Quantum dipoles go liquid: Quantum spin liquids do not achieve an ordered magnetic state, even at the lowest temperatures. Hassan et al. studied an organic compound that may be both a spin liquid and a dipole liquid (see the Perspective by Powell). In the layered material κ-(BEDT-TTF) ₂ Hg(SCN) ₂ Br, molecules form charged dimers whose sites are arranged on a triangular lattice. The extra charge associated with each dimer can “live” on one of the two molecules in the dimer, resulting in a nonzero electric dipole moment for the dimer. Raman spectroscopy and heat capacity measurements revealed that, like spins in a quantum spin liquid, these dimers remained disordered down to the lowest temperatures. Science , this issue p. 1101 ; see also p. 1073

We explore the stability of the hybridization gap in ${\mathrm{SmB}}_6$ to the presence of a small number of Sm vacancies typical for this material, and demonstrate the extreme fragility of the Kondo insulating state. For the most stoichiometric sample we detect the hybridization gap which appears below 50 K as a depressed electronic Raman intensity below about 30 meV. The spectral weight that shifts to higher frequencies on the opening of the hybridization gap forms two electronic maxima at 100 and 41 meV. We assign these maxima to the excitations between hybridized $4f-5d$ bands using recent band structure calculations. Below 30 K, in-gap exciton modes with long lifetimes protected by the hybridization gap develop at 16–18 meV. With the increase of the number of Sm vacancies the exciton features broaden, evidencing a decrease in the lifetime due to the presence of electronic states in the gap. At a concentration of Sm vacancies of only about 1% the in-gap exciton is completely quenched, and the hybridization gap is not fully opened. We suggest that only the most stoichiometric ${\mathrm{SmB}}_6$ samples possess a bulk gap necessary for the topological Kondo insulator state.

Strong electronic interactions can drive a system into a state with a symmetry breaking. Lattice frustration or competing interactions tend to prevent symmetry breaking, leading to quantum disordered phases. In spin systems frustration can produce a spin liquid state. Frustration of a charge degree of freedom also can result in various exotic states, however, experimental data on these effects is scarce. In this work we demonstrate how in a Mott insulator on a weakly anisotropic triangular lattice a charge ordered state melts on cooling down to low temperatures. Raman scattering spectroscopy finds that $$\kappa$$κ -(BEDT-TTF) $${}_{2}$$2 Hg(SCN) $${}_{2}$$2 Cl enters an insulating “dipole solid” state at $$T=30\,{\mathrm{K}}$$T = 30 K , but below $$T=15\,{\mathrm{K}}$$T = 15 K the order melts, while preserving the insulating energy gap. Based on these observations, we suggest a phase diagram relevant to other quantum paraelectric materials.

${\mathrm{NiGa}}_2{\mathrm{S}}_4$ is a triangular lattice $S=1$ system with strong two dimensionality of the lattice, actively discussed as a candidate to host spin-nematic order brought about by strong quadrupole coupling. Using Raman scattering spectroscopy we identify a phonon of $E_g$ symmetry which can modulate magnetic exchange $J_1$ and produce quadrupole coupling. Additionally, our Raman scattering results demonstrate a loss of local inversion symmetry on cooling, which we associate with sulfur vacancies. This will lead to disordered Dzyaloshinskii-Moriya interactions, which can prevent long-range magnetic order. Using magnetic Raman scattering response we identify 160 K as a temperature of an upturn of magnetic correlations. The temperature range below 160 K, but above 50 K where antiferromagnetic correlations start to increase, is a candidate for spin-nematic regime.

Co-based materials have recently been explored because of their potential to realize complex bond-dependent anisotropic magnetism. Prominent examples include ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6, {\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2, {\mathrm{Na}}_2\mathrm{BaCo}{\left({\mathrm{PO}}_4\right)}_2$, and ${\mathrm{Co}\mathit{X}}_2$ $(X=\mathrm{Cl},\mathrm{Br},\mathrm{I})$. In order to provide insight into the magnetic interactions in these compounds, we make a comparative analysis of their local crystal electric field excitations spectra via Raman scattering measurements. Combining these measurements with theoretical analysis confirms the validity of $j_{\mathrm{eff}}=1/2$ single-ion ground states for all compounds, and provides accurate experimental estimates of the local crystal distortions, which play a prominent role in the magnetic couplings between spin-orbital coupled Co moments.

Using Raman spectroscopy, we investigate the lattice phonons, magnetic excitations, and magnetoelastic coupling in the distorted triangular-lattice Heisenberg antiferromagnet $\alpha {\text{-SrCr}}_2{\text{O}}_4$, which develops helical magnetic order below 43 K. Temperature-dependent phonon spectra are compared to predictions from density functional theory calculations which allows us to assign the observed modes and identify weak effects arising from coupled lattice and magnetic degrees of freedom. Raman scattering associated with two-magnon excitations is observed at 20 and 40 meV. These energies are in general agreement with our ab initio calculations of exchange interactions and earlier theoretical predictions of the two-magnon Raman response of triangular-lattice antiferromagnets. The temperature dependence of the two-magnon excitations indicates that spin correlations persist well above the Néel temperature.