8,696 research outputs found
Very Old Isolated Compact Objects as Dark Matter Probes
Very old isolated neutron stars and white dwarfs have been suggested to be
probes of dark matter. To play such a role, two requests should be fulfilled,
i.e., the annihilation luminosity of the captured dark matter particles is
above the thermal emission of the cooling compact objects (request-I) and also
dominate over the energy output due to the accretion of normal matter onto the
compact objects (request-II). Request-I calls for very dense dark matter medium
and the critical density sensitively depends on the residual surface
temperature of the very old compact objects. The accretion of
interstellar/intracluster medium onto the compact objects is governed by the
physical properties of the medium and by the magnetization and rotation of the
stars and may outshine the signal of dark matter annihilation. Only in a few
specific scenarios both requests are satisfied and the compact objects are dark
matter burners. The observational challenges are discussed and a possible way
to identify the dark matter burners is outlined.Comment: 9 pages including 1 Figure, to appear in Phys. Rev.
Atomic spatial coherence with spontaneous emission in a strong coupling cavity
The role of spontaneous emission in the interaction between a two-level atom
and a pumped micro-cavity in the strong coupling regime is discussed in this
paper. Especially, using a quantum Monte-Carlo simulation, we investigate
atomic spatial coherence. It is found that atomic spontaneous emission destroys
the coherence between neighboring lattice sites, while the cavity decay does
not. Furthermore, our computation of the spatial coherence function shows that
the in-site locality is little affected by the cavity decay, but greatly
depends on the cavity pump amplitude.Comment: 4 pages, 5 figures, accepted by PR
Resolving and Tuning Mechanical Anisotropy in Black Phosphorus via Nanomechanical Multimode Resonance Spectromicroscopy
Black phosphorus (P) has emerged as a layered semiconductor with a unique
crystal structure featuring corrugated atomic layers and strong in-plane
anisotropy in its physical properties. Here, we demonstrate that the crystal
orientation and mechanical anisotropy in free-standing black P thin layers can
be precisely determined by spatially resolved multimode nanomechanical
resonances. This offers a new means for resolving important crystal orientation
and anisotropy in black P device platforms in situ beyond conventional optical
and electrical calibration techniques. Furthermore, we show that
electrostatic-gating-induced straining can continuously tune the mechanical
anisotropic effects on multimode resonances in black P electromechanical
devices. Combined with finite element modeling (FEM), we also determine the
Young's moduli of multilayer black P to be 116.1 and 46.5 GPa in the zigzag and
armchair directions, respectively.Comment: Main Text: 13 Pages, 4 Figures; Supplementary Information: 5 Pages, 2
Figures, 2 Table
Dynamics and delocalisation transition for an interface driven by a uniform shear flow
We study the effect of a uniform shear flow on an interface separating the
two broken-symmetry ordered phases of a two-dimensional system with
nonconserved scalar order parameter. The interface, initially flat and
perpendicular to the flow, is distorted by the shear flow. We show that there
is a critical shear rate, \gamma_c, proportional to 1/L^2, (where L is the
system width perpendicular to the flow) below which the interface can sustain
the shear. In this regime the countermotion of the interface under its
curvature balances the shear flow, and the stretched interface stabilizes into
a time-independent shape whose form we determine analytically. For \gamma >
\gamma_c, the interface acquires a non-zero velocity, whose profile is shown to
reach a time-independent limit which we determine exactly. The analytical
results are checked by numerical integration of the equations of motion.Comment: 5 page
Maximum level and time to peak of dam-break waves on mobile horizontal bed
Journal of Hydraulic Engineering, Vol. 135, No. 11, November 1, 2009This experimental study focuses the influence of bed material mobility and initial downstream water level on maximum water level and time to peak of dam-break waves. It covers horizontal bed conditions on fixed bed, sand bed, and pumice bed. Results include water surface level time evolution, maxima wave levels and time to peak. The influence of bed material mobility and downstream water level was identified and characterized, stressing the importance of using mathematical models with appropriate sediment transport formulations instead of purely hydrodynamic models to simulate dam-break waves on mobile bed channels
The 2008 Nura earthquake sequence at the Pamir-Tian Shan collision zone, southern Kyrgyzstan
We analyzed the 5 October 2008, Mw 6.6 Nura earthquake, which occurred in the border triangle between Kyrgyzstan, Tajikistan, and China, and its aftershock series based on locally recorded seismic data. More than 3000 aftershocks were detected and located, using a double-difference technique and a regional 3-D velocity model. Moment tensors for the main event and the 42 largest aftershocks were determined by full-waveform inversion of long-period displacement seismograms. The Nura main shock was a shallow (∼3.4 km deep) reverse faulting event and occurred on an approximately east striking rupture plane situated east of the Alai Valley, along the Pamir Frontal Thrust of the Trans Alai Range, the leading edge of the Pamir Thrust System. Its presumed rupture plane dips steeply (∼59°) southward. The aftershocks constitute several distinct clusters that can be attributed to the activation of an array of individual faults including the one that was presumably broken by the main shock. Background seismicity occurred mainly further south, behind the crest of the Trans Alai Range, in an approximately east trending zone of dextral transpressional motion in the interior of the Pamir Thrust System. We show that nearly all reactivated structures lie in regions that experienced an increase in Coulomb stress due to the main shock rupture. The Nura earthquake sequence indicates slip partitioning between north-south shortening that creates large earthquakes along the Pamir Frontal Thrust, and lateral movement in the interior of the Pamir Thrust System.This research was funded by DFG
bundle TIPAGE (PAK 443), the CAME
project bundle TIPTIMON funded by
the German Federal Ministry of Education
and Research (support code
03G0809), and GFZ. We acknowledge
funding for the Earthquake Task Force
deployment by GFZ and the Hannover
Rück reinsurance company
The MobyDick Project: A Mobile Heterogeneous All-IP Architecture
Proceedings of Advanced Technologies, Applications and Market Strategies for 3G (ATAMS 2001). Cracow, Poland: 17-20 June, 2001.This paper presents the current stage of an IP-based architecture for heterogeneous environments, covering UMTS-like W-CDMA wireless access technology, wireless and wired LANs, that is being developed under the aegis of the IST Moby Dick project. This architecture treats all transmission capabilities as basic physical and data-link layers, and attempts to replace all higher-level tasks by IP-based strategies.
The proposed architecture incorporates aspects of mobile-IPv6, fast handover, AAA-control, and Quality of Service. The architecture allows for an optimised control on the radio link layer resources. The Moby dick architecture is currently under refinement for implementation on field trials. The services planned for trials are data transfer and voice-over-IP.Publicad
First-principles calculations of the structural, electronic, vibrational and magnetic properties of C_{60} and C_{48}N_{12}: a comparative study
In this work, we perform first-principles calculations of the structural,
electronic, vibrational and magnetic properties of a novel azafullerene. Full geometrical optimization shows that is characterized by several distinguishing features: only
one nitrogen atom per pentagon, two nitrogen atoms preferentially sitting in
one hexagon, symmetry, 6 unique nitrogen-carbon and 9 unique
carbon-carbon bond lengths. The highest occupied molecular orbital of is a doubly degenerate level of symmetry and its
lowest unoccupied molecular orbital is a nondegenerate level of
symmetry. Vibrational frequency analysis predicts that has in total 116 vibrational modes: 58 infrared-active and 58
Raman-active modes. is also characterized by 8
and 2 NMR spectral signals. Compared to , shows an enhanced third-order optical
nonlinearities which implies potential applications in optical limiting and
photonics.Comment: a long version of our manuscript submitted to J.Chem.Phy
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