Anisotropic materials with abnormal Poisson's ratios and acoustic velocities.

Isotropic materials are required to adhere to various mechanical principles due to their limited thermal stability. For instance, it is essential for Poisson's ratio to be within the range of -1 to 0.5, and the longitudinal wave velocity must exceed the transverse wave velocity. Nevertheless, perfect crystals, as anisotropic materials, have the ability to defy conventional rules. Through the integration of high-throughput processes and first-principles calculations, a comprehensive exploration of known materials was conducted, resulting in the establishment of a database featuring an extreme anisotropic mechanism. This included the identification of abnormal Poisson's ratios (with the directional Poisson's ratio ranging from -3.00 to 3.67), the discovery of extreme negative linear compressibility, the determination of the upper and lower limits of the sound velocity, and other associated properties. Several materials with abnormal Poisson's ratios (<-1 or >0.5) were listed, and their peculiar mechanical behavior, wherein the volume decreased counterintuitively with uniaxial tension, was discussed. Finally, this study focused on the velocities of longitudinal and transverse waves, with specific emphasis on materials exhibiting transverse wave velocities that exceeded the longitudinal wave velocities.

Electronegativity and chemical hardness of elements under pressure.

SignificanceOver the years, many unusual chemical phenomena have been discovered at high pressures, yet our understanding of them is still very fragmentary. Our paper addresses this from the fundamental level by exploring the key chemical properties of atoms-electronegativity and chemical hardness-as a function of pressure. We have made an appropriate modification to the definition of Mulliken electronegativity to extend its applicability to high pressures. The change in atomic properties, which we observe, allows us to provide a unified framework explaining (and predicting) many chemical phenomena and the altered behavior of many elements under pressure.

Ultrahigh-Pressure Magnesium Hydrosilicates as Reservoirs of Water in Early Earth.

The origin of water on the Earth is a long-standing mystery, requiring a comprehensive search for hydrous compounds, stable at conditions of the deep Earth and made of Earth-abundant elements. Previous studies usually focused on the current range of pressure-temperature conditions in the Earth's mantle and ignored a possible difference in the past, such as the stage of the core-mantle separation. Here, using ab initio evolutionary structure prediction, we find that only two magnesium hydrosilicate phases are stable at megabar pressures, α-Mg_{2}SiO_{5}H_{2} and ß-Mg_{2}SiO_{5}H_{2}, stable at 262-338 GPa and >338 GPa, respectively (all these pressures now lie within the Earth's iron core). Both are superionic conductors with quasi-one-dimensional proton diffusion at relevant conditions. In the first 30 million years of Earth's history, before the Earth's core was formed, these must have existed in the Earth, hosting much of Earth's water. As dense iron alloys segregated to form the Earth's core, Mg_{2}SiO_{5}H_{2} phases decomposed and released water. Thus, now-extinct Mg_{2}SiO_{5}H_{2} phases have likely contributed in a major way to the evolution of our planet.

Nonreciprocal optical diffraction by a single layer of gyromagnetic cylinders.

We study the diffraction of optical waves by a single layer of gyromagnetic cylinders. We show that a nonvanishing rotating dipole momentum is excited in a single gyromagnetic cylinder because of the classic analog of the Zeeman effect on photonic angular momentum states (PAMSs). Consequently, different collective dipole modes are excited in a gyromagnetic cylinder array at opposite incident angles. Nonreciprocal optical diffraction effects can be observed, where the transmission and reflection coefficients depend on the sign of the incident angle. A novel phenomenon of nonreciprocal negative directional transmission is demonstrated and numerically analyzed. This work highlights the potential of PAMSs in manipulating the propagation of optical waves for various applications.

Manipulation of dark photonic angular momentum states via magneto-optical effect for tunable slow-light performance.

We propose a novel scheme in realizing tunable slow-light performance by manipulating dark photonic angular momentum states (PAMSs) in metamaterials via the magneto-optical effect. We show that by applying a static magnetic field B, some pairs of sharp transmission dips can be observed in the background transparency window of a complex metamaterial design. Each pair of transmission dips are related to the excitation of dark PAMSs with opposite topological charges -m and +m, with a lifted degeneracy due to the classic analogue of Zeeman effect. Nonreciprocal characteristics can be observed in the distributions of field amplitude and transverse energy flux. The performance of slow light, including the group index ng, its abnormal feature, the associated strong absorption and the dependence with B are also discussed.

Exceptional points in extraordinary optical transmission through dual subwavelength metallic gratings.

Complex metamaterials with multiple optical resonances in constituent elements possess many similarities with open quantum systems that can be described by non-Hermitian Hamiltonian. By analogy with a two-state open quantum system, we show that a classic analogue of exceptional points can be observed in the transmission spectra of dual subwavelength metallic gratings. Anti-crossing (crossing) between the two branches λ(R) of extraordinary optical transmission, with crossing (anti-crossing) of the corresponding widths Γ(R), is observed in the parameter space spanned by the lateral displacement L and the angle of incidence φ0. Exchanges of field patterns and phases, and the variation of field profile when circling the exceptional point are discussed. This work highlights the potential to transfer the concepts and applications from open quantum systems to optical metamaterials.

Guided modes in magneto-optical waveguides and the role in resonant transmission.

Magneto-optical (MO) effect can break the reciprocal propagation of an optical wave along a MO-metal interface. We show that this nonreciprocal property also influences the guided modes in metal-MO-metal waveguides. Especially, the field profiles of the guided modes are neither symmetric nor anti-symmetric, but asymmetric. We then study the resonant optical transmission through a thin metal film with subwavelength MO slits. Magnetic field changes the transmission spectra of the structure, and a MO-induced transparent window is open, where the MO medium becomes extremely anisotropic. The guided-mode mediated high transmission is associated with an asymmetric field distribution and a circling energy flux.

Slow light from sharp dispersion by exciting dark photonic angular momentum states.

A photonic angular momentum state (PAMS) with a topological charge of m≠±1 is dipole forbidden at all polarizations of free-space incidence due to the existence of a unique helical phase. We show that by indirectly exciting dark PAMSs through coupling with a bright resonant element, a sharply variant transmission behavior and strong dispersion can be achieved. This behavior can subsequently be utilized in slow light. A metamaterial design, in which a group index n(g) greater than 500 can be achieved, is present.

Gouy phase of optical beams in anisotropic indefinite metamaterials.

The Gouy phase shift of optical beams in anisotropic indefinite metamaterials displays extraordinary characteristics because of the presence of anisotropic magnetic and electric responses. We propose a simple interpretation of these characteristics from the unusual dispersion relation of the optical waves in the metamaterials, which influences the expectation value of the axial propagation constant of the focused beam.

Asymmetric transmission for linearly polarized electromagnetic radiation.

Metamaterials have shown to support the intriguing phenomenon of asymmetric electromagnetic transmission in the opposite propagation directions, for both circular and linear polarizations. In the present article, we propose a criterion on the relationship among the elements of transmission matrix, which allows asymmetrical transmission for linearly polarized electromagnetic radiation only while the reciprocal transmission for circularly one. Asymmetric hybridized metamaterials are shown to satisfy this criterion. The influence from the rotation of the sample around the radiation propagation direction is discussed. A special structure design is proposed, and its characteristics are analyzed by using numerical simulation.