4,420 research outputs found
Can planetesimals form by collisional fusion?
As a test bed for the growth of protoplanetary bodies in a turbulent
circumstellar disk we examine the fate of a boulder using direct numerical
simulations of particle seeded gas flowing around it. We provide an accurate
description of the flow by imposing no-slip and non-penetrating boundary
conditions on the boulder surface using the immersed boundary method pioneered
by Peskin (2002). Advected by the turbulent disk flow, the dust grains collide
with the boulder and we compute the probability density function (PDF) of the
normal component of the collisional velocity. Through this examination of the
statistics of collisional velocities we test the recently developed concept of
collisional fusion which provides a physical basis for a range of collisional
velocities exhibiting perfect sticking. A boulder can then grow sufficiently
rapidly to settle into a Keplerian orbit on disk evolution time scales.Comment: Astrophysical Journal, in pres
First-principles study of ferroelectric domain walls in multiferroic bismuth ferrite
We present a first-principles density functional study of the structural,
electronic and magnetic properties of the ferroelectric domain walls in
multiferroic BiFeO3. We find that domain walls in which the rotations of the
oxygen octahedra do not change their phase when the polarization reorients are
the most favorable, and of these the 109 degree domain wall centered around the
BiO plane has the lowest energy. The 109 degree and 180 degree walls have a
significant change in the component of their polarization perpendicular to the
wall; the corresponding step in the electrostatic potential is consistent with
a recent report of electrical conductivity at the domain walls. Finally, we
show that changes in the Fe-O-Fe bond angles at the domain walls cause changes
in the canting of the Fe magnetic moments which can enhance the local
magnetization at the domain walls.Comment: 9 pages, 20 figure
Bound-state/elementary-particle duality in the Higgs sector and the case for an excited 'Higgs' within the standard model
Though being weakly interacting, QED can support bound states. In principle,
this can be expected for the weak interactions in the Higgs sector as well. In
fact, it has been argued long ago that there should be a duality between bound
states and the elementary particles in this sector, at least in leading order
in an expansion in the Higgs condensate. Whether this remains true beyond the
leading order is investigated using lattice simulations, and support is found.
This provides a natural interpretation of peaks in cross sections as bound
states. Unambiguously, this would imply the existence of (possibly very broad)
resonances of Higgs and W and Z bound states within the standard model.Comment: 15 pages, 3 figures v2: added appendix with technical details, some
minor improvement
Unconventional Magnetization below 25 K in Nitrogen-doped Diamond provides hints for the existence of Superconductivity and Superparamagnetism
The magnetization of nitrogen-doped single crystalline diamond bulk samples shows unconventional field and temperature hysteresis loops at T ≲ 25 K. The results suggest the existence of superparamagnetic and superconducting regions in samples with nitrogen concentration <200 ppm. Both phases vanish at temperatures above 25 K where the samples show diamagnetic behavior similar to undoped diamond. The observation of superparamagnetism and superconductivity is attributed to the nitrogen doping and to the existence of defective regions. From particle-induced X-ray emission with ppm resolution we rule out that the main observations below 25 K are due to magnetic impurities. We investigated also the magnetic properties of ferromagnetic/high-temperature superconducting oxide bilayers. The magnetization results obtained from those bilayers show remarkable similarities to the ones in nitrogen-doped diamond
Breaking the resilience of a two-dimensional Bose-Einstein condensate to fragmentation
A two-dimensional Bose-Einstein condensate (BEC) split by a radial potential
barrier is investigated. We determine on an accurate many-body level the
system's ground-state phase diagram as well as a time-dependent phase diagram
of the splitting process. Whereas the ground state is condensed for a wide
range of parameters, the time-dependent splitting process leads to substantial
fragmentation. We demonstrate for the first time the dynamical fragmentation of
a BEC despite its ground state being condensed. The results are analyzed by a
mean-field model and suggest that a large manifold of low-lying fragmented
excited states can significantly impact the dynamics of trapped two-dimensional
BECs.Comment: 5+eps pages, 4 figure
Dual Brane Pairs, Chains and the Bekenstein-Hawking Entropy
A proposal towards a microscopic understanding of the Bekenstein-Hawking
entropy for D=4 spacetimes with event horizon is made. Since we will not rely
on supersymmetry these spacetimes need not be supersymmetric. Euclidean
D-branes which wrap the event horizon's boundary will play an important role.
After arguing for a discretization of the Euclidean D-brane worldvolume based
on the worldvolume uncertainty relation, we count chainlike excitations on the
worldvolume of specific dual Euclidean brane pairs. Without the need for
supersymmetry it is shown that one can thus reproduce the D=4
Bekenstein-Hawking entropy and its logarithmic correction.Comment: 14 pages, 1 figur
Production of non-local quartets and phase-sensitive entanglement in a superconducting beam splitter
Three BCS superconductors S_a, S_b, and S and two short normal regions N_a
and N_b in a three-terminal S_aN_aSN_bS_b set-up provide a source of non-local
quartets spatially separated as two correlated pairs in S_a and S_b, if the
distance between the interfaces N_aS and SN_b is comparable to the coherence
length in S. Low-temperature dc-transport of non-local quartets from S to S_a
and S_b can occur in equilibrium, and also if S_a and S_b are biased at
opposite voltages. At higher temperatures, thermal excitations result in
correlated current fluctuations which depend on the superconducting phases
phi_a and phi_b in S_a and S_b. Phase-sensitive entanglement is obtained at
zero temperature if N_a and N_b are replaced by discrete levels.Comment: 4 pages, 2 figures; technical details attached in ancillary file
http://arxiv.org/src/1102.2355v4/anc/EPAPS_Freyn_2011.pdf; higher versions:
minor corrections, cleanup and corrected reference
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