Chiral Crystals Research Articles

Description of bandgaps opening in chiral phononic crystals by analogy with Thomson scattering

Chiral phononic crystals (PnCs) provide unique properties not offered by conventional metamaterial based on classic Bragg scattering and local resonance. However, it is insufficient to only consider the inertial amplification effect to describe its bandgap mechanism due to the absence of the bandgap caused by the chirality in some specific chiral structures. Here, we theoretically and experimentally introduce an analogy with Thomson scattering in electromagnetic waves to characterize the bandgap phenomena in chiral PnCs with translation–rotation coupling. Another phononic structures with translation–translation coupling are proposed to illustrate the universality of the analogy. We evidence that the coupling motion in chiral unit cells is similar to the result of Thomson scattering, which quantitatively formulizing as inertial amplification in theory and, twice elastic Thomson scattering allows the waves in the same polarization mode to superpose in antiphase, which is essence of the bandgap formation. This finding sheds a new light on the physics of the elastodynamic wave manipulation in chiral PnCs, thus opening a definite route for the pragmatic exploitation of chiral PnCs as well as other structures with motion coupling in achieving low-frequency and broad bandgaps.

Circularly Polarized Luminescence from Chiral Photonic Crystals of Caesium Lead Halide Perovskites

AbstractThe generation and detection of circularly polarized light (CPL) are of importance in many modern technologies such as digital communications, machine vision, bio‐imaging, and 3D displays. In this study, inspired by the fact that various species of plants and animals can efficiently produce and/or perceive CPL with chiral photonic crystal structures, efforts are made to fabricate luminescent chiral photonic crystals with caesium lead halide perovskites, a class of solution‐processable semiconductors that have excellent luminescence properties and readily tunable electronic band gaps. Coupling between the emission frequency of the perovskite and the cavity resonance frequency of the photonic crystal structure can be established at any desired point in the visible region, giving rise to CPL output with a dissymmetry factor up to −0.34. These results suggest a new approach to chiral light‐emitting materials and may contribute to the development of semiconductors with ordered microstructures.

Surface barrier effect as evidence of chiral soliton lattice formation in chiral dichalcogenide CrTa3S6 crystals

The formation of chiral magnetic soliton lattice (CSL) is investigated in monoaxial chiral dichalcogenide CrTa3S6 crystals in terms of a surface barrier, which prevents the penetration of chiral solitons into the system and is an intrinsic origin of hysteresis for the continuous phase transition of nucleation-type, as discussed in the system of quantized vortices in type-II superconductors. The magnetoresistance (MR) was examined with microfabricated platelet samples in different dimensions with regard to the c-axis direction of the crystal. The CSL formation was confirmed by the discrete MR changes, reflecting the number of chiral solitons, as well as by the presence of a surface barrier, recognized as a fixed ratio of critical magnetic fields during the hysteresis field cycle. We also argue the influence of the surface barrier in bulk CrTa3S6 crystals.

Natural Optical Activity from Density-Functional Perturbation Theory.

We present an accurate and computationally efficient first-principles methodology to calculate natural optical activity. Our approach is based on the long-wave density-functional perturbation theory and includes self-consistent field terms naturally in the formalism, which are found to be of crucial importance. The final result is expressed exclusively in terms of response functions to uniform field perturbations and avoids troublesome summations over empty states. Our strategy is validated by computing the natural optical activity tensor in representative chiral crystals (trigonal Se, α-HgS, and α-SiO_{2}) and molecules (C_{4}H_{4}O_{2}), finding excellent agreement with experiment and previous theoretical calculations.

Achiral 2-pyridone and 4-aminopyridine act as chiral inducers of asymmetric autocatalysis with amplification of enantiomeric excess via the formation of chiral crystals.

Enantiomorphous crystals of achiral 2-pyridone and 4-aminopyridine served as sources of chirality, to induce the asymmetric autocatalysis of 5-pyrimidyl alkanol during the asymmetric addition of diisopropylzinc to the corresponding pyrimidine-5-carbaldehyde, that is, the Soai reaction. Following a significant amplification of enantiomeric excess through asymmetric autocatalysis, highly enantioenriched 5-pyrimidyl alkanol could be synthesized with their corresponding absolute configurations to those of chiral crystals of 2-pyridone and 4-aminopyridine.

Generalized Dzyaloshinskii–Moriya interaction and chirality-induced phenomena in chiral crystals

Generalized Dzyaloshinskii–Moriya interaction and chirality-induced phenomena in chiral crystals

Generalized Dzyaloshinskii–Moriya Interaction and Chirality-Induced Phenomena in Chiral Crystals

This paper is a review of recent advances in studies of the magnetic, electronic, spintronic, and phononic properties of chiral crystals, with a focus on the connection between the Dzyaloshinskii–Moriya interaction (DMI) and chirality in materials. Importantly, a microscopic DMI is elevated to a macroscopic Lifshitz invariant (LI) in chiral crystals, where mirror reflection and inversion symmetry are globally broken. As a consequence, chiral crystals exhibit nontrivial physical responses over a macroscopic scale. Indeed, in chiral magnetism, a chiral spin soliton lattice (CSL) appears in monoaxial chiral magnetic crystals and exhibits various nontrivial physical properties characterized by the coherent and topological nature of the CSL. This CSL isone of the prominent outcomes arising from the DMI and LI, as envisioned by Dzyaloshinskii in the 1960s. Furthermore, a giant spin polarization emerges over a macroscopic scale in a wide range of chiral materials from organic molecules to inorganic crystals. The first-order spatial dispersion, such as the LI, also appears in elastic or phonon degrees of freedom in chiral crystals. These examples show how a time-even pseudoscalar term, which corresponds to a generalized concept of the DMI and LI, is realized in chiral media.

Isotacticity in chiral phononic crystals for low-frequency bandgap

Realizing a low-frequency and broad bandgap is extremely challenging because the lattice constant, equivalent stiffness, and equivalent density must be taken into account in practice. Despite the advances of inertial amplification which is one of the ideal strategies to address this issue, the design ideas are mainly limited to the classic geometries featured by mirror symmetry (syndiotacticity), confining then progress in this topic. Here, we report numerical and experimental investigation of a chiral phononic crystal without mirror symmetry (isotacticity) according to the analogous Thomson scattering. Numerical analysis shows that due to the lack of one degree of freedom in isotactic lattice (ISL) compared to syndiotactic lattice (SYL), the bandgap starting-frequency of the ISL is significantly lower than that the one of the SYL by up to 66%. The bandgap properties are further demonstrated by simulation and experimental measurements. This research reveals that the condition of having alternating arrangement orientations between neighboring atoms is not indispensable for this kind of bandgap, and a rational geometry satisfying the Thomson scattering-based bandgap conditions can further reduce the bandgap frequency without affecting the stiffness and lightweight of the structure.

Spin polarization gate device based on the chirality-induced spin selectivity and robust nonlocal spin polarization.

Nonlocal spin polarization phenomena are thoroughly investigated in the devices made of chiral metallic single crystals of CrNb3S6 and NbSi2 as well as of polycrystalline NbSi2. We demonstrate that simultaneous injection of charge currents in the opposite ends of the device with the nonlocal setup induces the switching behavior of spin polarization in a controllable manner. Such a nonlocal spin polarization appears regardless of the difference in the materials and device dimensions, implying that the current injection in the nonlocal configuration splits spin-dependent chemical potentials throughout the chiral crystal even though the current is injected into only a part of the crystal. We show that the proposed model of the spin dependent chemical potentials explains the experimental data successfully. The nonlocal double-injection device may offer significant potential to control the spin polarization to large areas because of the nature of long-range nonlocal spin polarization in chiral materials.

Nucleation-mediated growth of chiral 3D organic-inorganic perovskite single crystals.

Although their zero- to two-dimensional counterparts are well known, three-dimensional chiral hybrid organic-inorganic perovskite single crystals have remained difficult because they contain no chiral components and their crystal phases belong to centrosymmetric achiral point groups. Here we report a general approach to grow single-crystalline 3D lead halide perovskites with chiroptical activity. Taking MAPbBr3 (MA, methylammonium) perovskite as a representative example, whereas achiral MAPbBr3 crystallized from precursors in solution by inverse temperature crystallization method, the addition of micro- or nanoparticles as nucleating agents promoted the formation of chiral crystals under a near equilibrium state. Experimental characterization supported by calculations showed that the chirality of the 3D APbX3 (where A is an ammonium ion and X is Cl, Br or mixed Cl-Br or Br-I) perovskites arises from chiral patterns of the A-site cations and their interaction with the [PbX6]4- octahedra in the perovskite structure. The chiral structure obeys the lowest-energy principle and thereby thermodynamically stable. The chiral 3D hybrid organic-inorganic perovskites served in a circularly polarized light photodetector prototype successfully.

Weyl Phonons in Chiral Crystals.

Chirality is an indispensable concept that pervades fundamental science and nature, manifesting itself in diverse forms, e.g., quasiparticles, and crystal structures. Of particular interest are Weyl phonons carrying specific Chern numbers and chiral phonons doing circular motions. Up to now, they have been studied independently and the interpretations of chirality seem to be different in these two concepts, impeding our understanding. Here, we demonstrate that they are entangled in chiral crystals. Employing a typical chiral crystal of elementary tellurium (Te) as a case study, we expound on the intrinsic relationship between Chern number of Weyl phonons and pseudoangular momentum (PAM, lph) of chiral phonons. We propose Raman scattering as a new technique to demonstrate the existence of Weyl phonons in Te, by detecting the chirality-induced energy splitting between the two constituent chiral phonon branches for Weyl phonons. Moreover, we also observe the obstructed phonon surface states for the first time.

Chiral Plastic Crystal of Solid-State Dual Rotators

Chiral Plastic Crystal of Solid-State Dual Rotators

Chirality reversal of magnetic solitons in chiral Cr1/3TaS2

Ferromagnetism in two-dimensional (2D) materials provides an ideal platform to study emergent electromagnetic phenomena in low dimensions for future spintronics. In magnetic-element intercalated transition metal dichalcogenides, topologically nontrivial spin textures, such as chiral helimagnetic spin states and chiral soliton lattices, are realized due to the chiral lattice distortions induced by intercalated magnetic ions. Consequently, the magnetic chirality is predictably determined by the sign of antisymmetric exchange interaction (or Dzyaloshinskii–Moriya interaction, DMI) vector that is coupled to the underlying crystal chirality. Here, using cryogenic Lorentz phase microscopy, we directly observed the chirality reversal behavior of the chiral soliton lattices in Cr1/3TaS2 across the structural defects. We show that a partial 1 T stacking in 2H-TaS2 locally reduces DMI, leading to magnetic chirality reversal with direct atomic resolution imaging. Our experimental results show that manipulation of stacking sequence provides a viable way to control the chirality of topologically nontrivial soliton lattices in 2D magnets.

In situ Growth Transition Towards Helical Crystals of Achiral Molecules

The synthesis of chiral crystals such as helical crystals is important due to their chirality‐dependent applications in various fields. Since chiral crystals of achiral molecules are getting more attention, it is important to establish synthetic strategies for helical crystals of achiral molecules. Quite surprisingly, there are numerous examples of helical crystals of achiral molecules, although they are mostly obtained without specific synthetic strategies. Herein, it is proposed that Eshelby twist can be applied to noncovalent molecular crystals by introducing screw dislocations and demonstrates a successful synthesis of helical tube crystals of achiral titanyl phthalocyanine by vapor‐phase crystal growth. The dislocations form where interfaces are created via growth transition from wire growth to tube growth upon growth temperature change. The screw dislocation is confirmed by Burgers vector analysis using transmission electron microscopy. The screw dislocation induces crystal twisting along the growth direction to dissipate the strain applied around the dislocation. The diffraction peak broadening of the (200) and (002) planes confirms the nonuniform strain, i.e., twisting force. This work offers a new approach for the synthesis of helical crystals from achiral molecules.

Vibrational Circular Dichroism Spectroscopy of Chiral Molecular Crystals: Insights from Theory.

Chirality is a curious phenomenon that appears in various forms. While the concept of molecular (RS-)chirality is ubiquitous in chemistry, there are also more intricate forms of structural chirality. One of them is the enantiomorphism of crystals, especially molecular crystals, that describes the lack of mirror symmetry in the unit cell. Its relation to molecular chirality is not obvious, but still an open question, which can be addressed with chiroptical tools. Vibrational circular dichroism (VCD) denotes chiral infrared (IR) spectroscopy that is susceptible to both, the molecular as well as the intermolecular space by means of vibrational transitions. When carried out in the solid state, VCD delivers a very rich set of non-local contributions that are determined by crystal packing and collective motion. Since its discovery in the 1970s, VCD has become the method of choice for the determination of absolute configurations, but its applicability reaches beyond towards the study of different crystal forms and polymorphism. This brief review summarises the theoretical concepts of crystal chirality and how computations of solid-state VCD can shed light into the intimate connection of chiral structure and vibrational optical activity.

Vibrational Circular Dichroism Spectroscopy of Chiral Molecular Crystals: Insights from Theory

AbstractChirality is a curious phenomenon that appears in various forms. While the concept of molecular (RS‐)chirality is ubiquitous in chemistry, there are also more intricate forms of structural chirality. One of them is the enantiomorphism of crystals, especially molecular crystals, that describes the lack of mirror symmetry in the unit cell. Its relation to molecular chirality is not obvious, but still an open question, which can be addressed with chiroptical tools. Vibrational circular dichroism (VCD) denotes chiral infrared (IR) spectroscopy that is susceptible to both, the molecular as well as the intermolecular space by means of vibrational transitions. When carried out in the solid state, VCD delivers a very rich set of non‐local contributions that are determined by crystal packing and collective motion. Since its discovery in the 1970s, VCD has become the method of choice for the determination of absolute configurations, but its applicability reaches beyond towards the study of different crystal forms and polymorphism. This brief review summarises the theoretical concepts of crystal chirality and how computations of solid‐state VCD can shed light into the intimate connection of chiral structure and vibrational optical activity.

Tunable circular dichroism based on chiral photonic crystals

Tunable circular dichroism based on chiral photonic crystals

Measurement of surface chirality at near-normal incidence

The chirality of a medium is typically measured either by transmitting a beam of light through it or by single or multiple interface reflection at large and/or special angles of incidence. We propose and demonstrate here the experimental measurement of surface chirality of z-cut quartz crystal by reflecting a focused beam of light at a near-normal angle of incidence. A small difference in the reflection coefficients between orthogonal elliptically polarized incident beam of 10−4 is measured in the dark-field region of the reflected light via the weak measurement method, taking advantage of the significant transverse spin-shift (TSS) that arises due to the interaction. The TSS behavior is simulated for different chiral parameters (±γ) of the material. The experimental results match well with the theoretically simulated behavior to quantify γ of quartz crystal used as an example interface. The significance of our method can be of interest for a wide variety of fundamental and applied investigations.

Switching Chirality in Arrays of Shape-Reconfigurable Spindle Microparticles.

The giant circular photo-galvanic effect is realized in chiral metals when illuminated by circularly polarized light. However, the structure itself is not switchable nor is the crystal chirality in the adjacent chiral domains. Here spindle-shaped liquid crystalline elastomer microparticles that can switch from prolate to spherical to oblate reversibly upon heating above the nematic to isotropic transition temperature are synthesized. When arranged in a honeycomb lattice, the continuous shape change of the microparticles leads to lattice reconfiguration, from a right-handed chiral state to an achiral one, then to a left-handed chiral state, without breaking the translational symmetry. Accordingly, the sign of rotation of the polarized light passing through the lattices changes as measured by time-domain terahertz spectroscopy. Further, it can locally alter the chirality in the adjacent domains using near-infrared light illumination. The reconfigurable chiral microarrays will allow us to explore non-trivial symmetry-protected transport modes of topological lattices at the light-matter interface. Specifically, the ability to controllably create chiral states at the boundary of the achiral/chiral domains will lead to rich structures emerging from the interplay of symmetry and topology.

Redox‐Modulated Circularly Polarized Luminescence in Chiral Photonic Films Derived from Cellulose Nanocrystals and Polyoxometalate

AbstractAchieving smart circularly polarized luminescence (CPL) materials with large luminescence dissymmetry factor (glum) is in high demand for potential technological applications in the chiroptical fields. Herein, a series of chiral photonic crystals (CPCs) derived from cellulose nanocrystals (CNCs) and photosensitive inorganic polyoxometalates (POMs) through an evaporation‐induced self‐assembly strategy are reported. The photonic bandgaps (PBGs) of CPC films are well regulated by controlling the amount of doped POMs, yielding the tunable right‐handed CPL emission with tailored glum. By locating the maximum emission peak of POMs at the center of the PBG of CPC films, the composite shows remarkable CPL performance with a |glum| value up to 0.53. Furthermore, the CPL can be reversibly switched on/off through a fluorescence resonance energy transfer (FRET) process controlled by alternate UV irradiation reduction and air oxidation. The CPC films hold the great promise as next‐generation chiroptical memory devices.