Jedrska magnetna resonanca superprevodnika i-FeSe

Majcen Hrovat, Matevž (author), Arčon, Denis (mentor), Jeglič, Peter (co-mentor)

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
• Typology: 2.09 - Master's Thesis
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2018
• PID: 20.500.12556/RUL-102258
• COBISS.SI-ID: 3225956
• Publication date in RUL: 28.07.2018

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