39,428 research outputs found
Enhancement of Quantum Tunneling for Excited States in Ferromagnetic Particles
A formula suitable for a quantitative evaluation of the tunneling effect in a
ferromagnetic particle is derived with the help of the instanton method. The
tunneling between n-th degenerate states of neighboring wells is dominated by a
periodic pseudoparticle configuration. The low-lying level-splitting previously
obtained with the LSZ method in field theory in which the tunneling is viewed
as the transition of n bosons induced by the usual (vacuum) instanton is
recovered. The observation made with our new result is that the tunneling
effect increases at excited states. The results should be useful in analyzing
results of experimental tests of macroscopic quantum coherence in ferromagnetic
particles.Comment: 18 pages, LaTex, 1 figur
The Growth in Size and Mass of Cluster Galaxies since z=2
We study the formation and evolution of Brightest Cluster Galaxies starting
from a population of quiescent ellipticals and following them to .
To this end, we use a suite of nine high-resolution dark matter-only
simulations of galaxy clusters in a CDM universe. We develop a scheme
in which simulation particles are weighted to generate realistic and
dynamically stable stellar density profiles at . Our initial conditions
assign a stellar mass to every identified dark halo as expected from abundance
matching; assuming there exists a one-to-one relation between the visible
properties of galaxies and their host haloes. We set the sizes of the luminous
components according to the observed relations for massive quiescent
galaxies. We study the evolution of the mass-size relation, the fate of
satellite galaxies and the mass aggregation of the cluster central. From ,
these galaxies grow on average in size by a factor 5 to 10 of and in mass by 2
to 3. The stellar mass growth rate of the simulated BCGs in our sample is of
1.9 in the range consistent with observations, and of 1.5 in the
range . Furthermore the satellite galaxies evolve to the present day
mass-size relation by . Assuming passively evolving stellar populations,
we present surface brightness profiles for our cluster centrals which resemble
those observed for the cDs in similar mass clusters both at and at .
This demonstrates that the CDM cosmology does indeed predict minor and
major mergers to occur in galaxy clusters with the frequency and mass ratio
distribution required to explain the observed growth in size of passive
galaxies since . Our experiment shows that Brightest Cluster Galaxies can
form through dissipationless mergers of quiescent massive galaxies,
without substantial additional star formation.Comment: submitted to MNRAS, 10 pages, 8 figures, 2 table
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Experimental and Numerical Investigation on Progressive Collapse Resistance of Post-tensioned Precast Concrete Beam-Column Sub-assemblages
In this paper, four 1/2 scaled precast concrete (PC) beam-column sub-assemblages with high performance connection were tested under push-down loading procedure to study the load resisting mechanism of PC frames subjected to different column removal scenarios. The parameters investigated include the location of column removal and effective prestress in tendons. The test results indicated that the failure modes of unbonded post-tensioned precast concrete (PTPC) frames were different from that of reinforced concrete (RC) frames: no cracks formed in the beams and wide opening formed near the beam to column interfaces. For specimens without overhanging beams, the failure of side column was eccentric compression failure. Moreover, the load resisting mechanisms in PC frames were significantly different from that of RC frames: the compressive arch action (CAA) developed in concrete during column removal was mainly due to actively applied pre-compressive stress in the concrete; CAA will not vanish when severe crush in concrete occurred. Thus, it may provide negative contribution for load resistance when the displacement exceeds one-beam depth; the tensile force developed in the tendons could provide catenary action from the beginning of the test. Moreover, to deeper understand the behavior of tested specimens, numerical analyses were carried out. The effects of concrete strength, axial compression ratio at side columns, and loading approaches on the behavior of the sub-assemblages were also investigated based on validated numerical analysis
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Random-walk simulation of non-conservative pollutant transport in shallow water flows
The random-walk method is inherently simple and numerically stable. However, when used in hydro-environmental analyses, most of the existing random-walk methods ignore the influence of the non-uniform water depth and only consider the transport of inert materials, hence the inability of modelling biochemical reactions. In addition, they mainly examine the instantaneous-release problems, with a fixed number of particles moving in the computational domain. This paper first presents examples to showcase the capability of the newly developed model in simulating the continuous source of non-conservative substances. Then, the method is applied to simulate the BOD-DO balance along a hypothetical river. The numerical results agree well with the analytical solutions. Finally, the developed model is used to study the pollutant transport in the Thames Estuary. The current model is illustrated to be able to accurately predict the interaction between multiple pollutants in real-world situations with uneven bathymetry and extensive intertidal floodplains.The work has been supported by the Royal Academy of Engineering UK-China Urban Flooding Research Impact Programme (No. UUFRIP\100051), the Ministry of Education and State Administration of Foreign Experts Affairs 111 Project (No. B17015), the Cambridge Tier-2 system operated by the University of Cambridge Research Computing Service (http://www.hpc.cam.ac.uk) funded by EPSRC Tier-2 capital grant EP/P020259/1 and the China Scholarship Council (No. 201708060090)
Probing non-Abelian statistics of Majorana fermions in ultracold atomic superfluid
We propose an experiment to directly probe the non-Abelian statistics of
Majorana fermions by braiding them in an s-wave superfluid of ultracold atoms.
We show different orders of braiding operations give orthogonal output states
that can be distinguished through Raman spectroscopy. Realization of Majorana
bound states in an s-wave superfluid requires strong spin-orbital coupling and
a controllable Zeeman field in the perpendicular direction. We present a simple
laser configuration to generate the artificial spin-orbital coupling and the
required Zeeman field in the dark state subspace.Comment: 4 pages; Add detailed discussion of feasibility of the scheme;add
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Low frequency noise peak near magnon emission energy in magnetic tunnel junctions
We report on the low frequency (LF) noise measurements in magnetic tunnel
junctions (MTJs) below 4 K and at low bias, where the transport is strongly
affected by scattering with magnons emitted by hot tunnelling electrons, as
thermal activation of magnons from the environment is suppressed. For both
CoFeB/MgO/CoFeB and CoFeB/AlO/CoFeB MTJs, enhanced LF noise is observed
at bias voltage around magnon emission energy, forming a peak in the bias
dependence of noise power spectra density, independent of magnetic
configurations. The noise peak is much higher and broader for unannealed
AlO-based MTJ, and besides Lorentzian shape noise spectra in the
frequency domain, random telegraph noise (RTN) is visible in the time traces.
During repeated measurements the noise peak reduces and the RTN becomes
difficult to resolve, suggesting defects being annealed. The Lorentzian shape
noise spectra can be fitted with bias-dependent activation of RTN, with the
attempt frequency in the MHz range, consistent with magnon dynamics. These
findings suggest magnon-assisted activation of defects as the origin of the
enhanced LF noise.Comment: 6 pages, 5 figure
Stability of Strutinsky Shell Correction Energy in Relativistic Mean Field Theory
The single-particle spectrum obtained from the relativistic mean field (RMF)
theory is used to extract the shell correction energy with the Strutinsky
method. Considering the delicate balance between the plateau condition in the
Strutinsky smoothing procedure and the convergence for the total binding
energy, the proper space sizes used in solving the RMF equations are
investigated in detail by taking 208Pb as an example. With the proper space
sizes, almost the same shell correction energies are obtained by solving the
RMF equations either on basis space or in coordinate space.Comment: 9 pages, 4 figure
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