20.500.12556/RUL-184468 Pasovna struktura in simetrija superprevodne reže v dopiranem FeSe Band structure and superconducting gap symmetry of doped FeSe Dopiran železov superprevodnik Li$_x$(C$_5$H$_5$N)$_y$Fe$_2$Se$_2$ z $x \sim 0.6$ in $y \sim 0.7-0.9$ je raziskan z uporabo jedrske magnetne resonance ali NMR. Meritve spinsko-mrežnega relaksacijskega časa $T_1$ na selenovih jedrih, ki so kot del FeSe ravnin ključnega pomena za superprevodnost, pokažejo nekonvencionalno superprevodnost pod kritično temperaturo $T_c = 39\,\mathrm{K}$. Temperaturna odvisnost $T_1$ v superprevodni fazi je skladna s tako imenovano $s^\pm$ simetrijo superprevodne reže. V normalni fazi selenov $T_1$ in frekvenčni premik njegovega NMR spektra sledita nenavadno močni temperaturni odvisnosti, ki ju opišemo z večpasovnim modelom, kjer je zgornji rob enega od pasov za $\tilde{\Delta} = 58\,\mathrm{meV}$ pod Fermijevo energijo. Meritve litijevih NMR spektrov pokažejo, da ima litij pasivno vlogo pri določanju elektronskih lastnosti v normalni in superprevodni fazi. Pomemben je zgolj z vidika dopiranja in strukturne stabilnosti materiala. Izredno počasna relaksacija litijevih jeder namiguje na popolnoma razklopljene ravnine FeSe in dvodimenzionalno fiziko v njih. A doped iron-based superconductor Li$_x$(C$_5$H$_5$N)$_y$Fe$_2$Se$_2$ with $x \sim 0.6$ and $y \sim 0.7-0.9$ is investigated using nuclear magnetic resonance (NMR). Measurements of the spin-lattice relaxation time $T_1$ on selenium nuclei, which are key to superconductivity as a part of the FeSe planes, reveal unconventional superconductivity below the critical temperature $T_c = 39\,\mathrm{K}$. Temperature dependence of $T_1$ in the superconducting state is consistent with the so-called $s^\pm$ symmetry of the superconducting gap. In the normal state, the selenium $T_1$ and the frequency shift of its NMR spectrum follow an unusually strong temperature dependence, which is described using a multiband model with one of the valence bands having its upper edge at $\tilde{\Delta} = 58\,\mathrm{meV}$ below the Fermi energy. Measurements of lithium NMR spectra show that lithium plays a passive role in determining electronic characteristics in the normal and the superconducting state. It is important solely from the perspective of doping and structural stability of the material. Extremely slow relaxation of lithium nuclei suggests completely decoupled FeSe planes and two-dimensional physics within them. nekonvencionalna superprevodnost železovi superprevodniki elektronska pasovna struktura simetrija superprevodne reže NMR unconventional superconductivity iron-based superconductors electron band structure superconducting gap symmetry NMR true false false Slovenski jezik Angleški jezik Magistrsko delo/naloga 2026-07-08 08:15:20 2026-07-08 08:15:29 2026-07-23 13:46:45 0000-00-00 00:00:00 2026 0 0 0000-00-00 NiDoloceno NiDoloceno NiDoloceno 0000-00-00 0000-00-00 0000-00-00 160609 285856771 22036.pdf 22036.pdf 1 C7DEE505C507728E1AA95E72E2F67982 de684cc3f3900f1bbfa4436068709c17af9dbd1338ddedbafcaa164ca2d3481f 4c5c5aca-7a94-11f1-9b0d-0050569b8976 https://repozitorij.uni-lj.si/Dokument.php?lang=slv&id=237297 Fakulteta za matematiko in fiziko 0 0 0