{"id":{"repo_id":"cambridge","oai_identifier":"oai:www.repository.cam.ac.uk:1810/301507"},"canonical_url":"https://search.dev.ndltd.org/etd/cambridge/oai:www.repository.cam.ac.uk:1810/301507","repository":{"repo_id":"cambridge","name":"Cambridge University","base_url":"https://api.repository.cam.ac.uk/server/oai/request"},"display":{"title":"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.","abstract_html":"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<span class=\"etd-inline-math\"><sub>2</sub></span>Ge<span class=\"etd-inline-math\"><sub>2</sub></span> 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<span class=\"etd-inline-math\"><sub>2</sub></span>Ge<span class=\"etd-inline-math\"><sub>2</sub></span> exhibits an unusually high Sommerfeld coefficient of <span class=\"etd-inline-math\">\\approx \\SI{100}{\\milli\\joule/\\mole\\kelvin<sup>2</sup>}</span>, signalling strong electronic correlations. Its low-temperature normal-state resistivity displays a <span class=\"etd-inline-math\">T<sup>1.5</sup></span> power-law temperature dependence, which is an indication of non-Fermi-liquid behaviour. While superconductivity in YFe<span class=\"etd-inline-math\"><sub>2</sub></span>Ge<span class=\"etd-inline-math\"><sub>2</sub></span> has been widely observed below <span class=\"etd-inline-math\">T<sub>c</sub> \\approx \\SI{1.9}{\\kelvin}</span> 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<span class=\"etd-inline-math\"><sub>2</sub></span>Ge<span class=\"etd-inline-math\"><sub>2</sub></span> 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, <span class=\"etd-inline-math\">T<sub>c</sub></span>, of YFe<span class=\"etd-inline-math\"><sub>2</sub></span>Ge<span class=\"etd-inline-math\"><sub>2</sub></span> by disorder suggests an unconventional pairing mechanism. Using a liquid transport flux method, single crystals with residual resistivity ratios (<span class=\"etd-inline-math\"><span class=\"etd-inline-math-roman\">RRR</span> = <span class=\"etd-inline-math-roman\">\\rho</span><sub>\\SI{300}{\\kelvin}</sub>/<span class=\"etd-inline-math-roman\">\\rho</span><sub>\\SI{2}{\\kelvin}</sub></span>) reaching 470 have been synthesised. These crystals exhibit clear bulk superconducting transitions. Low-temperature specific heat and <span class=\"etd-inline-math\">&mu;</span>SR measurements performed on these crystals provided evidence for multi-gap superconductivity, most likely of the <span class=\"etd-inline-math\">s<sup>\\</sup>pm</span>-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<span class=\"etd-inline-math\"><sub>2</sub></span>Ge<span class=\"etd-inline-math\"><sub>2</sub></span> 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.","abstract_has_math":true,"creators":["Chen, Jiasheng"],"institution":"University of Cambridge","degree_name":"Doctor of Philosophy (PhD)","degree_level":"Doctoral","degree_discipline":null,"degree_department":null,"school":null,"contributors":[],"advisors":["Grosche, Malte"],"committee_chairs":[],"committee_members":[],"year":2020,"date_issued":"2020-01-30","date_published":"2020-01-30","updated_at":"2026-07-22T22:24:30Z","subjects":["Iron-based superconductor","Unconventional superconductivity","YFe2Ge2","Iron germanide"],"languages":["en"],"rights":[],"rights_urls":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/975f7578-6f7e-4919-8798-ddf8402eb2e6/download","https://www.rioxx.net/licenses/all-rights-reserved/"],"identifier_entries":[]},"links":{"outbound_url":"https://doi.org/10.17863/CAM.48576","outbound_label":"DOI","outbound_source":"dc:identifier.doi"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["Grosche, Malte"]},{"key":"dc:contributor.sponsor","label":"Sponsor","values":["Trinity College; EPSRC of the UK (Grants No. EP/K012894 and EP/P023290/1)"]},{"key":"dc:creator","label":"Author","values":["Chen, Jiasheng"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date.issued","label":"Date","values":["2020-01-30"]},{"key":"dc:publisher.institution","label":"Dc Publisher Institution","values":["University of Cambridge"]},{"key":"dc:relation.isreferencedby.uri","label":"Dc Relation Isreferencedby URI","values":["https://www.repository.cam.ac.uk/handle/1810/301507"]},{"key":"dc:type","label":"Dc Type","values":["Thesis"]},{"key":"dc:type.qualificationlevel","label":"Dc Type Qualificationlevel","values":["Doctoral"]},{"key":"dc:type.qualificationname","label":"Dc Type Qualificationname","values":["Doctor of Philosophy (PhD)"]}]},{"id":"subjects_keywords","label":"Subjects and Keywords","entries":[{"key":"dc:subject","label":"Dc Subject","values":["Iron-based superconductor","Unconventional superconductivity","YFe2Ge2","Iron germanide"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language","label":"Dc Language","values":["en"]},{"key":"dc:rights","label":"Dc Rights","values":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/975f7578-6f7e-4919-8798-ddf8402eb2e6/download","https://www.rioxx.net/licenses/all-rights-reserved/"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier.doi","label":"DOI","values":["10.17863/CAM.48576"]},{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/9dfff4d6-343c-46b0-8b16-a93e13e0c174/download"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["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."]},{"key":"dc:format.checksum.md5","label":"Dc Format Checksum Md5","values":["5d89ddd84a041ec79733c7a5027f2f48","87eda9de84448d1f82354d60eee3eb5f"]},{"key":"dc:title","label":"Title","values":["Unconventional Superconductivity in the Layered Iron Germanide YFe2Ge2"]}]}],"canonical_facts":{"dc:contributor.advisor":["Grosche, Malte"],"dc:contributor.sponsor":["Trinity College; EPSRC of the UK (Grants No. EP/K012894 and EP/P023290/1)"],"dc:creator":["Chen, Jiasheng"],"dc:date.issued":["2020-01-30"],"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:format.checksum.md5":["5d89ddd84a041ec79733c7a5027f2f48","87eda9de84448d1f82354d60eee3eb5f"],"dc:identifier.doi":["10.17863/CAM.48576"],"dc:identifier.uri":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/9dfff4d6-343c-46b0-8b16-a93e13e0c174/download"],"dc:language":["en"],"dc:publisher.institution":["University of Cambridge"],"dc:relation.isreferencedby.uri":["https://www.repository.cam.ac.uk/handle/1810/301507"],"dc:rights":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/975f7578-6f7e-4919-8798-ddf8402eb2e6/download","https://www.rioxx.net/licenses/all-rights-reserved/"],"dc:subject":["Iron-based superconductor","Unconventional superconductivity","YFe2Ge2","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)"]},"updated_at":"2026-07-22T22:24:30Z"}