Effects of superstructuring on optical and transport properties of selected layered materials

doctoral thesis

• Title (croatian): Utjecaj superstrukturiranja na optička i transportna svojstva odabranih slojevitih materijala
• Author: Kristijan Velebit
• Mentor: Ana Smontara (mentor)
• Committee member: Mario Basletić (predsjednik povjerenstva)
• Committee member: Ana Smontara (član povjerenstva)
• Committee member: Laszlo Forro (član povjerenstva)
• Committee member: Neven Barišić (član povjerenstva)
• Committee member: Eduard Tutiš (član povjerenstva)
• Granter: University of Zagreb; Faculty of Science; (Department of Physics); Zagreb
• Defense date and country: 2015-10-29, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Physics
• Universal decimal classification (UDC): 53 - Physics

Abstract

Very special electronic phases brought upon by the superstructuring in layered materials have held the interest of researchers for decades. This thesis deals with the effects of the lattice superstructuring on optical and transport properties of pure and doped samples of 1T-TaS2 and 1T-TiSe2. This work brings the first ever analysis of the optical properties of the nearly-commensurate charge density wave (NCCDW) phase of 1T-TaS2 at temperatures below 200 K. Optical response of the NCCDW phase is for the first time analyzed with the nano-composite-like nature of the phase in mind. Modeling of the optical response using the Bruggeman effective medium theory is suggested and demonstrated. As a result, localized surface plasmon feature is identified in the optical spectrum, as well as the asymmetry of the optical phonon modes brought upon by the coupling of spatially distinct nano-sized domains which comprise the material. A novel analysis of the superstructure-related phonon peaks which allows the determination of the charge redistribution over the star-like superstructure is presented. Lastly, this work also presents the study of the 1T-TiSe2 compound, where the analysis is focused on the properties of the high temperature phase of the material. The contributions of electron and hole pockets are resolved for the first time, as well as the spectral weights and the scattering rates for each channel. Calculated energy ranges of the quasi-2D hole band settle the ongoing debate and determine that 1T-TiSe2 in the high-temperature phase is a semimetal, with scattering which gets stronger over a wide interval as the temperature approaches the phase transition.

Abstract (croatian)

Vrlo posebne elektronske faze koje nastaju superstrukturiranjem slojastih materijala već desetljećima zaokupljaju pažnju istraživača. Ovaj doktorski rad proučava efekte superstrukturiranja kristalne rešetke na optička i transportna svojstva čistih i dopiranih uzoraka 1T-TaS2 i 1T-TiSe2. Rad donosi prvu analizu optičkih svojstava faze približno sumjerljivog vala gustoće naboja (engl. NCCDW) 1T-TaS2 na temperaturama ispod 200 K. Po prvi put je optički odziv NCCDW faze analiziran uzimajući u obzir njenu nanokompozitnu prirodu, te je predloženo i provedeno modeliranje optičkog odziva pomoću Bruggemanove teorije efektivnog medija. Kao rezultat, u optičkom spektru je identificiran učinak lokalnog površinskog plazmona, kao i asimetrija fononskih modova do koje dolazi zbog vezanja optičkog odziva prostorno razdvojenih nano-domena. Nadalje, predložena je nova analiza intenziteta super-strukturnih fononskih vrhova koja omogućava određivanje redistribucije naboja u zvjezdastoj superstrukturi. Napose, ovaj rad predstavlja i istraživanje 1T-TiSe2, gdje je analiza usredotočena na svojstva visokotemperaturne faze materijala. Po prvi put su razlučeni doprinosi elektronskih i šupljinskih pobuđenja, te njihova spektralna težina i jačina raspršenja. Proračuni energijskog raspona kvazi-dvodimenzionalne šupljinske vrpce omogućuju zaključenje dugotrajne debate u korist zaključka da je 1T-TiSe2 u visokotemperaturnoj fazi “polumetal” (engl. semimetal), s raspršenjem koje jača preko vrlo širokog intervala u kojem se temperatura približava točki prijelaza.

• Keywords: layered materials; transition metal dichalcogenides; superstructuring; charge density wave; effective medium theory; localized surface plasmon; asymmetric phonon modes; charge redistribution; optical conductivity; infrared spectroscopy
• Keywords (croatian): slojeviti materijali; dihalkogenidi prijelaznih metala; superstrukturiranje; val gustoće naboja; teorija efektivnog medija; lokalizirani površinski plazmon; asimetrični fononski modovi; preraspodjela naboja; optička vodljivost; infracrvena spektroskopija
• Language: english
• URN:NBN: urn:nbn:hr:217:613872
• Study programme: Title: Physics; Study programme type: university; Study level: postgraduate; Academic / professional title: doktor/doktorica znanosti, područje prirodnih znanosti, polje fizika (doktor/doktorica znanosti, područje prirodnih znanosti, polje fizika)
• Type of resource: Text
• Extent: 151 str.
• File origin: Born digital
• Access conditions: Open access
• Created on: 2017-08-22 09:59:23