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D. Song

Publications and source records attributed to D. Song.

12 recordsLinked to original sources
arXiv2025

Enhanced coherence and layer-selective charge order in a trilayer cuprate superconductor

Trilayer cuprates hold the record for the highest superconducting critical temperatures ($T_{\text{c}}$), yet the underlying mechanism remains elusive. Using time- and angle-resolved photoemission spectroscopy (tr-ARPES), we uncover a striking interplay between charge order, superconducting gap magnitude, and quasiparticle coherence in Bi$_2$Sr$_2$Ca$_2$Cu$_3$O$_{10+\delta}$ (Bi2223). This constitutes ARPES-based evidence of charge order on the inner CuO$_2$ plane, as confirmed via resonant x-ray scattering (RXS); in addition, the same inner plane hosts a superconducting gap significantly larger than that of the overdoped outer planes, firmly establishing it as underdoped. Unexpectedly, despite its underdoped nature, the inner plane also exhibits an exceptional degree of quasiparticle coherence; suppressing charge-order fluctuations further enhances this, making it comparable to that of the overdoped outer planes at elevated electronic temperatures. These findings, supported by complementary three-layer single-band Hubbard calculations, reveal a unique interlayer mechanism in which both pairing strength and phase coherence are optimized when interfacing planes with distinct hole concentrations, providing new microscopic insight into the record $T_{\text{c}}$ of Bi2223.

cond-mat.str-el↗
arXiv2022

Quiet Sun Center to Limb Variation of the Linear Polarization Observed by CLASP2 Across the Mg II h & k Lines

The CLASP2 (Chromospheric LAyer SpectroPolarimeter 2) sounding rocket mission was launched on 2019 April 11. CLASP2 measured the four Stokes parameters of the Mg II h & k spectral region around 2800 Angstroms along a 200 arcsecond slit at three locations on the solar disk, achieving the first spatially and spectrally resolved observations of the solar polarization in this near ultraviolet region. The focus of the work presented here is the center-to-limb variation of the linear polarization across these resonance lines, which is produced by the scattering of anisotropic radiation in the solar atmosphere. The linear polarization signals of the Mg II h & k lines are sensitive to the magnetic field from the low to the upper chromosphere through the Hanle and magneto-optical effects. We compare the observations to theoretical predictions from radiative transfer calculations in unmagnetized semi-empirical models, arguing that magnetic fields and horizontal inhomogeneities are needed to explain the observed polarization signals and spatial variations. This comparison is an important step in both validating and refining our understanding of the physical origin of these polarization signatures, and also in paving the way toward future space telescopes for probing the magnetic fields of the solar upper atmosphere via ultraviolet spectropolarimetry.

astro-ph.SR↗
arXiv2022

Flaring-associated Complex Dynamics in Two M-dwarfs Revealed by Fast, Time-resolved Spectroscopy

Habitability of an exoplanet is believed to be profoundly affected by activities of the host stars, although the related coronal mass ejections (CMEs) are still rarely detected in solar-like and late-type stars. We here report an observational study on flares of two M-dwarfs triggered by the high-cadence survey performed by the Ground Wide-angle Camera system. In both events, the fast, time-resolved spectroscopy enables us to identify symmetric broad H$\alpha$ emission with not only a nearly zero bulk velocity, but also a large projected maximum velocity as high as $\sim700-800\ \mathrm{km\ s^{-1}}$. This broadening could be resulted from either Stark (pressure) effect or a flaring-associated CME at stellar limb. In the context of the CME scenario, the CME mass is estimated to be $\sim4\times10^{18}$ g and $2\times10^{19}$ g. In addition, our spectral analysis reveals a temporal variation of the line center of the narrow H$\alpha$ emission in both events. The variation amplitudes are at tens of $\mathrm{km\ s^{-1}}$, which could be ascribed to the chromospheric evaporation in one event, and to a binary scenario in the other one. With the total flaring energy determined from our photometric monitor, we show a reinforced trend in which larger the flaring energy, higher the CME mass is.

astro-ph.SR↗
arXiv2020

Detection of Acoustic Plasmons in Hole-Doped Lanthanum and Bismuth Cuprate Superconductors Using Resonant Inelastic X-Ray Scattering

High Tc superconductors show a rich variety of phases associated with their charge degrees of freedom. Valence charges can give rise to charge ordering or acoustic plasmons in these layered cuprate superconductors. While charge ordering has been observed for both hole- and electron-doped cuprates, acoustic plasmons have only been found in electron-doped materials. Here, we use resonant inelastic X-ray scattering (RIXS) to observe the presence of acoustic plasmons in two families of hole-doped cuprate superconductors [La2-xSrxCuO4 (LSCO) and Bi2Sr1.6La0.4CuO6+d (Bi2201)], crucially completing the picture. Interestingly, in contrast to the quasi-static charge ordering which manifests at both Cu and O sites, the observed acoustic plasmons are predominantly associated with the O sites, revealing a unique dichotomy in the behaviour of valence charges in hole-doped cuprates.

cond-mat.str-el↗
arXiv2020

Spectroscopic Evidence for Charge Order Melting via Quantum Fluctuations in a Cuprate

Copper-oxide high TC superconductors possess a number of exotic orders co-existing with or proximal to superconductivity, whose quantum fluctuations may account for the unusual behaviors of the normal state, even affecting superconductivity. Yet, spectroscopic evidence about such quantum fluctuations remains elusive. Here, we reveal spectroscopic fingerprints for such fluctuations associated with a charge order (CO) in nearly optimally-doped Bi2Sr2CaCu2O8+d, using resonant inelastic x-ray scattering (RIXS). In the superconducting state, while the quasi-elastic CO signal decreases with temperature, the interplay between CO fluctuations and bond-stretching phonons in the form of a Fano-like interference paradoxically increases, incompatible with expectations for competing orders. Invoking general principles, we argue that this behavior reflects the properties of a dissipative system near an order-disorder quantum critical point, where the dissipation varies with the opening of the pseudogap and superconducting gap at low temperatures, leading to the proliferation of quantum critical fluctuations which melt CO.

cond-mat.supr-con↗
arXiv2020

NWChem: Past, Present, and Future

Specialized computational chemistry packages have permanently reshaped the landscape of chemical and materials science by providing tools to support and guide experimental efforts and for the prediction of atomistic and electronic properties. In this regard, electronic structure packages have played a special role by using first-principledriven methodologies to model complex chemical and materials processes. Over the last few decades, the rapid development of computing technologies and the tremendous increase in computational power have offered a unique chance to study complex transformations using sophisticated and predictive many-body techniques that describe correlated behavior of electrons in molecular and condensed phase systems at different levels of theory. In enabling these simulations, novel parallel algorithms have been able to take advantage of computational resources to address the polynomial scaling of electronic structure methods. In this paper, we briefly review the NWChem computational chemistry suite, including its history, design principles, parallel tools, current capabilities, outreach and outlook.

physics.chem-ph↗
arXiv2018

Nematicity in the pseudogap state of cuprate superconductors revealed by angle-resolved photoemission spectroscopy

The nature of the pseudogap and its relationship with superconductivity are one of the central issues of cuprate superconductors. Recently, a possible scenario has been proposed that the pseudogap state is a distinct phase characterized by spontaneous rotational symmetry breaking called "nematicity" based on transport and magnetic susceptibility measurements, where the symmetry breaking was observed below the pseudogap temperature $T^*$. Here, we report a temperature-dependent ARPES study of nematicity in slightly overdoped Bi${}_{1.7}$Pb${}_{0.5}$Sr${}_{1.9}$CaCu${}_2$O${}_{8+\delta}$ triggered by a uniaxial strain applied along one of the Cu-O bond directions. While the nematicity was enhanced in the pseudogap state as in the previous studies, it was suppressed in the superconducting state. These results indicate that the pseudogap state is characterized by spontaneous rotational symmetry breaking and that the nematicity may compete with superconductivity. These new experimental insights may provide clues for the nature of the pseudogap and its relation to the superconductivity.

cond-mat.supr-con↗
arXiv2016

Object Recognition and Identification Using ESM Data

Recognition and identification of unknown targets is a crucial task in surveillance and security systems. Electronic Support Measures (ESM) are one of the most effective sensors for identification, especially for maritime and air--to--ground applications. In typical surveillance systems multiple ESM sensors are usually deployed along with kinematic sensors like radar. Different ESM sensors may produce different types of reports ready to be sent to the fusion center. The focus of this paper is to develop a new architecture for target recognition and identification when non--homogeneous ESM and possibly kinematic reports are received at the fusion center. The new fusion architecture is evaluated using simulations to show the benefit of utilizing different ESM reports such as attributes and signal level ESM data.

stat.AP↗
arXiv2014

The P versus NP Problem in Quantum Physics

Motivated by the fact that information is encoded and processed by physical systems, the P versus NP problem is examined in terms of physical processes. In particular, we consider P as a class of deterministic, and NP as nondeterministic, polynomial-time physical processes. Based on these identifications, we review a self-reference physical process in quantum theory, which belongs to NP but cannot be contained in P.

physics.gen-ph↗
arXiv2010

Einstein's Moon

An account of the subjective elements of quantum mechanics or of whether, as Einstein famously asked, the Moon exists when nobody is looking at it.

physics.gen-ph↗
arXiv2009

Geometric stabilization of extended S=2 vortices in two-dimensional photonic lattices: theoretical analysis, numerical computation and experimental results

In this work, we focus our studies on the subject of nonlinear discrete self-trapping of S=2 (doubly-charged) vortices in two-dimensional photonic lattices, including theoretical analysis, numerical computation and experimental demonstration. We revisit earlier findings about S=2 vortices with a discrete model, and find that S=2 vortices extended over eight lattice sites can indeed be stable (or only weakly unstable) under certain conditions, not only for the cubic nonlinearity previously used, but also for a saturable nonlinearity more relevant to our experiment with a biased photorefractive nonlinear crystal. We then use the discrete analysis as a guide towards numerically identifying stable (and unstable) vortex solutions in a more realistic continuum model with a periodic potential. Finally, we present our experimental observation of such geometrically extended S=2 vortex solitons in optically induced lattices under both self-focusing and self-defocusing nonlinearities, and show clearly that the S=2 vortex singularities are preserved during nonlinear propagation.

nlin.PS↗