Incipient ferroelectricity of water molecules confined to nano-channels of beryl

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.

Suppressed terahertz dynamics of water confined in nanometer gaps

Nanoconfined waters exhibit low static permittivity mainly due to interfacial effects that span about one nanometer. The characteristic length scale may be much longer in the terahertz (THz) regime where long-range collective dynamics occur; however, the THz dynamics have been largely unexplored because of the lack of a robust platform. Here, we use metallic loop nanogaps to sharply enhance light-matter interactions and precisely measure real and imaginary THz refractive indices of nanoconfined water at gap widths ranging from 2 to 20 nanometers, spanning mostly interfacial waters all the way to quasi-bulk waters. We find that, in addition to the well-known interfacial effect, the confinement effect also contributes substantially to the decrease in the complex refractive indices of the nanoconfined water by cutting off low-energy vibrational modes, even at gap widths as large as 10 nanometers. Our findings provide valuable insights into the collective dynamics of water molecules which is crucial to understanding water-mediated processes.

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.

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.

Fingerprints of Critical Phenomena in a Quantum Paraelectric Ensemble of Nanoconfined Water Molecules

We have studied the radio frequency dielectric response of a system consisting of separate polar water molecules periodically arranged in nanocages formed by the crystal lattice of the gemstone beryl. Below T = 20-30 K, quantum effects start to dominate the properties of the electric dipolar system as manifested by a crossover between the Curie-Weiss and the Barrett regimes in the temperature-dependent real dielectric permittivity ε'(T). When analyzing in detail the temperature evolution of the reciprocal permittivity (ε')^-1 down to T ≈ 0.3 K and comparing it with the data obtained for conventional quantum paraelectrics, like SrTiO₃, KTaO₃, we discovered clear signatures of a quantum-critical behavior of the interacting water molecular dipoles: Between T = 6 and 14 K, the reciprocal permittivity follows a quadratic temperature dependence and displays a shallow minimum below 3 K. This is the first observation of "dielectric fingerprints" of quantum-critical phenomena in a paraelectric system of coupled point electric dipoles.

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...

The Electrostatic Model of Homeopathy: The Mechanism of Physicochemical Activities of Homeopathic Medicines

This paper attempts to propose a model, called the electrostatic model of homeopathy, to explain a mechanism for the physicochemical activities of highly diluted homeopathic medicines (HMs). According to this proposed model, the source of HMs' action is dipole orientations as electrostatic imprints of the original molecules carried by diluent molecules (such as sugar molecules) or potentization-induced aqueous nanostructures. The nanoscale domains' contact charging and dielectric hysteresis play critical roles in the aqueous nanostructures' or sugar molecules' acquisition of the original molecules' dipole orientations. The mechanical stress induced by dynamization (vigorous agitation or trituration) is a crucial factor that facilitates these phenomena. After dynamization is completed, the transferred charges revert to their previous positions but, due to dielectric hysteresis, they leave a remnant polarization on the aqueous nanostructures or sugar molecules' nanoscale domains. This causes some nanoscale domains of the aqueous nanostructures or sugar molecules to obtain the original substance molecules' dipole orientations. A highly diluted HM may have no molecule of the original substance, but the aqueous nanostructures or sugar molecules may contain the original substance's dipole orientations. Therefore, HMs can precisely aim at the biological targets of the original substance molecules and electrostatically interact with them as mild stimuli.

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.

Abnormal in-plane permittivity and ferroelectricity of confined water: From sub-nanometer channels to bulk

Dielectric properties of nano-confined water are important in several areas of science, i.e., it is relevant in the dielectric double layer that exists in practically all heterogeneous fluid-based systems. Molecular dynamics simulations are used to predict the in-plane dielectric properties of confined water in planar channels of width ranging from sub-nanometer to bulk. Because of suppressed rotational degrees of freedom near the confining walls, the dipole of the water molecules tends to be aligned parallel to the walls, which results in a strongly enhanced in-plane dielectric constant (ε_∥) reaching values of about 120 for channels with height 8 Å < h < 10 Å. With the increase in the width of the channel, we predict that ε_∥ decreases nonlinearly and reaches the bulk value for h > 70 Å. A stratified continuum model is proposed that reproduces the h > 10 Å dependence of ε_∥. For sub-nanometer height channels, abnormal behavior of ε_∥ is found with two orders of magnitude reduction of ε_∥ around h ∼ 7.5 Å, which is attributed to the formation of a particular ice phase that exhibits long-time (∼μs) stable ferroelectricity. This is of particular importance for the understanding of the influence of confined water on the functioning of biological systems.

Rotational coherence of encapsulated ortho and para water in fullerene-C60 revealed by time-domain terahertz spectroscopy

We resolve the real-time coherent rotational motion of isolated water molecules encapsulated in fullerene-C60 cages by time-domain terahertz (THz) spectroscopy. We employ single-cycle THz pulses to excite the low-frequency rotational motion of water and measure the subsequent coherent emission of electromagnetic waves by water molecules. At temperatures below ~ 100 K, C60 lattice vibrational damping is mitigated and the quantum dynamics of confined water are resolved with a markedly long rotational coherence, extended beyond 10 ps. The observed rotational transitions agree well with low-frequency rotational dynamics of single water molecules in the gas phase. However, some additional spectral features with their major contribution at ~2.26 THz are also observed which may indicate interaction between water rotation and the C60 lattice phonons. We also resolve the real-time change of the emission pattern of water after a sudden cooling to 4 K, signifying the conversion of ortho-water to para-water over the course of 10s hours. The observed long coherent rotational dynamics of isolated water molecules confined in C60 makes this system an attractive candidate for future quantum technology.

One-Dimensional Glassy Behavior of Ultraconfined Water Strings

Water is renowned for its anomalous behaviors, which can be linked to a distributed H-bond network in bulk water. Ultraconfinement of the water molecule can remove H-bonding, leaving only molecular water. In natural cordierite crystals, water is trapped in an orthorhombic channel with an average diameter of 5.7 Å, running through the center of the unit cell parallel to the c-axis. Calorimetric measurements reveal the existence of a one-dimensional (1D) glass linked to this water. In these channels, water molecules in truncated, sparse 1D strings interact only via dipole-dipole correlations. A physical 1D glass is formed from these strings. This unusual state can be explained by a modified Ising model. This model predicts a dependence of the glass transition temperature, T_g, on the size of these domains. This is confirmed experimentally.

Dielectric ordering of water molecules arranged in a dipolar lattice

Intermolecular hydrogen bonds impede long-range (anti-)ferroelectric order of water. We confine H₂O 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 T₀ ≈ 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.

Despite the apparent simplicity of a H2O molecule, the mutual ferroelectric ordering of the molecules is unresolved. Here, the authors realize a macroscopic ferroelectric phase transition in a network of dipole-dipole coupled water molecules located in nanopores of gemstone.

Microwave probing of bulk dielectrics using superconducting coplanar resonators in distant-flip-chip geometry

Dielectric measurements on insulating materials at cryogenic temperatures can be challenging, depending on the frequency and temperature ranges of interest. We present a technique to study the dielectric properties of bulk dielectrics at GHz frequencies. A superconducting coplanar Nb resonator is deposited directly on the material of interest, and this resonator is then probed in distant-flip-chip geometry with a microwave feedline on a separate chip. Evaluating several harmonics of the resonator gives access to various probing frequencies in the present studies up to 20 GHz. We demonstrate the technique on three different materials (MgO, LaAlO₃, and TiO₂), at temperatures between 1.4 K and 7 K.

Rotational and Vibrational Excitations of a Single Water Molecule by Inelastic Electron Tunneling Spectroscopy

Two low-energy excitations of a single water molecule are observed via inelastic electron tunneling spectroscopy, where a significant enhancement is achieved by attaching the molecule to the tip apex in a scanning tunneling microscope. Density functional theory simulations and quantum mechanical calculations of an asymmetric top are carried out to reveal the origin of both excitations. Variations in tunneling junction separation give rise to the quantum confinement effect on the quantum state of a water molecule in the tunneling junction. Our results demonstrate a potential method for measuring the dynamic behavior of a single molecule confined in a tunneling junction, where the molecule-substrate interaction can be purposely tuned.

Extending the Range of Controlling Protein Adsorption via Subsurface Architecture

Recently, it has been shown that water, confined in a plasma polymer subsurface chemical gradient, nanometers below the surface, significantly reduced the amount of adsorbed protein bovine serum albumin (BSA). Relating to this effect, we proposed the hypothesis that oriented water molecules within the subsurface gradient generate a long-range dipolar field, which interacts with dipolar proteins such as BSA near the surface region. This study extends the above used in situ multistep plasma deposition process to introduce plasma oxidation modifications of the subsurface architecture with the aim to further control the effect on protein adsorption. Neutron reflectivity measurements reveal that the oxidation time increases the amount of matrix-confined water. There is, however, an optimal oxidation time to obtain minimal protein adsorption, which suggests that a minimal distance between confined water molecules plays an important role. Altogether we can extend the range of controlling the adsorbed protein mass by the introduction of this additional plasma oxidation step.

Room-Temperature Ferroelectricity in Group-IV Metal Chalcogenide Nanowires

The realization of low-dimensional ferroelectrics is both fundamentally intriguing and practically appealing, to be used in nanoscale devices. Here, GeS and SnS nanowires are predicted to be one-dimensional (1D) ferroelectrics with inversion symmetry spontaneously broken by soft optical modes. Despite the low dimensionality, the estimated Curie point for GeS nanowires is above room temperature, benefiting experimental detection and suggesting realistic applications. To this end, further aspects of these 1D ferroelectrics are also examined, revealing the domain wall localization, switchable carrier mobility, and practically effective shieling by confining the nanowires inside the carbon nanotubes, all together potentially useful for nanoscale ferroelectric devices of broad interest.

Electrically induced liquid-liquid phase transition in water at room temperature

In this work we expand on findings previously reported [Wexler et al., Phys. Chem. Chem. Phys., 2016, 18, 16281] on the experimental observation of a phase transition in a hydrogen bonded liquid manifesting in long range dipole-dipole interactions. The studied system, liquid water stressed by an electric field, exhibits collective oscillations brought about through spontaneous breakdown of symmetry. Raman spectroscopy identifies the primary excitation of the emergent phase as transverse optically active phonon-like sidebands that appear on the hydrogen bonded asymmetric stretch mode. The phase transition is observed throughout the entire volume of liquid. The system also exhibits a self-similarity relation between the scattered Raman intensity and the electric field strength which further supports the conclusion that collective behavior persists against thermal disruption. The experimental findings are discussed in terms of a quantum field theory for macroscopic quantum systems.

Effect of Hydrogen Bonds on the Dielectric Properties of Interfacial Water

The dielectric constant for water is reduced under confinement. Although this phenomenon is well known, the underlying physical mechanism for the reduction is still in debate. In this work, we investigate the effect of the orientation of hydrogen bonds on the dielectric properties of confined water using molecular dynamics simulations. We find a reduced rotational diffusion coefficient for water molecules close to the solid surface. The reduced rotational diffusion arises due to the hindered rotation away from the plane parallel to the channel walls. The suppressed rotation in turn affects the orientational polarization of water, leading to a low value for the dielectric constant at the interface. We attribute the constrained out-of-plane rotation to originate from a higher density of planar hydrogen bonds formed by the interfacial water molecules.

Quick Test for Transgenic Components in Rice Using Terahertz Spectra

The terahertz (THz) spectrum of 0.2-1.6 THz (6.6-52.8 cm^-1) was used to identify the existence of transgenic rice Bt63 contents in non-GMO rice using a THz time-domain spectroscopy system. Principal component analysis (PCA) was used to extract the feature data based on the cumulative rate of information contribution ( > 90%); the top four principal components were selected and a radial basis function neural network (RBFNN) method was then trained and used. Three selection radial basis functions including a Gaussian function were used to identify the three types (strong positive, weak positive, and negative). The results show that the samples were identified with an accuracy of nearly 90%; additionally, the positive identification rate was > 87.5% and the negative identification rate reached 100% using the proposed method (PCA-RBF). The proposed approach was then compared with other methods, including back propagation (BP) neural networks and support vector machine (SVM). The results of the comparison show that the accuracy of PCA-RBF method reaches 92% in total and all the rest are < 90% using 100 samples. Obviously, the proposed approach outperforms the other methods and also indicates that the proposed method, in combination with THz spectroscopy, is efficient and practical for transgenic ingredient identification in rice.

Near-infrared optical investigations of snow, ice, and water layers on diffuse reflecting surfaces

While most experiments on water or ice utilize rather complex, elaborate, and expensive apparatus in order to obtain reliable optical data, here we present a simple and affordable setup that enables us to perform near-infrared measurements on water, ice, and snow on top of rough diffuse reflecting surfaces such as concrete, stone, pavement, or asphalt. By using the properties of diffuse scattering instead of specular reflection, we are able to determine the imaginary part of the refraction index of water without using any liquid cells. In addition, we demonstrate that the snow spectra can be well described by newly developed two-dimensional ray tracing simulations.

A Graphene-Edge Ferroelectric Molecular Switch

We show that polar molecules (water, ammonia, and nitrogen dioxide) adsorbed solely at the exposed edges of an encapsulated graphene sheet exhibit ferroelectricity, collectively orienting and switching reproducibly between two available states in response to an external electric field. This ferroelectric molecular switching introduces drastic modifications to the graphene bulk conductivity and produces a large and ambipolar charge bistability in micrometer-size graphene devices. This system comprises an experimental realization of envisioned memory capacitive ("memcapacitive") devices whose capacitance is a function of their charging history, here conceived via confined and correlated polar molecules at the one-dimensional edge of a two-dimensional crystal.

Electrodynamics of quantum spin liquids

Quantum spin liquids attract great interest due to their exceptional magnetic properties characterized by the absence of long-range order down to low temperatures despite the strong magnetic interaction. Commonly, these compounds are strongly correlated electron systems, and their electrodynamic response is governed by the Mott gap in the excitation spectrum. Here we summarize and discuss the optical properties of several two-dimensional quantum spin liquid candidates. First we consider the inorganic material herbertsmithite ZnCu₃(OH)₆Cl₂ and related compounds, which crystallize in a kagome lattice. Then we turn to the organic compounds [Formula: see text]-EtMe₃Sb[Pd(dmit)₂]₂, κ-(BEDT-TTF)₂Ag₂(CN)₃ and κ-(BEDT-TTF)₂Cu₂(CN)₃, where the spins are arranged in an almost perfect triangular lattice, leading to strong frustration. Due to differences in bandwidth, the effective correlation strength varies over a wide range, leading to a rather distinct behavior as far as the electrodynamic properties are concerned. We discuss the spinon contributions to the optical conductivity in comparison to metallic quantum fluctuations in the vicinity of the Mott transition.

Nanoconfined water can orient and cause long-range dipolar interactions with biomolecules

Surface properties are generally determined by the top most surface layer also defining how molecules adsorb onto it. By exploring effects due to interactions with deeper subsurface layers, however, long-range interaction forces were found to also significantly contribute to molecular adsorption, in which hydration of the subsurface region is the key factor. Water molecules confined to a subsurface amphiphilic gradient are confirmed to cause these long-range dipolar interactions by preferential orientation, thus significantly changing the way how a protein interacts with the surface. These findings imply future exploitation of an additional factor to modulate adsorption processes.

Transient Phase of Ice Observed by Sum Frequency Generation at the Water/Mineral Interface During Freezing

We observed a transient noncentrosymmetric phase of ice at water/mineral interfaces during freezing, which enhanced the intensity of the IR-visible sum frequency generation intensity by up to 20-fold. The lifetime of the transient phase was several minutes. Since the most stable form of ice, hexagonal and cubic ice, are centrosymmetric, our study suggests the transient existence of stacking-disordered ice during the freezing process at water/mineral interfaces. Stacking-disordered ice, which has only been observed in bulk ice at temperatures lower than -20 °C, is a random mixture of layers of hexagonal ice and cubic ice. However, the transient phase at the ice/mineral interface was observed at temperatures as high as -1 °C. It suggests that the mineral surface may play a role in promoting and stabilizing the formation of stacking-disordered ice at the interface.

Vibrational states of nano-confined water molecules in beryl investigated by first-principles calculations and optical experiments

Using the DFT approach and optical spectroscopy, we provide a comprehensive analysis of IR excitation of water molecules confined in beryl nanopores.

Wide-aperture aspherical lens for high-resolution terahertz imaging

In this paper, we introduce wide-aperture aspherical lens for high-resolution terahertz (THz) imaging. The lens has been designed and analyzed by numerical methods of geometrical optics and electrodynamics. It has been made of high-density polyethylene by shaping at computer-controlled lathe and characterized using a continuous-wave THz imaging setup based on a backward-wave oscillator and Golay detector. The concept of image contrast has been implemented to estimate image quality. According to the experimental data, the lens allows resolving two points spaced at 0.95λ distance with a contrast of 15%. To highlight high resolution in the THz images, the wide-aperture lens has been employed for studying printed electronic circuit board containing sub-wavelength-scale elements. The observed results justify the high efficiency of the proposed lens design.