Incommensurate antiferromagnetic order in weakly frustrated two-dimensional van der Waals insulator CrPSe3_3

Although the magnetic order is suppressed by a strong magnetic frustration, it is maintained but appears in complex order forms such as a cycloid or spin density wave in weakly frustrated systems. Herein, we report a weakly magnetic-frustrated two-dimensional van der Waals material CrPSe$_3$. Polycrystalline CrPSe$_3$ was synthesized at an optimized temperature of 700$^\circ$C to avoid the formation of any secondary phases (e.g., Cr$_2$Se$_3$). The antiferromagnetic transition appeared at $T_N\sim 126$ K with a large Curie-Weiss temperature $T_{\rm CW} \sim -371$ via magnetic susceptibility measurements, indicating weak frustration in CrPSe$_3$ with a frustration factor $f (|T_{\rm CW}|/T_N) \sim 3$. Evidently, the formation of long-range incommensurate spin-density wave antiferromagnetic order with the propagation vector $k = (0, 0.04, 0)$ was revealed by neutron diffraction measurements at low temperatures (below 120K). The monoclinic crystal structure of C2/m symmetry is preserved over the studied temperature range down to 20K, as confirmed by Raman spectroscopy measurements. Our findings on the spin density wave antiferromagnetic order in two-dimensional (2D) magnetic materials, not previously observed in the MPX$_3$ family, are expected to enrich the physics of magnetism at the 2D limit, thereby opening opportunities for their practical applications in spintronics and quantum devices.Comment: 23 pages, 4 figures, 2 table

Selective Dynamical Imaging of Interferometric Data

Recent developments in very long baseline interferometry (VLBI) have made it possible for the Event Horizon Telescope (EHT) to resolve the innermost accretion flows of the largest supermassive black holes on the sky. The sparse nature of the EHT's (u, v)-coverage presents a challenge when attempting to resolve highly time-variable sources. We demonstrate that the changing (u, v)-coverage of the EHT can contain regions of time over the course of a single observation that facilitate dynamical imaging. These optimal time regions typically have projected baseline distributions that are approximately angularly isotropic and radially homogeneous. We derive a metric of coverage quality based on baseline isotropy and density that is capable of ranking array configurations by their ability to produce accurate dynamical reconstructions. We compare this metric to existing metrics in the literature and investigate their utility by performing dynamical reconstructions on synthetic data from simulated EHT observations of sources with simple orbital variability. We then use these results to make recommendations for imaging the 2017 EHT Sgr A* data set

Structural, Magnetic and Vibrational Properties of Van Der Waals Ferromagnet CrBr3 at High Pressure

The crystal and magnetic structures of van der Waals layered ferromagnet CrBr3 were studied using X-ray powder diffraction and neutron powder diffraction at pressures up to 23 GPa at ambient temperature and up to 2.8 GPa in the temperature range 6–300 K, respectively. The vibration spectra of CrBr3 were studied using Raman spectroscopy at pressures up to 23 GPa at ambient temperature. The anomalous pressure behavior of structural parameters and vibrational modes was observed, associated with a gradual isostructural phase transition in the pressure range 2.5–7 GPa. The Curie temperature TC reduced rapidly with a pressure coefficient dTC/dP=−4.1(4) K/GPa. A full suppression of the ferromagnetic state was expected at PC~8.4 GPa, where onset of the antiferromagnetic spin arrangement or magnetically disordered state may take place. Anomalies in Raman spectra at P~15 GPa point to another possible phase transformation in CrBr3, which may be related to the proximity of metallization of this van der Waals ferromagnet

Pore Segmentation Techniques for Low-Resolution Data: Application to the Neutron Tomography Data of Cement Materials

The development of neutron imaging facilities provides a growing range of applications in different research fields. The significance of the obtained structural information, among others, depends on the reliability of phase segmentation. We focused on the problem of pore segmentation in low-resolution images and tomography data, taking into consideration possible image corruption in the neutron tomography experiment. Two pore segmentation techniques are proposed. They are the binarization of the enhanced contrast data using the global threshold, and the segmentation using the modified watershed technique—local threshold by watershed. The proposed techniques were compared with a conventional marker-based watershed on the test images simulating low-quality tomography data and on the neutron tomography data of the samples of magnesium potassium phosphate cement (MKP). The obtained results demonstrate the advantages of the proposed techniques over the conventional watershed-based approach

Incommensurate Antiferromagnetic Order in Weakly Frustrated Two-Dimensional van der Waals Insulator CrPSe3

Copyright © 2023 American Chemical Society. The two-dimensional (2D) van der Waals(vdW) material MPX3 with a hexagonal-like arrangement oftransition metal atomsin the vdW layers has a diversity of commensurate antiferromagneticorders. Distinctly, CrPSe3 reveals a weakly frustratedincommensurate antiferromagnetic order without structural phase transition. Although magnetic order is suppressed by a strong frustration,it appears in complex forms such as a cycloid or spin density wavein weakly frustrated systems. Herein, we report a weakly magneticallyfrustrated two-dimensional (2D) van der Waals material CrPSe3. Polycrystalline CrPSe3 was synthesized at an optimizedtemperature of 700 & DEG;C to avoid the formation of any secondaryphases (e.g., Cr2Se3). The antiferromagnetictransition appeared at T (N) & AP; 127K with a large Curie-Weiss temperature & theta;(CW) & AP; -301 K via magnetic susceptibility measurements,indicating weak frustration in CrPSe3 with a frustrationfactor of f (|& theta;(CW)|/T (N)) & AP; 2.4. Evidently, the formation of a long-rangeincommensurate antiferromagnetic order was revealed by neutron diffractionmeasurements at low temperatures (below 120 K). The monoclinic crystalstructure of the C2/m symmetry ispreserved over the studied temperature range down to 20 K, as confirmedby Raman spectroscopy measurements. Our findings on the incommensurateantiferromagnetic order in 2D magnetic materials, not previously observedin the MPX3 family, are expected to enrich the physicsof magnetism at the 2D limit, thereby opening opportunities for theirpractical applications in spintronics and quantum devices.11Nsciescopu

Verification of Radiative Transfer Schemes for the EHT

The Event Horizon Telescope (EHT) Collaboration has recently produced the first resolved images of the central supermassive black hole in the giant elliptical galaxy M87. Here we report on tests of the consistency and accuracy of the general relativistic radiative transfer codes used within the collaboration to model M87∗ and Sgr A∗. We compare and evaluate (1) deflection angles for equatorial null geodesics in a Kerr spacetime; (2) images calculated from a series of simple, parameterized matter distributions in the Kerr metric using simplified emissivities and absorptivities; (3) for a subset of codes, images calculated from general relativistic magnetohydrodynamics simulations using different realistic synchrotron emissivities and absorptivities; (4) observables for the 2017 configuration of EHT, including visibility amplitudes and closure phases. The error in total flux is of order 1% when the codes are run with production numerical parameters. The dominant source of discrepancies for small camera distances is the location and detailed setup of the software "camera"that each code uses to produce synthetic images. We find that when numerical parameters are suitably chosen and the camera is sufficiently far away the images converge and that for given transfer coefficients, numerical uncertainties are unlikely to limit parameter estimation for the current generation of EHT observations. The purpose of this paper is to describe a verification and comparison of EHT radiative transfer codes. It is not to verify EHT models more generally