Magnetic wallpaper Dirac fermions and topological magnetic Dirac insulators

Topological crystalline insulators(TCIs)can host anomalous surface states which inherits the characteristics of crystalline symmetry that protects the bulk topology.Especially,the diversity of magnetic crystalline symmetries indicates the potential for novel magnetic TCIs with distinct surface characteristics.Here,we propose a topological magnetic Dirac insulator(TMDI),whose two-dimensional surface hosts fourfold-degenerate Dirac fermions protected by either the p'_(c)4mm or p40 g0 m magnetic wallpaper group.The bulk topology of TMDIs is protected by diagonal mirror symmetries,which give chiral dispersion of surface Dirac fermions and mirror-protected hinge modes.We propose candidate materials for TMDIs including Nd_(4)Te_(8)Cl_(4)O_(2)0 and DyB_(4) based on first-principles calculations,and construct a general scheme for searching TMDIs using the space group of paramagnetic parent states.Our theoretical discovery of TMDIs will facilitate future research on magnetic TCIs and illustrate a distinct way to achieve anomalous surface states in magnetic crystals.

Competition Growth of α and β Phases in Ti-50 at.%Al Peritectic Alloy during the Rapid Solidification by Laser Melting Technique

Rapid solidification of Ti-50 at.%Al peritectic alloy is realized by laser melting technique at diferent conditions of laser power and scanning speed. The temperature field and the cooling rate under the corresponding conditions are derived from the finite element simulation.℃omparing the measured pool size with the simulated result, the laser absorptivity of Ti-50 at.%Al peritectic alloy at diferent conditions can be deduced to establish the relationships between the laser absorptivity, the laser power and the scanning speed. The morphology evolution and the phase selection of Ti-50 at.%Al peritectic alloy are described by the temperature gradient and the cooling rate. With the increase of temperature gradient and cooling rate, β phase replaces α phase to become the leading growth phase. And the growth of α phase experiences the transition from facet to non-facet manner, while β phase is refined. To understand the underlying mechanism of the competition growth can bring benefit to the industrial application of Ti-Al alloy.

Surface coating copper powder with carbon nanotubes using traditional and stirred ball mills under various experimental conditions

We investigated the effect of the raw powder properties of carbon nanotube surface coatings on metal powder,with the goal of obtaining high-quality nanocomposites.The mechanical dry coating technique was used to fabricate CNT coatings (guest particles)on the surface of copper (Cu,host)particles using a traditional ball mill and a stirred ball mill.The coatings were produced under various experimental conditions (varying rotation speed and grinding duration,with a ball diameter of 5mm),and the effect of these conditions on the surface of the powder was determined.The coated surfaces were characterized using scanning electron microscopy and field emission scanning electron microscopy.We compared prolonged milling (48h)at a low rotation speed (50rpm),with a short milling period (12h)at high rotation speed.We found that for the TBM,successful CNT coating was achieved at 50rpm over 48h. In contrast,for the SBM,CNT coatings were obtained after a short milling period (12h)at low rotation speed (50rpm).

Coherent interface driven super-plastic elongation of brittle intermetallic nano-fibers at room temperature

The intermetallic compound such as Ni_(2)Si has a brittle nature.Therefore,monolithic intermetallic compounds have not yet been prepared by mechanical downsizing.During mechanical drawing of bulk CuNi_(2)Si alloy at room temperature,we observed more than 400%plastic elongation of hard and brittle Ni_(2)Si intermetallic nano-fibers.The calculation based on the density functional theory reveals that the fully coherent interface induces strain on the intermetallic compound surrounded by the matrix,and lowers the intrinsic stacking fault energy below the level required to break an interatomic bond.The new interface between the Ni_(2)Si intermetallic and Cu matrix formed by the plastic deformation is as stable as the original coherent interface formed by precipitation,and the activation energy of the newly formed interface to slip is similar to that of the Cu matrix.All of these make plastic deformation of brittle Ni_(2)Si intermetallic possible by slip without failure.