Tag Archives: Disordered Systems and Neural Networks

Anisotropic electron transport in the nuclear pasta phase [CL]

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http://arxiv.org/abs/2212.11817


The presence of nuclear pasta is expected to modify the transport properties in the mantle of neutron stars. The non-spherical geometry of the pasta nuclear clusters leads to anisotropies in the collision frequencies, impacting the thermal and electrical conductivity. We derive analytical expressions for the anisotropic collision frequencies using the Boltzmann equation in the relaxation time approximation. The average parallel, perpendicular and Hall electrical conductivities are computed in the high-temperature regime above crustal melting, considering incoherent elastic electron-pasta scattering and randomly oriented pasta structures. Numerical values are obtained at different densities and temperatures by using the IUFSU parametrization of the non-linear Walecka model to determine the crustal structure. We find that the anisotropy of the collision frequencies grows with the length of the pasta structures and, independently of the magnetic field, the presence of rod and slab phases decreases the conductivity by more than one order of magnitude. Our numerical results indicate that, even if the pasta structures might survive above the crustal melting point, no strong anisotropies are to be expected in the conduction properties in this temperature regime, even in the presence of a very high magnetic field.

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M. Pelicer, M. Antonelli, D. Menezes, et. al.
Fri, 23 Dec 22
9/58

Comments: 15 pages, 12 figures

Domain wall annihilation — a QFT perspective [CL]

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http://arxiv.org/abs/2212.11204


Domain wall networks in the early universe, formed upon spontaneous breaking of a discrete symmetry, have a rich impact on cosmology. Yet, they remain somewhat unexplored. We introduce a new analytic strategy to understand better the domain wall epoch, from formation to annihilation. Our method includes a quantum field theoretical treatment of the initial state at domain wall formation, as well as of the time evolution. We find that the domain wall area density for a network with biased initial condition in $d+1$ dimensional flat spacetime evolves as $t^{-1/2}\,\exp\big(- (t/t_{ann})^{d/2}\big)$. We comment on the relation between this and previous results obtained in condensed matter and in cosmology. The extrapolation of this law to an expanding universe applies to networks that are close to the domain wall `gas’ limit.

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O. Pujolas and G. Zahariade
Thu, 22 Dec 22
43/59

Comments: 14 pages, 5 figures

The Disordered Heterogeneous Universe: Galaxy Distribution and Clustering Across Length Scales [CEA]

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http://arxiv.org/abs/2207.00519


Studies of disordered heterogeneous media and galaxy cosmology share a common goal: analyzing the distribution of particles at microscales' to predict physical properties atmacroscales’, whether for a liquid, composite material, or entire Universe. The former theory provides an array of techniques to characterize a wide class of microstructures; in this work, we apply them to the distributions of galaxies. We focus on the lower-order correlation functions, void' andparticle’ nearest-neighbor functions, pair-connectedness functions, percolation properties, and a scalar order metric. Compared to homogeneous Poisson and typical disordered systems, the cosmological simulations exhibit enhanced large-scale clustering and longer tails in the nearest-neighbor functions, due to the presence of quasi-long-range correlations. On large scales, the system appears hyperuniform', due to primordial density fluctuations, whilst on the smallest scales, the system becomes almostantihyperuniform’, and, via the order metric, is shown to be a highly correlated disordered system. Via a finite scaling analysis, we show that the percolation threshold of the galaxy catalogs is significantly lower than for Poisson realizations; this is consistent with the observation that the galaxy distribution contains larger voids. However, the two sets of simulations share a fractal dimension, implying that they lie in the same universality class. Finally, we consider the ability of large-scale clustering statistics to constrain cosmological parameters using simulation-based inference. Both the nearest-neighbor distribution and pair-connectedness function considerably tighten bounds on the amplitude of cosmological fluctuations at a level equivalent to observing twenty-five times more galaxies. These are a useful alternative to the three-particle correlation, and are computable in much reduced time. (Abridged)

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O. Philcox and S. Torquato
Mon, 4 Jul 22
30/62

Comments: 27 pages, submitted to Phys. Rev. X

On the visible continuum and lines in the Interstellar Extinction Curve [GA]

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http://arxiv.org/abs/1905.07560


This work purports to help understand the InterStellar Extinction Curve in and near the visible range. In this range, crystalline materials are known to be transparent, so amorphous dust is needed. Molecular modeling experiments are used to compute the electronic spectra of various, relatively large, carbon and silicate structures. Hardly any transition shows up beyond 0.4 mum when the structure is in its ground state (the lowest, most stable state, usually crystalline). This is no longer the case as soon as the structure is distorted in any way. Examples of simulated distortions (or “defects”) are: angular or linear bond alteration, insertion of free radicals near the main structure, dangling bonds; their cumulative effects lead to the amorphous state. It is shown that, in this state, a structure bears a majority of weak transitions and a minority of strong ones. As the structure grows in size, the former ultimately form a weak continuum already detected experimentally, in the visible, on amorphous carbons and silicates. The stronger transitions will manage to emerge above the continuum, especially when they bunch together by chance near the same wavelength. Parallels are drawn between several properties of the computed continua and transitions and the observed continuum and Diffuse Interstellar Bands.

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R. Papoular
Tue, 21 May 19
52/71

Comments: N/A

Machine learning and the physical sciences [CL]

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http://arxiv.org/abs/1903.10563


Machine learning encompasses a broad range of algorithms and modeling tools used for a vast array of data processing tasks, which has entered most scientific disciplines in recent years. We review in a selective way the recent research on the interface between machine learning and physical sciences.This includes conceptual developments in machine learning (ML) motivated by physical insights, applications of machine learning techniques to several domains in physics, and cross-fertilization between the two fields. After giving basic notion of machine learning methods and principles, we describe examples of how statistical physics is used to understand methods in ML. We then move to describe applications of ML methods in particle physics and cosmology, quantum many body physics, quantum computing, and chemical and material physics. We also highlight research and development into novel computing architectures aimed at accelerating ML. In each of the sections we describe recent successes as well as domain-specific methodology and challenges.

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G. Carleo, I. Cirac, K. Cranmer, et. al.
Wed, 27 Mar 19
11/74

Comments: N/A

Applications of deep learning to relativistic hydrodynamics [CL]

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http://arxiv.org/abs/1801.03334


Relativistic hydrodynamics is a powerful tool to simulate the evolution of the quark gluon plasma (QGP) in relativistic heavy ion collisions. Using 10000 initial and final profiles generated from 2+1-d relativistic hydrodynamics VISH2+1 with MC-Glauber initial conditions, we train a deep neural network based on stacked U-net, and use it to predict the final profiles associated with various initial conditions, including MC-Glauber, MC-KLN and AMPT and TRENTo. A comparison with the VISH2+1 results shows that the network predictions can nicely capture the magnitude and inhomogeneous structures of the final profiles, and nicely describe the related eccentricity distributions $P(\varepsilon_n)$ (n=2, 3, 4). These results indicate that deep learning technique can capture the main features of the non-linear evolution of hydrodynamics, showing its potential to largely accelerate the event-by-event simulations of relativistic hydrodynamics.

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H. Huang, B. Xiao, H. Xiong, et. al.
Thu, 11 Jan 18
3/56

Comments: 4 pages, 3 figures

Quantum chaos of dark matter in the Solar System [CL]

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http://arxiv.org/abs/1711.07815


We perform time-dependent analysis of quantum dynamics of dark matter particles in the Solar System. It is shown that this problem has similarities with a microwave ionization of Rydberg atoms studied previously experimentally and analytically. On this basis it is shown that the quantum effects for chaotic dark matter dynamics become significant for dark matter mass ratio to electron mass being smaller than $2 \times 10^{-15}$. Below this border multiphoton diffusion over Rydberg states of dark matter atom becomes exponentially localized in analogy with the Anderson localization in disordered solids. The life time of dark matter in the Solar System is determined in dependence on mass ratio in the localized phase and a few photon ionization regime. Various implications of these quantum results are discussed for the capture of dark matter from Galaxy and its steady-state density distribution.

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D. Shepelyansky
Thu, 23 Nov 17
48/52

Comments: 5 pages, 2 figures

Quantum chaos of dark matter in the Solar System [CL]

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http://arxiv.org/abs/1711.07815


We perform time-dependent analysis of quantum dynamics of dark matter particles in the Solar System. It is shown that this problem has similarities with a microwave ionization of Rydberg atoms studied previously experimentally and analytically. On this basis it is shown that the quantum effects for chaotic dark matter dynamics become significant for dark matter mass ratio to electron mass being smaller than $2 \times 10^{-15}$. Below this border multiphoton diffusion over Rydberg states of dark matter atom becomes exponentially localized in analogy with the Anderson localization in disordered solids. The life time of dark matter in the Solar System is determined in dependence on mass ratio in the localized phase and a few photon ionization regime. Various implications of these quantum results are discussed for the capture of dark matter from Galaxy and its steady-state density distribution.

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D. Shepelyansky
Thu, 23 Nov 17
25/52

Comments: 5 pages, 2 figures

ENDOR study of nitrogen hyperfine and quadrupole tensors in vanadyl porphyrins of heavy crude oil [CL]

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http://arxiv.org/abs/1702.06066


We report the observation of pulsed electron-nuclear double resonance (ENDOR) spectrum caused by interactions of the nitrogen nuclei 14N with the unpaired electron of the paramagnetic vanadyl complexes VO2+ of vanadyl porphyrins in natural crude oil. We provide detailed experimental and theoretical characterization of the nitrogen hyperfine and quadrupole tensors.

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I. Gracheva, M. Gafurov, G. Mamin, et. al.
Tue, 21 Feb 17
29/70

Comments: 6 pages, 2 Figures

Accelerating cross-validation with total variation and its application to super-resolution imaging [CL]

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http://arxiv.org/abs/1611.07197


We develop an approximation formula for the cross-validation error (CVE) of a sparse linear regression penalized by $\ell_1$-norm and total variation terms, which is based on a perturbative expansion utilizing the largeness of both the data dimensionality and the model. The developed formula allows us to reduce the necessary computational cost of the CVE evaluation significantly. The practicality of the formula is tested through application to simulated black-hole image reconstruction on the event-horizon scale with super resolution. The results demonstrate that our approximation reproduces the CVE values obtained via literally conducted cross-validation with reasonably good precision.

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T. Obuchi, S. Ikeda, K. Akiyama, et. al.
Wed, 23 Nov 16
13/68

Comments: 5 pages, 1 figure

From Wires to Cosmology [CEA]

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http://arxiv.org/abs/1512.02637


We provide a statistical framework for characterizing stochastic particle production in the early universe via a precise correspondence to current conduction in wires with impurities. Our approach is particularly useful when the microphysics is uncertain and the dynamics are complex, but only coarse-grained information is of interest. We study scenarios with multiple interacting fields and derive the evolution of the particle occupation numbers from a Fokker-Planck equation. At late times, the typical occupation numbers grow exponentially which is the analog of Anderson localization for disordered wires. Some statistical features of the occupation numbers show hints of universality in the limit of a large number of interactions and/or a large number of fields. For test cases, excellent agreement is found between our analytic results and numerical simulations.

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M. Amin and D. Baumann
Thu, 10 Dec 15
45/63

Comments: 35 pages, 8 figures

The characterisation of irregularly-shaped particles: a re-consideration of finite-sized, porous and fractal grains [CL]

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http://arxiv.org/abs/1511.01663


Context. A porous and/or fractal description can generally be applied where particles have undergone coagulation into aggregates. Aims. To characterise finite-sized, porous and fractal particles and to understand the possible limitations of these descriptions. Methods. We use simple structure, lattice and network considerations to determine the structural properties of irregular particles. Results. We find that, for finite-sized aggregates, the terms porosity and fractal dimension may be of limited usefulness and show with some critical and limiting assumptions, that highly-porous aggregates (porosity > 80%) may not be constructable. We also investigate their effective cross-sections using a simple cubic model. Conclusions. In place of the terms porosity and fractal dimension, for finite-sized aggregates, we propose the readily-determinable quantities of inflation, I (a measure of the solid filling factor and size), and dimensionality, D (a measure of the shape). These terms can be applied to characterise any form of particle, be it an irregular, homogeneous solid or a highly-extended aggregate.

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A. Jones
Mon, 9 Nov 15
50/55

Comments: 13 pages, 8 figures

Variations on a theme – the evolution of hydrocarbon solids: I. Compositional and spectral modelling – the eRCN and DG models [GA]

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http://arxiv.org/abs/1511.01673


Context. The compositional properties of hydrogenated amorphous carbons are known to evolve in response to the local conditions. Aims. To present a model for low-temperature, amorphous hydrocarbon solids, based on the microphysical properties of random and defected networks of carbon and hydrogen atoms, that can be used to study and predict the evolution of their properties in the interstellar medium. Methods. We adopt an adaptable and prescriptive approach to model these materials, which is based on a random covalent network (RCN) model, extended here to a full compositional derivation (the eRCN model), and a defective graphite (DG) model for the hydrogen poorer materials where the eRCN model is no longer valid. Results. We provide simple expressions that enable the determination of the structural, infrared and spectral properties of amorphous hydrocarbon grains as a function of the hydrogen atomic fraction, XH. Structural annealing, resulting from hydrogen atom loss, results in a transition from H-rich, aliphatic-rich to H-poor, aromatic-rich materials. Conclusions. The model predicts changes in the optical properties of hydrogenated amorphous carbon dust in response to the likely UV photon-driven and/or thermal annealing processes resulting, principally, from the radiation field in the environment. We show how this dust component will evolve, compositionally and structurally in the interstellar medium in response to the local conditions.

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A. Jones
Fri, 6 Nov 15
10/69

Comments: 19 pages, 15 figures

Quantum path integral molecular dynamics simulations on transport properties of dense liquid helium [EPA]

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http://arxiv.org/abs/1502.04554


Transport properties of dense liquid helium under the conditions of planet’s core and cool atmosphere of white dwarfs have been investigated by using the improved centroid path-integral simulations combined with density functional theory. The self-diffusion is largely higher and the shear viscosity is notably lower predicted with the quantum mechanical description of the nuclear motion compared with the description by Newton equation. The results show that nuclear quantum effects (NQEs), which depends on the temperature and density of the matter via the thermal de Broglie wavelength and the ionization of electrons, are essential for the transport properties of dense liquid helium at certain astrophysical conditions. The Stokes-Einstein relation between diffusion and viscosity in strongly coupled regime is also examined to display the influences of NQEs.

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D. Kang, J. Dai, H. Sun, et. al.
Tue, 17 Feb 15
22/60

Comments: 6 figures