arXiv subjects

Junjie Zhang

Publications and source records attributed to Junjie Zhang.

114 records · Page 7Linked to original sources

arXiv2017

High pressure floating-zone growth of perovskite nickelate LaNiO3 single crystals

We report the first single crystal growth of the correlated metal LaNiO3 using a high-pressure optical-image floating zone furnace. The crystals were studied using single crystal/powder x-ray diffraction, resistivity, specific heat, and magnetic susceptibility. The availability of bulk LaNiO3 crystals will (i) promote deep understanding in this correlated material, including the mechanism of enhanced paramagnetic susceptibility, and (ii) provide rich opportunities as a substrate for thin film growth such as important ferroelectric and/or multiferroic materials. This study demonstrates the power of high pO2 single crystal growth of nickelate perovskites and correlated electron oxides more generally.

cond-mat.mtrl-sci↗

arXiv2017

Multi-Label Image Classification with Regional Latent Semantic Dependencies

Deep convolution neural networks (CNN) have demonstrated advanced performance on single-label image classification, and various progress also have been made to apply CNN methods on multi-label image classification, which requires to annotate objects, attributes, scene categories etc. in a single shot. Recent state-of-the-art approaches to multi-label image classification exploit the label dependencies in an image, at global level, largely improving the labeling capacity. However, predicting small objects and visual concepts is still challenging due to the limited discrimination of the global visual features. In this paper, we propose a Regional Latent Semantic Dependencies model (RLSD) to address this problem. The utilized model includes a fully convolutional localization architecture to localize the regions that may contain multiple highly-dependent labels. The localized regions are further sent to the recurrent neural networks (RNN) to characterize the latent semantic dependencies at the regional level. Experimental results on several benchmark datasets show that our proposed model achieves the best performance compared to the state-of-the-art models, especially for predicting small objects occurred in the images. In addition, we set up an upper bound model (RLSD+ft-RPN) using bounding box coordinates during training, the experimental results also show that our RLSD can approach the upper bound without using the bounding-box annotations, which is more realistic in the real world.

cs.CV↗

arXiv2016

Interference Evidence for Rashba-Type Spin-Split on Semimetallic WTe2 Surface

Semimetallic tungsten ditelluride (WTe2) displays an extremely large non-saturating magnetoresistance (XMR), which is the subject of intense interest. This phenomenon is thought to arise from the combination of perfect n-p charge compensation with low carrier densities in WTe2 and presumably details of its band structure. Recently, "spin texture" induced by strong spin-orbital coupling (SOC) has been observed in WTe2 by angle-resolved photoemission spectroscopy (ARPES). This provides a mechanism for protecting backscattering for the states involved and thus was proposed to play an important role in the XMR of WTe2. Here, based on our density functional calculations for bulk WTe2, we found a strong Rashba spin-orbit effect in the calculated band structure due to its non-centrosymmetric structure. This splits bands and two-fold spin degeneracy of bands is lifted. A prominent Umklapp interference pattern (a spectroscopic feature with involving reciprocal lattice vectors) can be observed by scanning tunneling microscopic (STM) measurements on WTe2 surface at 4.2 K. This differs distinctly from the surface atomic structure demonstrated at 77 K. The energy dependence of Umklapp interference shows a strong correspondence with densities of states integrated from ARPES measurement, manifesting a fact that the bands are spin-split on the opposites side of Gamma point. Spectroscopic survey reveals the ratio of electron/hole asymmetry changes alternately with lateral locations along b axis, providing a microscopic picture for double-carrier transport of semimetallic WTe2. The calculated band structure and Fermi surface is further supported by our ARPES results and Shubnikov-de Haas (SdH) oscillations measurements.

cond-mat.mtrl-sci↗

arXiv2016

Stacked Charge Stripes in Quasi-Two_Dimensional Trilayer Nickelate La4Ni3O8

The quasi-two-dimensional nickelate La4Ni3O8 (La-438) is an anion deficient n=3 Ruddlesden-Popper (R-P) phase that consists of trilayer networks of square planar Ni ions, formally assigned as Ni1+ and Ni2+ in a 2:1 ratio. While previous studies on polycrystalline samples have identified a 105 K phase transition with a pronounced electronic and magnetic response but weak lattice character, no consensus on the origin of this transition has been reached. Here we show using synchrotron x-ray diffraction on high-pO2 floating-zone grown single crystals that this transition is driven by a real space ordering of charge into a quasi-2D charge stripe ground state. The charge stripe superlattice propagation vector, q=(2/3, 0, 1), corresponds with that found in the related 1/3-hole doped single layer Ruddlesden-Popper nickelate, La5/3Sr1/3NiO4 (LSNO-1/3, Ni2.33+) with orientation at 45-degrees to the Ni-O bonds. Like LSNO-1/3, the charge stripes in La-438 are weakly correlated along the c axis to form a staggered ABAB stacking that minimizes the Coulomb repulsion among the stripes. Surprisingly, however, we find that the charge stripes within each trilayer of La-438 are stacked in phase from one layer to the next, at odds with any simple Coulomb repulsion argument.

cond-mat.str-el↗

arXiv2015

An Inference Attack Model for Flow Table Capacity and Usage: Exploiting the Vulnerability of Flow Table Overflow in Software-Defined Network

As the most competitive solution for next-generation network, software-defined network (SDN) and its dominant implementation OpenFlow, are attracting more and more interests. But besides convenience and flexibility, SDN/OpenFlow also introduces new kinds of limitations and security issues. Of these limitations, the most obvious and maybe the most neglected one, is the flow table capacity of SDN/OpenFlow switches. In this paper, we proposed a novel inference attack targeting at SDN/OpenFlow network, which is motivated by the limited flow table capacities of SDN/OpenFlow switches and the following measurable network performance decrease resulting from frequent interactions between data plane and control plane when the flow table is full. To our best knowledge, this is the first proposed inference attack model of this kind for SDN/OpenFlow. We also implemented an inference attack framework according to our model and examined its efficiency and accuracy. The simulation results demonstrate that our framework can infer the network parameters(flow table capacity and flow table usage) with an accuracy of 80% or higher. These findings give us a deeper understanding of SDN/OpenFlow limitations and serve as guidelines to future improvements of SDN/OpenFlow.

cs.NI↗

arXiv2014

Brownmillerite Ca2Co2O5: Synthesis, Stability, and Re-entrant Single-Crystal-to-Single-Crystal Structural Transitions

Ca2Co2O5 in the brownmillerite form was synthesized using a high-pressure optical-image floating zone furnace, and single crystals with dimensions up to 1.4x0.8x0.5 mm3 were obtained. At room temperature, Ca2Co2O5 crystallizes as a fully ordered brownmillerite variant in the orthorhombic space group Pcmb (No. 57) with unit cell parameters a=5.28960(10) Å, b=14.9240(2) Å, and c=10.9547(2) Å. With decreasing temperature, it undergoes a re-entrant sequence of first-order structural phase transitions (Pcmb to P2/c11 to P121/m1 to Pcmb) that is unprecedented among brownmillerites, broadening the family of space groups available to these materials and challenging current approaches for sorting the myriad variants of brownmillerite structures. Magnetic susceptibility data indicate antiferromagnetic ordering in Ca2Co2O5 occurs near 240 K, corroborated by neutron powder diffraction. Below 140 K, the specimen exhibits a weak ferromagnetic component directed primarily along the b axis that shows a pronounced thermal and magnetic history dependence.

cond-mat.mtrl-sci↗