25,010 research outputs found
Electrical and Hydrogen Reduction Enhances Kinetics in Doped Zirconia and Ceria: II. Mapping Electrode Polarization and Vacancy Condensation in YSZ
Knowing the correlation between grain boundary mobility and oxygen potential
in yttria stabilized zirconia (YSZ), we have utilized the grain size as a
microstructural marker to map local oxygen potential. Abrupt oxygen potential
transition is established under a large current density and in thicker samples.
Cathodically depressed oxygen potential can be easily triggered by poor
electrode kinetics or in an oxygen-lean environment. Widespread cavitation in
the presence of highly reducing oxygen potential suggests oxygen vacancy
condensation instead of oxygen bubble formation as commonly assumed for solid
oxide fuel/electrolysis cells. These results also suggest electrode kinetics
has a direct influence on the microstructure and properties of ceramics
sintered under a large electric current
Inversion of oxygen potential transitions at grain boundaries of SOFC/SOEC electrolytes
Solid oxide fuel/electrolyzer cell (SOFC/SOEC) converts energy between
chemical and electrical forms inversely. Yet electrolyte degradation takes
place much more severe for SOEC than SOFC during long-term operations. By
solving transport equations, we found very large oxygen potential gradients and
sharp oxygen potential transitions at grain boundaries of polycrystalline
SOFC/SOEC electrolyte. Surprisingly, an inversion of oxygen potential
transitions was identified, suggesting a fundamentally different transport
mechanism for minor electronic charge carriers. Such findings could be critical
to understand and eliminate SOFC/SOEC degradations in practical applications
X-ray transients from the accretion-induced collapse of white dwarfs
The accretion-induced collapse (AIC) of a white dwarf in a binary with a
nondegenerate companion can sometimes lead to the formation of a rapidly
rotating and highly magnetized neutron star (NS). The spin-down of this NS can
drive a powerful pulsar wind (PW) and bring out some detectable
multi-wavelength emissions. On the one hand, the PW can evaporate the companion
in a few days to form a torus surrounding the NS. Then, due to the blockage of
the PW by the torus, a reverse shock can be formed in the wind to generate
intense hard X-rays. This emission component disappears in a few weeks' time,
after the torus is broken down at its inner boundary and scoured into a very
thin disk. On the other hand, the interaction between the PW with an AIC ejecta
can lead to a termination shock of the wind, which can produce a long-lasting
soft X-ray emission component. In any case, the high-energy emissions from deep
inside the system can be detected only after the AIC ejecta becomes transparent
for X-rays. Meanwhile, by absorbing the X-rays, the AIC ejecta can be heated
effectively and generate a fast-evolving and luminous ultraviolet (UV)/optical
transient. Therefore, the predicted hard and soft X-ray emissions, associated
by an UV/optical transient, provide a clear observational signature for
identifying AIC events in current and future observations (e.g., AT 2018cow).Comment: 7 pages, 6 figure
Transverse angular momentum in topological photonic crystals
Engineering local angular momentum of structured light fields in real space
enables unprecedented applications in many fields, in particular for the
realization of unidirectional robust transport in topological photonic crystals
with non-trivial Berry vortex in momentum space. Here, we show transverse
angular momentum modes in silicon topological photonic crystals when
considering transverse electric polarization. Excited by a chiral external
source with either transverse spin or orbital angular momentum, robust light
flow propagating along opposite directions was observed in several kinds of
sharp-turn interfaces between two topologically-distinct silicon photonic
crystals. A transverse orbital angular momentum mode with alternating-sign
topological charge was found at the boundary of such two photonic crystals. In
addition, we also found that unidirectional transport is robust to the working
frequency even when the ring-size or location of pseudo-spin source varies in a
certain range, leading to the superiority of broadband photonic device. These
findings enable for making use of transverse angular momentum, a kind of degree
of freedom, to achieve unidirectional robust transport in telecom region and
other potential applications in integrated photonic circuits such as on-chip
robust delay line.Comment: 17 pages, 5 figure
Valley controlled propagation of pseudospin states in bulk metacrystal waveguides
Light manipulations such as spin-direction locking propagation, robust
transport, quantum teleportation and reconfigurable electromagnetic pathways
have been investigated at the boundaries of photonic systems. Recently by
breaking Dirac cones in time-reversal invariant photonic crystals,
valley-pseudospin coupled edge states have been employed to realize selective
propagation of light. Here, without photonic boundaries, we realize the
propagation of pseudospin states in three-dimensional bulk metacrystal
waveguides by employing the ubiquitous valley degree of freedom.
Valley-dependent pseudospin bands are achieved in three-dimensional metacrystal
waveguides without Dirac cones. Reconfigurable photonic valley Hall effect is
proposed after studying the variation of pseudospin states near K' and K
valleys. Moreover, a prototype of photonic blocker is realized by cascading two
inversion asymmetric metacrystal waveguides in which the pseudospin direction
locking propagation exists. In addition, valley-dependent pseudospin bands are
also discussed in a realistic metamaterials sample. These results show an
alternative way towards molding the pseudospin flow in photonic systems.Comment: 26 pages, 7 figure
Bosonic Integer Quantum Hall States without Landau Levels on Square Lattice
We study an interacting two-component hard-core bosons on square lattice for
which, in the presence of staggered magnetic flux, the ground state is a
bosonic integer quantum Hall (BIQH) state. Using a coupled-wire bosonization
approach, we analytically show this model exhibits a BIQH state at total charge
half filling associated with a symmetry-protected topological phase under
charge conservation. These theoretical expectations are verified, using
the infinite density matrix renormalization group method, by providing
numerical evidences for: (i) a quantized Hall conductance ,
and (ii) two counter-propagating gapless edge modes. Our model is a bosonic
cousin of the fermionic Haldane model and serves as an additional case of
analogy between bosonic and fermionic quantum Hall states.Comment: 5 pages, 4 figures, 1 tabl
On Three-dimensional CR Yamabe Solitons
In this paper, we investigate the geometry and classification of
three-dimensional CR Yamabe solitons. In the compact case, we show that any
3-dimensional CR Yamabe soliton must have constant Tanaka-Webster scalar
curvature; we also obtain a classification under the assumption that their
potential functions are in the kernel of the CR Paneitz operator. In the
complete case, we obtain a structure theorem on the diffeomorphism types of
complete 3-dimensional pseudo-gradient CR Yamabe solitons (shrinking, or
steady, or expanding) of vanishing torsion.Comment: Revised version;Theorem 1.3 improved; Section 5 rewritten; Appendix
adde
-positivity and a classification of closed three-dimensional CR torsion solitons
A closed CR 3-manifold is said to have -positive pseudohermitian
curvature if for any . We discover
an obstruction for a closed CR 3-manifold to possess -positive
pseudohermitian curvature. We classify closed three-dimensional CR Yamabe
solitons according to -positivity and -negativity whenever
and the potential function lies in the kernel of Paneitz operator.
Moreover, we show that any closed three-dimensional CR torsion soliton must be
the standard Sasakian space form. At last, we discuss the persistence of
-positivity along the CR torsion flow starting from a pseudo-Einstein
contact form.Comment: Texts in the preliminary section, where we recall some basic notions
in CR geometry, have some overlap with our previous wor
Entanglements in Systems with Multiple Degrees of Freedom
We present the entanglement properties of the spin-orbital coupling systems
with multiple degrees of freedom. After constructing the maximally entangled
spin-orbital basis of bipartite, we find that the quantum entanglement length
in the noninteracting itinerant Fermion system with spin and orbit is
considerably larger than that in the system with only spin. In the
SU(2)SU(2) spin-orbital interacting system, the entanglement,
expressed in terms of the spin-orbital correlation functions, clearly manifests
the close relationship with the quantum phases in strongly correlated systems;
and the entanglement phase diagram of the finite-size systems is in agreement
with the magnetic and orbital phase diagram of the infinite systems. The
application of the present theory on nucleon systems is suggested.Comment: 4 pages, 2 figures, submitte
Electron Localization Enhances Cation Diffusion in Reduced ZrO2, CeO2 and BaTiO3
According to defect chemistry, the experimental observations of enhanced
cation diffusion in a reducing atmosphere in zirconia, ceria and barium
titanate are in support of an interstitial mechanism. Yet previous
computational studies always found a much higher formation energy for cation
interstitials than for cation vacancies, which would rule out the interstitial
mechanism. The conundrum has been resolved via first-principles calculations
comparing migration of reduced cations and oxidized ones, in cubic ZrO2, CeO2
and BaTiO3. In nearly all cases, reduction alone lowers the migration barrier,
and pronounced lowering results if cation's electrostatic energy at the saddle
point decreases. The latter is most effectively realized when a Ti cation is
allowed to migrate via an empty Ba site thus being fully screened all the way
by neighboring anions. Since reduction creates oxygen vacancies as well, which
are highly mobile, we also studied their effect on cation migration, and found
it only marginally lowers the migration barrier. In several cases, however, a
large synergistic effect between cation reduction and oxygen vacancy is
revealed, causing an electron to localize in the saddle-point state at a much
lower energy than normal, signaling that the saddle point is a negative-U state
in which the soft environment enables a large electron-phonon interaction that
can over-compensate the on-site Coulomb repulsion. These general findings are
expected to be applicable to defect-mediated ion migration in most transitional
metal oxides.Comment: arXiv admin note: text overlap with arXiv:1808.0519
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