arXiv subjects

Hao Song

Publications and source records attributed to Hao Song.

At least 55 records · Page 3Linked to original sources

arXiv2022

Pressure-induced high-temperature superconductivity in ternary Y-Zr-H compounds

Compressed hydrogen-rich compounds have received extensive attention as appealing contenders for superconductors, and further challenges are maintaining the stability and superconductivity of hydrides at lower pressures. In this work, we found several novel hydrides YZrH6, YZrH8 and YZrH12 with excellent superconductivity in the Y-Zr-H ternary system. Interestingly, YZrH6 with an A15-type structure can maintain dynamic stability down to 0.01 GPa and still with a critical temperature (Tc) of 16 K. YZrH8 and YZrH12 have high Tc of 70 K and 183 K at 200 GPa and 160 GPa, respectively. The phonon modes associated with H atoms contribute significantly to the electron-phonon coupling, and the H-driven electronic density of states play an important role in superconductivity. These findings highlight relationship between the H-driven electronic density of states, electron-phonon coupling and the superconductivity in a distinct class of hydrides, opening new avenues for designing and optimizing new hydrogen-rich high temperature superconductors.

cond-mat.supr-con↗

arXiv2022

Room-temperature superconductivity in heavy rare earth metal substituted sodalite-like clathrate hexahydrides under moderate pressure

Room temperature superconductivity is a dream that mankind has been chasing for a century. In recent years, the synthesis of H3S, LaH10 and C-S-H system has gradually made this dream a reality. But the extreme pressures required for the metallization of hydrogen-based superconductors limit their applications. In this work, we design a series of high temperature superconductors that can be stable at moderate pressures by incorporating heavy rare earth elements Yb/Lu into sodalite-like clathrate hydrides. In particular, the critical temperatures of Y3LuH24, YLuH12 and YLu3H24 are 283 K, 275 K and 288 K, respectively, which are close to or have reached room temperature, and the required pressure for stabilization of these hydrides is about 120 GPa which is significantly lower than that of reported room temperature superconductors. Our work provides an effective method for the rational design of low-pressure stabilized hydrogen-based superconductors with high-Tc and will stimulate further experimental exploration.

cond-mat.supr-con↗

arXiv2022

Tunable bilayer dielectric metasurface via stacking magnetic mirrors

Functional tunability, environmental adaptability, and easy fabrication are highly desired properties in metasurfaces. Here we provide a tunable bilayer metasurface composed of two stacked identical dielectric magnetic mirrors, which are excited by the dominant electric dipole and other magnetic multipoles, exhibiting nonlocal electric field enhancement near the interface and high reflection. Differ from the tunability through the direct superposition of two structures with different functionalities, we achieve the reversible conversion between high reflection and high transmission by manipulating the interlayer coupling near the interface between the two magnetic mirrors. The magnetic mirror effect boosts the interlayer coupling when the interlayer spacing is small. Decreasing the interlayer spacing of the bilayer metasurface leads to stronger interlayer coupling and scattering suppression of the meta-atom, which results in high transmission. On the contrary, increasing the spacing leads to weaker interlayer coupling and scattering enhancement, which results in high reflection. The high transmission of the bilayer metasurface has good robustness due to that the meta-atom with interlayer coupling can maintain the scattering suppression against adjacent meta-atom movement and disordered position perturbation. This work provides a straightforward method (i.e. stacking magnetic mirrors) to design tunable metasurface, shed new light on high-performance optical switches applied in communication and sensing.

physics.app-ph↗

arXiv2022

Duality of generalized twisted Reed-Solomon codes and Hermitian self-dual MDS or NMDS codes

Self-dual MDS and NMDS codes over finite fields are linear codes with significant combinatorial and cryptographic applications. In this paper, firstly, we investigate the duality properties of generalized twisted Reed-Solomon (abbreviated GTRS) codes in some special cases. In what follows, a new systematic approach is proposed to draw Hermitian self-dual (+)-GTRS codes. The necessary and sufficient conditions of a Hermitian self-dual (+)-GTRS code are presented.With this method, several classes of Hermitian self-dual MDS and NMDS codes are constructed.

cs.IT↗

arXiv2021

Efficient Data Exchange in Unmanned Aerial Vehicle Networks Utilizing Unsupervised Learning-Based Clustering

An unmanned aerial vehicle (UAV) network can serve as an aerial relay to periodically receive packets from macro base stations (BSs). Severe packet loss may happen especially when UAVs have bad wireless connections to a BS. In this paper, a data exchange scheme is proposed utilizing unsupervised learning to enable efficient lost packet retrieval through reliable wireless transmissions between UAVs instead of through retransmissions of macro BSs with a longer delay and higher overhead. With the proposed scheme, all UAVs are assigned to multiple clusters and a UAV can only request its lost packets to UAVs in the same cluster. By this way, UAVs in different clusters could carry out their lost packets retrieval processes simultaneously to expedite data exchange. The agglomerative hierarchical clustering, a type of unsupervised learning, is used to conduct clustering, guaranteeing that UAVs clustered together could supply and supplement each other's lost packets. To further enhance data exchange efficiency, a data exchange mechanism is designed, where the priority of UAVs performing data exchange is determined by the number of their lost packets or the number of requested packets that they can provide. The introduced data exchange mechanism can make each request-reply process maximally beneficial to other UAVs' lost packet retrieval in the same cluster. A new random backoff procedure based on the carrier sense multiple access with collision avoidance (CSMA/CA) is designed to support the data exchange mechanism. Finally, simulation studies are conducted to demonstrate the effectiveness and superiority of our proposed data exchange scheme.

cs.IT↗

arXiv2021

Clustering-based Multicast Scheme for UAV Networks

When an unmanned aerial vehicle (UAV) network is utilized as an aerial small base station (BS), like a relay deployed far away from macro BSs, existing multicast methods based on acknowledgement (ACK) feedback and retransmissions may encounter severe delay and signaling overhead due to hostile wireless environments caused by a long-distance propagation and numerous UAVs. In this paper, a novel multicast scheme is designed for UAV networks serving as an aerial small BS, where a UAV experiencing a packet loss will request the packet from other UAVs in the same cluster rather than relying on retransmissions of BSs. The technical details of the introduced multicast scheme are designed with the carrier sense multiple access with collision avoidance (CSMA/CA) protocol for practicability and without loss of generality. Then, the Poisson cluster process is employed to model UAV networks to capture their dynamic network topology, based on which distance distributions are derived using tools of stochastic geometry for analytical tractability. Additionally, critical performance indicators of the designed multicast scheme are analyzed. Through extensive simulation studies, the superiority of the designed multicast scheme is demonstrated and the system design insight related to the proper number of clusters is revealed.

eess.SY↗

arXiv2021

Inverse-designed multi-dimensional silicon photonic transmitters

Modern microelectronic processors have migrated towards parallel computing architectures with many-core processors. However, such expansion comes with diminishing returns exacted by the high cost of data movement between individual processors. The use of optical interconnects has burgeoned as a promising technology that can address the limits of this data transfer. While recent pushes to enhance optical communication have focused on developing wavelength-division multiplexing technology, this approach will eventually saturate the usable bandwidth, and new dimensions of data transfer will be paramount to fulfill the ever-growing need for speed. Here we demonstrate an integrated intra- and inter-chip multi-dimensional communication scheme enabled by photonic inverse design. Using inverse-designed mode-division multiplexers, we combine wavelength- and mode- multiplexing and send massively parallel data through nano-photonic waveguides and optical fibres. Crucially, as we take advantage of an orthogonal optical basis, our approach is inherently scalable to a multiplicative enhancement over the current state of the art.

physics.app-ph↗

arXiv2021

Design Principles for High Temperature Superconductors with Hydrogen-based Alloy Backbone at Moderate Pressure

Hydrogen-based superconductors provide a route to the long-sought goal of room-temperature superconductivity, but the high pressures required to metallize these materials limit their immediate application. For example, carbonaceous sulfur hydride, the first room-temperature superconductor, can reach a critical temperature (Tc) of 288 K only at the extreme pressure of 267 GPa. The next recognized challenge is the realization of room-temperature superconductivity at significantly lower pressures. Here, we propose a strategy for the rational design of high-temperature superconductors at low pressures by alloying small-radius elements and hydrogen to form ternary hydride superconductors with alloy backbones. We identify a hitherto unknown fluorite-type backbone in compositions of the form AXH8, which exhibit high temperature superconductivity at moderate pressures. The Fm-3m phase of LaBeH8, with a fluorite-type H-Be alloy backbone, is predicted to be metastable and superconducting with a Tc ~ 191 K at 50 GPa; a substantially lower pressure than that required by the geometrically similar clathrate hydride LaH10 (170 GPa). Our approach paves the way for finding high-Tc ternary hydride superconductors at conditions close to ambient pressures.

cond-mat.supr-con↗

arXiv2021

Turbulence-Resilient Coherent Free-Space Optical Communications using Automatic Power-Efficient Pilot-Assisted Optoelectronic Beam Mixing of Many Modes

Atmospheric turbulence generally limits free-space optical (FSO) communications, and this problem is severely exacerbated when implementing highly sensitive and spectrally efficient coherent detection. Specifically, turbulence induces power coupling from the transmitted Gaussian mode to higher-order Laguerre-Gaussian (LG) modes, resulting in a significant decrease of the power that mixes with a single-mode local oscillator (LO). Instead, we transmit a frequency-offset Gaussian pilot tone along with the data signal, such that both experience similar turbulence and modal power coupling. Subsequently, the photodetector (PD) optoelectronically mixes all corresponding pairs of the beams' modes. During mixing, a conjugate of the turbulence experienced by the pilot tone is automatically generated and compensates the turbulence experienced by the data, and nearly all orders of the same corresponding modes efficiently mix. We demonstrate a 12-Gbit/s 16-quadrature-amplitude-modulation (16-QAM) polarization-multiplexed (PolM) FSO link that exhibits resilience to emulated turbulence. Experimental results for turbulence D/r_0~5.5 show up to ~20 dB reduction in the mixing power loss over a conventional coherent receiver. Therefore, our approach automatically recovers nearly all the captured data power to enable high-performance coherent FSO systems.

physics.optics↗

arXiv2020

New Phase Transition of Lanthanum at High Pressure

Lanthanum (La), the first member of the rare-earth elements, recently aroused strong interest due to its unique superhydride with superconducting properties. Although there is much theoretical and experimental work about phase transitions and superconductivity in metallic La, we got a new body-centred tetragonal ($bct$) phase in metallic La with space group $I4/mmm$ at $190$ GPa exchanging from face-centered cubic ($fcc$) phase in previous work, which expanded the phase transition sequence. The $bct$ phase shows an abnormal packing way that turned to non-closed packing at high pressure. And more detailed properties of the new phase are discussed.

cond-mat.mtrl-sci↗

arXiv2020

High Tc superconductivity in heavy Rare Earth Hydrides: correlation between the presence of the f states on the Fermi surface, nesting and the value of Tc

Lanthanum hydrides, containing hydrogen framework structures under compression, display a superconducting state with a high observed critical temperature. However, this phenomenon has so far only been observed at very high pressures. Here, we computationally search for superconductors with very high critical temperatures, but at much lower pressures. We uncover two such sodalite-type clathrate hydrides, YbH6 and LuH6 (Tc =145 K at P=70 GPa for YbH6 and, especially, Tc =273 K at P=100 GPa for LuH6). These striking properties are a consequence of the strong interrelationship between the f states present at the Fermi surface, structural stability and Tc value. For example, TmH6, with unfilled 4f orbitals, is stable at 50 GPa, while it has a relatively low value of Tc of 25 K. As for the YbH6 and LuH6 compounds, they have filled f-shells and the decrease of the f-energy below the Fermi level leads to formation of the nesting regions on the Fermi surface and, as a result, to phonon "softening" and an increase in Tc.

cond-mat.supr-con↗

arXiv2020

Disorder-immune metasurfaces with constituents exhibiting the anapole mode

Common optical metasurfaces are 2-dimensional functional devices composed of periodically arranged subwavelength constituents. Here, we achieved the positional-disorder-immune metasurfaces composed of core-shell cylinders which successively exhibit the magnetic dipole (MD) resonant, non-radiating anapole, and electric dipole (ED) resonant modes when their outer radii are fixed and the inner radii changes continuously in a range. The performances of the metasurfaces under a periodically structural design are not degraded even when the positions of the cylinders are subjected to random and considerable displacements. The positional-disorder-immunity is due to the weak non-local effect of the metasurfaces. Because the multiple scattering among cylinders is weak and insensitive to the spacing among the cylinders around the ED and MD resonant modes and vanishing irrespective of the spacing at the non-radiating anapole mode, the reflection properties including the reflection phase and reflectivity of the metasurfaces are insensitive to the spacing between neighboring cylinders for this entire variation range of the inner radius. The positional-disorder-immunity make the metasurface to be adapted to some unstable and harsh environments.

physics.optics↗

arXiv2020

High-Tc state of Lanthanum hydrides

The high critical temperature phase of lanthanum hydrides deserves special attention because this phase displays the highest observed critical temperature. We focus on the evaluation of the critical temperature, isotope coefficient, and their pressure dependence of this phase (Fm3m phase of the LaH10 compound). By combining the method of two coupling constants, describing the interaction with the optical and acoustic modes, and two characteristic frequencies with first-principle calculations we are able to analyze the role of the light hydrogen in the amplitude of the critical temperature in this unique compound. Specifically, the critical temperature of 254 K and the isotope coefficient of 0.45 are evaluated at high pressure, and the results are in good agreement with the experimental data. The peculiar decrease in the value of the critical temperature upon further increase in pressure is explained by the presence of a flat region on the Fermi surface and the appearance of a two-gap structure, which should be experimentally observable by tunneling spectroscopy.

cond-mat.supr-con↗

arXiv2020

High-Pressure Synthesis of Barium Superhydrides: Pseudocubic BaH12

Following the discovery of high-temperature superconductivity in the La-H system, where for the recently discovered fcc-LaH10 a record critical temperature Tc = 250 K was achieved [Drozdov et al., Nature, 569, 528 (2019) and Somayazulu et al., Phys. Rev. Lett. 122, 027001 (2019)], we studied the formation of new chemical compounds in the barium-hydrogen system at pressures up to 173 GPa. Using in situ generation of hydrogen from NH3BH3, we synthesized previously unknown superhydride BaH12 with a pseudocubic (fcc) Ba sublattice, which was observed in a wide range of pressures from 75 to 173 GPa in four independent experiments. DFT calculations indicate a close agreement between the theoretical and experimental equations of state. In addition to BaH12, we identified previously known P6/mmm BaH2 and possibly BaH10 and BaH6 as impurities in the samples. Ab initio calculations show that newly discovered semimetallic BaH12 contains H2, H3 molecular units and detached H12 chains. Barium dodecahydride is a unique molecular hydride with metallic conductivity which demonstrates a superconducting transition around 20 K at 140 GPa in agreement with calculations (19-32 K). The interpretation of the multiphase XRD data was possible thanks to the development of new Python scripts for postprocessing the results of evolutionary searches. These scripts help quickly identify the theoretical structures that explain the experimental data in the best way, among thousands of candidates.

cond-mat.supr-con↗

arXiv2020

Hydrogen "penta-graphene-like" structure stabilized by hafnium: a high-temperature conventional superconductor

The recent discovery of H3S and LaH10 superconductors with record high superconducting transition temperatures, Tc, at high pressure, has fueled the search for room-temperature superconductivity in the compressed superhydrides. Here we predict the existence of an unprecedented hexagonal HfH10, with an extraordinarily high Tc of around 213-234 K at 250 GPa. In HfH10, the H atoms are arranged in clusters to form a planar "penta-graphene-like" sublattice, in contrast to the covalent sixfold cubic structure in H3S and clathrate-like structure in LaH10. The Hf atom acts as a "precompressor" and electron donor to the hydrogen sublattice. This "penta-graphene-like" H10 structure is also found in ZrH10, ScH10 and LuH10 at high pressure, each material showing a high Tc ranging from 134 to 220 kelvin. Our study of dense superhydrides with "penta-graphene-like" layered structures opens the door to the exploration and exploitation of a new class of high Tc superconductors.

cond-mat.supr-con↗

arXiv2020

Superconductivity of $LaH_{10}$ and $LaH_{16}$ polyhydrides

We explore high-pressure phase stability and superconductivity of lanthanum hydrides $LaH_m$ (m=4-11,16). We predict stability of a hitherto unreported polyhydride $P6/mmm$-$LaH_{16}$ at pressures above 150 GPa; at 200 GPa its predicted superconducting $T_C$ is 156 K, critical field $μ_0$$H_C$(0) ~ 35 T and superconducting gap is up to 35 meV. We revisit superconductivity of the recently discovered $LaH_{10}$ and find its $T_C$ to be up to 259 K (170 GPa) from solving the Eliashberg equation and 271 K from solving the gap equation in SCDFT which also allowed us to compute the Coulomb pseudopotential $μ^*$ for $LaH_{10}$ and $LaH_{16}$. Presence of several polymorph modifications of LaH10 may explain the variety in the experimentally measured $T_C$ values for $LaH_{10}$ [1,2]

cond-mat.supr-con↗

arXiv2020

A Multi-oriented Chinese Keyword Spotter Guided by Text Line Detection

Chinese keyword spotting is a challenging task as there is no visual blank for Chinese words. Different from English words which are split naturally by visual blanks, Chinese words are generally split only by semantic information. In this paper, we propose a new Chinese keyword spotter for natural images, which is inspired by Mask R-CNN. We propose to predict the keyword masks guided by text line detection. Firstly, proposals of text lines are generated by Faster R-CNN;Then, text line masks and keyword masks are predicted by segmentation in the proposals. In this way, the text lines and keywords are predicted in parallel. We create two Chinese keyword datasets based on RCTW-17 and ICPR MTWI2018 to verify the effectiveness of our method.

cs.CV↗

arXiv2019

Beyond temperature scaling: Obtaining well-calibrated multiclass probabilities with Dirichlet calibration

Class probabilities predicted by most multiclass classifiers are uncalibrated, often tending towards over-confidence. With neural networks, calibration can be improved by temperature scaling, a method to learn a single corrective multiplicative factor for inputs to the last softmax layer. On non-neural models the existing methods apply binary calibration in a pairwise or one-vs-rest fashion. We propose a natively multiclass calibration method applicable to classifiers from any model class, derived from Dirichlet distributions and generalising the beta calibration method from binary classification. It is easily implemented with neural nets since it is equivalent to log-transforming the uncalibrated probabilities, followed by one linear layer and softmax. Experiments demonstrate improved probabilistic predictions according to multiple measures (confidence-ECE, classwise-ECE, log-loss, Brier score) across a wide range of datasets and classifiers. Parameters of the learned Dirichlet calibration map provide insights to the biases in the uncalibrated model.

cs.LG↗