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Topological Weyl and Node-Line Semimetals in Ferromagnetic Vanadium-Phosphorous-Oxide -VOPO Compound
We propose that the topological semimetal features can co-exist with
ferromagnetic ground state in vanadium-phosphorous-oxide -VOPO
compound from first-principles calculations. In this magnetic system with
inversion symmetry, the direction of magnetization is able to manipulate the
symmetric protected band structures from a node-line type to a Weyl one in the
presence of spin-orbital-coupling. The node-line semimetal phase is protected
by the mirror symmetry with the reflection-invariant plane perpendicular to
magnetic order. Within mirror symmetry breaking due to the magnetization along
other directions, the gapless node-line loop will degenerate to only one pair
of Weyl points protected by the rotational symmetry along the magnetic axis,
which are largely separated in momentum space. Such Weyl semimetal phase
provides a nice candidate with the minimum number of Weyl points in a condensed
matter system. The results of surface band calculations confirm the non-trivial
topology of this proposed compound. This findings provide a realistic candidate
for the investigation of topological semimetals with time-reversal symmetry
breaking, particularly towards the realization of quantum anomalous Hall effect
in Weyl semimetals.Comment: 5 pages, 4 figure
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Cathode chemistries and electrode parameters affecting the fast charging performance of li-ion batteries
Li-ion battery fast-charging technology plays an important role in popularizing electric vehicles (EV), which critically need a charging process that is as simple and quick as pumping fuel for conventional internal combustion engine vehicles. To ensure stable and safe fast charging of Li-ion battery, understanding the electrochemical and thermal behaviors of battery electrodes under high rate charges is crucial, since it provides insight into the limiting factors that restrict the battery from acquiring energy at high rates. In this work, charging simulations are performed on Li-ion batteries that use the LiCoO2 (LCO), LiMn2O4 (LMO), and LiFePO4 (LFP) as the cathodes. An electrochemical-thermal coupling model is first developed and experimentally validated on a 2.6Ah LCO based Li-ion battery and is then adjusted to study the LMO and LFP based batteries. LCO, LMO, and LFP based Li-ion batteries exhibited different thermal responses during charges due to their different entropy profiles, and results show that the entropy change of the LCO battery plays a positive role in alleviating its temperature rise during charges. Among the batteries, the LFP battery is difficult to be charged at high rates due to the charge transfer limitation caused by the low electrical conductivity of the LFP cathode, which, however, can be improved through doping or adding conductive additives. A parametric study is also performed by considering different electrode thicknesses and secondary particle sizes. It reveals that the concentration polarization at the electrode and particle levels can be weaken by using thin electrodes and small solid particles, respectively. These changes are helpful to mitigate the diffusion limitation and improve the performance of Li-ion batteries during high rate charges, but careful consideration should be taken when applying these changes since they can reduce the energy density of the batteries
Mass segregation in very young open clusters -- A case study of NGC 2244 and NGC 6530
We derive the proper motions, membership probabilities, and velocity
dispersions of stars in the regions of the young (about 2-4 Myr-old) open
clusters NGC 2244 (the central cluster in the Monoceros R2 association) and NGC
6530 (the dominant cluster in the Sgr OB1 association) from photographic plate
material obtained at Shanghai Astronomical Observatory, with time baselines of
34 and 87 years, respectively. Both clusters show clear evidence of mass
segregation, but they do not exhibit any significant velocity-mass (or,
equivalently, a velocity-luminosity) dependence. This provides strong support
for the suggestion that the observed mass segregation is -- at least partially
-- due to the way in which star formation has proceeded in these complex
star-forming regions (``primordial'' mass segregation). Based on arguments
related to the clusters' published initial mass functions, in conjunction with
our new measurements of their internal velocity dispersions (35 and 8 km/s for
NGC 2244 and NGC 6530, respectively), we provide strong arguments in favor of
the dissolution of NGC 2244 on very short time-scales, while we speculate that
NGC 6530 may be more stable against the effects of internal two-body
relaxation. However, this latter object may well be destroyed by the strong
tidal field prevalent at its location in the Galactic plane in the direction of
the Galactic Center.Comment: 36 pages, 10 figures, accepted to A
Magnon squeezing in an antiferromagnet: reducing the spin noise below the standard quantum limit
At absolute zero temperature, thermal noise vanishes when a physical system
is in its ground state, but quantum noise remains as a fundamental limit to the
accuracy of experimental measurements. Such a limitation, however, can be
mitigated by the formation of squeezed states. Quantum mechanically, a squeezed
state is a time-varying superposition of states for which the noise of a
particular observable is reduced below that of the ground state at certain
times. Quantum squeezing has been achieved for a variety of systems, including
the electromagnetic field, atomic vibrations in solids and molecules, and
atomic spins, but not so far for magnetic systems. Here we report on an
experimental demonstration of spin wave (i.e., magnon) squeezing. Our method
uses femtosecond optical pulses to generate correlations involving pairs of
magnons in an antiferromagnetic insulator, MnF2. These correlations lead to
quantum squeezing in which the fluctuations of the magnetization of a
crystallographic unit cell vary periodically in time and are reduced below that
of the ground state quantum noise. The mechanism responsible for this squeezing
is stimulated second order Raman scattering by magnon pairs. Such squeezed
states have important ramifications in the emerging fields of spintronics and
quantum computing involving magnetic spin states or the spin-orbit coupling
mechanism
Dynamics of self-organized driven particles with competing range interaction
Non-equilibrium self-organized patterns formed by particles interacting
through competing range interaction are driven over a substrate by an external
force. We show that, with increasing driving force, the pre-existed static
patterns evolve into dynamic patterns either via disordered phase or depinned
patterns, or via the formation of non-equilibrium stripes. Strikingly, the
stripes are formed either in the direction of the driving force or in the
transverse direction, depending on the pinning strength. The revealed dynamical
patterns are summarized in a dynamical phase diagram.Comment: 8 pages, 11 figure
Correction of the definition of mass-flow parameter in dynamic inflow modelling
No abstract available
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