ShantanuT01/DACTYL-Pretraining huggingface dataset:
Exact electromagnetic wave solutions to Maxwell equations on anisotropic Bianchi I cosmological spacetime backgrounds are studied. The waves evolving on Bianchi I spacetimes exhibit birefringence (associated to linear polarization) and dispersion.   The particular case of a vacuum--dominated anisotropic Universe, which reproduces a Friedmann-Robertson-Walker Universe (for late times) while for earlier times it matches a Kasner Universe, is studied. The electromagnetic waves do not, in general, follow null geodesics. This produces a modification of the cosmological redshift, which is now dependent on light polarization and dispersion and its non-null geodesic behavior. New results presented here may help to tackle some issues related to the "horizon" problem.
We give an introduction to a notion of weak dependence which is more general than mixing and allows to treat for example processes driven by discrete innovations as they appear with time series bootstrap. As a typical example, we analyze autoregressive processes and their bootstrap analogues in detail and show how weak dependence can be easily derived from a contraction property of the process. Furthermore, we provide an overview of classes of processes possessing the property of weak dependence and describe important probabilistic results under such an assumption.
We present a theory for the ab-initio computation of NMR chemical shifts (sigma) in condensed matter systems, using periodic boundary conditions. Our approach can be applied to periodic systems such as crystals, surfaces, or polymers and, with a super-cell technique, to non-periodic systems such as amorphous materials, liquids, or solids with defects. We have computed the hydrogen sigma for a set of free molecules, for an ionic crystal, LiH, and for a H-bonded crystal, HF, using density functional theory in the local density approximation. The results are in excellent agreement with experimental data.
In this note we analyze the large time behavior of solutions to a class of initial/boundary problems involving a damped nonlinear beam equation. We show that under mild conditions on the damping term of the equation of motions the solutions of the dynamical problem converge to the solution of the stationary problem. We also show that this convergence is exponential.
In this paper we consider Roche accretion in an Extreme Mass-Ratio Inspiral (EMRI) binary system formed by a star orbiting a massive black hole. The ultimate goal is to detect the mass and spin of the black hole and provide a test of general relativity in the strong-field regime from the resultant quasi-periodic signals. Before accretion starts, the stellar orbit is presumed to be circular and equatorial, and shrinks due to gravitational radiation. New fitting formulae are presented for the inspiral time and the radiation-reaction torque in the relativistic regime. If the inspiralling star fills its Roche lobe outside the Innermost Stable Circular Orbit (ISCO) of the hole, gas will flow through the inner Lagrange point (L1) to the hole. We give new relativistic interpolation formulae for the volume enclosed by the Roche lobe. If this mass-transfer happens on a time scale faster than the thermal time scale but slower than the dynamical time scale, the star will evolve adiabatically, and, in most cases, will recede from the hole while filling its Roche lobe. We calculate how the stellar orbital period and mass-transfer rate will change through the "Roche evolution" for various types of stars in the relativistic regime. We envisage that the mass stream eventually hits the accretion disc, where it forms a hot spot orbiting the hole and may ultimately modulate the luminosity with the stellar orbital frequency. The observability of such a modulation is discussed along with a possible interpretation of an intermittent 1 hour period in the X-ray emission of RE J1034+396.
Thermonuclear supernovae result when interaction with a companion reignites nuclear fusion in a carbon-oxygen white dwarf, causing a thermonuclear runaway, a catastrophic gain in pressure, and the disintegration of the whole white dwarf. It is usually thought that fusion is reignited in near-pycnonuclear conditions when the white dwarf approaches the Chandrasekhar mass. I briefly describe two long-standing problems faced by this scenario, and our suggestion that these supernovae instead result from mergers of carbon-oxygen white dwarfs, including those that produce sub-Chandrasekhar mass remnants. I then turn to possible observational tests, in particular those that test the absence or presence of electron captures during the burning.
Among its many outstanding properties, graphene supports terahertz surface plasma waves -- sub-wavelength charge density oscillations connected with electromagnetic fields that are tightly localized near the surface[1,2]. When these waves are confined to finite-sized graphene, plasmon resonances emerge that are characterized by alternating charge accumulation at the opposing edges of the graphene. The resonant frequency of such a structure depends on both the size and the surface charge density, and can be electrically tuned throughout the terahertz range by applying a gate voltage[3,4]. The promise of tunable graphene THz plasmonics has yet to be fulfilled, however, because most proposed optoelectronic devices including detectors, filters, and modulators[5-10] desire near total modulation of the absorption or transmission, and require electrical contacts to the graphene -- constraints that are difficult to meet using existing plasmonic structures. We report here a new class of plasmon resonance that occurs in a hybrid graphene-metal structure. The sub-wavelength metal contacts form a capacitive grid for accumulating charge, while the narrow interleaved graphene channels, to first order, serves as a tunable inductive medium, thereby forming a structure that is resonantly-matched to an incident terahertz wave. We experimentally demonstrate resonant absorption near the theoretical maximum in readily-available, large-area graphene, ideal for THz detectors and tunable absorbers. We further predict that the use of high mobility graphene will allow resonant THz transmission near 100%, realizing a tunable THz filter or modulator. The structure is strongly coupled to incident THz radiation, and solves a fundamental problem of how to incorporate a tunable plasmonic channel into a device with electrical contacts.
Given a number field, it is an important question in algorithmic number theory to determine all its subfields. If the search is restricted to abelian subfields, one can try to determine them by using class field theory. For this, it is necessary to know the ramified primes. We show that the ramified primes of the subfield can be computed efficiently. Using this information we give algorithms to determine all the quadratic and the cyclic cubic subfields of the initial field. The approach generalises to cyclic subfields of prime degree. In the case of quadratic subfields, our approach is much faster than other methods.
The present paper is devoted to the theoretical study of the spectrum of low-frequency electronic density oscillations running along the boundary of two contacting two-dimensional electronic systems in the perpendicular magnetic field. For the first time, such waves were predicted in the Institute of Radio-engineering and Electronics of RAS (V.A. Volkov, S.A. Mikhailov, 1992), studied experimentally in the papers of foreign investigations and called inter-edge magnetoplasmons (IEMP). When the two-dimensional system is finite and has the half-plane shape, the internal boundary becomes external and these plasmons go over into the well-known edge magnetoplasmons. The existing theory of edge and inter-edge magnetoplasmons is built with the neglect of spatial dispersion of the conductivity. Here we attempt to derive and analyze the IEMP dispersion equation involving spatial dispersion of conductivity. The inner boundary is assumed to be a straight line at which the electron concentration changes weakly but stepwise. The jump of the diagonal conductivity is considered small compared with that of the Hall magnetoconductivity. The IEMP frequency is small compared with the cyclotron frequency. The IEMP frequency as a function of wavevector oscillates with the period determined by a ratio of electron cyclotron radius to the plasmon wavelength. For long wavelengths, the IEMP dispersion curve coincides with the known result.
In this paper we present some new identities for multiple polylogarithms (abbr. MPLs) and multiple harmonic star sums (abbr. MHSSs) by using the methods of iterated integral computations of logarithm functions. Then, by applying these formulas obtained, we establish some explicit relations between Kaneko-Yamamoto type multiple zeta values (abbr. K-Y MZVs), multiple zeta values (abbr. MZVs) and MPLs. Further, we find some explicit relations between MZVs and multiple zeta star values (abbr. MZSVs). Furthermore, we define an Ap\'{e}ry-type variant of MZSVs $\zeta^\star_B({\bf k})$ (called multiple zeta $B$-star values, abbr. MZBSVs) which involve MHSSs and central binomial coefficients, and establish some explicit connections among MZVs, alternating MZVs and MZBSVs by using the method of iterated integrals. Finally, some interesting consequences and illustrative examples are presented.
For a real polynomial $f$ we present explicit zero-free angular sectors in the complex plane, symmetric with respect to the real axis, with angles depending only on the degree of $f$, and vertices expressed in terms of the coefficients of $f$. We also describe zero-free lens-shaped regions for $f$ that are associated to the zero-free sectors of the reciprocal of $f$. As an application, we use these zero-free regions to obtain irreducibility criteria for polynomials with integer coefficients that take a prime or a prime power value.
The behavior of spin propagation in metals in various measurement schemes is shown to be qualitatively different than a simple exponential decay - due to the backflow effect on spin diffusion in the presence of interfaces. To probe this effect we utilize the spin sensitivity of an Andreev contact between gold films of variable thickness deposited on top of a spin injector, Co$_{2}$Mn$_{0.5}$Fe$_{0.5}$Si, with the spin polarization of approximately 45\%, and Nb superconducting tip. While the results are consistent with gradually decaying spin polarization as the film thickness increases, the spin diffusion length in Au found to be 285 nm, is more than two times larger that one would have obtained without taking the backflow effect into account.
We study the process e^+ e^- -> chi^0_1 chi^0_2 and the subsequent decay chi^0_2 -> chi^0_1 l^{+} l^{-}$ with polarized beams, including the complete spin correlations of chi^0_2$. We present numerical results for the lepton energy and angular distributions, and for the distribution of the opening angle between the leptons for sqrt{s}=500 GeV. The effects of spin correlations are important in the lepton angular distribution, especially for gaugino-like neutralinos. The opening angle distribution is particularly suitable for distinguishing between higgsino- and gaugino-like neutralinos. The polarized cross sections and lepton angular distributions are very sensitive to the mixing of the neutralinos.
We study the superlattice minibands produced by the interplay between moir\'e pattern induced by hexagonal BN substrate on graphene layer and the interlayer coupling in bilayer graphene with Bernal stacking (BLG). We compare moir\'e miniband features in BLG, where they are affected by the interlayer asymmetry of BLG-hBN heterostructure and trigonal warping characteristic for electrons in Bernal-stacked bilayers with those found in monolayer graphene.
We present the Yano-Koonin-Podgoretskii parametrisation of the correlation function. Compared to the conventionally used Cartesian parametrisation, this one provides more straightforward measurement of the duration of the emission process in the fireball and a clearer signal of the longitudinal expansion, which is expected in ulrarelativistic heavy ion collisions.
This document contains additional information for the paper: J. Quer. Fields of definition of building blocks, Math. Comp. 78 (2009), no. 265, pp. 537--554.   It contains a description of the implementation in Magma of the algorithms and computational techniques described in that paper, and also several tables with data that were elaborated using this implementation.
Ring theory is one of the branches of the abstract algebra that has been broadly used in images. However, ring theory has not been very related with image segmentation. In this paper, we propose a new index of similarity among images using Zn rings and the entropy function. This new index was applied as a new stopping criterion to the Mean Shift Iterative Algorithm with the goal to reach a better segmentation. An analysis on the performance of the algorithm with this new stopping criterion is carried out. The obtained results proved that the new index is a suitable tool to compare images.
In the simplest model of single-file diffusion, $N$ point particles wander on a segment of the $x$ axis of length $L$, with hard core interactions, which prevent passing, and with overdamped Brownian dynamics, $\lambda\dot{x}=\eta(t)$, where $\eta(t)$ has the form of Gaussian white noise with zero mean. In 1965 Harris showed that in the limit $N\to\infty$, $L\to\infty$ with constant $\rho=N/L$, the mean square displacement of a tagged particle grows subdiffusively, as $t^{1/2}$, for long times. Recently, it has been shown that the proportionality constants of the $t^{1/2}$ law for randomly-distributed initial positions of the particles and for equally-spaced initial positions are not the same, but have ratio $\sqrt{2}$. In this paper we consider point particles on the $x$ axis, which collide elastically, and which move according to (i) random-acceleration dynamics $\ddot{x}=\eta(t)$ and (ii) Langevin dynamics $\ddot{x}+\lambda\dot{x}=\eta(t)$. The mean square displacement and mean-square velocity of a tagged particle are analyzed for both types of dynamics and for random and equally-spaced initial positions and Gaussian-distributed initial velocities. We also study tagged particle statistics, for both types of dynamics, in the spreading of a compact cluster of particles, with all of the particles initially at the origin.
Gravitational Wave (GW) astronomy severely narrowed down the theoretical space for scalar-tensor theories. We propose a new class of attractor models for Horndeski action in which GWs propagate at the speed of light in the nearby universe but not in the past. To do so we derive new solutions to the interacting dark sector in which the ratio of dark energy and dark matter remains constant, which we refer to as doppelg\"anger dark energy (DDE). We then remove the interaction between dark matter and dark energy by a suitable change of variables. The accelerated expansion that (we) baryons observe is due to a conformal coupling to the dark energy scalar field. We show how in this context it is possible to find a non trivial subset of solutions in which GWs propagate at the speed of light only at low red-shifts. The model is an attractor, thus reaching the limit $c_{T}\to1$ relatively fast. However, the effect of baryons turns out to be non-negligible and severely constrains the form of the Lagrangian. In passing, we found that in the simplest DDE models the no-ghost conditions for perturbations require a non-universal coupling to gravity. In the end, we comment on possible ways to solve the lack of matter domination stage for DDE models.
We study low energy hadronic interaction models based on BESS observed cosmic ray proton and antiproton spectra at medium high altitude. Among the three popular low energy interaction models, we find that FLUKA reproduces results of BESS observations on secondary proton spectrum reasonably well over the whole observed energy range, the model UrQMD works well at relatively higher energies whereas spectrum obtained with GHEISHA differs significantly from the measured spectrum. Simulated antiproton spectrum with FLUKA, however, exhibits significant deviations from the BESS observation wheras UrQMD and GHEISHA reproduce the BESS observations within the experimental error.
In this paper, we present the distribution of $H_2O$ masers associated with the semi-regular variable star W Hydrae (W Hya). We have collected the radio interferometric data of the maser distribution taken with the Very Large Array (VLA), the Kashima-Nobeyama InterFErometer (KNIFE), the Multi-Element Radio Link Network (MERLIN), the VLBI Exploration of Radio Astrometry (VERA), and the combined array of the Korean VLBI Network (KVN) and VERA (KaVA) in order to trace the maser distribution variation in two decades. Even though differences in the sensitivities and angular resolutions of the interferometric observations should be taken into account, we attempt to find possible correlation of the maser distribution with the stellar light curve. Our failure in the measurement of the annual parallax of the masers with VERA is likely caused by the properties of the maser features, which have been spatially resolved by the synthesized beam and survived for only half a year or less. No dependence of the maser spot flux density on its size is found in the KNIFE data, suggesting that maser spot size is determined by the physical boundary, as is expected for a clump affected by outward propagation of a stellar pulsation shock wave, rather than the (spherical) geometry of maser beaming in the maser gas clump.
A wide variety of phenomena such as gentle but persistent brightening, dynamic slender features (~100 km), and compact (~1'') ultraviolet (UV) bursts are associated with the heating of the solar chromosphere. High spatio-temporal resolution is required to capture the finer details of the likely magnetic reconnection-driven, rapidly evolving bursts. Such observations are also needed to reveal their similarities to large-scale flares, which are also thought to be reconnection driven, and more generally their role in chromospheric heating. Here we report observations of chromospheric heating in the form of a UV burst obtained with the balloon-borne observatory, SUNRISE. The observed burst displayed a spatial morphology similar to that of a large-scale solar flare with circular ribbon. While the co-temporal UV observations at 1.5'' spatial resolution and 24s cadence from the Solar Dynamics Observatory showed a compact brightening, the SUNRISE observations at diffraction-limited spatial resolution of 0.1'' at 7s cadence revealed a dynamic sub-structure of the burst that it is composed of extended ribbon-like features and a rapidly evolving arcade of thin (~0.1'' wide) magnetic loop-like features, similar to post-flare loops. Such a dynamic sub-structure reveals the small-scale nature of chromospheric heating in these bursts. Furthermore, based on magnetic field extrapolations, this heating event is associated with a complex fan-spine magnetic topology. Our observations strongly hint at a unified picture of magnetic heating in the solar atmosphere from some large-scale flares to small-scale bursts, all being associated with such a magnetic topology.
Magnetic fields appear to be present in all galaxies and galaxy clusters. Recent measurements indicate that a weak magnetic field may be present even in the smooth low density intergalactic medium. One explanation for these observations is that a seed magnetic field was generated by some unknown mechanism early in the life of the Universe, and was later amplified by various dynamos in nonlinear objects like galaxies and clusters. We show that a primordial magnetic field is expected to be generated in the early Universe on purely linear scales through vorticity induced by scale-dependent temperature fluctuations or equivalently, a spatially varying speed of sound of the gas. Residual free electrons left over after recombination tap into this vorticity to generate magnetic field via the Biermann battery process. Although the battery operates even in the absence of any relative velocity between dark matter and gas at the time of recombination, the presence of such a relative velocity modifies the predicted spatial power spectrum of the magnetic field. At redshifts of order a few tens, we estimate a root mean square field strength of order 1e-25 -1e-24 G on comoving scales~10 kpc. This field, which is generated purely from linear perturbations, is expected to be amplified significantly after reionization, and to be further boosted by dynamo processes during nonlinear structure formation.
We present VLT/UVES spectroscopy of the quasar Q0841+129, whose spectrum shows a proximate damped Lyman-alpha (PDLA) absorber at z=2.47621 and a proximate sub-DLA at z=2.50620, both lying close in redshift to the QSO itself at z_em=2.49510+/-0.00003. This fortuitous arrangement, with the sub-DLA acting as a filter that hardens the QSO's ionizing radiation field, allows us to model the ionization level in the foreground PDLA, and provides an interesting case-study on the origin of the high-ion absorption lines Si IV, C IV, and O VI in DLAs. The high ions in the PDLA show at least five components spanning a total velocity extent of ~160 km/s, whereas the low ions exist predominantly in a single component spanning just 30 km/s. We examine various models for the origin of the high ions. Both photoionization and turbulent mixing layer models are fairly successful at reproducing the observed ionic ratios after correcting for the non-solar relative abundance pattern, though neither model can explain all five components. We show that the turbulent mixing layer model, in which the high ions trace the interfaces between the cool PDLA gas and a hotter phase of shock-heated plasma, can explain the average high-ion ratios measured in a larger sample of 12 DLAs.
A non-singular, static spherically symmetric solution to the nonsymmetric gravitational and electromagnetic theory field equations is derived, which depends on the four parameters m, l^2, Q and s, where m is the mass, Q is the electric charge, l^2 is the NGT charge of a body and s is a dimensionless constant. The electromagnetic field invariants are also singularity-free, so that it is possible to construct regular particle-like solutions in the theory. All the curvature invariants are finite, there are no null surfaces in the spacetime and there are no black holes. A new stable, superdense object (SDO) replaces black holes.
A promising candidate for universal memory, which would involve combining the most favourable properties of both high-speed dynamic random access memory (DRAM) and non-volatile flash memory, is resistive random access memory (ReRAM). ReRAM is based on switching back and forth from a high-resistance state (HRS) to a low-resistance state (LRS). ReRAM cells are small, allowing for the creation of memory on the scale of terabits. One of the most promising materials for use as the active medium in resistive memory is hafnia (HfO$_2$).   However, an unresolved physics is the nature of defects and traps that are responsible for the charge transport in HRS state of resistive memory.   In this study, we demonstrated experimentally and theoretically that oxygen vacancies are responsible for the HRS charge transport in resistive memory elements based on HfO$_2$. We also demonstrated that LRS transport occurs through a mechanism described according to percolation theory.   Based on the model of multiphonon tunneling between traps, and assuming that the electron traps are oxygen vacancies, good quantitative agreement between the experimental and theoretical data of current-voltage characteristics were achieved. The thermal excitation energy of the traps in hafnia was determined based on the excitation spectrum and luminescence of the oxygen vacancies.   The findings of this study demonstrate that in resistive memory elements using hafnia, the oxygen vacancies in hafnia play a key role in creating defects in HRS charge transport.