Tak-Pong Woo

Understanding the Core-Halo Relation of Quantum Wave Dark Matter from 3D Simulations

We examine the nonlinear structure of gravitationally collapsed objects that form in our simulations of wavelike cold dark matter, described by the Schrödinger-Poisson (SP) equation with a particle mass ∼10(-22)  eV. A distinct gravitationally self-bound solitonic core is found at the center of every halo, with a profile quite different from cores modeled in the warm or self-interacting dark matter scenarios. Furthermore, we show that each solitonic core is surrounded by an extended halo composed of large fluctuating dark matter granules which modulate the halo density on a scale comparable to the diameter of the solitonic core. The scaling symmetry of the SP equation and the uncertainty principle tightly relate the core mass to the halo specific energy, which, in the context of cosmological structure formation, leads to a simple scaling between core mass (Mc) and halo mass (Mh), Mc∝a(-1/2)Mh(1/3), where a is the cosmic scale factor. We verify this scaling relation by (i) examining the internal structure of a statistical sample of virialized halos that form in our 3D cosmological simulations and by (ii) merging multiple solitons to create individual virialized objects. Sufficient simulation resolution is achieved by adaptive mesh refinement and graphic processing units acceleration. From this scaling relation, present dwarf satellite galaxies are predicted to have kiloparsec-sized cores and a minimum mass of ∼10(8)M⊙, capable of solving the small-scale controversies in the cold dark matter model. Moreover, galaxies of 2×10(12)M⊙ at z=8 should have massive solitonic cores of ∼2×10(9)M⊙ within ∼60  pc. Such cores can provide a favorable local environment for funneling the gas that leads to the prompt formation of early stellar spheroids and quasars.

HIGH-RESOLUTION SIMULATION ON STRUCTURE FORMATION WITH EXTREMELY LIGHT BOSONIC DARK MATTER

A bosonic dark matter model is examined in detail via high-resolution simulations. These bosons have particle mass of the order of 10–22 eV and are noninteracting. If they do exist and can account for structure formation, these bosons must be condensed into the Bose-Einstein state and described by a coherent wave function. This matter, also known as fuzzy dark matter, is speculated to be able, first, to eliminate the subgalactic halos to solve the problem of overabundance of dwarf galaxies, and, second, to produce flat halo cores in galaxies suggested by some observations. We investigate this model with simulations up to 10243 resolution in a 1 h –1 Mpc box that maintains the background matter density Ω m = 0.3 and ΩΛ = 0.7. Our results show that the extremely light bosonic dark matter can indeed eliminate low-mass halos through the suppression of short-wavelength fluctuations, as predicted by the linear perturbation theory. But in contrast to expectation, our simulations yield singular cores in the collapsed halos, where the halo density profile is similar, but not identical, to the Navarro-Frenk-White profile. Such a profile arises regardless of whether the halo forms through accretion or merger. In addition, the virialized halos exhibit anisotropic turbulence inside a well-defined virial boundary. Much like the velocity dispersion of standard dark matter particles, turbulence is dominated by the random radial flow in most part of the halos and becomes isotropic toward the halo cores. Consequently, the three-dimensional collapsed halo mass distribution can deviate from spherical symmetry, as the cold dark matter halo does.

Effects of Preheated Clusters on the Cosmic Microwave Background Spectrum

Mounting evidence from x-ray observations reveals that bound objects should receive some form of energy in the past injected from non-gravitaional sources. We report that an instantaneous heating scheme, for which gases in dense regions were subjected to a temperature jump of 1keV at z = 2 whereas those in rarified regions remained intact, can produce bound objects obeying the observed mass-temperature and luminosity-temperature relations. Such preheating lowers the peak Sunyaev-Zeldovich (SZ) power by a factor of 2 and exacerbates the need for the normalization of matter fluctuations σ8 to assume an extreme high value (∼ 1.1) for the SZ signals to account for the excess anisotropy on 5-arcminute scale detected by the Cosmic Background Imager in the cosmic microwave background radiation. Subject headings: cosmology: theory – cosmic microwave background – intergalactic medium

Experimental evidence for direct insulator-quantum Hall transition in multi-layer graphene

We have performed magnetotransport measurements on a multi-layer graphene flake. At the crossing magnetic field Bc, an approximately temperature-independent point in the measured longitudinal resistivity ρxx, which is ascribed to the direct insulator-quantum Hall (I-QH) transition, is observed. By analyzing the amplitudes of the magnetoresistivity oscillations, we are able to measure the quantum mobility μq of our device. It is found that at the direct I-QH transition, μqBc ≈ 0.37 which is considerably smaller than 1. In contrast, at Bc, ρxx is close to the Hall resistivity ρxy, i.e., the classical mobility μBc is ≈ 1. Therefore, our results suggest that different mobilities need to be introduced for the direct I-QH transition observed in multi-layered graphene. Combined with existing experimental results obtained in various material systems, our data obtained on graphene suggest that the direct I-QH transition is a universal effect in 2D.

Mesoscopic conductance fluctuations in multi-layer graphene

Multi-layer graphene has many unique properties for realizing graphene-based nano-electronic device applications as well as for fundamental studies. This paper mainly focuses on the conductance fluctuations in multi-layer graphene. The low-temperature saturation of dephasing time in multi-layer graphene is one order magnitude shorter than that in single-layer graphene, and the onset temperature of the low-temperature saturation of dephasing time in multi-layer graphene was significantly lower than that in single-layer graphene, which is noteworthy in the low-temperature saturation of dephasing time. We speculate that the carrier transport is shielded by capping transport and bottom layer graphene due to the substrate impurities and air molecules scattering.

Multi-science applications with single codebase - GAMER - for massively parallel architectures

The growing need for power efficient extreme-scale high-performance computing (HPC) coupled with plateauing clock-speeds is driving the emergence of massively parallel compute architectures. Tens to many hundreds of cores are increasingly made available as compute units, either as the integral part of the main processor or as coprocessors designed for handling massively parallel workloads. In the case of many-core graphics processing units (GPUs) hundreds of SIMD cores primarily designed for image and video rendering are used for high-performance scientific computations. The new architectures typically offer ANSI standard programming models such as CUDA (NVIDIA) and OpenCL. However, the wide-ranging adoption of these parallel architectures is steeped in difficult learning curve and requires reengineering of existing applications that mostly leads to expensive and error prone code rewrites without prior guarantee and knowledge of any speedups. Broad range of complex scientific applications across many domains use common algorithms and techniques, such as adaptive mesh refinements (AMR), advanced hydrodynamics partial differential equation (PDE) solvers, Poisson-Gravity solvers etc, that have demonstrably performed highly efficiently on GPU based systems. Taking advantage of the commonalities, we use GPU-aware AMR code, GAMER [1], to examine the unique approach of solving multi-science problems in astrophysics, hydrodynamics and particle physics with single codebase. We demonstrate significant speedups in disparate class of scientific applications on 3 separate clusters, viz., Dirac, Laohu and Mole 8.5. By extensively reusing the extendable single codebase we mitigate the impediments of significant code rewrites. We also collect performance and energy consumption benchmark metrics on 50-nodes NVIDIA C2050 GPU and Intel 8-core Nehalem CPU on Dirac cluster at the National Energy Research Supercomputing Center (NERSC). In addition, we propose a strategy and framework for legacy and new applications to successfully leverage the evolving GAMER codebase on massively parallel architectures. The framework and the benchmarks are aimed to help quantify the adoption strategies for legacy and new scientific applications.

Dirac fermion heating, current scaling, and direct insulator-quantum Hall transition in multilayer epitaxial graphene

We have performed magnetotransport measurements on multilayer epitaxial graphene. By increasing the driving current I through our graphene devices while keeping the bath temperature fixed, we are able to study Dirac fermion heating and current scaling in such devices. Using zero-field resistivity as a self thermometer, we are able to determine the effective Dirac fermion temperature (TDF) at various driving currents. At zero field, it is found that TDF ∝ I≈1/2. Such results are consistent with electron heating in conventional two-dimensional systems in the plateau-plateau transition regime. With increasing magnetic field B, we observe an I-independent point in the measured longitudinal resistivity ρxx which is equivalent to the direct insulator-quantum Hall (I-QH) transition characterized by a temperature-independent point in ρxx. Together with recent experimental evidence for direct I-QH transition, our new data suggest that such a transition is a universal effect in graphene, albeit further studies are required to obtain a thorough understanding of such an effect.

Observation of Quantum Hall Plateau-Plateau Transition and Scaling Behavior of the Zeroth Landau Level in Graphene p-n-p Junction

Electrical transport studies of graphene heterostructures (Young and Kim in Ann Rev Conden Ma P 2:101 [1]) have revealed the quantum Hall effect (QHE) (Ozyilmaz et al. in Phys Rev Lett 99:166804 [2]), quantum interference behaviors (Young and Kim in Nat Phys 5:222; [3]; Satoru et al. in Jpn J Appl Phys 52:110105 [4]), Klein tunneling (Katsnelson et al. in Nat Phys 2:620 [5]; Shytov et al. in Phys Rev Lett 101:156804 [6]), and the split closed-loop resonator (Zheng et al. in Nat Nanotechnol 8:119 [7]), hence convincingly demonstrating the advantages of constructing in-plane heterostructures of graphene.

High Current-Induced Electron Redistribution in a CVD-Grown Graphene Wide Constriction

Investigating the charge transport behavior in one-dimensional quantum confined system such as the localized states and interference effects due to the nanoscale grain boundaries and merged domains in wide chemical vapor deposition graphene constriction is highly desirable since it would help to realize industrial graphene-based electronic device applications. Our data suggests a crossover from interference coherent transport to carriers flushing into grain boundaries and merged domains when increasing the current. Moreover, many-body fermionic carriers with disordered system in our case can be statistically described by mean-field Gross-Pitaevskii equation via a single wave function by means of the quantum hydrodynamic approximation. The novel numerical simulation method supports the experimental results and suggests that the extreme high barrier potential regions on graphene from the grain boundaries and merged domains can be strongly affected by additional hot charges. Such interesting results could pave the way for quantum transport device by supplying additional hot current to flood into the grain boundaries and merged domains in one-dimensional quantum confined CVD graphene, a great advantage for developing graphene-based coherent electronic devices.

Imaging coherent transport in chemical vapor deposition graphene wide constriction by scanning gate microscopy

We use a scanning gate microscopy to perturb coherent transport in chemical vapor deposition (CVD) graphene wide constriction. Particularly, we observe conductance oscillations in the wide constriction region (W ∼ 800 nm) characterized by spatial conductance variations, which imply formation of the nanometer-scale ring structure due to the merged domains and intrinsic grain boundaries. Moreover, additional hot charges from high current can suppress the coherent transport, suggesting that the hot carriers with a wide spreading kinetic energy could easily tunnel merged domains and intrinsic grain boundaries in CVD-grown graphene due to the heating effect, a great advantage for applications in graphene-based interference-type nano-electronics.