The hidden (H) quantum state in 1T-TaS₂ has sparked considerable interest in the field of correlated electron systems. Here, we investigate ultrafast switches to stable H charge density wave (H-CDW) states observed in 1T-TaS_2−xSe_x, with x = 0 and 0.5 crystals, upon excitation with a single femtosecond laser pulse. In situ cooling transmission electron microscopy observations, initiated by a single femtosecond laser pumping with a low fluence, reveal a clear transition from a commensurate CDW phase (q_C) to a new CDW order with q_H = (1 − δ)q_C for the H-CDW state (δ = 1/9) accompanied by an evident phase separation. H-CDW domain relaxation then occurs and yields a stable metallic phase under a high-fluence excitation. Furthermore, electrical resistivity measurements show that the notable drop in x = 0 and 0.5 samples associated with the appearance of H-CDW states depend on laser fluence and temperature. These results potentially provide a new perspective on the photodoping mechanism for the emergence of H-CDW states in the 1T-TaS_2−xSe_x family.

Four-dimensional ultrafast transmission electron microscopy measurements reveal a rich variety of structural dynamic phenomena at a phase transition in the charge density wave (CDW) of $1T\text{−}{\mathrm{TaSe}}_2$. Under photoexcitation, remarkable changes in both the CDW intensity and orientation are clearly observed, associated with the transformation from a commensurate (C) phase into an incommensurate (IC) phase in a time scale of picoseconds. Moreover, the transient states reveal a notable “structural isosbestic point” at a wave vector of $q_{\mathrm{iso}}$ where the C and IC phases yield their diffracting efficiencies in equal ratio. This observation reveals that the crystal planes parallel to $q_{\mathrm{iso}}$ adopt a common dynamic feature in the CDW phases throughout whole nonequilibrium transition. The second-order characteristics observed in this nonequilibrium phase transition have also been analyzed based on time-resolved structural data.

Resistive switching effect in conductor/insulator/conductor thin-film stacks is promising for resistance random access memory with high-density, fast speed, low power dissipation and high endurance, as well as novel computer logic architectures. NiO is a model system for the resistive switching effect and the formation/rupture of Ni nanofilaments is considered to be essential. However, it is not clear how the nanofilaments evolve in the switching process. Moreover, since Ni nanofilaments should be ferromagnetic, it provides an opportunity to explore the electromagnetic coupling in this system. Here, we report a direct observation of Ni nanofilaments and their specific evolution process for the first time by a combination of various measurements and theoretical calculations. We found that multi-nanofilaments are involved in the low resistance state and the nanofilaments become thin and rupture separately in the RESET process with subsequent increase of the rupture gaps. Theoretical calculations reveal the role of oxygen vacancy amount in the evolution of Ni nanofilaments. We also demonstrate electromagnetic coupling in this system, which opens a new avenue for multifunctional devices.