Session R24: 2D Frustrated Spin Systems: YbMgGaO4 and Kagome

Spin Liquid vs. Spin-Orbit Coupling on the Triangular Lattice

In recent years it has been realized that strong spin-orbit coupling can provide an new avenue in the search for spin liquid phases in experimentally realizable materials. We explore the relationship between strong spin-orbit coupling and spin liquid physics. We study a very general model on the triangular lattice where spin-orbit coupling leads to the presence of highly anisotropic interactions. We use variational Monte Carlo to study both U(1) quantum spin liquid states and ordered ones, via the Gutzwiller projected fermion construction. We thereby obtain the ground state phase diagram in this phase space. We further develop a novel method to go beyond the simple Gutzwiller wave functions for the spinon Fermi surface quantum spin liquid. This modification results in the appearance of several qualitatively new phenomena. Of particular interest is the presence of a large thermal Hall conductivity in this system.   

Structural Disorder in the Quantum Spin-Liquid Candidate YbMgGaO4

The rare-earth oxide YbMgGaO₄ is an exciting candidate for a quantum spin-liquid state based on a triangular lattice of Yb³⁺ ions. Experimental evidence for spin-liquid-like behavior includes a continuous magnetic excitation spectrum and the absence of long-range magnetic order to sub-100mK temperatures. Yet the origin of this behavior remains a subject for debate, with proposals ranging from resonating-valence-bond states and U(1) spin liquids, to disorder-induced mimicry of a quantum spin liquid. Disorder plays an intrinsic role in the structure of YbMgGaO₄, because magnetic Yb³⁺ layers are separated by layers with disordered Mg²⁺/Ga³⁺ occupancy. In this talk, I will discuss experimental evidence for the importance of structural disorder in YbMgGaO₄. Our X-ray and neutron scattering data reveal that structural disorder is not restricted to random Mg²⁺/Ga³⁺ occupancies, but is actually strongly correlated. I will explore the implications of our experimental results for the spin-liquid-like physics of YbMgGaO₄.   

Magnetic diffuse scattering of the triangular lattice antiferromagnet YbMgGaO4

YbMgGaO₄, a rare-earth S=1/2 triangular lattice antiferromagnet, has been to focus of recent studies with debate from several different groups about the origin of seemingly spin-liquid behavior. The interplay of strong spin-orbit coupling and site disorder of interplane Mg2+ and Ga3+ leads to interesting magnetic diffuse scattering signatures and lack of long range magnetic ordering even down to tens of mK. In this talk, I will present recent neutron scattering data measured at Oak Ridge National Laboratory on single crystals of YbMgGaO₄ using polarized neutrons on HYSPEC and diffuse scattering on CORELLI. These results will be compared to different exchange models proposed for this topical material.   

Topography and Mimicry of a Spin Liquid on a Triangular Lattice

Motivated by recent interest in strongly-anisotropic spin-spin interactions, we have explored an extended three-dimensional phase diagram of a class of such models on an ideal triangular lattice using density-matrix renormalization group and quasiclassical approaches. In the case appropriate for the rare-earth-based materials with interactions only among the nearest-neighbor spins, we have mapped out all phases and identified the topography of the region that can harbor a spin liquid. A four-dimensional extension of this phase diagram with the next-nearest-neighbor coupling J₂ allows for a natural continuity of the spin liquid in the strongly-anisotropic model to the previously well-studied spin-liquid state of the J₁-J₂ Heisenberg model. Their respective spin-liquid states are shown to exhibit nearly identical spin-spin correlations, suggesting that the nature of spin liquids in both models is the same.
For YbMgGaO₄, an excellent material realization of the rare-earth-based triangular-lattice antiferromagnet with the highly anisotropic effective spin-spin interactions, our analysis finds no transitions to a spin liquid near experimentally relevant range of parameters, putting YbMgGaO₄ firmly in the stripe-ordered state. At the same time, in the regime with small or subleading anisotropic pseudo-dipolar terms, stripe states are selected by an order-by-disorder fluctuations, making stripes fragile toward orientational disorder. Then, the randomization of the pseudo-dipolar interactions due to spatially-fluctuating charge environment of the magnetic ions can successfully mimic a spin liquid by forming short-range stripe or stripe-superposition domains, producing the structure factor that is in agreement with experiment. This spin-liquid mimicry scenario is relevant to other quantum magnets with fragile ground states and random environments.   

Hierarchy of exchange interactions in the triangular-lattice spin-liquid YbMgGaO4

The spin-1/2 triangular lattice antiferromagnet YbMgGaO₄ has attracted recent attention as a quantum spin-liquid candidate. Whether a quantum spin-liquid is stabilized or not depends on the interplay of various exchange interactions with chemical disorder that is inherent to the layered structure of the compound. We combine time-domain terahertz spectroscopy and inelastic neutron scattering measurements in the field polarized state of YbMgGaO₄ to obtain better microscopic insights on its exchange interactions. Terahertz spectroscopy in this fashion probes the spin-wave excitations at the Brillouin zone center, ideally complementing neutron scattering. A global spin-wave fit to all our spectroscopic data at fields over 4 T, informed by the analysis of the terahertz spectroscopy linewidths, yields stringent constraints on g-factors and exchange interactions. Our results paint YbMgGaO₄ as an easy-plane XXZ antiferromagnet with the combined and necessary presence of sub-leading next nearest neighbor and weak anisotropic off-diagonal nearest-neighbor interactions. This work establishes the hierarchy of exchange interactions in YbMgGaO₄ and thus strongly constrains possible mechanisms responsible for the observed spin-liquid phenomenology.   

Absence of Magnetic Thermal Conductivity in the Quantum Spin-Liquid Candidate YbMgGaO4

We present the ultralow-temperature specific heat and thermal conductivity measurements on single crystals of YbMgGaO₄, which was recently argued to be a promising candidate for a quantum spin liquid (QSL). In a zero magnetic field, a large magnetic contribution of specific heat is observed, and exhibits a power-law temperature dependence (C_m ∼ T^0.74). On the contrary, we do not observe any significant contribution of thermal conductivity from magnetic excitations. In magnetic fields H ≥ 6 T, the exponential T dependence of C_m and the enhanced thermal conductivity indicate a magnon gap of the fully polarized state. More crucially, from ultralow-temperature a.c. susceptibility measurements, we find evidence for a spin-glass ground state with the observation of frequency-dependent peaks around 0.1 K. Therefore, we conclude that instead of a QSL, the ground state of YbMgGaO₄ is likely a disorder-induced spin glass [1, 2].
[1] Y. Xu, et al., Physical Review Letters 117, 267202 (2016).
[2] Z. Ma, et al., arXiv:1709.00256.   

Investigating the Quantum vs Classical Nature of the Spin Liquid State in Yb(Mg,Zn)GaO4

Recently the rare earth RMM’O4 systems (where R = rare earth, M and M’ = transition or main group) have been suggested as strong spin-orbit coupled quantum spin liquid candidates. In this talk we present our recent field-dependent diffuse neutron scattering, torque magnetometry, and magneto-transport results performed on Yb(Mg,Zn)GaO4 single crystal samples. Further, we discuss the role of disorder and thermal fluctuations in explaining the observed spin liquid phenomena.   

Spin Frustration and Spin Liquid on a Triangular Lattice

Searching for Spin liquid has been one of major subjects of strongly correlated systems in last two decades for both theoretician and experimentists. Spin frustrations are introduced to enhance quantum fluctuations for materials with triangular, kogome and pyrochlore structures. The progresses on the material synthesis were made in the last decade. In particular, a new triangular inorganics sample was made as YbMgGaO₄ which is nearly an ideal triangular lattice without Dyzaloshinskii-Moriya interaction. This is an effective spin-half triangular lattice with a strong spin-orbit coupling originating from the splitting the rare each element Yb the crystal field, which gives rise to extra competition imposed to the geometric frustration.
In this talk, I will give a simple introduction on this material and discuss some experimental results and present our theoretical results on the Hamiltonian proposed for this material, including the spin wave approaches and density matrix renormalization group computations. Although one highly considers the material to be a good candidate of a gapless spin liquid, our theoretical results cannot provide enough evidence on the presents of the spin liquid with the proposed Hamiltonian.   

Muon Spin Relaxation Study on quantum spin liquid candidate YbMgGaO4

Quantum spin liquid (QSL) state has been a crucial topic in condensed matter physics in the last three decades, due to its potential application on quantum computation and its relation between exotic phenomenons such as high temperature superconductivity[1–3]. Recently, the QSL state was proposed to be in triangular-lattice structured YbMgGaO₄[4–9], but it was still under debates[10, 11]. We performed new muon spin relaxation experiments on high quality and small amount of single crystals of YbMgGaO_4. Detal study of the ground state of this compound will be reported.

Reference:
[1] P. W. Anderson, Science 235, 1196 (1987).
[2] P. A. Lee, Science 321, 1306 (2008).
[3] L. Balents, Nature 464, 199 (2010).
[4] Y. Li et al., Sci Rep 5, 16419 (2015).
[5] Y. Li et al., Phys. Rev. Lett. 115, 167203 (2015).
[6] Y. Shen et al., Nature 540, 559 (2016).
[7] J. A. M. Paddison et al., Nat Phys 13, 117 (2017).
[8] Y. Li et al., Phys Rev Lett 118, 107202 (2017).
[9] Y. Li et al., Physical Review Letters 117, 097201 (2016).
[10] Y. Xu et al., Phys Rev Lett 117, 267202 (2016).
[11] Z. Ma et al., arXiv: 1709. 00256 (2017).   

Fractionalized excitations in the partially magnetized spin liquid candidate YbMgGaO4

Quantum spin liquids (QSLs) are exotic states of matter characterized by emergent gauge structures and fractionalized elementary excitations. In the recently discovered triangular lattice QSL candidate YbMgGaO₄, dispersive spinon excitations with a clear upper excitation edge has been observed, but the nature of its ground state is still under debate. In this talk, we present neutron scattering measurement on YbMgGaO₄ under various magnetic fields. Our data reveal a dispersive spin excitation continuum with clear upper and lower excitation edges under a weak magnetic field (H = 2.5 T). Moreover, a spectral crossing emerges at the Gamma point at the Zeeman-split energy. The corresponding redistribution of the spectral weight and its field-dependent evolution is inconsistent with a magnon behavior, but instead resembles the theoretical prediction based on the inter-band and intra-band spinon particle-hole excitations associated with the Zeeman-split spinon bands. These results provide a strong evidence for fractionalized excitations and spinon Fermi surfaces in the partially magnetized QSL state in YbMgGaO₄.   

Crystal growth and magnetic properties of ErMgGaO4

Triangular lattices with antiferromagnetic nearest neighbour exchange interactions are some of the most popular examples of geometrically frustrated systems. Recently, YbMgGaO₄ has been considered as a potential quantum spin liquid candidate, where the magnetic Yb³⁺ ions are forming a quasi two dimensional triangular lattice while Mg²⁺/Ga³⁺ are randomly mixed between the Yb layers. The variation of exchange interactions produced by the disordered ions and the frustration from the triangular planes are expected to produce the spin liquid ground state. By replacing Yb with Er, different interaction strengths of J₁ and J₂ are expected, resulting in an interesting unknown ground state. Here, we will show our results of single crystal growth of ErMgGaO₄ and its magnetic properties.   

Magnetic Ground State of the Kagome Lattice in Fe4Si2Sn7O16

Bulk Characterisation, neutron powder diffraction and muon spin relaxation data has been used to determine the magnetic ground state of Fe₄Si₂Sn₇O₁₆. The only magnetic ions in this compound are layers of high-spin Fe²⁺ (d⁶, S=2) arranged on a perfectly hexagonal kagome lattice. Below T_N=3.5 K, the spins partiall order into canted antiferromagnetic chains, separated by paramagnetic spis on the geometrically frustrated lattice. No static long-range order was observed down to at least T=0.1~K.   

Persistence of the flat band in a kagome magnet with dipolar interactions

The weathervane modes of the classical Heisenberg antiferromagnet on the kagome lattice constitute possibly the earliest and certainly the most celebrated example of a flat band of zero-energy excitations. Such modes arise from the underconstraint that has since become a defining criterion of strong geometrical frustration. We focus on the fate of this flat band in the presence of additional dipolar interactions. These change the nearest-neighbour model fundamentally as they remove the Heisenberg spin-rotational symmetry while also introducing a long- range component to the interaction. Curiously, the modes continue to remain approximately dispersionless, while being lifted to finite energy. This phenomenon provides interesting connections between concepts such as constraint counting and self-screening underpinning the field of frustrated magnetism. Moreover, this property is found to be remarkably stable to a wide range of additional interactions.