Single-layer intrinsic 2H-phase LuX_(2)(X=Cl,Br,I)with large valley polarization and anomalous valley Hall effect

Manipulation of the valley degree of freedom provides a new path for quantum information technology,but the real intrinsic large valley-polarization materials are rarely reported up to date.Here,we perform first-principles calculations to predict a class of 2H-phase single layer(SL)materials LuX_(2)(X=Cl,Br,I)to be ideal candidates.SL-Lu X_(2)are ferrovalley materials with a giant valley-polarization of 55 meV–148 meV as a result of its large spin–orbital coupling(SOC)and intrinsic ferromagnetism(FM).The magnetic transition temperatures of SL-LuI_(2)and SL-LuCl2are estimated to be 89 K–124 K,with a sizable magnetic anisotropy at out-of-plane direction.Remarkably,the anomalous valley Hall effect(AVHE)can be controlled in SL-LuX_(2)when an external electric field is applied.Moreover,the intrinsic valleypolarization of SL-LuI_(2)is highly robust for biaxial strain.These findings provide a promising ferrovalley material system for the experimentation of valleytronics and subsequent applications.

First-principles prediction of quantum anomalous Hall effect in two-dimensional Co2Te lattice

The quantum anomalous Hall effect(QAHE) has special quantum properties that are ideal for possible future spintronic devices. However, the experimental realization is rather challenging due to its low Curie temperature and small non-trivial bandgap in two-dimensional(2D) materials. In this paper, we demonstrate through first-principles calculations that monolayer Co2Te material is a promising 2D candidate to realize QAHE in practice. Excitingly, through Monte Carlo simulations, it is found that the Curie temperature of single-layer Co2Te can reach 573 K. The band crossing at the Fermi level in monolayer Co2Te is opened when spin–orbit coupling is considered, which leads to QAHE with a sizable bandgap of Eg= 96 me V, characterized by the non-zero Chern number(C = 1) and a chiral edge state. Therefore, our findings not only enrich the study of quantum anomalous Hall effect, but also broaden the horizons of the spintronics and topological nanoelectronics applications.

More than softer-when-brighter: The X-ray powerlaw spectral variability in NGC 4051

The powerlaw X-ray spectra of active galactic nuclei at moderate to high accretion rates normally appear softer when they brighten,for which the underlying mechanisms are yet unclear.Utilizing XMM-Newton observations and excluding photons<2 keV to avoid contamination from the soft excess,in this work we scrutinize the powerlaw spectral variability of NCG 4051 from two new aspects.We first find that a best-fit"softer-when-brighter"relation is statistically insufficient to explain the observed spectral variabilities,and intervals deviated from the empirical relation are clearly visible in the light curve of 2-4 ke V/4-10 keV count rate ratio.The deviations are seen not only between but also within individual XMM-Newton exposures,consistent with random variations of the corona geometry or inner structure(with timescales as short as^1 ks),in addition to those behind the smooth"softer-when-brighter"trend.We further find the"softer-when-brighter"trend gradually weakens with the decreasing timescale(from^100 ks down to 0.5 ks).These findings indicate that the powerlaw spectral slope is not solely determined by its brightness.We propose a two-tier geometry,including flares/nano-flares on top of the inner disc and an embedding extended corona(heated by the flares,in analogy to solar corona)to explain the observations together with other observational clues in literature.Rapid spectral variabilities could be due to individual flares/nano-flares,while slow ones are driven by the variations in the global activity of inner disc region(akin to the variation of solar activity,but not the accretion rate)accompanied with heating/cooling and inflation/contraction of the extended corona.