arXiv subjects

X. Hu

Publications and source records attributed to X. Hu.

At least 19 recordsLinked to original sources

arXiv2026

Exotic Pressure-Driven Band Gap Widening in Carbon Chain-Filled KFI Zeolite and Its Pathway to High-Pressure Semiconducting Electronics and High-Temperature Superconductivity

Semiconducting devices face persistent challenges in operating at high pressure, as the band theory predicts that materials transition to a more metallic state under compression. However, our findings with carbon chains in KFI substrates reveal a conditional deviation from this norm. We not only witness the transition from polyyne (semiconductor) to cumulene (metal) at medium pressure, but we also observe an unexpected re-entrance of the polyyne at high pressures, where the band gap in the polyyne increases with pressure. In addition, the synthesis of long cumulene chains has posed a longstanding challenge in the quest for high-temperature organic superconductivity. We have identified critical conditions for synthesizing extended cumulene chains within zeolite frameworks, highlighting the interplay between unconventional charge density waves and significant torsions. The KFI zeolite facilitates the formation of carbon chains exceeding 5,000 atoms, in stark contrast to around 100 other zeolites that are limited to ~10 atoms. The cumulene@KFI system demonstrates a superconducting transition temperature reaching ~62 K, surpassing the highest reported values for bulk iron-based superconductors. This interplay between carbon structures and superconductivity not only advances our understanding of charge density waves but also heralds a new era in the study of novel applications

cond-mat.supr-con↗

arXiv2025

Learning from Watching: Scalable Extraction of Manipulation Trajectories from Human Videos

Collecting high-quality data for training large-scale robotic models typically relies on real robot platforms, which is labor-intensive and costly, whether via teleoperation or scripted demonstrations. To scale data collection, many researchers have turned to leveraging human manipulation videos available online. However, current methods predominantly focus on hand detection or object pose estimation, failing to fully exploit the rich interaction cues embedded in these videos. In this work, we propose a novel approach that combines large foundation models for video understanding with point tracking techniques to extract dense trajectories of all task-relevant keypoints during manipulation. This enables more comprehensive utilization of Internet-scale human demonstration videos. Experimental results demonstrate that our method can accurately track keypoints throughout the entire manipulation process, paving the way for more scalable and data-efficient robot learning.

cs.RO↗

arXiv2024

MU-MIMO Symbol-Level Precoding for QAM Constellations with Maximum Likelihood Receivers

In this paper, we investigate symbol-level precoding (SLP) and efficient decoding techniques for downlink transmission, where we focus on scenarios where the base station (BS) transmits multiple QAM constellation streams to users equipped with multiple receive antennas. We begin by formulating a joint symbol-level transmit precoding and receive combining optimization problem. This coupled problem is addressed by employing the alternating optimization (AO) method, and closed-form solutions are derived by analyzing the obtained two subproblems. Furthermore, to address the dependence of the receive combining matrix on the transmit signals, we switch to maximum likelihood detection (MLD) method for decoding. Notably, we have demonstrated that the smallest singular value of the precoding matrix significantly impacts the performance of MLD method. Specifically, a lower value of the smallest singular value results in degraded detection performance. Additionally, we show that the traditional SLP matrix is rank-one, making it infeasible to directly apply MLD at the receiver end. To circumvent this limitation, we propose a novel symbol-level smallest singular value maximization problem, termed SSVMP, to enable SLP in systems where users employ the MLD decoding approach. Moreover, to reduce the number of variables to be optimized, we further derive a more generic semidefinite programming (SDP)-based optimization problem. Numerical results validate the effectiveness of our proposed schemes and demonstrate that they significantly outperform the traditional block diagonalization (BD)-based method.

eess.SP↗

arXiv2023

RelJoin: Relative-cost-based Selection of Distributed Join Methods for Query Plan Optimization

Selecting appropriate distributed join methods for logical join operations in a query plan is crucial for the performance of data-intensive scalable computing (DISC). Different network communication patterns in the data exchange phase generate varying network communication workloads and significantly affect the distributed join performance. However, most cost-based query optimizers focus on the local computing cost and do not precisely model the network communication cost. We propose a cost model for various distributed join methods to optimize join queries in DISC platforms. Our method precisely measures the network and local computing workloads in different execution phases, using information on the size and cardinality statistics of datasets and cluster join parallelism. Our cost model reveals the importance of the relative size of the joining datasets. We implement an efficient distributed join selection strategy, known as RelJoin in SparkSQL, which is an industry-prevalent distributed data processing framework. RelJoin uses runtime adaptive statistics for accurate cost estimation and selects optimal distributed join methods for logical joins to optimize the physical query plan. The evaluation results on the TPC-DS benchmark show that RelJoin performs best in 62 of the 97 queries and can reduce the average query time by 21% compared with other strategies.

cs.DB↗

arXiv2023

Local Contrastive Learning for Medical Image Recognition

The proliferation of Deep Learning (DL)-based methods for radiographic image analysis has created a great demand for expert-labeled radiology data. Recent self-supervised frameworks have alleviated the need for expert labeling by obtaining supervision from associated radiology reports. These frameworks, however, struggle to distinguish the subtle differences between different pathologies in medical images. Additionally, many of them do not provide interpretation between image regions and text, making it difficult for radiologists to assess model predictions. In this work, we propose Local Region Contrastive Learning (LRCLR), a flexible fine-tuning framework that adds layers for significant image region selection as well as cross-modality interaction. Our results on an external validation set of chest x-rays suggest that LRCLR identifies significant local image regions and provides meaningful interpretation against radiology text while improving zero-shot performance on several chest x-ray medical findings.

cs.CV↗

arXiv2023

The New Small Wheel electronics

The increase in luminosity, and consequent higher backgrounds, of the LHC upgrades require improved rejection of fake tracks in the forward region of the ATLAS Muon Spectrometer. The New Small Wheel upgrade of the Muon Spectrometer aims to reduce the large background of fake triggers from track segments that are not originated from the interaction point. The New Small Wheel employs two detector technologies, the resistive strip Micromegas detectors and the "small" Thin Gap Chambers, with a total of 2.45 Million electrodes to be sensed. The two technologies require the design of a complex electronics system given that it consists of two different detector technologies and is required to provide both precision readout and a fast trigger. It will operate in a high background radiation region up to about 20 kHz/cm$^{2}$ at the expected HL-LHC luminosity of $\mathcal{L}$=7.5$\times10^{34}$cm$^{-2}$s$^{-1}$. The architecture of the system is strongly defined by the GBTx data aggregation ASIC, the newly-introduced FELIX data router and the software based data handler of the ATLAS detector. The electronics complex of this new detector was designed and developed in the last ten years and consists of multiple radiation tolerant Application Specific Integrated Circuits, multiple front-end boards, dense boards with FPGA's and purpose-built Trigger Processor boards within the ATCA standard. The New Small Wheel has been installed in 2021 and is undergoing integration within ATLAS for LHC Run 3. It should operate through the end of Run 4 (December 2032). In this manuscript, the overall design of the New Small Wheel electronics is presented.

hep-ex↗

arXiv2021

Energy deposition in Saturn's equatorial upper atmosphere

We construct Saturn equatorial neutral temperature and density profiles of H, H$_2$, He, and CH$_4$, between 10$^{-12}$ and 1 bar using measurements from Cassini's Ion Neutral Mass Spectrometer (INMS) taken during the spacecraft's final plunge into Saturn's atmosphere on 15 September 2017, combined with previous deeper atmospheric measurements from the Cassini Composite InfraRed Spectrometer (CIRS) and from the UltraViolet Imaging Spectrograph (UVIS). These neutral profiles are fed into an energy deposition model employing soft X-ray and Extreme UltraViolet (EUV) solar fluxes at a range of spectral resolutions ($\Delta\lambda=4\times10^{-3}$ nm to 1 nm) assembled from TIMED/SEE, from SOHO/SUMER, and from the Whole Heliosphere Interval (WHI) quiet Sun campaign. Our energy deposition model calculates ion production rate profiles through photo-ionisation and electron-impact ionisation processes, as well as rates of photo-dissociation of CH$_4$. The ion reaction rate profiles we determine are important to obtain accurate ion density profiles, meanwhile methane photo-dissociation is key to initiate complex organic chemical processes. We assess the importance of spectral resolution in the energy deposition model by using a high-resolution H$_2$ photo-absorption cross section, which has the effect of producing additional ionisation peaks near 800 km altitude. We find that these peaks are still formed when using low-resolution ($\Delta\lambda=1$ nm) or mid-resolution ($\Delta\lambda=0.1$ nm) solar spectra, as long as high-resolution cross sections are included in the model.

astro-ph.EP↗

arXiv2021

Hourly Warning for Strong Earthquakes

A promising perspective is presented that humans can provide hourly warning for strong land earthquakes (EQs, Ms6). Two important atmospheric electrostatic signal features are described. A table that lists 9 strong land EQs with shock time, epicenter, magnitude, weather in the region near the epicenter, precursor beginning time, and precursor duration demonstrates that at approximately several hours to one day before a strong land EQ, the weather conditions are fair near the epicenter, and an abnormal negative atmospheric electrostatic signal is very obvious. Moreover, the mechanism is explained. A method by which someone could determine the epicenter and the magnitude of a forthcoming strong EQ is suggested. Finally, the possibility of realizing hourly warning for strong land EQs in the near future is pointed out.

physics.geo-ph↗

arXiv2020

Optical transceivers for event triggers in the ATLAS phase-I upgrade

The ATLAS phase-I upgrade aims to enhance event trigger performance in the Liquid Argon (LAr) calorimeter and the forward muon spectrometer. The trigger signals are transmitted by optical transceivers at 5.12 Gbps per channel in a radiation field. We report the design, quality control in production and ageing test of the transceivers fabricated with the LOCld laser driver and multi-mode 850 nm vertical-cavity surface-emitting laser (VCSEL). The modules are packaged in miniature formats of dual-channel transmitter (MTx) and transceiver (MTRx) for the LAr. The transmitters are also packaged in small form-factor pluggable (SFP) for the muon spectrometer. In production, the LOCld chips and VCSELs in TOSA package were examined before assembly. All of the modules were tested and selected during production for quality control based on the eye-diagram parameters measured at 5.12 Gbps. The yield is 98 % for both the MTx and MTRx on a total 4.7k modules. The uniformity of transmitter channels of a MTx was assured by choosing the TOSA components with approximately equal light powers. The ageing effect is monitored in burn-in of a small batch of transmitter modules with bit-error test and eye-diagrams measured periodically. The observables are stable with the light power degradation within 5 % over a period of more than 6k hours.

physics.ins-det↗

arXiv2020

The near surface vertical atmospheric electric field abnormality could be as a promising imminent precursor of major earthquakes

A promising short term precursor of major earthquakes (EQ) is very crucial in saving people and preventing huge losses. Ez, atmospheric electrostatic field vertical component, under fair air conditions, is generally oriented downwards (positive). Anomalous negative Ez signals could be used as an indicator of a great number of radioactive gases which are released from great number of rock clefts just before major earthquakes. Enhanced emission of radon radioactive decay will produce an anomalously large number of ion pairs. The positive particles will be transported downward by the fair weather electrostatic field and pile up near the surface. Finally, obviously and abnormally, an oriented upward atmospheric electric field Ez near the ground could be formed. Therefore, monitoring this Ez may be applied effectively in earthquake warning.

physics.geo-ph↗

arXiv2019

Observation of incoherently coupled dark-bright vector solitons in single-mode fibers

We report experimental observation of incoherently coupled dark-bright vector solitons in single-mode fibers. Properties of the vector solitons agree well with those predicted by the respective systems of incoherently coupled nonlinear Schroedinger equations. To the best of our knowledge, this is the first experimental observation of temporal incoherently coupled dark-bright solitons in single-mode fibers.

physics.optics↗

arXiv2019

Diurnal variation of dust and gas production in comet 67P/Churyumov-Gerasimenko at the inbound equinox as seen by OSIRIS and VIRTIS-M on board Rosetta

On 27 Apr 2015, when 67P/C-G was at 1.76 au from the Sun and moving towards perihelion, the OSIRIS and VIRTIS-M instruments on Rosetta observed the evolving dust and gas coma during a complete rotation of the comet. We aim to characterize the dust, H2O and CO2 gas spatial distribution in the inner coma. To do this we performed a quantitative analysis of the release of dust and gas and compared the observed H2O production rate with the one calculated using a thermo-physical model. For this study we selected OSIRIS WAC images at 612 nm (dust) and VIRTIS-M image cubes at 612 nm, 2700 nm (H2O) and 4200 nm (CO2). We measured the average signal in a circular annulus, to study spatial variation around the comet, and in a sector of the annulus, to study temporal variation in the sunward direction with comet rotation, both at a fixed distance of 3.1 km from the comet centre. The spatial correlation between dust and water, both coming from the sun-lit side of the comet, shows that water is the main driver of dust activity in this time period. The spatial distribution of CO2 is not correlated with water and dust. There is no strong temporal correlation between the dust brightness and water production rate as the comet rotates. The dust brightness shows a peak at 0deg sub-solar longitude, which is not pronounced in the water production. At the same epoch, there is also a maximum in CO2 production. An excess of measured water production, with respect to the value calculated using a simple thermo-physical model, is observed when the head lobe and regions of the Southern hemisphere with strong seasonal variations are illuminated. A drastic decrease in dust production, when the water production (both measured and from the model) displays a maximum, happens when typical Northern consolidated regions are illuminated and the Southern hemisphere regions with strong seasonal variations are instead in shadow.

astro-ph.EP↗

arXiv2018

Near-field magneto-caloritronic nanoscopy on ferromagnetic nanostructures

Near-field optical microscopy by means of infrared photocurrent mapping has rapidly developed in recent years. In this letter we introduce a near-field induced contrast mechanism arising when a conducting surface, exhibiting a magnetic moment, is exposed to a nanoscale heat source. The magneto-caloritronic response of the sample to near-field excitation of a localized thermal gradient leads to a contrast determined by the local state of magnetization. By comparing the measured electric response of a magnetic reference sample with numerical simulations we derive an estimate of the field enhancement and the corresponding temperature profile induced on the sample surface.

cond-mat.mes-hall↗

arXiv2017

Constraints on cometary surface evolution derived from a statistical analysis of 67P's topography

We present a statistical analysis of the distribution of large scale topographic features on comet 67P/Churyumov-Gerasimenko. We observe that the cumulative cliff height distribution across the surface follows a power law with a slope equal to -1.69 +- 0.02. When this distribution is studied independently for each region, we find a good correlation between the slope of the power law and the orbital erosion rate of the surface. For instance, the northern hemisphere topography is dominated by structures on the 100~m scale while the southern hemisphere topography, illuminated at perihelion, is dominated by 10~m scale terrain features. Our study suggest that the current size of a cliff is controlled not only by material cohesion but by the dominant erosional process in each region. This observation can be generalized to other comets, where we argue that primitive nuclei are characterized by the presence of large cliffs with a cumulative height power index equal to or above -1.5, while older, eroded cometary surfaces have a power index equal to or below -2.3. In effect, our model shows that a measure of the topography provides a quantitative assessment of a comet's erosional history, i.e. its evolutionary age.

astro-ph.EP↗

arXiv2017

Modeling of the outburst on July 29th, 2015 observed with OSIRIS cameras in the southern hemisphere of comet 67P/Churyumov-Gerasimenko

Images of the nucleus and the coma (gas and dust) of comet 67P/Churyumov- Gerasimenko have been acquired by the OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) cameras since March 2014 using both the Wide Angle Camera (WAC) and the Narrow Angle Camera (NAC). We use images from the NAC camera to study a bright outburst observed in the southern hemisphere on July 29, 2015. The high spatial resolution of the NAC is needed to localize the source point of the outburst on the surface of the nucleus. The heliocentric distance is 1.25 au and the spacecraft-comet distance is 186 km. Aiming to better understand the physics that led to the outgassing, we used the Direct Simulation Monte Carlo (DSMC) method to study the gas flow close to the nucleus and the dust trajectories. The goal is to understand the mechanisms producing the outburst. We reproduce the opening angle of the outburst in the model and constrain the outgassing ratio between the outburst source and the local region. The outburst is in fact a combination of both gas and dust, in which the active surface is approximately 10 times more active than the average rate found in the surrounding areas. We need a number of dust particles 7.83 $\times$ 10$^{11}$ - 6.90 $\times$ 10$^{15}$ (radius 1.97 - 185 {\mu}m), which corresponds to a mass of dust 220 - 21 $\times$ 10$^{3}$kg.

astro-ph.EP↗

arXiv2016

Sublimation of icy aggregates in the coma of comet 67P/Churyumov-Gerasimenko detected with the OSIRIS cameras onboard Rosetta

Beginning in March 2014, the OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) cameras began capturing images of the nucleus and coma (gas and dust) of comet 67P/Churyumov-Gerasimenko using both the wide angle camera (WAC) and the narrow angle camera (NAC). The many observations taken since July of 2014 have been used to study the morphology, location, and temporal variation of the comet's dust jets. We analyzed the dust monitoring observations shortly after the southern vernal equinox on May 30 and 31, 2015 with the WAC at the heliocentric distance Rh = 1.53 AU, where it is possible to observe that the jet rotates with the nucleus. We found that the decline of brightness as a function of the distance of the jet is much steeper than the background coma, which is a first indication of sublimation. We adapted a model of sublimation of icy aggregates and studied the effect as a function of the physical properties of the aggregates (composition and size). The major finding of this article was that through the sublimation of the aggregates of dirty grains (radius a between 5 microm and 50 microm) we were able to completely reproduce the radial brightness profile of a jet beyond 4 km from the nucleus. To reproduce the data we needed to inject a number of aggregates between 8.5 x $10^{13}$ and 8.5 x $10^{10}$ for a = 5 microm and 50 microm respectively, or an initial mass of $H_2O$ ice around 22kg.

astro-ph.EP↗

arXiv2016

Precision measurements of Linear Scattering Density using Muon Tomography

We demonstrate that muon tomography can be used to precisely measure the properties of various materials. The materials which have been considered have been extracted from an experimental blast furnace, including carbon (coke) and iron oxides, for which measurements of the linear scattering density relative to the mass density have been performed with an absolute precision of 10%. We report the procedures that are used in order to obtain such precision, and a discussion is presented to address the expected performance of the technique when applied to heavier materials. The results we obtain do not depend on the specific type of material considered and therefore they can be extended to any application.

physics.ins-det↗

arXiv2015

Are fractured cliffs the source of cometary dust jets ? Insights from OSIRIS/Rosetta at 67P

Dust jets, i.e. fuzzy collimated streams of cometary material arising from the nucleus, have been observed in-situ on all comets since the Giotto mission flew by comet 1P/Halley in 1986. Yet their formation mechanism remains unknown. Several solutions have been proposed, from localized physical mechanisms on the surface/sub-surface (see review in Belton (2010)) to purely dynamical processes involving the focusing of gas flows by the local topography (Crifo et al. 2002). While the latter seems to be responsible for the larger features, high resolution imagery has shown that broad streams are composed of many smaller features (a few meters wide) that connect directly to the nucleus surface. We monitored these jets at high resolution and over several months to understand what are the physical processes driving their formation, and how this affects the surface. Using many images of the same areas with different viewing angles, we performed a 3-dimensional reconstruction of collimated jets, and linked them precisely to their sources on the nucleus. Results.We show here observational evidence that the Northern hemisphere jets of comet 67P arise from areas with sharp topographic changes and describe the physical processes involved. We propose a model in which active cliffs are the main source of jet-like features, and therefore the regions eroding the fastest on comets. We suggest that this is a common mechanism taking place on all comets.

astro-ph.EP↗