Stability of quantum spin liquids in two dimensions

Arh, Tina (author), Zorko, Andrej (mentor)

Quantum spin liquid is a disordered but strongly quantum-entangled ground state of matter that can occur in layered materials with geometrically frustrated lattices of magnetic ions, such as the triangular and the kagome lattice. Although quantum spin liquid can arise as a ground state of an isotropic nearest-neighbour Heisenberg model, it can be stabilized or destabilized by various perturbations of this model, such as magnetic anisotropy or further-neighbour interactions. In this work, we first study the quantum magnet YCu$_3$(OH)$_6$Cl$_3$, in which copper ions form well-separated kagome layers. This material is magnetically ordered at low temperatures, although a quantum spin liquid ground state is expected due to dominant nearest-neighbour interactions. Using electron spin resonance, magnetic susceptibility, and specific heat measurements, we show that the magnetic order is stabilized by the Dzyaloshinskii-Moriya magnetic anisotropy. Nuclear magnetic resonance is used to characterize the local magnetic fields and their fluctuations. In zinc-doped barlowite, another representative of quantum kagome antiferromagnets, experiments suggest a quantum spin liquid ground state, but its characterization is challenging due to the presence of magnetic defects. Using nuclear magnetic resonance and magnetic susceptibility measurements, we show that the amount of defects in this material is higher than previously reported. We successfully separate the contributions of intrinsic spins and magnetic defects in the nuclear magnetic resonance spectra and determine the local magnetic susceptibility. Finally, we study rare-earth heptatantalates in which neodymium or erbium ions form the triangular lattice in well-separated layers. Rare earths are known for anisotropic exchange interactions, which we show to have a strong Ising-type anisotropy using inelastic neutron scattering, magnetic susceptibility and electron spin resonance. In neodymium heptatantalate, measurements also reveal the short-range spin correlations of Ising character. Nevertheless, the material is not ordered to the lowest experimentally accessible temperatures and is a quantum spin liquid candidate.

• Language: English
• Keywords: quantum spin liquids, frustrated magnetism, quantum materials, kagome lattice, triangular lattice, YCu$_3$(OH)$_6$Cl$_3$, Zn-barlowite, rare-earth heptatantalates
• Work type: Doctoral dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2024
• PID: 20.500.12556/RUL-159093
• Publication date in RUL: 29.06.2024

Secondary language

• Language: Slovenian
• Title: Stabilnost kvantnih spinskih tekočin v dveh dimenzijah

Abstract

Kvantna spinska tekočina je neurejeno, a močno kvantno prepleteno osnovno stanje snovi, ki se lahko pojavi v plastovitih materialih, v katerih magnetni ioni tvorijo geometrijsko frustrirane mreže, kot sta trikotna mreža in mreža kagome. Takšno stanje lahko sledi že iz preprostega izotropnega Heisenbergova modela za interakcije med najbližjimi sosedi. Različne perturbacije tega modela, kot sta denimo magnetna anizotropija ali dodatne interakcije med bolj oddaljenimi ioni, lahko kvantno spinsko tekočino stabilizirajo ali destabilizirajo. V tem delu najprej obravnavamo kvantni magnet YCu$_3$(OH)$_6$Cl$_3$, v katerem bakrovi ioni tvorijo dobro ločene plasti mreže kagome. Pri nizkih temperaturah se ta material magnetno ureja, čeprav bi ob dominantni interakciji med najbližjimi sosedi pričakovali osnovno stanje kvantne spinske tekočine. Z analizo rezultatov elektronske paramagnetne resonance, magnetne susceptibilnosti ter specifične toplote pokažemo, da je razlog za magnetno urejanje magnetna anizotropija tipa Dzyaloshinskii-Moriya. Z meritvami jedrske magnetne resonance karakteriziramo lokalno magnetno susceptibilnost in fluktuacije lokalnih magnetnih polj. V Zn-barlowitu, še enemu predstavniku kvantnih antiferomagnetov z mrežo kagome, eksperimenti kažejo na osnovno stanje kvantne spinske tekočine, vendar je njegova karakterizacija zahtevna zaradi prisotnosti magnetnih defektov. S pomočjo meritev jedrske magnetne resonance in magnetne susceptibilnosti pokažemo, da je količina defektov v tem materialu višja, kot je bilo do sedaj poročano v literaturi. V spektrih fluorove jedrske magnetne resonance uspešno ločimo prispevka intrinzičnih spinov in magnetnih defektov, kar nam omogoča določitev lokalne magnetne susceptibilnosti. Nazadnje obravnavamo heptatantalate redkih zemelj, v katerih neodim ali erbij tvorita popolno trikotno mrežo v dobro ločenih plasteh. Za redke zemlje je značilna magnetno anizotropna izmenjalna interakcija, za katero s pomočjo neelastičnega nevtronskega sipanja, magnetne susceptibilnosti in elektronske spinske resonance pokažemo, da ima močno anizotropijo Isingovega tipa, prav tako pa imajo Isingov značaj spinske korelacije kratkega dosega. Kljub temu se material ne ureja do najnižjih eksperimentalno dostopnih temperatur in je kandidat za kvantno spinsko tekočino.

• Keywords: kvantne spinske tekočine, frustrirani magnetizem, kvantni materiali, mreža kagome, trikotna mreža, YCu$_3$(OH)$_6$Cl$_3$, Zn-barlowite, heptatantalati redkih zemelj