Title: Electron-phonon interaction, magnetic phase transition, charge density waves and resistive switching in VS2 and VSe2 revealed by Yanson point contact spectroscopy

Abstract: VS2 and VSe2 have attracted particular attention among the transition metals dichalcogenides because of their promising physical properties concerning magnetic ordering, charge density wave (CDW), emergent superconductivity, etc., which are very sensitive to stoichiometry and dimensionality reduction. Yanson point contact (PC) spectroscopic study reveals metallic and nonmetallic states in VS2 PCs, as well as a magnetic phase transition was detected below 25 K. Analysis of PC spectra of VS2 testifies the realization of the thermal regime in PCs. At the same time, rare PC spectra, where the magnetic phase transition was not visible, shows a broad maximum of around 20 mV, likely connected with electron-phonon interaction (EPI). On the other hand, PC spectra of VSe2 demonstrate metallic behavior, which allowed us to detect features associated with EPI and CDW transition. The Kondo effect appeared for both compounds, apparently due to interlayer vanadium ions. Besides, the resistive switching was observed in PCs on VSe2 between a low resistive, mainly metallic-type state, and a high resistive nonmetallic-type state by applying bias voltage (about 0.4V). In contrast, reverse switching occurs by applying a voltage of opposite polarity (about 0.4V). The reason may be the alteration of stoichiometry in the PC core due to the displacement of V ions to interlayer under a high electric field. The observed resistive switching characterize VSe2 as a potential material, e.g., for non-volatile resistive RAM, neuromorphic engineering, and for other nanoelectronic applications. At the same time, VSe2 attracts attention as a rare layered van der Waals compound with magnetic transition.