Second-order Reflections Research Articles

Coupling between Charge Density Wave Ordering and Magnetism in Ho2Ir3Si5

Ho2Ir3Si5 belongs to the family of three-dimensional (3D) R2Ir3Si5 (R = Lu, Er, Ho) compounds that exhibit a first-order, charge-density-wave (CDW) phase transition, where there is a strong orthorhombic-to-triclinic distortion of the lattice accompanied by superlattice reflections. The analysis by single-crystal X-ray diffraction (SXRD) has revealed that the Ir–Ir zigzag chains along c are responsible for the CDW in all three compounds. The replacement of the rare earth element from nonmagnetic Lu to magnetic Er or Ho lowers TCDW, where TCDWLu = 200 K, TCDWEr = 150 K, and TCDWHo = 90 K. Out of the three compounds, Ho2Ir3Si5 is the only system where second-order superlattice reflections could be observed, indicative of an anharmonic shape of the modulation wave. The CDW transition is observed as anomalies in the temperature dependencies of the specific heat, electrical conductivity, and magnetic susceptibility, which includes a large hysteresis of 90 to 130 K for all measured properties, thus corroborating the SXRD measurements. Similar to previously reported Er2Ir3Si5, there appears to be a coupling between CDW and magnetism such that the Ho3+ magnetic moments are influenced by the CDW transition, even in the paramagnetic state. Moreover, earlier investigations on polycrystalline material revealed antiferromagnetic (AFM) ordering at TN = 5.1 K, whereas AFM order is suppressed and only the CDW is present down to at least 0.1 K in our highly ordered single crystal. First-principles calculations predict Ho2Ir3Si5 to be a metal with coexisting electron and hole pockets at the Fermi level. The Ho and Ir atoms have spherically symmetric metallic-type charge density distributions that are prone to CDW distortion. Phonon calculations affirm that the Ir atoms are primarily responsible for the CDW distortion, which is in agreement with the experiment.

Behaving Badly in Early and Medieval China

Behaving Badly in Early and Medieval China

БЕЗЭХОВАЯ КАМЕРА, ВСТРАИВАЕМАЯ В ПОМЕЩЕНИЯ ПРОМЫШЛЕННОГО НАЗНАЧЕНИЯ

The issues of designing anechoic chambers for antenna measurements of a certain type – em-bedded in the premises of industrial purpose are considered. The advantage of such chambers is a positive economic effect associated with both the reduction of costs for construction work, and with the possibility of joint use of auxiliary room systems in the process of operation. Known ap-proaches to the design of chambers for antenna measurements are based either on ensuring a min-imum level of aesthetics, or minimum overall dimensions. In this case, it is necessary to provide a compromise between the parameters of anechoic stability and overall dimensions while ensuring the technological accessibility of the entire usable area of the room. Aim. The aim of the work is to justify the form and geometric dimensions of the chamber. Research Methods. In the process of re-search used the methods of geometric optics. When justifying the form of the chamber, practical aspects were taken into account, namely, the common form of industrial premises and workshops, as well as the possibility of effective use of common radio-absorbing materials to cover the cham-ber from the inside. In the process of finding the optimal effective geometric dimensions, the quality functionals were assumed to be aechoic and dimensional parameters. Results. A chamber in the form of a rectangular trapezoid is optimal for embedding in industrial premises. The expressions for the geometric dimensions of the chamber, ensuring the absence of first- and second-order reflections in the working area, have been found. The optimum value of the deflection angle of the back wall of the anechoic chamber was found. Conclusion. Based on the above technique, an anechoic chamber of a compact range for antenna measurements has been realized.

Spontaneous chiral self-assembly of CdSe@CdS nanorods

Spontaneous chiral self-assembly of CdSe@CdS nanorods

Recent enhancements in the performance of the Orion high-resolution x-ray spectrometers.

During the past few years, the Orion high-resolution x-ray spectrometers have been successful tools for measuring x-ray spectra from plasmas generated in the Orion laser facility. Duplicate spectrometers also operate successfully at the Livermore EBIT-I and SuperEBIT electron beam ion traps for measuring x-ray polarization. We have recently implemented very high-quality, optically bonded, spherically bent quartz crystals to remove the structure in the x-ray image that had been observed in earlier measurements. The structure had been caused by focusing defects and limited the accuracy of our measurements. We present before and after images that show a drastic improvement. We, furthermore, have implemented a spherically bent potassium acid phthalate (KAP) crystal on one of our spectrometers. The KAP crystal was prepared in a similar fashion, and we present measurements of the N Ly-β and Ne Lyβ lines taken in first- and second-order reflections at 600 and 1200eV, respectively. These measurements confirm that KAP crystals can be produced at a quality suitable for extending the spectral coverage to wavelengths longer than those accessible by different quartz crystals, especially those that cover the astrophysically important lines of iron.

Nonlinear signal errors in homodyne laser Doppler vibrometry induced by strong second-order ghost reflections and their mitigation.

A variety of mechanisms can induce distortions in the output signals of a homodyne laser Doppler vibrometer (LDV). In this paper, the nonlinear LDV distortions caused by a strong second-order ghost reflection originating from lens flares are theoretically explained and analyzed. We propose a simple compensation method to mitigate this distortion. The performance and limitations of this method are also explained both in simulation and in experiment.

Moral Theorizing and the Source of Normativity in Classical Chinese Philosophy: An Outline

When engaging with classical Chinese ethics, we might end up wondering what kind of moral theorizing we ultimately are confronted with. The accounts and answers to specific practical problems are dispersed throughout the texts and expressed via various codes of composition, ranging from sayings to theoretical reflections to poems. However, what exactly the aim of these theories consists in is not explicitly addressed by systematic second-order reflections. In this article I try to shed some light on the understanding of moral theorizing that underlies the transmitted texts. For this, I adopt an issue-centered approach that attempts to indirectly address the understanding of moral theorizing via the question about the source of normativity. That approach will be tested by drawing on the accounts of Mengzi 孟子 and Cheng Yi 程頤. First conclusions will show that moral theorizing can be understood as the simultaneous business of a moral anatomist and a moral painter.

Influence of second-order reflections during polarimetric calibration with two wire-grid polarizers

In polarimetric instruments, it is often necessary to characterize the polarimetric dependence at various polarization states. Frequently, this is done by placing a polarizer between the instrument and light source. Certain polarizer materials (e.g., wire grids as opposed to polymer-based materials) tend to reflect a significant amount of light, which can cause second-order reflections in the region between the two polarizers. We characterize the reflections using Jones calculus and discuss their significance for polarization instruments.

Design and performance of the variable-wavelength Bonse–Hart ultra-small-angle neutron scattering diffractometer KOOKABURRA at ANSTO

The double-crystal ultra-small-angle neutron scattering (USANS) diffractometer KOOKABURRA at ANSTO was made available for user experiments in 2014. KOOKABURRA allows the characterization of microstructures covering length scales in the range of 0.1–10 µm. Use of the first- and second-order reflections coming off a doubly curved highly oriented mosaic pyrolytic graphite premonochromator at a fixed Bragg angle, in conjunction with two interchangeable pairs of Si(111) and Si(311) quintuple-reflection channel-cut crystals, permits operation of the instrument at two individual wavelengths, 4.74 and 2.37 Å. This unique feature among reactor-based USANS instruments allows optimal accommodation of a broad range of samples, both weakly and strongly scattering, in one sample setup. The versatility and capabilities of KOOKABURRA have already resulted in a number of research papers, clearly demonstrating that this instrument has a major impact in the field of large-scale structure determination.

Study on structure variations of incommensurately modulated labradorite feldspars with different cooling histories

The incommensurately modulated structures of three intermediate plagioclase feldspars with compositions of ~An 51 are determined by single-crystal X-ray diffraction analyses. The samples selected cover a range of different cooling rate, from relatively fast to extremely slow. The structures show various ordering states that are directly correlated with the cooling histories of the samples. The slowest cooled sample shows an e 1 structure with strong density modulation, along with nanoscale exsolution lamellae. The fastest cooled sample displays an e 2 structure, without second-order satellite reflections ( f -reflections) and density modulation. The sample with intermediate cooling rate shows a less ordered e 1 structure with weak density modulation, but the modulation period and orientation are the same as in e 2 structure. The comparison of the structures with the same composition reveals the ordering process and phase transitions during the cooling of plagioclase within the compositional range of Boggild intergrowth. New parameters from modulation waves can be used for quantifying the ordering state of plagioclase feldspars. Proposed phase relationship and T-T-T diagram for ~An 51 plagioclase feldspars are illustrated for explaining the relationship among C 1, e 1 and e 2 structures, and relative cooling rates of their host rocks.

Incommensurate density modulation in a Na-rich plagioclase feldspar: Z-contrast imaging and single-crystal X-ray diffraction study.

Plagioclase feldspars are the most abundant mineral in the Earth's crust. Intermediate plagioclase feldspars commonly display incommensurately modulated or aperiodic structures. Z-contrast images show both Ca-Na ordering and density modulation. The local structure of lamellae domains has I1-like symmetry. The neighboring lamellae domains are in an inversion twinning relationship. With a state-of-the-art X-ray diffraction unit, second-order satellite reflections (f-reflections) are observed for the first time in andesine (An45), a Na-rich e-plagioclase. The f-reflections indicate a structure with a density modulation which is close to a Ca-rich e-plagioclase. The similarity between this e-andesine structure and previously solved e-labradorite structure is confirmed. Refinement of the structure shows density modulation of ∼ 7 mol % in compositional variation of the anorthite (An) component. The results from Z-contrast imaging and low-temperature single X-ray diffraction (XRD) provide a structure consistent with density modulation. The discovery of f-reflections in Na-rich e-plagioclase extends the composition range of e1 structure with density modulation to as low as at least An45, which is the lower end of the composition range of Bøggild intergrowth. The new result supports the loop-shaped solvus for Bøggild intergrowth, below which is a homogenous stable area for e1 structure in the phase diagram. The phase transition between e2 structure without density modulation and e1 structure with density modulation should happen at low temperature. There is a change in modulation period accompanying the phase transition, as well as higher occupancy of Al in the T1o site. The andesine with density modulation also indicates extremely slow cooling of its host rock.

On the robustness of upper limits for circular auditory motion perception

We developed a new rendering system that consists of a image-source reflection model followed by a two-step strategy for modeling moving phantom sound sources (combining a physical propagation model and a spatialization method). This rendering system was used for simulations of pressure signals and acoustical localization cues, and for stimulus generation. We report three perceptual experiments measuring the upper limits, defined as auditory velocity thresholds beyond which listeners are no longer able to perceptually, resolve a smooth circular trajectory. These thresholds were measured in different reverberation conditions using white noise, band-limited white noise and band-limited white noise with a pure tone. Experiment 1 took place in a hemi-anechoic room and compared two conditions: dry and with simulated first-order reflections. In Experiment 2 we compare the thresholds measured in the hemi-anechoic room (dry and simulated second-order reflections) and in a reverberant room using two different loudspeaker configurations. Experiment 3 investigated the effect of audio source type in the dry condition at high velocities. No significant effects were observed among reverberation conditions, loudspeaker configurations and audio source type, suggesting that the upper limit is robust against reverberation.

Enhanced Bragg Reflection in Non-Bravais Superlattice Formed by a Dislocated Teeth-Shaped Plasmonic Waveguide

A periodically teeth-shaped surface plasmonic waveguide with dislocation is theoretically investigated. We propose an equivalent solvable model, a four-section non-Bravais lattice with infinite length to describe the dislocated structure. Based on the Bloch theorem, the band gap and Bragg condition of non-Bravais lattice are both derived, and the effect of dislocation on Bragg reflection is discussed. The analysis indicates that the dislocation has a remarkable effect on the distribution of band gaps and gap width and can cause an enhancement of odd-order or even-order Bragg reflection at different filling factors. In addition, the Bragg wavelength can be linearly adjusted by the dislocation. The transmission spectra of a finite-period plasmonic waveguide are studied via both transfer matrix method and numerical simulation at last. The calculation results present two cases for the enhancement of the first-order and the second-order Bragg reflections with different geometric parameters, respectively. The simulation results verify the enhanced Bragg reflection with the increase of dislocation and are well matched with our theoretical analysis.

Reflection spectra of distorted cholesteric liquid crystal structures in cells with interdigitated electrodes

We studied the appearance of second- and third-order Bragg reflections in cholesteric liquid crystals (CLCs) in cells where the electric field was perpendicular to the helical axis. Second-order reflections with reflectance values as large as 80% of the first-order one were observed in the gap regions of alignment cells with interdigitated electrodes for CLC mixtures with pitches in the range 0.5-1.0 μm upon application of a field. The characterization was enabled by local probing of the CLC using a microspectrophotometer. LC cells that are transparent in the visible spectrum in the off-state and become colored upon application of a field due the second- or third-order reflection band appearance were demonstrated. The spectral position of the higher-order Bragg reflections can also be tuned by adjusting the magnitude of the electric field.

Analysis of Dispersion Characteristics of Planar Waveguides via Multi-Order Interrogation of Integrated Bragg Gratings

We demonstrate experimentally a simple technique to measure the wavelength-dependent effective refractive index of a waveguide utilizing integrated Bragg grating structures. A broadband measurement of the Bragg wavelengths yields the effective index of the waveguide and, thus, an accurate total dispersion relationship. An empirical calculation of the waveguide component of the dispersion yields both the waveguide and material dispersion components of the measured total dispersion. The technique allows direct measurement of the effective index of the waveguide and yields a zero dispersion wavelength at 1220.5 nm in our silica-on-silicon platform. Importantly, inclusion of second-order Bragg reflections improves the accuracy of modal refractive index for near-visible wavelengths.

Monochromatization of femtosecond X-ray free-electron laser pulses by means of quasi-forbidden bragg reflections from periodic multilayer structures

It is shown that the use of quasi-forbidden second-order reflections from a periodic multilayer structure Al2O3/B4C allows monochromatization of femtosecond X-ray free-electron laser pulses at level ΔE/E ≈ 0.04% with an efficiency of ∼60%. The intensity, shape, duration, and statistical characteristics of the reflected pulses are studied.

A Fast Hybrid Method for Scattering From a Large Object With Dihedral Effects Above a Large Rough Surface

A new hybrid numerical method is described for scattering from an electrically large perfectly-conducting object with dihedral effects above a very long one-dimensional rough surface (two-dimensional problem). Such a problem involves a large number of unknowns and cannot be solved easily with a conventional method of moments by using a direct LU inversion. Thus, to solve this issue, the Extended-PILE method is combined with the forward-backward spectral acceleration (FBSA) for the local interactions on the rough surface and with the second-order physical optics (PO2) approximation for the local interactions on the object. Classically objects under test do not present dihedral effects and in the high frequency domain the first-order PO (PO1) provides the main contribution. In opposite, in this paper since a cross is considered, the second-order inner reflections contribute significantly and the PO2 must be included. By assuming a Gaussian process with a Gaussian height spectrum, this new hybrid method, E-PILE+FBSA+PO2, is tested against the rigorous E-PILE+ FBSA method (direct LU inversion on the object) as functions of the object inclination, the polarization and the incidence angle.

Impact of Polarization Characteristics on 60-GHz Indoor Radio Communication Systems

This letter presents experimental results for the polarization impact on 60-GHz indoor radio communication systems. Different transmit and receive antenna polarizations (linear polarizations with different electrical field vector orientations and circular polarizations with different handedness) were used to analyze the degradation in the received signal power due to nonoptimally matched polarization characteristics between the transmit antenna, the propagation channel, and the received antenna. Different signal propagation paths including line-of-sight (LOS) propagation and first- and second-order reflections were investigated. It was found that the degradation due to polarization characteristics mismatch could be as large as 10-20 dB. Polarization characteristics of the transmit and receive antennas providing maximum received signal power were found to be dependent on the type of the signal propagation path. The most robust antenna polarization configuration was identified as a configuration using linear polarization at one end of the communication link and circular polarization at the other end, giving the degradation relative to the optimally matched case of 2-3 dB only.

Accurate Sound Localization in Reverberant Environments Is Mediated by Robust Encoding of Spatial Cues in the Auditory Midbrain

In reverberant environments, acoustic reflections interfere with the direct sound arriving at a listener's ears, distorting the spatial cues for sound localization. Yet, human listeners have little difficulty localizing sounds in most settings. Because reverberant energy builds up over time, the source location is represented relatively faithfully during the early portion of a sound, but this representation becomes increasingly degraded later in the stimulus. We show that the directional sensitivity of single neurons in the auditory midbrain of anesthetized cats follows a similar time course, although onset dominance in temporal response patterns results in more robust directional sensitivity than expected, suggesting a simple mechanism for improving directional sensitivity in reverberation. In parallel behavioral experiments, we demonstrate that human lateralization judgments are consistent with predictions from a population rate model decoding the observed midbrain responses, suggesting a subcortical origin for robust sound localization in reverberant environments.

Millimeter-Wave Propagation in Indoor Corridors

In this letter, ray-based models for both line-of-sight (LOS) and non-line-of-sight (NLOS) indoor corridors are presented and compared with results of propagation measurements performed at millimeter (mm)-waves in the 60-GHz frequency band. It is shown that in the LOS corridor case, signals propagate like in free space and in a guided fashion in near and far zones, respectively. A nine-ray model (LOS path plus four first-order and four second-order reflections from the corridor walls, ground, and ceiling) is found to be suitable in characterizing propagation signals in far zone in this environment. In the NLOS corridor case, multiray models with a heuristic diffraction coefficient are used in the field simulation. Results show that a model with eight rays is adequate in characterizing both average power and variation of signals. However, in characterizing the average decay of signal power, only two diffracted rays are needed. Also, empirical models given for NLOS corridors cases indicate that diffraction is the dominant propagation phenomenon in the environment. Furthermore, multipath power distributions are investigated. It is found that Rayleigh and double-Rayleigh distributions are the bounds for both the LOS and NLOS data. Multiple-Rayleigh distribution can fit the measurement data very accurately.