Študija kvazi-enodimenzionalnega superprevodnika Cs$_2$Mo$_3$As$_3$ z mionsko spinsko relaksacijo

Jaksetič, Kevin (author), Arčon, Denis (mentor), Arh, Tina (comentor)

Za enodimenzionalne elektronske sisteme so značilne močne nabojne in spinske fluktuacije, ki v realnih sistemih zaradi prisotnosti sklopitev med posameznimi žičkami povzročijo nestabilnosti v različne urejene faze pri dovolj nizkih temperaturah. Še posebej so zanimivi primeri, kjer kvazi-enodimenzionalni sistemi postanejo superprevodni, ker ima tam lahko superprevodni ureditveni parameter različne simetrije. V svojem magistrskem delu sem se posvetil nedavno odkritemu sistemu Cs$_2$Mo$_3$As$_3$, v katerem Mo in As atomi tvorijo prevodne „nanožičke“ vzdolž kristalografske $c$-osi. Sistem Cs$_2$Mo$_3$As$_3$ je superprevoden pod dokaj visoko kritično temperaturo $T_\mathrm{c} = 11.5~\text{K}$ – najvišjo med znanimi kvazi-enodimenzionalnimi superprevodniki. Simetrijo superprevodnega ureditvenega parametra sem poskušal določiti z mionsko spinsko relaksacijo ($\mathrm{\mu SR}$) v šibkem polju, pravokotnem na začetno polarizacijo mionskih spinov. Tako v normalnem kot tudi v superprevodnem stanju mionski signal kaže značilne oscilacije zaradi Larmorjeve precesije mionskih momentov, ki pa so v superprevodnem stanju še dodatno zadušene. To dušenje Gaussovske oblike sem pripisal vzpostavitvi mreže vrtinčnih niti, ki močno razširi statično porazdelitev polj v katerih mionski momenti precesirajo. Primerjava z modelnimi računi, kjer sem vzel v obzir tri različne možne simetrije ureditvenega parametra ($s$-, $p$-, ali $d$-simetrije), pokaže najboljše ujemanje za scenarija s simetrijo $s$ in $p$, ter bistveno slabše ujemanje za tretjo možnost. Iz analiz sem tudi določil velikost ureditvenega parametra pri temperaturi 0 ($\Delta_0$) ter izračunal razmerji $2\Delta_0/k_\mathrm{B}T_\mathrm{c} = 4.57(1)$ in $5.98(1)$ za modela s $s$- in $p$-simetrijo. Predstavljene $\mathrm{\mu SR}$ meritve ne morejo nedvoumno določiti narave superprevodnega stanja v sistemu Cs$_2$Mo$_3$As$_3$. Kljub temu pa smo pokazali, da v tem primeru superprevodnega stanja ne moremo enostavno opisati znotraj standardne Bardeen-Cooper-Schrieffer teorije.

• Language: Slovenian
• Keywords: kvazi-enodimenzionalni superprevodniki, nekonvencionalna superprevodnost, simetrija superprevodnega ureditvenega parametra, mionska spinska relaksacija
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2024
• PID: 20.500.12556/RUL-161811
• COBISS.SI-ID: 207835651
• Publication date in RUL: 14.09.2024

Secondary language

• Language: English
• Title: Muon spin relaxation study of a quasi-one-dimensional superconductor Cs$_2$Mo$_3$As$_3$

Abstract

One-dimensional electronic systems are characterized by strong charge and spin fluctuations, which due to coupling between individual wires in real systems lead to instabilities into various ordered phases at sufficiently low temperatures. Especially interesting are the cases where quasi-one-dimensional systems before superconducting, since the superconducting order parameter can have different symmetries. In this master’s thesis I focused on a recently discovered system Cs$_2$Mo$_3$As$_3$, in which Mo and As atoms form conductive “nanowires” along the crystallographic $c$-axis. The material is superconducting below a relatively high critical temperature $T_\mathrm{c} = 11.5~\text{K}$ – the highest among the known quasi-one-dimensional superconductors. I attempted to determine the superconducting order parameter symmetry using muon spin relaxation ($\mathrm{\mu SR}$) in a weak field perpendicular to the initial polarisation of muon spins. In both the normal and superconducting state the muon signal shows characteristic oscillations due to the Larmor precession of the muon magnetic moments, which is additionally dampened in the superconducting state. I ascribed this Gaussian-shaped damping to the formation of a vortex lattice, which significantly broadens the static field distribution in which the muons precess. The comparison of different order parameter symmetries, where I considered the $s$-, $p$-, $d$-symmetries, showed best agreement for scenarios with $s$- and $p$- symmetry and significantly worse agreement for the third option. From the analyses I also determined the magnitude of the order parameter at zero temperature $\Delta_0$ and calculated the ratios $2\Delta_0/k_\mathrm{B}T_\mathrm{c} = 4.57(1)$ in $5.98(1)$ for the $s$- and $p$- symmetry models, respectively. The presented $\mathrm{\mu SR}$ data cannot unambiguously determine the nature of the superconducting state of Cs$_2$Mo$_3$As$_3$, but have nevertheless shown that this case of superconductivity cannot be described simply by the standard Bardeen-Cooper-Schrieffer theory.

• Keywords: quasi-one-dimensional superconductors, unconventional superconductivity, superconducting order parameter symmetry, muon spin relaxation