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Chao Yao

Chao Yao appears in the imported research catalog. Authorship, coauthor and topic links are available while profile ownership is still unclaimed.

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cond-mat.supr-conComputer VisionArtificial Intelligencecond-mat.str-el

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Published work

27 published item(s)

preprint2026arXiv

PDEAgent-Bench: A Multi-Metric, Multi-Library Benchmark for PDE Solver Generation

PDE-to-solver code generation aims to automatically synthesize executable numerical solvers from partial differential equation (PDE) specifications. This task requires not only understanding the mathematical structure of PDEs, but also selecting appropriate discretization schemes and solver configurations, and correctly implementing the resulting formulations in finite-element method (FEM) libraries. Existing code generation benchmarks mainly evaluate syntactic correctness, or success on predefined test cases. To our knowledge, there is currently no publicly available benchmark specifically for PDE-to-solver code generation, and general-purpose code benchmarks do not fully capture the unique challenges of numerical PDE solution, such as ensuring solver accuracy, efficiency, and compatibility with professional FEM libraries. We introduce PDEAgent-Bench, to the best of our knowledge, the first multi-metric, multi-library benchmark for PDE-to-solver code generation. PDEAgent-Bench contains 645 instances across 6 mathematical categories and 11 PDE families, with common FEM libraries for DOLFINx, Firedrake, and deal.II. Each instance provides an agent-facing problem specification, a reference solution on a prescribed evaluation grid, and case-specific accuracy and runtime targets. PDEAgent-Bench adopts a staged evaluation framework in which generated solvers must sequentially pass executability, numerical accuracy, and computational efficiency checks. Experiments with representative LLMs and code agents show that models can often produce runnable code, but their pass rate drops substantially once accuracy and efficiency requirements are enforced. These results indicate that current agents remain limited in producing numerically reliable and efficient PDE solvers, and that PDEAgent-Bench provides a reproducible testbed grounded in the practical requirements of numerical PDE solving.

0 citations0 reviewsNo code linkedOpen access
preprint2023arXiv

Giant Nernst effect in the crossover between Fermi liquid and strange metal

The strange-metal state is a crucial problem in condensed matter physics highlighted by its ubiquity in almost all major correlated systems[1-7]. Its understanding could provide important insight into high-Tc superconductivity[2] and quantum criticality[8]. However, with the Fermi liquid theory failing in strange metals, understanding the highly unconventional behaviors has been a long-standing challenge. Fundamental aspects of strange metals remain elusive, including the nature of their charge carriers[1]. Here, we report the observation of a giant Nernst response in the strange-metal state in a two-dimensional superconductor 2M-WS2. A giant Nernst coefficient comparable to the vortex Nernst signal in superconducting cuprates, and its high sensitivity to carrier mobility, are found when the system enters the strange-metal state from the Fermi liquid state. The temperature and magnetic field dependence of the giant Nernst peak rule out the relevance of both Landau quasiparticles and superconductivity. Instead, the giant Nernst peak at the crossover indicates a dramatic change in carrier entropy when entering the strange-metal state. The presence of such an anomalous Nernst response is further confirmed in other iconic strange metals, suggesting its universality and places stringent experimental constraints on the mechanism of strange metals.

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preprint2022arXiv

Dense residual Transformer for image denoising

Image denoising is an important low-level computer vision task, which aims to reconstruct a noise-free and high-quality image from a noisy image. With the development of deep learning, convolutional neural network (CNN) has been gradually applied and achieved great success in image denoising, image compression, image enhancement, etc. Recently, Transformer has been a hot technique, which is widely used to tackle computer vision tasks. However, few Transformer-based methods have been proposed for low-level vision tasks. In this paper, we proposed an image denoising network structure based on Transformer, which is named DenSformer. DenSformer consists of three modules, including a preprocessing module, a local-global feature extraction module, and a reconstruction module. Specifically, the local-global feature extraction module consists of several Sformer groups, each of which has several ETransformer layers and a convolution layer, together with a residual connection. These Sformer groups are densely skip-connected to fuse the feature of different layers, and they jointly capture the local and global information from the given noisy images. We conduct our model on comprehensive experiments. Experimental results prove that our DenSformer achieves improvement compared to some state-of-the-art methods, both for the synthetic noise data and real noise data, in the objective and subjective evaluations.

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preprint2019arXiv

Deep Image-to-Video Adaptation and Fusion Networks for Action Recognition

Existing deep learning methods for action recognition in videos require a large number of labeled videos for training, which is labor-intensive and time-consuming. For the same action, the knowledge learned from different media types, e.g., videos and images, may be related and complementary. However, due to the domain shifts and heterogeneous feature representations between videos and images, the performance of classifiers trained on images may be dramatically degraded when directly deployed to videos. In this paper, we propose a novel method, named Deep Image-to-Video Adaptation and Fusion Networks (DIVAFN), to enhance action recognition in videos by transferring knowledge from images using video keyframes as a bridge. The DIVAFN is a unified deep learning model, which integrates domain-invariant representations learning and cross-modal feature fusion into a unified optimization framework. Specifically, we design an efficient cross-modal similarities metric to reduce the modality shift among images, keyframes and videos. Then, we adopt an autoencoder architecture, whose hidden layer is constrained to be the semantic representations of the action class names. In this way, when the autoencoder is adopted to project the learned features from different domains to the same space, more compact, informative and discriminative representations can be obtained. Finally, the concatenation of the learned semantic feature representations from these three autoencoders are used to train the classifier for action recognition in videos. Comprehensive experiments on four real-world datasets show that our method outperforms some state-of-the-art domain adaptation and action recognition methods.

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preprint2016arXiv

Vortex pinning and dynamics in high performance Sr0.6K0.4Fe2As2 superconductor

We have studied vortex pinning and dynamics in a Sr0.6K0.4Fe2As2 superconducting tape with critical current density Jc~0.1 MA/cm2 at 4.2 K and 10 T. It is found that grain boundary pinning is dominant in the vortex pinning mechanism. Furthermore, we observe large density of dislocations which can also serve as effective pinning centers. We find that the temperature dependence of critical current density is in agreement with the model of vortices pinned via spatial fluctuation of charge carrier mean free path. Magnetic relaxation measurement indicates that the magnetization depends on time in a logarithmic way. The relaxation rate in the low and intermediate temperature region is small, and it exhibits a weak temperature and field dependence. A crossover from elastic creep to plastic creep regime is observed. Finally, we conclude a vortex phase diagram for the high performance Sr0.6K0.4Fe2As2 superconducting tape.

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preprint2015arXiv

Large transport Jc in Cu-sheathed Sr0.6K0.4Fe2As2 superconducting tape conductors

Copper sheath is the first choice for manufacturing high-Tc superconducting wires and tapes because of its high electrical and thermal conductivities, low cost and good mechanical properties. However, Cu can easily react with superconducting cores, such as BSCCO, MgB2 and pnictides, during high-temperature sintering process and therefore drastically decrease the transport Jc. Here, we report the fabrication of Cu-sheathed Sr1-xKxFe2As2 tapes with superior Jc performance using a simple hot pressing method that is capable of eliminating the lengthy high-temperature sintering required by conventional process. We obtained high-quality Sr1-xKxFe2As2 tapes with processing at 800oC for 30 minutes and measured high Tc and sharp transition. This rapid fabrication method can effectively thwart the diffusion of Cu into polycrystalline Sr-122 core. As a consequence, we achieved high transport Jc of 3.1x10^4 A/cm^2 in 10 T and 2.7x10^4 A/cm^2 in 14 T at 4.2 K. The in-field Jc performance is by far the highest reported for Cu-sheathed high-Tc conductors. Our results demonstrate the potential of Cu sheath for practical application of pnictide wires and tapes.

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preprint2014arXiv

Enhancement of transport critical current density of SmFeAsO1-xFx tapes fabricated by an ex-situ powder-in-tube method with a Sn-presintering process

SmFeAsO1-xFx (Sm1111) tapes were prepared by an ex-situ powder-in-tube method with a Sn-presintering process. Scanning electron microscopy revealed apparent difference in microstructure between Sn-presintered tapes and the previously reported polycrystalline Sm1111 bulk, since Sn has reduced FeAs wetting phase and filled the voids between Sm1111 grains. The Sn-presintered tapes showed significant enhanced field dependences of transport Jc compared with Sn-added tapes. A highest transport critical current density (Jc) of 3.45x10^4 A cm-2 at 4.2 K and self-field is achieved. Magneto-optical (MO) imaging further confirmed large and well-distributed global and intergranular Jc in Sn-presintered Sm1111 tapes.

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preprint2014arXiv

High critical current density in textured Ba-122/Ag tapes fabricated by scalable rolling process

The industrial manufacturing of the long-length iron-pnictide wires and tapes requires simple and low-cost technology. Although the transport critical current density of the FeAs-122 tapes has already achieved the practical application level, their fabrication procedures are relatively complicated or required specialized instruments. Ag-sheathed Ba0.6K0.4Fe2As2 superconducting tapes were fabricated by the scalable rolling process. At 4.2 K and 10 T, the critical current density Jc has achieved 5.4x10^4 A/cm2. We ascribe the excellent performance to the high purity and homogeneity of the superconducting phase, high density and high c-axis texture of the superconducting core. Our method provides a simple way to scale up the production of the long-length iron-based superconducting tapes with high Jc.

0 citations0 reviewsNo code linkedOpen access
preprint2014arXiv

Hot pressing to enhance the transport Jc of Sr0.6K0.4Fe2As2 superconducting tapes

High-performance Sr0.6K0.4Fe2As2 (Sr-122) tapes have been successfully fabricated using hot pressing (HP) process. The effect of HP temperatures (850-925 C) on the c-axis texture, resistivity, Vickers micro-hardness, microstructure and critical current properties has been systematically studied. Taking advantage of high degree of c-axis texture, well grain connectivity and and large concentration of strong-pinning defects, we are able to obtain an excellent Jc of 1.2x10^5 A/cm^2 at 4.2 K and 10 T for Sr-122 tapes. More importantly, the field dependence of Jc turns out to be very weak, such that in 14 T the Jc remains ~1.0x10^5 A/cm^2. These Jc values are the highest ever reported so far for iron-pnictide wires and tapes, achieving the level desired for practical applications. Our results clearly strengthen the position of iron-pnictide conductors as a competitor to the conventional and MgB2 superconductors for high field applications.

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preprint2014arXiv

Realization of practical level current densities in Sr0.6K0.4Fe2As2 tape conductors for high-field applications

122 type pnictide superconductors are of particular interest for high-field applications because of their large upper critical fields Hc2 (> 100 T), low Hc2 anisotropy r (< 2), and high thin-film critical current densities Jc (> 1 MA/cm2 at 4.2 K). Successful magnet applications require fabrication of polycrystalline superconducting wires that exhibit large in-field Jc, which is limited by poor grain coupling and weak-link behavior at grain boundaries. Here we report our recent achievement in the developing Sr0.6K0.4Fe2As2 tapes with transport Jc up to 0.1 MA/cm2 at 10 T and 4.2 K. This value is by far the highest ever recorded for iron based superconducting wires and has surpassed the threshold for practical application for the first time. The synergy effects of enhanced grain connectivity, alleviation of the weak-link behavior at grain boundaries, and the strong intrinsic pinning characteristics led to the superior Jc performance exhibited in our samples. This advanced Jc result opens up the possibility for iron-pnictide superconducting wires to win the race in high-field magnet applications.

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preprint2014arXiv

Strongly enhanced current densities in Sr0.6K0.4Fe2As2 superconducting tapes

Improving transport current has been the primary issue for practical application of superconducting wires and tapes. However, the porous nature of powder-in-tube (PIT) processed iron-based superconducting tapes is thought to be one of the important reasons for the low Jc values. In this work, the superconducting core density of ex-situ Sr0.6K0.4Fe2As2 tapes was significantly improved by employing a simple hot pressing as an alternative route for final sintering. At 4.2 K, the transport Jc values showed excellent values of 5.1x10^4 A/cm^2 at 10 T and 4.3x10^4 A/cm^2 at 14 T, respectively, which attain the Jc level desired for practical applications. Moreover, the Jc values exhibited extremely weak magnetic field dependence. These results clearly demonstrate that PIT pnictide wire conductors are very promising for high field magnet applications.

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preprint2013arXiv

High critical current density and low anisotropy in textured Sr1-xKxFe2As2 tapes for high field applications

From the application point of view, large critical current densities Jc (H) for superconducting wires are required, preferably for magnetic fields higher than 5 T. Here we show that strong c-axis textured Sr1-xKxFe2As2 tapes with nearly isotropic transport Jc were fabricated by an ex-situ powder-in-tube (PIT) process. At 4.2 K, the Jc values show extremely weak magnetic field dependence and reach high values of 1.7x10^4 A/cm^2 at 10 T and 1.4x10^4 A/cm^2 at 14 T, respectively, these values are by far the highest ever reported for iron based wires and approach the Jc level desired for practical applications. Transmission electron microscopy investigations revealed that amorphous oxide layers at grain boundaries were significantly reduced by Sn addition which resulted in greatly improved intergranular connectivity. Our results demonstrated the strong potential of using iron based superconductors for high field applications.

0 citations0 reviewsNo code linkedOpen access
preprint2013arXiv

Large transport critical currents and magneto-optical imaging of textured Sr1-xKxFe2As2 superconducting tapes

We report the temperature dependence of the transport critical current density (Jc) in textured Sr1-xKxFe2As2/Fe (Sr122) tapes fabricated by an ex situ powder-in-tube process. Critical currents were measured in magnetic fields up to 0-15 T and/or the temperature range 4.2-30 K by using a dc four-probe method. It was found that textured Sr122 tapes heat-treated at low temperatures showed higher transport Jc performance due to much improved intergrain connections. At temperatures of 20 K, easily obtained using a cryocooler, Jc reached ~ 10^4 A/cm^2 in self field, which is the highest transport value of ferropnictide wires and tapes reported so far. Magneto-optical imaging observations further revealed significant and well distributed global Jc at 20 K in our tapes. These results demonstrate that 122 type superconducting tapes are promising for high-field applications at around 20 K.

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preprint2012arXiv

Effect of starting materials on the superconducting properties of SmFeAsO1-xFx tapes

SmFeAsO1-xFx tapes were prepared using three kinds of starting materials. It shows that the starting materials have an obvious effect on the impurity phases in final superconducting tapes. Compared with the other samples, the samples fabricated by SmAs, FeO, Fe2As, and SmF3 have the smallest arsenide impurity phase and voids. As a result, these samples possess much denser structure and better grain connectivity. Moreover, among the three kinds of samples fabricated in this work, this kind of sample has the highest zero-resistivity temperature ~40 K and largest critical current density ~4600 A/cm^2 in self-field at 4.2 K. This is the highest Jc values reported so far for SmFeAsO1-xFx wires and tapes.

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preprint2012arXiv

Fabrication and transport properties of Sr0.6K0.4Fe2As2 multifilamentary superconducting wires

Seven-core Ag/Fe sheathed Sr0.6K0.4Fe2As2 (Sr-122) superconducting wires were produced by the ex situ powder-in-tube (PIT) method. The relationship between the cold-work deformation process and the superconducting properties of wires were systematically studied. It was found that flat rolling can efficiently increase the density of the superconducting core and largely improve the transport critical current density (Jc) of as-drawn wires. The Jc of the best sample achieved 21.1 kA/cm^2 at 4.2 K in self field, and showed very weak magnetic field dependence in high fields. Our result suggested a promising future of multifilamentary iron-based superconductors in practical application.

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preprint2012arXiv

Improved transport critical currents in Ag and Pb co-doped BaxK1-xFe2As2 superconducting tapes

Fe-clad BaxK1-xFe2As2 superconducting tapes were fabricated by the ex situ powder-in-tube method combined with a short high-temperature annealing technique. The effect of annealing time and different dopants on the transport properties of the BaxK1-xFe2As2 tapes were systematically studied. By co-doping with Ag and Pb, the transport critical current density Jc of BaxK1-xFe2As2 tapes was significantly improved in whole field region and the highest transport Jc was up to 1.4x10^4 A/cm^2 (Ic=100 A) at 4.2K in self field. It is proposed that the superior Jc in the co-doped samples are due to the combine effect of Pb doping at low fields and Ag doping at high fields.

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preprint2011arXiv

Direct observation of nanometer-scale amorphous layers and oxide crystallites at grain boundaries in polycrystalline Sr1-xKxFe2As2 superconductors

We report here an atomic resolution study of the structure and composition of the grain boundaries in polycrystalline Sr0.6K0.4Fe2As2 superconductor. A large fraction of grain boundaries contain amorphous layers larger than the coherence length, while some others contain nanometer-scale particles sandwiched in between amorphous layers. We also find that there is significant oxygen enrichment at the grain boundaries. Such results explain the relatively low transport critical current density (Jc) of polycrystalline samples with respect to that of bicrystal films.

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preprint2011arXiv

Enhanced critical current properties in Ba0.6K0.4+xFe2As2 superconductor by over-doping of potassium

Phase-pure polycrystalline Ba0.6K0.4+xFe2As2 with were prepared using a one-step solid-state reaction method. We found that over-doping of potassium can improve critical current density (Jc). High-field Jc for samples with x = 0.1 is three times higher than that for samples with x = 0. Over-doping of K has minimal effect on the critical transition temperature (Tc). Less than 0.5 K degradations in Tc was measured for samples with x = 0.1. TEM revealed high concentration of dislocations in samples with x = 0.1, resulting in enhanced flux pining. Further analyses on magnetization loops for powder samples confirm that K over-doping can promote intra-grain Jc. Our results indicate that slight excess of K in Ba0.6K0.4Fe2As2 samples is beneficial to high-field applications.

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preprint2011arXiv

High transport critical current densities in textured Fe-sheathed Sr1-xKxFe2As2+Sn superconducting tapes

We report the realization of grain alignment in Sn-added Sr1-xKxFe2As2 superconducting tapes prepared by ex-situ powder-in-tube method. At 4.2 K, high transport critical current densities Jc of 2.5x10^4 A/cm^2 (Ic = 180 A) in self-field and 3.5x10^3 A/cm^2 (Ic = 25.5 A) in 10 T have been measured. These values are the highest ever reported so far for Fe-based superconducting wires and tapes. We believe the superior Jc in our tape samples are due to well textured grains and strengthened intergrain coupling achieved by Sn addition. Our results demonstrated an encouraging prospect for application of iron based superconductors.

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preprint2011arXiv

Superconducting properties of FeSe wires and tapes prepared by gas diffusion technique

Superconducting FeSe in the form of wires and tapes were successfully fabricated using a novel gas diffusion procedure. Structural analysis by mean of x-ray diffraction shows that themain phase of tetragonal PbO-type FeSe was obtained by this synthesis method. The zero resistivity transition temperature of the FeSe was confirmed to be 9.3 K. The critical current density as high as 137 A/cm^2 (4 K, self field) has been observed. The results suggest that the diffusion procedure is promising in preparing high-quality FeSe wires and tapes.

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preprint2011arXiv

Superconductivity induced by doping Rh in CaFe2-xRhxAs2

In this paper we report the synthesis of iron-based superconductors CaFe2-xRhxAs2 using one-step solid state reaction method, which crystallizes in the ThCr2Si2-type structure with a space group I4/mmm. The systematic evolution of the lattice constants demonstrates that the Fe ions are successfully replaced by the Rh. By increasing the doping content of Rh, the spin-density-wave (SDW) transition in the parent compound is suppressed and superconductivity emerges. The maximum superconducting transition temperature is found at 18.5 K with the doping level of x = 0.15. The temperature dependence of DC magnetization confirms superconducting transitions at around 15 K. The general phase diagram was obtained and found to be similar to the case of Rh-doping Sr122 system. Our results explicitly demonstrate the feasibility of inducing superconductivity in Ca122 compounds by higher d-orbital electrons doping, however, different Rh-doping effect between FeAs122 compounds and FeAs1111 systems still remains an open question.

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preprint2011arXiv

Synthesis and properties of La-doped CaFe2As2 single crystals with Tc = 42.7 K

Large single crystals of La-doped CaFe2As2 were successfully synthesized by the FeAs self-flux method. The x-ray diffraction patterns suggest high crystalline quality and c-axis orientation. By substitution of trivalent La3+ ions for divalent Ca2+, the resistivity anomaly in the parent compound CaFe2As2 is completely suppressed and a superconducting transition reaches the value of 42.7 K, which is higher than that of about 30 K reported in Saha S. R. et al., arXiv:1105.4798v1. The upper critical field has been determined with the magnetic field along ab-plane and c-axis, yielding an anisotropy of about 3.3.

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preprint2011arXiv

Transport properties and anisotropy in rare earth doped CaFe2As2 single crystals with Tc above 40 K

In this paper we report the superconductivity above 40 K in the electron doping single crystal Ca1-xRexFe2As2 (Re = La, Ce, Pr). The x-ray diffraction patterns indicate high crystalline quality and c-axis orientation. the resistivity anomaly in the parent compound CaFe2As2 is completely suppressed by partial replacement of Ca by rare earth and a superconducting transition reaches as high as 43 K, which is higher than the value in electron doping FeAs-122 compounds by substituting Fe ions with transition metal, even surpasses the highest values observed in hole doping systems with a transition temperature up to 38 K. The upper critical field has been determined with the magnetic field along ab-plane and c-axis, yielding the anisotropy of 2~3. Hall-effect measurements indicate that the conduction in this material is dominated by electron like charge carriers. Our results explicitly demonstrate the feasibility of inducing superconductivity in Ca122 compounds via electron doping using aliovalent rare earth substitution into the alkaline earth site, which should add more ingredients to the underlying physics of the iron-based superconductors.

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preprint2011arXiv

Upper fields and critical current density of K0.58Fe1.56Se2 single crystals grown by one step technique

Single crystals of K0.58Fe1.56Se2 were successfully synthesized by a new single step process with the onset superconducting transition temperatures 31.9 K. The x-ray diffraction patterns suggest that they have high crystalline quality and c-axis orientation. A possible modulation structure of Fe-vacancy along c axis was observed. The upper critical field has been determined with the magnetic field along ab-plane and c-axis, yielding an anisotropy of about 3.3. It has also been shown that the critical current density of the K0.58Fe1.56Se2 is about 1.7x10^4 A/cm^2 at 5 K.

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preprint2010arXiv

Influence of Pb addition on the superconducting properties of polycrystalline Sr0.6K0.4Fe2As2

Polycrystalline Sr0.6K0.4Fe2As2 samples with various Pb additions (0-20 wt%) were prepared using a one-step solid state reaction. X-ray diffraction analysis shows no evidence for chemical reaction between the Pb and the FeAs-based superconductor. However, the presence of the Pb can affect the microstructure and superconducting properties of the final products. The critical transition temperature Tc indicates no degradation up to 20 wt% Pb addition, and dramatic improvements of magnetic Jc and irreversibility field Hirr were observed for appropriate Pb concentration. Transport critical current property of pure and Pb-added Sr0.6K0.4Fe2As2 tapes was also measured by a four-probe technique, and a remarkable enhancement of Jc at low fields was detected for the Pb added tapes.

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preprint2010arXiv

Low-temperature synthesis of SmO0.8F0.2FeAs superconductor with Tc = 56.1K

We report a systematic study on the effect of sintering temperature on the structural and superconducting properties of nominal SmO0.8F0.2FeAs fabricated by simple one-step solid state reaction method. A detailed correlation between the sintering temperature, structure, onset transition temperature (Tc) and critical current density (Jc) has been found in all samples of each batch. Most importantly, samples sintered at a low temperature clearly shows high Tc, for example the Tc of the samples sintered at 850C is even above 53 K, and the samples prepared at 1000C display the highest Tc of 56.1 K reported so far. Furthermore, the samples sintered at 1000C also show the highest RRR and the lowest resistivity(57K), indicating the low impurity scattering and enhanced carrier density. However, the maximum Jc of 10510 A/cm^2 at 5 K in self field was achieved in the samples sintered at 1100oC. The dependence of Tc on a-axis lattice constant indicates that the sintering temperature has strong influence on the effective substitution level of F for O. This result suggests that annealing at a temperature of ~1000C seems much better for obtaining high quality 1111 phase oxypnictides, compared to commonly used temperatures of around 1200C.

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preprint2010arXiv

One-step method to grow Ba0.6K0.4Fe2As2 single crystals without fluxing agent

Single crystals of Ba0.6K0.4Fe2As2 with excellent quality have been successfully grown without fluxing agent through a simple one-step method for the first time. X-ray diffraction patterns demonstrate that the samples have high crystalline quality and c-axis orientation. The onset transition temperature is up to 38 K with the zero resistivity temperature about 36.7 K. Both the R-T and M-T data show a very sharp superconducting transition with transition width 0.4 K. We also found that the samples possess very large current carrying ability and high upper critical fields, indicating potential applications requiring very high field. The above simple and safe one-step technique of single crystal growth can be effective in other systems of Fe-based superconductors.

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