arXiv subjects

Xiao Hu

Publications and source records attributed to Xiao Hu.

At least 199 records · Page 11Linked to original sources

arXiv2020

A Tale of Two Grains: impact of grain size on ring formation via nonideal MHD processes

Substructures in PPDs, whose ubiquity was unveiled by recent ALMA observations, are widely discussed regarding their possible origins. We carry out global full magnetohydrodynamic (MHD) simulations in axisymmetry, coupled with self-consistent ray-tracing radiative transfer, thermochemistry, and non-ideal MHD diffusivities. The abundance profiles of grains are also calculated based on the global dust evolution calculation, including sintering effects. We found that dust size plays a crucial role in the ring formation around the snow lines of protoplanetary disks (PPDs) through the accretion process. Disk ionization structures and thus tensorial conductivities depend on the size of grains.When grains are significantly larger than PAHs, the non-ideal MHD conductivities change dramatically across each snow line of major volatiles, leading to a sudden change of the accretion process across the snow lines and the subsequent formation of gaseous rings/gaps there. On the other hand,the variations of conductivities are a lot less with only PAH sized grains in disks and then these disks retain smoother radial density profiles across snow lines.

astro-ph.EP↗

arXiv2019

Non-ideal MHD simulation of HL Tau disk: formation of rings

Recent high resolution observations unveil ring structures in circumstellar disks. The origin of these rings has been widely investigated under various theoretical scenarios. In this work we perform global 3D non-ideal MHD simulations including effects from both Ohmic resistivity and ambipolar diffusion (AD) to model the HL Tau disk. The non-ideal MHD diffusion profiles are calculated based on the global dust evolution calculation including sintering effects. Disk ionization structure changes dramatically across the snow line due to the change of dust size distribution close to snow line of major volatiles. We find that accretion is mainly driven by disk wind. Gaps and rings can be quickly produced from different accretion rates across snow line. Furthermore, ambipolar diffusion (AD) leads to highly preferential accretion at midplane, followed by magnetic reconnection. This results a local zone of decretion that drains of mass in the field reconnection area, which leaves a gap and an adjacent ring just outside it. Overall, under the favorable condition, both snow lines and non-ideal MHD effects can lead to gaseous gaps and rings in protoplanetary disks.

astro-ph.EP↗

arXiv2019

Hetero-Center Loss for Cross-Modality Person Re-Identification

Cross-modality person re-identification is a challenging problem which retrieves a given pedestrian image in RGB modality among all the gallery images in infrared modality. The task can address the limitation of RGB-based person Re-ID in dark environments. Existing researches mainly focus on enlarging inter-class differences of feature to solve the problem. However, few studies investigate improving intra-class cross-modality similarity, which is important for this issue. In this paper, we propose a novel loss function, called Hetero-Center loss (HC loss) to reduce the intra-class cross-modality variations. Specifically, HC loss can supervise the network learning the cross-modality invariant information by constraining the intra-class center distance between two heterogenous modalities. With the joint supervision of Cross-Entropy (CE) loss and HC loss, the network is trained to achieve two vital objectives, inter-class discrepancy and intra-class cross-modality similarity as much as possible. Besides, we propose a simple and high-performance network architecture to learn local feature representations for cross-modality person re-identification, which can be a baseline for future research. Extensive experiments indicate the effectiveness of the proposed methods, which outperform state-of-the-art methods by a wide margin.

cs.CV↗

arXiv2019

Low-Power Computer Vision: Status, Challenges, Opportunities

Computer vision has achieved impressive progress in recent years. Meanwhile, mobile phones have become the primary computing platforms for millions of people. In addition to mobile phones, many autonomous systems rely on visual data for making decisions and some of these systems have limited energy (such as unmanned aerial vehicles also called drones and mobile robots). These systems rely on batteries and energy efficiency is critical. This article serves two main purposes: (1) Examine the state-of-the-art for low-power solutions to detect objects in images. Since 2015, the IEEE Annual International Low-Power Image Recognition Challenge (LPIRC) has been held to identify the most energy-efficient computer vision solutions. This article summarizes 2018 winners' solutions. (2) Suggest directions for research as well as opportunities for low-power computer vision.

cs.CV↗

arXiv2019

Instance and Output Optimal Parallel Algorithms for Acyclic Joins

Massively parallel join algorithms have received much attention in recent years, while most prior work has focused on worst-optimal algorithms. However, the worst-case optimality of these join algorithms relies on hard instances having very large output sizes, which rarely appear in practice. A stronger notion of optimality is {\em output-optimal}, which requires an algorithm to be optimal within the class of all instances sharing the same input and output size. An even stronger optimality is {\em instance-optimal}, i.e., the algorithm is optimal on every single instance, but this may not always be achievable. In the traditional RAM model of computation, the classical Yannakakis algorithm is instance-optimal on any acyclic join. But in the massively parallel computation (MPC) model, the situation becomes much more complicated. We first show that for the class of r-hierarchical joins, instance-optimality can still be achieved in the MPC model. Then, we give a new MPC algorithm for an arbitrary acyclic join with load $O ({\IN \over p} + {\sqrt{\IN \cdot \OUT} \over p})$, where $\IN,\OUT$ are the input and output sizes of the join, and $p$ is the number of servers in the MPC model. This improves the MPC version of the Yannakakis algorithm by an $O (\sqrt{\OUT \over \IN} )$ factor. Furthermore, we show that this is output-optimal when $\OUT = O(p \cdot \IN)$, for every acyclic but non-r-hierarchical join. Finally, we give the first output-sensitive lower bound for the triangle join in the MPC model, showing that it is inherently more difficult than acyclic joins.

cs.DB↗

arXiv2018

Data-augmented modeling of intracranial pressure

Precise management of patients with cerebral diseases often requires intracranial pressure (ICP) monitoring, which is highly invasive and requires a specialized ICU setting. The ability to noninvasively estimate ICP is highly compelling as an alternative to, or screening for, invasive ICP measurement. Most existing approaches for noninvasive ICP estimation aim to build a regression function that maps noninvasive measurements to an ICP estimate using statistical learning techniques. These data-based approaches have met limited success, likely because the amount of training data needed is onerous for this complex applications. In this work, we discuss an alternative strategy that aims to better utilize noninvasive measurement data by leveraging mechanistic understanding of physiology. Specifically, we developed a Bayesian framework that combines a multiscale model of intracranial physiology with noninvasive measurements of cerebral blood flow using transcranial Doppler. Virtual experiments with synthetic data are conducted to verify and analyze the proposed framework. A preliminary clinical application study on two patients is also performed in which we demonstrate the ability of this method to improve ICP prediction.

physics.med-ph↗

arXiv2018

Ultrahigh Magnetic Fields Produced by Shearing Carbon Nanotubes

In laboratories, ultrahigh magnetic fields are usually produced with very large currents through superconducting, resistive or hybrid magnets, which require extreme conditions, such as low temperature, huge cooling water or tens of megawatts of power. In this work we report that when single walled carbon nanotubes (SWNTs) are cut, there are magnetic moments at the shearing end of SWNTs. The average magnetic moment is found to be 41.5+-9.8uB per carbon atom in the end states with a width of 1 nm at temperature of 300.0K, suggesting ultrahigh magnetic fields can be produced. The dangling sigma and pi bonds of the carbon atoms at the shearing ends play important roles for this unexpectedly high magnetic moments because the oxidation temperature of cut SWNTs is found to be as low as 312 in dry air. Producing ultrahigh magnetic field with SWNTs has the advantage of working at higher working temperature and with low energy consumption, suggesting great potentials of applications.

cond-mat.mtrl-sci↗

arXiv2018

Discovering Student Behavior Patterns from Event Logs: Preliminary Results on A Novel Probabilistic Latent Variable Model

Digital platforms enable the observation of learning behaviors through fine-grained log traces, offering more detailed clues for analysis. In addition to previous descriptive and predictive log analysis, this study aims to simultaneously model learner activities, event time spans, and interaction levels using the proposed Hidden Behavior Traits Model (HBTM). We evaluated model performance and explored their capability of clustering learners on a public dataset, and tried to interpret the machine recognized latent behavior patterns. Quantitative and qualitative results demonstrated the promising value of HBTM. Results of this study can contribute to the literature of online learner modeling and learning service planning.

cs.CY↗

arXiv2018

Inside-Out Planet Formation. IV. Pebble Evolution and Planet Formation Timescales

Systems with tightly-packed inner planets (STIPs) are very common. Chatterjee & Tan proposed Inside-Out Planet Formation (IOPF), an in situ formation theory, to explain these planets. IOPF involves sequential planet formation from pebble-rich rings that are fed from the outer disk and trapped at the pressure maximum associated with the dead zone inner boundary (DZIB). Planet masses are set by their ability to open a gap and cause the DZIB to retreat outwards. We present models for the disk density and temperature structures that are relevant to the conditions of IOPF. For a wide range of DZIB conditions, we evaluate the gap opening masses of planets in these disks that are expected to lead to truncation of pebble accretion onto the forming planet. We then consider the evolution of dust and pebbles in the disk, estimating that pebbles typically grow to sizes of a few cm during their radial drift from several tens of AU to the inner, $\lesssim1\:$AU-scale disk. A large fraction of the accretion flux of solids is expected to be in such pebbles. This allows us to estimate the timescales for individual planet formation and entire planetary system formation in the IOPF scenario. We find that to produce realistic STIPs within reasonable timescales similar to disk lifetimes requires disk accretion rates of $\sim10^{-9}\:M_\odot\:{\rm yr}^{-1}$ and relatively low viscosity conditions in the DZIB region, i.e., Shakura-Sunyaev parameter of $α\sim10^{-4}$.

astro-ph.EP↗

arXiv2018

Generating electromagnetic modes with fine tunable orbital angular momentum by planar topological circuits

Metamaterials made of periodic arrangements of electric permittivity and magnetic permeability and arrays of resonators can provide optic properties unexperienced in conventional materials, such as negative refractive index which can be used to achieve superlensing and cloaking. Developing new simpler structures with richer electromagnetic (EM) properties and wider working frequency bands is highly demanded for fast light-based information transfer and information processing. In the present work, we propose theoretically that a honeycomb-structured LC circuit with short-range textures in inductance can generate a topological photonic state accompanied by inversion between p- and d-orbital like EM modes carrying optic orbital angular momenta (OAM). We realize experimentally the topological photonic state in planar microstrips, a sandwich structure of bottom metallic substrate, middle dielectric film and top metallic strips with hexagonal patterns in strip width, in microwave frequency. Measuring accurately the amplitude and phase of EM fields, we demonstrate explicitly that topological interfacial EM waves launched from a linearly polarized dipole source propagate in opposite directions according to the sign of OAM, with the relative weight of p- and d-orbital like EM modes tunable precisely by sweeping the frequency in the topological band gap, which can hardly be achieved in other photonic systems and condensed-matter systems. Achieving the topological photonic state in a simple and easy fabricable structure, the present work uncovers a new direction for generating and manipulating EM modes with rich internal degrees of freedom, which can be exploited for various applications.

physics.optics↗

arXiv2018

Topological Aspect of Graphene Patchwork with Regular Arrays of Nano Holes

Triangular and honeycomb lattices are dual to each other -- if we puncture holes into a featureless plane in a regular triangular alignment, the remaining body looks like a honeycomb lattice, and vice versa, if the holes are in a regular honeycomb alignment, the remaining body has a feature of triangular lattice. In this work, we reveal that the electronic states in graphene sheets with nano-sized holes in triangular and honeycomb alignments are also dual to each other in a topological sense. Namely, a regular hole array perforated in graphene can open a band gap in the energy-momentum dispersion of relativistic electrons in the pristine graphene, and the insulating states induced by triangular and honeycomb hole arrays are distinct in topology. In a graphene patchwork with regions of these two hole arrays put side by side counterpropagating topological currents emerge at the domain wall. This observation indicates that the cerebrated atomically thin sheet is where topological physics and nanotechnology meet.

cond-mat.mes-hall↗

arXiv2017

Ultra-low Power Nanoelectromechanical Memory Based on Location-controllable Nanogap System

Nanogap engineering of low-dimensional nanomaterials, has received considerable interest in a variety of fields, ranging from molecular electronics to memories. Creating nanogaps at a certain position is of vital importance for the repeatable fabrication of the devices. In this work, we report a rational design of non-volatile memories based on sub-5 nm nanogaped single-walled carbon nanotubes (SWNTs) via the electromechanical motion. The nanogaps are readily realized by electroburning in a partially suspended SWNTs device with nanoscale region. The SWNT memory devices are applicable for both metallic and semiconducting SWNTs, resolving the challenge of separation of semiconducting SWNTs from metallic ones. Meanwhile, the memory devices exhibit excellent performance: ultra-low writing energy (4.1*10-19 J per bit), ON/OFF ratio of 105, stable switching ON operations and over 30 hours retention time in ambient conditions, which are of great potential for applications.

cond-mat.mes-hall↗

arXiv2016

Topological Light Nodal Lines in FCC Lattice

Light cone with the speed of light independent of its wavelength in vacuum has been known for long time. In the present work, we unveil that in a face-centered-cubic (fcc) lattice of dielectric spheres novel light cones can be created over closed loops in momentum space, dubbed as nodal lines (NL), and that as a consequence of the nontrivial topology of NL interface states with a drumhead-shaped band structure appear where light can be slowed down significantly. We discuss that photonic pseudogaps found in previous experimental and theoretical studies for fcc photonic crystals are consistent with the present finding of NL. This offers a unique chance to confirm the existence of NL as a novel topological state.

cond-mat.mes-hall↗

arXiv2016

Visualization of unidirectional optical waveguide using topological photonic crystals made of dielectric material

The introduction of topology unravels a new chapter of physics. Topological systems provide unique edge/interfacial quantum states which are expected to contribute to the development of novel spintronics and open the door to robust quantum computation. Optical systems can also benefit from topology. Engineering locally in real space a honeycomb photonic crystal with double Dirac cone in its photonic dispersion, topology transition in photonic band structure is induced and a pseudospin unidirectional optical channel is created and demonstrated by the backscattering immune electromagnetic transportation. The topological photonic crystal made of dielectric material can pave the road towards steering light propagations and contribute to novel communication technology.

physics.optics↗

arXiv2016

Landau-Zener-Stückelberg Interferometry for Majorana Qubit

Stimulated by a very recent experiment observing successfully two superconducting states with even- and odd-number of electrons in a nanowire topological superconductor as expected from the existence of two end Majorana quasiparticles (MQs) [Albrecht \textit{et al.}, Nature \textbf{531}, 206 (2016)], we propose a way to manipulate Majorana qubit exploiting quantum tunneling effects. The prototype setup consists of two one-dimensional (1D) topological superconductors coupled by a tunneling junction which can be controlled by gate voltage. We show that, upon current injection, the time evolution of superconducting phase difference at the junction induces an oscillation in energy levels of the Majorana parity states, whereas the level-crossing is avoided by a small coupling energy of MQs in the individual 1D superconductors. This results in a Landau-Zener-Stückelberg (LZS) interference between the Majorana parity states. Adjusting the current pulse and gate voltage, one can build a LZS interferometry which provides an arbitrary manipulation of the Majorana qubit. The LZS rotation of Majorana qubit can be monitored by the microwave radiated from the junction.

cond-mat.supr-con↗

arXiv2016

Mirror winding number and helical edge modes in honeycomb lattice with hopping-energy texture

We illustrate possible topological phases in honeycomb lattice with textures in electron hopping energy between nearest-neighboring sites and show that they are characterized by the mirror winding number intimately related to the chiral (or sublattice) symmetry. Analytic wave functions of zero-energy edge modes in ribbon geometry are provided, which are classified into even and odd sectors with respect to the mirror operation with the mirror plane perpendicular to the edge, and evolve into the topological helical edge states at finite momenta. Intriguingly our results demonstrate that in order to achieve the topological phase one can decorate the edge in a way adaptive to the bulk hopping texture. This paves a new way to tailoring graphene in the topological point of view.

cond-mat.mes-hall↗

arXiv2015

Fractional Flux Plateau in Magnetization Curve of Multicomponent Superconductor Loop

Time-reversal symmetry (TRS) may be broken in superconductors with three or more condensates interacting repulsively, yielding two degenerate states specified by chirality of gap functions. We consider a loop of such superconductor with two halves occupied by the two states with opposite chiralities. Fractional flux plateaus are found in magnetization curve associated with free-energy minima, where the two domain walls between the two halves accommodate different inter-component phase kinks leading to finite winding numbers in a part of the whole condensates around the loop. Fractional flux plateaus form pairs with their heights related to the flux quantum Φ0 = hc/2e. This phenomenon is a clear evidence of time-reversal symmetry broken (TRSB) superconductivity, which in a general point of view provides a novel chance to explore relative phase difference, phase kink and soliton in ubiquitous multi-component superconductivity such as that in iron pnicitides.

cond-mat.supr-con↗

arXiv2015

Quantum Anomalous Hall Effect in a Perovskite and Inverse-Perovskite Sandwich Structure

Based on first-principles calculations, we propose a sandwich structure composed of a G-type anti-ferromagnetic (AFM) Mott insulator LaCrO$_3$ grown along the [001] direction with one atomic layer replaced by an inverse-perovskite material Sr$_3$PbO. We show that the system is in a topologically nontrivial phase characterized by simultaneous nonzero charge and spin Chern numbers, which can support a spin-polarized and dissipationless edge current in a finite system. Since these two materials are stable in bulk and match each other with only small lattice distortions, the composite material is expected easy to synthesize.

cond-mat.mes-hall↗