Quantum Magnetism in π-electron Molecular Systems

Knaflič, Tilen (author), Arčon, Denis (mentor)

In this Thesis, we study quantum magnetism in molecular systems of $\pi$-orbital electrons. We use spectroscopic methods such as electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR) and muon spin rotation/relaxation ($\mu$SR) to probe the static and dynamic properties of the magnetic state in selected compounds at the local scale. The studied systems include alkali superoxides and sesquioxides (AO$_2$ and A$_4$O$_6$, where A = Cs, Rb, Na), with the O$_2^-$ superoxide ion as the source of the principal magnetic moment, and the aromatic crystal systems, where the unpaired electrons occupying their $\pi$ molecular orbitals form quasi one-dimensional structures. Orbital ordering leading to strong antiferromagnetic interactions along $b$ crystal axis was suggested for the CsO$_2$ system. With the use of theoretical models based on quantum field theory and bosonization technique, the CsO$_2$ EPR data analysis shows good agreement with one-dimensional spin chain and the underlying Tomonaga-Luttinger liquid framework. Additionally, we discuss the presence of Ising-type anisotropy. The Cs$_4$O$_6$ and Rb$_4$O$_6$ systems' oxygen units are at room temperature in an average charge state O$_2^{-4/3}$. Extensive EPR, NMR and $\mu$SR measurements on both samples lead us to the discovery of a Verwey-type structural and charge ordering transition from high-temperature cubic to low-temperature tetragonal phase. Additionally, another subtle structural transition is discovered in Rb$_4$O$_6$ that further breaks the symmetry, and together with orbital ordering enables the formation of low-temperature quantum ground state of weakly coupled spin dimers. The coupling between lattice, charge, orbital and spin degrees of freedom is discussed. Geometric frustrations of nearest-neighbour exchange interactions are the most probable cause for the absence of long-range magnetic order in the Rb$_4$O$_6$ system. Finally, as the representative of low-dimensional organic compounds, we study the alkali-metal-doped triphenylene family. The three compounds with intercalated K, Rb or Cs alkali metals exhibit low-dimensional magnetism with a spin gap in the excitation spectrum. By the use of $\mu$SR, slowing down of collective spin fluctuations is observed. These quantum spin fluctuations prevent the formation of long-range magnetic order and are proposed to be a signature of a quantum spin liquid.

• Language: English
• Keywords: quantum magnetism, molecular systems, electrons, exchange interaction, anisotropy, spin chain, spin dimer, Tomonaga-Luttinger liquid, superoxide, paramagnet, quantum spin liquid, orbital order, charge order, structural transition, frustration
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2020
• PID: 20.500.12556/RUL-124057
• COBISS.SI-ID: 45783555
• Publication date in RUL: 23.12.2020

Secondary language

• Language: Slovenian
• Title: Kvantni magnetizem v molekularnih sistemih π-elektronov

Abstract

V tej doktorski disertaciji raziskujemo kvantni magnetizem v molekularnih kristalih z elektroni v $\pi$ orbitalah. V ta namen uporabljamo eksperimentalne spektroskopske metode, kot so elektronska paramagnetna resonanca (EPR), jedrska magnetna resonanca (JMR) in mionska spinska rotacija/relaksacija ($\mu$SR), s katerimi na lokalni skali merimo statične in dinamične lastnosti magnetnega stanja v izbranih spojinah. Obravnavamo sisteme alkalijskih superoksidov in seskvioksidov (AO$_2$ in A$_4$O$_6$, kjer je A = Cs, Rb, Na), v katerih kot nosilci osnovnega magnetnega momenta nastopajo superoksidni ioni O$_2^-$, ter na aromatske kristalne sisteme, kjer nesparjeni elektroni v njihovih molekularnih $\pi$ orbitalah tvorijo kvazi enodimenzionalne strukture. Za sistem CsO$_2$ je bilo predlagano, da orbitalno urejanje vodi do močnih interakcij vzdolž $b$ kristalnih osi. Analiza meritev EPR odziva na tem sistemu znotraj modelov, ki uporabljajo kvantno teorijo polja in tehniko bozonizacije, pokaže dobro ujemanje z napovedmi za spinske verige in Tomonaga-Luttingerjeve tekočine. Dodatno zaznamo tudi prisotnost anizotropije Isingovega tipa. Za vzorca Rb$_4$O$_6$ in Cs$_4$O$_6$ je značilno, da imamo v povprečju 4/3 naboja na O$_2$ molekulo. Obsežne meritve z metodami EPR, JMR in $\mu$SR na obeh vzorcih nas vodijo do odkritja strukturnega prehoda Verwey-evega tipa z nabojnim urejanjem iz visokotemperaturne kubične v nizkotemperaturno tetragonalno fazo. Poleg tega v Rb$_4$O$_6$ odkrijemo še dodaten strukturni prehod, ki skupaj z orbitalnim urejanjem pri tej temperaturi omogoči obstoj nizkotemperaturnega stanja šibko sklopljenih dimerov. Znotraj tega stanja se ne uspe razviti magnetni red dolgega dosega, kar pripišemo vplivu geometrijske frustracije na izmenjalne interakcije med najbližjimi sosedi. V sistemih Rb$_4$O$_6$ in Cs$_4$O$_6$ se obravnava sklopitve med mrežnimi, nabojnimi, orbitalnimi in spinskimi prostostnimi stopnjami. Na koncu pa obravnavamo tudi nizkodimenzionalne organske sisteme trifenilena, interkaliranega z različnimi alkalijskimi kovinami, kot so kalij, rubidij in cezij. Ti sistemi kažejo na obstoj nizkodimenzionalnega kvantnega osnovnega stanja z energijsko režo v spinskem vzbuditvenem spektru. Dodatne $\mu$SR meritve razkrijejo, da pri nizkih temperaturah pride do upočasnitve kolektivnih spinskih fluktuacij, ki nasprotujejo magnetnemu urejanju dolgega dosega. To nakazuje na zelo verjeten obstoj kvantne spinske tekočine.

• Keywords: kvantni magnetizem, molekularni sistemi, elektroni, izmenjalna interakcija, anizotropija, spinska veriga, spinski dimer, superoksid, paramagnet, kvantna spinska tekočina, orbitalni red, nabojna ureditev, strukturni prehod, frustracija