Vpliv orientacije magnetnega polja na stanja kvantnega magneta na mreži satovja

Mejak, Maj (author), Klanjšek, Martin (mentor)

Kvantni materiali, v katerih fiziko narekujejo frustrirane spinske prostostne stopnje na mreži satovja, imajo velik potencial za realizacijo eksotičnega stanja kvantne spinske tekočine, za katerega je poleg kvantne prepletenosti ter odsotnosti magnetne ureditve značilna predvsem frakcionalizacija spinskih prostostnih stopenj. Tovrstno obnašanje v svojem osnovnem stanju napoveduje tudi analitično rešljivi spinski model Kitaeva na mreži satovja. Posebne lastnosti frakcijskih vzbuditev v modelu Kitaeva ter morebitne aplikacije modela v kvantnem računalništvu so motivirale eksperimentalno iskanje njegovih materialnih realizacij. Med potencialne realizacije modela Kitaeva sodijo tudi kobaltovi sistemi, kjer kristalno polje in močna sklopitev spin-tir vodi do anizotropnih izmenjalnih interakcij med efektivnimi spini kobaltovih ionov Co$^{2+}$ na mreži satovja. Poleg modela Kitaeva bi fiziko lokaliziranih spinov v kobaltovih sistemih morda še bolje lahko opisal Heisenbergov XXZ model z dodatno interakcijo med tretjimi najbližjimi sosedi na mreži satovja. Na stanja kobaltovih sistemov poleg temperature pomembno vpliva tudi zunanje magnetno polje, s katerim je mogoče izničiti magnetno ureditev, ki pri nizkih temperaturah izpodrine stanje kvantne spinske tekočine. V magistrskem delu je eksperimentalno analiziran vpliv orientacije magnetnega polja na stanja kobaltovega magneta BaCo$_2$(AsO$_4$)$_2$ z metodo jedrske magnetne resonance. Rezultati meritev kažejo, da je možno z orientacijo magnetnega polja glede na vzorec spreminjati kritično vrednost polja, ki sicer izpodrine magnetno ureditev pri nizkih temperaturah, vendar ne vodi v stanje kvantne spinske tekočine, temveč v magnetno polarizirano fazo. Kljub temu je mogoče pri nekoliko višjih temperaturah v korelirani paramagnetni fazi opaziti značilne lastnosti frakcijskih vzbuditev, ki jim ustreza širok maksimum v temperaturni odvisnosti spinsko-mrežne relaksacijske hitrosti $T_{1}^{-1}(T)$. Dobljeni eksperimentalni rezultati sicer potrdijo navzočnost frakcijskih vzbuditev v korelirani paramagnetni fazi, vendar ne morejo nedvoumno potrditi, ali magnetne lastnosti kobaltovega sistema BaCo$_2$(AsO$_4$)$_2$ bolje opiše model Kitaeva ali $J_{1}$-$J_{3}$ XXZ Heisenbergov model.

• Language: Slovenian
• Keywords: kvantni magnetizem, magnetna frustracija, kvantna spinska tekočina, model Kitaeva, mreža satovja, frakcionalizacija, kristalno polje, $\alpha$-RuCl$_3$, BaCo$_2$(AsO$_4$)$_2$, jedrska magnetna resonanca, hiperfina sklopitev, kvadrupolna interakcija
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2023
• PID: 20.500.12556/RUL-151095
• COBISS.SI-ID: 164261635
• Publication date in RUL: 29.09.2023

Secondary language

• Language: English
• Title: The influence of magnetic field orientation on the states of a quantum magnet on a honeycomb lattice

Abstract

Quantum materials, whose physics is governed by frustrated spin degrees of freedom, hold significant potential for realizing exotic states like quantum spin liquids. Such a behaviour is also predicted in the ground state of the analytically solvable Kitaev model on a honeycomb lattice. The specific properties of fractional excitations in the Kitaev model and their potential applications in quantum computing have motivated the search for its experimental realizations. Among promising realizations of the model are cobaltates, where crystal field effects and strong spin-orbit coupling lead to anisotropic exchange interactions between the effective spin degrees of freedom of the cobalt ions. Alternatively, the physics of localized spins in cobaltates could potentially be better described by the Heisenberg XXZ model with additional interactions between third-nearest neighbours on a honeycomb lattice. The states of cobalt-based systems are significantly influenced by an external magnetic field, which suppresses the magnetic order that replaces the quantum spin liquid state at low temperatures. This master's thesis analyses the influence of magnetic field orientation on the states of the cobalt-based magnet BaCo$_2$(AsO$_4$)$_2$ using nuclear magnetic resonance. Experimental results demonstrate that varying the orientation of the magnetic field can change the critical value of the magnetic field for the suppression of the magnetic order, although this does not lead to a quantum spin liquid but rather to a magnetically polarized state. Nevertheless, at slightly higher temperatures, in the correlated paramagnetic phase, distinctive characteristics of fractional spin excitations can be observed. These excitations lead to a broad maximum in the temperature-dependent spin-lattice relaxation rate $T_{1}^{-1}(T)$. The obtained experimental results confirm the presence of fractional excitations in the correlated paramagnetic phase. However, they cannot conclude whether the magnetism of the cobalt-based system BaCo$_2$(AsO$_4$)$_2$ is better described by the Kitaev model or the $J_{1}$-$J_{3}$ XXZ Heisenberg model.

• Keywords: quantum magnetism, magnetic frustration, quantum spin liquid, Kitaev model, honeycomb lattice, fractionalization, crystal field, $\alpha$-RuCl$_3$, BaCo$_2$(AsO$_4$)$_2$, nuclear magnetic resonance, hyperfine coupling, quadrupole coupling