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[en] Complete text of publication follows. In the preparation phase of the Cluster mission, reciprocal vectors were introduced into the space physics community as a generic and convenient tool to estimate spatial gradients from four-point measurements. They have been used also in other applications such as discontinuity analysis and error estimation. In the recently developed wave surveyor technique, we take the reciprocal vector concept to ease the formulation of a direct wave identification method based on the eigendecomposition of the cross spectral density matrix. The wave analysis scheme extracts only the dominant wave mode but is much faster to apply than existing techniques, hence it is expected to ease survey-type detection of waves in large data sets. For three-spacecraft configurations where the tetrahedral reciprocal vector approach fails, we define a set of planar reciprocal vectors that allows to address key multi-point analysis problems such as gradient estimation, wave identification, and discontinuity analysis in the same way as in the four-spacecraft case. The new multi-point analysis tools are demonstrated using synthetic data and Cluster measurements.
[en] This paper proposes a non parametric discrete estimate of evolutionary spectral density of non-stationary process with continuous time. To solve a problem related to aliasing effects, we assume that the evolutionary spectral density has a compact support and we use a specific uniform sampling depending on the width of the compact support. An asymptotically unbiased and consistent estimate is given.
[en] In this paper we introduce and analyze a model of a random collection of random oscillators. The model has a random number of oscillators, the oscillators have random amplitudes and random frequencies, and the model’s output is the aggregate of its oscillators’ outputs. Also, the model has two time scales: a ‘human’ time scale, over which the spectral density of the model’s output is measured; and a ‘cosmic’ or a ‘geological’ time scale, over which the model’s random parameters slowly evolve. Analyzing the model we establish that, with respect to the evolution of the oscillators’ frequencies: (i) general random-walk dynamics universally yield white noise, i.e. flat spectral densities; (ii) general geometric random-walk dynamics universally yield 1/f noise, i.e. harmonic spectral densities; and (iii) general Gaussian geometric random-walk dynamics universally yield white noise and flicker noise, i.e. inverse power-law spectral densities. (paper)
[en] In a recent paper a new method, called displaced spectra techniques, was presented for distinguishing between sinusoidal components and narrowband random noise contributions in otherwise random noise data. It is based on Fourier transform techniques, and uses the power spectral density (PSD) and a newly-introduced second-order displaced power spectra density (SDPSD) function. In order to distinguish between the two peak types, a validation criterion has been established. In this note, three topics are covered: a) improved numerical data for the validation criterion are given by using the refined estimation procedure of the PSD and SDPSD functions by the Welch method; b) the validation criterion requires the subtraction of the background below the peaks. A semiautomatic procedure is described; c) it was observed that peaks in the real part of the SDPSD function can be accompanied by fine structure phenomena which are unresolved in the PSD function. A few remarks are made about this problem. (author)
[en] We analytically evaluate the moments of the spectral density of the -body Sachdev-Ye-Kitaev (SYK) model, and obtain order corrections for all moments, where is the total number of Majorana fermions. To order , moments are given by those of the weight function of the Q-Hermite polynomials. Representing Wick contractions by rooted chord diagrams, we show that the correction for each chord diagram is proportional to the number of triangular loops of the corresponding intersection graph, with an extra grading factor when is odd. Therefore the problem of finding corrections is mapped to a triangle counting problem. Since the total number of triangles is a purely graph-theoretic property, we can compute them for the and SYK models, where the exact moments can be obtained analytically using other methods, and therefore we have solved the moment problem for any to accuracy. The moments are then used to obtain the spectral density of the SYK model to order . We also obtain an exact analytical result for all contraction diagrams contributing to the moments, which can be evaluated up to eighth order. This shows that the Q-Hermite approximation is accurate even for small values of .
[en] Measurements have been made of the EAS array density spectrum for densities close to the knee of the EAS size spectrum. A four scintillator array with a typical spacing of approximately 30 m has been used and measurements have also been made with pairs of detectors from the Buckland Park EAS array
[en] A central challenge in modern condensed matter physics is developing the tools for understanding nontrivial yet unordered states of matter. One important idea to emerge in this context is that of a ‘pseudogap’: the fact that under appropriate circumstances the normal state displays a suppression of the single particle spectral density near the Fermi level, reminiscent of the gaps seen in ordered states of matter. While these concepts arose in a solid state context, they are now being explored in cold gases. This article reviews the current experimental and theoretical understanding of the normal state of strongly interacting Fermi gases, with particular focus on the phenomonology which is traditionally associated with the pseudogap. (report on progress)