13,579 research outputs found
Chiral Fermi liquid approach to neutron matter
We present a microscopic calculation of the complete quasiparticle
interaction, including central as well as noncentral components, in neutron
matter from high-precision two- and three-body forces derived within the
framework of chiral effective field theory. The contributions from two-nucleon
forces are computed in many-body perturbation theory to first and second order
(without any simplifying approximations). In addition we include the
leading-order one-loop diagrams from the N2LO chiral three-nucleon force, which
contribute to all Fermi liquid parameters except those associated with the
center-of-mass tensor interaction. The relative-momentum dependence of the
quasiparticle interaction is expanded in Legendre polynomials up to L=2.
Second-order Pauli blocking and medium polarization effects act coherently in
specific channels, namely for the Landau parameters f_1, h_0 and g_0, which
results in a dramatic increase in the quasiparticle effective mass as well as a
decrease in both the effective tensor force and the neutron matter spin
susceptibility. For densities greater than about half nuclear matter saturation
density \rho_0, the contributions to the Fermi liquid parameters from the
leading-order chiral three-nucleon force scale in all cases approximately
linearly with the nucleon density. The largest effect of the three-nucleon
force is to generate a strongly repulsive effective interaction in the
isotropic spin-independent channel. We show that the leading-order chiral
three-nucleon force leads to an increase in the spin susceptibility of neutron
matter, but we observe no evidence for a ferromagnetic spin instability in the
vicinity of the saturation density \rho_0. This work sets the foundation for
future studies of neutron matter response to weak and electromagnetic probes
with applications to neutron star structure and evolution.Comment: 21 pages, 6 figures, 5 table
Efficient algorithm for mobile multicast using anycast group
The authors present a novel and efficient multicast algorithm that aims to reduce delay and communication cost for the registration between mobile nodes and mobility agents and solicitation for foreign agent services based on the mobile IP. The protocol applies anycast group technology to support multicast transmissions for both mobile nodes and home/foreign agents. Mobile hosts use anycast tunnelling to connect to the nearest available home/foreign agent where an agent is able to forward the multicast messages by selecting an anycast route to a multicast router so as to reduce the end-to-end delay. The performance analysis and experiments demonstrated that the proposed algorithm is able to enhance the performance over existing remote subscription and bidirectional tunnelling approaches regardless of the locations of mobile nodes/hosts<br /
Microscopic optical potential from chiral nuclear forces
The energy- and density-dependent single-particle potential for nucleons is
constructed in a medium of infinite isospin-symmetric nuclear matter starting
from realistic nuclear interactions derived within the framework of chiral
effective field theory. The leading-order terms from both two- and
three-nucleon forces give rise to real, energy-independent contributions to the
nucleon self-energy. The Hartree-Fock contribution from the two-nucleon force
is attractive and strongly momentum dependent, in contrast to the contribution
from the three-nucleon force which provides a nearly constant repulsive mean
field that grows approximately linearly with the nuclear density. Together, the
leading-order perturbative contributions yield an attractive single-particle
potential that is however too weak compared to phenomenology. Second-order
contributions from two- and three-body forces then provide the additional
attraction required to reach the phenomenological depth. The imaginary part of
the optical potential, which is positive (negative) for momenta below (above)
the Fermi momentum, arises at second-order and is nearly inversion-symmetric
about the Fermi surface when two-nucleon interactions alone are present. The
imaginary part is strongly absorptive and requires the inclusion of an
effective mass correction as well as self-consistent single-particle energies
to attain qualitative agreement with phenomenology.Comment: 12 pages, 7 figures, added references, corrected typo
Lambda-nuclear interactions and hyperon puzzle in neutron stars
Brueckner theory is used to investigate the in-medium properties of a
-hyperon in nuclear and neutron matter, based on hyperon-nucleon
interactions derived within SU(3) chiral effective field theory (EFT). It is
shown that the resulting single-particle potential
becomes strongly repulsive for densities
of two-to-three times that of normal nuclear matter. Adding a density-dependent
effective -interaction constructed from chiral
three-body forces increases the repulsion further. Consequences of these
findings for neutron stars are discussed. It is argued that for hyperon-nuclear
interactions with properties such as those deduced from the SU(3) EFT
potentials, the onset for hyperon formation in the core of neutron stars is
expected to be shifted to extremely high baryon density, thus potentially
resolving the so-called hyperon puzzle.Comment: 6 pages, two figures; longer discussion about uncertainties adde
"Oxide-free" tip for scanning tunneling microscopy
We report a new tip for scanning tunneling microscopy and a tip repair procedure that allows one to reproducibly obtain atomic images of highly oriented pyrolytic graphite with previously inoperable tips. The tips are shown to be relatively oxide-free and highly resistant to oxidation. The tips are fabricated with graphite by two distinct methods
Scanning tunneling microscopy investigation of 2H-MoS_2: A layered semiconducting transition‐metal dichalcogenide
Scanning tunneling microscopy (STM) has been enormously
successful in solving several important problems in the geometric and electronic structure of homogeneous metallic and semiconducting surfaces. A central question which remains to be answered with respect to the study of
compound surfaces, however, is the extent to which the
chemical identity of constituent atoms may be established.
Recently, progress in this area was made by Feenstra et al.
who succeeded in selectively imaging either Ga or As atoms
on the GaAs (110) surface. So far this is the only case where such selectivity has been achieved. In an effort to add to our understanding of compound surface imaging we have undertaken a vacuum STM study of 2H-MoS_2, a material which has two structurally and electronically different atomic species at its surface
Electron tunnel sensor technology
Researchers designed and constructed a novel electron tunnel sensor which takes advantage of the mechanical properties of micro-machined silicon. For the first time, electrostatic forces are used to control the tunnel electrode separation, thereby avoiding the thermal drift and noise problems associated with piezoelectric actuators. The entire structure is composed of micro-machined silicon single crystals, including a folded cantilever spring and a tip. The application of this sensor to the development of a sensitive accelerometer is described
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