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Jedrska magnetna resonanca superprevodnika i-FeSe
Majcen Hrovat, Matevž (Author), Arčon, Denis (Mentor) More about this mentor... This link opens in a new window, Jeglič, Peter (Co-mentor)

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Abstract
V magistrskem delu smo z jedrsko magnetno resonanco (NMR) na jedrih 77Se, 7Li in 1H preučevali magnetne, strukturne in superprevodne lastnosti praškastega vzorca Lix(C2H8N2)y Fe(2−z)Se2 in jih primerjali z lastnostmi železovega superprevodnika FeSe. Kritična temperatura Tc, pod katero material preide v superprevodno stanje, se močno poveča v materialu FeSe z vstavljanjem dodatnih elementov med FeSe ravnine. FeSe ima Tc = 8 K, medtem ko je v našem vzorcu kritična temperatura dvignjena na skoraj optimalen Tc = 45 K. Odsotnost premika NMR spektra 7Li in 1H nakazuje, da so elektroni v prevodni ravnini FeSe popolnoma ločeni od Lix(C2H8N2)y izolatorskih ravnin. Meritve 7Li in 1H spinsko-mrežnih relaksacijskih časov pokažejo šibko temperaturno odvisnost, kar nakazuje, da imamo v izolatorskih ravninah nezanemarljive količine Fe nečistoč. Meritve 77Se NMR spektra in spinsko-mrežnih relaksacijskih časov 1/77T1 pokažejo močno temperaturno odvisnost, ki jo lahko pojasnimo s pasovi blizu Fermijeve energije. Pri temperaturah nad Tc spinsko-mrežni relaksacijski časi 1/77T1 ne kažejo anizotropije, kar nakazuje na odsotnost elektronskega nematičnega reda. Odsotnost ojačanja 1/77T1 v normalnem stanju pa nakazuje, da spinske fluktuacije nimajo znatnega vpliva na spinsko-mrežno relaksacijo. Kljub temu pa je značilna potenčna odvisnost 1/77T1 ∝ T^(4,5) v superprevodnem stanju v skladu s teorijo, da so Cooperjevi pari vezani preko spinskih fluktuacij.

Language:Slovenian
Keywords:jedrska magnetna resonanca, železovi superprevodniki, FeSe
Work type:Master's thesis/paper (mb22)
Tipology:2.09 - Master's Thesis
Organization:FMF - Faculty of Mathematics and Physics
Year:2018
COBISS.SI-ID:3225956 This link opens in a new window
Views:1533
Downloads:395
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Secondary language

Language:English
Title:Nuclear magnetic resonance in superconductor i-FeSe
Abstract:
In the thesis, we report a 77Se, 7Li and 1H nuclear magnetic resonance (NMR) study of magnetic, structural and superconducting properties of powdered Lix(C2H8N2)yFe(2−z)Se2 sample in comparison to iron-based superconductor FeSe. The critical superconducting temperature, Tc, of FeSe can be dramatically enhanced by intercalation of layers between FeSe layers. In our sample critical temperature rises from initial Tc = 8 K to nearly optimal Tc = 45 K. Absence of shift in 7Li and 1H NMR spectra indicates a total decoupling of conducting FeSe layer and insulating Lix(C2H8N2)y layers. Measurements of 7Li and 1H spin-lattice relaxation rates show weak temperature dependence which indicates that there are non-negligible Fe impurities in insulating layer. Measurements of 77Se NMR spectra and spin-lattice relaxation rates, 1/77T1, show strong temperature dependence which can be attributed to the hole-like bands near Fermi energy. Relaxation rates 1/77T1 show no anisotropy which would indicate the onset of electronic nematic order. Similarly, enhancements in 1/77T1 due to spin fluctuations in normal state are absent. However, a characteristic power-law 1/77T1 ∝ T^(4,5) indicates that Cooper pair bonding is mediated by spin fluctuations.

Keywords:nuclear magnetic resonance, iron-based superconductors, FeSe

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