Studies of ultrafast processes in correlated materials using scanning tunnelling microscope

Ravnik, Jan (author), Mihailović, Dragan (mentor)

This thesis is focused on combining the atomically resolved microscopic picture with the ultrafast sub picosecond picture of laser induced phase transitions in 1T-TaS2. The combination of scanning tunneling microscopy (STM) and pump-probe measurements, gives insight not only into the microscopic ordering and topological nature of different laser induced states in 1T-TaS2, but also into the conditions at which they are formed and the timescales at which they appear. The layered transition metal dichalcogenide 1T-TaS2 has undergone a recent resurgence of interest with the discovery of a stable hidden state, which can be induced by an ultrashort laser or electrical pulse. Since the transition is fast, the material became a very promising candidate for future cryogenic memory elements. I performed multiple excitation intensity dependent switching experiments at different temperatures in order to construct a phase diagram of photoinduced states in 1T-TaS2. It turns out that optical switching to the hidden state is very robust, while on the other hand switching to the other observed states cannot be controllably reproduced using an 800 nm laser. The threshold fluence for switching to the hidden state appears to be temperature independent, suggesting that only the density of the photoinduced carriers is important for the transition. Other photoinduced states such as the amorphous state and states with different chiralities seem to appear more randomly. At high laser powers, I discovered a novel kind of laser induced transformation that is a single layer polytype transformation from 1T to 1H polytype, where local heating and fast quench play a crucial role in formation of the new state. The polytype transformed layer shows a phase transition between the stripe phase and a network of hexagonal vertices, as seen with STM, which show interesting topological nature. I also examined the domain walls between the domains of different chiralities in 1T-TaS2, which appear to have a non-trivial structure. In order to combine the ultrafast measurements with atomically resolved STM scans, I built a prototype THz gated STM. I built and characterized several THz single cycle pulse sources and used them to excite the STM tip. I observed up to 20 pA of rectified THz current, which is expected enough to observe changes in time resolved measurements. The device was not finalized during my research work, but I am nevertheless showing the interesting and helpful experimental observations in order to make the future optimization of the THz resolved STM easier.

• Language: English
• Keywords: scanning tunneling microscopy, photoinduced phase transition, hidden state, polytype transformation, 1T-TaS2
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2019
• PID: 20.500.12556/RUL-108715
• COBISS.SI-ID: 3335780
• Publication date in RUL: 14.07.2019

Secondary language

• Language: Slovenian
• Title: Raziskave ultrahitrih procesov v koreliranih snoveh s tunelskim vrstičnim mikroskopom

Abstract

Doktorsko delo je posvečeno povezavi med atomsko ločljivimi mikrokopskimi slikami in ultrahitrimi pod pikosekundnimi meritvami lasersko vzbujenih faznih prehodov v 1T-TaS2. Kombinacija vrstične tunelske mikroskopije (STM) in vzbuditveno testne tehnike nam da vpogled ne le v mikroskopsko sliko urejanja in topološko naravo različnih lasersko vzbujenih stanj, temveč tudi v pogoje pod katerimi se ta stanja pojavijo in v časovno skalo na kateri se oblikujejo. 1T-TaS2 je plastoviti dihalkogenid prehodnih kovin, ki je vzbudil veliko zanimanje z odkritjem stabilnega skritega stanja, ki ga lahko vzbudimo z ultrahitrim laserskim ali električnim sunkom. Ker je prehod hiter, je 1T-TaS2 postal zelo dober kandidat za ultrahitre nizkotemperaturne spominske elemente prihodnosti. Izvedel sem več meritev, kjer sem spreminjal moč in fluenco laserskega snopa pri različnih temperaturah z namenom sestaviti fazni diagram fotoinduciranih stanj v 1T-TaS2. Izkaže se, da je optični preklop v skrito stanje zelo robusten in da je mejna fluenca za preklop enaka pri vseh temperaturah. Po drugi strani pa je preklop med ostalimi stanji težko ponovljiv pri uporabi laserja z valovno dolžino 800 nm. Tako se amorfno stanje in stanja z domenami različnih kiralnosti pri preklapljanju pojavljajo naključno. Pri uporabi visokih moči laserja sem opazil novo vrsto laserske transformacije v kristalu, in sicer gre za politipno transformacijo vrhnje plasti materiala iz 1T v 1H politip, kjer ključno vlogo igra lokalno segrevanje in hitro ohlajanje. Na transformirani površini lahko opazujemo fazni prehod med fazama s črtami in trikotno mrežo, ki kažeta zanimive topološke lastnosti. Podrobneje sem preučil tudi domenske stene med domenami različne kiralnosti, ki nimajo trivialne strukture. Z namenom združiti ultrahitre meritve z atomsko ločljivimi STM slikami sem sestavil teraherčno (THz) prožen STM. Sestavil in karakteriziral sem nekaj virov ultrahitrih teraherčnih sunkov in jih uporabil za vzbujanje konice tunelskega mikroskopa. Opazil sem do 20 pA usmerjenega THz toka, kar je dovolj za opazovanje sprememb v časovno ločljivih meritvah. Naprava v času mojega dela ni bila popolnoma dokončana. Izsledki so kljub temu predstavljeni kot pomoč pri nadaljnji izgradnji in optimizaciji naprave.

• Keywords: vrstična tunelska mikroskopija, fotovzbujeni fazni prehodi, skrito stanje, politipna transformacija, 1T-TaS2