22 research outputs found
Superradiant instability of the charged scalar field in stringy black hole mirror system
It has been shown that the mass of the scalar field in the charged stringy
black hole is never able to generate a potential well outside the event horizon
to trap the superradiant modes. This is to say that the charged stringy black
hole is stable against the massive charged scalar perturbation. In this paper
we will study the superradiant instability of the massless scalar field in the
background of charged stringy black hole due to a mirror-like boundary
condition. The analytical expression of the unstable superradiant modes is
derived by using the asymptotic matching method. It is also pointed out that
the black hole mirror system becomes extremely unstable for a large charge
of scalar field and the small mirror radius .Comment: 5 pages, no figure, published versio
Momentum relaxation of holographic Weyl semimetal from massive gravity
We consider the effects of momentum relaxation on the topological quantum
phase transitions in holographic Weyl semimetals. The translational symmetry
breaking in the field theory is realized in the framework of massive gravity.
We find that the critical value of the phase transition, characterized by the
anomalous Hall conductivity, decreases with the increasing of graviton mass,
i.e. the momentum relaxation strength. There exists a critical value of
graviton mass above which the topological phase transition disappears and
therefore the Weyl points are destroyed. All these phenomena are qualitatively
similar to that of axion fields induced momentum relaxation, indicating that a
universal feature emerges in the momentum relaxed holographic Weyl semimetals,
which is also consistent with the predictions from weakly coupled field theory.Comment: minor clarifications and references adde
Time domain analysis of superradiant instability for the charged stringy black hole-mirror system
It has been proved that the charged stringy black holes are stable under the
perturbations of massive charged scalar fields. However, superradiant
instability can be generated by adding the mirror-like boundary condition to
the composed system of charged stringy black hole and scalar field. The
unstable boxed quasinormal modes have been calculated by using both analytical
and numerical method. In this paper, we further provide a time domain analysis
by performing a long time evolution of charged scalar field configuration in
the background of the charged stringy black hole with the mirror-like boundary
condition imposed. We have used the ingoing Eddington-Finkelstein coordinates
to derive the evolution equation, and adopted Pseudo-spectral method and the
forth-order Runge-Kutta method to evolve the scalar field with the initial
Gaussian wave packet. It is shown by our numerical scheme that Fourier
transforming the evolution data coincides well with the unstable modes computed
from frequency domain analysis. The existence of the rapid growth mode makes
the charged stringy black hole a good test ground to study the nonlinear
development of superradiant instability.Comment: 7 pages, 6 figures, and 5 tables. References adde
Numerical study of superradiant instability for charged stringy black hole-mirror system
We numerically study the superradiant instability of charged massless scalar
field in the background of charged stringy black hole with mirror-like boundary
condition. We compare the numerical result with the previous analytical result
and show the dependencies of this instability upon various parameters of black
hole charge , scalar field charge , and mirror radius . Especially,
we have observed that imaginary part of BQN frequencies grows with the scalar
field charge rapidly.Comment: 5 pages, 5 figures, accepted by PLB. arXiv admin note: text overlap
with arXiv:1403.727
Image Encryption Algorithm Based on Plane-Level Image Filtering and Discrete Logarithmic Transform
Image encryption is an effective way to protect image data. However, existing image encryption algorithms are still unable to strike a good balance between security and efficiency. To overcome the shortcomings of these algorithms, an image encryption algorithm based on plane-level image filtering and discrete logarithmic transformation (IEA-IF-DLT) is proposed. By utilizing the hash value more rationally, our proposed IEA-IF-DLT avoids the overhead caused by repeated generations of chaotic sequences and further improves the encryption efficiency through plane-level and three-dimensional (3D) encryption operations. Aiming at the problem that common modular addition and XOR operations are subject to differential attacks, IEA-IF-DLT additionally includes discrete logarithmic transformation to boost security. In IEA-IF-DLT, the plain image is first transformed into a 3D image, and then three rounds of plane-level permutation, plane-level pixel filtering, and 3D chaotic image superposition are performed. Next, after a discrete logarithmic transformation, a random pixel swapping is conducted to obtain the cipher image. To demonstrate the superiority of IEA-IF-DLT, we compared it with some state-of-the-art algorithms. The test and analysis results show that IEA-IF-DLT not only has better security performance, but also exhibits significant efficiency advantages
The Origin of Threshold Reduction in Random Lasers Based on MoS<sub>2</sub>/Au NPs: Charge Transfer
Random lasers have attracted much attention in recent years owing to their advantages of a simple fabrication process, low processing cost, and material flexibility for any lasing wavelengths. They provide a roadmap for the design of ultra-bright lighting, displays, etc. However, the threshold reduction in random nanolasers remains a challenge in practical applications. In this work, lower-threshold random laser action from monolayer molybdenum disulfide film-encapsulated Au nanoparticles (MoS2/Au NPs) is demonstrated. The observed laser action of the MoS2/Au NPs shows a lower threshold of about 0.564 µJ/mm2, which is about 46.2% lower than the threshold of random lasers based on Au NPs. We proposed that the charge transfer between MoS2 and the gain material is the main reason for the reduction in the random laser threshold. The finite-difference time-domain (FDTD) method was used to calculate the lasing action of these two nanostructures. When charge transfer is taken into account, the theoretically calculated threshold of the MoS2/Au NPs is reduced by 46.8% compared to Au NP samples, which is consistent with the experimental results. This study provides a new mechanism to achieve low-threshold and high-quality random lasers, which has the potential to facilitate the application of random lasers and the development of high-performance optoelectronic devices