Studies of non-equilibrium phenomena using scanning tunneling microscopy

Vaskivskyi, Yevhenii (author), Mihailović, Dragan (mentor)

This thesis reports on the electron dynamics in the metastable states of 1T-TaS$_2$, which is studied using scanning tunnelling microscopy, time-resolved techniques and ultrashort laser pulses. Such a combination allows for creating fine-tuned microscopic electronic structures in the material and studying their time evolution. 1T-TaS$_2$ has been a widely-studied material in the last decade due to its promising metastable metallic hidden state, which has a potential for applications in ultrafast and efficient low-temperature electronics. This state can be reached with a single femtosecond laser pulse or a picosecond electrical pulse. At the same time, the size of the created structures and the microscopic processes, which occur on different time scales, remains an open question. The possibility of creating other metastable states in the material, stable at higher temperatures, such as the amorphous electronic order, opens even more opportunities. These states present a good playground for studying the properties and interplay between electronic orders in strongly correlated materials. This offers a unique insight into electron dynamics by using a single prototype material as an experimental model. This work reveals the collective dynamics of the electronic structure and single electron motion on the material surface at previously unexpected time scales by using novel experimental techniques such as fast scanning tunnelling microscopy. The applied systematic approach allowed for a careful definition of the phase diagram of the states which were considered random or were not precisely defined before. These states include the electronic amorphous and the chiral hidden orders. In addition, the relaxation between different states was studied in detail. These studies deepen our understanding of the processes and properties of 1T-TaS$_2$. Moreover, the proposed experimental techniques have the potential to be used in future to study a broad range of materials and systems, by providing both spatial and temporal resolution which was not accessible before.

• Language: English
• Keywords: scanning tunnelling microscopy, photoinduced phase transitions, metastable states, single electron dynamics, strongly correlated materials, 1T-TaS$_2$, electronic amorphous state, charge density waves
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2024
• PID: 20.500.12556/RUL-161879
• COBISS.SI-ID: 209894915
• Publication date in RUL: 15.09.2024

Secondary language

• Language: Slovenian
• Title: Raziskave neravnovesnih pojavov z vrstično tunelsko mikroskopijo

Abstract

Doktorsko delo poroča o dinamiki elektronov v metastabilnih stanjih 1T-TaS$_2$, ki jo raziskujemo z uporabo metode vrstične tunelske mikroskopije, časovno-ločljivih tehnik in ultra-hitrih laserskih pulzov. Taka kombinacija omogoča ustvarjanje mikroskopskih elektronskih struktur in študij njihove časovne evolucije. V zadnjem desetletju, 1T-TaS$_2$ je bil široko raziskovan zaradi svojega metastabilnega kovinskega skritega stanja, ki ima potencial za uporabo v ultrahitri in energijsko učinkoviti elektroniki pri nizkih temperaturah. Z možnostjo preklopa v to stanje s samo enim femtosekundnim laserskim pulzom ali pikosekundnim električnim pulzom, glavno vprašanje ostaja o velikosti ustvarjenih struktur in mikroskopskih procesih v njih na različnih časovnih skalah. Možnost ustvarjanja drugih metastabilnih stanj v materialu, stabilnih pri višjih temperaturah, kot je elektronsko amorfno stanje, odpira še več možnosti. Velika raznolikost stanj, induciranih pri neravnovesnih pogojih, omogoča študij procesov in lastnosti, opaženih v drugih močno koreliranih materialih, tako v ravnovesju kot izven njega. Na tak način, lahko uporabimo prototipni material kot eksperimentalni model za raziskave dinamike urejanja elektronov. Z uporabo novih tehnik tunelske mikroskopije v kombinaciji z lasersko vzbuditvijo, doktorsko delo prikazuje kolektivno dinamiko elektronske strukture in premikanje posameznih elektronov na površini materiala na doslej nepričakovanih časovnih lestvicah. Sistematičen pristop uporabljen v delu je omogočil natančno opredelitev faznega diagrama prehodov v stanja, kjer se je elektronska urejenost v preteklosti štela za naključno, ter tudi relaksacijo med različnimi stanji. Ta stanja vključujejo elektronsko amorfno in kiralno skrita stanja. Te ugotovitve so razširile naše razumevanje procesov in lastnosti preučevanega sistema. Poleg tega predlagane eksperimentalne metode imajo potencial za uporabo v prihodnih študijah širokega spektra materialov in sistemov, saj omogočajo doslej nedosegljivo prostorsko in časovno ločljivost pri študiju neravnovesnih pojavov.

• Keywords: vrstična tunelska mikroskopija, fotoinducirani fazni prehodi, metastabilna stanja, dinamika posameznih elektronov, močno korelirani materiali, 1T-TaS2, elektronsko amorfno stanje, valovi gostote naboja