Experimental Detection of a Quantum Spin Liquid

Janša, Nejc (author), Klanjšek, Martin (mentor)

Quantum spin liquids are disordered states which feature entanglement and fractional quasiparticles with a lot of potential for study of quantum physics and for technological applications, for example in topologically protected quantum computation. The Kitaev model predicts an exact realization of the quantum spin liquid ground state. We search for experimental evidence of such states in real materials predicted to host the necessary Kitaev interactions. Our studies are done primarily using magnetic resonance on the candidates α-RuCl3 and BaCo2(AsO4)2. We identify the observables which show a unique response characteristic of the Kitaev quantum spin liquid as simulated by theoretical calculations. Although the pure ground state is disturbed by magnetic order, we observe that properties of the quantum spin liquid persist at higher temperatures in the so called Kitaev paramagnetic phase. In α-RuCl3 we show the fractionalization of a spin-flip into two types of anyons, a Majorana fermion and a pair of gauge fluxes in a wide range of temperatures and fields. The identity of the particles is confirmed by matching the spin excitation gap to the predictions of the pure Kitaev model with a cubic field dependence. The sample is studied in depth determining the electric field gradient and hyperfine tensors, properties of the crystal structure and finding effects of additional interactions where the pure Kitaev picture breaks down. In BaCo2(AsO4)2 we also find characteristics of the Kitaev paramagnetic phase in a phase diagram that is more sensitive to the external magnetic field. The spin excitation gap is found to have a linear field dependence as expected for models with additional interactions. A region in the phase diagram is found, where a different type of fractionalization into pairs of Majorana fermions might be present.

• Language: English
• Keywords: quantum spin liquid, Kitaev model, quantum computation, nuclear magnetic resonance, nuclear quadrupole resonance, Kitaev paramagnet, α-RuCl3, BaCo2(AsO4)2, Majorana fermion, gauge flux, fractionalization, anyon, spin excitation gap
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2022
• PID: 20.500.12556/RUL-138622
• COBISS.SI-ID: 112435715
• Publication date in RUL: 03.08.2022

Secondary language

• Language: Slovenian
• Title: Eksperimentalna detekcija kvantne spinske tekočine

Abstract

Kvantne spinske tekočine so neurejena stanja, ki vsebujejo kvanto prepletenost in frakcijske kvazidelce in obljubljajo veliko potenciala v raziskovanju kvantne fizike in v tehnoloških aplikacijah, kot naprimer topološko zaščiteno kvantno računalništvo. Model Kitaeva napoveduje točno realizacijo osnovnega stanja kvantne spinske tekočine. V tej tezi iščemo eksperimentalne dokaze takih stanj v realnih materialih, v katerih so pričakovane interakcije Kitaeva. Naše raziskave so opravljene primarno z uporabo magnetne resonance na kandidatih α-RuCl3 in BaCo2(AsO4)2. Prepoznamo opazljivke, ki kažejo unikaten odziv značilen za Kitaevo kvantno spinsko tekočino v skladu z napovedmi teoretičnih simulacij. Kljub temu, da magnetni red prepreči čisto osnovno stanje, opazimo, da se lastnosti kvantne spinske tekočine ohranijo pri višjih temperaturah v tako imenovani paramagnetni fazi Kitaeva. V α-RuCl3 pokažemo frakcionalizacijo spinskega obrata na dva tipa anyonov, Majoranovega fermiona in para umeritvenih fluksov za širok razpon temperature in magnetnega polja. Identiteta delcev je potrjena z ujemanjem vrzeli spinske vzbuditve z napovedmi za čisti model Kitaeva, ki kaže kubično poljsko odvisnost. Vzorec natančneje preučimo in določimo tenzor gradienta električnega polja in hiperfini tenzor, lastnosti kristalne strukture in najdemo učinke dodatnih interakcij, kjer se pokvari čista slika Kitaeva. V BaCo2(AsO4)2 tudi najdemo karakteristike paramagnetne faze Kitaeva s faznim diagramom bolj občutljivim na magnetno polje. Izkaže se, da ima vrzel spinske vzbuditve linearno odvisnost od polja, kot je napovedano za modele z dodatnimi interakcijami. V faznem diagramu najdemo območje, kjer je možna drugačna vrsta frakcionalizacije na pare Majoranovih fermionov.

• Keywords: kvantna spinska tekočina, model Kitaeva, kvantno računalništvo, jedrska magnetna resonanca, jedrska kvadrupolna resonanca, paramagnet Kitaeva, α-RuCl3, BaCo2(AsO4)2, Majoranov fermion, umeritveni fluks, frakcionalizacija, anyon