313,361 research outputs found
Destruction of invariant circles for Gevrey area-preserving twist maps
In this paper, we show that for exact area-preserving twist maps on annulus,
the invariant circles with a given rotation number can be destroyed by
arbitrarily small Gevrey- perturbations of the integrable generating
function in the topology with .Comment: to appear in Journal of Dynamics and Differential Equations (JDDE).
arXiv admin note: text overlap with arXiv:1208.2840, arXiv:1106.613
Dirac spin gapless semiconductors: Ideal platforms for massless and dissipationless spintronics and new (quantum) anomalous spin Hall effects
It is proposed that the new generation of spintronics should be ideally
massless and dissipationless for the realization of ultra-fast and
ultra-low-power spintronic devices. We demonstrate that the spin-gapless
materials with linear energy dispersion are unique materials that can realize
these massless and dissipationless states. Furthermore, we propose four new
types of spin Hall effects which consist of spin accumulation of equal numbers
of electrons and holes having the same or opposite spin polarization at the
sample edge in Hall effect measurements, but with vanishing Hall voltage. These
new Hall effects can be classified as (quantum) anomalous spin Hall effects.
The physics for massless and dissipationless spintronics and the new spin Hall
effects are presented for spin-gapless semiconductors with either linear or
parabolic dispersion. New possible candidates for Dirac-type or parabolic type
spin-gapless semiconductors are demonstrated in ferromagnetic monolayers of
simple oxides with either honeycomb or square lattices.Comment: 5 pages, 7 figue
Intrinsic Defects and Electronic Conductivity of TaON: First-Principles Insights
As a compound in between the tantalum oxide and nitride, the tantalum
oxynitride TaON is expected to combine their advantages and act as an efficient
visible-light-driven photocatalyst. In this letter, using hybrid functional
calculations we show that TaON has different defect properties from the binary
tantalum oxide and nitride: (i) instead of O or N vacancies or Ta
interstitials, the antisite is the dominant defect, which determines its
intrinsic n-type conductivity and the p-type doping difficulty; (ii) the
antisite has a shallower donor level than O or N vacancies, with a delocalized
distribution composed mainly of the Ta orbitals, which gives rise to
better electronic conductivity in the oxynitride than in the oxide and nitride.
The phase stability analysis reveals that the easy oxidation of TaON is
inevitable under O rich conditions, and a relatively O poor condition is
required to synthesize stoichiometric TaON samples
Thermodynamic Oxidation and Reduction Potentials of Photocatalytic Semiconductors in Aqueous Solution
We introduce an approach to calculate the thermodynamic oxidation and
reduction potentials of semiconductors in aqueous solution. By combining a
newly-developed ab initio calculation for compound formation energy and band
alignment with electrochemistry experimental data, this approach can be used to
predict the stability of almost any compound semiconductor in aqueous solution.
30 photocatalytic semiconductors have been studied, and a graph (a simplified
Pourbaix diagram) showing their valence/conduction band levels and
oxidation/reduction potentials is produced. Based on this graph, we have
studied the stabilities and trends against the oxidative and reductive
photocorrosion for compound semiconductors. We found that, only metal oxides
can be thermodynamically stable when used as the n-type photoanodes. All the
non-oxides are unstable due to easy oxidation by the photogenerated holes, but
they can be resistant to the reduction by electrons, thus stable as the p-type
photocathodes
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Role of initial magnetic disorder: A time-dependent ab initio study of ultrafast demagnetization mechanisms.
Despite more than 20 years of development, the underlying physics of the laser-induced demagnetization process is still debated. We present a fast, real-time time-dependent density functional theory (rt-TDDFT) algorithm together with the phenomenological atomic Landau-Lifshitz-Gilbert model to investigate this problem. Our Hamiltonian considers noncollinear magnetic moment, spin-orbit coupling (SOC), electron-electron, electron-phonon, and electron-light interactions. The algorithm for time evolution achieves hundreds of times of speedup enabling calculation of large systems. Our simulations yield a demagnetization rate similar to experiments. We found that (i) the angular momentum flow from light to the system is not essential and the spin Zeeman effect is negligible. (ii) The phonon can play a role but is not essential. (iii) The initial spin disorder and the self-consistent update of the electron-electron interaction play dominant roles and enhance the demagnetization to the experimentally observed rate. The spin disorder connects the electronic structure theory with the phenomenological three-temperature model
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