ShantanuT01/DACTYL-Pretraining huggingface dataset:
DyFeO3 exhibits a weak ferromagnetism (TNFe ~ 645 K) that disappears below a spin-reorientation (Morin) transition at TSRFe ~ 50 K. It is also known that applied magnetic field induces ferroelectricity at the magnetic ordering temperature of Dy-ions (TNDy ~ 4.5 K). Here, we show that the ferroelectricity exists in the weak ferromagnetic state (TSRFe < T < TN,C) without applying magnetic field, indicating the crucial role of weak ferromagnetism in inducing ferroelectricity. 57Fe M\"ossbauer studies show that hyperfine field (Bhf) deviates from mean field-like behaviour that is observed in the weak ferromagnetic state and decreases below the onset of spin-reorientation transition (80 K), implying that the Bhf above TSR had additional contribution from Dy-ions due to induced magnetization by the weak ferromagnetic moment of Fe-sublattice and below TSR, this contribution decreases due to collinear ordering of Fe-sublattice. These results clearly demonstrate the presence of magnetic interactions between Dy(4f) and Fe(3d) and their correlation with ferroelectricity in the weak ferromagnetic state of DyFeO3.
The theory of general relativity, for which we celebrate the centennial at this Symposium, is based on the Einstein equivalence principle. This principle could be violated through a pseudoscalar-photon interaction, which would also produce a rotation of the polarization angle for radiation traveling over very long distances. Therefore, if we could show that this cosmic polarization rotation does not exist, our confindence in general relativity would be greatly increased. We review here the astrophysical searches for cosmic polarization rotation, which have been made in the past 26 years using the polarization of radio galaxies and of the cosmic microwave background. So far no rotation has been detected within about 1 degree. We discuss current problems and future prospects for cosmic polarization rotation measurements.
We attempt to produce low temperature opacity data incorporating the effects of varied abundances of the elements carbon and nitrogen. For our temperature range of interest, molecules represent the dominant opacity source. Our dataset covers a wide metallicity range and is meant to provide important input data for stellar evolution models and other applications. We conduct chemical equilibrium calculations to evaluate the partial pressures of neutral atoms, ions, and molecules. Based on a large dataset containing atomic line and continuum data and, most importantly, a plethora of molecular lines, we calculate Rosseland mean opacity coefficients not only for a number of different metallicities, but also for varied abundances of the isotopes ^{12}C and ^{14}N at each metallicity. The molecular data comprise the main opacity sources for either an oxygen-rich or carbon-rich chemistry. We tabulate the opacity coefficients as a function of temperature and, basically, density. Due to the special role of the CO molecule, within a certain chemistry regime an alteration to the carbon abundance causes considerable changes in the Rosseland opacity. The transition from a scaled solar (i. e. oxygen-rich) mixture to a carbon-rich regime results in opacities that can, at low temperatures, differ by orders of magnitude from to the initial situation. The reason is that the mean opacity in either case is due to different molecular absorbers. Variations in the abundance of nitrogen have less pronounced effects but, nevertheless, cannot be neglected. [abridged]
Infrared photometry and spectroscopy (1-25 um) of background stars reddened by the Lupus molecular cloud complex are used to determine the properties of the grains and the composition of the ices before they are incorporated into circumstellar envelopes and disks. H2O ices form at extinctions of Ak=0.25+/-0.07 mag (Av=2.1+/-0.6). Such a low ice formation threshold is consistent with the absence of nearby hot stars. Overall, the Lupus clouds are in an early chemical phase. The abundance of H2O ice (2.3+/-0.1*10^-5 relative to Nh) is typical for quiescent regions, but lower by a factor of 3-4 compared to dense envelopes of YSOs. The low solid CH3OH abundance (<3-8% relative to H2O) indicates a low gas phase H/CO ratio, which is consistent with the observed incomplete CO freeze out. Furthermore it is found that the grains in Lupus experienced growth by coagulation. The mid-infrared (>5 um) continuum extinction relative to Ak increases as a function of Ak. Most Lupus lines of sight are well fitted with empirically derived extinction curves corresponding to Rv~ 3.5 (Ak=0.71) and Rv~5.0 (Ak=1.47). For lines of sight with Ak>1.0 mag, the tau9.7/Ak ratio is a factor of 2 lower compared to the diffuse medium. Below 1.0 mag, values scatter between the dense and diffuse medium ratios. The absence of a gradual transition between diffuse and dense medium-type dust indicates that local conditions matter in the process that sets the tau9.7/Ak ratio. This process is likely related to grain growth by coagulation, as traced by the A7.4/Ak continuum extinction ratio, but not to ice mantle formation. Conversely, grains acquire ice mantles before the process of coagulation starts.
We present the analysis of simultaneous NuSTAR and XMM-Newton data of 8 Compton-thick (CT-) active galactic nuclei (AGN) candidates selected in the Swift-Burst Alert Telescope (BAT) 100 month survey. This work is part of an ongoing effort to find and characterize all CT-AGN in the local ($z\leq$0.05) Universe. We used two physically motivated models, MYTorus and borus02, to characterize the sources in the sample, finding 5 of them to be confirmed CT-AGN. These results represent an increase of $\sim19$% over the previous NuSTAR-confirmed, BAT-selected CT-AGN at $z\leq0.05$, bringing the total number to 32. This corresponds to an observed fraction of $\sim 8$\% of all AGN within this volume-limited sample, although it increases to $20\pm5$% when limiting the sample to $z\leq0.01$. Out of a sample of 48 CT-AGN candidates, selected using BAT and soft (0.3$-$10 keV) X-ray data, only 24 are confirmed as CT-AGN with the addition of the NuSTAR data. This highlights the importance of NuSTAR when classifying local obscured AGN. We also note that most of the sources in our full sample of 48 Seyfert 2 galaxies with NuSTAR data have significantly different line-of-sight and average torus column densities, favouring a patchy torus scenario.
The X-ray light curves of hundreds of bursts are now available, thanks to the X-ray Telescope on board the Swift satellite, on time scales from ~1 minute up to weeks and in some cases months from the burst explosion. These data allow us to investigate the physics of the highly relativistic fireball outflow and its interaction with the circumburst environment. Here we review the main results of the XRT observations, with particular regard to the evolution of the X-ray light curves in the early phases. Unexpectedly, they are characterised by different slopes, with a very steep decay in the first few hundred of seconds, followed by a flatter decay and, a few thousand of seconds later, by a somewhat steeper decay. Often strong flare activity up to few hours after the burst explosion is also seen. These flares, most likely, are still related to the central engine activity, that last much longer than expected and it is still dominating the X-ray light curve well after the prompt phase, up to a few thousand of seconds. The real afterglow emission (external shock) is dominating the X-ray light curve only after the flatter phase ends. The flatter phase is probably the combination of late-prompt emission and afterglow emission. When the late-prompt emission ends the light curve steepens again. Some flare activity can still be detected during these later phases. Finally, even the late evolution of the XRT light curves is puzzling, in particular many of them do not show a ``jet-break''. There are various possibilities to explain these observations (e.g.time evolution of the microphysical parameters, structured jet). However, a clear understanding of the formation and evolution of the jet and of the afterglow emission is still lacking.
This work is devoted to prove uniqueness result for the positive solution to a strongly competing system of Lotka - Volterra type in the limiting configuration, when the competition rate tends to infinity.
An approach to computing, withing the framework of distribution theory, the distributional valued energy-momentum tensor for the Schwarzschild spacetime is disscused. This approach avoids the problems associated with the regularization of singularities in the curvature tensors and shares common features with the by now standard treatment of discontinuities in General Relativity. Finally, the Reissner-Nordstrom spacetime is also considered using the same approach.
We present a computer code written in C that is designed to simulate structure formation from collisionless matter. The code is purely grid-based and uses a recursively refined Cartesian grid to solve Poisson's equation for the potential, rather than obtaining the potential from a Green's function. Refinements can have arbitrary shapes and in practice closely follow the complex morphology of the density field that evolves. The timestep shortens by a factor two with each successive refinement. It is argued that an appropriate choice of softening length is of great importance and that the softening should be at all points an appropriate multiple of the local inter-particle separation. Unlike tree and P3M codes, multigrid codes automatically satisfy this requirement. We show that at early times and low densities in cosmological simulations, the softening needs to be significantly smaller relative to the inter-particle separation than in virialized regions. Tests of the ability of the code's Poisson solver to recover the gravitational fields of both virialized halos and Zel'dovich waves are presented, as are tests of the code's ability to reproduce analytic solutions for plane-wave evolution. The times required to conduct a LCDM cosmological simulation for various configurations are compared with the times required to complete the same simulation with the ART, AP3M and GADGET codes. The power spectra, halo mass functions and halo-halo correlation functions of simulations conducted with different codes are compared.
We perform a comprehensive survey of the potential energy landscapes of 13-atom Morse clusters, and describe how they can be characterized and visualized. Our aim is to detail how the global features of the funnel-like surface change with the range of the potential, and to relate these changes to the dynamics of structural relaxation. We find that the landscape becomes rougher and less steep as the range of the potential decreases, and that relaxation paths to the global minimum become more complicated.
We present near-IR surface photometry (2D-profiling) for a sample of 29 nearby galaxies for which super-massive black hole (SMBH) masses are constrained. The data is derived from the UKIDSS-LASS survey representing a significant improvement in image quality and depth over previous studies based on 2MASS data. We derive the spheroid luminosity and spheroid S\'ersic index for each galaxy with GALFIT3 and use these data to construct SMBH mass -bulge luminosity ($M_{\rm bh}$--$L$) and SMBH - S\'ersic index ($M_{\rm bh}$--$n$) relations. The best fit K-band relation for elliptical and disk galaxies is $\log(M_{\rm bh}/M_{\odot})= -0.36(\pm 0.03) (M_{\rm K} + 18) + 6.17(\pm 0.16)$ with an intrinsic scatter of 0.4$^{+0.09}_{-0.06}$dex whilst for elliptical galaxies we find $\log(M_{\rm bh}/M_{\odot})= -0.42(\pm 0.06) (M_{\rm K} + 22) + 7.5(\pm 0.15)$ with an intrinsic scatter of 0.31$^{+0.087}_{-0.047}$dex. Our revised $M_{\rm bh}$--$L$ relation agrees closely with the previous near-IR constraint by \citet{tex:G07}. The lack of improvement in the intrinsic scatter in moving to higher quality near-IR data suggests that the SMBH relations are not currently limited by the quality of the imaging data but is either intrinsic or a result of uncertainty in the precise number of required components required in the profiling process. Contrary to expectation (see \citealt{tex:GD07a}) a relation between SMBH mass and the S\'ersic index was not found at near-IR wavelengths. This latter outcome is believed to be explained by the generic inconsistencies between 1D and 2D galaxy profiling which are currently under further investigation.
With conformal-invariance methods, Burkhardt, Guim, and Xue studied the critical Ising model, defined on the upper half plane $y>0$ with different boundary conditions $a$ and $b$ on the negative and positive $x$ axes. For $ab=-+$ and $f+$, they determined the one and two-point averages of the spin $\sigma$ and energy $\epsilon$. Here $+$, $-$, and $f$ stand for spin-up, spin-down, and free-spin boundaries, respectively. The case $+-+-+\dots$, where the boundary conditions switch between $+$ and $-$ at arbitrary points, $\zeta_1$, $\zeta_2$, $\dots$ on the $x$ axis was also analyzed.   In this paper the alternating boundary conditions $+f+f+\dots$ and the case $-f+$ of three different boundary conditions are considered. Exact results for the one and two-point averages of $\sigma$, $\epsilon$, and the stress tensor $T$ are derived. Using the results for $\langle T\rangle$, the critical Casimir interaction with the boundary of a wedge-shaped inclusion is analyzed for mixed boundary conditions.   The paper also includes a comprehensive discussion of boundary-operator expansions in two-dimensional critical systems with mixed boundary conditions. Two types of expansions - away from switching points of the boundary condition and at switching points - are considered. The asymptotic behavior of two-point averages is expressed in terms of one-point averages with the help of the expansions. We also consider the strip geometry with mixed boundary conditions and derive the distant-wall corrections to one-point averages near one edge due to the other edge using the boundary-operator expansions. The predictions of the boundary-operator expansions are consistent with exact results for Ising systems.
We report the observation of a frequency ($\omega $) and temperature (T) -dependent loss peak in $\chi''$ and accompanying dispersion in $\chi'$ from microwave (2-18GHz) measurements of the complex susceptibility $\tilde{\chi}(\omega, T)=\chi'+i\chi''$ of $Sr_{14-x}Ca_{x}Cu_{24}O_{41}$. We associate this phenomenon with a rapid decrease of spin disorder and corresponding spin relaxation rate, representing a ``spin freezing transition'' accompanying charge ordering which occurs at temperatures $\sim 250K$. Our results enable direct quantitative measurements of the spin relaxation rate, and yield information on spin dynamics in these materials and the related compounds $SrCuO_{2}$ and $Sr_{2}CuO_{3}$.
The knowledge of the intrinsic three-dimensional (3D) structure of galaxy components provides crucial information about the physical processes driving their formation and evolution. In this paper I discuss the main developments and results in the quest to better understand the 3D shape of galaxy bulges. I start by establishing the basic geometrical description of the problem. Our understanding of the intrinsic shape of elliptical galaxies and galaxy discs is then presented in a historical context, in order to place the role that the 3D structure of bulges play in the broader picture of galaxy evolution. Our current view on the 3D shape of the Milky Way bulge and future prospects in the field are also depicted.
As a consequence of the approximate spin-valley symmetry in graphene, the ground state of electrons in graphene at charge neutrality is a particular SU(4) quantum-Hall ferromagnet to minimize their exchange energy. If only the Coulomb interaction is taken into account, this ferromagnet can appeal either to the spin degree of freedom or equivalently to the valley pseudo-spin degree of freedom. This freedom in choice is then limited by subleading energy scales that explicitly break the SU(4) symmetry, the simplest of which is given by the Zeeman effect that orients the spin in the direction of the magnetic field. In addition, there are also valley symmetry breaking terms that can arise from short-range interactions or electron-phonon couplings. Here, we build upon the phase diagram, which has been obtained by Kharitonov [Phys. Rev. B \textbf{85}, 155439 (2012)], in order to identify the different skyrmions that are compatible with these types of quantum-Hall ferromagnets. Similarly to the ferromagnets, the skyrmions at charge neutrality are described by the $\text{Gr}(2,4)$ Grassmannian at the center, which allows us to construct the skyrmion spinors. The different skyrmion types are then obtained by minimizing their energy within a variational approach, with respect to the remaining free parameters that are not fixed by the requirement that the skyrmion at large distances from their center must be compatible with the ferromagnetic background. We show that the different skyrmion types have a clear signature in the local, sublattice-resolved, spin magnetization, which is in principle accessible in scanning-tunneling microscopy and spectroscopy.
From a recent study of a stationary cylindrical solution for a relativistic two-constituent superfluid at low temperature limit, we propose to specify this solution under the form of a relativistic generalisation of a Rankine vortex (Potential vortex whose the core has a solid body rotation).Then we establish the dynamics of the central line of this vortex by supposing that the deviation from the cylindrical configuration is weak in the neighbourhood of the core of the vortex. In "stiff" material the Nambu-Goto equations are obtained.
In this short letter we present a some rigorous vacuum solutions of the D-dimensional Jordan-Brans-Dicke field equations. In contrast with the well known Brans-Dicke solutions, to the search of static and spherically symmetric space-time we choose the widespread Hilbert coordinates.
The first objects to arise in a cold dark matter universe present a daunting challenge for models of structure formation. In the ultra small-scale limit, CDM structures form nearly simultaneously across a wide range of scales. Hierarchical clustering no longer provides a guiding principle for theoretical analyses and the computation time required to carry out credible simulations becomes prohibitively high. To gain insight into this problem, we perform high-resolution (N=720^3 - 1584^3) simulations of an Einstein-de Sitter cosmology where the initial power spectrum is P(k) propto k^n, with -2.5 < n < -1. Self-similar scaling is established for n=-1 and n=-2 more convincingly than in previous, lower-resolution simulations and for the first time, self-similar scaling is established for an n=-2.25 simulation. However, finite box-size effects induce departures from self-similar scaling in our n=-2.5 simulation. We compare our results with the predictions for the power spectrum from (one-loop) perturbation theory and demonstrate that the renormalization group approach suggested by McDonald improves perturbation theory's ability to predict the power spectrum in the quasilinear regime. In the nonlinear regime, our power spectra differ significantly from the widely used fitting formulae of Peacock & Dodds and Smith et al. and a new fitting formula is presented. Implications of our results for the stable clustering hypothesis vs. halo model debate are discussed. Our power spectra are inconsistent with predictions of the stable clustering hypothesis in the high-k limit and lend credence to the halo model. Nevertheless, the fitting formula advocated in this paper is purely empirical and not derived from a specific formulation of the halo model.
The connection between gravity and thermodynamics is explored. Examining a perfect fluid in gravitational equilibrium we find that the entropy is extremal only if Einstein's equations are satisfied. Conversely, one can derive part of Einstein's equations from ordinary thermodynamical considerations. This allows the theory of this system to be recast in such a way that a sector of general relativity is purely thermodynamical and should not be quantized.
We present optical spectroscopy (photoluminescence and reflectance) studies of thin layers of the transition metal dichalcogenide WSe2, with thickness ranging from mono- to tetra-layer and in the bulk limit. The investigated spectra show the evolution of excitonic resonances as a function of layer thickness, due to changes in the band structure and, importantly, due to modifications of the strength of Coulomb interaction as well. The observed temperature-activated energy shift and broadening of the fundamental direct exciton are well accounted for by standard formalisms used for conventional semiconductors. A large increase of the photoluminescence yield with temperature is observed in WSe2 monolayer, indicating the existence of competing radiative channels. The observation of absorption-type resonances due to both neutral and charged excitons in WSe2 monolayer is reported and the effect of the transfer of oscillator strength from charged to neutral exciton upon increase of temperature is demonstrated.
The Fourier method is used to find conditions on the right-hand side and on the initial data in the Rayleigh-Stokes problem, which ensure the existence and uniqueness of the solution. Then, in the Rayleigh-Stokes problem, instead of the initial condition, consider the non-local condition: $u(x,T)=\beta u(x,0)+\varphi(x)$, where $\beta$ is either zero or one. It is well known that if $\beta=0$, then the corresponding problem, called the backward problem, is ill-posed in the sense of Hadamard, i.e. a small change in $u(x,T)$ leads to large changes in the initial data. Nevertheless, we will show that if we consider sufficiently smooth current information, then the solution exists and it is unique and stable. It will also be shown that if $\beta=1$, then the corresponding non-local problem is well-posed and coercive type inequalities are valid.
The Hubbard and Su-Schrieffer-Heeger Hamiltonians (SSH) are iconic models for understanding the qualitative effects of electron-electron and electron-phonon interactions respectively. In the two-dimensional square lattice Hubbard model at half filling, the on-site Coulomb repulsion, $U$, between up and down electrons induces antiferromagnetic (AF) order and a Mott insulating phase. On the other hand, for the SSH model, there is an AF phase when the electron-phonon coupling $\lambda$ is less than a critical value $\lambda_c$ and a bond order wave when $\lambda > \lambda_c$. In this work, we perform numerical studies on the square lattice optical Su-Schrieffer-Heeger-Hubbard Hamiltonian (SSHH), which combines both interactions. We use the determinant quantum Monte Carlo (DQMC) method which does not suffer from the fermionic sign problem at half filling. We map out the phase diagram and find that it exhibits a direct first-order transition between an antiferromagnetic phase and a bond-ordered wave as $\lambda$ increases. The AF phase is characterized by two different regions. At smaller $\lambda$ the behavior is similar to that of the pure Hubbard model; the other region, while maintaining long range AF order, exhibits larger kinetic energies and double occupancy, i.e. larger quantum fluctuations, similar to the AF phase found in the pure SSH model.