Metastable states in two-dimensional charge density wave materials

Venturini, Rok (author), Mihailović, Dragan (mentor)

A 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.

• Language: English
• Keywords: charge density wave, transition metal dichalcogenides, 1T-TaS2, picosecond electrical pulse, 2H-NbSe2, 1T-NbSe2, 4Hb-TaSe2, memory device, electrical switching, polytype transformation, scanning tunnelling microscopy
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2023
• PID: 20.500.12556/RUL-152648
• COBISS.SI-ID: 176619779
• Publication date in RUL: 02.12.2023

Secondary language

• Language: Slovenian
• Title: Metastabilna stanja v dvodimenzionalnih materialih z valom gostote naboja

Abstract

Stanje z modulirano elektronsko gostoto - val gostote naboja (VGN) - je značilno za številne dvodimenzionalne materiale iz skupine prehodnokovinskih dihalkogenidov. Nedavno odkritje prehoda iz ravnovesnih stanj VGN v neravnovesna metastabilna stanja VGN je odprlo vrata preučevanju nove fizike in razvoju obetavnih možnosti za praktično uporabo. V prvem delu te doktorske naloge raziskujemo, kako lahko preklapljanje med stanji z visokim in nizkim električnim uporom v kristalu 1T-TaS2 uporabimo za izdelavo energetsko učinkovitega pomnilniškega elementa. Še posebej se posvetimo električnemu preklapljanju kristala 1T-TaS2 s pikosekundnimi električnimi sunki, za kar izdelamo ustrezno optoelektronsko vezje s prenosno linijo. Če v vezje integriramo napravo 1T-TaS2, lahko dosežemo učinkovit preklop vrednosti upora z le 1.9 ps dolgim električnim sunkom. Z optično detekcijo električnega sunka in numeričnim modeliranjem prenosne linije lahko ocenimo energijo preklopa na površino naprave, ki je izjemno majhna in je le 9.4 fJ/μm2. V naslednjem delu na femtosekundni časovni skali z uporabo ultrahitre časovno ločljive optične spektroskopije preučujemo odziv VGN v 2H-NbSe2. Čeprav v politipu 2H ne opazimo dolgoživih metastabilnih stanj, lahko s konico tunelskega mikroskopa izvedemo transformacijo iz politipa 2H v politip 1T na velikostni skali nanometra. V 1T-NbSe2 opazimo bogato fiziko VGN z domenami in domenskimi stenami ter raznoliko elektronsko strukturo. Nazadnje ustvarimo tudi nanometrsko transformacijo politipa v 4Hb-TaSe2, kjer ponovno opazimo domene in domenske stene stanja VGN.

• Keywords: val gostote naboja, prehodnokovinski dihalkogenidi, 1T-TaS2, pikosekundni električni sunek, 2H-NbSe2, 1T-NbSe2, 4Hb-TaSe2, pomnilniški element, električni preklop, politipna transformacija, tunelski mikroskop