1,654 research outputs found
Massive field contributions to the QCD vacuum tunneling amplitude
For the one-loop contribution to the QCD vacuum tunneling amplitude by quarks
of generic mass value, we make use of a calculational scheme exploiting a large
mass expansion together with a small mass expansion. The large mass expansion
for the effective action is given by a series involving higher-order
Seeley-DeWitt coefficients, and we carry this expansion up to order
, where denotes mass of the quark and the instanton
size parameter. For the small mass expansion, we use the known exact expression
for the particle propagation functions in an instanton background and evaluate
explicitly the effective action to order . A smooth interpolation of
the results from both expansions suggests that the quark contribution to the
instanton tunneling amplitude have a relatively simple -dependent
behavior.Comment: revtex, 4figures, 33page
Spin wave assisted current induced magnetic domain wall motion
The interaction between the propagating spin waves and the current driven
motion of a transverse domain wall in magnetic nanowires is studied by
micromagnetic simulations. If the speed of domain walls due to current induced
spin transfer torque is comparable to the velocity driven by spin waves, the
speed of domain wall is improved by applying spin waves. The domain wall
velocity can be manipulated by the frequency and amplitude of spin waves. The
effect of spin waves is suppressed in the high current density regime in which
the domain wall is mostly driven by current induced spin transfer torque
Analysis of Carbon Dioxide and Cloud Effects on Temperature in Northeast China
With the observed rise in temperature, many researchers have tried to identify the causes of such climate change to help mitigate its effects. The objective of this study is to determine whether, under the same carbon dioxide (CO2) concentrations, CO2 with lower cloud coverage would raise the temperature at a greater rate than CO2 with higher cloud coverage. The hypothesis was tested through data analysis and modeling. The relationships between the temperature and the CO2 emissions, the temperature and the cloud coverage, and the CO2 emissions and the cloud coverage were identified using Pearson\u27s correlation test. The data analysis concluded that the relationship between the temperature and the CO2 emission is positively proportional with a significant correlation. The relationship between the cloud coverage and the temperature and the relationship between the CO2 emissions and the cloud coverage were determined to be negatively proportional with significant correlations. For modeling, the temperature increased more rapidly as cloud coverage shrank. The results supported the hypothesis that the cloud coverage mitigates warming effects created by carbon dioxide emissions. Further research is anticipated to reduce the uncertainties in the data along with specification of cloud types
The Role of Grain Boundaries in the Tensile Deformation Behavior of CoCrFeMnNi High Entropy Alloys
High Entropy alloys (HEAs) are metal alloys consisting of multiple base metals in equimolar or near equimolar concentrations. HEAs exhibit unique combinations of properties that render them an attractive choice in many engineering applications. Among HEAs, a single phase face centered cubic (FCC) CoCrFeMnNi alloy, known as the Cantor alloy, shows simultaneous strength and ductility specifically at cryogenic temperatures. This has been attributed to the activation of deformation twinning as an additional mode of plastic deformation. Experimentally it has been observed that grain boundaries (GBs) facilitate the nucleation of deformation twins in HEAs. However, the role of GB geometry in the deformation behavior of HEAs remains unexplored. In this thesis work, we leverage atomistic simulations to systematically investigate the role of GB geometry in the deformation behavior of the Cantor alloy at 77 K. To this end, a series of Cantor alloy bicrystals with \u3c110\u3e and \u3c111\u3e symmetric twist GBs are constructed and used in tensile deformation simulations. Simulation results reveal that plastic deformation proceeds by the nucleation of partial dislocations from GBs, which then grow with further loading by bowing into the bulk crystals leaving behind stacking faults. Variations in the nucleation stress exist as function of GB character, defined in this work by the twist angle. Our results provide future avenues to explore GBs as a microstructure design tool to develop HEAs with tailored mechanical properties
Instanton Determinant with Arbitrary Quark Mass: WKB Phase-shift Method and Derivative Expansion
The fermion determinant in an instanton background for a quark field of
arbitrary mass is studied using the Schwinger proper-time representation with
WKB scattering phase shifts for the relevant partial-wave differential
operators. Previously, results have been obtained only for the extreme small
and large quark mass limits, not for intermediate interpolating mass values. We
show that consistent renormalization and large-mass asymptotics requires up to
third-order in the WKB approximation. This procedure leads to an almost
analytic answer, requiring only modest numerical approximation, and yields
excellent agreement with the well-known extreme small and large mass limits. We
estimate that it differs from the exact answer by no more than 6% for generic
mass values. In the philosophy of the derivative expansion the same amplitude
is then studied using a Heisenberg-Euler-type effective action, and the leading
order approximation gives a surprisingly accurate answer for all masses.Comment: 15 pages, 3 figure
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