

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/975f7578-6f7e-4919-8798-ddf8402eb2e6/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

en

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.48576

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/301507