Metastable states in two-dimensional charge density wave materialsVenturini, Rok (Avtor) Mihailović, Dragan (Mentor) charge density wavetransition metal dichalcogenides1T-TaS2picosecond electrical pulse2H-NbSe21T-NbSe24Hb-TaSe2memory deviceelectrical switchingpolytype transformationscanning tunnelling microscopyA state with modulated electron density - a charge density wave (CDW) state - is common in many two-dimensional materials belonging to the group of transition metal dichalcogenides. The recent discovery of switching from the equilibrium CDW states to the non-equilibrium metastable CDW states has opened the doors to studying new physics as well as unveiled promising avenues for practical applications. In the first part of this thesis, we explore how switching between high-resistance and low-resistance states of a 1T-TaS2 crystal can be used for fabrication of an efficient memory device. In particular, we investigate in great detail the electrical switching of a 1T-TaS2 crystal with picosecond electrical pulses for which a custom optoelectronic circuit is built. By combining the circuit that also acts as a transmission line with a 1T-TaS2 device we achieve high-contrast resistance switching with only 1.9 ps long electrical pulses. By using both optical detection of the electrical pulse and numerical modelling of the transmission line circuit we are able to estimate the switching energy per unit area which is exceptionally small, only 9.4 fJ/μm2. Next, we examine the non-equilibrium response of the CDW in 2H-NbSe2 to the optical excitation with a transient reflectivity experiment. Although no long-lived metastable states are observed in the 2H polytype, we are able to perform nanoscale polytype transformation from the 2H to the 1T polytype with the scanning tunnelling microscope tip. In 1T-NbSe2 we observe rich physics of CDW domains and domain walls and a diverse electronic structure. Finally, we also create a nanoscale polytype transformation in 4Hb-TaSe2 where we again observe domains and domain walls of the CDW state.20232023-12-02 08:15:03Doktorsko delo/naloga152648sl