7,013 research outputs found
Ultracold, radiative charge transfer in hybrid Yb ion - Rb atom traps
Ultracold hybrid ion-atom traps offer the possibility of microscopic
manipulation of quantum coherences in the gas using the ion as a probe.
However, inelastic processes, particularly charge transfer can be a significant
process of ion loss and has been measured experimentally for the Yb ion
immersed in a Rb vapour. We use first-principles quantum chemistry codes to
obtain the potential energy curves and dipole moments for the lowest-lying
energy states of this complex. Calculations for the radiative decay processes
cross sections and rate coefficients are presented for the total decay
processes. Comparing the semi-classical Langevin approximation with the quantum
approach, we find it provides a very good estimate of the background at higher
energies. The results demonstrate that radiative decay mechanisms are important
over the energy and temperature region considered. In fact, the Langevin
process of ion-atom collisions dominates cold ion-atom collisions. For spin
dependent processes \cite{kohl13} the anisotropic magnetic dipole-dipole
interaction and the second-order spin-orbit coupling can play important roles,
inducing couplingbetween the spin and the orbital motion. They measured the
spin-relaxing collision rate to be approximately 5 orders of magnitude higher
than the charge-exchange collision rate \cite{kohl13}. Regarding the measured
radiative charge transfer collision rate, we find that our calculation is in
very good agreement with experiment and with previous calculations.
Nonetheless, we find no broad resonances features that might underly a strong
isotope effect. In conclusion, we find, in agreement with previous theory that
the isotope anomaly observed in experiment remains an open question.Comment: 7 figures, 1 table accepted for publication in J. Phys. B: At. Mol.
Opt. Phys. arXiv admin note: text overlap with arXiv:1107.114
Electron release of rare gas atom clusters under an intense laser pulse
Calculating the energy absorption of atomic clusters as a function of the
laser pulse length we find a maximum for a critical . We show that
can be linked to an optimal cluster radius . The existence of this
radius can be attributed to the enhanced ionization mechanism originally
discovered for diatomic molecules. Our findings indicate that enhanced
ionization should be operative for a wide class of rare gas clusters. From a
simple Coulomb explosion ansatz, we derive an analytical expression relating
the maximum energy release to a suitably scaled expansion time which can be
expressed with the pulse length .Comment: 4 pages, 5 figure
Energy spectrum and Landau levels in bilayer graphene with spin-orbit interaction
We present a theoretical study of the bandstructure and Landau levels in
bilayer graphene at low energies in the presence of a transverse magnetic field
and Rashba spin-orbit interaction in the regime of negligible trigonal
distortion. Within an effective low energy approach (L\"owdin partitioning
theory) we derive an effective Hamiltonian for bilayer graphene that
incorporates the influence of the Zeeman effect, the Rashba spin-orbit
interaction, and inclusively, the role of the intrinsic spin-orbit interaction
on the same footing. Particular attention is spent to the energy spectrum and
Landau levels. Our modeling unveil the strong influence of the Rashba coupling
in the spin-splitting of the electron and hole bands. Graphene
bilayers with weak Rashba spin-orbit interaction show a spin-splitting linear
in momentum and proportional to , but scales inversely proportional
to the interlayer hopping energy . However, at robust spin-orbit
coupling the energy spectrum shows a strong warping behavior near
the Dirac points. We find the bias-induced gap in bilayer graphene to be
decreasing with increasing Rashba coupling, a behavior resembling a topological
insulator transition. We further predict an unexpected assymetric
spin-splitting and crossings of the Landau levels due to the interplay between
the Rashba interaction and the external bias voltage. Our results are of
relevance for interpreting magnetotransport and infrared cyclotron resonance
measurements, including also situations of comparatively weak spin-orbit
coupling.Comment: 25 pages, 5 figure
Recycling cellular downlink energy for overlay self-sustainable IoT networks
This paper investigates the self-sustainability of an overlay Internet of Things (IoT) network that relies on harvest- ing energy from a downlink cellular network. Using stochastic geometry and queueing theory, we develop a spatiotemporal model to derive the steady state distribution of the number of packets in the bu ff ers and energy levels in the batteries of IoT devices given that the IoT and cellular communications are allocated disjoint spectrum. Particularly, each IoT device is modeled via a two-dimensional discrete-time Markov Chain (DTMC) that jointly tracks the evolution of data bu ff er and energy battery. In this context, stochastic geometry is used to derive the energy generation at the batteries and the packet transmission probability from bu ff ers taking into account the mutual interference from other active IoT devices. To this end, we show the Pareto-Frontiers of the sustainability region, which defines the network parameters that ensure stable network operation and finite packet delay. The results provide several insights to design self-sustainable IoT networks. Index Terms —Spatiotemporal models, stochastic geometry, queuing theory, energy harvesting, packet transmission success probability, two-dimensional discrete-time Markov chain, sta- bility conditions
Gate-tunable bandgap in bilayer graphene
The tight-binding model of bilayer graphene is used to find the gap between
the conduction and valence bands, as a function of both the gate voltage and as
the doping by donors or acceptors. The total Hartree energy is minimized and
the equation for the gap is obtained. This equation for the ratio of the gap to
the chemical potential is determined only by the screening constant. Thus the
gap is strictly proportional to the gate voltage or the carrier concentration
in the absence of donors or acceptors. In the opposite case, where the donors
or acceptors are present, the gap demonstrates the asymmetrical behavior on the
electron and hole sides of the gate bias. A comparison with experimental data
obtained by Kuzmenko et al demonstrates the good agreement.Comment: 6 pages, 5 figure
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