789 research outputs found
A homopolar disc dynamo experiment with liquid metal contacts
We present experimental results of a homopolar disc dynamo constructed at
CICATA-Quer\'etaro in Mexico. The device consists of a flat, multi-arm spiral
coil which is placed above a fast-spinning metal disc and connected to the
latter by sliding liquid-metal electrical contacts. Theoretically,
self-excitation of the magnetic field is expected at the critical magnetic
Reynolds number Rm~45, which corresponds to a critical rotation rate of about
10 Hz. We measured the magnetic field above the disc and the voltage drop on
the coil for the rotation rate up to 14 Hz, at which the liquid metal started
to leak from the outer sliding contact. Instead of the steady magnetic field
predicted by the theory we detected a strongly fluctuating magnetic field with
a strength comparable to that of Earth's magnetic field which was accompanied
by similar voltage fluctuations in the coil. These fluctuations seem to be
caused by the intermittent electrical contact through the liquid metal. The
experimental results suggest that the dynamo with the actual electrical
resistance of liquid metal contacts could be excited at the rotation rate of
around 21 Hz provided that the leakage of liquid metal is prevented.Comment: 6 pages, 5 figures (to appear in Magnetohydrodynamics
Suppression of electron spin decoherence of the diamond NV center by a transverse magnetic field
We demonstrate that the spin decoherence of nitrogen vacancy (NV) centers in
diamond can be suppressed by a transverse magnetic field if the electron spin
bath is the primary decoherence source. The NV spin coherence, created in "a
decoherence-free subspace" is protected by the transverse component of the
zero-field splitting, increasing the spin-coherence time about twofold. The
decoherence due to the electron spin bath is also suppressed at magnetic fields
stronger than ~25 gauss when applied parallel to the NV symmetry axis. Our
method can be used to extend the spin-coherence time of similar spin systems
for applications in quantum computing, field sensing, and other metrologies.Comment: 20 pages, 4 figure
Coupled charge and spin dynamics in high-density ensembles of nitrogen-vacancy centers in diamond
We studied the spin depolarization of ensembles of nitrogen-vacancy (NV)
centers in nitrogen-rich single crystal diamonds. We found a strong dependence
of the evolution of the polarized state in the dark on the concentration of NV
centers. At low excitation power, we observed a simple exponential decay
profile in the low-density regime and a paradoxical inverted exponential
profile in the high-density regime. At higher excitation power, we observed
complex behavior, with an initial sharp rise in luminescence signal after the
preparation pulse followed by a slower exponential decay. Magnetic field and
excitation laser power-dependent measurements suggest that the rapid initial
increase of the luminescence signal is related to recharging of the
nitrogen-vacancy centers (from neutral to negatively charged) in the dark. The
slow relaxing component corresponds to the longitudinal spin relaxation of the
NV ensemble. The shape of the decay profile reflects the interplay between two
mechanisms: the NV charge state conversion in the dark and the longitudinal
spin relaxation. These mechanisms, in turn, are influenced by ionization,
recharging and polarization dynamics during excitation. Interestingly, we found
that charge dynamics are dominant in NV-dense samples even at very feeble
excitation power. These observations may be important for the use of ensembles
of NV centers in precession magnetometry and sensing applications.Comment: 7 pages, 6 figure
Particle-Based Mesoscale Hydrodynamic Techniques
Dissipative particle dynamics (DPD) and multi-particle collision (MPC)
dynamics are powerful tools to study mesoscale hydrodynamic phenomena
accompanied by thermal fluctuations. To understand the advantages of these
types of mesoscale simulation techniques in more detail, we propose new two
methods, which are intermediate between DPD and MPC -- DPD with a multibody
thermostat (DPD-MT), and MPC-Langevin dynamics (MPC-LD). The key features are
applying a Langevin thermostat to the relative velocities of pairs of particles
or multi-particle collisions, and whether or not to employ collision cells. The
viscosity of MPC-LD is derived analytically, in very good agreement with the
results of numerical simulations.Comment: 7 pages, 2 figures, 1 tabl
A reduced model for shock and detonation waves. II. The reactive case
We present a mesoscopic model for reactive shock waves, which extends a
previous model proposed in [G. Stoltz, Europhys. Lett. 76 (2006), 849]. A
complex molecule (or a group of molecules) is replaced by a single
mesoparticle, evolving according to some Dissipative Particle Dynamics.
Chemical reactions can be handled in a mean way by considering an additional
variable per particle describing a rate of reaction. The evolution of this rate
is governed by the kinetics of a reversible exothermic reaction. Numerical
results give profiles in qualitative agreement with all-atom studies
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