91,247 research outputs found
Spin-correlation functions in ultracold paired atomic-fermion systems: sum rules, self-consistent approximations, and mean fields
The spin response functions measured in multi-component fermion gases by
means of rf transitions between hyperfine states are strongly constrained by
the symmetry of the interatomic interactions. Such constraints are reflected in
the spin f-sum rule that the response functions must obey. In particular, only
if the effective interactions are not fully invariant in SU(2) spin space, are
the response functions sensitive to mean field and pairing effects. We
demonstrate, via a self-consistent calculation of the spin-spin correlation
function within the framework of Hartree-Fock-BCS theory, how one can derive a
correlation function explicitly obeying the f-sum rule. By contrast, simple
one-loop approximations to the spin response functions do not satisfy the sum
rule. As we show, the emergence of a second peak at higher frequency in the rf
spectrum, as observed in a recent experiment in trapped , can be
understood as the contribution from the paired fermions, with a shift of the
peak from the normal particle response proportional to the square of the BCS
pairing gap.Comment: 7 pages, 1 figure, content adde
Developments in electromagnetic tomography instrumentation.
A new EMT sensor and instrumentation is described which combines the best features of previous systems and has a modular structure to allow for future system expansion and development
Microwave Nanotube Transistor Operation at High Bias
We measure the small signal, 1 GHz source-drain dynamical conductance of a
back-gated single-walled carbon nanotube field effect transistor at both low
and high dc bias voltages. At all bias voltages, the intrinsic device dynamical
conductance at 1 GHz is identical to the low frequency dynamical conductance,
consistent with the prediction of a cutoff frequency much higher than 1 GHz.
This work represents a significant step towards a full characterization of a
nanotube transistor for RF and microwave amplifiers.Comment: 3 pages, 4 figure
Multifractal analysis of complex networks
Complex networks have recently attracted much attention in diverse areas of
science and technology. Many networks such as the WWW and biological networks
are known to display spatial heterogeneity which can be characterized by their
fractal dimensions. Multifractal analysis is a useful way to systematically
describe the spatial heterogeneity of both theoretical and experimental fractal
patterns. In this paper, we introduce a new box covering algorithm for
multifractal analysis of complex networks. This algorithm is used to calculate
the generalized fractal dimensions of some theoretical networks, namely
scale-free networks, small world networks and random networks, and one kind of
real networks, namely protein-protein interaction networks of different
species. Our numerical results indicate the existence of multifractality in
scale-free networks and protein-protein interaction networks, while the
multifractal behavior is not clear-cut for small world networks and random
networks. The possible variation of due to changes in the parameters of
the theoretical network models is also discussed.Comment: 18 pages, 7 figures, 4 table
Radio Emission from Pulsar Wind Nebulae without Surrounding Supernova Ejecta: Application to FRB 121102
In this paper, we propose a new scenario in which a rapidly-rotating
strongly-magnetized pulsar without any surrounding supernova ejecta produces
fast radio bursts (FRBs) repeatedly via some mechanisms, and meanwhile, an
ultra-relativistic electron/positron pair wind from the pulsar sweeps up its
ambient dense interstellar medium, giving rise to a non-relativistic pulsar
wind nebula (PWN). We show that the synchrotron radio emission from such a PWN
is bright enough to account for the recently-discovered persistent radio source
associated with the repeating FRB 121102 in reasonable ranges of the model
parameters. In addition, our PWN scenario is consistent with the non-evolution
of the dispersion measure inferred from all the repeating bursts observed in
four years.Comment: 6 pages, 1 figure, ApJ Letters in pres
Formation of Hydrogenated Graphene Nanoripples by Strain Engineering and Directed Surface Self-assembly
We propose a new class of semiconducting graphene-based nanostructures:
hydrogenated graphene nanoripples (HGNRs), based on continuum-mechanics
analysis and first principles calculations. They are formed via a two-step
combinatorial approach: first by strain engineered pattern formation of
graphene nanoripples, followed by a curvature-directed self-assembly of H
adsorption. It offers a high level of control of the structure and morphology
of the HGNRs, and hence their band gaps which share common features with
graphene nanoribbons. A cycle of H adsorption/desorption at/from the same
surface locations completes a reversible metal-semiconductor-metal transition
with the same band gap.Comment: 11 pages, 5 figure
- …