Session G24: Dynamics of New and Old Triangular Antiferromagnets

Complete field-induced spectral response of the spin-1/2 triangular-lattice antiferromagnet CsYbSe2

Fifty years after Anderson’s resonating valence-bond proposal, the spin-1/2 triangular-lattice Heisenberg antiferromagnet (TLHAF) remains the ultimate platform to explore highly entangled quantum spin states in proximity to magnetic order. Yb-based delafossites are ideal candidate TLHAF materials, which allow experimental access to the full range of applied in-plane magnetic fields. We perform a systematic neutron scattering study of CsYbSe₂, first proving the Heisenberg character of the interactions and quantifying the second-neighbor coupling. We then measure the complex evolution of the excitation spectrum, finding extensive continuum features near the 120°-ordered state, throughout the 1/3-magnetization plateau and beyond this up to saturation. We perform cylinder matrix-product-state (MPS) calculations to obtain an unbiased numerical benchmark for the TLHAF and spectacular agreement with the experimental spectra. The measured and calculated longitudinal spectral functions reflect the role of multi-magnon bound and scattering states. These results provide valuable insight into unconventional field-induced spin excitations in frustrated quantum materials.   

Phase diagrams and neutron scattering spectra of triangular lattice antiferromagnets from theory and matrix product state computations

Ever since the pioneering work of Anderson, the possibility of exotic magnetic phases on the triangular lattice has attracted interest.  Phase diagrams for spin and Hubbard models are presented, together with intuitive explanations where those are available.  We then turn to recent advances in the ability to predict inelastric neutron scattering spectra by dynamical calculations using matrix product state methods.  While computational methods for frustrated two-dimensional quantum magnets are still evolving, they are able to provide relatively unbiased approximate spectra for comparison to experiment, as demonstrated for materials in the YbSe2 and YbZnGaO families.  Neutron scattering results on these materials are shown to be consistent with predicted spectra for various points in the phase diagram of the anisotropic J1-J2 model, near the boundary between a Dirac spin liquid and conventional three-sublattice order.  We close with a discussion of the implications for theory and applications of finding either chiral spin liquids or U(1) Dirac quantum spin liquids on the triangular lattice.

The work presented resulted from theoretical collaborations with Berkeley colleagues T. Cookmeyer, J. Motruk, N. Sherman, A. Szasz, K. Wang, and M. Zaletel, and experimental collaborations with groups led by S. Haravifard, A. Scheie, and A. Tennant.   

Experiments exploring the dynamics of KYbSe2 and NaYbSe2 in field near their magnetization plateaux.

The AYbSe2 delafossite family is a near perfect realization of a triangular lattice Heisenberg model, with a ratio between the first and second neighbor in-plane exchanges J2/J1 very close to a theoretically predicted quantum spin liquid phase. In this talk I present neutron scattering measurements in zero field and the 1/3 magnetization plateau phase. In this "up-up-down" magnetic ordered phase we find coherent magnons which are describable via nonlinear spin wave theory, and can be fitted to a magnetic exchange model. The fittted J2/J1 of KYbSe2 lies within the 120 degree ordered phase, consistent with other measurements. However, the fitted J2/J1 of NaYbSe2 indicates larger J2/J1, potentially inside the theoretical quantum spin liquid phase, consistent with reports of no magnetic order in NaYbSe2. This indicates that the periodic table can be used to tune the triangular lattice across this phase diagram, and enter the long-sought quantum spin liquid phase in a controlled manner.   

The curse of Midas: when touching triangular and kagome 2d magnets turn them into 1d spin ladders and tubes.

Cs₂CoBr₄ is a new member of an old and famous family of distorted triangular-lattice systems. It differs from its thoroughly studied Cu-based counterparts by the presence of strong bond-dependent XY anisotropy. In applied fields it shows as may as 5 different ordered phase, including a m=1/3 plateau state [1], an incommensurate spin density wave with a field-dependent period [2], an incommensurate fan-structure with a period defined entirely by the frustration ratio, and two distinct commensurate phases. The most spectacular excitation spectrum is observed actually in zero field. Here semiclassical spin wave theory fails spectacularly. Instead, the spectrum is a series of bound fractional excitations [3]. These are reminiscent of Zeeman ladders in Ising spin chains, but have a far  more complex structure and are able to propagate in two dimensions.

But is the distorted-triangular-lattice model indeed a good starting point for understanding this material? The latest high-field neutron and THz spectroscopy experiments suggest  that a  zig-zag spin ladder description may actually be mode adequate.

Much excitement was recently caused by the discovery of a new family of rare earth boratotungstates, believed to realize the breathing-kagome antiferromagnetic model. Of these materials, Nd₃BWO₉, is highly frustrated, but orders magnetically at very low temperatures (TN/qW<12)and has a bewilderingly complex phase diagram in applied fields [4]. On the other hand, Pr₃BWO₉ does not order down to at least 50 mK. Yet it shows power-law magnetic specific heat and is thus a prime candidate for being a two-dimensional spin  liquid candidate.

But is the breathing-kagome-lattice model a indeed good starting point for understanding these materials? The latest neutron spectroscopy experiments suggest a  quasi one dimensional frustrated "hexagonal spin-tube" description may actually be mode adequate.   

Dynamics of New and Old Triangular Antiferromagnets

The dynamics of magnets stems from magnetic excitations above the ground state, yielding crucial insights into the microscopic mechanisms governing a material's physical properties. Our presentation centers on an old member of the antiferromagnetic family on triangular lattices from the 1970s. This member has garnered renewed attention in recent years, thanks to cutting-edge experimental instruments that allow high-precision measurement of its dynamics. Furthermore, this member reveals intriguing quantum effects, encompassing energy spectrum renormalization, quasiparticle instability, and the emergence of higher-order bound states, etc.. To elucidate these phenomena, we present theoretical methods designed to provide a comprehensive physical picture and a quantitative description. Significantly, the theoretical calculations successfully account for several recent experimental findings.