cssi/SciSciGPT-SciSciNet huggingface dataset:
Site Characterization at Downhole Arrays by Joint Inversion of Dispersion Data and Acceleration Time Series

We present a sequential data assimilation algorithm based on the ensemble Kalman inversion to estimate the near‐surface shear‐wave velocity profile and damping; this is applicable when heterogeneous data and a priori information that can be represented in forms of (physical) equality and inequality constraints in the inverse problem are available. Although noninvasive methods, such as surface‐wave testing, are efficient and cost‐effective methods for inferring an V_S profile, one should acknowledge that site characterization using inverse analyses can yield erroneous results associated with the lack of inverse problem uniqueness. One viable solution to alleviate the unsuitability of the inverse problem is to enrich the prior knowledge and/or the data space with complementary observations. In the case of noninvasive methods, the pertinent data are the dispersion curve of surface waves, typically resolved by means of active source methods at high frequencies and passive methods at low frequencies. To improve the inverse problem suitability, horizontal‐to‐vertical spectral ratio data are commonly used jointly with the dispersion data in the inversion. In this article, we show that the joint inversion of dispersion and strong‐motion downhole array data can also reduce the margins of uncertainty in the V_S profile estimation. This is because acceleration time series recorded at downhole arrays include both body and surface waves and therefore can enrich the observational data space in the inverse problem setting. We also show how the proposed algorithm can be modified to systematically incorporate physical constraints that further enhance its suitability. We use both synthetic and real data to examine the performance of the proposed framework in estimation of the V_S profile and damping at the Garner Valley downhole array and compare them against the V_S estimations in previous studies.
PHONON TRANSPORT AND THERMAL CONDUCTIVITY IN TWO-DIMENSIONAL MATERIALS

Two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, have attracted increased interest due to their potential applications in electronics and optoelectronics. Thermal transport in two-dimensional materials could be quite different from three-dimensional bulk materials. This article reviews the progress on experimental measurements and theoretical modeling of phonon transport and thermal conductivity in two-dimensional materials. We focus our review on a few typical two-dimensional materials, including graphene, boron nitride, silicene, transition metal dichalcogenides, and black phosphorus. The effects of different physical factors, such as sample size, strain and defects, on thermal transport in Two-dimensional materials are summarized. We also discuss the environmental effect on the thermal transport of two-dimensional materials, such as substrate and when two-dimensional materials are presented in heterostructures and intercalated with inorganic components or organic molecules.
Lower semicontinuity of the ADM mass in dimensions two through seven

The semicontinuity phenomenon of the ADM mass under pointed (i.e., local) convergence of asymptotically flat metrics is of interest because of its connections to nonnegative scalar curvature, the positive mass theorem, and Bartnik's mass-minimization problem in general relativity. In this paper, we extend a previously known semicontinuity result in dimension three for $C^2$ pointed convergence to higher dimensions, up through seven, using recent work of S. McCormick and P. Miao (which itself builds on the Riemannian Penrose inequality of H. Bray and D. Lee). For a technical reason, we restrict to the case in which the limit space is asymptotically Schwarzschild. In a separate result, we show that semicontinuity holds under weighted, rather than pointed, $C^2$ convergence, in all dimensions $n \geq 3$, with a simpler proof independent of the positive mass theorem. Finally, we also address the two-dimensional case for pointed convergence, in which the asymptotic cone angle assumes the role of the ADM mass.
White Holes as the Asymptotic Limit of Evaporating Primordial Black Holes

This paper examines the interaction of an intense fermion field with all of the particle species of an attometer primordial black hole’s (PBH) high energy Hawking radiation spectrum. By extrapolating to Planck-sized PBHs, it is shown that although Planck-sized PBHs closely simulate the zero absorption requirement of white holes, the absorption probability is not truly zero, and therefore, thermodynamically, Planck-sized primordial black holes are not true white holes.
Symmetries of the hydrogen atom and algebraic families

We show how the Schrodinger equation for the hydrogen atom in two dimensions gives rise to an algebraic family of Harish-Chandra pairs that codifies hidden symmetries. The hidden symmetries vary continuously between SO(3), SO(2, 1), and the Euclidean group O(2)⋉R2. We show that the solutions of the Schrodinger equation may be organized into an algebraic family of Harish-Chandra modules. Furthermore, we use Jantzen filtration techniques to algebraically recover the spectrum of the Schrodinger operator. This is a first application to physics of the algebraic families of Harish-Chandra pairs and modules developed in the work of Bernstein et al. [Int. Math. Res. Notices, rny147 (2018); rny146 (2018)].We show how the Schrodinger equation for the hydrogen atom in two dimensions gives rise to an algebraic family of Harish-Chandra pairs that codifies hidden symmetries. The hidden symmetries vary continuously between SO(3), SO(2, 1), and the Euclidean group O(2)⋉R2. We show that the solutions of the Schrodinger equation may be organized into an algebraic family of Harish-Chandra modules. Furthermore, we use Jantzen filtration techniques to algebraically recover the spectrum of the Schrodinger operator. This is a first application to physics of the algebraic families of Harish-Chandra pairs and modules developed in the work of Bernstein et al. [Int. Math. Res. Notices, rny147 (2018); rny146 (2018)].
Computational screening of high-performance optoelectronic materials using OptB88vdW and TB-mBJ formalisms

We perform high-throughput density functional theory (DFT) calculations for optoelectronic properties (electronic bandgap and frequency dependent dielectric function) using the OptB88vdW functional (OPT) and the Tran-Blaha modified Becke Johnson potential (MBJ). This data is distributed publicly through JARVIS-DFT database. We used this data to evaluate the differences between these two formalisms and quantify their accuracy, comparing to experimental data whenever applicable. At present, we have 17,805 OPT and 7,358 MBJ bandgaps and dielectric functions. MBJ is found to predict better bandgaps and dielectric functions than OPT, so it can be used to improve the well-known bandgap problem of DFT in a relatively inexpensive way. The peak positions in dielectric functions obtained with OPT and MBJ are in comparable agreement with experiments. The data is available on our websites http://www.ctcms.nist.gov/~knc6/JVASP.html and https://jarvis.nist.gov.
Some stochastic inequalities for weighted sums

We compare weighted sums of i.i.d. positive random variables according to the usual stochastic order. The main inequalities are derived using majorization techniques under certain log-concavity assumptions. Specifically, let $Y_i$ be i.i.d. random variables on R+. Assuming that $\log Y_i$ has a log-concave density, we show that $∑ a_iY_i$ is stochastically smaller than ∑ biYi, if ($\log a_1$, …, $\log a_n$) is majorized by ($\log b_1$, …, $\log b_n$). On the other hand, assuming that $Y_i^p$ has a log-concave density for some $p > 1$, we show that $∑a_iY_i$ is stochastically larger than $∑ b_iY_i$, if ($a_1^q$, …, $a_n^q$) is majorized by ($b_1^q$, …, $b_n^q$), where $p^{−1} + q^−1 = 1$. These unify several stochastic ordering results for specific distributions. In particular, a conjecture of Hitczenko [Sankhyā A 60 (1998) 171–175] on Weibull variables is proved. Potential applications in reliability and wireless communications are mentioned.
Does Singlet Fission Enhance the Performance of Organic Solar Cells

Singlet fission, in which the optical spin-singlet exciton dissociates into two low energy triplet excitons, has been proposed as a viable approach to enhance the quantum efficiency of organic solar cells. We show that even when singlet fission is occurring in the donor molecule, the electronic structure at the donor–acceptor interface must satisfy specific requirements for the solar cell performance to be enhanced by this process. We focus on the pentacene–C60 solar cell, and on the basis of our calculations and available experimental data, we conclude that there is not enough evidence that these requirements are met by the donor–acceptor interface here. We propose experiments that can determine whether the minimal requirement for enhanced performance driven by singlet fission is met in this and other solar cells.
TACKLING RADIO POLARIZATION OF ENERGETIC PULSARS

The traditional, geometrical rotating vector model (RVM) has proved particularly poor at capturing the polarization sweeps of the young energetic and millisecond pulsars detected by Fermi. We augment this model by including finite altitude effects using a swept back vacuum dipole geometry. By further including the effects of orthogonal mode jumps, multiple emission altitudes, open zone growth via y-point lowering, and interstellar scattering, we show that a wide range of departures from RVM can be modeled well while retaining a geometrical picture. We illustrate these effects by fitting six Fermi-detected pulsars (J0023+0923, J1024–0719, J1744–1134, J1057–5226, J1420–6048, and J2124–3358) and we describe how such modeling can improve our understanding of their emission geometry.
Structural phase diagram and magnetic properties of Sc-substituted rare earth ferrites R1−xScxFeO3 (R = Lu, Yb, Er, and Ho)

We have studied the structural stability of Sc-substituted rare earth (R) ferrites R1−xScxFeO3 and constructed a structural phase diagram for different R and x. While RFeO3 and ScFeO3 adopt the orthorhombic and bixbyite structure, respectively, the substituted compound R1−xScxFeO3 may be stable in a different structure. Specifically, for R0.5Sc0.5FeO3, the hexagonal structure can be stable for small R, such as Lu and Yb, while the garnet structure is stable for larger R, such as Er and Ho. The formation of the garnet structure of R0.5Sc0.5FeO3 compounds, which requires that Sc occupies both the rare earth and the Fe sites, is corroborated by their magnetic properties.We have studied the structural stability of Sc-substituted rare earth (R) ferrites R1−xScxFeO3 and constructed a structural phase diagram for different R and x. While RFeO3 and ScFeO3 adopt the orthorhombic and bixbyite structure, respectively, the substituted compound R1−xScxFeO3 may be stable in a different structure. Specifically, for R0.5Sc0.5FeO3, the hexagonal structure can be stable for small R, such as Lu and Yb, while the garnet structure is stable for larger R, such as Er and Ho. The formation of the garnet structure of R0.5Sc0.5FeO3 compounds, which requires that Sc occupies both the rare earth and the Fe sites, is corroborated by their magnetic properties.
Conformal Invariance and Stochastic Loewner Evolution Predictions for the 2D Self-Avoiding Walk—Monte Carlo Tests

Simulations of the two-dimensional self-avoiding walk (SAW) are performed in a half-plane and a cut-plane (the complex plane with the positive real axis removed) using the pivot algorithm. We test the conjecture of Lawler, Schramm, and Werner that the scaling limit of the two-dimensional SAW is given by Schramm's stochastic Loewner evolution (SLE). The agreement is found to be excellent. The simulations also test the conformal invariance of the SAW since conformal invariance implies that if we map infinite length walks in the cut-plane into the half plane using the conformal map \(z \to \sqrt z\), then the resulting walks will have the same distribution as the SAW in the half plane. The simulations show excellent agreement between the distributions.
Dark matter search at a linear collider: effective operator approach

Experiments at electron-positron colliders can search for dark matter particle pair-production in association with a photon. We estimate the sensitivity of this search at the proposed International Linear Collider (ILC), under a variety of run scenarios. We employ the effective operator formalism to provide a quasi-model-independent theoretical description of the signal, and present the reach of the ILC in terms of the scale Λ suppressing the dark matter-electron coupling operator. We find that at the 250 GeV center-of-mass energy, the ILC can probe Λ up to 650–900 GeV, approximately a factor of 2 above the best current bounds from LEP-2. With 1 TeV energy and polarized beams, the reach can be extended to 2–3.5TeV. The ILC can discover this signature even if annihilation to electrons provides only a small fraction of the total dark matter annihilation rate in the early universe. We also argue that large regions of parameter space allowed by current LHC and direct detection bounds will be accessible at the ILC.
KEPLER FLARES. I. ACTIVE AND INACTIVE M DWARFS

We analyzed Kepler short-cadence M dwarf observations. Spectra from the Astrophysical Research Consortium 3.5 m telescope identify magnetically active (Hα in emission) stars. The active stars are of mid-M spectral type, have numerous flares, and have well-defined rotational modulation due to starspots. The inactive stars are of early M type, exhibit less starspot signature, and have fewer flares. A Kepler to U-band energy scaling allows comparison of the Kepler flare frequency distributions with previous ground-based data. M dwarfs span a large range of flare frequency and energy, blurring the distinction between active and inactive stars designated solely by the presence of Hα. We analyzed classical and complex (multiple peak) flares on GJ 1243, finding strong correlations between flare energy, amplitude, duration, and decay time, with only a weak dependence on rise time. Complex flares last longer and have higher energy at the same amplitude, and higher energy flares are more likely to be complex. A power law fits the energy distribution for flares with log 31 erg, but the predicted number of low-energy flares far exceeds the number observed, at energies where flares are still easily detectable, indicating that the power-law distribution may flatten at low energy. There is no correlation of flare occurrence or energy with starspot phase, the flare waiting time distribution is consistent with flares occurring randomly in time, and the energies of consecutive flares are uncorrelated. These observations support a scenario where many independent active regions on the stellar surface are contributing to the observed flare rate.
A family of minimal and renormalizable rectangle exchange maps

A domain exchange map (DEM) is a dynamical system defined on a smooth Jordan domain which is a piecewise translation. We explain how to use cut-and-project sets to construct minimal DEMs. Specializing to the case in which the domain is a square and the cut-and-project set is associated to a Galois lattice, we construct an infinite family of DEMs in which each map is associated to a Pisot–Vijayaraghavan (PV) number. We develop a renormalization scheme for these DEMs. Certain DEMs in the family can be composed to create multistage, renormalizable DEMs.
Asking without Telling: Exploring Latent Ontologies in Contextual Representations

The success of pretrained contextual encoders, such as ELMo and BERT, has brought a great deal of interest in what these models learn: do they, without explicit supervision, learn to encode meaningful notions of linguistic structure? If so, how is this structure encoded? To investigate this, we introduce latent subclass learning (LSL): a modification to classifier-based probing that induces a latent categorization (or ontology) of the probe’s inputs. Without access to fine-grained gold labels, LSL extracts emergent structure from input representations in an interpretable and quantifiable form. In experiments, we find strong evidence of familiar categories, such as a notion of personhood in ELMo, as well as novel ontological distinctions, such as a preference for fine-grained semantic roles on core arguments. Our results provide unique new evidence of emergent structure in pretrained encoders, including departures from existing annotations which are inaccessible to earlier methods.
Calculation of nonperturbative terms in open string models

Abstract Nonperturbative corrections in type II string theory corresponding to Riemann surfaces with one boundary are calculated in several noncompact geometries of desingularized orbifolds. One of these models has a complicated phase structure which is explored. A general condition for integrality of the numerical invariants is discussed.
Regimes of the Pomeron and its intrinsic entropy

Abstract We suggest that the perturbative and non-perturbative descriptions of the Pomeron can be viewed as complementary descriptions of different phases in the Pomeron phase diagram, with a phase boundary where the proper description of the produced systems is “string balls”. Their intrinsic entropy is calculated and turned out to be the same, as the recently reported perturbative entanglement entropy. The distribution of large multiplicities stemming from the string balls is also wide, with its moments close to those reported for hadrons in p p collisions at the LHC. At low-x, the quantum string is so entangled that sufficiently weak string self-attraction can cause it to turn to a string ball dual to a black hole. We suggest that low-x saturation occurs when the density of wee-strings reaches the Bekenstein bound, with a proton size that freezes with increasing rapidity. Some of these observations maybe checked at the future eIC.
Resolving the Image of Gamma‐Ray Burst Afterglows with Gravitational Microlensing

Microlensing of a gamma-ray burst afterglow by an intervening star can be used to infer the radial structure of the afterglow image. Near the peak of the microlensing event, the outer edge of the image is more highly magnified than its central region, whereas the situation is reversed at later times because of the rapid radial expansion of the image on the sky. Thus, the microlensed afterglow light curve can be inverted to recover the self-similar radial intensity profile of the afterglow image. We calculate the expected errors in the recovered intensity profile as a function of the number of resolution elements, under the assumption that the afterglow and microlensing event parameters are known. For a point-mass lens and uniform source, we derive a simple scaling relation between these parameters and the resultant errors. We find that the afterglow need not be monitored for its entire duration; rather, observations from the peak magnification time tpeak of the microlensing event until ~7tpeak are sufficient to resolve the majority of the afterglow image. Thus, microlensing events can be alerted by relatively infrequent observations of afterglows and then monitored intensively, without significant loss of information about the afterglow intensity profile. The relative intensity profile of ~1% of all afterglows can be measured with 10 resolution elements to an accuracy of (1%) in the optical and (10%) in the infrared, using 4 m class telescopes. Weak microlensing events with large impact parameters are more common; we estimate that for ~10% of afterglows the image profile may be inverted to a fractional accuracy 20% through frequent optical observations. We also calculate the effects of external shear due to the host galaxy or a binary companion, as well as contamination by background light from the host galaxy.
Lithium Subhydrides under Pressure and Their Superatom-like Building Blocks

Evolutionary structure searches are used to predict a new class of compounds in the lithium--rich region of the lithium/hydrogen phase diagram under pressure. First principles computations show that LimH, 4<m<9, are stabilized with respect to LiH and Li between 50-100 GPa. The building block of all of the lithium subhydrides is an Li8H cluster, which can be thought of as a superalkali. The geometries and electronic structures of these phases is analogous to that of the well-known alkali metal suboxides.
Interpretable subgroup discovery in treatment effect estimation with application to opioid prescribing guidelines

The dearth of prescribing guidelines for physicians is one key driver of the current opioid epidemic in the United States. In this work, we analyze medical and pharmaceutical claims data to draw insights on characteristics of patients who are more prone to adverse outcomes after an initial synthetic opioid prescription. Toward this end, we propose a generative model that allows discovery from observational data of subgroups that demonstrate an enhanced or diminished causal effect due to treatment. Our approach models these sub-populations as a mixture distribution, using sparsity to enhance interpretability, while jointly learning nonlinear predictors of the potential outcomes to better adjust for confounding. The approach leads to human interpretable insights on discovered subgroups, improving the practical utility for decision support.
INTERSTELLAR SODIUM AND CALCIUM ABSORPTION TOWARD SN 2011dh IN M51

We present high-resolution echelle observations of SN 2011dh, which exploded in the nearby, nearly face-on spiral galaxy M51. Our data, acquired on three nights when the supernova was near maximum brightness, reveal multiple absorption components in Na I D and Ca II H and K, which we identify with gaseous material in the Galactic disk or low halo and in the disk and halo of M51. The M51 components span a velocity range of over 140 km s–1, extending well beyond the range exhibited by H I 21 cm emission at the position of the supernova. Since none of the prominent Na I or Ca II components appear to coincide with the peak in H I emission, the supernova may lie just in front of the bulk of the H I disk. The Na I/Ca II ratios for the components with the most extreme positive and negative velocities relative to the disk are ~1.0, similar to those for more quiescent components, suggesting that the absorption originates in relatively cool gas. Production scenarios involving a galactic fountain and/or tidal interactions between M51 and its companion would be consistent with these results. The overall weakness of Na I D absorption in the direction of SN 2011dh confirms a low foreground and host galaxy extinction for the supernova.
Higher melonic theories

We classify a large set of melonic theories with arbitrary q-fold interactions, demonstrating that the interaction vertices exhibit a range of symmetries, always of the form ℤ 2 for some n, which may be 0. The number of different theories proliferates quickly as q increases above 8 and is related to the problem of counting one-factorizations of complete graphs. The symmetries of the interaction vertex lead to an effective interaction strength that enters into the Schwinger-Dyson equation for the two-point function as well as the kernel used for constructing higher-point functions.
Evaluation and selection of models for out-of-sample prediction when the sample size is small relative to the complexity of the data-generating process

In regression with random design, we study the problem of selecting a model that performs well for outof-sample prediction. We do not assume that any of the candidate models under consideration are correct. Our analysis is based on explicit finite-sample results. Our main findings differ from those of other analyses that are based on traditional large-sample limit approximations because we consider a situation where the sample size is small relative to the complexity of the data-generating process, in the sense that the number of parameters in a ‘good’ model is of the same order as sample size. Also, we allow for the case where the number of candidate models is (much) larger than sample size.
Time-dependent Electron Acceleration in Blazar Transients: X-Ray Time Lags and Spectral Formation

Electromagnetic radiation from blazar jets often displays strong variability, extending from radio to $\gamma$-ray frequencies. In a few cases, this variability has been characterized using Fourier time lags, such as those detected in the X-rays from Mrk~421 using BeppoSAX. The lack of a theoretical framework to interpret the data has motivated us to develop a new model for the formation of the X-ray spectrum and the time lags in blazar jets based on a transport equation including terms describing stochastic Fermi acceleration, synchrotron losses, shock acceleration, adiabatic expansion, and spatial diffusion. We derive the exact solution for the Fourier transform of the electron distribution, and use it to compute the Fourier transform of the synchrotron radiation spectrum and the associated X-ray time lags. The same theoretical framework is also used to compute the peak flare X-ray spectrum, assuming that a steady-state electron distribution is achieved during the peak of the flare. The model parameters are constrained by comparing the theoretical predictions with the observational data for Mrk~421. The resulting integrated model yields, for the first time, a complete first-principles physical explanation for both the formation of the observed time lags and the shape of the peak flare X-ray spectrum. It also yields direct estimates of the strength of the shock and the stochastic MHD wave acceleration components in the Mrk~421 jet.
Extending the Thermal Near Field Through Compensation in Hyperbolic Waveguides

A promising method to leverage near-field power densities without the use of nanoscale vacuum gaps is through hyperbolic metamaterial (HMM) waveguides. When placed between a hot and cold reservoir, an ideal HMM can transmit surface waves across several microns, enabling an extension of near-field enhancements. However, when accounting for transmission loss due to realistic levels of absorption within the waveguide, previous studies have shown that the enhancements are significantly curtailed at wide separations. In our study, we investigate the role of internal sources within nonisothermal HMMs. We demonstrate that, in some cases, the emission from the HMM accounts for over 90% of the total radiative heat transfer to the receiver, and that these additional sources can largely compensate for optical losses associated with decreased transmission from the emitter to the receiver. Lastly, we investigate the spectral transport in a realistic three-body system that has mismatching optical properties between the boundaries (emitter and receiver) and the waveguide (HMM). Our model shows that the near-field thermal transport remains spectrally selective to the boundaries, even as major radiative contributions come from the waveguide. This work may enable the design of nonisothermal emitter-waveguide-receiver systems that transmit near-field power levels over wider separations.
SMOOTHED ESTIMATING EQUATIONS FOR INSTRUMENTAL VARIABLES QUANTILE REGRESSION

The moment conditions or estimating equations for instrumental vari- ables quantile regression involves the discontinuous indicator function. We instead use smoothed estimating equations, with bandwidthh. This is known to allow higher- order expansions that justify bootstrap renements for inference. Computation of the estimator also becomes simpler and more reliable, especially with (more) endogenous regressors. We show that the mean squared error of the vector of estimating equa- tions is minimized for some h > 0, which also reduces the mean squared error of the parameter estimators. The same h also minimizes higher-order type I error for a 2 test, leading to improved size-adjusted power. Our plug-in bandwidth consistently reproduces all of these properties in simulations.
Correlations far from equilibrium in charged strongly coupled fluids subjected to a strong magnetic field

Within a holographic model, we calculate the time evolution of 2-point and 1-point correlation functions (of selected operators) within a charged strongly coupled system of many particles. That system is thermalizing from an anisotropic initial charged state far from equilibrium towards equilibrium while subjected to a constant external magnetic field. One main result is that thermalization times for 2-point functions are significantly (approximately three times) larger than those of 1-point functions. Magnetic field and charge amplify this difference, generally increasing thermalization times. However, there is also a competition of scales between charge density, magnetic field, and initial anisotropy, which leads to an array of qualitative changes on the 2- and 1-point functions. There appears to be a strong effect of the medium on 2-point functions at early times, but approximately none at later times. At strong magnetic fields, an apparently universal thermalization time emerges, at which all 2-point functions appear to thermalize regardless of any other scale in the system. Hence, this time scale is referred to as saturation time scale. As extremality is approached in the purely charged case, 2- and 1-point functions appear to equilibrate at infinitely late time. We also compute 2-point functions of charged operators. Our results can be taken to model thermalization in heavy ion collisions, or thermalization in selected condensed matter systems.
Bar Instability in Disk-Halo Systems

We show that the exponential growth rate of a bar in a stellar disk is substantially greater when the disk is embedded in a live halo than in a rigid one having the same mass distribution. We also find that the vigor of the instability in disk-halo systems varies with the shape of the halo velocity ellipsoid. Disks in rigid halos that are massive enough to be stable by the usual criteria, quickly form bars in isotropic halos and much greater halo mass is needed to avoid a strong bar; thus stability criteria derived for disks in rigid halos do not apply when the halo is responsive. The study presented here is of an idealized family of models with near uniform central rotation and that lack an extended halo; we present more realistic models with extended halos in a companion paper. The puzzle presented by the absence of strong bars in some galaxies having gently rising inner rotation curves is compounded by the results presented here.
All-optical spin switching: A new frontier in femtomagnetism — A short review and a simple theory

Using an ultrafast laser pulse to manipulate the spin degree of freedom has broad technological appeal. It allows one to control the spin dynamics on a femtosecond time scale. The discipline, commonly called femtomagnetism, started with the pioneering experiment by Beaurepaire and coworkers in 1996, who showed subpicosecond demagnetization occurs in magnetic Ni thin films. This finding has motivated extensive research worldwide. All-optical helicity-dependent spin switching (AO-HDS) represents a new frontier in femtomagnetism, where a single ultrafast laser pulse can permanently switch spin without any assistance from a magnetic field. This review summarizes some of the crucial aspects of this new discipline: key experimental findings, leading mechanisms, controversial issues, and possible future directions. The emphasis is on our latest investigation. We first develop the all-optical spin switching (AOS) rule that determines how the switchability depends on the light helicity. This rule allows one to understand microscopically how the spin is reversed and why the circularly polarized light appears more powerful than the linearly polarized light. Then we invoke our latest spin-orbit coupled harmonic oscillator model to simulate single spin reversal. We consider both continuous wave (cw) excitation and pulsed laser excitation. The results are in a good agreement with the experimental result (a MatLab code is available upon request from the author). We then extend the code to include the exchange interaction among different spin sites. We show where the “inverse-Faraday field” comes from and how the laser affects the spin reversal nonlinearly. Our hope is that this review will motivate new experimental and theoretical investigations and discussions.
VERSION 1 OF THE HUBBLE SOURCE CATALOG

The Hubble Source Catalog is designed to help optimize science from the Hubble Space Telescope by combining the tens of thousands of visit-based source lists in the Hubble Legacy Archive into a single master catalog. Version 1 of the Hubble Source Catalog includes WFPC2, ACS/WFC, WFC3/UVIS, and WFC3/IR photometric data generated using SExtractor software to produce the individual source lists. The catalog includes roughly 80 million detections of 30 million objects involving 112 different detector/filter combinations, and about 160 thousand HST exposures. Source lists from Data Release 8 of the Hubble Legacy Archive are matched using an algorithm developed by Budavari & Lubow (2012). The mean photometric accuracy for the catalog as a whole is better than 0.10 mag, with relative accuracy as good as 0.02 mag in certain circumstances (e.g., bright isolated stars). The relative astrometric residuals are typically within 10 mas, with a value for the mode (i.e., most common value) of 2.3 mas. The absolute astrometric accuracy is better than $$\sim$$0.1 arcsec for most sources, but can be much larger for a fraction of fields that could not be matched to the PanSTARRS, SDSS, or 2MASS reference systems. In this paper we describe the database design with emphasis on those aspects that enable the users to fully exploit the catalog while avoiding common misunderstandings and potential pitfalls. Here, we provide usage examples to illustrate some of the science capabilities and data quality characteristics, and briefly discuss plans for future improvements to the Hubble Source Catalog.
The Fraction of Stars That Form in Clusters in Different Galaxies

We estimate the fraction of stars that form in compact clusters (bound and unbound), , in a diverse sample of eight star-forming galaxies, including two irregulars, two dwarf starbursts, two spirals, and two mergers. The average value for our sample is . We also calculate the fraction of stars in clusters that survive to ages between and , denoted by , and find and , significantly lower than for the same galaxies. We do not find any systematic trends in or with the star formation rate (SFR), the SFR per unit area (), or the surface density of molecular gas () within the host galaxy. Our results are consistent with those found previously from the CMF/SFR statistic (where CMF is the cluster mass function), and with the quasi-universal model in which clusters in different galaxies form and disrupt in similar ways. Our results, however, contradict many previous claims that the fraction of stars in bound clusters increases strongly with and . We find that the previously reported trends are largely driven by comparisons that mixed and , where was systematically used for galaxies with higher and , and for galaxies with lower and .
Helioseismic Investigation of Modeled and Observed Supergranule Structure

The subsurface structure of an “average” supergranule is derived using existing data products from the Helioseismic and Magnetic Imager (HMI) time–distance pipeline and compared to the best helioseismic flow model detailed by Duvall and Hanasoge (Solar Phys. 287, 71, 2013). We find that significant differences exist between them. Unlike the shallow structure predicted by the model, the average HMI supergranule is very extended in depth, exhibiting horizontal outflow down to 7 – 10 Mm, followed by a weak inflow reaching a depth of ≈ 20 Mm below the photosphere. The maximal velocities in the horizontal direction for the average supergranule are much lower than in the model, and its near-surface flow field RMS value is about an order of magnitude lower than the often-quoted values of ≈ 250 – 350 m s−1 for supergranulation. Much of the overall HMI supergranule structure and its weak flow amplitudes can be explained by examining the HMI pipeline averaging kernels for the near-surface inversions, which are found to be very broad in depth, and nearly identical to one another in terms of sensitivity along the z-direction. We also show that forward-modeled travel times in the Born approximation using the model (derived from a ray-theory approach) are inconsistent with measured travel times for an average supergranule at any distance. Our findings suggest systematic inaccuracies in the typical techniques used to study supergranulation, confirming some of the results of Duvall and Hanasoge (Solar Phys. 287, 71, 2013).
Development of tearing instability in a current sheet forming by sheared incompressible flow

National Science Foundation (U.S.). Graduate Research Fellowship Program (grant no. DGE-1122374)
The Dwarf Nova Outbursts of Nova Her 1960 (=V446 Her)

V446 Her is the best example of an old nova which has developed dwarf nova (DN) eruptions in the post-nova state. We report on observed properties of the long-term light curve of V446 Her, using photometry over 19 years. Yearly averages of the outburst (OB) magnitudes show a decline of {approx}0.013 mag yr{sup -1}, consistent with the decline of other post-novae that do not have DN OBs. Previous suggestions of bimodal distributions of the amplitudes and widths of the OBs are confirmed. The OBs occur at a mean spacing of 18 days but the range of spacings is large (13-30 days). From simulations of DN OBs, it has been predicted that the OB spacing in V446 Her will increase as M-dot from the red dwarf companion slowly falls following the nova; however, the large intrinsic scatter in the spacings serves to hide any evidence of this effect. We do find a systematic change in the OB pattern in which the brighter, wider type of OBs disappeared after late 2003, and this phenomenon is suggested to be due to falling M-dot following the nova.
Prediction of Li intercalation voltages in rechargeable battery cathode materials: Effects of exchange-correlation functional, van der Waals interactions, and Hubbard U

Quantitative predictions of the Li intercalation voltage and of the electronic properties of rechargeable battery cathode materials are a substantial challenge for first-principles theory due to the possibility of (1) strong correlations associated with localized transition metal $d$ electrons and (2) significant van der Waals (vdW) interactions in layered systems, both of which are not accurately captured by standard approximations to density functional theory (DFT). Here, we perform a systematic benchmark of electronic structure methods based on the widely used generalized-gradient approximation of Perdew, Burke, and Ernzerhof (PBE) and the new strongly constrained and appropriately normed (SCAN) meta-generalized-gradient approximation for battery cathode materials. Studying layered ${\mathrm{Li}}_{x}{\mathrm{TiS}}_{2}, {\mathrm{Li}}_{x}{\mathrm{NiO}}_{2}$, and ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$, olivine ${\mathrm{Li}}_{x}{\mathrm{FePO}}_{4}$, and spinel ${\mathrm{Li}}_{x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{4}$, we compute the voltage, crystal structure, and electronic structure with and without extensions to incorporate onsite Hubbard interactions and vdW interactions. Within pure DFT (i.e., without corrections for onsite Hubbard interactions), SCAN is a significant improvement over PBE for describing cathode materials, decreasing the mean absolute voltage error by more than 50%. Although explicit vdW interactions are not critical and in cases even detrimental when applied in conjunction with SCAN, Hubbard-$U$ corrections are still in general necessary to achieve reasonable agreement with experiment. We show that no single method considered here can accurately describe the voltage and overall structural, electronic, and magnetic properties (i.e., errors no more than 5% for voltage, volume, band gap, and magnetic moments) of battery cathode materials, motivating a strong need for improved electronic structure approaches for such systems.
Trading drift and fluctuations in entropic dynamics: quantum dynamics as an emergent universality class

Entropic Dynamics (ED) is a framework that allows the formulation of dynamical theories as an application of entropic methods of inference. In the generic application of ED to derive the Schrodinger equation for N particles the dynamics is a non-dissipative diffusion in which the system follows a "Brownian" trajectory with fluctuations superposed on a smooth drift. We show that there is a family of ED models that differ at the "microscopic" or sub-quantum level in that one can enhance or suppress the fluctuations relative to the drift. Nevertheless, members of this family belong to the same universality class in that they all lead to the same emergent Schrodinger behavior at the "macroscopic" or quantum level. The model in which fluctuations are totally suppressed is of particular interest: the system evolves along the smooth lines of probability flow. Thus ED includes the Bohmian or causal form of quantum mechanics as a special limiting case. We briefly explore a different universality class - a nondissipative dynamics with microscopic fluctuations but no quantum potential. The Bohmian limit of these hybrid models is equivalent to classical mechanics. Finally we show that the Heisenberg uncertainty relation is unaffected either by enhancing or suppressing microscopic fluctuations or by switching off the quantum potential.
Apodized vortex coronagraph designs for segmented aperture telescopes

Current state-of-the-art high contrast imaging instruments take advantage of a number of elegant coronagraph designs to suppress starlight and image nearby faint objects, such as exoplanets and circumstellar disks. The ideal performance and complexity of the optical systems depends strongly on the shape of the telescope aperture. Unfortunately, large primary mirrors tend to be segmented and have various obstructions, which limit the performance of most conventional coronagraph designs. We present a new family of vortex coronagraphs with numerically-optimized gray-scale apodizers that provide the sensitivity needed to directly image faint exoplanets with large, segmented aperture telescopes, including the Thirty Meter Telescope (TMT) as well as potential next-generation space telescopes.
Cluster structures in Schubert varieties in the Grassmannian

In this article we explain how the coordinate ring of each (open) Schubert variety in the Grassmannian can be identified with a cluster algebra, whose combinatorial structure is encoded using (target labelings of) Postnikov's plabic graphs. This result generalizes a theorem of Scott (Scott 2006) for the Grassmannian, and proves a folklore conjecture for Schubert varieties that has been believed by experts since (Scott 2006), though the statement was not formally written down until Muller-Speyer explicitly conjectured it (Muller-Speyer 2016). To prove this conjecture we use a result of Leclerc (Leclerc 2016), who used the module category of the preprojective algebra to prove that coordinate rings of many Richardson varieties in the complete flag variety can be identified with cluster algebras. Our proof also uses a construction of Karpman (Karpman 2016) to build plabic graphs associated to reduced expressions. We additionally generalize our result to the setting of skew Schubert varieties; the latter result uses generalized plabic graphs, i.e. plabic graphs whose boundary vertices need not be labeled in cyclic order.
Anisotropic swim stress in active matter with nematic order

Active Brownian particles (ABPs) transmit a swim pressure to the container boundaries, where ζ is the drag coefficient, D swim is the swim diffusivity and n is the uniform bulk number density far from the container walls. In this work we extend the notion of the isotropic swim pressure to the anisotropic tensorial swim stress , which is related to the anisotropic swim diffusivity . We demonstrate this relationship with ABPs that achieve nematic orientational order via a bulk external field. The anisotropic swim stress is obtained analytically for dilute ABPs in both 2D and 3D systems. The anisotropy, defined as the ratio of the maximum to the minimum of the three principal stresses, is shown to grow exponentially with the strength of the external field. We verify that the normal component of the anisotropic swim stress applies a pressure on a wall with normal vector , and, through Brownian dynamics simulations, this pressure is shown to be the force per unit area transmitted by the active particles. Since ABPs have no friction with a wall, the difference between the normal and tangential stress components—the normal stress difference—generates a net flow of ABPs along the wall, which is a generic property of active matter systems.
The delicate dance of orbital rendezvous

The meeting of two spacecraft in orbit around a planet or moon involves a delicate dance that must carefully balance the gravitational, Coriolis, and centrifugal forces acting on the spacecraft. The intricacy of the relative motion between the two spacecraft caused problems for the Gemini missions in the mid-1960s. Although now mastered, the problem of how to bring two orbiting objects together continues to be misrepresented in popular movies and books. In this article, I will consider the case when the two spacecraft are in close proximity (compared with the radii of their orbits), and examine the counter-intuitive trajectories that are needed to bring them together. I will examine how a stranded astronaut might use an impulsive force to return to her ship in Earth orbit, how and when line-of-sight targeting may be used for a rendezvous, and how the Apollo 11 lunar module executed a Terminal Phase Initiation maneuver to rendezvous with the command/service module as they both circled the Moon.
An unstable change of rings for Morava E–theory

The Bousfield–Kan (or unstable Adams) spectral sequence can be constructed for various homology theories, such as Brown–Peterson homology theory BP, Johnson–Wilson theory E(n) or Morava E–theory En. For nice spaces the E2–term is given by Ext in a category of unstable comodules. We establish an unstable Morava change of rings isomorphism between Ext𝒰ΓB(B,M) and Ext𝒰En∗En∕In(En∗∕In,En∗⊗BP∗M), where (B,ΓB) denotes the Hopf algebroid (vn−1BP∗∕In,vn−1BP∗BP∕In). We show that the latter groups can be interpreted as Ext in the category of continuous modules over the profinite monoid of endomorphisms of the Honda formal group law. By comparing this with the cohomology of the Morava stabilizer group we obtain an unstable Morava vanishing theorem when p−1∤n.
Parking functions and tree inversions revisited

Kreweras proved that the reversed sum enumerator for parking functions of length n is equal to the inversion enumerator for labeled trees on n + 1 vertices. Recently, Perkinson, Yang, and Yu gave a bijective proof of this equality that moreover generalizes to graphical parking functions. Using a depth-first search variant of Dhar's burning algorithm they proved that the reversed sum enumerator for G-parking functions equals the ź-number enumerator for spanning trees of G. The ź-number is a kind of generalized tree inversion number originally defined by Gessel. We extend the work of Perkinson-Yang-Yu to what are referred to as "generalized parking functions" in the literature, but which we prefer to call vector parking functions because they depend on a choice of vector x ź N n . Specifically, we give an expression for the reversed sum enumerator for x-parking functions in terms of inversions in rooted plane trees with respect to certain admissible vertex orders. Along the way we clarify the relationship between graphical and vector parking functions.