886 research outputs found
Large magneto-optical Kerr effect in noncollinear antiferromagnets Mn ( = Rh, Ir, or Pt)
Magneto-optical Kerr effect, normally found in magnetic materials with
nonzero magnetization such as ferromagnets and ferrimagnets, has been known for
more than a century. Here, using first-principles density functional theory, we
demonstrate large magneto-optical Kerr effect in high temperature noncollinear
antiferromagnets Mn ( = Rh, Ir, or Pt), in contrast to usual wisdom.
The calculated Kerr rotation angles are large, being comparable to that of
transition metal magnets such as bcc Fe. The large Kerr rotation angles and
ellipticities are found to originate from the lifting of the band
double-degeneracy due to the absence of spatial symmetry in the Mn
noncollinear antiferromagnets which together with the time-reversal symmetry
would preserve the Kramers theorem. Our results indicate that Mn would
provide a rare material platform for exploration of subtle magneto-optical
phenomena in noncollinear magnetic materials without net magnetization
Topological magneto-optical effects and their quantization in noncoplanar antiferromagnets
Reflecting the fundamental interactions of polarized light with magnetic
matter, magneto-optical effects are well known since more than a century. The
emergence of these phenomena is commonly attributed to the interplay between
exchange splitting and spin-orbit coupling in the electronic structure of
magnets. Using theoretical arguments, we demonstrate that topological
magneto-optical effects can arise in noncoplanar antiferromagnets due to the
finite scalar spin chirality, without any reference to exchange splitting or
spin-orbit coupling. We propose spectral integrals of certain magneto-optical
quantities that uncover the unique topological nature of the discovered effect.
We also find that the Kerr and Faraday rotation angles can be quantized in
insulating topological antiferromagnets in the low-frequency limit, owing to
nontrivial global properties that manifest in quantum topological
magneto-optical effects. Although the predicted topological and quantum
topological magneto-optical effects are fundamentally distinct from
conventional light-matter interactions, they can be measured by readily
available experimental techniques.Comment: 10 pages, 5 figure
Mechanism for Degradation of Nafion in PEM Fuel Cells from Quantum Mechanics Calculations
We report results of quantum mechanics (QM) mechanistic studies of Nafion membrane degradation in a polymer electrolyte membrane (PEM) fuel cell. Experiments suggest that Nafion degradation is caused by generation of trace radical species (such as OH^●, H^●) only when in the presence of H_2, O_2, and Pt. We use density functional theory (DFT) to construct the potential energy surfaces for various plausible reactions involving intermediates that might be formed when Nafion is exposed to H_2 (or H^+) and O_2 in the presence of the Pt catalyst. We find a barrier of 0.53 eV for OH radical formation from HOOH chemisorbed on Pt(111) and of 0.76 eV from chemisorbed OOH_(ad), suggesting that OH might be present during the ORR, particularly when the fuel cell is turned on and off. Based on the QM, we propose two chemical mechanisms for OH radical attack on the Nafion polymer: (1) OH attack on the S–C bond to form H_2SO_4 plus a carbon radical (barrier: 0.96 eV) followed by decomposition of the carbon radical to form an epoxide (barrier: 1.40 eV). (2) OH attack on H_2 crossover gas to form hydrogen radical (barrier: 0.04 eV), which subsequently attacks a C–F bond to form HF plus carbon radicals (barrier as low as 1.00 eV). This carbon radical can then decompose to form a ketone plus a carbon radical with a barrier of 0.86 eV. The products (HF, OCF_2, SCF_2) of these proposed mechanisms have all been observed by F NMR in the fuel cell exit gases along with the decrease in pH expected from our mechanism
Magnetar giant flares in multipolar magnetic fields. III. Multipolar magnetic field structure variations
We have analyzed the multipolar magnetic field structure variation at neutron
star surface by means of the catastrophic eruption model, and find that the
variation of the geometry of multipolar fields on the magnetar surface could
result in the catastrophic rearrangement of the magnetosphere, which provides
certain physical mechanism for the outburst of giant flares. The magnetospheric
model we adopted consists of two assumptions: a helically twisted flux rope is
suspended in an ideal force-free magnetosphere around the magnetar, and a
current sheet emerges during the flux rope evolution. Magnetic energy
accumulates during the flux rope's gradual evolution along with the variation
of magnetar surface magnetic structure before the eruption. The two typical
behaviors, either state transition or catastrophic escape, would take place
once the flux rope loses equilibrium, thus tremendous accumulated energy is
radiated. We have investigated the equilibrium state of the flux rope and the
energy release affected by different multipolar structures, and find structures
that could trigger violent eruption and provide the radiation approximately
0.5 of the total magnetic energy during the giant flare outburst. Our
results provide certain multipolar structures of the neutron star's magnetic
field with an energy release percentage 0.42 in the state transition and
0.51 in the catastrophic escape case, which are sufficient for the
previously reported energy release from SGR 1806-20 giant flares
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