Next-generation non-volatile memories with ultrafast speed, low power consumption, and high density are highly desired in the era of big data. Here, we report a high performance memristor based on a Ag/BaTiO 3 /Nb:SrTiO 3 ferroelectric tunnel junction (FTJ) with the fastest operation speed (600 ps) and the highest number of states (32 states or 5 bits) per cell among the reported FTJs. The sub-nanosecond resistive switching maintains up to 358 K, and the write current density is as low as 4 × 10 3 A cm −2. The functionality of spike-timingdependent plasticity served as a solid synaptic device is also obtained with ultrafast operation. Furthermore, it is demonstrated that a Nb:SrTiO 3 electrode with a higher carrier concentration and a metal electrode with lower work function tend to improve the operation speed. These results may throw light on the way for overcoming the storage performance gap between different levels of the memory hierarchy and developing ultrafast neuromorphic computing systems.

Recently, theoretical studies have indicated that BaFe2Se3 may host a ferrielectric polarization and an uncompensated ferroelectricity driven by the exchange striction in its magnetic block order. Here, structural, magnetic, electrical transport, dielectric, and magnetodielectric properties in the BaFe2Se3 single crystals were systematically investigated. Below 320 K, BaFe2Se3 is a semiconductor, and the thermal activated transport processes with different activation energies were used to describe the conductivities in high and low temperature ranges. Magnetization measurements show a crossover from a short-range antiferromagnetic correlation to a long-range antiferromagnetic order at around 230 K and another antiferromagnetic transition at 150 K. The dielectric constant can be changed by about 6% in a magnetic field of 8 T. However, the ferrielectric characteristics of BaFe2Se3 are difficult to be fully verified by electric polarization and dielectric measurements, which may be related to the quite narrow energy gap and low resistivity.

The abundant site occupancy and optical transitions of multivalence Mn dopants in luminescent materials have attracted much attention. Here, detailed first-principles calculations based on density functional theory have been carried out to clarify the multisite and multivalence nature of Mn ions in solids and predict their optical transition properties by using garnets as prototype systems. The formation energies of dodecahedral, octahedral, and tetrahedral coordinated Mn dopants are evaluated with chemical potential environments, and the preferable site occupancy and valence state of Mn ions in three garnet systems are clarified. The results show that even in a fixed atmosphere, taking Ca3Al2Ge3O12 in air as an example, not only can the preference of Mn ions switch between dodecahedral and octahedral sites, but also can the valence state change from Mn2+ to Mn3+ and Mn4+. Furthermore, for all of the three garnet systems, the calculation results of the energy-level structure and photoluminescence of Mn ions at different sites in the different valence states provide a reliable interpretation of the available spectroscopic data. The proposed first-principles scheme, with general applicability and encouraging predictive power, provides an effective approach for elucidating and characterizing the site occupancy, valence state, and optical transition of Mn activators in insulators, and will greatly benefit the design and optimization of related materials.

Luminescent ns2 centers have shown great potential for the applications as phosphors and scintillators. First-principles calculations based on density functional theory are performed to systematically analyze the luminescent centers of...