arXiv subjects

Liang Dai

Publications and source records attributed to Liang Dai.

At least 37 records · Page 2Linked to original sources

arXiv2025

Non-Gaussian Statistics of Nanohertz Stochastic Gravitational Waves

Recent detection of nHz stochastic gravitational wave background (SGWB) by multiple pulsar timing arrays (PTAs) has stimulated intensive discussions about its physical origin. In principle, either supermassive black hole binaries (SMBHBs) or processes in the early universe may be the sources. One key difference between the two lies in the statistics of the SGWB frequency power spectrum. In particular, the often assumed Gaussian random SGWB does not accurately describe the distribution of the collective SMBHB emission. In this work, we present a semi-analytical framework for calculating the non-Gaussian statistics of SGWB power expected from SMBHBs. We find that (a) wave interference between individual SMBHBs with indistinguishable observed frequencies and (b) the Poisson fluctuation of the source numbers, together shape the non-Gaussian statistics. Implementing the non-Gaussian statistics developed in this work, we investigate the sensitivity of current and future PTA datasets in distinguishing the origin of the SGWB through non-Gaussian information. Additionally, we find an interesting approximation of the non-Gaussian statistics, which has implications for accurately and practically treating non-Gaussianity in PTA Bayesian analyses.

astro-ph.CO↗

arXiv2025

A Unified Knowledge-Distillation and Semi-Supervised Learning Framework to Improve Industrial Ads Delivery Systems

Industrial ads ranking systems conventionally rely on labeled impression data, which leads to challenges such as overfitting, slower incremental gain from model scaling, and biases due to discrepancies between training and serving data. To overcome these issues, we propose a Unified framework for Knowledge-Distillation and Semi-supervised Learning (UKDSL) for ads ranking, empowering the training of models on a significantly larger and more diverse datasets, thereby reducing overfitting and mitigating training-serving data discrepancies. We provide detailed formal analysis and numerical simulations on the inherent miscalibration and prediction bias of multi-stage ranking systems, and show empirical evidence of the proposed framework's capability to mitigate those. Compared to prior work, UKDSL can enable models to learn from a much larger set of unlabeled data, hence, improving the performance while being computationally efficient. Finally, we report the successful deployment of UKDSL in an industrial setting across various ranking models, serving users at multi-billion scale, across various surfaces, geological locations, clients, and optimize for various events, which to the best of our knowledge is the first of its kind in terms of the scale and efficiency at which it operates.

cs.LG↗

arXiv2025

Effects of Subhalos on Interpreting Highly Magnified Sources Near Lensing Caustics

Large magnification factors near gravitational lensing caustics of galaxy cluster lenses allow the study of individual stars or compact stellar associations at cosmological distances. We study how the presence of sub-galactic subhalos, an inevitable consequence of cold dark matter, can alter the property of caustics and hence change the interpretation of highly magnified sources that lie atop them. First, we consider a galaxy cluster halo populated with subhalos sampled from a realistic subhalo mass function calibrated to $N$-body simulations. Then, we compare a semi-analytical approximation and an adaptive ray-shooting method which we employ to quantify the property of the caustics. As a case study, we investigate Earendel, a $z = 6.2$ candidate of magnified single or multiple star system with a lone lensed image atop the critical curve in the Sunrise Arc. We find that the source size constraint ($\lesssim 0.3\, \mathrm{pc}$) previously derived from macro lens models should be relaxed by a factor of a few to ten when subhalos are accounted for, therefore allowing the possibility of a compact star cluster. The subhalos could introduce an astrometric perturbation that is $\lesssim 0.5''$, which does not contradict observation. These conclusions are largely robust to changes in the subhalo population. Subhalos therefore should be seriously accounted for when interpreting the astrophysical nature of similar highly magnified sources uncovered in recent high-$z$ observations.

astro-ph.GA↗

arXiv2025

Small polarons mediated near-room-temperature metal-insulator transition in vanadium dioxide and their hopping dynamics

Researchers pursuing advanced photoelectric devices have discovered near room-temperature metal-insulator transitions (MIT) in non-volatile VO2. Despite theoretical investigations suggesting that polaron dynamics mediate the MIT, direct experimental evidence remains scarce. In this study, we present direct evidence of the polaron state in insulating VO2 through high-resolution spectroscopic ellipsometry measurements and first-principles calculations. We illustrate the complementary role of polaron dynamics in facilitating Peierls and Mott transitions, thereby contributing to the MIT processes. Furthermore, our observations and characterizations of conventional metallic and correlated plasmons in the respective phases of the VO2 film offer valuable insights into their electron structures. This investigation enhances comprehension of the MIT mechanism in correlated systems and underscores the roles of polarons, lattice distortions, and electron correlations in facilitating phase transition processes in strongly-correlated systems. Additionally, the detailed detection of small polarons and plasmons serves as inspiration for the development of new device functionalities.

physics.app-ph↗

arXiv2025

Recognizing and generating knotted molecular structures by machine learning

Knotted molecules occur naturally and are designed by scientists to gain special biological and material properties. Understanding and utilizing knotting require efficient methods to recognize and generate knotted structures, which are unsolved problems in mathematics and physics. Here, we solve these two problems using machine learning. First, our Transformer-based neural network (NN) can recognize the knot types of given chain conformations with an accuracy of $>99\%$. We can use a single NN model to recognize knots with different chain lengths, and our computational speed is about 4500 times faster than the most popular mathematical method for knot recognition: the Alexander polynomials. Second, we for the first time design a diffusion-based NN model to generate conformations for given knot types. The generated conformations satisfy not only the desired knot types, but also the correct physical distributions of the radii of gyration and knot sizes. The results have several implications. First, the Transformer is suitable for handling knotting tasks, probably because of its strength in processing sequence information, a key component in knotting. Second, our NN can replace mathematical methods of knot recognition for faster speed on many occasions. Third, our models can facilitate the design of knotted protein structures. Lastly, analyzing how NN recognizes knot types can provide insight into the principle behind knots, an unsolved problem in mathematics. We provide an online website (http://144.214.24.236) for using our models.

physics.comp-ph↗

arXiv2025

Identification of $>$40 gravitationally magnified stars in a galaxy at redshift of 0.725

Strong gravitational magnifications enable to detect faint background sources, resolve their internal structures, and even identify individual stars in distant galaxies. Highly magnified individual stars allow various applications, including studies of stellar populations in distant galaxies and constraining dark matter structures in the lensing plane. However, these applications have been hampered by the small number of individual stars observed, as typically one or a few stars are identified from each distant galaxy. Here, we report the discovery of more than 40 microlensed stars in a single galaxy behind Abell 370 at redshift of 0.725 when the Universe was half of its current age (dubbed ``the Dragon arc''), using James Webb Space Telescope (JWST) observations with the time-domain technique. These events are found near the expected lensing critical curves, suggesting that these are magnified stars that appear as transients from intracluster stellar microlenses. Through multi-wavelength photometry, we constrain stellar types and find that many of them are consistent with red giants/supergiants magnified by factors of hundreds. This finding reveals an unprecedented high occurrence of microlensing events in the Dragon arc, and proves that {\it JWST}'s time-domain observations open up the possibility of conducting statistical studies of high-redshift stars.

astro-ph.GA↗

arXiv2024

SN H0pe: The First Measurement of $H_0$ from a Multiply-Imaged Type Ia Supernova, Discovered by JWST

The first James Webb Space Telescope ({\it JWST}) Near InfraRed Camera (NIRCam) imaging in the field of the galaxy cluster PLCK G165.7+67.0 ($z=0.35$) uncovered a Type Ia supernova (SN Ia) at $z=1.78$, called ``SN H0pe." Three different images of this one SN were detected as a result of strong gravitational lensing, each one traversing a different path in spacetime, thereby inducing a relative delay in the arrival of each image. Follow-up {\it JWST} observations of all three SN images enabled photometric and rare spectroscopic measurements of the two relative time delays. Following strict blinding protocols which oversaw a live unblinding and regulated post-unblinding changes, these two measured time delays were compared to the predictions of seven independently constructed cluster lens models to measure a value for the Hubble constant, $H_0=71.8^{+9.2}_{-8.1}$ km s$^{-1}$ Mpc$^{-1}$. The range of admissible $H_0$ values predicted across the lens models limits further precision, reflecting the well-known degeneracies between lens model constraints and time delays. It has long been theorized that a way forward is to leverage a standard candle, but this has not been realized until now. For the first time, the lens models are evaluated by their agreement with the SN absolute magnifications, breaking degeneracies and producing our best estimate, $H_0=75.7^{+8.1}_{-5.5}$ km s$^{-1}$ Mpc$^{-1}$. This is the first precision measurement of $H_0$ from a multiply-imaged SN Ia and only the second from any multiply-imaged SN.

astro-ph.CO↗

arXiv2024

Billion-Fold Enhancement of Room-Temperature Ionic Conductivity in h-RMnO3/YSZ Heterostructures via Electric-Field-Assisted Oxygen Deficiency Engineering

Oxide heterostructures provide versatile platforms for manipulating electronic and ionic conductive states. In this study, we demonstrate a remarkable billion-fold enhancement in room-temperature ionic conductivity within h-RMnO3/YSZ heterostructures, achieved through electric-field-assisted oxygen deficiency engineering. This enhancement is closely linked to substantial oxygen depletion in YSZ and is tunable by varying the thickness of the h-RMnO3 film layer and the applied voltage bias. Our findings underscore the critical importance of interfacial design and vacancy control in enhancing ionic transport capabilities, paving the way for advanced applications in low-temperature energy harvesting, storage, and conversion technologies.

cond-mat.mtrl-sci↗

arXiv2024

A Gravitational Waveform Model for Detecting Accelerating Inspiraling Binaries

We present an analytic frequency-domain gravitational waveform model for an inspiraling binary whose center-of-mass undergoes a small acceleration, assumed to be constant during the detection, such as when it orbits a distant tertiary mass. The center-of-mass acceleration along the line of sight is incorporated as a new parameter that perturbs the standard TaylorF2 model. We calculate the wave phase to 3rd post-Newtonian order and first order in the acceleration. It is shown that acceleration most significantly modifies the wave phase in the low frequency portion of the signal, so ground-based detectors with a good sensitivity at low frequencies are the most effective at detecting this effect. We present a Fisher information calculation to quantify the detectability of this effect at advanced LIGO A Plus, Cosmic Explorer, and Einstein Telescope over the mass range of neutron stars and stellar-mass black holes, and discuss degeneracy between acceleration and other parameters. We also determine the parameter space where the acceleration is large enough that the wave phase model would have to be extended to nonlinear orders in the acceleration.

gr-qc↗

arXiv2024

GLoW: novel methods for wave-optics phenomena in gravitational lensing

Wave-optics phenomena in gravitational lensing occur when the signal's wavelength is commensurate to the gravitational radius of the lens. Although potentially detectable in lensed gravitational waves, fast radio bursts and pulsars, accurate numerical predictions are challenging to compute. Here we present novel methods for wave-optics lensing that allow the treatment of general lenses. In addition to a general algorithm, specialized methods optimize symmetric lenses (arbitrary number of images) and generic lenses in the single-image regime. We also develop approximations for simple lenses (point-like and singular isothermal sphere) that drastically outperform known solutions without compromising accuracy. These algorithms are implemented in Gravitational Lensing of Waves (GLoW): an accurate, flexible, and fast code. GLoW efficiently computes the frequency-dependent amplification factor for generic lens models and arbitrary impact parameters in O(1 ms) to O(10 ms) depending on the lens configuration and complexity. GLoW is readily applicable to model lensing diffraction on gravitational-wave signals, offering new means to investigate the distribution of dark-matter and large-scale structure with signals from ground and space detectors.

gr-qc↗

arXiv2024

A Young Super Star Cluster Powering a Nebula of Retained Massive Star Ejecta

We suggest that "Godzilla" of the lensed Sunburst galaxy ($z=2.37$) is a young super star cluster powering a nebula of gravitationally trapped stellar ejecta. Employing HST photometry and spectroscopy from VLT/MUSE and VLT/X-Shooter, we infer physical and chemical properties of the cluster and nebula, finding Godzilla is young $4$-$6$Myr, massive $2 \times 10^{6} M_\odot (1000/μ)$, a stellar metallicity $Z \simeq 0.25 Z_\odot$, and has a compact FUV component $\lesssim 1{\rm pc}(1000/μ)$, where $μ$ is the flux magnification factor. The gas is significantly enriched with N and He, indicating stellar wind material, and has highly elevated O relative to the stellar metallicity, indicating entrainment of CCSNe ejecta. The high density $n_{\rm e} \simeq 10^{7-8}{\rm cm}^{-3}$ implies a highly pressurized intracluster environment. We propose the pressure results from CCSN-driven supersonic turbulence in warm, self-shielding gas, which has accumulated in the cluster center after runaway radiative cooling and is dense enough to resist removal by CCSNe. The nebula gas shows sub-solar C/O, Ne/O and Si/O, which may reflect the CCSN element yields for initial stellar masses $>40 M_\odot$. A comparison to element yield synthesis models for young star clusters shows the gas abundances are consistent with complete retention and mixture of stellar winds and CCSNe ejecta until the inferred cluster age. The inferred O and He enhancement may have implications for the formation of multiple stellar populations in globular clusters, as stars formed from this gas would contradict the observed abundances of second-population stars.

astro-ph.GA↗

arXiv2024

Detecting Dark Matter Substructures on Small Scales with Fast Radio Bursts

We propose measuring the arrival time difference of Fast Radio Bursts (FRBs) along two adjacent sightlines as a new probe to dark matter substructures on scales down to $\sim 1\,$AU. We discuss two observational scenarios in which it may be possible to place interesting constraints on such models through monitoring repeating FRB sources: 1) By sending radio receivers to space to form a baseline of tens of AU or more and measuring the temporal variation of the arrival time difference between receivers. 2) By measuring the temporal variation of the arrival time difference between two lensed images of one strongly lensed repeater. In both scenarios, obtaining interesting constraints requires correlating the voltage time series to measure the radio-signal arrival time to sub-nanosecond precision. We find that two radio dishes separated by $20\,$AU may be sensitive to the enhancement of small-scale structures at $\sim 10^{-8}M_\odot$ masses in the QCD axion dark matter scenario or from an early epoch of matter-domination with a reheating temperature up to 60 MeV. Other dark matter models such as those composed of $\sim 10^{-13}M_{\odot}$ primordial black holes produced during inflation would also be probed by this method. We further show that a strong lensing situation of multiple images provides an equivalent $\sim 2000\,$AU baseline, which can be much more sensitive but with the uncertainty that intervening ISM decoherence may degrade the timing precision and that spatial variation in the FRB emission spot may result in confounding signals. We show that the lensing magnifications of Type Ia supernovea constrain a similar quantity to such FRB timing, with present limits being equivalent to ruling out the same parameter space that would be probed by a $0.14~$AU baseline.

astro-ph.CO↗

arXiv2024

Tunable Collective Excitations in Epitaxial Perovskite Nickelates

The formation of plasmons through the collective excitation of charge density has generated intense discussions, offering insights to fundamental sciences and potential applications. While the underlying physical principles have been well-established, the effects of many-body interactions and orbital hybridization on plasmonic dynamics remain understudied. In this work, we present the observation of conventional metallic and correlated plasmons in epitaxial La1-xSrxNiO3 (LSNO) films with varying Sr doping concentrations (x = 0, 0.125, 0.25), unveiling their intriguing evolution. Unlike samples at other doping concentrations, the x = 0.125 intermediate doping sample does not exhibit the correlated plasmons despite showing high optical conductivity. Through a comprehensive experimental investigation using spectroscopic ellipsometry and X-ray absorption spectroscopy, the O2p-Ni3d orbital hybridization for LSNO with a doping concentration of x = 0.125 is found to be significantly enhanced, alongside a considerable weakening of its effective correlation U*. These factors account for the absence of correlated plasmons and the high optical conductivity observed in LSNO (0.125). Our results underscore the profound impact of orbital hybridization on the electronic structure and the formation of plasmon in strongly-correlated systems. This in turn suggest that LSNO could serve as a promising alternative material in optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2024

Unraveling Anisotropic Hybridizations of Solid-state Electrolyte Nano-films in Li-ion Batteries

Li2WO4 (LWO) is recognized for its potential as a solid-state electrolyte and it has demonstrated the ability to enhance the electrochemical performance of LiCoO2 (LCO) cathodes in Li-ion batteries. However, prior investigations into LWO have predominantly involved polycrystalline structures, thereby lacking a comprehensive understanding of its behavior when interfaced with single crystal systems, particularly those intricately connected to LCO. In this study, we employ pulsed laser deposition (PLD) to epitaxially synthesize LWO nano-films on LCO layers with different orientations. Based on a series of high-resolution synchrotron-based techniques including X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS), the electronic structure of LWO is carefully scrutinized where a higher main energy level of W5d(eg)-O2p orbitals hybridization in LWO/LCO(104) as compared to LWO/LCO(003) has been observed. This experimental finding is further validated by a comprehensive set of density of states calculations. Furthermore, detailed polarized XAS characterization unveils distinct anisotropy between the two oriented LWO configurations. This comprehensive scientific investigation, harnessing the capabilities of synchrotron-based techniques, provides invaluable insights for future studies, offering guidance for the optimized utilization of LWO as a solid-state electrolyte or modification layer for LCO cathodes in high-powered Li-ion batteries.

cond-mat.mtrl-sci↗

arXiv2024

Dissecting the Stochastic Gravitational Wave Background with Astrometry

Astrometry, the precise measurement of star motions, offers an alternative avenue to investigate low-frequency gravitational waves through the spatial deflection of photons, complementing pulsar timing arrays reliant on timing residuals. Upcoming data from Gaia and Roman can not only cross-check pulsar timing array findings but also explore the uncharted frequency range bridging pulsar timing arrays and LISA. We present an analytical framework to evaluate the feasibility of detecting a gravitational wave background, considering measurement noise and the intrinsic variability of the stochastic background. Furthermore, we highlight astrometry's crucial role in uncovering key properties of the gravitational wave background, such as spectral index and chirality, employing information-matrix analysis. Finally, we simulate the emergence of quadrupolar correlations, commonly referred to as the generalized Hellings-Downs curves.

gr-qc↗

arXiv2024

Imaging dark matter at the smallest scales with $z\approx1$ lensed stars

Observations of caustic-crossing galaxies at redshift $0.7 2$ and the number density of events is greater around substructureand the number density of events is greater around substructures, and (ii) negative imaging regime where $β<2$. We study the particular case of seven microlensing events found by HST in the Dragon arc (at z=0.725). We find that a population of supergiant stars with a steep LF with $β=2.55$ fits the distribution of these events. We identify a small region of high density of microlensing events, and interpret it as evidence of a possible invisible substructure, for which we derive a mass of $\sim 1.3 \times 10^8\,\Msun$ (within its Einstein radius).

astro-ph.CO↗

arXiv2024

Under Einstein's Microscope: Measuring Properties of Individual Rotating Massive Stars From Extragalactic Micro Caustic Crossings

Highly magnified stars residing in caustic crossing lensed galaxies at z ~ 0.7-1.5 in galaxy cluster lensing fields inevitably exhibit recurrent brightening events as they traverse a micro caustic network cast down by foreground intracluster stars. The detectable ones belong to Nature's most massive and luminous class of stars, with evolved blue supergiants being the brightest ones at optical wavelengths. Considering single stars in this work, we study to what extent intrinsic stellar parameters are measurable from multi-filter lightcurves, which can be obtained with optical/near-IR space telescopes during one or multiple caustic crossing events. We adopt a realistic model for the axisymmetric surface brightness profiles of rotating O/B stars and develop a numerical lensing code that treats finite-source-size effects. With a single micro caustic crossing, the ratio of the surface rotation velocity to the breakup value is measurable to an precision of ~ 0.1-0.2 for feasible observation parameters with current space telescopes, with all unknown intrinsic and extrinsic parameters marginalized over and without a degeneracy with inclination. Equatorial radius and bolometric luminosity can be measured to 1/3 and 2/3 of the fractional uncertainty in the micro caustic strength, for which the value is not known at each crossing but an informative prior can be obtained from theory. Parameter inference precision may be further improved if multiple caustic crossing events for the same lensed star are jointly analyzed. Our results imply new opportunities to survey individual massive stars in star-formation sites at z ~ 0.7-1.5 or beyond.

astro-ph.CO↗

arXiv2024

Enhancement of cell membrane poration by the antimicrobial peptide Melp5

Melittin, a natural antimicrobial peptide comprising 26 amino acid residues, can kill bacteria by inducing pores in cell membranes. Clinical applications of melittin as an antibiotic require a thorough understanding of its poration mechanism and mutations that enhance its antimicrobial activity. Previous experiments showed Melp5, a variant of melittin with five mutations, exhibits a higher poration ability. However, the mechanism of the enhanced poration ability is not fully understood. Here, we investigated the mechanism by comparing the poration of melittin and Melp5 using coarse-grained (CG) and all-atom (AA) molecular dynamics (MD) simulations. We observe that Melp5 is likely to form a pore with 5 peptides (pentameric), while melittin is likely to form a pore with 4 peptides (tetrameric). Our atomistic MD simulations show that the pentameric pore of Melp5 has a higher water permeability than the tetrameric pore of melittin. We also analyze the stability of the pores of melittin and Melp5 by calculating the interaction energies of the pores. In particular, we investigate the effects of mutant residues on pore stability by calculating electrostatic and LJ interactions. These results should provide insights on the enhanced poration ability of Melp5 and push it toward applications.

physics.bio-ph↗