Conical Crystals Research Articles

Giant and negative magnetoresistances in conical magnets in the nonequilibrium Boltzmann equation approach

We study magnetotransport in conical helimagnet crystals using the nonequilibriun Boltzmann equation approach. Spin dependent magnetoresistance exhibits dramatic properties for high and low electron concentrations at different temperatures. For spin up electrons we find negative magnetoresistance despite only considering a single carrier type. For spin down electrons we observe giant magnetoresistance due to depletion of spin down electrons with an applied magnetic field. For spin up carriers, the magnetoresistance is negative, due to the increase in charge carriers with a magnetic field. In addition, we investigate the spin dependent Hall effect. If a magnetic field reaches some critical value for spin down electrons, giant Hall resistance occurs, i.e. Hall current vanishes. This effect is explained by the absence of spin down carriers. For spin up carriers, the Hall constant dramatically decreases with field, due to the increase in spin up electron density. Because of the giant spin dependent magnetoresistance and Hall resistivity, conical helimagnets could be useful in spin switching devices.

New readout system of the FATIMA detectors based on Silicon Photomultipliers arrays

We present a new readout system based on large area Silicon Photomultipliers arrays coupled with cylindrical 1.5 × 2” LaBr3(Ce) FATIMA-type crystals. This achieves the fast-timing capabilities of such crystals coupled to the usual Photomultiplier Tubes. Energy calibration was measured with 137Cs and 152Eu standard radioactive sources, while the timing performances were investigated with a 60Co radioactive source. We benchmark our results against the corresponding results obtained with the fast R9779 Photomultiplier tubes and highlight the importance of achieving similar energy and timing performances for nuclear structure experiments. The FWHM energy resolution for the 1.5 × 2” LaBr3(Ce) crystal coupled with the SiPM and PMT was found to be 3.31(2)% and 3.85(1)%, respectively, for the 661.6 keV transition of the 137Cs source. The timing resolution was measured between a conical crystal known for its fast response and a cylindrical crystal coupled to a SiPM or PMT readout. We obtain values of FWHM = 230(1) ps and 232(1) ps, respectively, between the 1173.2-1332.5 keV transitions of the 60Co source. In addition, a temperature effect compensation circuit was developed to maintain a good stability of the gain during long measurements, typical for in-beam/off-beam experiments in nuclear physics.

Geometric frustration of hard-disk packings on cones.

Conical surfaces pose an interesting challenge to crystal growth: A crystal growing on a cone can wrap around and meet itself at different radii. We use a disk-packing algorithm to investigate how this closure constraint can geometrically frustrate the growth of single crystals on cones with small opening angles. By varying the crystal seed orientation and cone angle, we find that-except at special commensurate cone angles-crystals typically form a seam that runs along the axial direction of the cone, while near the tip, a disordered particle packing forms. We show that the onset of disorder results from a finite-size effect that depends strongly on the circumference and not on the seed orientation or cone angle. This finite-size effect occurs also on cylinders, and we present evidence that on both cylinders and cones, the defect density increases exponentially as circumference decreases. We introduce a simple model for particle attachment at the seam that explains the dependence on the circumference. Our findings suggest that the growth of single crystals can become frustrated even very far from the tip when the cone has a small opening angle. These results may provide insights into the observed geometry of conical crystals in biological and materials applications.

Self-rolling and circling of a conical liquid crystal elastomer rod on a hot surface

Self-oscillation refers to the steady-state periodic motion maintained by the mutual coupling of parts within system itself, which can compensate for damping dissipation through periodically absorbing energy from non-periodic external stimuli. Recent experiments have demonstrated that the uneven thermal contraction in the cylindrical rod can drive its forward self-rolling on a hot surface. In current paper, by introducing a cone angle to the rod, we fabricate a conical liquid crystal elastomer (LCE) rod to investigate its rolling on a hot surface. Our experiments show that the thermally-responsive conical LCE rod can self-roll and circle on a flat hot surface at a uniform temperature. We propose a thermomechanical model to elucidate the mechanism of self-rolling and circling and calculate the rolling and circling angular velocities of the conical LCE rod. The theoretical results demonstrate that the rolling angular velocity depends on the Biot number, cone angle, rod length and heat inflow. The theoretical predictions are proved to be consistent with the experimental results. The self-rolling and circling of the conical LCE rod can retrace its original position through a curved trajectory, offering the benefit of circumventing extensive heated surfaces and maneuvering around obstacles or traps. Such self-rolling and circling system based on conical LCE rod may constitute novel designs for self-cleaning sweepers, energy harvesters, active machinery, mass transport devices and so on.

A focusing X-ray spectrometer based on continuously conical crystal.

X-ray optics with good focusing ability and high spectral resolution are required in X-ray spectroscopy for the diagnosis of high temperature and density plasmas. In our study, a novel X-ray spectrometer is developed to provide the ability to record spectra with excellent focusing performance and high energy resolution. It is accomplished by using a continuously conical crystal (CCC) that is formed by circles with different curvatures. In this paper, we present the foundational work of the design and development of continuously conical crystal spectrometer (CCCS) along with initial results obtained with a titanium (Ti) target as the object source. First, the spectrometer based on such a continuously conical crystal is used to measure X-ray spectra on Ti target X-ray Tube device. The spectral resolution (λ/Δλ) is around 615 with the source size of 1 mm. Then, we test the capability of the spectrometer on Xingguang-III Laser Facility with Ti target. He-like and Li-like Ti lines are recorded based on which the spectrometer performance is evaluated. The experiment result shows that the spectrometer provides a high spectral resolving power up to 1000, while acquiring a one-dimensional image of the source.

Anomalous Hall effect in conical helimagnetic crystals

Spin-spiral texture can substantially change charge transport properties in helimagnets. We find the anomalous Hall effect (AHE) exhibiting the dramatic behavior with respect to chemical potential $\ensuremath{\mu}$ in conical magnetic structures. The direct conductivity demonstrates kinks, and the anomalous Hall current exhibits minima and maxima changing the sign. We analytically derive the expression for energy bands and eigenstates in the most general case. Because of the conical potential, the energy bands are split into two nonparabolic bands where the lower band can have one- or two-minima shapes in the ${k}_{z}$ direction ($\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{\mathbit{z}}$ is a direction of the spiral axis). We prove that the origin of the anomalous Hall effect is not topological and is due to the interplay between the asymmetry of energy bands in the $x$ and $z$ directions and spin restrictions in the phase space due to the conical potential. We also investigate the dependence of transport properties on cone half- angle $\ensuremath{\theta}$, and find that the effects are most pronounced at $\ensuremath{\theta}=\ensuremath{\pi}/2$ (a helical state). Electric current is calculated using the Boltzmann equation where the relaxation is caused by electron-acoustic phonon interaction. The transition probability is found to be a $2\ifmmode\times\else\texttimes\fi{}2$ matrix with nonvanishing off-diagonal elements indicating the strong interband transitions. The origin of interband transitions is because of the nature of the conical potential where conduction electron spins interact with localized magnetic moments. To verify the proposed theory, we calculate the temperature dependence of resistivity for $\mathrm{MnSi}$ crystals and find the discontinuity at the phase transition between conical and paramagnetic phases. The calculations are in the excellent agreement with the experimental data. In addition, we predict the discontinuity behavior for the anomalous Hall resistivity at the phase transition where the resistivity exhibits the abrupt change at $T={T}_{C}$, (a) to zero if the relativistic effects for the conduction electrons are small or (b) to a nonzero value if Rashba/Dresselhaus effects are taken into account.

Partially coherent conical refraction promises new counter-intuitive phenomena

In this paper, we extend the paraxial conical refraction model to the case of the partially coherent light using the unified optical coherence theory. We demonstrate the decomposition of conical refraction correlation functions into well-known conical refraction coherent modes for a Gaussian Schell-model source. Assuming randomness of the electrical field phase of the input beam, we reformulated and significantly simplified the rigorous conical refraction theory. This approach allows us to consider the propagation of light through a conical refraction crystal in exactly the same way as in the classical case of coherent radiation. Having this in hand, we derive analytically the conical refraction intensity both in the focal plane and in the far field, which allows us to explain and rigorously justify earlier experimental findings and predict new phenomena. The last include the counterintuitive effect of narrowing of the conical refraction ring width, disappearance of the dark Poggendorff’s ring in the Lloyd’s plane, and shift of Raman spots for the low-coherent conical refraction light. We also demonstrate a universal power-law dependence of conical refraction cones coherence degree on the input correlation length and diffraction-free propagation of the low-coherent conical refraction light in the far field.

Absolute calibration of the conical crystal configuration of the zinc spectrometer (ZSPEC) at the OMEGA laser facility.

In this study, we present the absolute calibration of the conical crystal for the zinc spectrometer (ZSPEC), an x-ray spectrometer at the OMEGA laser facility at the Laboratory for Laser Energetics. The ZSPEC was originally designed to measure x-ray Thomson scattering using flat or cylindrically curved highly oriented pyrolytic graphite crystals centered around Zn He-alpha emission at 9keV. To improve the useful spectral range and collection efficiency of the ZSPEC, a conical highly annealed pyrolytic graphite crystal was fabricated for the ZSPEC. The conically bent crystal in the Hall geometry produces a line focus perpendicular to the spectrometer axis, corresponding to the detector plane of electronic detectors at large scale laser facilities such as OMEGA, extending the useful range of the spectrometer to 7-11keV. Using data collected using a microfocus Mo x-ray source, we determine important characteristics of ZSPEC such as the dispersion, spatial resolution, and absolute sensitivity of the instrument. A ray-trace model of ZSPEC provides another point of agreement in calculations of the ZSPEC dispersion and crystal response.

Studying Surfactant Effects on Hydrate Crystallization at Oil-Water Interfaces Using a Low-Cost Integrated Modular Peltier Device

We introduce an approach to study the formation and growth of hydrates under the influence of nonionic surfactants. The experimental system includes a temperature regulator, visualization techniques, and inner pressure measurements. The temperature control system contains a low-cost, programmable temperature regulator made with solid-state Peltier components. Along with the temperature control system, we incorporated visualization techniques and internal pressure measurements to study hydrate formation and inhibition in the presence of nonionic surfactants. We studied the hydrate-inhibiting ability of nonionic surfactants (sorbitane monolaurate, sorbitane monooleate, PEG-PPG-PEG, and polyoxyethylenesorbitan tristearate) at low (i.e., 0.1 CMC), medium (i.e., CMC), and high (i.e., 10 CMC) concentrations. Two types of crystals were formed: planar and conical. Planar crystals were formed in plain water and low surfactant concentrations. Conical crystals were formed in high surfactant concentrations. The results of the study show that conical crystals are the most effective in terms of hydrate inhibition. Because conical crystals cannot grow past a certain size, the hydrate growth rate as a conical crystal is slower than the hydrate growth rate as planar crystal. Hence, surfactants that force hydrates to form conical crystals are the most efficient. The goal of the protocol is to provide a detailed description of an experimental system that is capable of investigating the cyclopentane hydrate crystallization process on the surface of a water droplet in the presence of surfactant molecules.

Controllable synthesis of conical BiVO4 for photocatalytic water oxidation

Free-standing conical BiVO4 crystals featuring a curved surface and sharp tips were synthesized. The sample exhibited an enlarged visible light absorption region and good photocatalytic water oxidation ability.

Conical-shaped hexagonal barium ferrite nanodot arrays on an alumina substrate based on an ultrathin alumina mask method

Abstract In this work, conical-shaped M-type ferrite (BaFe12O19, BaM) magnetic nanodot arrays have been prepared on alumina substrate by pulsed laser deposition (PLD) through an ultrathin alumina mask (UTAM). The UTAM has periodically ordered cylindrical holes with a diameter of 350 nm and an interpore distance of 450 nm. Nanodots with a shape distribution in which the diameter is centred at 327 nm with a standard deviation of 23 nm have been fabricated. The microstructures, morphologies and magnetic properties of the nanodot arrays were investigated. Scanning electron microscopy and X-ray diffraction revealed that the prepared nanodots are c-axis-oriented conical crystals. The hysteresis loops indicated that the BaM nanodot arrays have a 63% squareness ratio, which is significantly higher than that of the film (45%). The first-order reversal curve (FORC) diagram showed the nanodots have small distributions of coercivity (Hc) and interactive field (Hu) between particles, with the centre at Hc = 1800 Oe and Hu = −280 Oe. Furthermore, the micromagnetic simulation revealed that this unique structure greatly impacts the distribution of demagnetizing field and effectively reduces the intensity of demagnetizing field.

Absolute calibration of a time-resolved high resolution x-ray spectrometer for the National Ignition Facility (invited).

A high resolution, Diagnostic Instrument Manipulator (DIM)-based x-ray Bragg crystal spectrometer has been calibrated for and deployed at the National Ignition Facility (NIF) to diagnose plasma conditions in ignition capsules near stagnation times. The spectrometer has two conical crystals in the Hall geometry focusing rays from the Kr Heα, Lyα, and Heβ complexes onto a streak camera, with the physics objectives of measuring time-resolved electron density and temperature through observing Stark broadening and the relative intensities of dielectronic satellites. A third von Hámos crystal that time-integrates the Kr Heα, Heβ and intervening energy range provides in situ calibration for the streak camera signals. The spectrometer has been absolutely calibrated using a microfocus x-ray source, an array of CCD and single-photon-counting detectors, and multiple K- and L-absorption edge filters at the Princeton Plasma Physics Laboratory (PPPL) x-ray laboratory. Measurements of the integrated reflectivity, energy range, and energy resolution for each crystal are discussed. These calibration data provide absolute x-ray signal levels for NIF measurements, enabling precise filter selection and comparisons to simulations.

Polytype Identification in Trioctahedral 1:1 Layer Silicates Using Electron Diffraction with Application to a Chronstedtite That was Synthesized Using Metallic Iron-Clay Interactions

AbstractThe present study presents a generalized procedure to accurately identify trioctahedral 1:1 layer silicates. The reciprocal space (RS) sections were obtained by the precession method by “unwarping” frames that were recorded using diffractometers with area detectors or from electron diffraction tomography (EDT) patterns. Distributions of subfamily reflections along the reciprocal lattice rows [21̄l]* / [11l]* / [1̄2l]* in (2hh̄lhex)* / (hhlhex)* / (h̄2hlhex)* RS planes were used to determine OD (Ordered — Disordered) subfamilies (Bailey’s groups A, B, C, D). The distributions along the [10l]* / [01l]* / [1̄1l]* rows in the (h0lhex)* / (0klhex)* / (h̄hlhex) RS planes allow determination of the polytypes. The use of traditional identification diagrams for the determination of OD subfamilies and polytypes was generalized in order to identify monoclinic and orthorhombic MDO (Maximum Degree of Order) polytypes. In these polytypes, the distribution of characteristic reflections along rows is different in RS sections that are perpendicular and diagonal to the symmetry plane of the polytype. The identification diagram of non-MDO polytype 6T2 is also presented. The method was applied to the identification of single crystals of cronstedtite that were synthesized during interactions of Fe metal with a natural claystone during experiments over a temperature range of 90-60°C. Conical and pyramidal crystals with a maximum size of a few micrometers were studied using electron diffraction tomography (EDT). The following polytypes were identified: 1M, 2M1, an apparently ninetuple polytype that was interpreted as triclinic 3A (group A), 1T (group C), and disordered crystals of group D. The 1M polytype was the most abundant. Some 1M crystals were twinned by reticular merohedry with a 120° rotation along the chex axis as the twin operation. The 2M1 occurred as isolated crystals as well as in mixed crystals. Intergrown 1T and 1M polytypes of the C and A group, respectively, were identified in one mixed crystal. The possible stacking sequence and the 3A polytype identification diagram was presented and discussed. Example RS sections of all polytypes were identified and demonstrated.

Development of a high resolution x-ray spectrometer for the National Ignition Facility (NIF).

A high resolution (E/ΔE = 1200-1800) Bragg crystal x-ray spectrometer is being developed to measure plasma parameters in National Ignition Facility experiments. The instrument will be a diagnostic instrument manipulator positioned cassette designed mainly to infer electron density in compressed capsules from Stark broadening of the helium-β (1s2-1s3p) lines of krypton and electron temperature from the relative intensities of dielectronic satellites. Two conically shaped crystals will diffract and focus (1) the Kr Heβ complex and (2) the Heα (1s2-1s2p) and Lyα (1s-2p) complexes onto a streak camera photocathode for time resolved measurement, and a third cylindrical or conical crystal will focus the full Heα to Heβ spectral range onto an image plate to provide a time integrated calibration spectrum. Calculations of source x-ray intensity, spectrometer throughput, and spectral resolution are presented. Details of the conical-crystal focusing properties as well as the status of the instrumental design are also presented.

A streaked X-ray spectroscopy platform for rapidly heated, near-solid density plasmas.

A picosecond, time-resolved, x-ray spectroscopy platform was developed to study the thermal line emission from rapidly heated solid targets containing buried aluminum or iron layers. The targets were driven by high-contrast 1ω or 2ω laser pulses at focused intensities up to 1 × 1019 W/cm2. The experimental platform combines time-integrating and time-resolved x-ray spectrometers. Picosecond time resolution was achieved with a pair of ultrafast x-ray streak cameras coupled to high-throughput Hall spectrometers. Time-integrated spectra were collected on each shot to correct the streaked data for variations in x-ray photocathode spectral sensitivity. The time-integrated spectrometer uses three elliptical crystals to disperse x rays with energies between 800 and 2100 eV with moderate (E/ΔE ∼ 450) resolving power. The streaked spectrometers accept four interchangeable conical crystals with higher resolving power (E/ΔE ∼ 650) to measure the brightest thermal lines in the 1300 to 1700 eV spectral range.

Conical photonic crystals for enhancing light extraction efficiency from high refractive index materials.

We propose, analyze and optimize a two-dimensional conical photonic crystal geometry to enhance light extraction from a high refractive index material, such as an inorganic scintillator. The conical geometry suppresses Fresnel reflections at an optical interface due to adiabatic impedance matching from a gradient index effect. The periodic array of cone structures with a pitch larger than the wavelength of light diffracts light into higher-order modes with different propagating angles, enabling certain photons to overcome total internal reflection (TIR). The numerical simulation shows simultaneous light yield gains relative to a flat surface both below and above the critical angle and how key parameters affect the light extraction efficiency. Our optimized design provides a 46% gain in light yield when the conical photonic crystals are coated on an LSO (cerium-doped lutetium oxyorthosilicate) scintillator.

Possible improvements of alumina–magnesia castable by lightweight microporous aggregates

This paper focuses on the properties of lightweight alumina–magnesia castable prepared with homemade microporous corundum aggregate, as well as on the investigation of the effects due to the introduction of microporous corundum aggregate on alumina–magnesia castable. The results showed that, in comparison to common alumina–magnesia castable, due to the small pore size and low apparent porosity of microporous corundum aggregate, its introduction leads to an improvement in volume stability, strength, heat insulation and thermal shock resistance of alumina–magnesia castable. The slag resistance of lightweight alumina–magnesia castable is significantly better than that of common alumina–magnesia castable. Microstructure and energy dispersive analyses show that the formation of conically crystallizing CA2 and CA6 is the main reason for the difference in slag resistance. The conical crystals, interlaced and distributed around the aggregate, prevent the sample from further corrosion and penetration of the slag. In addition, since the CaO content of the slag is largely absorbed by the refractory, the viscosity of slag increases, and a solidified layer is formed and adhered on the hot face of sample, thus further deterring the penetration of the slag.

Hydrophobic Particle Effects on Hydrate Crystal Growth at the Water–Oil Interface

This study introduced hydrophobic silica nanoparticles (SiNPs) into an interface of aqueous and hydrate-forming oil phases and analyzed the inhibition of hydrate crystal growth after seeding the hydrate slurry. The hydrate inhibition performance was quantitatively identified by micro-differential scanning calorimetry (micro-DSC) experiments. Through the addition of 1.0 wt% of SiNPs into the water-oil interface, the hydrate crystal growth only occurred around the seeding position of cyclopentane (CP) hydrate slurry, and the growth of hydrate crystals was retarded. Upon a further increase in the SiNP concentration up to 2.0 wt%, the SiNP-laden interface completely prevented hydrate growth. We observed a hollow conical shape of hydrate crystals with 0.0 and 1.0 wt% of SiNPs, respectively, but the size and shape of the conical crystals was shrunken at 1.0 wt% of silica nanoparticles. However, the conical shape did not appear with an increased nanoparticle concentration of 2 wt%. These findings can provide insight into hydrate inhibition in oil and gas delivery lines, possibly with nanoparticles.

Surfactant Effects on Hydrate Crystallization at the Water–Oil Interface: Hollow-Conical Crystals

Clathrate hydrates are icelike crystalline compounds with small guest molecules trapped inside the cages of hydrogen bonded water molecules. Clathrate hydrate crystal growth is studied for the specific case of the guest molecule cyclopentane. Cyclopentane hydrate formation is visualized at a millimeter-scale water drop. Crystal formation takes place at the water–organic interface and has been shown in prior work to occur in a three-step sequence of nucleation, lateral surface growth, and radial growth. This study describes cyclopentane hydrate crystal characteristics during the lateral surface growth and demonstrates the effect of the oil-soluble surfactant sorbitan monooleate (Span 80) on the hydrate crystal growth. A faceted polycrystalline hydrate shell is formed around the water drop in the absence of surfactant. A unique hollow-conical crystal is observed at Span 80 concentrations greater than 0.01% by volume in cyclopentane; the critical micelle concentration is 0.03% Span 80. The conical crystals h...

Double conical crystal x-ray spectrometer for high resolution ultrafast x-ray absorption near-edge spectroscopy of Al K edge

An x-ray spectrometer devoted to dynamical studies of transient systems using the x-ray absorption fine spectroscopy technique is presented in this article. Using an ultrafast laser-induced x-ray source, this optical device based on a set of two potassium acid phthalate conical crystals allows the extraction of x-ray absorption near-edge spectroscopy structures following the Al absorption K edge. The proposed experimental protocol leads to a measurement of the absorption spectra free from any crystal reflectivity defaults and shot-to-shot x-ray spectral fluctuation. According to the detailed analysis of the experimental results, a spectral resolution of 0.7 eV rms and relative fluctuation lower than 1% rms are achieved, demonstrated to be limited by the statistics of photon counting on the x-ray detector.