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$\alpha -{\mathrm{RuCl}}_3$ has attracted enormous attention since it has been proposed as a prime candidate to study fractionalized magnetic excitations akin to Kitaev's honeycomb-lattice spin liquid. We have performed a detailed specific-heat investigation at temperatures down to $0.4\mathrm{K}$ in applied magnetic fields up to $9\mathrm{T}$ for fields parallel to the $ab$ plane. We find a suppression of the zero-field antiferromagnetic order, together with an increase of the low-temperature specific heat, with increasing field up to ${\mu }_0{H}_{\text{c}}\approx 6.9$ T. Above $H_{\text{c}}$, the magnetic contribution to the low-temperature specific heat is strongly suppressed, implying the opening of a spin-excitation gap. Our data point toward a field-induced quantum critical point at $H_{\text{c}}$; this is supported by universal scaling behavior near $H_{\text{c}}$. Remarkably, the data also reveal the existence of a small characteristic energy scale well below 1 meV, above which the excitation spectrum changes qualitatively. We relate the data to theoretical calculations based on a $J_1-K_1-{\mathrm{\Gamma }}_1-J_3$ honeycomb model.

Magnetic properties of the substitution series ${\mathrm{Ru}}_{1-x}{\mathrm{Cr}}_x{\mathrm{Cl}}_3$ were investigated to determine the evolution from the anisotropic Kitaev magnet $\alpha \text{−}{\mathrm{RuCl}}_3$ with $J_{\mathrm{eff}}=1/2$ magnetic ${\mathrm{Ru}}^{3+}$ ions to the isotropic Heisenberg magnet ${\mathrm{CrCl}}_3$ with $S=3/2$ magnetic ${\mathrm{Cr}}^{3+}$ ions. Magnetization measurements on single crystals revealed a reversal of the magnetic anisotropy under doping, which we argue to arise from the competition between anisotropic Kitaev and off-diagonal interactions on the Ru-Ru links and approximately isotropic Cr-Ru and isotropic Cr-Cr interactions. In addition, combined magnetization, ac susceptibility, and specific-heat measurements clearly show the destabilization of the long-range magnetic order of $\alpha \text{−}{\mathrm{RuCl}}_3$ in favor of a spin-glass state of ${\mathrm{Ru}}_{1-x}{\mathrm{Cr}}_x{\mathrm{Cl}}_3$ for a low doping of $x\backsimeq 0.1$. The corresponding freezing temperature as a function of Cr content shows a broad maximum around $x\backsimeq 0.45$.

Water is a unique solvent with strong, yet highly dynamic, intermolecular interactions. Many insights into this distinctive liquid have been obtained using ultrafast vibrational spectroscopy of water’s O-H stretch vibration. However, it has been challenging to separate the different contributions to the dynamics of the O-H stretch vibration in ${\mathrm{H}}_2\mathrm{O}$. Here, we present a novel nonequilibrium molecular dynamics (NEMD) algorithm that allows for a detailed picture of water vibrational dynamics by generating nonequilibrium vibrationally excited states at targeted vibrational frequencies. Our ab initio NEMD simulations reproduce the experimentally observed time scales of vibrational dynamics in ${\mathrm{H}}_2\mathrm{O}$. The approach presented in this work uniquely disentangles the effects on the vibrational dynamics of four contributions: the delocalization of the O-H stretch mode, structural dynamics of the hydrogen bonded network, intramolecular coupling within water molecules, and intermolecular coupling between water molecules (near-resonant energy transfer between O-H groups). Our results illustrate that intermolecular energy transfer and the delocalization of the O-H stretch mode are particularly important for the spectral diffusion in ${\mathrm{H}}_2\mathrm{O}$.