Crystal Axes Research Articles

Dimensional Analysis of Double-Track Microstructures in a Lithium Niobate Crystal Induced by Ultrashort Laser Pulses

Double-track microstructures were induced in the bulk of a z-cut lithium niobate crystal by 1030 nm 240 fs ultrashort laser pulses with a repetition rate of 100 kHz at variable pulse energies exceeding the critical Kerr self-focusing power. The microstructure topography was characterized by atomic force microscopy in piezoelectric response mode. The spatial positions of laser-induced modification regions inside lithium niobate in the case of laser beam propagation along the crystal optical axis can be directly predicted by simple analytical expressions under the paraxial approximation. A dimensional analysis of the morphology of the double-track structures revealed that both their length and width exhibit a monotonous increase with the pulse energy. The presented results have important implications for direct laser writing technology in crystalline dielectric birefringent materials, paving the way to control the high spatial resolution by means of effective energy deposition in modified regions.

Study on the Interface Microstructure of TaC/GCr15 Steel Surface Reinforced Layer Formed by In-Situ Reaction.

In this study, a micro-nano TaC ceramic steel matrix reinforced layer was prepared by an in situ reaction between a pure tantalum plate and GCr15 steel. The microstructure and phase structure of the in situ reaction reinforced layer of the sample at 1100 °C and reaction time 1 h were characterized with FIB micro-section, TEM transmission, SAED diffraction pattern, SEM and EBSD. The phase composition, phase distribution, grain size, grain orientation and grain boundary deflection, phase structure and lattice constant of the sample were characterized in detail. The results show that the phase composition of the Ta sample is Ta, TaC, Ta2C and α-Fe. TaC is formed after Ta and carbon atoms meet, and the orientation changes in the X and Z directions. The grain size of TaC is widely in the range of 0~0.4 μm, and the angular deflection of TaC grain is not obvious. The high-resolution transmission structure, diffraction pattern and interplanar spacing of the phase were characterized, and the crystal planes of different crystal belt axes were determined. The study provides technical and theoretical support for further research on the preparation technology and microstructure of the TaC ceramic steel matrix reinforcement layer.

Tuning crystal orientation and chiral spin order in Mn3Ge by annealing process and ion implantation

The non-collinear antiferromagnetic Weyl semimetal Mn3X (X = Ga, Ge, Sn) system has attracted a lot of attentions owing to its robust anomalous Hall effect (AHE), large spin Hall angle and small net magnetization at room temperature. The high spin-charge interconversion efficiency makes it a super candidate in topological antiferromagnetic spintronic devices, which could facilitate ultra-fast operation of high-density devices with low energy consumption. In this work, we have realized to obtain different chiral spin structures in Heusler alloy Mn3Ge thin films, which originate from different crystalline orientations. The high-quality (0002)- and (200)-oriented single phase hexagonal Mn3Ge films are achieved by controllable growth, annealing process and ion implantation. The various magnetic properties and AHE behaviors are observed along a and c crystal axes, equivalent to magnetic field in and out of the inverse triangular spin plane. The observation demonstrates the manipulation of crystal structure accompanied with chiral spin order in a non-collinear antiferromagnetic Mn3Ge film, which is induced by energy conversion and defect introduction. The in situ thermal treatment induces crystal phase rotation up to 90° and robust AHE modulation, which is significantly important and highly desirable for flexible spin memory device applications.

Temperature-dependent elastic constants of thorium dioxide probed using time-domain Brillouin scattering

We report the adiabatic elastic constants of single-crystal thorium dioxide over a temperature range of 77–350 K. Time-domain Brillouin scattering, an all-optical, non-contact picosecond ultrasonic technique, is used to generate and detect coherent acoustic phonons that propagate in the bulk perpendicular to the surface of the crystal. These coherent acoustic lattice vibrations have been monitored in two hydrothermally grown single-crystal thorium dioxide samples along the (100) and (311) crystallographic directions. The three independent elastic constants of the cubic crystal (C11, C12, and C44) are determined from the measured bulk acoustic velocities. The longitudinal wave along the (100) orientation provided a direct measurement of C11. Measurement of C44 and C12 was achieved by enhancing the intensity of quasi-shear mode in a (311) oriented crystal by adjusting the polarization angle relative to the crystal axes. We find the magnitude of softening of the three elastic constants to be ∼2.5% over the measured temperature range. Good agreement is found between the measured elastic constants with previously reported values at room temperature, and between the measured temperature-dependent bulk modulus with calculated values. We find that semi-empirical models capturing lattice anharmonicity adequately reproduce the observed trend. We also determine the acoustic Grüneisen anharmonicity parameter from the experimentally derived temperature-dependent bulk modulus and previously reported temperature-dependent values of volumetric thermal expansion coefficient and heat capacity. This work presents measurements of the temperature-dependent elasticity in single-crystal thorium dioxide at cryogenic temperature and provides a basis for testing ab initio theoretical models and evaluating the impact of anharmonicity on thermophysical properties.

Effects of the vacuum hot pressing process on the microstructures, strengthening mechanisms, and mechanical properties of Ni-35Cr-12Mo-3Nb alloys

This research fabricates Ni-35Cr-12Mo-3Nb alloys using the vacuum hot pressing process of PM technology. The experiments used various hot pressing temperatures (1050, 1100, 1150, and 1200 °C) and hot pressing pressures (20, 35, and 50 MPa) to determine the optimal process parameters. The experimental results show that the optimal hot pressing parameters for this alloy were 1100 °C at 35 MPa for 1 h. Among them, the sintered density was enhanced to 8.15 ± 0.01 g cm−3, and the hardness, transverse rupture strength (TRS) and flexural modulus reached 78.0 ± 0.1 HRA, 1404.2 ± 36.8 MPa, and 75.4 ± 3.3 GPa, respectively, and the electrical conductivity was 1.92 ± 0.14 × 104 S cm−1. TEM observation proves that the MoNi and Ni3Nb intermetallic compounds do indeed exist in the Ni-35Cr-12Mo-3Nb alloy after 1100 °C hot pressing at 35 MPa for 1 h. MoNi is an orthorhombic system with diffraction along the [3 4‾ 4] crystal axis, and Ni3Nb is the same system, but along the [100] crystal axis. EBSD analysis results also confirm that this alloy has no obvious grain orientation preference.

Three-Dimensional Anisotropic Carrier Mobility and Structure–Property Relationships for [1]Benzothieno[3,2-b][1]benzothiophene Derivatives: A Theoretical Study

Through the Marcus electron-transfer theory combined with the random walk technique for the charge-carrier diffusion process, we simulated the three-dimensional (3D) distributions of hole and electron mobilities for [1]benzothieno[3,2-b][1]benzothiophene (BTBT) and its derivatives. Our predicted mobility ranges agree well with the measured field-effect mobility of the BTBT derivatives. We further analyzed the charge-transfer mobility anisotropy of the studied compounds, and the optimum conducting-channel direction relative to the crystal axis was determined, which provides a reliable reference to assist in the performance optimization of field-effect transistors (FETs). Moreover, we analyzed in detail the influences of different substituents on the reorganization energies, ionization energies, electron affinities, frontier molecular orbital charge distributions, and solid-state packing motifs of the BTBT. It was found that the reorganization energies and energy barrier of charge injection effectively decreased with the fusion of the thiophene ring. However, the herringbone packing of BTBT is transformed to π stacking at a local site; as a result, the hole and electron mobilities of BTBT decreased slightly. In comparison, attaching electron-withdrawing −COPhF to BTBT not only increases the electron affinities significantly but also increases the electronic couplings and decreases the reorganization energy related to the electron transfer. It provides a promising way to design n-type or ambipolar organic semiconducting materials.

Electro-optics of blue phase liquid crystal in field-perpendicular direction

The electro-optic effect is the working principle of blue phase (BP) liquid crystals, and it describes the relationship between the field-induced birefringence of BPs and the field strength. Due to the electrostriction of BP crystals under the electric field, an orthorhombic or tetragonal crystal is usually obtained when the field is applied along one of the twofold axes of a BP crystal, leading to the optical biaxiality under electric field. Such field-induced optical biaxiality of BPs has been predicted and observed, but its dependence on the field strength has not been investigated. In this research, we analyze the electro-optics in the field-perpendicular direction by measuring the birefringence in highly ordered BP I(110) crystals perpendicular to the electric field. Results reveal that BP I crystals in the field-perpendicular direction show an electro-optic coefficient of the order of 10−10 m/V2 that may result from the large lattice deformation of BP crystals perpendicular to the electric field. Our research provides important experimental evidence for the tensorial properties of BP Kerr effect and may have important implications on the engineering of BP electro-optical devices in practical applications.

Effects of polymers on the regulation of p-aminobenzoic acid crystal habit

This study focuses on how polymers regulate the crystal habit of p-aminobenzoic acid (PABA) from long rodlike to blocklike. To propose the regulation mechanism of polymers on the crystal habit of PABA, experiments include cooling crystallization and single crystal growth as well as molecular simulation have been carried out. Firstly, crystal form of PABA from cooling crystallization was determined as α form. Secondly, among four 1.0 wt% polymers, polyvinylpyrrolidone (PVP) played a more significant role in regulating crystal habit, which inhibited the growth of long axis resulting the growth rates of the long axis from 92.1 μm/h down to 6.7 μm/h. Furthermore, the effect of PVP chain length and concentration on crystal habit has also been studied, 0.2 wt% PVP-K17 regulated the crystal habit of PABA from long rodlike to blocklike but the effect of PVP was weakened with increasing chain length and decreasing concentration. Molecular simulation results show that the interaction between PVP-K17 and (110)/(011) face, which determines the axial growth rate, is stronger compared with other crystal faces. The mechanism of PVP-K17 regulating crystal habit was that it can be selectively adsorbed on (110) and (011) faces, hindering the diffusion and adsorption of solute molecules, inhibiting the growth of the long axis of PABA crystals, so the crystal habit of obtained crystal is blocklike.

Unveiling the Intrinsic Structure and Intragrain Defects of Organic-Inorganic Hybrid Perovskites by Ultralow Dose Transmission Electron Microscopy.

Transmission electron microscopy (TEM) is a powerful tool for unveiling the structural, compositional, and electronic properties of organic-inorganic hybrid perovskites (OIHPs) at the atomic to micrometer length scales. However, the structural and compositional instability of OIHPs under electron beam radiation results in misunderstandings of the microscopic structure-property-performance relationship in OIHP devices. Here, ultralow dose TEM is utilized to identify the mechanism of the electron-beam-induced changes in OHIPs and clarify the cumulative electron dose thresholds (critical dose) of different commercially interesting state-of-the-art OIHPs, including methylammonium lead iodide (MAPbI3 ), formamidinium lead iodide (FAPbI3 ), FA0.83 Cs0.17 PbI3 , FA0.15 Cs0.85 PbI3 , and MAPb0.5 Sn0.5 I3 . The critical dose is related to the composition of the OIHPs, with FA0.15 Cs0.85 PbI3 having the highest critical dose of ≈84 e Å-2 and FA0.83 Cs0.17 PbI3 having the lowest critical dose of ≈4.2 e Å-2 . The electron beam irradiation results in the formation of a superstructure with ordered I and FA vacancies along <110>c , as identified from the three major crystal axes in cubic FAPbI3 , <100>c , <110>c , and <111>c . The intragrain planar defects in FAPbI3 are stable, while an obvious modification is observed in FA0.83 Cs0.17 PbI3 under continuous electron beam exposure. This information can serve as a guide for ensuring a reliable understanding of the microstructure of OIHP optoelectronic devices by TEM.

Anisotropic Van Der Waals 2D GeAs Integrated on Silicon Four-Waveguide Crossing

In-plane optical anisotropy plays a critical role in manipulating light in a wide range of planner photonic devices. In this study, the strong anisotropy of multilayer 2D GeAs is leveraged and utilized to validate the technical feasibility of on-chip light management. A 2D GeAs is stamped into an ultra-compact silicon waveguide four-way crossing optimized for operation in the O-optical band. The measured optical transmission spectra indicated a remarkable discrepancy between the in-plane crystal optical axes with an attenuation ratio of ∼3.5 (at 1330 nm). Additionally, the effect of GeAs crystal orientation on the electro-optic transmission performance is demonstrated on a straight waveguide. A notable 50% reduction in responsivity was recorded for devices constructed with cross direction compared to devices with a crystal <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</i> -direction parallel to the light polarization. This extraordinary optical anisotropy, combined with a high refractive index ∼4 of 2D GeAs, opens possibilities for efficient on-chip light manipulation in photonic devices.

Chiral phonons: circularly polarized Raman spectroscopy and ab initio calculations in a chiral crystal tellurium.

Recently, phonons with chirality (chiral phonons) have attracted significant attention. Chiral phonons exhibit angular and pseudoangular momenta. In circularly polarized Raman spectroscopy, the peak split of the mode is detectable along the principal axis of the chiral crystal in the backscattering configuration. In addition, peak splitting occurs when the pseudoangular momenta of the incident and scattered circularly polarized light are reversed. Until now, chiral phonons in binary crystals have been observed, whereas those in unary crystals have not been observed. Here, we observe chiral phonons in a chiral unary crystal Te. The pseudoangular momentum of the phonon is obtained in Te by an ab initio calculation. From this calculation, we verified the conservation law of pseudoangular momentum in Raman scattering. From this conservation law, we determined the handedness of the chiral crystals. We also evaluated the true chirality of the phonons using a measure with symmetry similar to that of an electric toroidal monopole.

Controlling the spin Hall effect of grafted vortex beams propagating in uniaxial crystal.

Though numerous studies of spin-orbit interaction (SOI) of light beams propagating along the optic axis of uniaxial crystals have been carried out, in previous studies, the initial input beams have cylindrical symmetry. In this case, the total system preserves cylindrical symmetry so that the output light after passing through the uniaxial crystal doesn't exhibit spin dependent symmetry breaking. Therefore, no spin Hall effect (SHE) occurs. In this paper, we investigate the SOI of a kind of novel structured light beam, grafted vortex beam (GVB) in uniaxial crystal. The cylindrical symmetry of the system is broken by the spatial phase structure of the GVB. As a result, a SHE determined by the spatial phase structure emerges. It is found that the SHE and evolution of the local angular momentum are controllable both by changing the grafted topological charge of the GVB and by employing linear electro-optic effect of the uniaxial crystal. This can open a new perspective to investigate the SHE of light beams in uniaxial crystals via constructing and manipulating the spatial structure of the input beams artificially, hence offers novel regulation capabilities of spin photon.

Nonlinear Optical Properties in an Epitaxial YbFe2O4 Film Probed by Second Harmonic and Terahertz Generation.

An epitaxial film of YbFe2O4, a candidate for oxide electronic ferroelectrics, was fabricated on yttrium-stabilized zirconia (YSZ) substrate by magnetron sputtering technique. For the film, second harmonic generation (SHG), and a terahertz radiation signal were observed at room temperature, confirming a polar structure of the film. The azimuth angle dependence of SHG shows four leaves-like profiles and is almost identical to that in a bulk single crystal. Based on tensor analyses of the SHG profiles, we could reveal the polarization structure and the relationship between the film structure of YbFe2O4 and the crystal axes of the YSZ substrate. The observed terahertz pulse showed anisotropic polarization dependence consistent with the SHG measurement, and the intensity of the emitted terahertz pulse reached about 9.2% of that emitted from ZnTe, a typical nonlinear crystal, implying that YbFe2O4 can be applied as a terahertz wave generator in which the direction of the electric field can be easily switched.

1D Hybrid Semiconductor Silver 2,6-Difluorophenylselenolate.

Organic-inorganic hybrid materials present new opportunities for creating low-dimensional structures with unique light-matter interaction. In this work, we report a chemically robust yellow emissive one-dimensional (1D) semiconductor, silver 2,6-difluorophenylselenolate─AgSePhF2(2,6), a new member of the broader class of hybrid low-dimensional semiconductors, metal-organic chalcogenolates. While silver phenylselenolate (AgSePh) crystallizes as a two-dimensional (2D) van der Waals semiconductor, introduction of fluorine atoms at the (2,6) position of the phenyl ring induces a structural transition from 2D sheets to 1D chains. Density functional theory calculations reveal that AgSePhF2 (2,6) has strongly dispersive conduction and valence bands along the 1D crystal axis. Visible photoluminescence centered around λp ≈ 570 nm at room temperature exhibits both prompt (110 ps) and delayed (36 ns) components. The absorption spectrum exhibits excitonic resonances characteristic of low-dimensional hybrid semiconductors, with an exciton binding energy of approximately 170 meV as determined by temperature-dependent photoluminescence. The discovery of an emissive 1D silver organoselenolate highlights the structural and compositional richness of the chalcogenolate material family and provides new insights for molecular engineering of low-dimensional hybrid organic-inorganic semiconductors.

Elastic Properties of the Iron(II)–Triazole Spin Crossover Complexes [Fe(Htrz)2trz]BF4 and [Fe(NH2trz)3]SO4

Two benchmark Fe(II)-triazole spin-crossover (SCO) compounds, [Fe(Htrz)2trz]BF4 (1) and [Fe(NH2trz)3]SO4 (2), have been investigated using variable temperature and pressure powder X-ray diffraction, dynamical mechanical analysis, and differential scanning calorimetry techniques. We assessed the transformation strain, the compressibility, and their anisotropy in both compounds. The measured bulk moduli are relatively high (20 and 13 GPa in 1 and 2, respectively), but against expectations, the Fe–triazole chain direction is the most compressible crystal axis in 1. We revealed considerable elastic softening and associated mechanical damping at the SCO in both compounds, which we interpreted, using the Landau theory, as a consequence of the coupling between the spin state and the lattice strain. The magnitudes of the elastic potential energy (∼15%), stored elastic work (<2–3%), and frictional work (<3–4%) with respect to the latent heat of the spin transition (26.8 and 19.5 kJ/mol in 1 and 2, respectively) have been also assessed. The knowledge of these properties is important both for a better understanding of the SCO behavior of this important family of complexes and for guiding their integration into mechanical transducers and other technological applications.

Flexoelectro-optic properties of liquid crystal dimer CB7CB in the uniform standing helix structure under planar anchoring boundary conditions

Based on the theory of liquid crystal elasticity, flexoelectric effect and dielectric effect in uniform standing helix (USH) structure with the liquid crystal dimer CB7CB which has larger flexoelectric effect and smaller dielectric anisotropy under strong and weak planar anchoring boundary conditions are studied. The influence of flexoelectric effect on the distribution of director and the rotation of USH optical axis can be quantitatively analysed through numerical simulation. We find that the dielectric effect has little effect on the director and almost no effect on the rotation angle of the optical axis. The rotation angle of the optical axis under strong anchoring is consistent with the formulas in the literature. Under weak anchoring, the pitch of the cholesteric liquid crystal becomes shorter, and the difference between the flexoelectric coefficients e 1–e 3 and the single flexoelectric coefficient e 3 affect the rotation angle of the optical axis. Remarkably, our method provides a more accurate theory basis for the rotation of the optic axis in USH cholesteric liquid crystals.

Visualization of bulk and edge photocurrent flow in anisotropic Weyl semimetals

Materials that rectify light into current in their bulk are desired for optoelectronic applications. In inversion-breaking Weyl semimetals, bulk photocurrents may arise due to nonlinear optical processes that are enhanced near the Weyl nodes. However, the photoresponse of these materials is commonly studied by scanning photocurrent microscopy (SPCM), which convolves the effects of photocurrent generation and collection. Here, we directly image the photocurrent flow inside the type-II Weyl semimetals WTe2 and TaIrTe4 using high-sensitivity quantum magnetometry with nitrogen-vacancy center spins. We elucidate an unknown mechanism for bulk photocurrent generation termed the anisotropic photothermoelectric effect (APTE), where unequal thermopowers along different crystal axes drive intricate circulations of photocurrent around the photoexcitation. Using simultaneous SPCM and magnetic imaging at the sample's interior and edges, we visualize how the APTE stimulates the long-range photocurrent collected in our Weyl semimetal devices through the Shockley-Ramo theorem. Our results highlight an overlooked, but widely relevant source of current flow and inspire novel photodetectors using homogeneous materials with anisotropy.

Effects of optic axis rotation on the sensing properties of nematic liquid crystal based surface plasmon resonance (SPR) sensor

The effect of different orientations of the optic axis (OA) in the liquid crystal (LC) UCF-1 in two dimensions on the surface plasmon (SP) mode excited along the surface intermediate between a thin silver film and the LC at different temperatures is theoretically studied using the prism coupling technique and a TM-polarized, monochromatic laser with λ = 5890 Å. The shifts in the surface plasmon resonance (SPR) dip angle at different temperatures for varying alignments of the OA in a plane are determined. Changes in the SPR resonance angles with temperature are found to be significant and linear in a major part of the entire anisotropic phase of UCF-1 for the homeotropic orientation. A linear fit is obtained for the variation of SPR angle of resonance with the molecular order parameter over the entire anisotropic range of UCF-1. The effect of deviations of the OA of the LC from the homeotropic orientation in two dimensions on the temperature sensitivity of the SPR angle of resonance in the linear response temperature range is studied, and its influence on the accuracy of measurement by SPR corresponding to a fixed value of the molecular order parameter is investigated.

Hall Effect Anisotropy in the Paramagnetic Phase of Ho0.8Lu0.2B12 Induced by Dynamic Charge Stripes.

A detailed study of charge transport in the paramagnetic phase of the cage-cluster dodecaboride Ho0.8Lu0.2B12 with an instability both of the fcc lattice (cooperative Jahn-Teller effect) and the electronic structure (dynamic charge stripes) was carried out at temperatures 1.9-300 K in magnetic fields up to 80 kOe. Four mono-domain single crystals of Ho0.8Lu0.2B12 samples with different crystal axis orientation were investigated in order to establish the singularities of Hall effect, which develop due to (i) the electronic phase separation (stripes) and (ii) formation of the disordered cage-glass state below T*~60 K. It was demonstrated that a considerable intrinsic anisotropic positive component ρanxy appears at low temperatures in addition to the ordinary negative Hall resistivity contribution in magnetic fields above 40 kOe applied along the [001] and [110] axes. A relation between anomalous components of the resistivity tensor ρanxy~ρanxx1.7 was found for H||[001] below T*~60 K, and a power law ρanxy~ρanxx0.83 for the orientation H||[110] at temperatures T &lt; TS~15 K. It is argued that below characteristic temperature TS~15 K the anomalous odd ρanxy(T) and even ρanxx(T) parts of the resistivity tensor may be interpreted in terms of formation of long chains in the filamentary structure of fluctuating charges (stripes). We assume that these ρanxy(H||[001]) and ρanxy(H||[110]) components represent the intrinsic (Berry phase contribution) and extrinsic (skew scattering) mechanism, respectively. Apart from them, an additional ferromagnetic contribution to both isotropic and anisotropic components in the Hall signal was registered and attributed to the effect of magnetic polarization of 5d states (ferromagnetic nano-domains) in the conduction band of Ho0.8Lu0.2B12.

Kink band formation mechanism in α/β Ti-9Cr two-phase alloy

A combination of experimental and numerical studies was carried out to understand the microstructure and formation mechanism of kink bands in α/β two-phase titanium alloys. A deformation kink band was introduced in a Ti-9Cr alloy after uniaxial compression test. The strength changes of the α and β phases inside and outside the kink band were evaluated by nanoindentation tests. A potential hardening effect was revealed in the α phase inside the kink band, while a possible softening effect was located in the β phase at the kink boundaries. Microstructural examination of the kink band was conducted by electron backscatter diffraction measurement. The crystal rotation axis was found to be perpendicular to both the (0001) basal plane of the α phase and one of the (110) planes of the β phase. The crystallographic features were characterized by transmission electron microscopy observation, revealing a prismatic 〈a〉 slip that occurred during the compression to accommodate the rotations around the c-axis. Nanoindentation tests were performed on an undeformed specimen to calibrate the crystal plasticity model. A crystal plasticity finite element model was developed and {110}β-rotation-type kink bands were reproduced in the simulation and confirmed the prismatic slip activity.