An introduction to a new space debris evolution model -- SOLEM

SOLEM is the first space debris long-term evolution model of China. This paper describes the principles, components and workflow of the SOLEM. The effects of different mitigation measures based on SOLEM model are analyzed and presented. The limitation of the model is pointed out and its future improvement work-plan is prospected.Comment: 16 pages, 14 figure

Maximal quantum Fisher information for general su(2) parametrization processes

Quantum Fisher information is a key concept in the field of quantum metrology, which aims to enhance the parameter accuracy by using quantum resources. In this paper, utilizing a representation of quantum Fisher information for a general unitary parametrization process, we study unitary parametrization processes governed by su(2) dynamics. We obtain the analytical expression for the Hermitian operator of the parametrization and the maximal quantum Fisher information. We find that the maximal quantum Fisher information over the parameter space consists of two parts, one is quadratic in the time and the other oscillates with the time. We apply our result to the estimation of a magnetic field and obtained the maximal quantum Fisher information. We further discuss a driving field with a time-dependent Hamiltonian and find the maximal quantum Fisher information of the driving frequency attains optimum when it is in resonance with the atomic frequency.Comment: 6 pages, 2 figures, published versio

Robustness of the semimetal state of Na3Bi and Cd3As2 against Coulomb interaction

We study the excitonic semimetal-insulator quantum phase transition in three-dimensional Dirac semimetal in which the fermion dispersion is strongly anisotropic. After solving the Dyson-Schwinger equation for the excitonic gap, we obtain a global phase diagram in the plane spanned by the parameter for Coulomb interaction strength and the parameter for fermion velocity anisotropy. We find that excitonic gap generation is promoted as the interaction becomes stronger, but is suppressed if the anisotropy increases. Applying our results to two realistic three-dimensional Dirac semimetals Na$_{3}$Bi and Cd$_{3}$As$_{2}$, we establish that their exact zero-temperature ground state is gapless semimetal, rather than excitonic insulator. Moreover, these two materials are far from the excitonic quantum critical point, thus there should not be any observable evidence for excitonic insulating behavior. This conclusion is in general agreement with the existing experiments of Na$_{3}$Bi and Cd$_{3}$As$_{2}$.Comment: 12 pages, 5 figure