6,913 research outputs found
On tidal capture of primordial black holes by neutron stars
The fraction of primordial black holes (PBHs) of masses g
in the total amount of dark matter may be constrained by considering their
capture by neutron stars (NSs), which leads to the rapid destruction of the
latter. The constraints depend crucially on the capture rate which, in turn, is
determined by the energy loss by a PBH passing through a NS. Two alternative
approaches to estimate the energy loss have been used in the literature: the
one based on the dynamical friction mechanism, and another on tidal
deformations of the NS by the PBH. The second mechanism was claimed to be more
efficient by several orders of magnitude due to the excitation of particular
oscillation modes reminiscent of the surface waves. We address this
disagreement by considering a simple analytically solvable model that consists
of a flat incompressible fluid in an external gravitational field. In this
model, we calculate the energy loss by a PBH traversing the fluid surface. We
find that the excitation of modes with the propagation velocity smaller than
that of PBH is suppressed, which implies that in a realistic situation of a
supersonic PBH the large contributions from the surface waves are absent and
the above two approaches lead to consistent expressions for the energy loss.Comment: 7 page
Cavity Quantum Electrodynamics with a Rydberg blocked atomic ensemble
We propose to implement the Jaynes-Cummings model by coupling a
few-micrometer large atomic ensemble to a quantized cavity mode and classical
laser fields. A two-photon transition resonantly couples the single-atom ground
state |g> to a Rydberg state |e> via a non-resonant intermediate state |i>, but
due to the interaction between Rydberg atoms only a single atom can be
resonantly excited in the ensemble. This restricts the state space of the
ensemble to the collective ground state |G> and the collectively excited state
|E> with a single Rydberg excitation distributed evenly on all atoms. The
collectively enhanced coupling of all atoms to the cavity field with coherent
coupling strengths which are much larger than the decay rates in the system
leads to the strong coupling regime of the resulting effective Jaynes-Cummings
model. We use numerical simulations to show that the cavity transmission can be
used to reveal detailed properties of the Jaynes-Cummings ladder of excited
states, and that the atomic nonlinearity gives rise to highly non-trivial
photon emission from the cavity. Finally, we suggest that the absence of
interactions between remote Rydberg atoms may, due to a combinatorial effect,
induce a cavity-assisted excitation blockade whose range is larger than the
typical Rydberg dipole-dipole interaction length.Comment: 9 pages, 6 figure
Proteomic Investigations of Complex I Composition: How to Define a Subunit?
Complex I is present in almost all aerobic species. Being the largest complex of the respiratory chain, it has a central role in energizing biological membranes and is essential for many organisms. Bacterial complex I is composed of 14 subunits that are sufficient to achieve the respiratory functions. Eukaryotic enzymes contain orthologs of the 14 bacterial subunits and around 30 additional subunits. This complexity suggests either that complex I requires more stabilizing subunits in mitochondria or that it fulfills additional functions. In many organisms recent work on complex I concentrated on the determination of its exact composition. This review summarizes the work done to elucidate complex I composition in the model plant Arabidopsis and proposes a model for the organization of its 44 confirmed subunits. The comparison of the different studies investigating the composition of complex I across species identifies sample preparation for the proteomic analysis as critical to differentiate between true subunits, assembly factors, or proteins associated with complex I. Coupling comparative proteomics with biochemical or genetic studies is thus required to define a subunit and its function within the complex
Interstate Vibronic Coupling Constants Between Electronic Excited States for Complex Molecules
In the construction of diabatic vibronic Hamiltonians for quantum dynamics in
the excited-state manifold of molecules, the coupling constants are often
extracted solely from information on the excited-state energies. Here, a new
protocol is applied to get access to the interstate vibronic coupling constants
at the time-dependent density functional theory level through the overlap
integrals between excited-state adiabatic auxiliary wavefunctions. We discuss
the advantages of such method and its potential for future applications to
address complex systems, in particular those where multiple electronic states
are energetically closely lying and interact. As examples, we apply the
protocol to the study of prototype rhenium carbonyl complexes
[Re(CO)(N,N)(L)] for which non-adiabatic quantum dynamics within the
linear vibronic coupling model and including spin-orbit coupling have been
reported recently.Comment: 36 pages, 7 figures, 4 table
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