arXiv subjects
Publications and source records attributed to Yan Zhou.
The {\it crossing number} of a graph $G$ is the minimum number of pairwise intersections of edges in a drawing of $G$. The {\it $n$-dimensional folded hypercube} $FQ_n$ is a graph obtained from $n$-dimensional hypercube by adding all complementary edges. In this paper, we obtain upper and lower bounds of the crossing number of $FQ_n$.
Systemic symmetry is introduced into our work to investigate the symmetries of different ZnO nanocrystals. Result shows that systemic symmetries obey the law of spontaneous symmetry breaking during the formation of ZnO nanocrystals. A unitary formation mechanism of ZnO nanocrystals is proposed accordingly. Our results provide new insight into both crystallization and spontaneous symmetry breaking at meso-scale.
We demonstrate that the transient non-autonomous dynamics of a spin torque nano-oscillator (STNO) under a radio-frequency (rf) driving signal is qualitatively different from the dynamics described by the Adler model. If the external rf current $I_{rf}$ is larger than a certain critical value $I_{cr}$ (determined by the STNO bias current and damping) strong oscillations of the STNO power and phase develop in the transient regime. The frequency of these oscillations increases with $I_{rf}$ as $\propto\sqrt{I_{rf} - I_{cr}}$ and can reach several GHz, whereas the damping rate of the oscillations is almost independent of $I_{rf}$. This oscillatory transient dynamics is caused by the strong STNO nonlinearity and should be taken into account in most STNO rf applications.
Using non-linear system theory and numerical simulations we map out the static and dynamic phase diagram in zero applied field of a spin torque oscillator with a tilted polarizer (TP-STO).We find that for sufficiently large currents, even very small tilt angles (beta>1 degree) will lead to steady free layer precession in zero field. Within a rather large range of tilt angles, 1 degree< beta <19 degree, we find coexisting static states and hysteretic switching between these using only current. In a more narrow window (1 degree<beta<5 degree) one of the static states turns into a limit cycle (precession). The coexistence of static and dynamic states in zero magnetic field is unique to the tilted polarizer and leads to large hysteresis in the upper and lower threshold currents for TP-STO operation.
The spin torque oscillator (STO), where the magnetization of the fixed layer is tilted out of the film plane, is capable of strong microwave signal generation in zero magnetic field. Through numerical simulations of the Landau-Lifshitz-Gilbert-Slonczewski equations, within a macro-spin approximation, we study the microwave signal generation as a function of drive current for two realistic tilt angles. The tilt magnetization of the fixed layer can be achieved by using a material with high out-of-plane magnetocrystalline anisotropy, such as L10 FePt.
Hyperspherical close-coupling method is used to calculate the elastic, positronium formation and excitation cross sections for positron collisions with atomic hydrogen at energies below the H(n=4) threshold for the J=2 partial wave. The resonances below each inelastic threshold are also analyzed. The adiabatic hyperspherical potential curves are used to identify the nature of these resonances.