arXiv subjects

Ke Yang

Publications and source records attributed to Ke Yang.

At least 163 records · Page 9Linked to original sources

arXiv2019

IF-TTN: Information Fused Temporal Transformation Network for Video Action Recognition

Effective spatiotemporal feature representation is crucial to the video-based action recognition task. Focusing on discriminate spatiotemporal feature learning, we propose Information Fused Temporal Transformation Network (IF-TTN) for action recognition on top of popular Temporal Segment Network (TSN) framework. In the network, Information Fusion Module (IFM) is designed to fuse the appearance and motion features at multiple ConvNet levels for each video snippet, forming a short-term video descriptor. With fused features as inputs, Temporal Transformation Networks (TTN) are employed to model middle-term temporal transformation between the neighboring snippets following a sequential order. As TSN itself depicts long-term temporal structure by segmental consensus, the proposed network comprehensively considers multiple granularity temporal features. Our IF-TTN achieves the state-of-the-art results on two most popular action recognition datasets: UCF101 and HMDB51. Empirical investigation reveals that our architecture is robust to the input motion map quality. Replacing optical flow with the motion vectors from compressed video stream, the performance is still comparable to the flow-based methods while the testing speed is 10x faster.

cs.CV↗

arXiv2019

Record-High Superconductivity in Niobium-Titanium Alloy

Here we report the observation of extraordinary superconductivity in a pressurized commercial niobium-titanium alloy. We find that its zero-resistance superconductivity persists from ambient pressure to the pressure as high as 261.7 GPa, a record high pressure up to which a known superconducting state can continuously survives. Remarkably, at such an ultra-high pressure, although the ambient pressure volume is shrunk by 45% without structural phase transition, the superconducting transition temperature (TC) increases to ~19.1 K from ~9.6 K, and the critical magnetic field (HC2) at 1.8 K has been enhanced to 19 T from 15.4 T. These results set new records for both of the TC and the HC2 among all the known alloy superconductors composed of only transition metal elements. The remarkable high pressure superconducting properties observed in the NbTi alloy not only expand our knowledge on this important commercial superconductor but also are helpful for a better understanding on the superconducting mechanism.

cond-mat.supr-con↗

arXiv2019

Exploring Frame Segmentation Networks for Temporal Action Localization

Temporal action localization is an important task of computer vision. Though many methods have been proposed, it still remains an open question how to predict the temporal location of action segments precisely. Most state-of-the-art works train action classifiers on video segments pre-determined by action proposal. However, recent work found that a desirable model should move beyond segment-level and make dense predictions at a fine granularity in time to determine precise temporal boundaries. In this paper, we propose a Frame Segmentation Network (FSN) that places a temporal CNN on top of the 2D spatial CNNs. Spatial CNNs are responsible for abstracting semantics in spatial dimension while temporal CNN is responsible for introducing temporal context information and performing dense predictions. The proposed FSN can make dense predictions at frame-level for a video clip using both spatial and temporal context information. FSN is trained in an end-to-end manner, so the model can be optimized in spatial and temporal domain jointly. We also adapt FSN to use it in weakly supervised scenario (WFSN), where only video level labels are provided when training. Experiment results on public dataset show that FSN achieves superior performance in both frame-level action localization and temporal action localization.

cs.CV↗

arXiv2018

Projective Truncation Approximation for Equations of Motion of Two-Time Green's Functions

In the equation of motion approach to the two-time Green's functions, conventional Tyablikov-type truncation of the chain of equations is rather arbitrary and apt to violate the analytical structure of Green's functions. Here, we propose a practical way to truncate the equations of motion using operator projection. The partial projection approximation is introduced to evaluate the Liouville matrix. It guarantees the causality of Green's functions, fulfills the time translation invariance and the particle-hole symmetry, and is easy to implement in a computer. To benchmark this method, we study the Anderson impurity model using the operator basis at the level of Lacroix approximation. Improvement over conventional Lacroix approximation is observed. The distribution of Kondo screening in the energy space is studied using this method.

cond-mat.str-el↗

arXiv2018

Observation of superconductivity in pressurized Weyl semimetal candidate TaIrTe4

Here we report the observation of superconductivity in pressurized type-II Weyl semimetal (WSM) candidate TaIrTe4 by means of complementary high-pressure transport and synchrotron X-ray diffraction measurements. We find that TaIrTe4 shows superconductivity with transition temperature (TC) of 0.57 K at the pressure of ~23.8 GPa. Then, the TC value increases with pressure and reaches ~2.1 K at 65.7 GPa. In situ high-pressure Hall coefficient (RH) measurements at low temperatures demonstrate that the positive RH increases with pressure until the critical pressure of the superconducting transition is reached, but starts to decrease upon further increasing pressure. Above the critical pressure, the positive magnetoresistance effect disappears simultaneously. Our high pressure X-ray diffraction measurements reveal that, at around the critical pressure the lattice of the TaIrTe4 sample is distorted by the application of pressure and its volume is reduced by ~19.2%, the value of which is predicted to result in the change of the electronic structure significantly. We propose that the pressure-induced distortion in TaIrTe4 is responsible for the change of topology of Fermi surface and such a change favors the emergence of superconductivity. Our results clearly demonstrate the correlation among the lattice distortion, topological physics and superconductivity in the WSM.

cond-mat.supr-con↗

arXiv2018

Describing the ADD model in a warped geometry

We propose a new description of the (4+N)-dimensional Arkani-Hamed-Dimopoulos-Dvali (ADD) model in a (4+1)-dimensional warped geometry to solve the gauge hierarchy problem. It has the same KK spectrum as in the ADD model and recovers its phenomenons that do not involve the interaction among the graviton KK modes. There is no hierarchy between the fundamental length and the size of the extra dimension. An explicit realization is constructed in the nonlocal gravity theory to give a warped description of the six-dimensional ADD model. Remarkably, the equivalent number N of the extra dimensions in this description may be non-integral, which provides a new mechanism to escape the experimental constrains.

hep-th↗

arXiv2018

Possible High TC Layered Ferromagnetic Insulator Sr2NiRuO4: An ab initio Study

Magnetic insulators are often antiferromagnetic (AFM) and layered AFM compounds usually show low ordering temperature. On the other hand, layered ferromagnetic (FM) insulators with high-TC are very rare although they could be quite useful for spintronic applications. Here, using crystal field level analysis in combination with density functional theory calculations as well as Monte Carlo simulations, we predict that the layered insulator Sr2NiRuO4 would have a strong FM coupling with TC as high as 240 K. The tetragonal crystal field in the Ni-O-Ru square plane stabilizes the S=1/2 Ni+ and S=3/2 Ru3+ states. The unique level ordering and occupation optimize the FM Ni-Ru superexchange interactions in the checkerboard arrangement, thus suggesting Sr2NiRuO4 as an unusual high-TC layered FM insulator. This work highlights the potential of charge-spin-orbital degrees of freedom for stabilizing strong FM coupling in layered oxides.

cond-mat.mtrl-sci↗

arXiv2018

The Hierarchy Problem and New Warped Extra Dimension

In this paper, we propose a new mechanism with warped extra dimension to solve the hierarchy problem, which is parallel to the Randall-Sundrum (RS) brane scenario. Different from the RS scenario, the fundamental scale is TeV scale and the four-dimensional Planck scale is generated from the exponential warped extra dimension at size of a few TeV$^{-1}$. The experimental consequences of this scenario are very different from that of the RS scenario. In the explicit realization in the nonlocal gravity theory, there is a tower of spin-2 excitations with mass gap $10^{-4}\text{eV}$ and they are coupled with the gravitational scale to the standard model particles. We further discuss the possible generalizations in other modified gravity theories. The experimental consequences are similar to $(4+N)$-dimensional large extra dimension but $N$ can be a non-integer, which satisfies the experimental constraints more easily than the integer large extra dimension model.

hep-th↗

arXiv2018

Universal superconductivity phase diagram for pressurized tetradymite topological insulators

We show that two different superconducting phases exist at high pressures in the optimized tetradymite topological insulators Bi2Te2Se (BTS) and Bi1.1Sb0.9Te2S (BSTS). The superconducting phases emerge at structural phase transitions, the first at 8.4 GPa for BTS and 12.4 GPa for BSTS, and the second at 13.6 GPa for BTS and 20.4 GPa, for BSTS. Electronic structure calculations show that these phases do not have topological character. Comparison of our results with prior work on Bi2Se3, Bi2Te3 and (Bi,Sb)2(Se,Te)3 allows us to uncover a universal phase diagram for pressure-induced superconductivity in tetradymites, providing a basis for understanding the relationships between topological behavior, crystal structure, and superconductivity for these materials.

cond-mat.supr-con↗

arXiv2018

Independence of topological surface state and bulk conductances in three-dimensional topological insulators

The archetypical 3D topological insulators Bi2Se3, Bi2Te3 and Sb2Te3 commonly exhibit high bulk conductivities, hindering the characterization of the surface state charge transport. The optimally doped topological insulators Bi2Te2Se and Bi2-xSbxTe2S, however, allow for such characterizations to be made. Here we report the first experimental comparison of the topological surface states and bulk conductances of Bi2Te2Se and Bi1.1Sb0.9Te2S, based on temperature-dependent high-pressure measurements. We find that the surface state conductance at low temperatures remains constant in the face of orders of magnitude increase in the bulk state conductance, revealing in a straightforward way that the topological surface states and bulk states are decoupled at low temperatures, consistent with theoretical models, and confirming topological insulators to be an excellent venue for studying charge transport in 2D Dirac electron systems.

cond-mat.str-el↗

arXiv2018

Substrate-induced magnetism and topological phase transition in silicene

Silicene has shown great application potential as a versatile material for nanoelectronics, particularly promising as building block for spintronic applications. Unfortunately, despite its intriguing properties, such as relatively large spin-orbit interactions, one of the biggest hurdles for silicene to be useful as a host spintronic material is the lack of magnetism or the topological phase transition owing to the silicene-substrate interactions, which influence its fundamental properties and has yet to be fully explored. Here, we show that when silicene is grown on CeO2 substrate, an appreciable robust magnetic moment appears in silicene covalently bonded to CeO2 (111), while a topological phase transition to a band insulator occurs regardless of van der Waals (vdWs) interaction or covalent bonding interaction at interface. The induced magnetism of silicene is due to the breaking of Si-Si π-bonding, also resulting in trivial topological phase. The silicene-substrate interaction, even weak vdWs force (equivalent to an electric field), can destroy quantum spin Hall effect (QSHE) of silicene. We propose a viable strategy --- constructing inverse symmetrical sandwich structure (protective layer/silicene/substrate) --- to preserve quantum spin Hall (QSH) state of silicene in weak vdWs interaction system. This work takes a critical step towards fundamental physics and realistic applications of silicene-based spintronic devices.

cond-mat.mes-hall↗

arXiv2018

Domain wall brane in a reduced Born-Infeld-$f(T)$ theory

The Born-Infeld $f(T)$ theory is reduced from the Born-Infeld determinantal gravity in Weitzenböck spacetime. We investigate a braneworld scenario in this theory and obtain an analytic domain wall solution by utilizing the first-order formalism. The model is stable against the linear tensor perturbation. It is shown that the massless graviton is localized on the brane, but the continuous massive gravitons are non-localized and will generate a tiny correction with the behavior of ${1}/{(k r)^{3}}$ to the Newtonian potential. The four-dimensional teleparallel gravity is recovered as an effective infrared theory on the brane. As a physical application, we consider the (quasi-)localization property of spin-1/2 Dirac fermion in this model.

hep-th↗

arXiv2018

Unusual layered order and charge disproportionation in double perovskite Ca2FeMnO6

While double perovskites A2BB'O6, if ordered, usually form a rock-salt-type structure with a checkerboard B/B' ordering, it is surprising that Ca2FeMnO6 has alternate FeO2 and MnO2 layers in its perovskite structure. Here we demonstrate, using density functional calculations, that this unusual layered ordering facilitates, and is largely helped by, the Fe3+-Fe5+ charge disproportionation (CD) of the formal Fe4+ ions, which would otherwise be frustrated in the common rock salt structure. To further verify the important role of the CD for stabilization of this layered ordering, we carry out a comparative study for the isostructural Ca2TiMnO6 which has a simple Ti4+-Mn4+ state free of the CD. Our calculations indicate that Ca2TiMnO6 instead prefers the standard rock salt structure to the layered one. Thus our study shows a nontrivial interplay between the CD and the type of ion ordering, and proves that the CD is strongly involved in stabilizing the unusual layered order of Ca2FeMnO6.

cond-mat.str-el↗

arXiv2018

A Nutritional Label for Rankings

Algorithmic decisions often result in scoring and ranking individuals to determine credit worthiness, qualifications for college admissions and employment, and compatibility as dating partners. While automatic and seemingly objective, ranking algorithms can discriminate against individuals and protected groups, and exhibit low diversity. Furthermore, ranked results are often unstable --- small changes in the input data or in the ranking methodology may lead to drastic changes in the output, making the result uninformative and easy to manipulate. Similar concerns apply in cases where items other than individuals are ranked, including colleges, academic departments, or products. In this demonstration we present Ranking Facts, a Web-based application that generates a "nutritional label" for rankings. Ranking Facts is made up of a collection of visual widgets that implement our latest research results on fairness, stability, and transparency for rankings, and that communicate details of the ranking methodology, or of the output, to the end user. We will showcase Ranking Facts on real datasets from different domains, including college rankings, criminal risk assessment, and financial services.

cs.CY↗

arXiv2018

Can the quantum spin Hall state of silicene be preserved on substrate

The substrate-induced topological phase transition of silience is a formidable obstacle for developing silicene-based materials and devices for compatibility with current electronics by using its topologically protected dissipationless edge states. First-principles calculations indicate that the substrate will result in a phase transition from topological nontrivial phase to trivial phase of silicene, although its Dirac cone is still obvious. The substrate effect (equivalent to an electric field) annihilates its spin-orbit coupling effect, the reason why its quantum spin Hall effect (QSHE) of silicene has not been experimentally observed. Unfortunately, external electric field seems impossible to recover the QSHE due to the screen effect of substrate. We here first propose a viable strategy (constructing inverse symmetrical sandwich structure (protective layer/silicene/substrate)) to preserve quantum spin Hall (QSH) state of silicene, which is demonstrated by using two representatives (CeO2(111)/silicene/CeO2(111) and CaF2(111)/silicene/CaF2(111)) through the calculated edge states and Z2 invariant. This work takes a critical step towards fundamental physics and realistic applications of silicene-based nanoelectronic devices.

physics.comp-ph↗

arXiv2018

Scalar particle production in a simple Horndeski theory

The scalar particle production through a scalar field non-minimally coupled with geometry is investigated in the context of a spatially homogeneous and isotropic universe. In this paper, in order to study the evolution of particle production over time in the case of analytical solutions, we focus on a simple Horndeski theory. We first suppose that the universe is dominated by a scalar field and derive the energy conservation condition. Then from the thermodynamic point of view, the macroscopic non-conservation of the scalar field energy-momentum tensor can be explained as an irreversible production of the scalar particles. Based on the explanation, we obtain a scalar particle production rate and the corresponding entropy. Finally, since the universe, in general, could be regarded as a closed system satisfying the laws of thermodynamics, we naturally impose some thermodynamic constraints on it. The thermodynamic properties of the universe can provide additional constraints on the simple Horndeski theory.

gr-qc↗

arXiv2017

Anomalous connection between antiferromagnetic and superconducting phases in pressurized non-centrosymmetric heavy fermion compound CeRhGe3

Unconventional superconductivity frequently emerges as the transition temperature of a magnetic phase, typically antiferromagnetic, is suppressed continuously toward zero temperature. Here, we report contrary behavior in pressurized CeRhGe3, a non-centrosymmetric heavy fermion compound. We find that its pressure-tuned antiferromagnetic transition temperature (TN) appears to avoid a continuous decrease to zero temperature by terminating abruptly above a dome of pressure-induced superconductivity. Near 21.5 GPa, evidence for TN suddenly vanishes, the electrical resistance becomes linear in temperature and the superconducting transition reaches a maximum. In light of X-ray absorption spectroscopy measurements, these characteristics appear to be related to a pressured-induced Ce valence instability, which reveals as a sharp increase in the rate of change of Ce valence with applied pressure.

cond-mat.supr-con↗

arXiv2017

Pressure-induced melting of magnetic order and emergence of new quantum state in alpha-RuCl3

Here we report the observation of pressure-induced melting of antiferromagnetic (AFM) order and emergence of a new quantum state in the honeycomb-lattice halide alpha-RuCl3, a candidate compound in the proximity of quantum spin liquid state. Our high-pressure heat capacity measurements demonstrate that the AFM order smoothly melts away at a critical pressure (Pc) of 0.7 GPa. Intriguingly, the AFM transition temperature displays an increase upon applying pressure below the Pc, in stark contrast to usual phase diagrams, for example in pressurized parent compounds of unconventional superconductors. Furthermore, in the high-pressure phase an unusual steady of magnetoresistance is observed. These observations suggest that the high-pressure phase is in an exotic gapped quantum state which is robust against pressure up to ~140 GPa.

cond-mat.str-el↗