Using magnetic, thermal, and neutron measurements on single-crystal samples, we show that ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$ is a spin-$1/2$ triangular-lattice antiferromagnet with the $c$ axis as the magnetic easy axis and two magnetic phase transitions bracketing an intermediate up-up-down phase in magnetic field applied along the $c$ axis. A pronounced extensive neutron-scattering continuum above spin-wave excitations, observed below $T_{\mathrm{N}}$, implies that the system is in close proximity to one of two spin-liquid states that have been predicted for a 2D triangular lattice.

The synthesis and characterization of ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$, which has a layered array of ${\mathrm{Cu}}^{2+}$ spins in a triangular lattice, are reported. The magnetic susceptibility and neutron scattering experiments of this material show no magnetic ordering down to 0.2 K with a ${\theta }_{\mathrm{CW}}=-55\mathrm{K}$. The magnetic specific heat reveals a $T$-linear dependence with a $\gamma =43.4\mathrm{mJ}{\mathrm{K}}^{-2}{\mathrm{mol}}^{-1}$ below 1.4 K. These observations suggest that ${\mathrm{Ba}}_3{\mathrm{CuSb}}_2{\mathrm{O}}_9$ is a new quantum spin liquid candidate with a $S=1/2$ triangular lattice.

By muon spin-relaxation measurements on single-crystal specimens, we show that superconductivity in the $A{\text{Fe}}_2{\text{As}}_2$ $\left(A=\text{Ca},\text{Ba},\text{Sr}\right)$ systems, in both the cases of composition and pressure tunings, coexists with a strong static magnetic order in a partial volume fraction. The superfluid response from the remaining paramagnetic volume fraction of $\left({\text{Ba}}_{0.5}{\text{K}}_{0.5}\right){\text{Fe}}_2{\text{As}}_2$ exhibits a nearly linear variation in $T$ at low temperatures, suggesting an anisotropic energy gap with line nodes and/or multigap effects.

In this Letter, we report a new spin ice—${\mathrm{Pr}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$—which appears to have enhanced residual entropy due to the dynamic nature of the spins. Neutron scattering experiments show that at 200 mK, there is a significant amount of magnetic diffuse scattering which can be fit to the dipolar spin-ice model. However, these short-ranged ordered spins have a quasielastic response that is atypical of the canonical spin ices, and suggests that the ground state is dynamic (i.e., composed of locally ordered two-in–two-out spin configurations that can tunnel between energetically equivalent orientations). We report this as an example of a dynamic spin ice down to 200 mK.

A comparison among the two sets of studied pyrochlore spin ices, ${\mathrm{Ho}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$, ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, and ${\mathrm{Ho}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ with ${\mathrm{Ho}}^{3+}$ spins and ${\mathrm{Dy}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$, ${\mathrm{Dy}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, and ${\mathrm{Dy}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ with ${\mathrm{Dy}}^{3+}$ spins, shows that the application of chemical pressure through each set drives the system toward the antiferromagnetic phase boundary from the spin ice region, which agrees with the prediction of the “dipolar spin ice” model of den Hertog and Gingras. Among all the studied pyrochlore spin ices, ${\mathrm{Dy}}_2{\mathrm{Ge}}_2{\mathrm{O}}_7$ has the smallest ratio of $J_{nn}/D_{nn}=-0.73$.