Intermolecular hydrogen bonds impede long-range (anti-)ferroelectric order of water. We confine H
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
≈ 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.
Recently, the low-temperature phase of water molecules confined within nanocages formed by the crystalline lattice of water-containing cordierite crystals has been reported to comprise domains with ...ferroelectrically ordered dipoles within the
a
,
b
-planes which are antiferroelectrically alternating along the
c
-axis. In the present work, comprehensive broad-band dielectric spectroscopy is combined with specific heat studies and molecular dynamics and Monte Carlo simulations in order to investigate in more detail the collective modes and single-particle excitations of nanoconfined water molecules. From DFT-MD simulations we reconstruct the potential-energy landscape experienced by the H
2
O molecules. A rich set of anisotropic temperature-dependent excitations is observed in the terahertz frequency range. Their origin is associated with the complex rotational/translational vibrations of confined H
2
O molecules. A strongly temperature dependent relaxational excitation, observed at radio-microwave frequencies for the electric field parallel to the crystallographic
a
-axis,
E
||
a
is analyzed in detail. The temperature dependences of loss-peak frequency and dielectric strength of the excitation together with specific heat data confirm a ferroelectric order-disorder phase transition at
T
0
3 K in the network of H
2
O dipoles. Additional dielectric data are also provided for polarization
E
||
b
, too. Overall, these combined experimental investigations enable detailed conclusions concerning the dynamics of the confined water molecules that develop within their microscopic energy landscapes.
Dielectric spectroscopy along with MD and MC simulations was used to study the excitations of nanoconfined water molecules in cordierite nanocages.
Abstract
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, Ba
1−
x
Pb
x
Fe
12
O
19
, 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 Ba
1−
x
Pb
x
Fe
12
O
19
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 Fe
2+
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 Ba
1−
x
Pb
x
Fe
12
O
19
compounds provide information that makes the entire class of hexaferrites attractive for manufacturing electronic devices for the terahertz range.
We measure the real part ε^{′} of the dielectric permittivity of beryl crystals with heavy water molecules D_{2}O confined in nanosized cages formed by an ionic crystal lattice. The experiments are ...performed at a frequency of 1 MHz in the temperature interval from 300 down to 4 K under different hydrostatic pressures up to P=6.3 GPa. At high temperatures, a Curie-Weiss-like increase of ε^{′}(T) is observed upon cooling. Application of pressure leads to flattening of ε^{′}(T) at low temperatures due to quantum effects, i.e., tunneling of deuterium atoms in the hexagonal localizing potential. Analyzing the temperature behavior of ε^{′} with the Barrett expression allows us to obtain pressure dependencies of the quantum temperature T_{1}, the Curie-Weiss temperature T_{C}, and the Barrett constant C. The increase of T_{1} observed up to 4 GPa is associated with an enhanced azimuthal tunneling of the confined water molecules through the barriers of the potential. For P>4 GPa, T_{1}(P) levels off since the barriers disappear. Any further pressure increase does not affect the tunneling rate because of the absence of a barrier. The behavior is modeled by solving the Schrödinger equation for the water molecule in the azimuthal potential numerically. Small negative values of T_{C}≈−10 K obtained for P<4 GPa indicate the antiferroelectric ordering tendency of the water dipoles localized in the crystalline nanochannels. For higher pressure, a strong decrease of T_{C} toward negative values is observed that would correspond to the enhanced interdipole coupling strength, which is however hard to explain in the present case, and thus calls for additional theoretical and experimental studies.
We study the J_{1}-J_{2} Ising model on the square lattice using the random local field approximation (RLFA) and Monte Carlo (MC) simulations for various values of the ratio p=J_{2}/|J_{1}| with ...antiferromagnetic coupling J_{2}, ensuring spin frustration. RLFA predicts metastable states with zero order parameter (polarization) at low temperature for p∈(0,1). This is supported by our MC simulations, in which the system relaxes into metastable states with not only zero, but also with arbitrary polarization, depending on its initial value, external field, and temperature. We support our findings by calculating the energy barriers of these states at the level of individual spin flips relevant to the MC calculation. We discuss experimental conditions and compounds appropriate for experimental verification of our predictions.
We have performed the first study of the effect of an external electric field on the quantum paraelectric state of the electric dipolar lattice of polar water molecules confined within nano-sized ...cages of beryl crystal lattice. The complex dielectric permittivity ε* = ε'+iε" of a water subsystem in hydrous beryl is measured at frequencies 1 Hz-1 MHz in the temperature interval 10–300 K both in zero and 7 kV/cm external field. For comparison, we measure dielectric response of conventional quantum paraelectric KTaO3. In KTaO3, the application of electric field of several kV/cm suppresses quantum fluctuations and leads to a shallow maximum of ε′(T) around 15 K that could indicate a diffuse phase transition. In hydrous beryl, however, a bias electric field of up to 7 kV/cm has no effect on the dielectric response of the water molecules network. This result is confirmed by Monte Carlo simulations. We attribute the enhanced rigidity of the water dipolar subsystem to the strong long-range dipole-dipole interaction between separate H2O molecules. The field that could affect the polarization state of the dipoles is estimated to be of the order of 100 kV/cm.
•Electric field up to 7 kV/cm has no effect on water paraelectricity in beryl.•Enhanced rigidness of water dipolar network is related to dipole-dipole interaction.•Field of ≈105 V/cm can affect the polarization state of the dipoles.
SciPost Phys. 16, 151 (2024) Free energy as a function of polarization is calculated for the
square-lattice $J_1$-$J_2$ Ising model for $J_2 < |J_1|/2$ using the random
local field approximation ...(RLFA) and Monte Carlo (MC) simulations. Within RLFA,
it reveals a metastable state with zero polarization in the ordered phase. In
addition, the Landau free energy calculated within RLFA indicates a geometric
slab-droplet phase transition at low temperature, which cannot be predicted by
the mean field approximation. In turn, restricted free energy calculations for
finite-size samples, exact and using MC simulations, reveal metastable states
with a wide range of polarization values, but with only two domains. Taking
into account the dependence of the restricted free energy on the
nearest-neighbor correlations allows us to identify several more metastable
states. The calculations also reveal additional slab-droplet transitions at
$J_2 > |J_1|/4$. These findings enrich our knowledge of the $J_1$-$J_2$ Ising
model and the RLFA as a useful theoretical tool to study phase transitions in
spin systems.
We study the J1-J2 Ising model on the square lattice using the random local field approximation (RLFA) and Monte Carlo (MC) simulations for various values of the ratio p = J2/|J1| with ...antiferromagnetic coupling J2, ensuring spin frustration. RLFA predicts metastable states with zero order parameter (polarization) at low temperature for p \in (0,1). This is supported by our MC simulations, in which the system relaxes into metastable states with not only zero, but also with arbitrary polarization, depending on its initial value, external field, and temperature. We support our findings by calculating the energy barriers of these states at the level of individual spin flips relevant to the MC calculation. We discuss experimental conditions and compounds appropriate for experimental verification of our predictions.
Free energy as a function of polarization is calculated for the square-lattice \(J_1\)-\(J_2\) Ising model for \(J_2 < |J_1|/2\) using the random local field approximation (RLFA) and Monte Carlo (MC) ...simulations. Within RLFA, it reveals a metastable state with zero polarization in the ordered phase. In addition, the Landau free energy calculated within RLFA indicates a geometric slab-droplet phase transition at low temperature, which cannot be predicted by the mean field approximation. In turn, restricted free energy calculations for finite-size samples, exact and using MC simulations, reveal metastable states with a wide range of polarization values, but with only two domains. Taking into account the dependence of the restricted free energy on the nearest-neighbor correlations allows us to identify several more metastable states. The calculations also reveal additional slab-droplet transitions at \(J_2 > |J_1|/4\). These findings enrich our knowledge of the \(J_1\)-\(J_2\) Ising model and the RLFA as a useful theoretical tool to study phase transitions in spin systems.
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