dc.contributor.author Chen, Jiasheng dc.date.accessioned 2020-01-31T14:37:51Z dc.date.available 2020-01-31T14:37:51Z dc.date.issued 2020-01-30 dc.date.submitted 2019-10-31 dc.identifier.uri https://www.repository.cam.ac.uk/handle/1810/301507 dc.description.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. dc.description.sponsorship Trinity College; EPSRC of the UK (Grants No. EP/K012894 and EP/P023290/1) dc.language.iso en dc.rights All rights reserved dc.rights All Rights Reserved en dc.rights.uri https://www.rioxx.net/licenses/all-rights-reserved/ en dc.subject Iron-based superconductor dc.subject Unconventional superconductivity dc.subject YFe2Ge2 dc.subject Iron germanide dc.title Unconventional Superconductivity in the Layered Iron Germanide YFe2Ge2 dc.type Thesis dc.type.qualificationlevel Doctoral dc.type.qualificationname Doctor of Philosophy (PhD) dc.publisher.institution University of Cambridge dc.publisher.department Physics dc.date.updated 2020-01-29T10:38:11Z dc.identifier.doi 10.17863/CAM.48576 dc.publisher.college Trinity College dc.type.qualificationtitle Doctor of Philosophy in Physics cam.supervisor Grosche, Malte cam.thesis.funding false rioxxterms.freetoread.startdate 2021-01-31
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