11,417 research outputs found
Quantum Antiferromagnetism of Fermions in Optical Lattices with Half-filled p-band
We study Fermi gases in a three-dimensional optical lattice with five
fermions per site, i.e. the s-band is completely filled and the p-band with
three-fold degeneracy is half filled. We show that, for repulsive interaction
between fermions, the system will exhibit spin-3/2 antiferromagnetic order at
low temperature. This conclusion is obtained in strong interaction regime by
strong coupling expansion which yields an isotropic spin-3/2 Heisenberg model,
and also in weak interaction regime by Hatree-Fock mean-field theory and
analysis of Fermi surface nesting. We show that the critical temperature for
this antiferromagnetism of a p-band Mott insulator is about two orders of
magnitudes higher than that of an -band Mott insulator, which is close to
the lowest temperature attainable nowadays
Valley-dependent Brewster angles and Goos-Hanchen effect in strained graphene
We demonstrate theoretically how local strains in graphene can be tailored to
generate a valley polarized current. By suitable engineering of local strain
profiles, we find that electrons in opposite valleys (K or K') show different
Brewster-like angles and Goos-H\"anchen shifts, exhibiting a close analogy with
light propagating behavior. In a strain-induced waveguide, electrons in K and
K' valleys have different group velocities, which can be used to construct a
valley filter in graphene without the need for any external fields.Comment: 5 pages, 4 figure
Spinon-Holon binding in model
Using a phenomenological model, we discuss the consequences of spinon-holon
binding in the U(1) slave-boson approach to model. Within a small
( hole concentration) expansion, we show that spinon-holon binding produces
a pseudo-gap normal state with a segmented Fermi surface and the
superconducting state is formed by opening an "additional" d-wave gap on the
segmented Fermi surface. The d-wave gap merge with the pseudo-gap smoothly as
temperature . The quasi-particles in the superconducting state are
coupled to external electromagnetic field with a coupling constant of order
where , depending on the strength of the
effective spinon-holon binding potential.Comment: 9 pages, 3 figure
Nearly Degenerate Gauginos and Dark Matter at the LHC
Motivated by dark-matter considerations in supersymmetric theories, we
investigate in a fairly model-independent way the detection at the LHC of
nearly degenerate gauginos with mass differences between a few GeV and about 30
GeV. Due to the degeneracy of gaugino states, the conventional leptonic signals
are likely lost. We first consider the leading signal from gluino production
and decay. We find that it is quite conceivable to reach a large statistical
significance for the multi-jet plus missing energy signal with an integrated
luminosity about 50 pb^-1 (50 fb^-1) for a gluino mass of 500 GeV (1 TeV). If
gluinos are not too heavy, less than about 1.5 TeV, this channel can typically
probe gaugino masses up to about 100 GeV below the gluino mass. We then study
the Drell-Yan type of gaugino pair production in association with a hard QCD
jet, for gaugino masses in the range of 100-150 GeV. The signal observation may
be statistically feasible with about 10 fb^-1, but systematically challenging
due to the lack of distinctive features for the signal distributions. By
exploiting gaugino pair production through weak boson fusion, signals of large
missing energy plus two forward-backward jets may be observable at a 4-6\sigma
level above the large SM backgrounds with an integrated luminosity of 100-300
fb^-1. Finally, we point out that searching for additional isolated soft muons
in the range p_T ~3-10 GeV in the data samples discussed above may help to
enrich the signal and to control the systematics. Significant efforts are made
to explore the connection between the signal kinematics and the relevant masses
for the gluino and gauginos, to probe the mass scales of the superpartners, in
particular the LSP dark matter.Comment: 35 pages, 32 figure
Evolved embodied phase coordination enables robust quadruped robot locomotion
Overcoming robotics challenges in the real world requires resilient control
systems capable of handling a multitude of environments and unforeseen events.
Evolutionary optimization using simulations is a promising way to automatically
design such control systems, however, if the disparity between simulation and
the real world becomes too large, the optimization process may result in
dysfunctional real-world behaviors. In this paper, we address this challenge by
considering embodied phase coordination in the evolutionary optimization of a
quadruped robot controller based on central pattern generators. With this
method, leg phases, and indirectly also inter-leg coordination, are influenced
by sensor feedback.By comparing two very similar control systems we gain
insight into how the sensory feedback approach affects the evolved parameters
of the control system, and how the performances differs in simulation, in
transferal to the real world, and to different real-world environments. We show
that evolution enables the design of a control system with embodied phase
coordination which is more complex than previously seen approaches, and that
this system is capable of controlling a real-world multi-jointed quadruped
robot.The approach reduces the performance discrepancy between simulation and
the real world, and displays robustness towards new environments.Comment: 9 page
Leptogenesis after Chaotic Sneutrino Inflation and the Supersymmetry Breaking Scale
We discuss resonant leptogenesis arising from the decays of two
nearly-degenerate right-handed neutrinos, identified as the inflaton and
stabiliser superfields in a model of chaotic sneutrino inflation. We compare an
analytical estimate of the baryon asymmetry in the Boltzmann
approximation to a numerical solution of the full density matrix equations, and
find that the analytical result fails to capture the correct physics in certain
regions of parameter space. The observed baryon asymmetry can be realised for a
breaking of the mass degeneracy as small as . The
origin of such a small mass splitting is explained by considering supersymmetry
(SUSY) breaking in supergravity, which requires a constant in the
superpotential of the order of the gravitino mass to cancel the
cosmological constant. This yields additional terms in the (s)neutrino mass
matrices, lifting the degeneracy and linking to the SUSY breaking
scale. We find that achieving the correct baryon asymmetry requires a gravitino
mass TeV.Comment: v2: 25 pages, 4 figures; version published in NPB, minor corrections.
v1: 24 pages, 4 figure
Rashba spin splitting in biased semiconductor quantum wells
Rashba spin splitting (RSS) in biased semiconductor quantum wells is
investigated theoretically based on the eight-band envelope function model. We
find that at large wave vectors, RSS is both nonmonotonic and anisotropic as a
function of in-plane wave vector, in contrast to the widely used linear and
isotropic model. We derive an analytical expression for RSS, which can
correctly reproduce such nonmonotonic behavior at large wave vectors. We also
investigate numerically the dependence of RSS on the various band parameters
and find that RSS increases with decreasing band gap and subband index,
increasing valence band offset, external electric field, and well width. Our
analytical expression for RSS provides a satisfactory explanation to all these
features.Comment: 5 pages, 4 figures, author names corrected, submitted to Phys. Rev.
Resonant Tunneling through S- and U-shaped Graphene Nanoribbons
We theoretically investigate resonant tunneling through S- and U-shaped
nanostructured graphene nanoribbons. A rich structure of resonant tunneling
peaks are found eminating from different quasi-bound states in the middle
region. The tunneling current can be turned on and off by varying the Fermi
energy. Tunability of resonant tunneling is realized by changing the width of
the left and/or right leads and without the use of any external gates.Comment: 6 pages, 7 figure
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