ShantanuT01/DACTYL-Pretraining huggingface dataset:
We have measured the spin injection efficiency and spin lifetime in Co$_2$FeSi/$n$-GaAs lateral nonlocal spin valves from 20 to 300 K. We observe large ($\sim$40 $\mu$V) spin valve signals at room temperature and injector currents of $10^3~$A/cm$^2$, facilitated by fabricating spin valve separations smaller than the 1 $\mu$m spin diffusion length and applying a forward bias to the detector contact. The spin transport parameters are measured by comparing the injector-detector contact separation dependence of the spin valve signal with a numerical model accounting for spin drift and diffusion. The apparent suppression of the spin injection efficiency at the lowest temperatures reflects a breakdown of the ordinary drift-diffusion model in the regime of large spin accumulation. A theoretical calculation of the D'yakonov-Perel spin lifetime agrees well with the measured $n$-GaAs spin lifetime over the entire temperature range.
Any account of the emergence of classicality from quantum theory must address the fact that the quantum operators representing positions and momenta do not commute, whereas their classical counterparts suffer no such restrictions. To address this, we revive an old idea of von Neumann, and seek a pair of commuting operators $X,P$ which are, in a specific sense, "close" to the canonical non-commuting position and momentum operators, $x,p$. The construction of such operators is related to the problem of finding complete sets of orthonormal phase space localized states, a problem severely limited by the Balian-Low theorem. Here these limitations are avoided by restricting attention to situations in which the density matrix is reasonably decohered (i.e., spread out in phase space).
In this paper we will focus on the potential and on the challenges associated with the development of an integrated brain and spinal cord modelling framework for processing MR neuroimaging data. The aim of the work is to explore how a hierarchical generative model of imaging data, which captures simultaneously the distribution of signal intensities and the variability of anatomical shapes across a large population of subjects, can serve to quantitatively investigate, in vivo, the morphology of the central nervous system (CNS). In fact, the generality of the proposed Bayesian approach, which extends the hierarchical structure of the segmentation method implemented in the SPM software, allows processing simultaneously information relative to different compartments of the CNS, namely the brain and the spinal cord, without having to resort to organ specific solutions (e.g. tools optimised only for the brain, or only for the spinal cord), which are inevitably harder to integrate and generalise.
In this work we consider a dissipative reaction-diffusion equation in a $d$-dimensional thin domain shrinking to a one dimensional segment and obtain good rates for the convergence of the attractors. To accomplish this, we use estimates on the convergence of inertial manifolds as developed previously in \cite{Arrieta-Santamaria-C0} and Shadowing theory.
We present the results of one and three-dimensional radiative transfer calculations of polarized spectra emerging from snapshots of radiation magnetohydrodynamical simulations of the local vertical structure of black hole accretion disks. The simulations cover a wide range of physical regimes relevant for the high/soft state of black hole X-ray binaries. We constrain the uncertainties in theoretical spectral color correction factors due to the presence of magnetic support of the disk surface layers and strong density inhomogeneities. For the radiation dominated simulation, magnetic support increases the color correction factor by about ten percent, but this is largely compensated by a ten percent softening due to inhomogeneities. We also compute the effects of inhomogeneities and Faraday rotation on the resulting polarization. Magnetic fields in the simulations are just strong enough to produce significant Faraday depolarization near the spectral peak of the radiation field. X-ray polarimetry may therefore be a valuable diagnostic of accretion disk magnetic fields, being able to directly test simulations of magnetorotational turbulence.
Multiple teams participate in a random competition. In each round the winner receives one point. We study the times until ties occur among teams. We construct martingales and supermartingales that enable us to prove the results regarding these stopping times. The problems studied in this paper are motivated by their applications to databases and their storage engines that are based on augmented balanced search trees. The ties in the competitions are related to the necessary rebalancing operations that have to be executed on the database.
Nestohedra are a family of convex polytopes that includes permutohedra, associahedra, and graph associahedra. In this paper, we study an extension of such polytopes, called extended nestohedra. We show that these objects are indeed the boundaries of simple polytopes, answering a question of Lam and Pylyavskyy. We also study the duals of (extended) nestohedra, giving a complete characterization of isomorphisms (as simplicial complexes) between the duals of extended nestohedra and a partial characterization of isomorphisms between the duals of nestohedra and extended nestohedra. In addition, we give formulas for their $f$-, $h$-, and $\gamma$-vectors. This includes showing that the $f$-vectors of the extended nestohedron corresponding to a forest $F$ and the nestohedron corresponding to the line graph of $F$ are the same, as well as showing that all flag extended nestohedra have nonnegative $\gamma$-vectors, thus proving Gal's conjecture for a large class of flag simple polytopes. We also relate the $f$- and $h$-vectors of the nestohedra and extended nestohedra, as well as give explicit formulas for the $h$- and $\gamma$-vectors in terms of descent statistics for a certain class of flag extended nestohedra. Finally, we define a partial ordering on partial permutations that is a join semilattice quotient of the weak Bruhar order on the symmetric group, and such that any linear extension of the partial order provides a shelling of the dual of the stellohedron.
A thermodynamic model for multifragmentation which is frequently used appears to give very different values for specific heat at constant volume depending upon whether canonical or grand canonical ensemble is used. The cause for this discrepancy is analysed.
Quantum cosmology is usually studied quantizing symmetry-reduced variables. Is it possible, instead, to define quantum cosmology starting from the full quantum gravity theory? In Loop Quantum Gravity (LQG), it is possible to cut the degrees of freedom in a suitable way, in order to define a cosmological model. Such a model provides a tool for describing general fluctuations of the quantum geometry at the bounce that replaces the initial singularity. I focus on its simplest version, a "dipole" formed by two tetrahedra. This has been shown to describe a universe with anisotropic and inhomogenous degrees of freedom. Its dynamics can be given using the spinfoam formalism. I briefly review the present state of this approach.
Correlations in the nuclear wave-function beyond the mean-field or Hartree-Fock approximation are very important to describe basic properties of nuclear structure. Various approaches to account for such correlations are described and compared to each other. This includes the hole-line expansion, the coupled cluster or ``exponential S'' approach, the self-consistent evaluation of Greens functions, variational approaches using correlated basis functions and recent developments employing quantum Monte-Carlo techniques. Details of these correlations are explored and their sensitivity to the underlying nucleon-nucleon interaction. Special attention is paid to the attempts to investigate these correlations in exclusive nucleon knock-out experiments induced by electron scattering. Another important issue of nuclear structure physics is the role of relativistic effects as contained in phenomenological mean field models. The sensitivity of various nuclear structure observables on these relativistic features are investigated. The report includes the discussion of nuclear matter as well as finite nuclei.
We present and analyze large-scale simulation results of a hybrid quantum-classical variational method to calculate the ground state energy of the anti-ferromagnetic Heisenberg model. Using a massively parallel universal quantum computer simulator, we observe that a low-depth-circuit ansatz advantageously exploits the efficiently preparable N\'{e}el initial state, avoids potential barren plateaus, and works for both one- and two-dimensional lattices. The analysis reflects the decisive ingredients required for a simulation by comparing different ans\"{a}tze, initial parameters, and gradient-based versus gradient-free optimizers. Extrapolation to the thermodynamic limit accurately yields the analytical value for the ground state energy, given by the Bethe ansatz. We predict that a fully functional quantum computer with 100 qubits can calculate the ground state energy with a relatively small error.
We study the thermoelectric properties of a double-Weyl fermion system, possibly realized in $\mathrm{HgCr_2Se_4}$ and $\mathrm{SrSi_2}$, by a semi-classical Boltzmann transport theory. We investigate different relaxation processes including short-range disorder and electron-electron interaction on the thermoelectric transport coefficients. It is found that the anisotropy of the band dispersion for in-plane and out-of-plane momentum directions affects the relaxation time for transport in different directions. The transport also exhibits an interesting directional dependence on the chemical potential and model parameters, differing from a simple isotropic quadratic or linearly dispersing electron gas. By applying a static magnetic field along the linearly dispersing direction, the longitudinal and transverse electrical and thermal magnetoconductivity show a similar dependence on the in-plane cyclotron frequency to the linear dispersing Weyl nodes. By including internode scattering, we find that the chiral anomaly contribution to the thermoelectric coefficients doubles that of a linearly dispersing Weyl node in both the semi-classical and quantum regimes. A magnetic field applied along the quadratically dispersing direction will split the double Weyl point into two single Weyl points with the same chirality.
Does a world that contains chemistry entail the validity of both the standard model of elementary particle physics and general relativity, at least as effective theories? This article shows that the answer may very well be affirmative. It further suggests that the very existence of stable, spatially extended material objects, if not the very existence of the physical world, may require the validity of these theories.
Although lattice gases composed by $k$NN particles, forbidding up to their $k$th nearest neighbors of being occupied, have been widely investigated in literature, the location and the universality class of the fluid-columnar transition in the 2NN model on the square lattice is still a topic of debate. Here, we present grand-canonical solutions of this model on Husimi lattices built with diagonal square lattices, with $2L(L+1)$ sites, for $L \leqslant 7$. The systematic sequence of mean-field solutions confirms the existence of a continuous transition in this system and extrapolations of the critical chemical potential $\mu_{2,c}(L)$ and particle density $\rho_{2,c}(L)$ to $L \rightarrow \infty$ yield estimates of these quantities in close agreement with previous results for the 2NN model on the square lattice. To confirm the reliability of this approach we employ it also for the 1NN model, where very accurate estimates for the critical parameters $\mu_{1,c}$ and $\rho_{1,c}$ -- for the fluid-solid transition in this model on the square lattice -- are found from extrapolations of data for $L \leqslant 6$. The non-classical critical exponents for these transitions are investigated through the coherent anomaly method (CAM), which in the 1NN case yields $\beta$ and $\nu$ differing by at most 6\% from the expected Ising exponents. For the 2NN model, the CAM analysis is somewhat inconclusive, because the exponents sensibly depend on the value of $\mu_{2,c}$ used to calculate them. Notwithstanding, our results suggest that $\beta$ and $\nu$ are considerably larger than the Ashkin-Teller exponents reported in numerical studies of the 2NN system.
We analyze the general class of supersymmetry preserving orbifolds of strong/weak Type IIA/heterotic dual pairs in six dimensions and below. A unified treatment is given by considering compactification to two spacetime dimensions and constructing orbifolds by subgroups of the Fischer-Greiss monster, utilizing the moonshine results of Conway and Norton. Duality requires nontrivial Ramond-Ramond fluxes on the Type IIA side which are localized at the fixed points. Further orbifolding by (-1)^{F_L} gives examples of new four dimensional N=2 Type IIA vacua which are not conformal field theory backgrounds.
We present a new approach to the question of properly defining energy and momenta for non asymptotically Minkowskian spaces in general relativity, in the case where these energy and momenta are conserved. In order to do this, we first prove that there always exist some special Gauss coordinates for which the conserved linear and angular three-momenta vanish. This allows us to consider the case of creatable universes (the universes whose proper 4-momenta vanish) in a consistent way, which is the main interest of the paper. When applied to the Friedmann-Lema{\^{\i}}tre-Robertson-Walker case, perturbed or not, our formalism leads to previous results, according to most literature on the subject. Some future work that should be done is mentioned.
We obtained relatively high signal-to-noise X-ray spectral data of the early massive star tau Sco (B0.2V) with the Suzaku XIS instrument. This source displays several unusual features that motivated our study: (a) redshifted absorption in UV P Cygni lines to approximately +250 km/s suggestive of infalling gas, (b) unusually hard X-ray emission requiring hot plasma at temperatures in excess of 10 MK whereas most massive stars show relatively soft X-rays at a few MK, and (c) a complex photospheric magnetic field of open and closed field lines. In an attempt to understand the hard component better, X-ray data were obtained at six roughly equally spaced phases within the same epoch of tau Sco's 41 day rotation period. The XIS instrument has three operable detectors: XIS1 is back illuminated with sensitivity down to 0.2 keV; XIS0 and XIS2 are front illuminated with sensivitity only down to 0.4 keV and have overall less effective area than XIS1. The XIS0 and XIS3 detectors show relatively little variability. In contrast, there is a 4sigma detection of a 4% drop in the count rate of the XIS1 detector at one rotational phase. In addition, all three detectors show a 3% increase in count rate at the same phase. The most optimistic prediction of X-ray variability allows for a 40% change in the count rate, particularly near phases where we have pointings. Observed modulations in the X-ray light curve on the rotation cycle is an order of magnitude smaller than this, which places new stringent constraints on future modeling of this interesting magnetic massive star.
We show that using character sum estimates due to H. Iwaniec leads to an improvement of recent results about the distribution and finding RSA moduli $M=pl$, where $p$ and $l$ are primes, with prescribed bit patterns. We are now able to specify about $n$ bits instead of about $n/2$ bits as in the previous work. We also show that the same result of H. Iwaniec can be used to obtain an unconditional version of a combinatorial result of W. de Launey and D. Gordon that was originally derived under the Extended Riemann Hypothesis.
We discuss the relativistic top theory from the point of view of the de Sitter (or anti de Sitter) group. Our treatment rests on Hanson-Regge's spherical relativistic top lagrangian formulation. We propose an alternative method for studying spinning objects via Kaluza-Klein theory. In particular, we derive the relativistic top equations of motion starting with the geodesic equation for a point particle in 4+N dimensions. We compare our approach with the Fukuyama's formulation of spinning objects, which is also based on Kaluza-Klein theory. We also report a generalization of our approach to a 4+N+D dimensional theory.
The elastic electron scattering form factors, longitudinal and transverse, from the He and Li isotopes and from 8B have been studied. Large space shell model functions have been assumed. The precise distribution of the neutron excess has little effect on the form factors of the isotopes though there is a mass dependence in the charge densities. However, the form factors of the proton-rich nucleus, 8B, are significantly changed by the presence of the proton halo.
Standard cosmological models rely on an approximate treatment of gravity, utilizing solutions of the linearized Einstein equations as well as physical approximations. In an era of precision cosmology, we should ask: are these approximate predictions sufficiently accurate for comparison to observations, and can we draw meaningful conclusions about properties of our Universe from them? In this work we examine the accuracy of linearized gravity in the presence of collisionless matter and a cosmological constant utilizing fully general relativistic simulations. We observe the gauge-dependence of corrections to linear theory, and note the amplitude of these corrections. For perturbations whose amplitudes are in line with expectations from the standard $\Lambda$CDM model, we find that the full, general relativistic metric is well-described by linear theory in Newtonian and harmonic gauges, while the metric in comoving-synchronous gauge is not. For the largest observed structures in our Universe, our results suggest that corrections to linear gravitational theory can reach or surpass the percent-level.
Here I will present an introduction to the results that have been recently obtained in constraint optimization of random problems using statistical mechanics techniques. After presenting the general results, in order to simplify the presentation I will describe in details the problems related to the coloring of a random graph.
Thermodynamic uncertainty relations have emerged as universal bounds on current fluctuations in non-equilibrium systems. Here we derive a new bound for a particular class of run-and-tumble type processes using the mathematical framework of renewal-reward theory which can be applied to both Markovian and non-Markovian systems. We demonstrate the results for selected single-particle models as well as a variant of the asymmetric simple exclusion process with collective tumbles. Our bound is relatively tight for a broad parameter regime and only requires knowledge of the statistics of run lengths and the mean entropy production rate of tumbles.
(transcript of a talk given by Prof.~Feshbach:) Nuclear physics has been and will be a major factor in science and technology. It makes unique and important contributions to medicine, to industry and to other sciences. Interaction with other physics has been strong. Astrophysics and mesoscopic physics are notable examples. But what I shall talk about today are labeled {\bf``universals"}. This refers to results which transcend the limits of a given subject providing fundamental principles which inform not only nuclear physics but all of science. It is the universals which take nuclear physics research from the parochial albeit fascinating studies of nuclear reactions and structure to the development and formulation of concepts of significance for all of the physical sciences.
Aims. The aim of this paper is to analyse the previously discovered discontinuity of the periodicity of the GeV $\gamma$-ray emission of the radio-loud X-ray binary LS I +61{\deg}303 and to determine its physical origin. Methods. We used wavelet analysis to explore the temporal development of periodic signals. The wavelet analysis was first applied to the whole data set of available Fermi-LAT data and then to the two subsets of orbital phase intervals $\Phi = 0.0 - 0.5$ and $\Phi = 0.5 - 1.0$. We also performed a Lomb-Scargle timing Analysis. We investigated the similarities between GeV $\gamma$-ray emission and radio emission by comparing the folded curves of the Fermi-LAT data and the Green Bank Interferometer radio data. Results. During the epochs when the timing analysis fails to determine the orbital periodicity, the periodicity is present in the two orbital phase intervals $\Phi = 0.0 - 0.5$ and $\Phi = 0.5 - 1.0$. That is, there are two periodical signals, one towards periastron (i.e., $\Phi = 0.0 - 0.5$) and another one towards apoastron ($\Phi = 0.5 - 1.0$). The apoastron peak seems to be affected by the same orbital shift as the radio outbursts and, in addition, reveals the same two periods $P_1$ and $P_2$ that are present in the radio data. Conclusions. The $\gamma$-ray emission of the apoastron peak normally just broadens the emission of the peak around periastron. Only when it appears at $\Phi \approx 0.8 - 1.0$, because of the orbital shift, it is enough detached from the first peak to become recognisable as a second orbital peak, which is the reason why the timing analysis fails. Two $\gamma$-ray peaks along the orbit are predicted by the two-peak accretion model for an eccentric orbit, that was proposed by several authors for LS I +61{\deg}303.
A new search for the gravitational lens effects of a significant cosmological density of supermassive compact objects (SCOs) on gamma-ray bursts has yielded a null result. We inspected the timing data of 774 BATSE-triggered GRBs for evidence of millilensing: repeated peaks similar in light-curve shape and spectra. Our null detection leads us to conclude that, in all candidate universes simulated, $\Omega_{SCO} < 0.1$ is favored for $10^5 < M_{SCO}/M_{\odot} < 10^9$, while in some universes and mass ranges the density limits are as much as 10 times lower. Therefore, a cosmologically significant population of SCOs near globular cluster mass neither came out of the primordial universe, nor condensed at recombination.
Two non-directly interacting qubits with equal frequencies can become entangled via a Markovian, dissipative dynamics through the action of a weakly coupled Ohmic heat bath. In the standard weak-coupling limit derivation, this purely dissipative effect disappears if the frequencies are different because of the "ergodic average" used by this approach. However, there are physical situations where this technique is too rough to capture all the relevant aspects of the dissipative dynamics. In these cases, in order to better describe the physical behavior of the open system, it is necessary to go beyond the "ergodic average". We show that, in this more refined framework, the entanglement capability of the environment persists also in the case of different frequencies.
We study the extension of our translationally invariant treatment of few-body nuclear systems to include tensor forces and correlations. It is shown that a direct application of our method is not as successful for realistic V6 interactions as our previous results for V4 potentials suggested. We investigate the cause in detail for the case of $^4$He, and show that a combination of our method with that of Jastrow-correlated wave functions seems to be a lot more powerful, thereby suggesting that for mildly to strongly repulsive forces such a hybrid procedure may be an appropriate description.