The Research on the Complexity of 1T-TaS2 at Ultra-low Temperatures

Graphene, as a successfully industrialized two-dimensional material, has greatly promoted the development of other two-dimensional materials, such as transition metal dichalcogenide (TMDs). 1T-TaS2 is a classical TMDs material, which presents metallicity at high temperature. It undergoes a variety of charge density wave (CDW) phase transitions during the temperature declining process, and presents insulating properties at low temperature. During the temperature rise period, 1T-TaS2 goes through a phase transition, from an energy band insulator to Mott insulator, followed by an insulation-metal phase transition. The complexity of 1T-TaS2 phase diagram encourages researchers to conduct extensive research on it. This paper, via means of resistance, magnetic susceptibility and other technical methods, finds out that the ultra-low temperature of 1T-TaS2 suggests additional complexity. In addition, with the angle resolved photoemission spectroscopy (ARPES) technique of in-situ alkali metal evaporation, this paper proposes that the 1T-TaS2 ultra-low temperature ground state may exist a combination of state and surface state. Our findings provide more experimental evidence for the physical mechanism of this system.

Single-nanoantenna driven nanoscale control of the VO2 insulator to metal transition

The ultrafast concentration of electromagnetic energy in nanoscale volumes is one of the key features of optical nanoantennas illuminated at their surface plasmon resonances. Here, we drive the insulator to metal phase transition in vanadium dioxide (VO2) using a laser-induced pumping effect obtained by positioning a single gold nanoantenna in proximity to a VO2 thermochromic material. We explore how the geometry of the single nanoantenna affects the size and permittivity of the nanometer-scale VO2 regions featuring phase transition under different pumping conditions. The results reveal that a higher VO2 phase transition effect is obtained for pumping of the longitudinal or transversal localized surface plasmon depending on the antenna length. This characterization is of paramount importance since the single nanoantennas are the building blocks of many plasmonic nanosystems. Finally, we demonstrate the picosecond dynamics of the VO2 phase transition characterizing this system, useful for the realization of fast nano-switches. Our work shows that it is possible to miniaturize the hybrid plasmonic-VO2 system down to the single-antenna level, still maintaining a controllable behavior, fast picosecond dynamics, and the features characterizing its optical and thermal response.

Modulation Electronic Properties of Silicane/SnSe2 Van der Waals Heterostructures Using External Force and Electric Field

In recent years, much interest in the study of Van der Waals heterostructures (vdWhs) has arisen. This has led to a significant amount of fundamental research being produced, from which novel optoelectronic applications have been established. By using first principles, we analyze the electronic structure of silicane/SnSe2 vdWhs in the response to an externally applied electric field and a normal strain. The results show that the silicane/SnSe2 vdWh acts as an indirect semiconductor when it is subjected to an applied electric field between −1 and 0.1 V/Å and becomes a metal in the 0.2 to 1 V/Å range. Significantly, the electronic band alignments of the silicane/SnSe2 vdWhs are modified from a type-II to a type-I when a field of −0.7 V/Å is applied. Furthermore, it is determined that the silicane/SnSe2 vdWhs appears to have a semiconductor-metal phase transition at a strain of −5%. Our results indicate that the silicane/SnSe2 vdWhs have the potential for applications in novel high-performance optoelectronic devices.

­­­­A THZ Tunable Metamaterial Reflective Polarization Converter Based Vanadium Oxide Film

In this paper a simple and tunable reflective polarization converter has been investigated numerically based on metamaterial which composes of a two-corner-cut square patch resonator with a slit embedded into Vanadium dioxide film (VO2) and reflective ground layer. All the results obtained by the CST Microwave Studio show that the polarization conversion ratio (PCR) above 90% is achieved from 2.22-5.42THz at the temperature about 25°C under the linearly and circularly polarized wave incidence normally. In addition, the influences on electromagnetic polarization properties have been demonstrated with the insulator-to-metal phase transition of the Vanadium dioxide film (VO2) film by the method of varying the temperature. At the same time, to be demonstrated, the physical mechanism of changeable polarization conversion has been discussed by the distributions of current densities. According to the results, the designed metamaterial could be applied in the area of temperature-controlled sensing, THz wireless communication, tunable polarized devices.

Photoinduced multistage phase transitions in Ta2NiSe5

Ultrafast control of material physical properties represents a rapidly developing field in condensed matter physics. Yet, accessing the long-lived photoinduced electronic states is still in its early stages, especially with respect to an insulator to metal phase transition. Here, by combining transport measurement with ultrashort photoexcitation and coherent phonon spectroscopy, we report on photoinduced multistage phase transitions in Ta2NiSe5. Upon excitation by weak pulse intensity, the system is triggered to a short-lived state accompanied by a structural change. Further increasing the excitation intensity beyond a threshold, a photoinduced steady new state is achieved where the resistivity drops by more than four orders at temperature 50 K. This new state is thermally stable up to at least 350 K and exhibits a lattice structure different from any of the thermally accessible equilibrium states. Transmission electron microscopy reveals an in-chain Ta atom displacement in the photoinduced new structure phase. We also found that nano-sheet samples with the thickness less than the optical penetration depth are required for attaining a complete transition.