Gregory V. Brown - LLNL

Gregory V. Brown
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Gregory V. Brown
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LLNL
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Physics - Statistical Mechanics (15)
 
Physics - Materials Science (11)
 
High Energy Astrophysical Phenomena (5)
 
Physics - Chemical Physics (2)
 
Astrophysics of Galaxies (2)
 
Cosmology and Nongalactic Astrophysics (2)
 
Physics - Computational Physics (2)
 
Astrophysics (1)
 
Physics - Soft Condensed Matter (1)

Publications Authored By Gregory V. Brown

A macroscopically constrained Wang-Landau Monte Carlo method was recently proposed to calculate the joint density of states (DOS) for systems with multiple order parameters. Here we demonstrate results for a nearest-neighbor Ising antiferromagnet with ferromagnetic long-range interactions (a model spin-crossover material). Its two relevant order parameters are the magnetization $M$ and the staggered magnetization $M_{\rm s}$. Read More

A generalized approach to Wang-Landau simulations, macroscopically constrained Wang-Landau, is proposed to simulate the density of states of a system with multiple macroscopic order parameters. The method breaks a multidimensional random-walk process in phase space into many separate, one-dimensional random-walk processes in well-defined subspaces. Each of these random walks is constrained to a different set of values of the macroscopic order parameters. Read More

2016Jul
Authors: Hitomi Collaboration, Felix A. Aharonian, Hiroki Akamatsu, Fumie Akimoto, Steven W. Allen, Lorella Angelini, Keith A. Arnaud, Marc Audard, Hisamitsu Awaki, Magnus Axelsson, Aya Bamba, Marshall W. Bautz, Roger D. Blandford, Laura W. Brenneman, Gregory V. Brown, Esra Bulbul, Edward M. Cackett, Maria Chernyakova, Meng P. Chiao, Paolo Coppi, Elisa Costantini, Jelle de Plaa, Jan-Willem den Herder, Chris Done, Tadayasu Dotani, Ken Ebisawa, Megan E. Eckart, Teruaki Enoto, Yuichiro Ezoe, Andrew C. Fabian, Carlo Ferrigno, Adam R. Foster, Ryuichi Fujimoto, Yasushi Fukazawa, Akihiro Furuzawa, Massimiliano Galeazzi, Luigi C. Gallo, Poshak Gandhi, Margherita Giustini, Andrea Goldwurm, Liyi Gu, Matteo Guainazzi, Yoshito Haba, Kouichi Hagino, Kenji Hamaguchi, Ilana Harrus, Isamu Hatsukade, Katsuhiro Hayashi, Takayuki Hayashi, Kiyoshi Hayashida, Junko Hiraga, Ann E. Hornschemeier, Akio Hoshino, John P. Hughes, Yuto Ichinohe, Ryo Iizuka, Hajime Inoue, Shota Inoue, Yoshiyuki Inoue, Kazunori Ishibashi, Manabu Ishida, Kumi Ishikawa, Yoshitaka Ishisaki, Masayuki Itoh, Masachika Iwai, Naoko Iyomoto, Jelle S. Kaastra, Timothy Kallman, Tuneyoshi Kamae, Erin Kara, Jun Kataoka, Satoru Katsuda, Junichiro Katsuta, Madoka Kawaharada, Nobuyuki Kawai, Richard L. Kelley, Dmitry Khangulyan, Caroline A. Kilbourne, Ashley L. King, Takao Kitaguchi, Shunji Kitamoto, Tetsu Kitayama, Takayoshi Kohmura, Motohide Kokubun, Shu Koyama, Katsuji Koyama, Peter Kretschmar, Hans A. Krimm, Aya Kubota, Hideyo Kunieda, Philippe Laurent, Francois Lebrun, Shiu-Hang Lee, Maurice Leutenegger, Olivier Limousin, Michael Loewenstein, Knox S. Long, David Lumb, Grzegorz M. Madejski, Yoshitomo Maeda, Daniel Maier, Kazuo Makishima, Maxim Markevitch, Hironori Matsumoto, Kyoko Matsushita, Dan McCammon, Brian R. McNamara, Missagh Mehdipour, Eric D. Miller, Jon M. Miller, Shin Mineshige, Kazuhisa Mitsuda, Ikuyuki Mitsuishi, Takuya Miyazawa, Tsunefumi Mizuno, Hideyuki Mori, Koji Mori, Harvey Moseley, Koji Mukai, Hiroshi Murakami, Toshio Murakami, Richard F. Mushotzky, Takao Nakagawa, Hiroshi Nakajima, Takeshi Nakamori, Toshio Nakano, Shinya Nakashima, Kazuhiro Nakazawa, Kumiko Nobukawa, Masayoshi Nobukawa, Hirofumi Noda, Masaharu Nomachi, Steve L. O'Dell, Hirokazu Odaka, Takaya Ohashi, Masanori Ohno, Takashi Okajima, Naomi Ota, Masanobu Ozaki, Frits Paerels, Stephane Paltani, Arvind Parmar, Robert Petre, Ciro Pinto, Martin Pohl, F. Scott Porter, Katja Pottschmidt, Brian D. Ramsey, Christopher S. Reynolds, Helen R. Russell, Samar Safi-Harb, Shinya Saito, Kazuhiro Sakai, Hiroaki Sameshima, Toru Sasaki, Goro Sato, Kosuke Sato, Rie Sato, Makoto Sawada, Norbert Schartel, Peter J. Serlemitsos, Hiromi Seta, Megumi Shidatsu, Aurora Simionescu, Randall K. Smith, Yang Soong, Lukasz Stawarz, Yasuharu Sugawara, Satoshi Sugita, Andrew E. Szymkowiak, Hiroyasu Tajima, Hiromitsu Takahashi, Tadayuki Takahashi, Shin'ichiro Takeda, Yoh Takei, Toru Tamagawa, Keisuke Tamura, Takayuki Tamura, Takaaki Tanaka, Yasuo Tanaka, Yasuyuki Tanaka, Makoto Tashiro, Yuzuru Tawara, Yukikatsu Terada, Yuichi Terashima, Francesco Tombesi, Hiroshi Tomida, Yohko Tsuboi, Masahiro Tsujimoto, Hiroshi Tsunemi, Takeshi Tsuru, Hiroyuki Uchida, Hideki Uchiyama, Yasunobu Uchiyama, Shutaro Ueda, Yoshihiro Ueda, Shiro Ueno, Shin'ichiro Uno, C. Meg Urry, Eugenio Ursino, Cor P. de Vries, Shin Watanabe, Norbert Werner, Daniel R. Wik, Dan R. Wilkins, Brian J. Williams, Shinya Yamada, Hiroya Yamaguchi, Kazutaka Yamaoka, Noriko Y. Yamasaki, Makoto Yamauchi, Shigeo Yamauchi, Tahir Yaqoob, Yoichi Yatsu, Daisuke Yonetoku, Atsumasa Yoshida, Irina Zhuravleva, Abderahmen Zoghbi

High-resolution X-ray spectroscopy with Hitomi was expected to resolve the origin of the faint unidentified E=3.5 keV emission line reported in several low-resolution studies of various massive systems, such as galaxies and clusters, including the Perseus cluster. We have analyzed the Hitomi first-light observation of the Perseus cluster. Read More

2016Jul
Authors: Hitomi Collaboration, Felix Aharonian, Hiroki Akamatsu, Fumie Akimoto, Steven W. Allen, Naohisa Anabuki, Lorella Angelini, Keith Arnaud, Marc Audard, Hisamitsu Awaki, Magnus Axelsson, Aya Bamba, Marshall Bautz, Roger Blandford, Laura Brenneman, Gregory V. Brown, Esra Bulbul, Edward Cackett, Maria Chernyakova, Meng Chiao, Paolo Coppi, Elisa Costantini, Jelle de Plaa, Jan-Willem den Herder, Chris Done, Tadayasu Dotani, Ken Ebisawa, Megan Eckart, Teruaki Enoto, Yuichiro Ezoe, Andrew Fabian, Carlo Ferrigno, Adam Foster, Ryuichi Fujimoto, Yasushi Fukazawa, Akihiro Furuzawa, Massimiliano Galeazzi, Luigi Gallo, Poshak Gandhi, Margherita Giustini, Andrea Goldwurm, Liyi Gu, Matteo Guainazzi, Yoshito Haba, Kouichi Hagino, Kenji Hamaguchi, Ilana Harrus, Isamu Hatsukade, Katsuhiro Hayashi, Takayuki Hayashi, Kiyoshi Hayashida, Junko Hiraga, Ann Hornschemeier, Akio Hoshino, John Hughes, Ryo Iizuka, Hajime Inoue, Yoshiyuki Inoue, Kazunori Ishibashi, Manabu Ishida, Kumi Ishikawa, Yoshitaka Ishisaki, Masayuki Itoh, Naoko Iyomoto, Jelle Kaastra, Timothy Kallman, Tuneyoshi Kamae, Erin Kara, Jun Kataoka, Satoru Katsuda, Junichiro Katsuta, Madoka Kawaharada, Nobuyuki Kawai, Richard Kelley, Dmitry Khangulyan, Caroline Kilbourne, Ashley King, Takao Kitaguchi, Shunji Kitamoto, Tetsu Kitayama, Takayoshi Kohmura, Motohide Kokubun, Shu Koyama, Katsuji Koyama, Peter Kretschmar, Hans Krimm, Aya Kubota, Hideyo Kunieda, Philippe Laurent, Francois Lebrun, Shiu-Hang Lee, Maurice Leutenegger, Olivier Limousin, Michael Loewenstein, Knox S. Long, David Lumb, Grzegorz Madejski, Yoshitomo Maeda, Daniel Maier, Kazuo Makishima, Maxim Markevitch, Hironori Matsumoto, Kyoko Matsushita, Dan McCammon, Brian McNamara, Missagh Mehdipour, Eric Miller, Jon Miller, Shin Mineshige, Kazuhisa Mitsuda, Ikuyuki Mitsuishi, Takuya Miyazawa, Tsunefumi Mizuno, Hideyuki Mori, Koji Mori, Harvey Moseley, Koji Mukai, Hiroshi Murakami, Toshio Murakami, Richard Mushotzky, Ryo Nagino, Takao Nakagawa, Hiroshi Nakajima, Takeshi Nakamori, Toshio Nakano, Shinya Nakashima, Kazuhiro Nakazawa, Masayoshi Nobukawa, Hirofumi Noda, Masaharu Nomachi, Steve O'Dell, Hirokazu Odaka, Takaya Ohashi, Masanori Ohno, Takashi Okajima, Naomi Ota, Masanobu Ozaki, Frits Paerels, Stephane Paltani, Arvind Parmar, Robert Petre, Ciro Pinto, Martin Pohl, F. Scott Porter, Katja Pottschmidt, Brian Ramsey, Christopher Reynolds, Helen Russell, Samar Safi-Harb, Shinya Saito, Kazuhiro Sakai, Hiroaki Sameshima, Goro Sato, Kosuke Sato, Rie Sato, Makoto Sawada, Norbert Schartel, Peter Serlemitsos, Hiromi Seta, Megumi Shidatsu, Aurora Simionescu, Randall Smith, Yang Soong, Lukasz Stawarz, Yasuharu Sugawara, Satoshi Sugita, Andrew Szymkowiak, Hiroyasu Tajima, Hiromitsu Takahashi, Tadayuki Takahashi, Shin'ichiro Takeda, Yoh Takei, Toru Tamagawa, Keisuke Tamura, Takayuki Tamura, Takaaki Tanaka, Yasuo Tanaka, Yasuyuki Tanaka, Makoto Tashiro, Yuzuru Tawara, Yukikatsu Terada, Yuichi Terashima, Francesco Tombesi, Hiroshi Tomida, Yohko Tsuboi, Masahiro Tsujimoto, Hiroshi Tsunemi, Takeshi Tsuru, Hiroyuki Uchida, Hideki Uchiyama, Yasunobu Uchiyama, Shutaro Ueda, Yoshihiro Ueda, Shiro Ueno, Shin'ichiro Uno, Meg Urry, Eugenio Ursino, Cor de Vries, Shin Watanabe, Norbert Werner, Daniel Wik, Dan Wilkins, Brian Williams, Shinya Yamada, Hiroya Yamaguchi, Kazutaka Yamaok, Noriko Y. Yamasaki, Makoto Yamauchi, Shigeo Yamauchi, Tahir Yaqoob, Yoichi Yatsu, Daisuke Yonetoku, Atsumasa Yoshida, Takayuki Yuasa, Irina Zhuravleva, Abderahmen Zoghbi

Clusters of galaxies are the most massive gravitationally-bound objects in the Universe and are still forming. They are thus important probes of cosmological parameters and a host of astrophysical processes. Knowledge of the dynamics of the pervasive hot gas, which dominates in mass over stars in a cluster, is a crucial missing ingredient. Read More

2016Apr
Affiliations: 1Remeis-Observatory and ECAP, FAU Erlangen, 2Remeis-Observatory and ECAP, FAU Erlangen, 3Remeis-Observatory and ECAP, FAU Erlangen, 4MIT Kavli Institute, 5CRESST-UMBC, 6MIT Kavli Institute, 7Remeis-Observatory and ECAP, FAU Erlangen, 8Remeis-Observatory and ECAP, FAU Erlangen, 9IMAPP, Radboud-University Nijmegen, 10IoA, University of Cambridge, 11CEA Saclay, 12Max Planck Computing and Data Facility, 13Dept. of Chemistry, Physics, and Astronomy, Georgia College and State University, 14SSL UC Berkeley, 15Harvard John A. Paulson School of Engineering and Applied Sciences and CfA, 16LLNL, 17Remeis-Observatory and ECAP, FAU Erlangen

The accretion onto the black hole in the system HDE 226868/Cygnus X-1 is powered by the strong line driven stellar wind of the O-type donor star. We study the X-ray properties of the stellar wind in the hard state of Cyg X-1 as determined with data from the Chandra High Energy Transmission Gratings. Large density and temperature inhomogeneities are present in the wind, with a fraction of the wind consisting of clumps of matter with higher density and lower temperature embedded in a photoionized gas. Read More

2015Oct
Affiliations: 1University of Michigan, 2SRON, 3University of Maryland, College Park, 4University of Michigan, 5University of Warwick, 6Joint Space-Science Institute, University of Maryland, College Park, 7Smithsonian Astrophysical Observatory, 8University of Maryland, College Park, 9Harvard University, 10University of Michigan, 11University of Alabama, 12University of Warwick, 13Wheaton College, 14University of Alabama, 15University of Maryland, College Park, 16University of Leicester, 17Columbia University, 18SRON, 19University of California, Santa Cruz, 20Joint Space-Science Institute, University of Maryland, College Park, 21University of Leicester

Tidal forces close to massive black holes can violently disrupt stars that make a close approach. These extreme events are discovered via bright X-ray and optical/UV flares in galactic centers. Prior studies based on modeling decaying flux trends have been able to estimate broad properties, such as the mass accretion rate. Read More

We report a multiwavelength (X-ray, ultraviolet/optical/infrared, radio) analysis of the relativistic tidal disruption event candidate Sw J2058+05 from 3 months to 3 yr post-discovery in order to study its properties and compare its behavior with that of Sw J1644+57. Our main results are as follows. (1) The long-term X-ray light curve of Sw J2058+05 shows a remarkably similar trend to that of Sw J1644+57. Read More

The unusual thermodynamic properties of the Ising antiferromagnet supplemented with a ferromagnetic, mean-field term are outlined. This simple model is inspired by more realistic models of spin-crossover materials. The phase diagram is estimated using Metropolis Monte Carlo methods, and differences with preliminary Wang-Landau Monte Carlo results for small systems are noted. Read More

We have observed the diffuse X-ray emission from the Galactic center (GC) using the X-ray Imaging Spectrometer (XIS) on Suzaku. The high-energy resolution and the low-background orbit provide excellent spectra of the GC diffuse X-rays (GCDX). The XIS found many emission lines in the GCDX near the energy of K-shell transitions of iron and nickel. Read More

A numerical model of single-domain nanoscale iron magnets fabricated using scanning-microscope-assisted chemical vapor deposition is simulated using finite-temperature micromagnetics. A Projective-dynamics method is used to determine the magnetization at the saddle point as a function of temperature. This magnetization is found to decrease linearly as the temperature is raised. Read More

We discuss numerical and theoretical results for models of magnetization switching in nanoparticles and ultrathin films. The models and computational methods include kinetic Ising and classical Heisenberg models of highly anisotropic magnets which are simulated by dynamic Monte Carlo methods, and micromagnetics models of continuum-spin systems that are studied by finite-temperature Langevin simulations. The theoretical analysis builds on the fact that a magnetic particle or film that is magnetized in a direction antiparallel to the applied field is in a metastable state. Read More

Results for the late-time regime of phase ordering in three dimensions are reported, based on numerical integration of the time-dependent Ginzburg-Landau equation with nonconserved order parameter at zero temperature. For very large systems ($700^3$) at late times, $t \ge 150,$ the characteristic length grows as a power law, $R(t) \sim t^n$, with the measured $n$ in agreement with the theoretically expected result $n=1/2$ to within statistical errors. In this time regime $R(t)$ is found to be in excellent agreement with the analytical result of Ohta, Jasnow, and Kawasaki [Phys. Read More

Dynamic quantities related to hysteresis have been measured in micromagnetic simulations of single-domain nanoscale magnets at nonzero temperature. The hysteresis-loop area and magnetization-field correlation display the characteristics of resonance, and the resonance frequency is found to be temperature dependent. The period-averaged magnetization displays symmetry breaking at high frequencies. Read More

Numerical solutions of the Landau-Lifshitz-Gilbert micromagnetic model incorporating thermal fluctuations and dipole-dipole interactions (calculated by the Fast Multipole Method) are presented for systems composed of nanoscale iron pillars of dimension 9 nm x 9 nm x 150 nm. Hysteresis loops generated under sinusoidally varying fields are obtained, while the coercive field is estimated to be 1979 $\pm$ 14 Oe using linear field sweeps at T=0 K. Thermal effects are essential to the relaxation of magnetization trapped in a metastable orientation, such as happens after a rapid reversal of an external magnetic field less than the coercive value. Read More

2000Oct
Affiliations: 1Florida State Univ, 2Florida State Univ, 3Florida State Univ

The results of large-scale simulations investigating the dynamics of magnetization reversal in arrays of single-domain nanomagnets after a rapid reversal of the applied field at nonzero temperature are presented. The numerical micromagnetic approach uses the Landau-Lifshitz-Gilbert equation including contributions from thermal fluctuations and long-range dipole-dipole demagnetizing effects implemented using a fast-multipole expansion. The individual model nanomagnets are 9 nm x 9 nm x 150 nm iron pillars similar to those fabricated on a surface with STM-assisted chemical vapor deposition [S. Read More

Numerical integration of a stochastic Landau-Lifshitz-Gilbert equation is used to study dynamic processes in single-domain nanoscale magnets at nonzero temperatures. Special attention is given to including thermal fluctuations as a Langevin term, and the Fast Multipole Method is used to calculate dipole-dipole interactions. It is feasible to simulate these dynamics on the nanosecond time scale for spatial discretizations that involve on the order of 10000 nodes using a desktop workstation. Read More

1999May

Time-dependent properties of the speckled intensity patterns created by scattering coherent radiation from materials undergoing spinodal decomposition are investigated by numerical integration of the Cahn-Hilliard-Cook equation. For binary systems which obey a local conservation law, the characteristic domain size is known to grow in time $\tau$ as $R = [B \tau]^n$ with n=1/3, where B is a constant. The intensities of individual speckles are found to be nonstationary, persistent time series. Read More

The universal form for the average scattering intensity from systems undergoing order-disorder transitions is found by numerical integration of the Langevin dynamics. The result is nearly identical for simulations involving two different forms of the local contribution to the free energy, supporting the idea that the Model A dynamical universality class includes a wide range of local free-energy forms. An absolute comparison with no adjustable parameters is made to the forms predicted by the theories of Ohta-Jasnow-Kawasaki and Mazenko. Read More

We present a tutorial discussion of Monte Carlo methods for equilibrium and nonequilibrium problems in interfacial electrochemistry. The discussion is illustrated with results from simulations of three specific systems: bromine adsorption on silver (100), underpotential deposition of copper on gold (111), and electrodeposition of urea on platinum (100). Read More

Numerical studies of lattice-gas models are well suited to describe multi-adsorbate systems. One example is the underpotential deposition of Cu on Au(111) in the presence of sulfuric acid. Preliminary results from dynamic Monte Carlo simulations of the evolution of the adsorbed layer during potential-step experiments across phase transitions are presented for this particular system. Read More

1997Jun
Affiliations: 1Florida State, McGill, Kyoto, 2Florida State, McGill, Kyoto, 3Florida State, McGill, Kyoto, 4Florida State, McGill, Kyoto

The statistical properties of coherent radiation scattered from phase-ordering materials are studied in detail using large-scale computer simulations and analytic arguments. Specifically, we consider a two-dimensional model with a nonconserved, scalar order parameter (Model A), quenched through an order-disorder transition into the two-phase regime. For such systems it is well established that the standard scaling hypothesis applies, consequently the average scattering intensity at wavevector _k and time t' is proportional to a scaling function which depends only on a rescaled time, t ~ |_k|^2 t'. Read More

We discuss applications of statistical-mechanical lattice-gas models to study static and dynamic aspects of electrochemical adsorption. The strategy developed to describe specific systems includes microscopic model formulation, calculation of zero-temperature phase diagrams, numerical simulation of thermodynamic and structural quantities at nonzero temperatures, and estimation of effective, lateral interactions. We briefly review earlier work, including studies by Monte Carlo simulation of the adsorption of urea on Pt(100). Read More

The scattering of coherent X-rays from dynamically evolving systems is currently becoming experimentally feasible. The scattered beam produces a pattern of bright and dark speckles, which fluctuate almost independently in time and can be used to study the dynamics of the system. Here we report large-scale computer simulations of the speckle dynamics for a phase ordering system, using a two-dimensional model quenched through an order--disorder transition into the two-phase regime. Read More