Intermolecular hydrogen bonds impede long-range (anti-)ferroelectric order of water. We confine H 2 O molecules in nanosized cages formed by ions of a dielectric crystal. Arranging them in channels at a distance of~5 Å with an interchannel separation of~10 Å prevents the formation of hydrogen networks while electric dipole-dipole interactions remain effective. Here, we present measurements of the temperature-dependent dielectric permittivity, pyrocurrent, electric polarization and specific heat that indicate an order-disorder ferroelectric phase transition at T 0 ≈ 3 K in the water dipolar lattice. Ab initio molecular dynamics and classical Monte Carlo simulations reveal that at low temperatures the water molecules form ferroelectric domains in the ab-plane that order antiferroelectrically along the channel direction. This way we achieve the long-standing goal of arranging water molecules in polar order. This is not only of high relevance in various natural systems but might open an avenue towards future applications in biocompatible nanoelectronics.

We calculate the ferroelectric polarization dynamics induced by a femtosecond midinfrared pulse as measured in the recent experiment by R. Mankowsky et al., Phys. Rev. Lett. 118, 197601 (2017). It is due to the nonlinear coupling of the excited infrared-active phonon with the ferroelectric mode or to the excitation of the ferroelectric mode itself depending on the pulse frequency. To begin with, we write the LiNbO3 crystal symmetry invariant thermodynamic potential including electric field and nonlinear phonon coupling terms. We solve the equations of motion determined by this potential for phonon coordinates numerically in classical approximation. We explain the transient polarization reversal observed in the experiment by action of the depolarizing electric field which is due to bound charges at the polarization domain boundaries and give a reasonable estimate for its value. We argue that the polarization could be ultimately reversed when this field is screened.

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Due to their outstanding dielectric and magnetic properties, hexaferrites are attracting ever-increasing attention for developing electronic components of next-generation communication systems. The complex crystal structure of hexaferrites and the critical dependences of their electric and magnetic properties on external factors, such as magnetic/electric fields, pressure, and doping, open ample opportunities for targeted tuning of these properties when designing specific devices. Here we explored the electromagnetic properties of lead-substituted barium hexaferrite, Ba1−xPbxFe12O19, a compound featuring an extremely rich set of physical phenomena that are inherent in the dielectric and magnetic subsystems and can have a significant effect on its electromagnetic response at terahertz frequencies. We performed the first detailed measurements of the temperature-dependent (5–300 K) dielectric response of single-crystalline Ba1−xPbxFe12O19 in an extremely broad spectral range of 1 Hz–240 THz. We fully analyzed numerous phenomena with a corresponding wide distribution of specific energies that can affect the terahertz properties of the material. The most important fundamental finding is the observation of a ferroelectric-like terahertz excitation with an unusual temperature behavior of its frequency and strength. We suggest microscopic models that explain the origin of the excitation and its nonstandard temperature evolution. Several narrower terahertz excitations are associated with electronic transitions between the fine-structure components of the Fe2+ ground state. The discovered radio-frequency relaxations are attributed to the response of magnetic domains. Gigahertz resonances are presumably of magnetoelectric origin. The obtained data on diverse electromagnetic properties of Ba1−xPbxFe12O19 compounds provide information that makes the entire class of hexaferrites attractive for manufacturing electronic devices for the terahertz range.

Recently, the low-temperature phase of water molecules confined within nanocages formed by the crystalline lattice of water-containing cordierite crystals was reported to comprise domains with ferroelectrically ordered dipoles within the...