High-quality single crystals of ${\mathrm{NaYbSe}}_2$, which resembles a perfect triangular-lattice antiferromagnet without intrinsic disorder, are investigated by magnetization and specific heat, as well as the local probe techniques nuclear magnetic resonance (NMR) and electron spin resonance. The low-field measurements confirm the absence of any spin freezing or long-range magnetic order down to 50 mK, which suggests a quantum spin liquid ground (QSL) state with gapless excitations. Instability of the QSL state is observed upon applying magnetic fields. For the $H\perp c$ direction, a field-induced magnetic phase transition is observed above 2 T from the $C_{\mathrm{p}}\left(T\right)$ data, agreeing with a clear $\frac{M_s}{3}$ plateau of $M\left(H\right)$, which is associated with an up-up-down spin arrangement. For the $H\parallel c$ direction, a field-induced transition could be evidenced at a much higher field range (9–21 T). The ${}^{23}\mathrm{Na}$ NMR measurements provide microscopic evidence of field-induced ordering for both directions. A reentrant behavior of $T_{\mathrm{N}}$, originating from the thermal and quantum spin fluctuations, is observed for both directions. The anisotropic exchange interactions $J_{\perp }\simeq 4.7$ K and $J_z\simeq 2.33$ K are extracted from the modified bond-dependent XXZ model for the spin-$\frac{1}{2}$ triangular-lattice antiferromagnet. The absence of magnetic long-range order at zero fields is assigned to the effect of strong bond frustration, arising from the complex spin-orbit entangled $4f$ ground state. Finally, we derive the highly anisotropic magnetic phase diagram, which is discussed in comparison with the existing theoretical models for spin-$\frac{1}{2}$ triangular-lattice antiferromagnets.

We report an electron spin resonance (ESR) study on single crystals of the heavy fermion metal ${\mathrm{Y}\mathrm{b}\mathrm{R}\mathrm{h}}_2{\mathrm{S}\mathrm{i}}_2$ which shows pronounced non-Fermi liquid behavior related to a close antiferromagnetic quantum critical point. It is shown that the observed ESR spectra can be ascribed to a bulk ${\mathrm{Y}\mathrm{b}}^{3+}$ resonance. This is the first observation of ESR of the Kondo ion itself in a dense Kondo lattice system. The ESR signal occurs below the Kondo temperature $(T_K)$ which thus indicates the existence of large unscreened ${\mathrm{Y}\mathrm{b}}^{3+}$ moments below $T_K$. We observe the spin dynamics as well as the static magnetic properties of the ${\mathrm{Y}\mathrm{b}}^{3+}$ spins to be consistent with the results of nuclear magnetic resonance and magnetic susceptibility.

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.

Electron spin resonance (ESR) measurements of the ferromagnetic (FM) Kondo lattice system CeRuPO show a well defined ESR signal which is related to the ${\mathrm{Ce}}^{3+}$ magnetism. In contrast, no ESR could be observed in the antiferromagnetic (AFM) homologue CeOsPO. Additionally, we detect an ESR signal in ferromagnetic YbRh while it was absent in a number of Ce or Yb intermetallic compounds with dominant AFM exchange. Thus, the observation of an ESR signal in a Kondo lattice is neither specific to Yb nor to the proximity to a quantum critical point, but seems to be connected to the presence of FM fluctuations. These conclusions not only provide a basic concept to understand the ESR in Kondo lattice systems even well below the Kondo temperature (as observed in ${\mathrm{YbRh}}_2{\mathrm{Si}}_2$) but point out ESR as a prime method to investigate directly the spin dynamics of the Kondo ion.

While the Heisenberg model for magnetic Mott insulators on planar lattice structures is comparatively well understood in the case of transition metal ions, the intrinsic spin-orbit entanglement of $4f$ magnetic ions on such lattices shows fascinating new physics largely due to corresponding strong anisotropies both in their single-ion and their exchange properties. We show here that the Yb delafossites, containing perfect magnetic ${\mathrm{Yb}}^{3+}$ triangular lattice planes with pseudospin $s=\frac{1}{2}$ at low temperatures, are an ideal platform to study these phenomena. Competing frustrated interactions may lead to an absence of magnetic order associated to a gapless spin liquid ground state with a huge linear specific heat exceeding that of many heavy fermions, whereas the application of a magnetic field induces anisotropic magnetic order with successive transitions into different long-range ordered structures. In this comparative study, we discuss our experimental findings in terms of a unified crystal-field and exchange model. We combine electron paramagnetic resonance (EPR) experiments and results from neutron scattering with measurements of the magnetic susceptibility, isothermal magnetization up to full polarization, and specific heat to determine the relevant model parameters. The impact of the crystal field is discussed as well as the symmetry-compatible form of the exchange tensor, and we give explicit expressions for the anisotropic $g$ factor, the temperature dependence of the susceptibility, the exchange-narrowed EPR linewidth, and the saturation field.

The delafossite structure of NaYbS2 contains a planar spin-1/2 triangular lattice of Yb3+ ions and features a possible realisation of a quantum spin-liquid state. We investigated the Yb3+ spin dynamics by electron spin resonance (ESR) in single-crystalline samples of NaYbS2. Very clear spectra with a well-resolved and large anisotropy could be observed down to the lowest accessible temperature of 2.7 K. In contrast to the ESR properties of other known spin-liquid candidate systems, the resonance seen in NaYbS2 is accessible at low fields (<1 T) and is narrow enough for accurate characterisation of the relaxation rate as well as the g factor of the Yb3+ spins.