Thermodynamic measurements on frustrated triangular and honeycomb lattices in the Millikelvin range

Frustrated magnets potentially host new phase of matters such as Quantum Spin Liquids (QSL). In contrast to conventional magnets, QSL do not show magnetic ordering even at 0K, and they have been suggested to be relevant for high-temperature superconductivity or fault-tolerant quantum computing. Therefore, it is one of the central tasks in solid state physics to synthesize a material exhibiting this peculiar phase. In general, the characterization of potential QSL candidate materials is extremely challenging because it requires temperatures as close as possible to 0K. In this thesis, several promising frustrated magnets on honeycomb and triangular lattices have been investigated in a He3/He4-dilution refrigerator. Thermodynamic measurements such as specific heat, magnetic Grüneisen parameter and magnetization have been performed over a broad temperature and magnetic field range down to 40mK and up to 15T. First, this thesis focuses on four different Yb{3+} based triangular lattice materials (YbMgGaO4, KYbS2, NaYbO2, KBaYb(BO3)2). None of these materials undergoes a magnetic phase transition down to the lowest accessible temperature, and potentially emerging QSL phases are discussed. The field-evolution turns out to be non-trivial in several materials and is investigated in detail for comparison with theoretical predictions on triangular lattices. Furthermore, a potential application of KBaYBb(BO3)2 for adiabatic demagnetization down to the 20mK has been shown. The second central part focuses on the Kitaev magnet alpha-RuCl3 on a honeycomb lattice, which shows zigzag magnetic order in zero field. However, a potential field-induced QSL has been vividly debated in the last years. In this thesis, measurements of the field-dependent magnetic Grüneisen parameter in combination with the specific heat has been applied to identify the phase transitions in this Kitaev material. The phase boundaries and the putative existence of a QSL in the phase diagram of alpha-RuCl3 is carefully discussed.In frustrierten Magneten können neuartige und exotische Phasen auftreten wie Quantenspinflüssigkeiten (Quantum Spin Liquids: QSL). Im Gegensatz zu konventionellen Magneten weisen QSL keine magnetische Ordnung auf, nicht einmal am absoluten Nullpunkt von 0K. Die Realisierung solch einer QSL in Festkörpern ist seit Jahrzehnten ein zentrales Unterfangen in der Festkörperphysik, unter anderem wegen ihrer möglichen Verbindung zu Hochtemperatursupraleitern oder fehlertoleranten Quantencomputern. Die Charakterisierung möglicher QSL Kandidaten ist extrem herausfordernd, da in der Regel Temperaturen möglichst nahe am absoluten Nullpunkt erforderlich sind. In der vorliegenden Arbeit wurden mehrere vielversprechende frustrierte Magnete, bei denen die magnetischen Momente auf einem Dreiecks- bzw einem Honigwabengitter angeordnet sind, in einem He3/He4-Entmischungskryostaten untersucht. Es wurden thermodynamische Messungen wie die spezifische Wärme, magnetischer Grüneisen Parameter und Magnetisierung in einem Temperaturbereich bis hinab zu 40mK in magnetischen Feldern bis zu 15T durchgeführt. Der Fokus liegt zunächst auf vier verschiedenen Yb{3+}-basierten Materialien auf einem Dreiecksgitter (YbMgGaO4, KYbS2, NaYbO2, KBaYb(BO3)2). Keines dieser Materialien weist einen Phasenübergang bis zu den tiefsten erreichbaren Temperaturen auf. In diesem Kontext werden potentielle QSL Phasen diskutiert. Einige der Materialien zeigen eine nicht-triviale Feldabhängigkeit, die im Detail untersucht und mit theoretischen Vorhersagen für zweidimensionale Dreiecksgitter verglichen wird. Zudem wird die mögliche Anwendung von KBaYb(BO3)2 im Bereich der adiabatischen Demagnetisierung bis hinab zu 20mK gezeigt. Der zweite Teil der Arbeit handelt vom Kitaev Magneten alpha-RuCl3 mit einem Honigwabengitter, der im Nullfeld einen magnetischen Phasenübergang zu einer geordneten Phase aufweist und somit keine QSL ist. Dennoch wurde in den letzten Jahren lebhafte Diskussionen über eine mögliche feldinduzierte QSL Phase in alpha-RuCl3 geführt. In der vorliegenden Arbeit werden Messungen des magnetischen Grüneisen Parameters an alpha-RuCl3 in Kombination mit der spezifischen Wärme präsentiert, um mögliche feldinduzierte Phasenübergänge zu identifizieren. Die Phasengrenzen sowie die vermeintliche Existenz einer QSL Phase im Phasendiagramm werden sorgfältig diskutiert

Gapped ground state in the zigzag pseudospin-1/2 quantum antiferromagnetic chain compound PrTiNbO6

We report a single-crystal study on the magnetism of the rare-earth compound PrTiNbO$_6$ that experimentally realizes the zigzag pseudospin-$\frac{1}{2}$ quantum antiferromagnetic chain model. Random crystal electric field caused by the site mixing between non-magnetic Ti$^{4+}$ and Nb$^{5+}$, results in the non-Kramers ground state quasi-doublet of Pr$^{3+}$ with the effective pseudospin-$\frac{1}{2}$ Ising moment. Despite the antiferromagnetic intersite coupling of about 4 K, no magnetic freezing is detected down to 0.1 K, whilst the system approaches its ground state with almost zero residual spin entropy. At low temperatures, a sizable gap of about 1 K is observed in zero field. We ascribe this gap to off-diagonal anisotropy terms in the pseudospin Hamiltonian, and argue that rare-earth oxides open an interesting venue for studying magnetism of quantum spin chains.Comment: 11 pages, 10 figures, 1D correlated magnetism of non-Kramers Ising quasi-doublets in PrTiNbO

Gapless spin-liquid state in the structurally disorder-free triangular antiferromagnet NaYbO2_2

We present the structural characterization and low-temperature magnetism of the triangular-lattice delafossite NaYbO$_2$. Synchrotron x-ray diffraction and neutron scattering exclude both structural disorder and crystal-electric-field randomness, whereas heat-capacity measurements and muon spectroscopy reveal the absence of magnetic order and persistent spin dynamics down to at least 70\,mK. Continuous magnetic excitations with the low-energy spectral weight accumulating at the $K$-point of the Brillouin zone indicate the formation of a novel spin-liquid phase in a triangular antiferromagnet. This phase is gapless and shows a non-trivial evolution of the low-temperature specific heat. Our work demonstrates that NaYbO$_2$ practically gives the most direct experimental access to the spin-liquid physics of triangular antiferromagnets.Comment: 6 pages, 4figure

Role of alkaline metal in the rare-earth triangular antiferromagnet KYbO2_2

We report crystal structure and magnetic behavior of the triangular antiferromagnet KYbO$_2$, the A-site substituted version of the quantum spin liquid candidate NaYbO$_2$. The replacement of Na by K introduces an anisotropic tensile strain with 1.6% in-plane and 12.1% out-of-plane lattice expansion. Compared to NaYbO$_2$, both Curie-Weiss temperature and saturation field are reduced by about 20% as the result of the increased Yb--O--Yb angles, whereas the $g$-tensor of Yb$^{3+}$ becomes isotropic with $g=3.08(3)$. Field-dependent magnetization shows the plateau at 1/2 of the saturated value and suggests the formation of the up-up-up-down field-induced order in the triangular AYbO$_2$ oxides (A = alkali metal), in contrast to the isostructural selenides that exhibit the 1/3 plateau and the up-up-down field-induced order

Field evolution of the spin-liquid candidate YbMgGaO4

We report magnetization, heat capacity, thermal expansion, and magnetostriction measurements down to mK temperatures on the triangular antiferromagnet YbMgGaO$_4$. Our data exclude the formation of the distinct $\frac13$-plateau phase observed in other triangular antiferromagnets, but reveal plateau-like features in second derivatives of the free energy, magnetic susceptibility and specific heat, at $\mu_0H$ = 1.0 - 2.5 T for $H\parallel{}c$ and 2 - 5 T for $H\perp{}c$. Using Monte-Carlo simulations of a realistic spin Hamiltonian, we ascribe these features to non-monotonic changes in the magnetization and the $\frac12$-plateau that is smeared out by the random distribution of exchange couplings in YbMgGaO$_4$

Partial up-up-down order with the continuously distributed order parameter in the triangular antiferromagnet TmMgGaO4

Frustrated quasidoublets without time-reversal symmetry can host highly unconventional magnetic structures with continuously distributed order parameters even in a single-phase crystal. Here, we report the comprehensive thermodynamic and neutron diffraction investigation on the single crystal of TmMgGaO$_4$, which entails non-Kramers Tm$^{3+}$ ions arranged on a geometrically perfect triangular lattice. The crystal electric field (CEF) randomness caused by the site-mixing disorder of the nonmagnetic Mg$^{2+}$ and Ga$^{3+}$ ions, merges two lowest-lying CEF singlets of Tm$^{3+}$ into a ground-state (GS) quasidoublet. Well below $T_c$ $\sim$ 0.7 K, a small fraction of the antiferromagnetically coupled Tm$^{3+}$ Ising quasidoublets with small inner gaps condense into two-dimensional (2D) up-up-down magnetic structures with continuously distributed order parameters, and give rise to the \emph{columnar} magnetic neutron reflections below $\mu_0H_c$ $\sim$ 2.6 T, with highly anisotropic correlation lengths, $\xi_{ab}$ $\geq$ 250$a$ in the triangular plane and $\xi_c$ $<$ $c$/12 between the planes. The remaining fraction of the Tm$^{3+}$ ions remain nonmagnetic at 0 T and become uniformly polarized by the applied longitudinal field at low temperatures. We argue that the similar model can be generally applied to other compounds of non-Kramers rare-earth ions with correlated GS quasidoublets.Comment: Accepted in Physical Review X. The present manuscript is significantly different from the earlier preprint, arXiv:1804.00696, in the analysis, results, and contains additional neutron diffraction dat

Rearrangement of Uncorrelated Valence Bonds Evidenced by Low-Energy Spin Excitations in YbMgGaO 4

dc-magnetization data measured down to 40 mK speak against conventional freezing and reinstateYbMgGaO4as a triangular spin-liquid candidate. Magnetic susceptibility measured parallel andperpendicular to thecaxis reaches constant values below 0.1 and 0.2 K, respectively, thus indicatingthe presence of gapless low-energy spin excitations. We elucidate their nature in the triple-axis inelasticneutron scattering experiment that pinpoints the low-energy (E≤J0∼0.2meV) part of the excitationcontinuum present at low temperatures (TJ0that is rooted in the breaking ofnearest-neighbor valence bonds and persists to temperatures well aboveJ0=kB, the low-energy oneoriginates from the rearrangement of the valence bonds and thus from the propagation of unpaired spins.We further extend this picture to herbertsmithite, the spin-liquid candidate on the kagome lattice, and arguethat such a hierarchy of magnetic excitations may be a universal feature of quantum spin liquids