The Experts below are selected from a list of 87963 Experts worldwide ranked by ideXlab platform
Kazunori Kohri - One of the best experts on this subject based on the ideXlab platform.
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primordial black hole abundance from random gaussian curvature perturbations and a local Density Threshold
Progress of Theoretical and Experimental Physics, 2018Co-Authors: Chulmoon Yoo, Tomohiro Harada, Jaume Garriga, Kazunori KohriAbstract:The production rate of primordial black holes (PBHs) is often calculated by considering a nearly Gaussian distribution of cosmological perturbations, and assuming that black holes will form in regions where the amplitude of such perturbations exceeds a certain Threshold. A Threshold |$\zeta_{\rm th}$| for the curvature perturbation is somewhat inappropriate for this purpose, because it depends significantly on environmental effects, not essential to the local dynamics. By contrast, a Threshold |$\delta_{\rm th}$| for the Density perturbation at horizon crossing seems to provide a more robust criterion. On the other hand, the Density perturbation is known to be bounded above by a maximum limit |$\delta_{\rm max}$| at the horizon entry and, given that |$\delta_{\rm th}$| is comparable to |$\delta_{\rm max}$|, the Density perturbation will be far from Gaussian near or above the Threshold. In this paper, we provide a new plausible estimate for the primordial black hole abundance based on peak theory. In our approach, we assume that the curvature perturbation is given as a random Gaussian field with the power spectrum characterized by a single scale, while an optimized criterion for PBH formation is imposed, based on the locally averaged Density perturbation around the nearly spherically symmetric high peaks. Both variables are related by the full non-linear expression derived in the long-wavelength approximation of general relativity. We do not introduce a window function, which is usually introduced to obtain the scale dependence of the spectrum. The scale of the inhomogeneity is introduced as a random variable in the peak theory, and the scale-dependent PBH fraction is automatically induced. We find that the mass spectrum is shifted to larger mass scales by one order of magnitude or so, compared to a conventional calculation. The abundance of PBHs becomes significantly larger than the conventional one, by many orders of magnitude, mainly due to the optimized criterion for PBH formation and the removal of the suppression associated with a window function.
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pbh abundance from random gaussian curvature perturbations and a local Density Threshold
arXiv: Cosmology and Nongalactic Astrophysics, 2018Co-Authors: Chulmoon Yoo, Tomohiro Harada, Jaume Garriga, Kazunori KohriAbstract:The production rate of primordial black holes is often calculated by considering a nearly Gaussian distribution of cosmological perturbations, and assuming that black holes will form in regions where the amplitude of such perturbations exceeds a certain Threshold. A Threshold $\zeta_{\rm th}$ for the curvature perturbation is somewhat inappropriate for this purpose, because it depends significantly on environmental effects, not essential to the local dynamics. By contrast, a Threshold $\delta_{\rm th}$ for the Density perturbation at horizon crossing seems to provide a more robust criterion. On the other hand, the Density perturbation is known to be bounded above by a maximum limit $\delta_{\rm max}$, and given that $\delta_{\rm th}$ is comparable to $\delta_{\rm max}$, the Density perturbation will be far from Gaussian near or above the Threshold. In this paper, we provide a new plausible estimate for the primordial black hole abundance based on peak theory. In our approach, we assume that the curvature perturbation is given as a random Gaussian field with the power spectrum characterized by a single scale, while an optimized criterion for PBH formation is imposed, based on the locally averaged Density perturbation. Both variables are related by the full nonlinear expression derived in the long-wavelength approximation of general relativity. We do not introduce a window function, and the scale of the inhomogeneity is introduced as a random variable in the peak theory. We find that the mass spectrum is shifted to larger mass scales by one order of magnitude or so, compared to a conventional calculation. The abundance of PBHs becomes significantly larger than the conventional one, by many orders of magnitude, mainly due to the optimized criterion for PBH formation and the removal of the suppresion associated with a window function.
X R Wang - One of the best experts on this subject based on the ideXlab platform.
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breaking the current Density Threshold in spin orbit torque magnetic random access memory
Physical Review B, 2018Co-Authors: Yin Zhang, H Y Yuan, Xiansi Wang, X R WangAbstract:Spin-orbit-torque magnetic random access memory (SOT-MRAM) is a promising technology for the next generation of data storage devices. The main bottleneck of this technology is the high reversal current Density Threshold. This outstanding problem is now solved by a new strategy in which the magnitude of the driven current Density is fixed while the current direction varies with time. The theoretical limit of minimal reversal current Density is only a fraction (the Gilbert damping coefficient) of the Threshold current Density of the conventional strategy. The Euler-Lagrange equation for the fastest magnetization reversal path and the optimal current pulse is derived for an arbitrary magnetic cell and arbitrary spin-orbit torque. The theoretical limit of minimal reversal current Density and current Density for a GHz switching rate of the new reversal strategy for CoFeB/Ta SOT-MRAMs are, respectively, of the order of ${10}^{5}$ ${\mathrm{A}/\mathrm{cm}}^{2}$ and ${10}^{6}$ ${\mathrm{A}/\mathrm{cm}}^{2}$ far below ${10}^{7}$ ${\mathrm{A}/\mathrm{cm}}^{2}$ and ${10}^{8}$ ${\mathrm{A}/\mathrm{cm}}^{2}$ in the conventional strategy. Furthermore, no external magnetic field is needed for a deterministic reversal in the new strategy.
Xiansi Wang - One of the best experts on this subject based on the ideXlab platform.
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breaking the current Density Threshold in spin orbit torque magnetic random access memory
Physical Review B, 2018Co-Authors: Yin Zhang, H Y Yuan, Xiansi Wang, X R WangAbstract:Spin-orbit-torque magnetic random access memory (SOT-MRAM) is a promising technology for the next generation of data storage devices. The main bottleneck of this technology is the high reversal current Density Threshold. This outstanding problem is now solved by a new strategy in which the magnitude of the driven current Density is fixed while the current direction varies with time. The theoretical limit of minimal reversal current Density is only a fraction (the Gilbert damping coefficient) of the Threshold current Density of the conventional strategy. The Euler-Lagrange equation for the fastest magnetization reversal path and the optimal current pulse is derived for an arbitrary magnetic cell and arbitrary spin-orbit torque. The theoretical limit of minimal reversal current Density and current Density for a GHz switching rate of the new reversal strategy for CoFeB/Ta SOT-MRAMs are, respectively, of the order of ${10}^{5}$ ${\mathrm{A}/\mathrm{cm}}^{2}$ and ${10}^{6}$ ${\mathrm{A}/\mathrm{cm}}^{2}$ far below ${10}^{7}$ ${\mathrm{A}/\mathrm{cm}}^{2}$ and ${10}^{8}$ ${\mathrm{A}/\mathrm{cm}}^{2}$ in the conventional strategy. Furthermore, no external magnetic field is needed for a deterministic reversal in the new strategy.
S H Glenzer - One of the best experts on this subject based on the ideXlab platform.
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observation of the Density Threshold behavior for the onset of stimulated raman scattering in high temperature hohlraum plasmas
Physical Review Letters, 2009Co-Authors: D H Froula, T Doppner, L Divol, R A London, R L Berger, N B Meezan, J S Ross, L J Suter, C Sorce, S H GlenzerAbstract:We show that the measured stimulated Raman scattering (SRS) in a large-scale high-temperature plasma scales strongly with the plasma Density, increasing by an order of magnitude when the electron Density is increased by 20%. This is consistent with linear theory, including pump depletion, in a uniform plasma and, as the Density is typically constrained by other processes, this effect will set a limit on drive laser beam intensity for forthcoming ignition experiments at the National Ignition Facility. Control of SRS at laser intensities consistent with 285 eV ignition hohlraums is achieved by using polarization smoothing which increases the intensity Threshold for the onset of SRS by 1.6 +/- 0.2. These results were quantitatively predicted by full beam three-dimensional numerical laser-plasma interaction simulations.
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observation of the Density Threshold behavior for the onset of stimulated raman scattering in high temperature hohlraum plasmas
Physical Review Letters, 2009Co-Authors: D H Froula, T Doppner, L Divol, R A London, R L Berger, N B Meezan, J S Ross, L J Suter, C Sorce, S H GlenzerAbstract:We show that the measured stimulated Raman scattering (SRS) in a large-scale high-temperature plasma scales strongly with the plasma Density, increasing by an order of magnitude when the electron Density is increased by 20%. This is consistent with linear theory, including pump depletion, in a uniform plasma and, as the Density is typically constrained by other processes, this effect will set a limit on drive laser beam intensity for forthcoming ignition experiments at the National Ignition Facility. Control of SRS at laser intensities consistent with 285 eV ignition hohlraums is achieved by using polarization smoothing which increases the intensity Threshold for the onset of SRS by $1.6\ifmmode\pm\else\textpm\fi{}0.2$. These results were quantitatively predicted by full beam three-dimensional numerical laser-plasma interaction simulations.
Yin Zhang - One of the best experts on this subject based on the ideXlab platform.
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breaking the current Density Threshold in spin orbit torque magnetic random access memory
Physical Review B, 2018Co-Authors: Yin Zhang, H Y Yuan, Xiansi Wang, X R WangAbstract:Spin-orbit-torque magnetic random access memory (SOT-MRAM) is a promising technology for the next generation of data storage devices. The main bottleneck of this technology is the high reversal current Density Threshold. This outstanding problem is now solved by a new strategy in which the magnitude of the driven current Density is fixed while the current direction varies with time. The theoretical limit of minimal reversal current Density is only a fraction (the Gilbert damping coefficient) of the Threshold current Density of the conventional strategy. The Euler-Lagrange equation for the fastest magnetization reversal path and the optimal current pulse is derived for an arbitrary magnetic cell and arbitrary spin-orbit torque. The theoretical limit of minimal reversal current Density and current Density for a GHz switching rate of the new reversal strategy for CoFeB/Ta SOT-MRAMs are, respectively, of the order of ${10}^{5}$ ${\mathrm{A}/\mathrm{cm}}^{2}$ and ${10}^{6}$ ${\mathrm{A}/\mathrm{cm}}^{2}$ far below ${10}^{7}$ ${\mathrm{A}/\mathrm{cm}}^{2}$ and ${10}^{8}$ ${\mathrm{A}/\mathrm{cm}}^{2}$ in the conventional strategy. Furthermore, no external magnetic field is needed for a deterministic reversal in the new strategy.