ShantanuT01/DACTYL-Pretraining huggingface dataset:
Dirac fermions are at the forefront of modern condensed matter physics research. They are known to occur in materials as diverse as graphene, topological insulators, and transition metal dichalcogenides, while closely related Weyl fermions have been discovered in other materials. They have been predicted to lend themselves to a variety of technological applications, while the recent prediction and discovery of the quantized anomalous Hall effect of massive Dirac fermions is regarded as a potential gateway towards low-energy electronics. Some materials hosting Dirac fermions are natural platforms for interlayer coherence effects such as Coulomb drag and exciton condensation. The top and bottom surfaces of a thin topological insulator film provide such a prototype system. Here we describe recent insights into Coulomb drag between two layers of Dirac fermions relying primarily on topological insulator films as a minimal model. We consider both non-magnetic topological insulators, hosting massless Dirac fermions, and magnetic topological insulators, in which the fermions are massive. We discuss in general terms the dynamics of the thin-film spin density matrix, outlining numerical results and approximate analytical expressions where appropriate for the drag resistivity \r{ho}D at low temperatures and low electron densities. In magnetic topological insulators with out-of-plane magnetizations in both the active and passive layers we analyze the role of the anomalous Hall effect in Coulomb drag.
We calculate the nucleon parameters in isospin asymmetric nuclear matter using the QCD sum rules. The nucleon self-energies are expressed in terms of the in-medium values of QCD condensates. The simple approximate expressions for the self-energies are obtained in terms of these condensates. Relation between successive inclusion of the condensates and the meson-exchange picture of the nucleon interaction with medium is analyzed. The values of the self-energies and of the symmetry energy agree with those obtained by the methods of nuclear physics.
We place firm upper limits on the global accretion history of massive black holes at z>5 from the recently measured unresolved fraction of the cosmic X-ray background. The maximum allowed unresolved intensity observed at 1.5 keV implies a maximum accreted-mass density onto massive black holes of rho_acc < 1.4E4 M_sun Mpc^{-3} for z>5. Considering the contribution of lower-z AGNs, the value reduces to rho_acc < 0.66E4 M_sun Mpc^{-3}. The tension between the need for the efficient and rapid accretion required by the observation of massive black holes already in place at z>7 and the strict upper limit on the accreted mass derived from the X-ray background may indicate that black holes are rare in high redshift galaxies, or that accretion is efficient only for black holes hosted by rare galaxies.
In recent years, the Extremely Low Mass White Dwarf (ELM WD) survey has quintupled the number of known close, detached double WD binaries (DWD). The tightest such DWD, SDSS J065133.33+284423.3 (J0651), harbors a He WD eclipsing a C/O WD every $\simeq\,12\,$min. The orbital decay of this source was recently measured to be consistent with general relativistic (GR) radiation. Here we investigate the role of dynamic tides in a J0651-Like binary and we uncover the potentially new phenomenon of "antiresonance" locking. In the most probable scenario of an asynchronous binary at birth, we find that dynamic tides play a key role in explaining the measured GR-driven orbital decay, as they lock the system at stable antiresonances with the star's eigenfrequencies. We show how such locking is naturally achieved and how, while locked at an antiresonance, GR drives the evolution of the orbital separation, while dynamic tides act to synchronize the spin of the He WD with the companion's orbital motion, but \emph{only on the GR timescale}. Given the relevant orbital and spin evolution timescales, the system is clearly on its way to synchronism, if not already synchronized.
Why and how may some active regions (ARs) frequently produce coronal mass ejections (CMEs)? It is one of the key questions to deepen our understanding of the mechanisms and processes of energy accumulation and sudden release in ARs and to improve our capability of space weather prediction. Although some case studies have been made, the question is still far from fully answered. This issue is now being tried to address statistically through an investigation of waiting times of quasi-homologous CMEs from super ARs in solar cycle 23. It is found that the waiting times of quasi-homologous CMEs have a two-component distribution with a separation at about 18 hours. The first component is a Gaussian-like distribution with a peak at about 7 hours, which indicates a tight physical connection between these quasi-homologous CMEs. The likelihood of occurrences of two or more CMEs faster than 1200 km s-1 from the same AR within 18 hours is about 20%. Furthermore, the correlation analysis among CME waiting times, CME speeds and CME occurrence rates reveals that these quantities are independent to each other, suggesting that the perturbation by preceding CMEs rather than free energy input be the direct cause of quasi-homologous CMEs. The peak waiting time of 7 hours probably characterize the time scale of the growth of instabilities triggered by preceding CMEs. This study uncovers more clues from a statistical perspective for us to understand quasi-homologous CMEs as well as CME-rich ARs.
Suppose we are given an infinite, finitely generated group $G$ and a transient random walk on the wreath product $(\mathbb{Z}/ 2\mathbb{Z})\wr G$, such that its projection on $G$ is transient and has finite first moment. This random walk can be interpreted as a lamplighter random walk on $G$. Our aim is to show that the random walk on the wreath product escapes to infinity with respect to a suitable (pseudo-)metric faster than its projection onto $G$. We also address the case where the pseudo-metric is the length of a shortest ``travelling salesman tour''. In this context, and excluding some degenerate cases if $G=\mathbb{Z}$, the linear rate of escape is strictly bigger than the rate of escape of the lamplighter random walk's projection on $G$.
The mysterious attractive constant radial force acted in the past on Pioneer spacecrafts - the so-called Pioneer anomaly - is considered within the context of Rindler acceleration. As an idea this is tempting since it is reminiscent of the cosmological constant. Fortunately the anomalous force acts radially toward the sun so that it differs from the mission of a cosmological constant. Without resorting to the physical source responsible for such a term we investigate the modified Schwarzschild geodesics. The Rindler acceleration naturally affects all massive / massless particle orbits. Stable orbits may turn unstable and vice versa with a finely-tuned acceleration parameter. The overall role of the extra term, given its attractive feature is to provide confinement in the radial geodesics.
We show that $k$-th iterated accumulation points of pseudo-effective thresholds of $n$-dimensional varieties are bounded by $n-k+1$.
We consider the unique determinations of impenetrable obstacles or diffraction grating profiles in $\mathbb{R}^3$ by a single far-field measurement within polyhedral geometries. We are particularly interested in the case that the scattering objects are of impedance type. We derive two new unique identifiability results for the inverse scattering problem in the aforementioned two challenging setups. The main technical idea is to exploit certain quantitative geometric properties of the Laplacian eigenfunctions which were initiated in our recent works [8,9]. In this paper, we derive novel geometric properties that generalize and extend the related results in [9], which further enable us to establish the new unique identifiability results. It is pointed out that in addition to the shape of the obstacle or the grating profile, we can simultaneously recover the boundary impedance parameters.
A Born-Green-Yvon type model for adatom density correlations is combined with a model for adatom interactions mediated by the strain in elastic anisotropic substrates. The resulting nonlinear integral equation is solved numerically for coverages from zero to a limit given by stability constraints. W, Nb, Ta and Au surfaces are taken as examples to show the effects of different elastic anisotropy regions. Results of the calculation are shown by appropriate plots and discussed. A mapping to superstructures is tried. Corresponding adatom configurations from Monte Carlo simulations are shown.
We have compiled a significantly updated and comprehensive census of massive stars in the nearby Cygnus OB2 association by gathering and homogenising data from across the literature. The census contains 169 primary OB stars, including 52 O-type stars and 3 Wolf-Rayet stars. Spectral types and photometry are used to place the stars in a Hertzprung-Russell diagram, which is compared to both non-rotating and rotating stellar evolution models, from which stellar masses and ages are calculated. The star formation history and mass function of the association are assessed, and both are found to be heavily influenced by the evolution of the most massive stars to their end states. We find that the mass function of the most massive stars is consistent with a `universal' power-law slope of Gamma = 1.3. The age distribution inferred from stellar evolutionary models with rotation and the mass function suggest the majority of star formation occurred more or less continuously between 1 and 7 Myr ago, in agreement with studies of low- and intermediate mass stars in the association. We identify a nearby young pulsar and runaway O-type star that may have originated in Cyg OB2 and suggest that the association has already seen its first supernova. Finally we use the census and mass function to calculate the total mass of the association of 16500^+3800_-2800 Msun, at the low end, but consistent with, previous estimates of the total mass of Cyg OB2. Despite this Cyg OB2 is still one of the most massive groups of young stars known in our Galaxy making it a prime target for studies of star formation on the largest scales.
We present a code for solving the nonlinear force-free equations in spherical polar geometry, with the motivation of modeling the magnetic field in the corona. The code is an implementation of the Grad-Rubin method. Our method is applicable to a spherical domain of arbitrary angular size. The implementation is based on a global spectral representation for the magnetic field which makes no explicit assumptions about the form of the magnetic field at the transverse boundaries of the domain. We apply the code to a bipolar test case with analytic boundary conditions, and we demonstrate the convergence of the Grad-Rubin method, and the self-consistency of the resulting numerical solution.
The Wiman-Edge pencil is a pencil of genus $6$ curves for which the generic member has automorphism group the alternating group $\mathfrak{A}_5$. There is a unique smooth member, the Wiman sextic, with automorphism group the symmetric group $\mathfrak{S}_5$. Farb and Looijenga proved that the monodromy of the Wiman-Edge pencil is commensurable with the Hilbert modular group $\mathrm{SL}_2(\mathbb{Z}[\sqrt{5}])$. In this note, we give a complete description of the monodromy by congruence conditions modulo $4$ and $5$. The congruence condition modulo $4$ is new, and this answers a question of Farb-Looijenga. We also show that the smooth resolution of the Baily-Borel compactification of the locally symmetric manifold associated with the monodromy is a projective surface of general type. Lastly, we give new information about the image of the period map for the pencil.
Spectral monitoring of the yellow hypergiant $\rho$ Cas with the by 6-m telescope of the Special Astrophysical Observatory with a spectral resolution of R$\ge$60000 has led to the detection of new features in the kinematic state of its extended atmosphere following the ejection of matter in 2013. Significant changes in the profile of the H$\alpha$ line were detected: the line had a doubled core for the first time in a 2014 spectrum, an inverse P Cygni profile on December 13, 2017, and the profile was again doubled on August 6, 2017 and September 5, 2017, but its core was strongly shifted toward longer wavelengths, indicating a rapid infall of matter. Splitting of the profiles of strong, low-excitation absorptions into three components was first detected in 2017. There is no correlation between the evolution of the profiles of H$\alpha$ and the splitted absorptions. Pulsation-like variability with an amplitude of about 10 km/s is characteristic only of symmetric weak and moderate-intensity absorption lines. Shell emissions of iron-group elements can be identified in the long-wavelength part of a spectrum obtained in 2013, whose intensity decreased until they completely disappeared in 2017. In the absence of emission in the cores of the H and K lines of CaII, emissions of shell metals are visible in the wings of these lines.
Using the diagram technique for Matsubara Green's functions it is shown that the dynamics of the localized spin subsystem in the s-d(f) model can be described in terms of an effective spin model with multi-site spin-spin interactions. An exact representation of the action for the effective purely spin model is derived as an infinite series in powers of s-d(f) exchange interaction $J$. The indirect interactions of the 2-nd, 3-rd and 4-th order are discussed.
We present the statistics of the ratio, ${\mathrm R}$, between the prompt and afterglow "plateau" fluxes of GRB. This we define as the ratio between the mean prompt energy flux in the {\em Swift} BAT and the {\em Swift} XRT, immediately following the steep transition between these two states and the beginning of the afterglow stage referred to as the "plateau". Like the distribution of other GRB observables, the histogram of ${\mathrm R}$ is close to log-normal, with maximum at ${\mathrm R = R}_{\rm m} \simeq 2,000$, FWHM of about 2 decades and with the entire distribution spanning about 6 decades in the value of ${\mathrm R}$. We note that the peak of the distribution is close to the proton-to-electron mass ratio $({\mathrm R}_{\rm m} \simeq m_p/m_e = 1836)$, as proposed by us earlier, on the basis of a specific model for the conversion of the GRB blast wave kinetic energy into radiation, before any similar analysis were made. It therefore appears that, in addition to the values of the energy of peak luminosity ${E_{\rm pk}\sim m_{e} c^2}$, GRB present us with one more quantity with an apparently characteristic value. The fact that the values of both these quantities (i.e. $E_{\rm pk}$ and ${\mathrm R}$) comply with those implied by the same specific model devised to account for an altogether different issue, namely the efficient conversion of the GRB blast wave kinetic energy into radiation, argues favorably for its underlying assumptions.
Brain age prediction based on neuroimaging data could help characterize both the typical brain development and neuropsychiatric disorders. Pattern recognition models built upon functional connectivity (FC) measures derived from resting state fMRI (rsfMRI) data have been successfully used to predict the brain age. However, most existing studies focus on coarse-grained FC measures between brain regions or intrinsic connectivity networks (ICNs), which may sacrifice fine-grained FC information of the rsfMRI data. Whole brain voxel-wise FC measures could provide fine-grained FC information of the brain and may improve the prediction performance. In this study, we develop a deep learning method to use convolutional neural networks (CNNs) to learn informative features from the fine-grained whole brain FC measures for the brain age prediction. Experimental results on a large dataset of resting-state fMRI demonstrate that the deep learning model with fine-grained FC measures could better predict the brain age.
We have measured the magnetic field (H<90 kOe) and pressure (P<10 kbar) dependence of the magnetic ordering temperature, Tmag, in single crystal samples of NaxCoO2 for a range of Na concentrations (0.60<x<0.72). We show that in zero field, Tmag remains constant with decreasing x before magnetic order disappears at x=0.65. Heat capacity and magnetization data show that for x=0.70, Tmag is unchanged in an applied field. In contrast, magnetization data collected under hydrostatic pressure show that Tmag increases from 22.0 K at 1 bar to 25.4 K at 10 kbar. This rise is at odds with the behaviour expected for model spin density wave systems.
We analyze VLT/X-shooter data for 7 quasars, where we study the relationships between their broad absorption line (BAL) and emission line outflows. We find: 1) the luminosity of the [OIII] $\lambda$5007 emission profile decreases with increasing electron number density (n$_e$) derived from the BAL outflow in the same quasar, 2) the measured velocity widths from the [OIII] emission features and CIV absorption troughs in the same object are similar, and 3) the mean radial velocity derived from the BAL outflow is moderately larger than the one from the [OIII] emission outflow. These findings can be explained by the physical interpretation that the [OIII] and BAL outflow are different manifestations of the same wind. When we have outflows with smaller distances to the central source, their n$_e$ is higher. Therefore, the [OIII] emission is collisionally de-excited and the [OIII] luminosity is then suppressed. Comparisons to previous studies show that the objects in our sample exhibit broad [OIII] emission features similar to the ones in extremely red quasars (ERQs). This might imply that BAL quasars and ERQs have the same geometry of outflows or are at a similar evolutionary stage. We found that the physical parameters derived from the BAL outflows can explain the amount of observed [OIII] luminosity, which strengthens our claim of both BAL and [OIII] outflows are from the same wind. These estimates can be tested with upcoming James Webb Space Telescope observations.
We report on the first observation of a single hybrid magnetic structure that contains both a pseudostreamer and a double helmet streamer. This structure was originally observed by the SWAP instrument aboard the PROBA2 satellite between 5 and 10~May~2013. It consists of a pair of filament channels near the south pole of the sun. On the western edge of the structure, the magnetic morphology above the filaments is that of a side-by-side double helmet streamer, with open field between the two channels. On the eastern edge, the magnetic morphology is that of a coronal pseudostreamer without the central open field. We investigated this structure with multiple observations and modelling techniques. We describe the topology and dynamic consequences of such a unified structure.
We prove an analogue in higher dimensions of the classical Narasimhan-Seshadri theorem for strongly stable vector bundles of degree 0 on a smooth projective variety $X$ with a fixed ample line bundle $\Theta$. As applications, over fields of characteristic zero, we give a new proof of the main theorem in a recent paper of Balaji and Koll\'ar and derive an effective version of this theorem; over uncountable fields of positive characteristics, if $G$ is a simple and simply connected algebraic group and the characteristic of the field is bigger than the Coxeter index of $G$, we prove the existence of strongly stable principal $G$ bundles on smooth projective surfaces whose holonomy group is the whole of $G$.
We show that all eccentric timelike orbits in Reissner-Nordstr\"{o}m spacetime can be classified using a taxonomy that draws upon an isomorphism between periodic orbits and the set of rational numbers. By virtue of the fact that the rationals are dense, the taxonomy can be used to approximate aperiodic orbits with periodic orbits. This may help reduce computational overhead for calculations in gravitational wave astronomy. Our dynamical systems approach enables us to study orbits for both charged and uncharged particles in spite of the fact that charged particle orbits around a charged black hole do not admit a simple one-dimensional effective potential description. Finally, we show that comparing periodic orbits in the RN and Schwarzschild geometries enables us to distinguish charged and uncharged spacetimes by looking only at the orbital dynamics.
Recently, counting the number of people for crowd scenes is a hot topic because of its widespread applications (e.g. video surveillance, public security). It is a difficult task in the wild: changeable environment, large-range number of people cause the current methods can not work well. In addition, due to the scarce data, many methods suffer from over-fitting to a different extent. To remedy the above two problems, firstly, we develop a data collector and labeler, which can generate the synthetic crowd scenes and simultaneously annotate them without any manpower. Based on it, we build a large-scale, diverse synthetic dataset. Secondly, we propose two schemes that exploit the synthetic data to boost the performance of crowd counting in the wild: 1) pretrain a crowd counter on the synthetic data, then finetune it using the real data, which significantly prompts the model's performance on real data; 2) propose a crowd counting method via domain adaptation, which can free humans from heavy data annotations. Extensive experiments show that the first method achieves the state-of-the-art performance on four real datasets, and the second outperforms our baselines. The dataset and source code are available at https://gjy3035.github.io/GCC-CL/.
The quantum theory of Ur-alternatives of Carl Friedrich von Weizsaecker tries to constitute general quantum theory based on the concept of logical alternatives in time. Based on this interpretation of quantum theory the existence of free objects in space, their symmetry properties and their interactions shall be inferred. The alternatives are represented by binary alternatives, which are called Ur-alternatives because of their logically fundamental character. Through Ur-alternatives as elementary quantum theoretical units of information the Copernican revolution with respect to the question of space is realized in a consequent way. This means that not the objects of nature are in a given space, but the existence of space arises as a kind of indirect representation of relations between abstract quantum theoretical objects. The Ur-alternatives do not exist in a given physical reality, but the existence of space is constituted by Ur-alternatives at all. Such a concept of reality is implicitly contained within the uncertainty relation and can be seen especially in the EPR-paradoxon. It is shown in this thesis in a mathematical consistent way that a state in the tensor space of many Ur-alternatives can directly be mapped into a real three dimensional space which means that together with the dynamics a representation in a (3+1)-dimensional space-time becomes possible. By considering the $G_2$ an approach for the incorporation of the internal symmetries can be suggested. Furthermore the Ur-alternatives enable the constitution of a concept of interaction, which is based on quantum theoretical entanglement. By using this concept it is tried to obtain a purely quantum theoretical description of electromagnetism and gravity. This corresponds to a much more principle and in a radical sense background independent way of quantization.