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Reducing the switching energy of ferroelectric thin films remains an important goal in the pursuit of ultralow-power ferroelectric memory and logic devices. Here, we elucidate the fundamental role of lattice dynamics in ferroelectric switching by studying both freestanding bismuth ferrite (BiFeO3) membranes and films clamped to a substrate. We observe a distinct evolution of the ferroelectric domain pattern, from striped, 71° ferroelastic domains (spacing of ~100 nm) in clamped BiFeO3 films, to large (10's of micrometers) 180° domains in freestanding films. By removing the constraints imposed by mechanical clamping from the substrate, we can realize a ~40% reduction of the switching voltage and a consequent ~60% improvement in the switching speed. Our findings highlight the importance of a dynamic clamping process occurring during switching, which impacts strain, ferroelectric, and ferrodistortive order parameters and plays a critical role in setting the energetics and dynamics of ferroelectric switching.

Phase-change alloy Ge 2 Sb 2 Te 5 (GST) forms a favorable material platform for modern optics, photonics, and electronics thanks to a pronounced increase in conductivity with thermally induced phase transitions from amorphous (a-GST) into cubic (c-GST) and then hexagonal (h-GST) crystalline states at the temperatures of ≃ 150 and ≃ 300 ° C, respectively. Nevertheless, the data on broadband electrodynamic response of distinct GST phases are still missing, which hamper the design and implementation of related devices and technologies. In this paper, a-, c-, and h-GST films on a sapphire substrate are studied using broadband dielectric spectroscopy. For all GST phases, complex dielectric permittivity is retrieved using Drude and Lorentz models in the frequency range of 0.06 – 50 THz or the wavelength range of ≃ 5000 – 6 μ m. A contribution from the free charge-carriers conductivity and vibrational modes to the broadband response of an analyte is quantified. In this way, the Drude model allows for estimation of the static (direct current—DC) and dynamic (at 1.0 THz) conductivity values, caused by motions of free charges only, which are as high as σ DC ≃ 15 and 40 S/cm and σ 1.0 THz ≃ 8.8 and 28.6 S/cm for the c- and h-GSTs, respectively. This overall agrees with the results of electrical measurements of GST conductivity using the four-point probe technique. The broadband electrodynamic response models obtained for the three GST phases are important for further research and developments of GST-based devices and technologies. [ABSTRACT FROM AUTHOR]

The terahertz(THz) wave is located in the transition region of photonics to electronics, and has important application prospects in high-speed broadband communication, radar, imaging and other fields. However, the devices currently used for dynamic regulation of THz waves are still lacking, which limits the development of THz technology to some extent. VO2 has an unique metal-insulator phase change characteristic, and the phase change process can be applied to dynamically regulate THz wave transmission. It is also a recent research hotspot to explore the combination of metamaterials and VO2 to prepare efficient, dynamic and flexible terahertz functional devices. In this paper, the phase transition characteristics of VO2 are briefly described, and the effects of microstructure and chemical composition on phase transition characteristics are analyzed. The research progress of VO2 film in THz band regulation performance is reviewed systematically, and the different combinations of VO2 and metamaterials are summarized. In the application of THz wave dynamic regulation, the development prospects and challenges of THz wave regulation function devices based on VO2 phase transition characteristics are prospected.