Ultrafast reorientation of the N\'eel vector in antiferromagnetic Dirac semimetals

Antiferromagnets exhibit distinctive characteristics such as ultrafast dynamics and robustness against perturbative fields, thereby attracting considerable interest in fundamental physics and technological applications. Recently, it was revealed that the N\'eel vector can be switched by a current-induced staggered (N\'eel) spin-orbit torque in antiferromagnets with the parity-time symmetry, and furthermore, a nonsymmorphic symmetry enables the control of Dirac fermions. However, the real-time dynamics of the magnetic and electronic structures remain largely unexplored. Here, we propose a theory of the ultrafast dynamics in antiferromagnetic Dirac semimetals and show that the N\'eel vector is rotated in the picosecond timescale by the terahertz-pulse-induced N\'eel spin-orbit torque and other torques originating from magnetic anisotropies. This reorientation accompanies the modulation of the mass of Dirac fermions and can be observed in real time by the magneto-optical effects. Our results provide a theoretical basis for emerging ultrafast antiferromagnetic spintronics combined with the topological aspects of materials.Comment: 8 pages, 4 figure

Solar zenith angle and solar activity dependences of vertical profile of electron number density in the nightside auroral region

Solar zenith angle and solar activity dependences of electron number density in the nightside auroral region from the topside ionosphere to the magnetosphere within a geocentric radial distance of 2.6 R_E were statistically investigated based on analysis of 7-years of plasma wave data measured by the plasma wave instrument onboard the Akebono (EXOS-D) satellite. The results are summarized as follows: (1) Electron number density N_e changes depending on solar zenith angle and solar activity: N_e in sunlight is about 3 times larger than that in darkness, and N_e during solar maximum is about 10 times larger than that during solar minimum. (2) During solar maximum, geopotential scale height is almost constant within a range from 250km to 400km. During solar minimum, geopotential scale height is drastically changes at a geopotential height around 2000-2500km, or an actual height of 3000-4000km: Geopotential scale height is 250-400km below the transition height and larger than 500km above the transition height. In order to discuss the auroral phenomena in various seasonal and solar activity conditions, the variations of ambient electron number density, as obviously shown in this study, should be taken into consideration in future studies

Seasonal and solar cycle dependences of the correlation between auroral kilometric radiation and the AE index

Seasonal and solar cycle dependences of the correlation between auroral kilometric radiation (AKR) and the auroral electrojet (AE) index have been investigated based on the plasma wave data obtained by the Akebono satellite. Under any seasonal and solar activity conditions, a clear correlation has been found between the AKR power flux and the AE index. The properties of the correlation, however, vary depending on season and solar activity. AKR power flux increases as about the 1.2 power of AE index in all seasonal and solar activity conditions. However, even for the same AE index, AKR power flux during solar minimum is 5dB larger than that during solar maximum. As for the seasonal variations, the AKR power flux in winter is 22dB larger than that in summer even for the same AE index. The results suggest that long-term variations of AKR depend not only on auroral current variations but also on factors associated with the total energy flux of auroral electrons and the generation process of AKR