Chiral Crystal Structure Research Articles

Phonon Angular Momentum Induced by the Temperature Gradient.

Phonon modes in crystals can have angular momenta in general. It nevertheless cancels in equilibrium when the time-reversal symmetry is preserved. In this Letter, we show that when a temperature gradient is applied and heat current flows in the crystal, the phonon distribution becomes off equilibrium, and a finite angular momentum is generated by the heat current. This mechanism is analogous to the Edelstein effect in electronic systems. This effect requires crystals with sufficiently low crystallographic symmetries, such as polar or chiral crystal structures. Because of the positive charges of the nuclei, this phonon angular momentum induces magnetization. In addition, when the crystal can freely rotate, this generated phonon angular momentum is converted to a rigid-body rotation of the crystal, due to the conservation of the total angular momentum. Furthermore, in metallic crystals, the phonon angular momentum will be partially converted into spin angular momentum of electrons.

Coupled Chiral Structure in Graphene-Based Film for Ultrahigh Thermal Conductivity in Both In-Plane and Through-Plane Directions.

The development of high-performance thermal management materials to dissipate excessive heat both in plane and through plane is of special interest to maintain efficient operation and prolong the life of electronic devices. Herein, we designed and constructed a graphene-based composite film, which contains chiral liquid crystals (cellulose nanocrystals, CNCs) inside graphene oxide (GO). The composite film was prepared by annealing and compacting of self-assembled GO-CNC, which contains chiral smectic liquid crystal structures. The helical arranged nanorods of carbonized CNC act as in-plane connections, which bridge neighboring graphene sheets. More interestingly, the chiral structures also act as through-plane connections, which bridge the upper and lower graphene layers. As a result, the graphene-based composite film shows extraordinary thermal conductivity, in both in-plane (1820.4 W m-1 K-1) and through-plane (4.596 W m-1 K-1) directions. As a thermal management material, the heat dissipation and transportation behaviors of the composite film were investigated using a self-heating system and the results showed that the real-time temperature of the heater covered with the film was 44.5 °C lower than a naked heater. The prepared film shows a much higher efficiency of heat transportation than the commonly used thermal conductive Cu foil. Additionally, this graphene-based composite film exhibits excellent mechanical strength of 31.6 MPa and an electrical conductivity of 667.4 S cm-1. The strategy reported here may open a new avenue to the development of high-performance thermal management films.

TaRh2B2 and NbRh2B2: Superconductors with a chiral noncentrosymmetric crystal structure.

It is a fundamental truth in solid compounds that the physical properties follow the symmetry of the crystal structure. Nowhere is the effect of symmetry more pronounced than in the electronic and magnetic properties of materials-even the projection of the bulk crystal symmetry onto different crystal faces is known to have a substantial impact on the surface electronic states. The effect of bulk crystal symmetry on the properties of superconductors is widely appreciated, although its study presents substantial challenges. The effect of a lack of a center of symmetry in a crystal structure, for example, has long been understood to necessitate that the wave function of the collective electron state that gives rise to superconductivity has to be more complex than usual. However, few nonhypothetical materials, if any, have actually been proven to display exotic superconducting properties as a result. We introduce two new superconductors that in addition to having noncentrosymmetric crystal structures also have chiral crystal structures. Because the wave function of electrons in solids is particularly sensitive to the host material's symmetry, crystal structure chirality is expected to have a substantial effect on their superconducting wave functions. Our two experimentally obtained chiral noncentrosymmetric superconducting materials have transition temperatures to superconductivity that are easily experimentally accessible, and our basic property characterization suggests that their superconducting properties may be unusual. We propose that their study may allow for a more in-depth understanding of how chirality influences the properties of superconductors and devices that incorporate them.

Eu2P7X and Ba2As7X (X = Br, I): Chiral double-Zintl salts containing heptapnictotricyclane clusters

Chiral double Zintl salts present tunable crystal structures with enhanced structural flexibilities and potential for applications requiring chiral control and enantioselectivity. To accompany the chiral Sr2P7I and Sr2P7Br double Zintl salts reported by us previously, six new chiral Zintl salts of the form Ba2−ySryAs7I (y = 0, 0.23, 2), Eu2P7I, Eu2P7Br, and Eu1.3Ba0.7P7Br have been synthesized and characterized by single crystal X-ray diffraction and SEM-EDS analyses. All new compounds crystallize in the Sohncke space group P213 (No. 198) with variations of P73- (heptaphosphanortricyclane) or As73- (heptaarsanortricyclane) clusters surrounded by alkaline-earth or Eu cations and halogen anions. Band structure calculations predict semiconducting properties for all synthesized compounds. Diffuse reflectance UV–vis spectroscopy indicates that Eu2P7I is a direct bandgap semiconductor with Eg of 1.7 eV.

Revolving supramolecular chiral structures powered by light in nanomotor-doped liquid crystals.

Molecular machines operated by light have been recently shown to be able to produce oriented motion at the molecular scale1,2 as well as do macroscopic work when embedded in supramolecular structures3-5. However, any supramolecular movement irremediably ceases as soon as the concentration of the interconverting molecular motors or switches reaches a photo-stationary state6,7. To circumvent this limitation, researchers have typically relied on establishing oscillating illumination conditions-either by modulating the source intensity8,9 or by using bespoke illumination arrangements10-13. In contrast, here we report a supramolecular system in which the emergence of oscillating patterns is encoded at the molecular level. Our system comprises chiral liquid crystal structures that revolve continuously when illuminated, under the action of embedded light-driven molecular motors. The rotation at the supramolecular level is sustained by the diffusion of the motors away from a localized illumination area. Above a critical irradiation power, we observe a spontaneous symmetry breaking that dictates the directionality of the supramolecular rotation. The interplay between the twist of the supramolecular structure and the diffusion 14 of the chiral molecular motors creates continuous, regular and unidirectional rotation of the liquid crystal structure under non-equilibrium conditions.

Elemental chalcogens as a minimal model for combined charge and orbital order

Helices of increased electron density can spontaneously form in materials containing multiple, interacting density waves. Although a macroscopic order parameter theory describing this behaviour has been proposed and experimentally tested, a detailed microscopic understanding of spiral electronic order in any particular material is still lacking. Here, we present the elemental chalcogens Selenium and Tellurium as model materials for the development of chiral charge and orbital order. We formulate minimal models capturing the formation of spiral structures both in terms of a macroscopic Landau theory and a microscopic Hamiltonian. Both reproduce the known chiral crystal structure and are consistent with its observed thermal evolution and behaviour under applied pressure. The combination of microscopic and macroscopic frameworks allows us to distil the essential ingredients in the emergence of helical charge order, and may serve as a guide to understanding spontaneous chirality both in other specific materials and throughout materials classes.

Thermal deformations of crystal structures formed in the systems of malic acid enantiomers and l-valine–l-isoleucine enantiomers

The anisotropy of thermal deformations in seven studied chiral crystal structures is attributed to the different numbers and organizations of intermolecular contacts.

Chiral Soliton Lattice Formation in Monoaxial Helimagnet Yb(Ni1−xCux)3Al9

Helical magnetic structures and its responses to external magnetic fields in Yb(Ni$_x$Cu$_{1-x}$)$_3$Al$_9$, with a chiral crystal structure of the space group $R32$, have been investigated by resonant X-ray diffraction. It is shown that the crystal chirality is reflected to the helicity of the magnetic structure by a one to one relationship, indicating that there exists an antisymmetric exchange interaction mediated via the conduction electrons. When a magnetic field is applied perpendicular to the helical axis ($c$ axis), the second harmonic peak of $(0, 0, 2q)$ develops with increasing the field. The third harmonic peak of $(0, 0, 3q)$ has also been observed for the $x$=0.06 sample. This result provides a strong evidence for the formation of a chiral magnetic soliton lattice state, a periodic array of the chiral twist of spins, which has been suggested by the characteristic magnetization curve. The helical ordering of magnetic octupole moments, accompanying with the magnetic dipole order, has also been detected.

Reversible Broad-Spectrum Control of Selective Reflections of Chiral Nematic Phases by Closed-/Open-Type Axially Chiral Azo Dopants.

Invited for this month's cover picture is the groups of Professor Hirotsugu Kikuchi and Dr. Yasushi Okumura at the Institute for Materials Chemistry and Engineering at Kyushu University (Japan). External dynamic control of molecular self‐organized superstructures with unique features has been researched, as these structures are applicable to chiral molecular devices. The cover picture shows photocontrol of selective reflection color that originated from the self‐organized helical structure of chiral nematic liquid crystals (N*LCs) using closed‐/open‐type dopants. The drastic structure changes of the dopants by light stimuli enable reversible broad‐spectrum control of selective reflections of N*LCs with opposite helical sign. Read the full text of the Full Paper at 10.1002/open.201700121.

Characterization of Crystal Chirality in Amino Acids Using Low-Frequency Raman Spectroscopy.

We present a new method for differentiating racemic crystals from enantiopure crystals. Recently, developments in optical filters have enabled the facile use of Raman spectroscopy to detect low-frequency vibrational (LFV) modes. Here, for the first time, we use Raman spectroscopy to characterize the LFV modes for crystalline organic materials composed of chiral molecules. The LF-Raman spectra of racemic and enantiopure crystals exhibit a significant variation, which we attribute to different hydrogen-bond networks in the chiral crystal structures. Across a representative set of amino acids, we observed that when comparing racemic versus enantiopure crystals, the available LFV modes and their relative scattering intensity are strong functions of side chain polarity. Thus, LF-Raman can be used as a method that is complementary to the currently used methods for characterizing crystal chirality due to simpler, faster, and more sensitive measurements, along with the small sample size required, which is limited by the laser-beam diameter in the focus.

Spin-wave spectroscopy of the Dzyaloshinskii-Moriya interaction in room-temperature chiral magnets hosting skyrmions

Propagation character of spin wave was investigated for chiral magnets FeGe and Co-Zn-Mn alloys, which can host magnetic skyrmions near room temperature. On the basis of the frequency shift between counter-propagating spin waves, the magnitude and sign of Dzyaloshinskii-Moriya (DM) interaction were directly evaluated. The obtained magnetic parameters quantitatively account for the size and helicity of skyrmions as well as their materials variation, proving that the DM interaction plays a decisive role in the skyrmion formation in this class of room-temperature chiral magnets. The propagating spin-wave spectroscopy can thus be an efficient tool to study DM interaction in bulk single-phase compounds. Our results also demonstrate a function of spin-wave diode based on chiral crystal structures at room temperature.

The Origin and Limit of Asymmetric Transmission in Chiral Resonators.

We observe that the asymmetric transmission (AT) through photonic systems with a resonant chiral response is strongly related to the far-field properties of eigenmodes of the system. This understanding can be used to predict the AT for any resonant system from its complex eigenmodes. We find that the resonant chiral phenomenon of AT is related to, and is bounded by, the nonresonant scattering properties of the system. Using the principle of reciprocity, we determine a fundamental limit to the maximum AT possible for a single mode in any chiral resonator. We propose and follow a design route for a highly chiral dielectric photonic crystal structure that reaches this fundamental limit for AT.

Transformation of Dipeptide‐Based Organogels into Chiral Crystals by Cryogenic Treatment

AbstractControlled molecular assembly is an important approach for the synthesis of single‐component materials with diverse functions. Unlike traditional heat treatment or solvent modulation, cryogenic treatment at 77 K enabled the tunable transition of a self‐assembled diphenylalanine organogel into a hexagonal crystal. Under these conditions, the assembled molecules undergo an internal rearrangement in the solid state to form a well‐defined chiral crystal structure. Moreover, these assemblies exhibit enhanced emission. This strategy for the synthesis of single‐component supramolecular assemblies can create new functions by manipulating phase transitions.

Transformation of Dipeptide-Based Organogels into Chiral Crystals by Cryogenic Treatment.

Controlled molecular assembly is an important approach for the synthesis of single-component materials with diverse functions. Unlike traditional heat treatment or solvent modulation, cryogenic treatment at 77 K enabled the tunable transition of a self-assembled diphenylalanine organogel into a hexagonal crystal. Under these conditions, the assembled molecules undergo an internal rearrangement in the solid state to form a well-defined chiral crystal structure. Moreover, these assemblies exhibit enhanced emission. This strategy for the synthesis of single-component supramolecular assemblies can create new functions by manipulating phase transitions.

Supramolecular chirality and symmetry breaking of fluoride complexes of achiral foldamers

Fluoride binding of achiral oligoamide foldamers produces polar and chiral crystal structures and induces exceptional symmetry breaking of the bulk sample.

Molecular model for chirality phenomena.

Chirality is a hallmark feature for molecular recognition in biology and chemical physics. We present a three-dimensional continuum model for studying chirality phenomena in condensed phases using molecular simulations. Our model system is based upon a simple four-site molecule and incorporates non-trivial kinetic behavior, including the ability to switch chirality or racemize, as well as thermodynamics arising from an energetic preference for specific chiral interactions. In particular, we introduce a chiral renormalization parameter that can locally favor either homochiral or heterochiral configurations. Using this model, we explore a range of chirality-specific phenomena, including the kinetics of chiral inversion, the mechanism of spontaneous chiral symmetry breaking in the liquid, chirally driven liquid-liquid phase separation, and chiral crystal structures.

Optically probed symmetry breaking in the chiral magnetCu2OSeO3

We report on the linear optical properties of the chiral magnet Cu2OSeO3, specifically associated with the absence of inversion symmetry, the chiral crystallographic structure, and magnetic order. Through spectroscopic ellipsometry, we observe local crystal-field excitations below the charge-transfer gap. These crystal-field excitations are optically allowed due to the lack of inversion symmetry at the Cu sites. Optical polarization rotation measurements were used to study the structural chirality and magnetic order. The temperature dependence of the natural optical rotation, originating in the chiral crystal structure, provides evidence for a finite magneto-electric effect in the helimagnetic phase. We find a large magneto-optical susceptibility on the order of V(540nm)~10^4 rad/(T*m) in the helimagnetic phase and a maximum Faraday rotation of ~165deg/mm in the ferrimagnetic phase. The large value of V can be explained by considering spin cluster formation and the relative ease of domain reorientation in this metamagnetic material. The magneto-optical activity allows us to map the magnetic phase diagram, including the skyrmion lattice phase. In addition to this, we probe and discuss the nature of the various magnetic phase transitions in Cu2OSeO3.

Absolute Structure Determination of Chiral Crystals Consisting of Achiral Benzophenone with Single-crystal X-ray Diffraction and Its Correlation with Solid-state Circular Dichroism

Achiral benzophenone is one of the simplest organic aromatic ketones that crystallize in the chiral form. The absolute structure of chiral crystals consisting of achiral benzophenone was firstly de...

A(2+) Cation Control of Chiral Domain Formation in A(TiO)Cu4(PO4)4 (A = Ba, Sr).

Single crystals of two novel tetragonal chiral materials, A(TiO)Cu4(PO4)4 (A = Ba, Sr), were grown from Na2Mo2O7 flux, and their crystal and chiral domain structures were characterized. Polarized-light microscopy studies of the chiral domain structures in the crystals show that Ba(TiO)Cu4(PO4)4 mostly hosts a multidomain state, while a monodomain state predominantly appears in Sr(TiO)Cu4(PO4)4. To explain this striking difference, we quantified the chirality strength of these materials by comparing atomic positions in the chiral and nearest-achiral crystal structures, revealing larger chirality strength in Sr(TiO)Cu4(PO4)4 than in Ba(TiO)Cu4(PO4)4. Our proposed mechanisms linking the chirality strength and domain formation can account for the different occurrence frequency of chiral domains in this system.

The synthesis and chiral crystal structures of two enantiomers of a Ag helical coordination polymer based on argentophilicity

Achiral ligand- and Ag(I)-driven spontaneous resolution occurred simultaneously in the formation of two enantiomers 1P and 1M of [Ag2(5-nip)(mpyz)] (5-nip = 5-nitroisophthalic, mpyz = 2,3,5,6-tetramethylpyrazine), whose absolute sense of a helix made of achiral components is induced by the metal Ag(I) through argentophilic interactions.