Abstract This review summarizes experimental evidence for the freezing of reorienting moments in solids. The moments may be of dipolar or quadrupolar nature, or both; they belong to one of the constituents of a mixed-crystal solid. Extensive results are reported for the following systems: KCl doped with hydroxyl, potassium tantalate doped with Li, Na and Nb, alkali halide cyanides and alkali-alkali cyanides, rubidium ammonium dihydrogen phosphate, solid ortho-para hydrogen and argon-nitrogen mixtures. These have clearly glass-like properties. In other systems, results are limited to one or two methods hinting at glass formation; some of those are also reported. Clustering phenomena and the slow-down of reorientations at the freezing temperature are observed in susceptibility measurements and by local probing on nuclear spins. The modulation of the structure by cluster formation is revealed by diffraction experiments. These phenomena are confronted with model predictions and numerical simulations.
Using neutron powder diffraction and Monte Carlo simulations we show that a spin-liquid regime emerges at all compositions in the diamond-lattice antiferromagnets $\text{Co}{({\text{Al}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{O}}_{4}$. This spin-liquid regime induced by frustration due to the second-neighbor exchange coupling ${J}_{2}$ is gradually superseded by antiferromagnetic collinear long-range order $(\mathbf{k}=0)$ at low temperatures. Upon substitution of ${\text{Al}}^{3+}$ by ${\text{Co}}^{3+}$ in the octahedral B site the temperature range occupied by the spin-liquid regime narrows and ${T}_{N}$ increases. To explain the experimental observations we considered magnetic anisotropy $D$ or third-neighbor exchange coupling ${J}_{3}$ as degeneracy-breaking perturbations. We conclude that $\text{Co}{({\text{Al}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{O}}_{4}$ is below the theoretical critical point ${J}_{2}/{J}_{1}=1/8$, and that magnetic anisotropy assists in selecting a collinear long-range ordered ground state, which becomes more stable with increasing $x$ due to a higher efficiency of ${\text{O-Co}}^{3+}\text{-O}$ as an interaction path compared to ${\text{O-Al}}^{3+}\text{-O}$.
We have conducted comprehensive electron spin resonance (ESR) investigations on single crystals of the one-dimensional organic compounds $(\mathrm{TMTTF}{)}_{2}{\mathrm{PF}}_{6}, (\mathrm{TMTTF}{)}_{2}{\mathrm{ClO}}_{4},$ $(\mathrm{TMTTF}{)}_{2}\mathrm{Br},$ $(\mathrm{TMTSF}{)}_{2}{\mathrm{PF}}_{6},$ and $(\mathrm{TMTSF}{)}_{2}{\mathrm{AsF}}_{6}$ in the temperature range from 4 to 500 K and additionally, $(\mathrm{TMTSF}{)}_{2}{\mathrm{ReO}}_{4}$ and $(\mathrm{TMTSF}{)}_{2}{\mathrm{ClO}}_{4}$ at room temperature. In contrast to the selenium analogs TMTSF which are one-dimensional metals, the sulfur salts are semiconductors with localized spins on the TMTTF dimers. Taking into account the thermal expansion of the crystals at high temperature $(T>20 \mathrm{K})$ the ESR intensity of all sulfur compounds can be described as a spin-1/2 antiferromagnetic Heisenberg chain with exchange constants $420<~J<~500 \mathrm{K}.$ Although the TMTSF compounds are one-dimensional organic metals down to 10 K, the temperature dependence of the spin susceptibility can also be described within the framework of the Hubbard model in the limit of strong Coulomb repulsion with $J\ensuremath{\approx}1400 \mathrm{K}.$ By modeling $(\mathrm{TMTTF}{)}_{2}{\mathrm{ClO}}_{4}$ as an alternating spin chain, the change of the alternation parameter at the first-order phase transition ${(T}_{\mathrm{AO}}=72.5 \mathrm{K})$ indicates a tetramerization of the chain. $(\mathrm{TMTTF}{)}_{2}{\mathrm{PF}}_{6}$ undergoes a spin-Peierls transition at ${T}_{\mathrm{SP}}=19 \mathrm{K}$ which can be well described by Bulaevskii's model with a singlet-triplet gap ${\ensuremath{\Delta}}_{\ensuremath{\sigma}}(0)=32.3 \mathrm{K}.$ We find evidence of antiferromagnetic fluctuations at temperatures well above the magnetic ordering in $(\mathrm{TMTTF}{)}_{2}\mathrm{Br},$ $(\mathrm{TMTSF}{)}_{2}{\mathrm{PF}}_{6},$ and $(\mathrm{TMTSF}{)}_{2}{\mathrm{AsF}}_{6}$ which follow the critical behavior expected for three-dimensional ordering. $(\mathrm{TMTTF}{)}_{2}{\mathrm{PF}}_{6}$ and $(\mathrm{TMTTF}{)}_{2}\mathrm{Br}$ show one-dimensional lattice fluctuations.
Why does a microwave oven work? How does biological tissue absorb electromagnetic radiation? Astonishingly, we do not have a definite answer to these simple questions because the microscopic processes governing the absorption of electromagnetic waves by water are largely unclarified. This absorption can be quantified by dielectric loss spectra, which reveal a huge peak at a frequency of the exciting electric field of about 20 GHz and a gradual tailing off towards higher frequencies. The microscopic interpretation of such spectra is highly controversial and various superpositions of relaxation and resonance processes ascribed to single-molecule or molecule-cluster motions have been proposed for their analysis. By combining dielectric, microwave, THz, and far-infrared spectroscopy, here we provide nearly continuous temperature-dependent broadband spectra of water. Moreover, we find that corresponding spectra for aqueous solutions reveal the same features as pure water. However, in contrast to the latter, crystallization in these solutions can be avoided by supercooling. As different spectral contributions tend to disentangle at low temperatures, this enables to deconvolute them when approaching the glass transition under cooling. We find that the overall spectral development, including the 20 GHz feature (employed for microwave heating), closely resembles the behavior known for common supercooled liquids. Thus, water's absorption of electromagnetic waves at room temperature is not unusual but very similar to that of glass-forming liquids at elevated temperatures, deep in the low-viscosity liquid regime, and should be interpreted along similar lines.
