811 research outputs found
Hydrodynamic and Brownian Fluctuations in Sedimenting Suspensions
We use a mesoscopic computer simulation method to study the interplay between
hydrodynamic and Brownian fluctuations during steady-state sedimentation of
hard sphere particles for Peclet numbers (Pe) ranging from 0.1 to 15. Even when
the hydrodynamic interactions are an order of magnitude weaker than Brownian
forces, they still induce backflow effects that dominate the reduction of the
average sedimentation velocity with increasing particle packing fraction.
Velocity fluctuations, on the other hand, begin to show nonequilibrium
hydrodynamic character for Pe > 1Comment: 4 pages 4 figures, RevTex, to appear in Phys. Rev. Lett. New version
with some minor correction
Exploring More-Coherent Quantum Annealing
In the quest to reboot computing, quantum annealing (QA) is an interesting
candidate for a new capability. While it has not demonstrated an advantage over
classical computing on a real-world application, many important regions of the
QA design space have yet to be explored. In IARPA's Quantum Enhanced
Optimization (QEO) program, we have opened some new lines of inquiry to get to
the heart of QA, and are designing testbed superconducting circuits and
conducting key experiments. In this paper, we discuss recent experimental
progress related to one of the key design dimensions: qubit coherence. Using
MIT Lincoln Laboratory's qubit fabrication process and extending recent
progress in flux qubits, we are implementing and measuring QA-capable flux
qubits. Achieving high coherence in a QA context presents significant new
engineering challenges. We report on techniques and preliminary measurement
results addressing two of the challenges: crosstalk calibration and qubit
readout. This groundwork enables exploration of other promising features and
provides a path to understanding the physics and the viability of quantum
annealing as a computing resource.Comment: 7 pages, 3 figures. Accepted by the 2018 IEEE International
Conference on Rebooting Computing (ICRC
A Numerical Model for Brownian Particles Fluctuating in Incompressible Fluids
We present a numerical method that consistently implements thermal
fluctuations and hydrodynamic interactions to the motion of Brownian particles
dispersed in incompressible host fluids. In this method, the thermal
fluctuations are introduced as random forces acting on the Brownian particles.
The hydrodynamic interactions are introduced by directly resolving the fluid
motions with the particle motion as a boundary condition to be satisfied. The
validity of the method has been examined carefully by comparing the present
numerical results with the fluctuation-dissipation theorem whose analytical
form is known for dispersions of a single spherical particle. Simulations are
then performed for more complicated systems, such as a dispersion composed of
many spherical particles and a single polymeric chain in a solvent.Comment: 6 pages, 8 figure
Anneal-path correction in flux qubits
Quantum annealers require accurate control and optimized operation schemes to
reduce noise levels, in order to eventually demonstrate a computational
advantage over classical algorithms. We study a high coherence four-junction
capacitively shunted flux qubit (CSFQ), using dispersive measurements to
extract system parameters and model the device. Josephson junction asymmetry
inherent to the device causes a deleterious nonlinear cross-talk when annealing
the qubit. We implement a nonlinear annealing path to correct the asymmetry
in-situ, resulting in a substantial increase in the probability of the qubit
being in the correct state given an applied flux bias. We also confirm the
multi-level structure of our CSFQ circuit model by annealing it through small
spectral gaps and observing quantum signatures of energy level crossings. Our
results demonstrate an anneal-path correction scheme designed and implemented
to improve control accuracy for high-coherence and high-control quantum
annealers, which leads to an enhancement of success probability in annealing
protocols.Comment: v2 published versio
The MOSDEF survey: a stellar mass-SFR-metallicity relation exists at
We investigate the nature of the relation among stellar mass, star-formation
rate, and gas-phase metallicity (the M-SFR-Z relation) at high redshifts
using a sample of 260 star-forming galaxies at from the MOSDEF
survey. We present an analysis of the high-redshift M-SFR-Z relation based
on several emission-line ratios for the first time. We show that a M-SFR-Z
relation clearly exists at . The strength of this relation is similar
to predictions from cosmological hydrodynamical simulations. By performing a
direct comparison of stacks of and galaxies, we find that
galaxies have dex lower metallicity at fixed M and
SFR. In the context of chemical evolution models, this evolution of the
M-SFR-Z relation suggests an increase with redshift of the mass-loading
factor at fixed M, as well as a decrease in the metallicity of infalling
gas that is likely due to a lower importance of gas recycling relative to
accretion from the intergalactic medium at high redshifts. Performing this
analysis simultaneously with multiple metallicity-sensitive line ratios allows
us to rule out the evolution in physical conditions (e.g., N/O ratio,
ionization parameter, and hardness of the ionizing spectrum) at fixed
metallicity as the source of the observed trends with redshift and with SFR at
fixed M at . While this study highlights the promise of
performing high-order tests of chemical evolution models at high redshifts,
detailed quantitative comparisons ultimately await a full understanding of the
evolution of metallicity calibrations with redshift.Comment: 19 pages, 8 figures, accepted to Ap
The MOSDEF Survey: Kinematic and Structural Evolution of Star-Forming Galaxies at
We present ionized gas kinematics for 681 galaxies at from
the MOSFIRE Deep Evolution Field survey, measured using models which account
for random galaxy-slit misalignments together with structural parameters
derived from CANDELS Hubble Space Telescope (HST) imaging. Kinematics and sizes
are used to derive dynamical masses. Baryonic masses are estimated from stellar
masses and inferred gas masses from dust-corrected star formation rates (SFRs)
and the Kennicutt-Schmidt relation. We measure resolved rotation for 105
galaxies. For the remaining 576 galaxies we use models based on HST imaging
structural parameters together with integrated velocity dispersions and
baryonic masses to statistically constrain the median ratio of intrinsic
ordered to disordered motion, . We find that
increases with increasing stellar mass and decreasing specific SFR (sSFR).
These trends may reflect marginal disk stability, where systems with higher gas
fractions have thicker disks. For galaxies with detected rotation we assess
trends between their kinematics and mass, sSFR, and baryon surface density
(). Intrinsic dispersion correlates most with
and velocity correlates most with mass. By comparing
dynamical and baryonic masses, we find that galaxies at are
baryon dominated within their effective radii (), with Mdyn/Mbaryon
increasing over time. The inferred baryon fractions within ,
, decrease over time, even at fixed mass, size, or surface
density. At fixed redshift, does not appear to vary with
stellar mass but increases with decreasing and increasing
. For galaxies at , the median inferred baryon
fractions generally exceed 100%. We discuss possible explanations and future
avenues to resolve this tension.Comment: Accepted to ApJ. Added Figure 9, corrected sample size (main results
unchanged). 28 pages, 13 figure
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