Hexagonal Single Crystals Research Articles

Hexagonal CdS single crystals coupled with layered CoAl LDH—a step-scheme heterojunction for efficient photocatalytic hydrogen evolution

Hexagonal CdS single crystals coupled with layered CoAl LDH—a step-scheme heterojunction for efficient photocatalytic hydrogen evolution

Unsupervised Identification of Topological Domain Defect Nodes of YbMnO3 Based on Radar Density Algorithm

When the ferroelectric hexagonal manganese oxide single crystal is near the Curie temperature and generates the phase transformation, the topological defects of the vortex and anti-vortex structure will be generated. Applying the Radar Density Algorithm, the representative fragment of the vortex and anti-vortex structure is analyzed. By the Radar Density Algorithm analysis, the feature of the vortex and anti-vortex structure can be translated and identified as a mathematic model and judged by the mathematic program. This provides a new algorithm and approach for intelligent identification of vortex structure nodes and the phase transition.

Building {0001} and {10[formula omitted]1} facet heterojunctions on hexagonal pyramid CdS single crystals with high photoactivity and photostability for hydrogen evolution

CdS has suitable band gap and high photoelectric conversion efficiency, which has attracted widespread attention. However, its severe photocorrosion and low photocatalytic activity limits its application. Generally, constructing heterojunction structure can effectively improve the separation efficiency of CdS photogenerated carriers, and its photocatalytic activity and stability can be further improved. Here, we prepared CdS hexagonal pyramid single crystal with co-exposed {0001} and {101¯1} crystal planes by the urothermal method. Type II heterojunction can be formed between {0001} and {101¯1} crystal planes, which can effectively improve the photocatalytic hydrogen production activity of CdS hexagonal pyramid. The photocatalytic hydrogen production activity of CdS hexagonal pyramid is 10.64 mmol·h-1·g-1 with an apparent quantum efficiency of 15.6% at 420 nm, which is 5.8 times and 13.2 times as high as CdS nanorod and CdS nanoparticles, respectively. Even after the 100 h cycle test, the CdS hexagonal pyramid shows still high photocatalytic activity. This work showed that the photocatalytic activity can be effectively improved by adjusting the crystal faces, which can further generaliza to other photocatalysts.

Switchable photovoltaic response in hexagonal LuMnO3 single crystals

Hexagonal manganites, such as h-LuMnO3, are ferroelectric with its polar axis along the hexagonal axis and have a narrow electronic bandgap (≈1.5 eV). Using Pt electrodes, h-LuMnO3 single crystals display a strong rectification, characteristic of a Schottky diode, and a large photoresponse. It is found that the short circuit photocurrent density Jsc along the polar axis is modulated (up to 25%) by the direction of the ferroelectric polarization P, leading to a short circuit photocurrent loop that mimics the ferroelectric polarization. However, a non-switchable Jsc persists. Diffusion photocurrent is shown to dominate current-in-plane measurements and contributes to the non-switchable Jsc. This observation illustrates the dramatic role of the large optical absorption in hexagonal manganites. The accompanying optical dichroism might challenge disentangling a genuine bulk photovoltaic response in h-LuMnO3 single crystals, contributing to the non-switchable Jsc. Epitaxial thin films may offer a suitable alternative.

Semi-empirical evaluation of linear coefficient of thermal expansion of metals

The paper considers various semi-empirical methods of the linear coefficient of thermal expansion (CTE), a, calculation. Debye-Grüneisen model was used as a basis and adjusted by taking account of thermodynamic (molar heat capacity, enthalpy) and elastic (Young, bulk, and shear moduli, Poison ratio) characteristics of metals. The formulas for numerical calculations of CTE are derived using: (1) elastic longitudinal modulus, (2) microscopic constants, and (3) enthalpy. CTE were calculated for 25 metals, Ge and Si, and five hexagonal single-crystals in directions perpendicular and parallel to the principle crystallographic axis. Theoretical values of a show good agreement with the experimental ones.

Planar topological Hall effect in a hexagonal ferromagnetic Fe5Sn3 single crystal

The planar topological Hall effect (PTHE), appearing when the magnetic field tended to be along the current, is believed to result from the real-space Berry curvature of the spin spiral structure and has been experimentally observed in skyrmion-hosting materials. In this paper, we report an experimental observation of the PTHE in a hexagonal ferromagnetic Fe5Sn3 single crystal. With a current along the c axis of Fe5Sn3, the transverse resistivity curves exhibited obvious peaks near the saturation field as the magnetic field rotated to the current and appeared more obvious with increasing temperature, which was related to the noncoplanar spin structure in Fe5Sn3. This spin structure induced nonzero scalar spin chirality, which acted as fictitious magnetic fields to conduction electrons and contributed the additional transverse signal. These findings deepen the understanding of the interaction between conduction electrons and complex magnetic structures and are instructive for the design of next-generation spintronic devices.

Amorphous Co3O4 quantum dots hybridizing with 3D hexagonal CdS single crystals to construct a 0D/3D p-n heterojunction for a highly efficient photocatalytic H2 evolution.

Herein, a novel amorphous monodisperse Co3O4 quantum dots/3D hexagonal CdS single crystals (0D/3D Co3O4 QDs/CdS) p-n heterojunction was constructed by a simple hydrothermal and electrostatic self-assembly method. The amorphous monodispersed Co3O4 QDs (≈4.5 nm) are uniformly and tightly attached to the surface of the hexagonal CdS single crystals. The sample, 0.5% CQDs/CdS exhibits outstanding hydrogen evolution activity of 17.5 mmol h-1 g-1 with a turnover number (TON) of 4214, up to 10.3 times higher than that of pure CdS. The enhanced photocatalytic activity can be attributed to the synergistic effect of the p-n heterostructure and the quantum confinement effect of Co3O4 QDs, which significantly promoted the separation efficiency of photo-generated electrons and holes. Additionally, the sulfur vacancy also can act as electron trappers to improve carrier separation and electron transfer. The photoelectrochemical and time-resolved fluorescence (TRPL) results further certify the effective spatial charge separation. This work gives an insight into the design of the 0D/3D Co3O4 QDs/CdS p-n heterostructure for a highly efficient photocatalysis.

Electron momentum density of hexagonal Zn studied by high-resolution Compton scattering.

High-resolution (0.12 a.u.) electron momentum density projections (Compton profiles) of a hexagonal Zn single crystal have been measured along five high-symmetry directions in reciprocal space. The experiment was performed with the use of 115.6 keV synchrotron radiation on the BL08W station at SPring-8. The quality of the measured Compton profiles is significantly better than that ofprevious medium- and high-resolution data. The experimental data were compared with the corresponding theoretical Korringa-Kohn-Rostoker (KKR) and density functional theory (DFT) calculations. Some minor and major differences between the two theoretical band-structure calculations have been observed. However, the good quality experimental results indicate their better agreement with DFT.

Biomineralization and Magnetism of Uncultured Magnetotactic Coccus Strain THC‐1 With Non‐chained Magnetosomal Magnetite Nanoparticles

AbstractMagnetotactic bacteria (MTB) have long fascinated geologists and biologists because they biomineralize intracellular single domain (SD) magnetite crystals within magnetosomes that are generally organized into single or multiple chains. MTB remains in the geological record (magnetofossils) are ideal magnetic carriers and are used to reconstruct paleomagnetic and paleoenvironmental information. Here we studied the biomineralization and magnetic properties of magnetosomal magnetite produced by uncultured magnetotactic coccus strain THC‐1, isolated from the Tanghe River, China, by combining transmission electron microscope (TEM) and rock magnetic approaches. Our results reveal that THC‐1 produces hexagonal prismatic magnetite single crystals that are elongated along the [111] crystallographic direction. Most of the magnetite crystals within THC‐1 are dispersed without obvious chain assembly. A whole‐cell THC‐1 sample yields a normal SD hysteresis loop and a Verwey transition temperature of ~112 K. In contrast to MTB cells with magnetosome chain(s), THC‐1 cells have a teardrop first‐order reversal curve distribution that is indicative of moderate interparticle interactions. Due to the absence of a magnetosome chain, THC‐1 has relatively high values of the difference between the saturation isothermal remanent magnetization (SIRM) below and above the Verwey transition temperature for field‐cooled and zero field‐cooled SIRM curves (δFC, δZFC) and a low δFC/δZFC value. Together with previous studies, our results demonstrate that some MTB species/strains can form magnetosomal magnetite without linear chain configurations. Magnetite produced by MTB has diverse magnetic properties, which are distinctive but not necessarily unique compared to other magnetite types. Therefore, combining bulk magnetic measurements and TEM observations remains necessary for identifying magnetofossils in the geological record.

High frequency atomic tunneling yields ultralow and glass-like thermal conductivity in chalcogenide single crystals

Crystalline solids exhibiting glass-like thermal conductivity have attracted substantial attention both for fundamental interest and applications such as thermoelectrics. In most crystals, the competition of phonon scattering by anharmonic interactions and crystalline imperfections leads to a non-monotonic trend of thermal conductivity with temperature. Defect-free crystals that exhibit the glassy trend of low thermal conductivity with a monotonic increase with temperature are desirable because they are intrinsically thermally insulating while retaining useful properties of perfect crystals. However, this behavior is rare, and its microscopic origin remains unclear. Here, we report the observation of ultralow and glass-like thermal conductivity in a hexagonal perovskite chalcogenide single crystal, BaTiS3, despite its highly symmetric and simple primitive cell. Elastic and inelastic scattering measurements reveal the quantum mechanical origin of this unusual trend. A two-level atomic tunneling system exists in a shallow double-well potential of the Ti atom and is of sufficiently high frequency to scatter heat-carrying phonons up to room temperature. While atomic tunneling has been invoked to explain the low-temperature thermal conductivity of solids for decades, our study establishes the presence of sub-THz frequency tunneling systems even in high-quality, electrically insulating single crystals, leading to anomalous transport properties well above cryogenic temperatures.

Generation of a simultaneous orthogonally polarized dual-wavelength Raman laser with power ratio tunability by a single hexagonal crystal: Cs2TeMo3O12.

A new generation of orthogonally polarized dual-wavelength lasers was demonstrated using a dye mode-locked neodymium-doped yttrium aluminum garnet laser for the first time. With a hexagonal Cs2TeMo3O12 as the Raman medium, efficient dual-wavelength stimulated Raman scattering was obtained at 1175 and 1154 nm with similar output power, corresponding to the stretching vibration of Mo-O and the asymmetric stretching vibrations of Mo-O and Te-O groups, respectively. The power ratio of two Raman components can be flexibly adjusted by tuning the polarization of the incident laser, which can be tuned from 0% to 100%. Laser sources with such a small wavelength separation could prove interesting for the difference-frequency generation of terahertz waves in the 4.6 THz range. Our study provides a simple and flexible method to achieve a promising dual-wavelength laser source in orthogonal polarization by Raman-based nonlinear frequency conversions.

Transformation of hollow-core screw dislocations: transitional configuration of superscrew dislocations

Hollow-core screw dislocations along [0001] in hexagonal silicon carbide single crystals, i.e. micropipes, are known to heal or close during annealing or epitaxial growth procedures. Although the healing and closing are understood to be caused by the dissociation of hollow-core superscrew dislocations into multiple closed-core elementary screw dislocations with Burgers vector of 1c, the transformation mechanism, especially in terms of elastic energy, is unclear. Here, we propose a unique transitional configuration of the super-screw dislocations based on theoretical considerations to well explain the transformation mechanism, which consists of a 1c-screw-dislocation cluster and dense basal-plane partial dislocation loops. An elastic energy calculation revealed that the activation energy for the loop creation based on the proposed model well explains experimentally derived tendencies of the healing process. Furthermore, we confirmed that the total elastic energy of a healed micropipe is lower than those of 1c-screw-dislocation clusters without dislocation loops and hollow-core micropipes.

Hexagonal Boron Nitride Single Crystal Growth from Solution with a Temperature Gradient

Hexagonal boron nitride (hBN) is attracting much attention due to its tremendous applications including nanophotonic and electronic devices, substrates for two-dimensional (2D) materials, heat mana...

Magnetostrictive properties of Tb0.2Gd0.8 single crystal in high magnetic fields

Temperature and field dependencies of the magnetostriction of the Tb0.2Gd0.8 single crystal were measured in strong magnetic fields up to 14 T in a temperature range of 4.2–300 K. The temperature dependencies of five magnetostriction constants λ1α,0, λ2α,0, λ1α,2, λ2α,2 and λγ,2 were determined, using Clark's theory for rare-earth hexagonal close-packed single crystals. Giant values of linear and volume magnetostriction due to forced magnetostriction were found in the Tb0.2Gd0.8 single crystal (λbc = 1.6 × 10−3) in the region of the Curie temperature (near room temperatures). It is shown that giant values of forced magnetostriction are caused by the sharp dependence of exchange interactions on interatomic distances along the hexagonal the c axis.

Large anisotropic topological Hall effect in a hexagonal non-collinear magnet Fe5Sn3

We report the observation of a large anisotropic topological Hall effect (THE) in the hexagonal non-collinear magnet Fe5Sn3 single crystals. It is found that the sign of the topological Hall resistivity ρTH is negative when a magnetic field H is perpendicular to the bc-plane (H ⊥ bc-plane); however, it changes form negative to positive when H is parallel to the c-axis (H ∥ c-axis). The value of ρTH increased with the increasing temperature and reached approximately −2.12 μΩ cm (H ⊥ bc-plane) and 0.5 μΩ cm (H ∥c-axis) at 350 K, respectively. Quantitative analyses of the measured data suggest that the observed anisotropic THE may originate from the opposite scalar spin chirality induced by the magnetic fields perpendicular and parallel to the c-axis, respectively.

(Invited) Atomic Resolution, Coordinated Electron Microscopy Characterization of 2D Layers Synthesized By Confinement Heteroepitaxy (CHet) Technique

Since the discovery of graphene in 2004 [1] there has been a flurry of activity in investigating two dimensional (2D) materials, because of their superlative properties: spin-orbit coupling, massless Dirac fermions, mechanical strength, thermal and chemical stability [2]. Remarkably, the in-plane hexagonal symmetry of graphene is largely responsible for its behavior. However, the zero-band gap of graphene presents hurdles to its use as a semiconductor. This has spurred interest in synthesis other van der Waals 2D structures, such as h-BN [3] and more recently 2D Gallium Nitride (2D GaN) [4].The synthesis of 2D Nitrides was first reported in 2016 by applying Confinement Heteroepitaxy (CHet) process to form a 2D Gallium Nitride (GaN) layer with 1-2 atom thickness. In this technique, a silicon carbide (SiC) substrate is graphitized and deliberately damaged using oxygen plasma to provide a two-step intercalation path. First a metal (like Gallium, or other group V metal) through evaporation is intercalated between the graphene and SiC substrate. Then, to complete the half reaction, nitrogen is introduced by ammonia annealing at 700 °C to form a 1-layer thick III-nitride.In this study, the growth method is extended from 2D GaN to variety of other 2D metals (Indium), Nitrides (InN) and alloys (InGa alloys). We employ a coordinated approach to understand the surface in both plan and cross-section view – linking through a variety of electron microscopy techniques, including auger electron spectroscopy (AES), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Bright-field, Dark-field, aberration-corrected STEM-HAADF imaging, and atomic-scale electron energy loss spectroscopy (EELS) mapping using a Gatan K2 SI direct electron detector and other techniques mapping were all applied to characterize the SiC/2D-layer/graphene interface. These electron microscopy techniques allow a precise, local measurement of the atomic structure of these new materials, including such growth parameters as the effect of graphene on the 2D layer stability, the effect of the SiC step edges on the layers structure, and the effect of the plasma type and source used in damaging the graphene on the 2D layer chemistry. Plasmon mapping also describes the unique bonding in the materials and their electronic structure. Ultimately, the measurements presented here characterize a whole new class of nanomaterials and provide insight on the fundamentals of the intercalation growth process. 1. Novoselov, K.S., et al., Electric field effect in atomically thin carbon films. Science, 2004. 306(5696): p. 666-669.2. Soldano, C., A. Mahmood, and E. Dujardin, Production, properties and potential of graphene. Carbon, 2010. 48(8): p. 2127-2150.3. Watanabe, K., T. Taniguchi, and H. Kanda, Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal. Nature Materials, 2004. 3(6): p. 404-4094. Al Balushi, Z.Y., et al., Two-dimensional gallium nitride realized via graphene encapsulation. Nature Materials, 2016. 15(11): p. 1166-1171. Figure 1

Large anomalous Hall effect in a hexagonal ferromagnetic Fe5Sn3 single crystal

In this paper, we report an experimental observation of the large anomalous Hall effect (AHE) in a hexagonal ferromagnetic Fe5Sn3 single crystal with current along the b axis and a magnetic field normal to the bc plane. The intrinsic contribution of the anomalous Hall conductance sigma_AH^int was approximately 613 {\Omega}-1 cm-1, which was more than 3 times the maximum value in the frustrated kagome magnet Fe3Sn2 and nearly independent of the temperature over a wide range between 5 and 350 K. The analysis results revealed that the large AHE was dominated by a common, intrinsic term, while the extrinsic contribution, i.e., the skew scattering and side jump, turned out to be small. In addition to the large AHE, it was found the types of majority carriers changed at approximately 275 and 30 K, consistent with the critical temperatures of the spin reorientation. These findings suggest that the hexagonal ferromagnetic Fe5Sn3 single crystal is an excellent candidate to use for the study of the topological features in ferromagnets.

The evolution of spin-glass behavior in hexagonal Lu1-xCaxMnO3 single crystals

In this research, we have successfully synthesized Lu1-xCaxMnO3 (x = 0.1, 0.3, and 0.5) single crystal samples with space group P63cm, and studied their magnetic properties with multiple methods. On the basis of AC susceptibility and magnetic relaxation rate, we can reveal the evolution of spin-glass (SG) behavior, which occurs in Ca doping with x = 0.1 and x = 0.3 and disappears at x = 0.5. Such SG behavior is induced by the competition between super-exchange and double-exchange interactions among Mn ions and the magnetic transition occurs at about 100 K. This study would be helpful to study the magnetism in RMnO3 compounds and the SG behavior in multi-valence doping.

Study of the elastic properties of hexagonal metal single crystals

Hexagonal metals (e.g., Be, Zr, Ti) are widely used in the nuclear industry, space and aircraft engineering (in manufacturing of the structural elements operating under extreme conditions). A promising way to improve the quality of products made of them is to improve the physical properties of materials using the natural anisotropy of metal single crystals. The results of studying anisotropy and a comparative analysis of the technical characteristics of the elastic properties of single crystals of hexagonal metals are presented. The equations of the elastic compliance matrix components are derived in the explicit form for arbitrary crystallographic direction proceeding from transformations of the elastic compliance tensor in the principal axes to a new arbitrary coordinate system with a subsequent use of Euler angles. Analytical expressions are presented for the technical characteristics of the elastic properties (shear and Young's moduli, Poisson's ratio) of the single crystals of 10 hep metals for an arbitrary crystallographic direction. The axial symmetry of the characteristics about the hexagonal axis is revealed. The sums of the elastic compliance coefficients which determine the shear moduli and the Poisson's ratios in two mutually perpendicular directions are constant in any crystallographic plane of the single crystal. A comparative analysis of the anisotropy of the elastic properties of single crystals of the studied group of metals revealed auxetic properties of Zn and Be single crystals and the region of crystallographic directions of uniaxia tension, leading to an auxetic effect The auxetic effect in Zn was observed under tension in the directions of the plane perpendicular to the hexagonal axis of the single crystal. The planes of the auxetic effect manifestation in Be single crystals are perpendicular to the directions making an angle of 45° with the hexagonal axis.

Experimental Realization of a Quaternary Bi-Chalcogenide Topological Insulator with Smaller Effective Mass

It is known that Sb2Se3 does not exhibit topological insulator behavior due to its orthorhombic structure. The introduction of a small amount of bismuth and tellurium may change its structure to hexagonal, leading to a stable topological insulator compound. We report here the synthesis and the structural, chemical, and electronic properties of the topological insulator BiSbSe2.5Te0.5. Combining X-ray and electron diffraction measurements, we demonstrate the formation of this stable quaternary hexagonal single crystal. We used X-ray photoelectron spectroscopy to determine quantitatively the exact chemical composition of the sample. The topological insulating behavior is similar to that of other bismuth chalcogenides, as probed by angle-resolved photoemission spectroscopy. A p-type doping, leading to a 0.15 eV shift of the Fermi level was found. This value compensates the intrinsically n-type doping produced by selenium vacancies. We also found a smaller effective mass and a higher electron group velocity for the electrons in the topological states compared with Bi2Se3.