Dielectric ordering of water molecules arranged in a dipolar lattice

Nonlinear Effects of Hydrophobic Confinement on the Electronic Structure and Dielectric Response of Water

Fundamental studies of the dielectrics of confined water are critical to understand the ion transport across biological and synthetic nanochannels. The relevance of these fundamental studies, however, surmounts the difficulty of probing water's dielectric constant as a function of a fine variation in confinement. In this work, we explore the computational efficiency of machine learning potentials to derive the confinement effects on the dielectric constant, polarization, and dipole moment of water. Our simulations predict an enhancement of the axial dielectric constant of water under extreme confinement, arising from either the formation of ferroelectric structures of ordered water or larger dipole fluctuations facilitated by the disruption of water's H-bond network. Our study highlights the impact of hydrophobic nanoconfinement on the dielectric constant and on the ionic and electronic structure of water molecules, pointing to the importance of geometric flexibility and electronic polarizability to properly model confinement effects on water.

Ground states of planar dipolar rotor chains with recurrent neural networks

In this contribution, we employ a recurrent neural network (RNN) architecture in a variational optimization to obtain the ground state of linear chains of planar, dipolar rotors. We test different local basis sets and discuss their impact on the sign structure of the many-body ground state wavefunction. It is demonstrated that the RNN ansatz we employ is able to treat systems with and without a sign problem in the ground state. For larger chains with up to 50 rotors, accurate properties, such as correlation functions and Binder parameters, are calculated. By employing quantum annealing, we show that precise entanglement properties can be obtained. All these properties allow one to identify a quantum phase transition between a paraelectric and a ferroelectric quantum phase.

Operator Formulation of Feynman Path Centroid Dynamics for Rotations

An operator formulation of centroid molecular dynamics (CMD) for rotational degrees of freedom is presented. The quasi-density operator concept was introduced by Jang and Voth [J. Chem. Phys 111, 2357 (1999)] and is used to obtain a phase-space mapping without the need for discretized path integrals. The approach allows the calculation of approximate Kubo-transformed time correlation functions. The particle on a ring is chosen as an illustrative example. Numerical results demonstrate that the proposed approach leads to accurate results when compared with exact diagonalization calculations for linear operators. At very low temperatures, it is found that rotational CMD yields results that are in exact agreement with the quantum dynamics of a spin-1 system.

Quantum criticality in chains of planar rotors with dipolar interactions

In this work, we perform a density matrix renormalization group study of chains of planar rotors interacting via dipolar interactions. By exploring the ground state from weakly to strongly interacting rotors, we find the occurrence of a quantum phase transition between a disordered and a dipole-ordered quantum state. We show that the nature of the ordered state changes from ferroelectric to antiferroelectric when the relative orientation of the rotor planes varies and that this change requires no modification of the overall symmetry. The observed quantum phase transitions are characterized by critical exponents and central charges, which reveal different universality classes ranging from that of the (1 + 1)D Ising model to the 2D classical XY model.

Pseudo-Goldstone Modes and Dynamical Gap Generation from Order by Thermal Disorder

Accidental ground state degeneracies-those not a consequence of global symmetries of the Hamiltonian-are inevitably lifted by fluctuations, often leading to long-range order, a phenomenon known as "order-by-disorder" (ObD). The detection and characterization of ObD in real materials currently lacks clear, qualitative signatures that distinguish ObD from conventional energetic selection. We show that for order by thermal disorder (ObTD) such a signature exists: a characteristic temperature dependence of the fluctuation-induced pseudo-Goldstone gap. We demonstrate this in a minimal two-dimensional model that exhibits ObTD, the ferromagnetic Heisenberg-compass model on a square lattice. Using spin-dynamics simulations and self-consistent mean-field calculations, we determine the pseudo-Goldstone gap, Δ, and show that at low temperatures it scales as the square root of temperature, sqrt[T]. We establish that a power-law temperature dependence of the gap is a general consequence of ObTD, showing that all key features of this physics can be captured in a simple model of a particle moving in an effective potential generated by the fluctuation-induced free energy.

Metastable states in the J_{1}-J_{2} Ising model

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.

Thermal immuno-nanomedicine in cancer

Immunotherapy has revolutionized the treatment of patients with cancer. However, promoting antitumour immunity in patients with tumours that are resistant to these therapies remains a challenge. Thermal therapies provide a promising immune-adjuvant strategy for use with immunotherapy, mostly owing to the capacity to reprogramme the tumour microenvironment through induction of immunogenic cell death, which also promotes the recruitment of endogenous immune cells. Thus, thermal immunotherapeutic strategies for various cancers are an area of considerable research interest. In this Review, we describe the role of the various thermal therapies and provide an update on attempts to combine these with immunotherapies in clinical trials. We also provide an overview of the preclinical development of various thermal immuno-nanomedicines, which are capable of combining thermal therapies with various immunotherapy strategies in a single therapeutic platform. Finally, we discuss the challenges associated with the clinical translation of thermal immuno-nanomedicines and emphasize the importance of multidisciplinary and inter-professional collaboration to facilitate the optimal translation of this technology from bench to bedside.

Radio frequency dielectric measurements in diamond anvil cells

We present the modifications, performance, and test of a diamond anvil cell for radio frequency dielectric spectroscopy studies of single crystals that can be used from room temperature down to 4 K and up to pressures of 5-6 GPa. Continuous frequency-dependent measurements between 5 Hz and 1 MHz can be performed with this modified pressure cell. The cell has an excellent performance with temperature-, frequency-, and pressure-independent stray capacitance of around 2 pF, enabling us to use relatively small samples with a weak dielectric response.

Quantum Phase Transition in the One-Dimensional Water Chain

The concept of quantum phase transitions (QPTs) plays a central role in the description of condensed matter systems. In this Letter, we perform high-quality wave-function-based simulations to demonstrate the existence of a quantum phase transition in a crucially relevant molecular system, namely, water, forming linear chains of rotating molecules. We determine various critical exponents and reveal the water chain QPT to belong to the (1+1)-dimensional Ising universality class. Furthermore, the effect of breaking symmetries is examined, and it is shown that, by breaking the inversion symmetry, the ground state degeneracy of the ordered quantum phase is lifted to yield two many-body states with opposite polarization. The possibility of forming ferroelectric phases together with a thermal stability of the quantum critical regime up to ∼10  K makes the linear water chain a promising candidate as a platform for quantum devices.

Ferroelectric water chains in carbon nanotubes: Creation and manipulation of ordered quantum phases

Systems composed of molecular rotors are promising candidates as quantum devices. In this work, we employ our recently developed density matrix renormalization group approach to study such a rotor system, namely, linear chains of rotating para-water molecules encapsulated in a (6,5)-carbon nanotube. We show that the anisotropic environment provided by the nanotube breaks the inversion symmetry of the chain. This symmetry breaking lifts the degeneracy of the ground state and leads to a splitting between the left- and right-polarized states. In turn, a ferroelectric phase in nanoscopic systems is created, with a polarization that can be switched in a manner analogous to that of a supramolecular qubit. We present results for a few low-lying states and discuss the effect of external electric fields on the energy splitting and the occurrence of a quantum phase transition.

Covalent organic framework membranes for efficient separation of monovalent cations

Covalent organic frameworks (COF), with rigid, highly ordered and tunable structures, can actively manipulate the synergy of entropic selectivity and enthalpic selectivity, holding great potential as next-generation membrane materials for ion separations. Here, we demonstrated the efficient separation of monovalent cations by COF membrane. The channels of COF membrane are decorated with three different kinds of acid groups. A concept of confined cascade separation was proposed to elucidate the separation process. The channels of COF membrane comprised two kinds of domains, acid-domains and acid-free-domains. The acid-domains serve as confined stages, rendering high selectivity, while the acid-free-domains preserve the pristine channel size, rendering high permeation flux. A set of descriptors of stage properties were designed to elucidate their effect on selective ion transport behavior. The resulting COF membrane acquired high ion separation performances, with an actual selectivity of 4.2–4.7 for K+/Li+ binary mixtures and an ideal selectivity of ~13.7 for K+/Li+.

Unusual features of lattice dynamics in lawsonite near its phase transitions

Lattice dynamics of a single crystal of lawsonite were studied over a broad range of frequencies (1 Hz to 20 THz) using impedance, THz time-domain and infrared spectroscopies. Based on polarized spectra of complex permittivity \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hat{\varepsilon }$$\end{document}ε^ measured as a function of temperature between 10 K and 500 K, we analyzed the properties of the two known phase transitions—an antiferrodistortive one near \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{\mathrm{c}1}=270\,\mathrm{K}$$\end{document}Tc1=270K and a ferroelectric one, occurring at \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{\mathrm{c}2}=124\,\mathrm{K}$$\end{document}Tc2=124K. The former one is accompanied by a flat maximum in the THz-range permittivity \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hat{\varepsilon }_{\mathrm{c}}$$\end{document}ε^c near \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{\mathrm{c}1}$$\end{document}Tc1, which is due to an overdamped polar excitation in the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathbf {E} \parallel c$$\end{document}E‖c spectra reflecting the dynamics of water and hydroxyl groups. The strength of this mode decreases on cooling below \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{\mathrm{c}1}$$\end{document}Tc1, and the mode vanishes below \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{\mathrm{c}2}$$\end{document}Tc2 due to hydrogen ordering. At the pseudoproper ferroelectric phase transition, two independent anomalies in permittivity were observed. First, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hat{\varepsilon }_a$$\end{document}ε^a exhibits a peak at \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{\mathrm{c}2}=124\,\mathrm{K}$$\end{document}Tc2=124K due to critical slowing down of a relaxation in the GHz range. Second, infrared and THz spectra revealed an optical phonon softening towards \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{\mathrm{c}2}$$\end{document}Tc2 which causes a smaller but pronounced maximum in \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hat{\varepsilon }_b$$\end{document}ε^b. Such anomaly, consisting in a soft mode polarized perpendicularly to the ferroelectric axis, is unusual in ferroelectrics.

Single-particle and collective excitations of polar water molecules confined in nano-pores within cordierite crystal lattice

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...

Dipole ordering of water molecules in cordierite: Monte Carlo simulations

Electric dipoles of water molecules, enclosed singly in regularly spaced nanopores of a cordierite crystal, become ordered at low temperature due to their mutual interaction and show the frequency dependence of their dielectric susceptibility, typical for relaxor ferroelectrics, according to recent experimental data. The corresponding phase transition is accompanied by anomalies in thermodynamic quantities, such as heat capacity and dielectric susceptibility, which are calculated here using the Monte Carlo method, and their agreement with the experimental data is discussed. Despite the increase in the correlation length, the partially filled dipole lattice at low temperatures, according to the calculations, does not have long-range order and corresponds to a dipole glass. This simulation gives a microscopical insight into the formation of polar nanoregions in relaxor ferroelectrics and the temperature dependence of their size.

Hydroxyl Defects in LiFePO4 Cathode Material: DFT+U and an Experimental Study

Lithium iron phosphate, LiFePO₄, a widely used cathode material in commercial Li-ion batteries, unveils a complex defect structure, which is still being deciphered. Using a combined computational and experimental approach comprising density functional theory (DFT)+U and molecular dynamics calculations and X-ray and neutron diffraction, we provide a comprehensive characterization of various OH point defects in LiFePO₄, including their formation, dynamics, and localization in the interstitial space and at Li, Fe, and P sites. It is demonstrated that one, two, and four (five) OH groups can effectively stabilize Li, Fe, and P vacancies, respectively. The presence of D (H) at both Li and P sites for hydrothermally synthesized deuterium-enriched LiFePO₄ is confirmed by joint X-ray and neutron powder diffraction structure refinement at 5 K that also reveals a strong deficiency of P of 6%. The P occupancy decrease is explained by the formation of hydrogarnet-like P/4H and P/5H defects, which have the lowest formation energies among all considered OH defects. Molecular dynamics simulation shows a rich structural diversity of these defects, with OH groups pointing both inside and outside vacant P tetrahedra creating numerous energetically close conformers, which hinders their explicit localization with diffraction-based methods solely. The discovered conformers include structural water molecules, which are only by 0.04 eV/atom H higher in energy than separate OH defects.

Infrared spectroscopy of an endohedral water in fullerene

An infrared absorption spectroscopy study of the endohedral water molecule in a solid mixture of H₂O@C₆₀ and C₆₀ was carried out at liquid helium temperature. From the evolution of the spectra during the ortho-para conversion process, the spectral lines were identified as para-H₂O and ortho-H₂O transitions. Eight vibrational transitions with rotational side peaks were observed in the mid-infrared: ω₁, ω₂, ω₃, 2ω₁, 2ω₂, ω₁ + ω₃, ω₂ + ω₃, and 2ω₂ + ω₃. The vibrational frequencies ω₂ and 2ω₂ are lower by 1.6% and the rest by 2.4%, as compared to those of free H₂O. A model consisting of a rovibrational Hamiltonian with the dipole and quadrupole moments of H₂O interacting with the crystal field was used to fit the infrared absorption spectra. The electric quadrupole interaction with the crystal field lifts the degeneracy of the rotational levels. The finite amplitudes of the pure v₁ and v₂ vibrational transitions are consistent with the interaction of the water molecule dipole moment with a lattice-induced electric field. The permanent dipole moment of encapsulated H₂O is found to be 0.50 ± 0.05 D as determined from the far-infrared rotational line intensities. The translational mode of the quantized center-of-mass motion of H₂O in the molecular cage of C₆₀ was observed at 110 cm^-1 (13.6 meV).

Protein dielectrophoresis: Key dielectric parameters and evolving theory

Globular proteins exhibit dielectrophoresis (DEP) responses in experiments where the applied field gradient factor ∇E² appears far too small, according to standard DEP theory, to overcome dispersive forces associated with the thermal energy kT of disorder. To address this a DEP force equation is proposed that replaces a previous empirical relationship between the macroscopic and microscopic forms of the Clausius-Mossotti factor. This equation relates the DEP response of a protein directly to the dielectric increment δε⁺ and decrement δε^- that characterize its β-dispersion at radio frequencies, and also indirectly to its intrinsic dipole moment by way of providing a measure of the protein's effective volume. A parameter Γ_pw , taken as a measure of cross-correlated dipole interactions between the protein and its water molecules of hydration, is included in this equation. For 9 of the 12 proteins, for which an evaluation can presently be made, Γ_pw has a value of ≈4600 ± 120. These conclusions follow an analysis of the failure of macroscopic dielectric mixture (effective medium) theories to predict the dielectric properties of solvated proteins. The implication of a polarizability greatly exceeding the intrinsic value for a protein might reflect the formation of relaxor ferroelectric nanodomains in its hydration shell.