Experimental Realization of an Intrinsic Magnetic Topological Insulator

An intrinsic magnetic topological insulator(TI) is a stoichiometric magnetic compound possessing both inherent magnetic order and topological electronic states. Such a material can provide a shortcut to various novel topological quantum effects but remained elusive experimentally for a long time. Here we report the experimental realization of thin films of an intrinsic magnetic TI, MnBi2Te4, by alternate growth of a Bi2Te3 quintuple layer and a MnTe bilayer with molecular beam epitaxy. The material shows the archetypical Dirac surface states in angle-resolved photoemission spectroscopy and is demonstrated to be an antiferromagnetic topological insulator with ferromagnetic surfaces by magnetic and transport measurements as well as first-principles calculations. The unique magnetic and topological electronic structures and their interplays enable the material to embody rich quantum phases such as quantum anomalous Hall insulators and axion insulators at higher temperature and in a well-controlled way.

Machine learning identification of impurities in the STM images

We train a neural network to identify impurities in the experimental images obtained by the scanning tunneling microscope(STM)measurements.The neural network is first trained with a large number of simulated data and then the trained neural network is applied to identify a set of experimental images taken at different voltages.We use the convolutional neural network to extract features from the images and also implement the attention mechanism to capture the correlations between images taken at different voltages.We note that the simulated data can capture the universal Friedel oscillation but cannot properly describe the non-universal physics short-range physics nearby an impurity,as well as noises in the experimental data.And we emphasize that the key of this approach is to properly deal with these differences between simulated data and experimental data.Here we show that even by including uncorrelated white noises in the simulated data,the performance of the neural network on experimental data can be significantly improved.To prevent the neural network from learning unphysical short-range physics,we also develop another method to evaluate the confidence of the neural network prediction on experimental data and to add this confidence measure into the loss function.We show that adding such an extra loss function can also improve the performance on experimental data.Our research can inspire future similar applications of machine learning on experimental data analysis.

Low-damage photolithography for magnetically doped(Bi,Sb)_(2)Te_(3) quantum anomalous Hall thin films

We have developed a low-damage photolithography method for magnetically doped(Bi,Sb)_(2)Te_(3)quantum anomalous Hall(QAH) thin films incorporating an additional resist layer of poly(methyl methacrylate)(PMMA). By performing control experiments on the transport properties of five devices at varied gate voltages(V_(g)s), we revealed that the modified photolithography method enables fabricating QAH devices with the transport and magnetic properties unaffected by fabrication process. Our experiment represents a step towards the production of novel micro-structured electronic devices based on the dissipationless QAH chiral edge states.

Temperature Dependence of the Electronic Structure of Ca_(3)Cu_(2)O_(4)Cl_(2) Mott Insulator

We use scanning tunneling microscopy to study the temperature evolution of electronic structure in Ca_(3)Cu_(2)O_(4)Cl_(2) parent Mott insulator of cuprates. It is found that the upper Hubbard band moves towards the Fermi energy with increasing temperature, while the charge transfer band remains basically unchanged. This leads to a reduction of the charge transfer gap size at high temperatures, and the rate of reduction is much faster than that of conventional semiconductors. Across the Neel temperature for antiferromagnetic order, there is no sudden change in the electronic structure. These results shed new light on the theoretical models about the parent Mott insulator of cuprates.

量子反常霍尔效应研究进展

量子反常霍尔效应是一种不需要外加磁场的量子霍尔效应.它不但为手征量子霍尔态的理解和应用提供了一个理想的研究平台,还可以用于构建多种新奇拓扑量子物态.本文回顾了近年来量子反常霍尔效应实验方面的研究进展,包括对磁性掺杂拓扑绝缘体中量子反常霍尔效应相关机理的深入研究、基于量子反常霍尔绝缘体的各种异质结构的制备及性质研究,以及可能具有更高工作温度的量子反常霍尔效应材料体系的探索等.这些研究进展对量子反常霍尔效应的未来发展方向和凝聚态物理的发展具有重要的启示作用.

Mycorrhizal symbiosis modulates the rhizosphere microbiota to promote rhizobia–legume symbiosis

Plants establish symbioses with mutualistic fungi,such as arbuscular mycorrhizal(AM)fungi,and bacteria,such as rhizobia,to exchange key nutrients and thrive.Plants and symbionts have coevolved and represent vital components of terrestrial ecosystems.Plants employ an ancestral AM signaling pathway to establish intracellular symbioses,including the legume–rhizobia symbiosis,in their roots.Nevertheless,the relationship between the AM and rhizobial symbioses in native soil is poorly understood.Here,we examined how these distinct symbioses affect root-associated bacterial communities in Medicago truncatula by performing quantitative microbiota profiling(QMP)of 16S rRNA genes.We found that M.truncatula mutants that cannot establish AM or rhizobia symbiosis have an altered microbial load(quantitative abundance)in the rhizosphere and roots,and in particular that AM symbiosis is required to assemble a normal quantitative root-associated microbiota in native soil.Moreover,quantitative microbial co-abundance network analyses revealed that AM symbiosis affects Rhizobiales hubs among plant microbiota and benefits the plant holobiont.Through QMP of rhizobial rpoB and AM fungal SSU rRNA genes,we revealed a new layer of interaction whereby AM symbiosis promotes rhizobia accumulation in the rhizosphere of M.truncatula.We further showed that AM symbiosis-conditioned microbial communities within the M.truncatula rhizosphere could promote nodulation in different legume plants in native soil.Given that the AM and rhizobial symbioses are critical for crop growth,our findings might inform strategies to improve agricultural management.Moreover,our work sheds light on the co-evolution of these intracellular symbioses during plant adaptation to native soil conditions.

Onset of the Meissner effect at 65 K in Fe Se thin film grown on Nb-doped Sr TiO_3 substrate

We report the Meissner effect studies on an Fe Se thin film grown on Nb-doped Sr Ti O3 substrate by molecular beam epitaxy. Two-coil mutual inductance measurement clearly demonstrates the onset of diamagnetic screening at 65 K, which is consistent with the gap opening temperature determined by previous angle-resolved photoemission spectroscopy results. The applied magnetic field causes a broadening of the superconducting transition near the onset temperature, which is the typical behavior for quasi-two-dimensional superconductors. Our results provide direct evidence that Fe Se thin film grown on Nb-doped Sr Ti O3 substrate has an onset TC* 65 K,which is the highest among all iron-based superconductors discovered so far.

Relationship between the parent charge transfer gap and maximum transition temperature in cuprates

One of the biggest puzzles concerning the cup- rate high temperature superconductors is what determines the maximum transition temperature （Tc,max）, which varies from less than 30 to above 130 K in different compounds. Despite this dramatic variation, a robust trend is that within each family, the double-layer compound always has higher Tc,max than the single-layer counterpart. Here we use scanning tunneling microscopy to investigate the electronic structure of four cuprate parent compounds belonging to two different families. We find that within each family, the double layer compound has a much smaller charge transfer gap size （ACT）, indicating a clear anticorrelation between AcT and Tc,max. These results suggest that the charge transfer gap plays a key role in the superconducting physics of cuprates, which shed important new light on the high To mechanism from doped Mott insulator perspective.

Atomic scale electronic structure of the ferromagnetic semiconductor Cr2Ge2Te6

Cr_2Ge_2Te_6is an intrinsic ferromagnetic semiconductor with van der Waals type layered structure,thus represents a promising material for novel electronic and spintronic devices.Here we combine scanning tunneling microscopy and first-principles calculations to investigate the electronic structure of Cr_2Ge_2Te_6.Tunneling spectroscopy reveals a surprising large energy level shift and change of energy gap size across the ferromagnetic to paramagnetic phase transition,as well as a peculiar double-peak electronic state on the Cr-site defect.These features can be quantitatively explained by density functional theory calculations,which uncover a close relationship between the electronic structure and magnetic order.These findings shed important new lights on the microscopic electronic structure and origin of magnetic order in Cr_2Ge_2Te_6.

Imaging the atomic-scale electronic states induced by a pair of hole dopants in Ca_(2)CuO_(2)Cl_(2) Mott insulator

We use scanning tunneling microscopy to visualize the atomic-scale electronic states induced by a pair of hole dopants in Ca_(2)CuO_(2)Cl_(2)parent Mott insulator of cuprates.We find that when the two dopants approach each other,the transfer of spectral weight from high energy Hubbard band to low energy ingap state creates a broad peak and nearly V-shaped gap around the Fermi level.The peak position shows a sudden drop at distance around 4 a_(0)and then remains almost constant.The in-gap states exhibit peculiar spatial distributions depending on the configuration of the two dopants relative to the underlying Cu lattice.These results shed important new lights on the evolution of low energy electronic states when a few holes are doped into parent cuprates.

Ambi-chiral anomalous Hall effect in magnetically doped topological insulators

The chirality associated with broken time-reversal symmetry in magnetically doped topological insulators has important implications for the quantum transport phenomena.Here we report anomalous Hall effect studies in Mn-and Cr-doped Bi_(2)Te_(3) topological insulators with varied thicknesses and doping contents.By tracing the magnitude of the anomalous Hall resistivity,we find that the Mn-type anomalous Hall effect characterized with clockwise chirality is strengthened by the reduction of film thickness,which is opposite to that of the Cr-type anomalous Hall effect with counterclockwise chirality.We provide a phenomenological physical picture to explain the evolution of the magnetic order and the anomalous Hall chirality in magnetically doped topological insulators.

Molecular characterization and expression of the feminization-1c (fem1c) in the freshwater mussel (Hyriopsis cumingii)

The feminization-1c(fem-1c)gene has been shown to be associated with sex differentiation and determination in many metazoan species.It belongs to the fem-1 family which is a member of the ANK superfamily.In this study,the full-length cDNA of the fem-1c(Hcfem-1c)gene was isolated from the freshwater mussel(Hypriopsis cumingii).The isolated Hcfem-1c cDNA was 2196 bp in length and encoded a putative protein of 621 amino acids that contains seven ANK domains.Phylogenetic analysis of the deduced HcFEM-1C protein showed that it clustered with the other invertebrates homologues,indicating that the sequence of HcFEM-1C was conserved during evolution.Quantitative real-time PCR(qPCR)expression revealed that the Hcfem-1c gene was expressed in the adductor muscle,foot,liver,gill,kidney,mantle,and gonads of male and female adult mussels(two years old).In the gonads Hcfem-1c was much less abundant in males than that in females.During early development of the gonads,Hcfem-1c transcripts were significantly increased in the primordial germ cell differentiation stage(5 months old).We hypothesized that Hcfem-1c probably regulates female gonad differentiation.In situ hybridization showed that a strong and specific signal concentrated in the female oocyte cell membrane and male follicular wall,indicating that Hcfem-1c gene may not only be involved in female gonad differentiation,but also participates in egg development.This study laid the foundations for a better understanding of gender differentiation mechanism in H.cumingii.